The mineralogy geochemistry and genesis of the alluvialplatinum-group minerals of the Freetown Layered ComplexSierra Leone

JOHN F W BOWLES1 SAIOA SUAacuteREZ23 HAZEL M PRICHARD2dagger AND PETER C FISHER2

1 School of Earth and Environmental Sciences University of Manchester Manchester M13 9PL UK2 School of Earth and Ocean Sciences Main College Park Place Cardiff University Cardiff CF10 3AT Wales3 Department of Mineralogy and Petrology UPVEHU 48940 Leioa and Ikerbasque 48011 Bilbao Spain

[Received 8 January 2017 Accepted 13 May 2017 Associate Editor Nigel Cook]

ABSTRACT

Heavy mineral concentrates from rivers and river terraces near York Freetown Peninsula Sierra Leone havebeen examined for their platinum-groupmineral (PGM) content The alluvial PGM are 01 to 15 mm in sizeand include Cu-bearing isoferroplatinum (Pt3Fe) and disordered Pt3ndashxFe (x le 038) tulameenite (Pt2FeCu)hongshiite (PtCu) cooperitendashvysotskite (PtSndashPdS) laurite (RuS2) erlichmanite (OsS2) Os-Ir alloy Os-Rualloy and native copper

Are the alluvial nuggets primary or a neoformation Comparison of the PGMmineralogy of fresh rocksweathered rocks and the saprolite with the alluvial suite shows strongly contrasting features highlighted bythe mineral assemblage Cooperite in the fresh rocks is rare in the alluvium whilst Pt-Fe alloys become moreabundant Oxidized PGM are a feature only of the weathering process and disordering of the Pt-Fe alloysdevelops during weathering Palladium is much less abundant in the alluvial suite than in the primaryminerals whereas Cu present as Cu-sulfides in the fresh rocks occurs in the alluvium as a minor componentof the Pt-Fe alloys and as hongshiite alteration to the Pt-Fe alloys The size difference is striking the primarymineralogy is micrometre-sized whereas the alluvial PGM are three orders of magnitude larger DelicatePGMwith alteration textures are seen only in theweathered rocks whilst delicate dendritic PGM are reportedonly from the alluvial suite An organic coating to the alluvial PGM may be indicative of an organic orbacterial involvement Some alluvial PGM occur in a drainage basin devoid of outcrops of PGE-bearinghorizons

Together these contrasting features of the primary and placer PGM support the proposal that the Freetownnuggets developed as a result of breakdown of the primary PGM during weathering movement of the PGEin solution and growth of new PGM in placers with a different mineral assemblage mineralogy and mineralchemistry

KEYWORDS platinum-group minerals Freetown Intrusion Sierra Leone alluvial nuggets

Introduction

ALLUVIAL platinum-group minerals (PGM) in thewestern hills of the Freetown Peninsula SierraLeone West Africa were first found in 1926(Junner 1929 1930 Pollet 1931 1951)

copy The Mineralogical Society 2018 This is an Open Access article distributed under the terms of the Creative CommonsAttribution licence (httpcreativecommonsorglicensesby40) which permits unrestricted re-use distribution andreproduction in any medium provided the original work is properly cited

E-mail johnbowlesmanchesteracukdaggerDied 1 January 2017httpsdoiorg101180minmag2017081032

This paper is part of a special issue entitled lsquoCritical-metal mineralogy and ore genesisrsquo The AppliedMineralogy Group of the Mineralogical Society and the IMACommission on Ore Mineralogy have contributedto the costs of Open Access publication for this paper

Mineralogical Magazine May 2018 Vol 82(S1) pp S223ndashS246

Artisanal mining recovered the PGM fromsmall rivers known locally as Big Water and theAlako in the York area (Fig 1) and subsequentlyfrom Guma Water and the Babadori further to thenorth ( Pollet 1931 1951 Barber 1962 Forde2011)The PGM from the Freetown Complex have been

examined in detail from the Babadori (Pollet 1951)

and GumaWater areas (Pollet 1931 Bowles 19811986 1988 2000a Bowles et al 1983 Hattoriet al 1991 1996 Cabri et al 1996) The alluvialPGM consist mainly of Pt-Fe alloys Euhedralcrystals of Os-Ir alloys and lauritendasherlichmanite(RuS2ndashOsS2) can be set in the Pt-Fe alloys or occurindependently Minor proportions of tulameenite(Pt2CuFe) are also present Sulfides of platinum-

FIG 1 Map of the York area Freetown Peninsula Sierra Leone showing the known occurrences of the primary PGE-bearing layers Horizon B Horizon C and Horizon D the main lateritized surfaces and the sampling sites in the Alako

and Big Water rivers either side of Mateki Ridge

S224

J F W BOWLES ETAL

group elements (PGE) and tellurides as well as Cu-sulfides have been reported as inclusions within thePt-Fe nuggets The nuggets have a coarse grain sizeusually in the range 05 to 2 mm but considerablylarger nuggets (1ndash2 cm) have been reportedincluding dendritic crystalline growths (Pollet1951)The processes that result in the presence of the

PGM in placer deposits have been the subject ofdebate Two contrasting models have been pro-posed for the formation of PGM and the most likelysituation is that both processes contribute depend-ing upon local conditions The first processinvolves primary formation of large PGE grainswithin an intrusion followed by weathering wherePGM are liberated and transported mechanicallywith no alteration apart from surface abrasion of thenuggets (Cabri et al 1996 Weiser 2002 andreferences therein) This process undoubtedlyoccurs where large PGM are present in the hostrocks and the weathering is purely mechanical as intemperate cold and polar climates The majorproblems with this proposal are the contrast in theprimary and the alluvial mineral assemblagesmineral chemistry as well as the three orders ofmagnitude difference in the size of the PGM placernuggets compared with those found in the igneousrock Several authors (eg Oberthuumlr et al 2003)have been able to find rare large PGM within thehost rocks and have shown that the size differencemight just be a question of appropriate sampling Todate no indication of any occurrence of large PGMhas been encountered in the Freetown rocks despitelithogeochemical and geomagnetic surveys of thearea Micrometre-sized PGM have been located inthe rocks (Bowles 2000b Bowles et al 20132017) but it is the differences between the mineralassemblages in the primary and placer occurrenceswhich are most significant (Bowles 1981 Bowleset al 2000a 2013 Oberthuumlr et al 2013)The second model involves solution of PGE

during weathering possibly aided by biochemicalleaching and high Eh and low pH conditions Thisis followed by transport of the PGE in solutionleading to supergene growth of nuggets withinlateritic soils where conditions such as lower Ehand neutral pH favour deposition (Fuchs and Rose1974 Bowles 1986 1988 Bowles et al 1994a2000 Talovina and Lazarenkov 2001 Aiglspergeret al 2015 2016) The problem with this model isthat the mechanisms of this process are largelyunknown and the evidence circumstantial There isclear evidence of alteration and weathering of theprimary PGM leading to transport of PGE in

solution (Bowles et al 2017) The Fe content ofthe altered product remains within the rangeencountered for the Pt-Fe alloys The change incomposition associated with the alteration of Pt-Fealloy is principally a loss of Pt and a gain of O orOH Bowles (1986) argued that the Eh and pH ofiron-rich lateritized soils are sufficiently extreme toallow the formation of Pt-oxide but Hattori et al(2010) have found that alteration in New Caledoniahas produced a very fine-grained mixture of relictPt-Fe alloy and Fe-oxides or hydroxides We havenot yet examined the Freetown material in thisdetail so this paper uses lsquooxidized PGMrsquo todescribe the alteration product How can neoformgrowth lead to the formation of large nuggets Thecontribution by Reith et al (2016) offers amechanism for bacterially-aided mineral growthThe nuggets are normally single crystals with nosign in polished section of the remains of grainboundaries that could suggest mechanical orchemical lsquoagglutinationrsquo The explanation fornugget formation must include early nuclei to bea preferred site for continued growthOccurrences of eluvial and alluvial PGM that

exhibit a contrasting mineralogy to their sourcerocks have been reported especially in areas of longduration intense weathering such as tropical weath-ering of ancient cratons Examples include placerdeposits derived from ophiolitic chromitites inBorneo (Burgath 1988) serpentinite lateritic coat-ings in Cameroon (Ndjigui and Bilong 2010)lateritic soils covering the deeply weathered Yubdoultramafic intrusion Ethiopia (Belete et al 20002002) the Uktus Complex in Central Urals Russia(Talovina and Lazarenkov 2001 Zaccarini et al2013) supergene leaching leading to neoformationin Minas Gerais Brazil (Cabral et al 2007 2008)and PGM neoformation in Ni-laterites DominicanRepublic (Aiglsperger et al 2015 2016 2017)Evidence from the rivers draining The Great Dykein Zimbabwe (Oberthuumlr et al 2013) provides aview of the process from start to finish Here PGE-bismuthotellurides and sulfarsenides found in thepristine sulfide-bearing zone and oxidized PGM inthe oxide zone (Oberthuumlr et al 2003) are absentfrom the alluvium Pt-Fe alloys occur instead themajority of which are considered to beneoformationsIt is relevant that the Freetown intrusion has

undergone a very long period of weathering Wave-cut platforms indicate erosion and weathering havebeen active since the Eocene (Gregory 1962) Inaddition the peninsula is exposed to very heavyrainfall which is currently 3000ndash5000 mmyear

S225

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

(Hughes and Hughes 1992) The source of theFreetown placer PGM was obscure until recentlyso comparison of the source the eluvial depositsand the placers was not possible Bowles (2000b)and Bowles et al (2013) defined four stratiformlayers of magnetite-bearing gabbros with PGEmineralization (up to 069 gt Pt) and Cu-richsulfides describing a primary source of theFreetown PGM and identifying primary PGM forthe first time The host rocks are fresh with coarse-grained olivine plagioclase clinopyroxene ortho-pyroxene and later interstitial amphibole (Bowleset al 2013) There has been re-equilibration of theFe-Ti-oxides during cooling but no indication ofsignificant metamorphism or serpentinization ThePGM occur in the interstices between the mainsilicate minerals and within the late amphibole ThePGM in the host rocks are considered here to belsquoprimary PGMrsquo the PGM that have been altered byweathering are described as lsquoweathered PGMrsquo andthe different alluvial suite are called lsquoneoformPGMrsquoBowles et al (2017) investigated the sequence of

mineralogical changes undergone by the primaryPGE mineralization during in situ weathering Thepresent contribution uses results from a systematicsurvey of the alluvial PGM from the river terracesand the beds of the Alako and the BigWater (Fig 1)that drain PGE-bearing horizons to compare thePGM in the primary andweathered rocks with thosein the alluvial assemblage to assess the origin of thesecondary PGM

Platinum from the Freetown Peninsula

In 1926 Pt-bearing grains were found in thewestward draining streams of the complex and by1949 5242 oz of platinum concentrate had beenrecovered Analysis of the concentrate showedPt 87 Pd 2 Os + Ir 13 and Fe 99 wt (Pollett1931) The extraction used artisanal methods withsluice boxes and local labour panning by hand It isworth quoting the description of the alluvialoccurrences from Pollett (1931) ldquoPlatinum in thenative state occurs in the gravels of streams drainingthe hills forming the peninsula of Sierra Leone To alimited extent it has also been found in beach gravelsin certain coast localities The most promisingdeposits are in the vicinity of York more specific-ally in the streams flowing into Whale River on itsnorthern banks Here the metal occurs as smallnuggets pitted and water worn grains delicatedendritic crystalline growths and dust Occasionally

cubic crystals are found Many of the small nuggetsand water-worn grains have a peculiar brass colourThe largest nugget yet recoveredweighed 1212 dwtand numerous nuggets ranging between 5 and1012 dwt have been foundrdquo [1 dwt or onepennyweight is 20 g so the weights mentionedrange from 10 to 250 g] ldquoThese larger nuggets areinvariably well water worn An interesting feature isthe presence of practically non water worn delicatedendritic crystalline growths of platinum in gravelsthat also contain well-rounded grains of the metalThis dendritic non water worn form however israrely seen in fragments larger than 12 dwt Anothernoticeable feature is the small percentage ofplatinum dustrdquoThe platinum described as being in the native state

is now known to be platinum-iron alloy occurring asPtFe and Pt3Fe (Bowles 1981) The density ofisoferroplatinum (Pt3Fe) is 182 gcc and that oftetraferroplatinum (PtFe) is 152 gcc For thepurposes of an illustrative calculation an intermedi-ate value of 16 gcc can be chosen so the lsquolargestnugget yet recoveredrsquo (1212 dwt) could berepresented by a cube with an edge of 25 cm Thenumerous nuggets of 5 to 1012 dwt would have hadavolume corresponding to cubeswith edges of 18 to24 cm and the dendritic growths reached a volumeequivalent to a cube with an edge of 085 cmExperiments with nuggets from the present study

have shown that the lsquobrass colourrsquo of the PGM is acoating of Fe- and Al-oxides and organic matterthat can be removed by washing in warm 6M HClfor 1 to 2 days and then reacting with 30H2O2 forabout one month The coating hides the otherwisesilvery colour of the nuggets making them difficultto locate in the sediments This coating appearscomparable with the biofilms identified by Reithet al (2016)The description of the morphology of the PGM

by Pollett (1931) is comparable with the PGM fromGuma Water Freetown described by Bowles(1981) The lsquosmall percentage of platinum dustrsquomay be a result of panning and hand picking of thegrains in the presence of a large amount of coarseblack sand so that fine-grained PGM would nothave been recoveredPollett (1951) suggested that artisanal mining

had ceased because of a low platinum price and itwas unclear if economic PGM concentrationsremained Therefore exploratory surveys of thealluvium were undertaken by panning in thestreams and by pitting of the river terraces toassess the present situation and also to aid thesearch for the hard-rock source

S226

J F W BOWLES ETAL

Methods

Alluvial sampling and analysis

The alluvial samples consist of black sandconcentrates collected from stream beds and pitsin the river terraces (Tables 1 and 2 Fig 1) Gravelfrom the beds of the Alako (sample A1) and BigWater (sample BW2) was panned by a team of localworkers At each site a volume of gravel corre-sponding to 30 lsquostandard head pansrsquowas reduced toblack sand The head pan is a shallow wooden dishsim70 cm in diameter that the local workers use forpanning (and for carrying goods on the head ndashhence the name) The volume of a head pan wasmeasured by filling it with water and weighing Theresult indicated a volume of 1168 litres althoughthat represents the pan filled to the brim whereas apan filled with gravel was often piled above thebrim For the purposes of estimation of a grade inthe source material a pan volume of 12 litres hasbeen used During panning of the streams rocks andboulders were avoided so the grade calculated inthis way is an overestimate of a grade that would beachieved by mechanical extraction The black sandfrom the rivers was split using a riffle splitter and asplit assayed for Pt Pd and Au by LakefieldResearch in CanadaThe river terraces either side of Big Water were

examined by pitting Square pits 15 m on eachside were dug and all of the material panned (PitsBW6 ndash BW13 only pits reporting significant Ptvalues are discussed here) The depth of the pitswas variable and constrained by reaching bedrockor rapid ingress of water through gravel below thewater table A lsquo4 inchrsquo (100 mm diameter input)pump only allowed limited progress below thewater table and an auger was used for deepersamples Each interval of 05 m in depth in the pitwas treated as a separate sample and panned toblack sand Each sample was dried on site andsubsequently subsampled using a riffle splitterbefore assay only for Pt by Lakefield Research ofCanada High Pt assay results were checked byrepeat assays

Recovery and study of the PGM

The black sand concentrates (60ndash500 g of eachsample) were panned again in the laboratory torecover the densest fraction and the PGM nuggetsfound during the process were hand picked Theremaining lightest fraction was processed with aFrantz LB-1 Magnetic Barrier Separator fitted with

a Low Field Control Unit which allowed asatisfactory separation of the most ferromagneticand diamagnetic fractions The majority of thealluvial nuggets were hand picked from the denseand the less magnetic fractions using a stereoscopicmicroscope All the PGM recovered from thesamples were mounted separately on stubs forexamination of their morphology and subse-quently in polished blocks for analysis using aZeiss SMT S360 analytical scanning electronmicroscope (SEM) with an Inca EDX system atCardiff University under the same analyticalconditions described by Bowles et al (2017) Theremainder of the densest fraction of each samplewas also examined by SEM to search for grainsoverlooked during hand picking

Assay results and their relationship todrainage

Assays of the samples described here are shown inTable 1 (panning in the rivers) and Table 2 (pittingthe river terraces) Repeated assay values show asignificant variation due to the lsquonugget effectrsquo Theassay values are reported as a proportion of theblack sand and they have been converted to lsquobankgradersquo to estimate the proportion of Pt in thesediment The results all relate to small volumes ofgravel and do not suggest an economic potentialPitting in the level river terrace of the east bank of

the BigWater (pits BW6 and BW8) revealed graveland lateritized soils to a depth of sim2 m overlyingweathered bedrock On the west bank soil overliesgravel in pit BW13 Soil and weathered bedrock arefound between the patches of gravel and theboulders in pit BW11 These pits are at the footof a steep hillside below the ridge on which thePGM-bearing horizons have been reported(Bowles 2000b Bowles et al 2013) and therehas clearly been some slumpingPlatinum concentrations appear in the upper 05 m

of sediment (up to 135 gt Pt in black sand) at thebottomof some pits inweathered bedrock (up to 583gt Pt in black sand) and in intermediate horizons(up to 395 gt in black sand) with no consistentpattern (Table 2) As a Pt concentration in thesediment the highest values (up to 1176 mg Ptm3)occur close to bedrock but the near surface sedimentcan contain up to 48 mg Ptm3 The Pt assays fromthe gravel in the Alako and the Big Water are similarand in the same range as the Big Water terraces(Table 1) The Pd and Au assay results are lowresulting in very high PtPd and PtAu ratios

S227

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The Alako and the Big Water drain entirelydifferent sequences of the layered rocks yet thealluvial PGM are similar The Alako runs for 2 kmalong the west side of Mateki Ridge in a small well

defined watershed that does not include knownPGM-bearing horizons Drilling in this area(Barber 1962) was not followed up There is noevidence of river capture or change of course that

TABLE 2 Results from pitting in the Big Water river terraces

Samplenumber

Depth(m) Lithology

Weight ofblack sand

(kg) Pt (gt)

Repeat assaysBank grade(mg Ptm3)Pt (gt) Pt (gt) Pt (gt)

Pits on the east bankBW 6A 0ndash05 10 cm soil + fine gravel 191 075 13BW 6B 05ndash10 Fine gravel 315 063 18BW 6C 10ndash15 Gravel with boulders 703 nd 0BW 6D 15ndash20 Gravel with big boulders at base 597 157 83BW 6E 20ndash22 Orange-red weathered bedrock 154 5830 1040 1176BW 8A 0ndash05 Dark brown soil 048 964 1630 1720 2590 75BW 8B 05ndash10 Red to orange brown soil 155 072 10BW 8C 10ndash15 Red to orange brown soil 224 035 07BW 8D 15ndash20 White clay amp gravel boulders

in pit wall 20 cm gravel atbase

242 021 05

BW 8E 20ndash215 White clay 037 286 31Pits on the west bankBW 9A 0ndash05 10 cm soil + orange soil below 059 5120 1030 769 1350 480BW 9B 05ndash10 Kharki-brown soil 043 051 02BW 9C 10ndash15 Auger sample below water

table030 011 003

BW 11A 0ndash05 Brown soil 081 015 01BW 11B 05ndash10 Orange soil with some boulders 121 3950 195 223BW 11C 10ndash15 Orange soil many big boulders 137 029 04BW 13A 0ndash05 20 cm soil gravel + boulders

below382 267 91

BW 13B 05ndash10 Gravel + abundant boulders 480 1930 823

The samples studied are shown in bold type

TABLE 1 Panning of river gravels using a lsquoheadpanrsquo

Sample numberWeight of

black sand (kg)

Assay results Bank grademg (Pt+Pd+Au)

per m3Pt(gt) Pd (gt) Au (gt) PtPd PtAu

AlakoA 1 220 2950 nd 008 gt1475 369 130

Big WaterBW 2 1220 1490 018 003 83 496 370

nd ndash less than detection limit (002 gt)The bank grade is an estimate of the in situ grade calculated using a lsquoheadpanrsquo volume of 12 litres and a factor of 138 toallow for the difference between the loose gravel in the pan and the undisturbed gravel It is an overestimate as boulderswere avoided when the gravel was collected

S228

J F W BOWLES ETAL

could have brought alluvial PGM from elsewhereThe west slope of Mateki Ridge above the Alako isthe dip slope of the layers above the PGM-bearinghorizons B C and D that outcrop on the east slopeof Mateki ridge There are no tributaries or dryvalleys on the west side of the ridge indicatingsufficient erosion to expose the PGE-enrichedlayers Therefore simple erosion of the PGE-enriched layers should only release PGM into theBig Water and cannot explain the occurrence ofPGM in the AlakoBig Water rises 5 km above the sampling points

(Fig 1) and flows along the eastern side of MatekiRidge with several dry valleys cutting into theridge The main stream comes from the eastcrossing the strike of the layered rocks including

the PGM-bearing HorizonMwhich has an As- Sb-PGE-bearing mineralogy (Bowles et al 2013)There could be some PGM input fromHorizonM atthe sampling sites but the nuggets do not showsignificant mineralogical differences between westbank stream bed and east bank (Table 3) and thereis no sign of As- or Sb-bearing phases It seemslikely that the majority of the PGM in BigWater arederived from the PGE-bearing horizons that followMateki Ridge

Mineralogy of the black sands

The black sands are mainly composed of magnet-ite ilmenite and hematite grains about 200 to

TABLE 3 Alluvial PGM in the Big Water and Alako

Big WaterAlakoRiver

West bank terraceRiver

East bank terrace

Sample BW13B BW9A BW2C BW8A BW6E Alako1 TotalsPit Pit Panned Pit Pit Panned

NuggetsNumber 1 5 13 1 12 4 36Size (microm) 263times318 80ndash1540 134ndash635 249times228 119ndash822 167ndash1030Shape

Euhedral 1 1 1 1 111Subrounded 1 2 3 1 194Rounded 2 2 111Elongate 2 3 3 1 250Irregular 2 5 1 3 1 333

Composition Pt3Fe 2 11 5 3 583Pt3ndashxFe (x = 016ndash038) 3 2 6 306Pt2FeCu 1 28(PtPd)S 1 1 56Cu0 1 28

Associated PGM No Nuggets(a) Grains(b)

OsplusmnIr-Ru alloy bull(3) bull(2) bull(1) 6Pt-Fe alloys (plates) bull(1) 4PtCu (coats plates) bull(2) bull(1) bull(3) 10RuS2 ndash OsS2 bull(2) bull(1) 4IrAsS bull(1) 1Cu(PtRhIr)2S4 bull(1) bull(1) bull(1) 3PtSndashPdS bull(1) 5Au-(PdAg) bull(1) 1Ag-(FeCu) bull(1) 1

(a)Number of nuggets on which these associated PGM and native metals were found(b)Total number of grains located (approx)Attached to ilmenite grains

S229

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

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J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

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J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

group elements (PGE) and tellurides as well as Cu-sulfides have been reported as inclusions within thePt-Fe nuggets The nuggets have a coarse grain sizeusually in the range 05 to 2 mm but considerablylarger nuggets (1ndash2 cm) have been reportedincluding dendritic crystalline growths (Pollet1951)The processes that result in the presence of the

PGM in placer deposits have been the subject ofdebate Two contrasting models have been pro-posed for the formation of PGM and the most likelysituation is that both processes contribute depend-ing upon local conditions The first processinvolves primary formation of large PGE grainswithin an intrusion followed by weathering wherePGM are liberated and transported mechanicallywith no alteration apart from surface abrasion of thenuggets (Cabri et al 1996 Weiser 2002 andreferences therein) This process undoubtedlyoccurs where large PGM are present in the hostrocks and the weathering is purely mechanical as intemperate cold and polar climates The majorproblems with this proposal are the contrast in theprimary and the alluvial mineral assemblagesmineral chemistry as well as the three orders ofmagnitude difference in the size of the PGM placernuggets compared with those found in the igneousrock Several authors (eg Oberthuumlr et al 2003)have been able to find rare large PGM within thehost rocks and have shown that the size differencemight just be a question of appropriate sampling Todate no indication of any occurrence of large PGMhas been encountered in the Freetown rocks despitelithogeochemical and geomagnetic surveys of thearea Micrometre-sized PGM have been located inthe rocks (Bowles 2000b Bowles et al 20132017) but it is the differences between the mineralassemblages in the primary and placer occurrenceswhich are most significant (Bowles 1981 Bowleset al 2000a 2013 Oberthuumlr et al 2013)The second model involves solution of PGE

during weathering possibly aided by biochemicalleaching and high Eh and low pH conditions Thisis followed by transport of the PGE in solutionleading to supergene growth of nuggets withinlateritic soils where conditions such as lower Ehand neutral pH favour deposition (Fuchs and Rose1974 Bowles 1986 1988 Bowles et al 1994a2000 Talovina and Lazarenkov 2001 Aiglspergeret al 2015 2016) The problem with this model isthat the mechanisms of this process are largelyunknown and the evidence circumstantial There isclear evidence of alteration and weathering of theprimary PGM leading to transport of PGE in

solution (Bowles et al 2017) The Fe content ofthe altered product remains within the rangeencountered for the Pt-Fe alloys The change incomposition associated with the alteration of Pt-Fealloy is principally a loss of Pt and a gain of O orOH Bowles (1986) argued that the Eh and pH ofiron-rich lateritized soils are sufficiently extreme toallow the formation of Pt-oxide but Hattori et al(2010) have found that alteration in New Caledoniahas produced a very fine-grained mixture of relictPt-Fe alloy and Fe-oxides or hydroxides We havenot yet examined the Freetown material in thisdetail so this paper uses lsquooxidized PGMrsquo todescribe the alteration product How can neoformgrowth lead to the formation of large nuggets Thecontribution by Reith et al (2016) offers amechanism for bacterially-aided mineral growthThe nuggets are normally single crystals with nosign in polished section of the remains of grainboundaries that could suggest mechanical orchemical lsquoagglutinationrsquo The explanation fornugget formation must include early nuclei to bea preferred site for continued growthOccurrences of eluvial and alluvial PGM that

exhibit a contrasting mineralogy to their sourcerocks have been reported especially in areas of longduration intense weathering such as tropical weath-ering of ancient cratons Examples include placerdeposits derived from ophiolitic chromitites inBorneo (Burgath 1988) serpentinite lateritic coat-ings in Cameroon (Ndjigui and Bilong 2010)lateritic soils covering the deeply weathered Yubdoultramafic intrusion Ethiopia (Belete et al 20002002) the Uktus Complex in Central Urals Russia(Talovina and Lazarenkov 2001 Zaccarini et al2013) supergene leaching leading to neoformationin Minas Gerais Brazil (Cabral et al 2007 2008)and PGM neoformation in Ni-laterites DominicanRepublic (Aiglsperger et al 2015 2016 2017)Evidence from the rivers draining The Great Dykein Zimbabwe (Oberthuumlr et al 2013) provides aview of the process from start to finish Here PGE-bismuthotellurides and sulfarsenides found in thepristine sulfide-bearing zone and oxidized PGM inthe oxide zone (Oberthuumlr et al 2003) are absentfrom the alluvium Pt-Fe alloys occur instead themajority of which are considered to beneoformationsIt is relevant that the Freetown intrusion has

undergone a very long period of weathering Wave-cut platforms indicate erosion and weathering havebeen active since the Eocene (Gregory 1962) Inaddition the peninsula is exposed to very heavyrainfall which is currently 3000ndash5000 mmyear

S225

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

(Hughes and Hughes 1992) The source of theFreetown placer PGM was obscure until recentlyso comparison of the source the eluvial depositsand the placers was not possible Bowles (2000b)and Bowles et al (2013) defined four stratiformlayers of magnetite-bearing gabbros with PGEmineralization (up to 069 gt Pt) and Cu-richsulfides describing a primary source of theFreetown PGM and identifying primary PGM forthe first time The host rocks are fresh with coarse-grained olivine plagioclase clinopyroxene ortho-pyroxene and later interstitial amphibole (Bowleset al 2013) There has been re-equilibration of theFe-Ti-oxides during cooling but no indication ofsignificant metamorphism or serpentinization ThePGM occur in the interstices between the mainsilicate minerals and within the late amphibole ThePGM in the host rocks are considered here to belsquoprimary PGMrsquo the PGM that have been altered byweathering are described as lsquoweathered PGMrsquo andthe different alluvial suite are called lsquoneoformPGMrsquoBowles et al (2017) investigated the sequence of

mineralogical changes undergone by the primaryPGE mineralization during in situ weathering Thepresent contribution uses results from a systematicsurvey of the alluvial PGM from the river terracesand the beds of the Alako and the BigWater (Fig 1)that drain PGE-bearing horizons to compare thePGM in the primary andweathered rocks with thosein the alluvial assemblage to assess the origin of thesecondary PGM

Platinum from the Freetown Peninsula

In 1926 Pt-bearing grains were found in thewestward draining streams of the complex and by1949 5242 oz of platinum concentrate had beenrecovered Analysis of the concentrate showedPt 87 Pd 2 Os + Ir 13 and Fe 99 wt (Pollett1931) The extraction used artisanal methods withsluice boxes and local labour panning by hand It isworth quoting the description of the alluvialoccurrences from Pollett (1931) ldquoPlatinum in thenative state occurs in the gravels of streams drainingthe hills forming the peninsula of Sierra Leone To alimited extent it has also been found in beach gravelsin certain coast localities The most promisingdeposits are in the vicinity of York more specific-ally in the streams flowing into Whale River on itsnorthern banks Here the metal occurs as smallnuggets pitted and water worn grains delicatedendritic crystalline growths and dust Occasionally

cubic crystals are found Many of the small nuggetsand water-worn grains have a peculiar brass colourThe largest nugget yet recoveredweighed 1212 dwtand numerous nuggets ranging between 5 and1012 dwt have been foundrdquo [1 dwt or onepennyweight is 20 g so the weights mentionedrange from 10 to 250 g] ldquoThese larger nuggets areinvariably well water worn An interesting feature isthe presence of practically non water worn delicatedendritic crystalline growths of platinum in gravelsthat also contain well-rounded grains of the metalThis dendritic non water worn form however israrely seen in fragments larger than 12 dwt Anothernoticeable feature is the small percentage ofplatinum dustrdquoThe platinum described as being in the native state

is now known to be platinum-iron alloy occurring asPtFe and Pt3Fe (Bowles 1981) The density ofisoferroplatinum (Pt3Fe) is 182 gcc and that oftetraferroplatinum (PtFe) is 152 gcc For thepurposes of an illustrative calculation an intermedi-ate value of 16 gcc can be chosen so the lsquolargestnugget yet recoveredrsquo (1212 dwt) could berepresented by a cube with an edge of 25 cm Thenumerous nuggets of 5 to 1012 dwt would have hadavolume corresponding to cubeswith edges of 18 to24 cm and the dendritic growths reached a volumeequivalent to a cube with an edge of 085 cmExperiments with nuggets from the present study

have shown that the lsquobrass colourrsquo of the PGM is acoating of Fe- and Al-oxides and organic matterthat can be removed by washing in warm 6M HClfor 1 to 2 days and then reacting with 30H2O2 forabout one month The coating hides the otherwisesilvery colour of the nuggets making them difficultto locate in the sediments This coating appearscomparable with the biofilms identified by Reithet al (2016)The description of the morphology of the PGM

by Pollett (1931) is comparable with the PGM fromGuma Water Freetown described by Bowles(1981) The lsquosmall percentage of platinum dustrsquomay be a result of panning and hand picking of thegrains in the presence of a large amount of coarseblack sand so that fine-grained PGM would nothave been recoveredPollett (1951) suggested that artisanal mining

had ceased because of a low platinum price and itwas unclear if economic PGM concentrationsremained Therefore exploratory surveys of thealluvium were undertaken by panning in thestreams and by pitting of the river terraces toassess the present situation and also to aid thesearch for the hard-rock source

S226

J F W BOWLES ETAL

Methods

Alluvial sampling and analysis

The alluvial samples consist of black sandconcentrates collected from stream beds and pitsin the river terraces (Tables 1 and 2 Fig 1) Gravelfrom the beds of the Alako (sample A1) and BigWater (sample BW2) was panned by a team of localworkers At each site a volume of gravel corre-sponding to 30 lsquostandard head pansrsquowas reduced toblack sand The head pan is a shallow wooden dishsim70 cm in diameter that the local workers use forpanning (and for carrying goods on the head ndashhence the name) The volume of a head pan wasmeasured by filling it with water and weighing Theresult indicated a volume of 1168 litres althoughthat represents the pan filled to the brim whereas apan filled with gravel was often piled above thebrim For the purposes of estimation of a grade inthe source material a pan volume of 12 litres hasbeen used During panning of the streams rocks andboulders were avoided so the grade calculated inthis way is an overestimate of a grade that would beachieved by mechanical extraction The black sandfrom the rivers was split using a riffle splitter and asplit assayed for Pt Pd and Au by LakefieldResearch in CanadaThe river terraces either side of Big Water were

examined by pitting Square pits 15 m on eachside were dug and all of the material panned (PitsBW6 ndash BW13 only pits reporting significant Ptvalues are discussed here) The depth of the pitswas variable and constrained by reaching bedrockor rapid ingress of water through gravel below thewater table A lsquo4 inchrsquo (100 mm diameter input)pump only allowed limited progress below thewater table and an auger was used for deepersamples Each interval of 05 m in depth in the pitwas treated as a separate sample and panned toblack sand Each sample was dried on site andsubsequently subsampled using a riffle splitterbefore assay only for Pt by Lakefield Research ofCanada High Pt assay results were checked byrepeat assays

Recovery and study of the PGM

The black sand concentrates (60ndash500 g of eachsample) were panned again in the laboratory torecover the densest fraction and the PGM nuggetsfound during the process were hand picked Theremaining lightest fraction was processed with aFrantz LB-1 Magnetic Barrier Separator fitted with

a Low Field Control Unit which allowed asatisfactory separation of the most ferromagneticand diamagnetic fractions The majority of thealluvial nuggets were hand picked from the denseand the less magnetic fractions using a stereoscopicmicroscope All the PGM recovered from thesamples were mounted separately on stubs forexamination of their morphology and subse-quently in polished blocks for analysis using aZeiss SMT S360 analytical scanning electronmicroscope (SEM) with an Inca EDX system atCardiff University under the same analyticalconditions described by Bowles et al (2017) Theremainder of the densest fraction of each samplewas also examined by SEM to search for grainsoverlooked during hand picking

Assay results and their relationship todrainage

Assays of the samples described here are shown inTable 1 (panning in the rivers) and Table 2 (pittingthe river terraces) Repeated assay values show asignificant variation due to the lsquonugget effectrsquo Theassay values are reported as a proportion of theblack sand and they have been converted to lsquobankgradersquo to estimate the proportion of Pt in thesediment The results all relate to small volumes ofgravel and do not suggest an economic potentialPitting in the level river terrace of the east bank of

the BigWater (pits BW6 and BW8) revealed graveland lateritized soils to a depth of sim2 m overlyingweathered bedrock On the west bank soil overliesgravel in pit BW13 Soil and weathered bedrock arefound between the patches of gravel and theboulders in pit BW11 These pits are at the footof a steep hillside below the ridge on which thePGM-bearing horizons have been reported(Bowles 2000b Bowles et al 2013) and therehas clearly been some slumpingPlatinum concentrations appear in the upper 05 m

of sediment (up to 135 gt Pt in black sand) at thebottomof some pits inweathered bedrock (up to 583gt Pt in black sand) and in intermediate horizons(up to 395 gt in black sand) with no consistentpattern (Table 2) As a Pt concentration in thesediment the highest values (up to 1176 mg Ptm3)occur close to bedrock but the near surface sedimentcan contain up to 48 mg Ptm3 The Pt assays fromthe gravel in the Alako and the Big Water are similarand in the same range as the Big Water terraces(Table 1) The Pd and Au assay results are lowresulting in very high PtPd and PtAu ratios

S227

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The Alako and the Big Water drain entirelydifferent sequences of the layered rocks yet thealluvial PGM are similar The Alako runs for 2 kmalong the west side of Mateki Ridge in a small well

defined watershed that does not include knownPGM-bearing horizons Drilling in this area(Barber 1962) was not followed up There is noevidence of river capture or change of course that

TABLE 2 Results from pitting in the Big Water river terraces

Samplenumber

Depth(m) Lithology

Weight ofblack sand

(kg) Pt (gt)

Repeat assaysBank grade(mg Ptm3)Pt (gt) Pt (gt) Pt (gt)

Pits on the east bankBW 6A 0ndash05 10 cm soil + fine gravel 191 075 13BW 6B 05ndash10 Fine gravel 315 063 18BW 6C 10ndash15 Gravel with boulders 703 nd 0BW 6D 15ndash20 Gravel with big boulders at base 597 157 83BW 6E 20ndash22 Orange-red weathered bedrock 154 5830 1040 1176BW 8A 0ndash05 Dark brown soil 048 964 1630 1720 2590 75BW 8B 05ndash10 Red to orange brown soil 155 072 10BW 8C 10ndash15 Red to orange brown soil 224 035 07BW 8D 15ndash20 White clay amp gravel boulders

in pit wall 20 cm gravel atbase

242 021 05

BW 8E 20ndash215 White clay 037 286 31Pits on the west bankBW 9A 0ndash05 10 cm soil + orange soil below 059 5120 1030 769 1350 480BW 9B 05ndash10 Kharki-brown soil 043 051 02BW 9C 10ndash15 Auger sample below water

table030 011 003

BW 11A 0ndash05 Brown soil 081 015 01BW 11B 05ndash10 Orange soil with some boulders 121 3950 195 223BW 11C 10ndash15 Orange soil many big boulders 137 029 04BW 13A 0ndash05 20 cm soil gravel + boulders

below382 267 91

BW 13B 05ndash10 Gravel + abundant boulders 480 1930 823

The samples studied are shown in bold type

TABLE 1 Panning of river gravels using a lsquoheadpanrsquo

Sample numberWeight of

black sand (kg)

Assay results Bank grademg (Pt+Pd+Au)

per m3Pt(gt) Pd (gt) Au (gt) PtPd PtAu

AlakoA 1 220 2950 nd 008 gt1475 369 130

Big WaterBW 2 1220 1490 018 003 83 496 370

nd ndash less than detection limit (002 gt)The bank grade is an estimate of the in situ grade calculated using a lsquoheadpanrsquo volume of 12 litres and a factor of 138 toallow for the difference between the loose gravel in the pan and the undisturbed gravel It is an overestimate as boulderswere avoided when the gravel was collected

S228

J F W BOWLES ETAL

could have brought alluvial PGM from elsewhereThe west slope of Mateki Ridge above the Alako isthe dip slope of the layers above the PGM-bearinghorizons B C and D that outcrop on the east slopeof Mateki ridge There are no tributaries or dryvalleys on the west side of the ridge indicatingsufficient erosion to expose the PGE-enrichedlayers Therefore simple erosion of the PGE-enriched layers should only release PGM into theBig Water and cannot explain the occurrence ofPGM in the AlakoBig Water rises 5 km above the sampling points

(Fig 1) and flows along the eastern side of MatekiRidge with several dry valleys cutting into theridge The main stream comes from the eastcrossing the strike of the layered rocks including

the PGM-bearing HorizonMwhich has an As- Sb-PGE-bearing mineralogy (Bowles et al 2013)There could be some PGM input fromHorizonM atthe sampling sites but the nuggets do not showsignificant mineralogical differences between westbank stream bed and east bank (Table 3) and thereis no sign of As- or Sb-bearing phases It seemslikely that the majority of the PGM in BigWater arederived from the PGE-bearing horizons that followMateki Ridge

Mineralogy of the black sands

The black sands are mainly composed of magnet-ite ilmenite and hematite grains about 200 to

TABLE 3 Alluvial PGM in the Big Water and Alako

Big WaterAlakoRiver

West bank terraceRiver

East bank terrace

Sample BW13B BW9A BW2C BW8A BW6E Alako1 TotalsPit Pit Panned Pit Pit Panned

NuggetsNumber 1 5 13 1 12 4 36Size (microm) 263times318 80ndash1540 134ndash635 249times228 119ndash822 167ndash1030Shape

Euhedral 1 1 1 1 111Subrounded 1 2 3 1 194Rounded 2 2 111Elongate 2 3 3 1 250Irregular 2 5 1 3 1 333

Composition Pt3Fe 2 11 5 3 583Pt3ndashxFe (x = 016ndash038) 3 2 6 306Pt2FeCu 1 28(PtPd)S 1 1 56Cu0 1 28

Associated PGM No Nuggets(a) Grains(b)

OsplusmnIr-Ru alloy bull(3) bull(2) bull(1) 6Pt-Fe alloys (plates) bull(1) 4PtCu (coats plates) bull(2) bull(1) bull(3) 10RuS2 ndash OsS2 bull(2) bull(1) 4IrAsS bull(1) 1Cu(PtRhIr)2S4 bull(1) bull(1) bull(1) 3PtSndashPdS bull(1) 5Au-(PdAg) bull(1) 1Ag-(FeCu) bull(1) 1

(a)Number of nuggets on which these associated PGM and native metals were found(b)Total number of grains located (approx)Attached to ilmenite grains

S229

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

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J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

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J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

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J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

(Hughes and Hughes 1992) The source of theFreetown placer PGM was obscure until recentlyso comparison of the source the eluvial depositsand the placers was not possible Bowles (2000b)and Bowles et al (2013) defined four stratiformlayers of magnetite-bearing gabbros with PGEmineralization (up to 069 gt Pt) and Cu-richsulfides describing a primary source of theFreetown PGM and identifying primary PGM forthe first time The host rocks are fresh with coarse-grained olivine plagioclase clinopyroxene ortho-pyroxene and later interstitial amphibole (Bowleset al 2013) There has been re-equilibration of theFe-Ti-oxides during cooling but no indication ofsignificant metamorphism or serpentinization ThePGM occur in the interstices between the mainsilicate minerals and within the late amphibole ThePGM in the host rocks are considered here to belsquoprimary PGMrsquo the PGM that have been altered byweathering are described as lsquoweathered PGMrsquo andthe different alluvial suite are called lsquoneoformPGMrsquoBowles et al (2017) investigated the sequence of

mineralogical changes undergone by the primaryPGE mineralization during in situ weathering Thepresent contribution uses results from a systematicsurvey of the alluvial PGM from the river terracesand the beds of the Alako and the BigWater (Fig 1)that drain PGE-bearing horizons to compare thePGM in the primary andweathered rocks with thosein the alluvial assemblage to assess the origin of thesecondary PGM

Platinum from the Freetown Peninsula

In 1926 Pt-bearing grains were found in thewestward draining streams of the complex and by1949 5242 oz of platinum concentrate had beenrecovered Analysis of the concentrate showedPt 87 Pd 2 Os + Ir 13 and Fe 99 wt (Pollett1931) The extraction used artisanal methods withsluice boxes and local labour panning by hand It isworth quoting the description of the alluvialoccurrences from Pollett (1931) ldquoPlatinum in thenative state occurs in the gravels of streams drainingthe hills forming the peninsula of Sierra Leone To alimited extent it has also been found in beach gravelsin certain coast localities The most promisingdeposits are in the vicinity of York more specific-ally in the streams flowing into Whale River on itsnorthern banks Here the metal occurs as smallnuggets pitted and water worn grains delicatedendritic crystalline growths and dust Occasionally

cubic crystals are found Many of the small nuggetsand water-worn grains have a peculiar brass colourThe largest nugget yet recoveredweighed 1212 dwtand numerous nuggets ranging between 5 and1012 dwt have been foundrdquo [1 dwt or onepennyweight is 20 g so the weights mentionedrange from 10 to 250 g] ldquoThese larger nuggets areinvariably well water worn An interesting feature isthe presence of practically non water worn delicatedendritic crystalline growths of platinum in gravelsthat also contain well-rounded grains of the metalThis dendritic non water worn form however israrely seen in fragments larger than 12 dwt Anothernoticeable feature is the small percentage ofplatinum dustrdquoThe platinum described as being in the native state

is now known to be platinum-iron alloy occurring asPtFe and Pt3Fe (Bowles 1981) The density ofisoferroplatinum (Pt3Fe) is 182 gcc and that oftetraferroplatinum (PtFe) is 152 gcc For thepurposes of an illustrative calculation an intermedi-ate value of 16 gcc can be chosen so the lsquolargestnugget yet recoveredrsquo (1212 dwt) could berepresented by a cube with an edge of 25 cm Thenumerous nuggets of 5 to 1012 dwt would have hadavolume corresponding to cubeswith edges of 18 to24 cm and the dendritic growths reached a volumeequivalent to a cube with an edge of 085 cmExperiments with nuggets from the present study

have shown that the lsquobrass colourrsquo of the PGM is acoating of Fe- and Al-oxides and organic matterthat can be removed by washing in warm 6M HClfor 1 to 2 days and then reacting with 30H2O2 forabout one month The coating hides the otherwisesilvery colour of the nuggets making them difficultto locate in the sediments This coating appearscomparable with the biofilms identified by Reithet al (2016)The description of the morphology of the PGM

by Pollett (1931) is comparable with the PGM fromGuma Water Freetown described by Bowles(1981) The lsquosmall percentage of platinum dustrsquomay be a result of panning and hand picking of thegrains in the presence of a large amount of coarseblack sand so that fine-grained PGM would nothave been recoveredPollett (1951) suggested that artisanal mining

had ceased because of a low platinum price and itwas unclear if economic PGM concentrationsremained Therefore exploratory surveys of thealluvium were undertaken by panning in thestreams and by pitting of the river terraces toassess the present situation and also to aid thesearch for the hard-rock source

S226

J F W BOWLES ETAL

Methods

Alluvial sampling and analysis

The alluvial samples consist of black sandconcentrates collected from stream beds and pitsin the river terraces (Tables 1 and 2 Fig 1) Gravelfrom the beds of the Alako (sample A1) and BigWater (sample BW2) was panned by a team of localworkers At each site a volume of gravel corre-sponding to 30 lsquostandard head pansrsquowas reduced toblack sand The head pan is a shallow wooden dishsim70 cm in diameter that the local workers use forpanning (and for carrying goods on the head ndashhence the name) The volume of a head pan wasmeasured by filling it with water and weighing Theresult indicated a volume of 1168 litres althoughthat represents the pan filled to the brim whereas apan filled with gravel was often piled above thebrim For the purposes of estimation of a grade inthe source material a pan volume of 12 litres hasbeen used During panning of the streams rocks andboulders were avoided so the grade calculated inthis way is an overestimate of a grade that would beachieved by mechanical extraction The black sandfrom the rivers was split using a riffle splitter and asplit assayed for Pt Pd and Au by LakefieldResearch in CanadaThe river terraces either side of Big Water were

examined by pitting Square pits 15 m on eachside were dug and all of the material panned (PitsBW6 ndash BW13 only pits reporting significant Ptvalues are discussed here) The depth of the pitswas variable and constrained by reaching bedrockor rapid ingress of water through gravel below thewater table A lsquo4 inchrsquo (100 mm diameter input)pump only allowed limited progress below thewater table and an auger was used for deepersamples Each interval of 05 m in depth in the pitwas treated as a separate sample and panned toblack sand Each sample was dried on site andsubsequently subsampled using a riffle splitterbefore assay only for Pt by Lakefield Research ofCanada High Pt assay results were checked byrepeat assays

Recovery and study of the PGM

The black sand concentrates (60ndash500 g of eachsample) were panned again in the laboratory torecover the densest fraction and the PGM nuggetsfound during the process were hand picked Theremaining lightest fraction was processed with aFrantz LB-1 Magnetic Barrier Separator fitted with

a Low Field Control Unit which allowed asatisfactory separation of the most ferromagneticand diamagnetic fractions The majority of thealluvial nuggets were hand picked from the denseand the less magnetic fractions using a stereoscopicmicroscope All the PGM recovered from thesamples were mounted separately on stubs forexamination of their morphology and subse-quently in polished blocks for analysis using aZeiss SMT S360 analytical scanning electronmicroscope (SEM) with an Inca EDX system atCardiff University under the same analyticalconditions described by Bowles et al (2017) Theremainder of the densest fraction of each samplewas also examined by SEM to search for grainsoverlooked during hand picking

Assay results and their relationship todrainage

Assays of the samples described here are shown inTable 1 (panning in the rivers) and Table 2 (pittingthe river terraces) Repeated assay values show asignificant variation due to the lsquonugget effectrsquo Theassay values are reported as a proportion of theblack sand and they have been converted to lsquobankgradersquo to estimate the proportion of Pt in thesediment The results all relate to small volumes ofgravel and do not suggest an economic potentialPitting in the level river terrace of the east bank of

the BigWater (pits BW6 and BW8) revealed graveland lateritized soils to a depth of sim2 m overlyingweathered bedrock On the west bank soil overliesgravel in pit BW13 Soil and weathered bedrock arefound between the patches of gravel and theboulders in pit BW11 These pits are at the footof a steep hillside below the ridge on which thePGM-bearing horizons have been reported(Bowles 2000b Bowles et al 2013) and therehas clearly been some slumpingPlatinum concentrations appear in the upper 05 m

of sediment (up to 135 gt Pt in black sand) at thebottomof some pits inweathered bedrock (up to 583gt Pt in black sand) and in intermediate horizons(up to 395 gt in black sand) with no consistentpattern (Table 2) As a Pt concentration in thesediment the highest values (up to 1176 mg Ptm3)occur close to bedrock but the near surface sedimentcan contain up to 48 mg Ptm3 The Pt assays fromthe gravel in the Alako and the Big Water are similarand in the same range as the Big Water terraces(Table 1) The Pd and Au assay results are lowresulting in very high PtPd and PtAu ratios

S227

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The Alako and the Big Water drain entirelydifferent sequences of the layered rocks yet thealluvial PGM are similar The Alako runs for 2 kmalong the west side of Mateki Ridge in a small well

defined watershed that does not include knownPGM-bearing horizons Drilling in this area(Barber 1962) was not followed up There is noevidence of river capture or change of course that

TABLE 2 Results from pitting in the Big Water river terraces

Samplenumber

Depth(m) Lithology

Weight ofblack sand

(kg) Pt (gt)

Repeat assaysBank grade(mg Ptm3)Pt (gt) Pt (gt) Pt (gt)

Pits on the east bankBW 6A 0ndash05 10 cm soil + fine gravel 191 075 13BW 6B 05ndash10 Fine gravel 315 063 18BW 6C 10ndash15 Gravel with boulders 703 nd 0BW 6D 15ndash20 Gravel with big boulders at base 597 157 83BW 6E 20ndash22 Orange-red weathered bedrock 154 5830 1040 1176BW 8A 0ndash05 Dark brown soil 048 964 1630 1720 2590 75BW 8B 05ndash10 Red to orange brown soil 155 072 10BW 8C 10ndash15 Red to orange brown soil 224 035 07BW 8D 15ndash20 White clay amp gravel boulders

in pit wall 20 cm gravel atbase

242 021 05

BW 8E 20ndash215 White clay 037 286 31Pits on the west bankBW 9A 0ndash05 10 cm soil + orange soil below 059 5120 1030 769 1350 480BW 9B 05ndash10 Kharki-brown soil 043 051 02BW 9C 10ndash15 Auger sample below water

table030 011 003

BW 11A 0ndash05 Brown soil 081 015 01BW 11B 05ndash10 Orange soil with some boulders 121 3950 195 223BW 11C 10ndash15 Orange soil many big boulders 137 029 04BW 13A 0ndash05 20 cm soil gravel + boulders

below382 267 91

BW 13B 05ndash10 Gravel + abundant boulders 480 1930 823

The samples studied are shown in bold type

TABLE 1 Panning of river gravels using a lsquoheadpanrsquo

Sample numberWeight of

black sand (kg)

Assay results Bank grademg (Pt+Pd+Au)

per m3Pt(gt) Pd (gt) Au (gt) PtPd PtAu

AlakoA 1 220 2950 nd 008 gt1475 369 130

Big WaterBW 2 1220 1490 018 003 83 496 370

nd ndash less than detection limit (002 gt)The bank grade is an estimate of the in situ grade calculated using a lsquoheadpanrsquo volume of 12 litres and a factor of 138 toallow for the difference between the loose gravel in the pan and the undisturbed gravel It is an overestimate as boulderswere avoided when the gravel was collected

S228

J F W BOWLES ETAL

could have brought alluvial PGM from elsewhereThe west slope of Mateki Ridge above the Alako isthe dip slope of the layers above the PGM-bearinghorizons B C and D that outcrop on the east slopeof Mateki ridge There are no tributaries or dryvalleys on the west side of the ridge indicatingsufficient erosion to expose the PGE-enrichedlayers Therefore simple erosion of the PGE-enriched layers should only release PGM into theBig Water and cannot explain the occurrence ofPGM in the AlakoBig Water rises 5 km above the sampling points

(Fig 1) and flows along the eastern side of MatekiRidge with several dry valleys cutting into theridge The main stream comes from the eastcrossing the strike of the layered rocks including

the PGM-bearing HorizonMwhich has an As- Sb-PGE-bearing mineralogy (Bowles et al 2013)There could be some PGM input fromHorizonM atthe sampling sites but the nuggets do not showsignificant mineralogical differences between westbank stream bed and east bank (Table 3) and thereis no sign of As- or Sb-bearing phases It seemslikely that the majority of the PGM in BigWater arederived from the PGE-bearing horizons that followMateki Ridge

Mineralogy of the black sands

The black sands are mainly composed of magnet-ite ilmenite and hematite grains about 200 to

TABLE 3 Alluvial PGM in the Big Water and Alako

Big WaterAlakoRiver

West bank terraceRiver

East bank terrace

Sample BW13B BW9A BW2C BW8A BW6E Alako1 TotalsPit Pit Panned Pit Pit Panned

NuggetsNumber 1 5 13 1 12 4 36Size (microm) 263times318 80ndash1540 134ndash635 249times228 119ndash822 167ndash1030Shape

Euhedral 1 1 1 1 111Subrounded 1 2 3 1 194Rounded 2 2 111Elongate 2 3 3 1 250Irregular 2 5 1 3 1 333

Composition Pt3Fe 2 11 5 3 583Pt3ndashxFe (x = 016ndash038) 3 2 6 306Pt2FeCu 1 28(PtPd)S 1 1 56Cu0 1 28

Associated PGM No Nuggets(a) Grains(b)

OsplusmnIr-Ru alloy bull(3) bull(2) bull(1) 6Pt-Fe alloys (plates) bull(1) 4PtCu (coats plates) bull(2) bull(1) bull(3) 10RuS2 ndash OsS2 bull(2) bull(1) 4IrAsS bull(1) 1Cu(PtRhIr)2S4 bull(1) bull(1) bull(1) 3PtSndashPdS bull(1) 5Au-(PdAg) bull(1) 1Ag-(FeCu) bull(1) 1

(a)Number of nuggets on which these associated PGM and native metals were found(b)Total number of grains located (approx)Attached to ilmenite grains

S229

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

S234

J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

Methods

Alluvial sampling and analysis

The alluvial samples consist of black sandconcentrates collected from stream beds and pitsin the river terraces (Tables 1 and 2 Fig 1) Gravelfrom the beds of the Alako (sample A1) and BigWater (sample BW2) was panned by a team of localworkers At each site a volume of gravel corre-sponding to 30 lsquostandard head pansrsquowas reduced toblack sand The head pan is a shallow wooden dishsim70 cm in diameter that the local workers use forpanning (and for carrying goods on the head ndashhence the name) The volume of a head pan wasmeasured by filling it with water and weighing Theresult indicated a volume of 1168 litres althoughthat represents the pan filled to the brim whereas apan filled with gravel was often piled above thebrim For the purposes of estimation of a grade inthe source material a pan volume of 12 litres hasbeen used During panning of the streams rocks andboulders were avoided so the grade calculated inthis way is an overestimate of a grade that would beachieved by mechanical extraction The black sandfrom the rivers was split using a riffle splitter and asplit assayed for Pt Pd and Au by LakefieldResearch in CanadaThe river terraces either side of Big Water were

examined by pitting Square pits 15 m on eachside were dug and all of the material panned (PitsBW6 ndash BW13 only pits reporting significant Ptvalues are discussed here) The depth of the pitswas variable and constrained by reaching bedrockor rapid ingress of water through gravel below thewater table A lsquo4 inchrsquo (100 mm diameter input)pump only allowed limited progress below thewater table and an auger was used for deepersamples Each interval of 05 m in depth in the pitwas treated as a separate sample and panned toblack sand Each sample was dried on site andsubsequently subsampled using a riffle splitterbefore assay only for Pt by Lakefield Research ofCanada High Pt assay results were checked byrepeat assays

Recovery and study of the PGM

The black sand concentrates (60ndash500 g of eachsample) were panned again in the laboratory torecover the densest fraction and the PGM nuggetsfound during the process were hand picked Theremaining lightest fraction was processed with aFrantz LB-1 Magnetic Barrier Separator fitted with

a Low Field Control Unit which allowed asatisfactory separation of the most ferromagneticand diamagnetic fractions The majority of thealluvial nuggets were hand picked from the denseand the less magnetic fractions using a stereoscopicmicroscope All the PGM recovered from thesamples were mounted separately on stubs forexamination of their morphology and subse-quently in polished blocks for analysis using aZeiss SMT S360 analytical scanning electronmicroscope (SEM) with an Inca EDX system atCardiff University under the same analyticalconditions described by Bowles et al (2017) Theremainder of the densest fraction of each samplewas also examined by SEM to search for grainsoverlooked during hand picking

Assay results and their relationship todrainage

Assays of the samples described here are shown inTable 1 (panning in the rivers) and Table 2 (pittingthe river terraces) Repeated assay values show asignificant variation due to the lsquonugget effectrsquo Theassay values are reported as a proportion of theblack sand and they have been converted to lsquobankgradersquo to estimate the proportion of Pt in thesediment The results all relate to small volumes ofgravel and do not suggest an economic potentialPitting in the level river terrace of the east bank of

the BigWater (pits BW6 and BW8) revealed graveland lateritized soils to a depth of sim2 m overlyingweathered bedrock On the west bank soil overliesgravel in pit BW13 Soil and weathered bedrock arefound between the patches of gravel and theboulders in pit BW11 These pits are at the footof a steep hillside below the ridge on which thePGM-bearing horizons have been reported(Bowles 2000b Bowles et al 2013) and therehas clearly been some slumpingPlatinum concentrations appear in the upper 05 m

of sediment (up to 135 gt Pt in black sand) at thebottomof some pits inweathered bedrock (up to 583gt Pt in black sand) and in intermediate horizons(up to 395 gt in black sand) with no consistentpattern (Table 2) As a Pt concentration in thesediment the highest values (up to 1176 mg Ptm3)occur close to bedrock but the near surface sedimentcan contain up to 48 mg Ptm3 The Pt assays fromthe gravel in the Alako and the Big Water are similarand in the same range as the Big Water terraces(Table 1) The Pd and Au assay results are lowresulting in very high PtPd and PtAu ratios

S227

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The Alako and the Big Water drain entirelydifferent sequences of the layered rocks yet thealluvial PGM are similar The Alako runs for 2 kmalong the west side of Mateki Ridge in a small well

defined watershed that does not include knownPGM-bearing horizons Drilling in this area(Barber 1962) was not followed up There is noevidence of river capture or change of course that

TABLE 2 Results from pitting in the Big Water river terraces

Samplenumber

Depth(m) Lithology

Weight ofblack sand

(kg) Pt (gt)

Repeat assaysBank grade(mg Ptm3)Pt (gt) Pt (gt) Pt (gt)

Pits on the east bankBW 6A 0ndash05 10 cm soil + fine gravel 191 075 13BW 6B 05ndash10 Fine gravel 315 063 18BW 6C 10ndash15 Gravel with boulders 703 nd 0BW 6D 15ndash20 Gravel with big boulders at base 597 157 83BW 6E 20ndash22 Orange-red weathered bedrock 154 5830 1040 1176BW 8A 0ndash05 Dark brown soil 048 964 1630 1720 2590 75BW 8B 05ndash10 Red to orange brown soil 155 072 10BW 8C 10ndash15 Red to orange brown soil 224 035 07BW 8D 15ndash20 White clay amp gravel boulders

in pit wall 20 cm gravel atbase

242 021 05

BW 8E 20ndash215 White clay 037 286 31Pits on the west bankBW 9A 0ndash05 10 cm soil + orange soil below 059 5120 1030 769 1350 480BW 9B 05ndash10 Kharki-brown soil 043 051 02BW 9C 10ndash15 Auger sample below water

table030 011 003

BW 11A 0ndash05 Brown soil 081 015 01BW 11B 05ndash10 Orange soil with some boulders 121 3950 195 223BW 11C 10ndash15 Orange soil many big boulders 137 029 04BW 13A 0ndash05 20 cm soil gravel + boulders

below382 267 91

BW 13B 05ndash10 Gravel + abundant boulders 480 1930 823

The samples studied are shown in bold type

TABLE 1 Panning of river gravels using a lsquoheadpanrsquo

Sample numberWeight of

black sand (kg)

Assay results Bank grademg (Pt+Pd+Au)

per m3Pt(gt) Pd (gt) Au (gt) PtPd PtAu

AlakoA 1 220 2950 nd 008 gt1475 369 130

Big WaterBW 2 1220 1490 018 003 83 496 370

nd ndash less than detection limit (002 gt)The bank grade is an estimate of the in situ grade calculated using a lsquoheadpanrsquo volume of 12 litres and a factor of 138 toallow for the difference between the loose gravel in the pan and the undisturbed gravel It is an overestimate as boulderswere avoided when the gravel was collected

S228

J F W BOWLES ETAL

could have brought alluvial PGM from elsewhereThe west slope of Mateki Ridge above the Alako isthe dip slope of the layers above the PGM-bearinghorizons B C and D that outcrop on the east slopeof Mateki ridge There are no tributaries or dryvalleys on the west side of the ridge indicatingsufficient erosion to expose the PGE-enrichedlayers Therefore simple erosion of the PGE-enriched layers should only release PGM into theBig Water and cannot explain the occurrence ofPGM in the AlakoBig Water rises 5 km above the sampling points

(Fig 1) and flows along the eastern side of MatekiRidge with several dry valleys cutting into theridge The main stream comes from the eastcrossing the strike of the layered rocks including

the PGM-bearing HorizonMwhich has an As- Sb-PGE-bearing mineralogy (Bowles et al 2013)There could be some PGM input fromHorizonM atthe sampling sites but the nuggets do not showsignificant mineralogical differences between westbank stream bed and east bank (Table 3) and thereis no sign of As- or Sb-bearing phases It seemslikely that the majority of the PGM in BigWater arederived from the PGE-bearing horizons that followMateki Ridge

Mineralogy of the black sands

The black sands are mainly composed of magnet-ite ilmenite and hematite grains about 200 to

TABLE 3 Alluvial PGM in the Big Water and Alako

Big WaterAlakoRiver

West bank terraceRiver

East bank terrace

Sample BW13B BW9A BW2C BW8A BW6E Alako1 TotalsPit Pit Panned Pit Pit Panned

NuggetsNumber 1 5 13 1 12 4 36Size (microm) 263times318 80ndash1540 134ndash635 249times228 119ndash822 167ndash1030Shape

Euhedral 1 1 1 1 111Subrounded 1 2 3 1 194Rounded 2 2 111Elongate 2 3 3 1 250Irregular 2 5 1 3 1 333

Composition Pt3Fe 2 11 5 3 583Pt3ndashxFe (x = 016ndash038) 3 2 6 306Pt2FeCu 1 28(PtPd)S 1 1 56Cu0 1 28

Associated PGM No Nuggets(a) Grains(b)

OsplusmnIr-Ru alloy bull(3) bull(2) bull(1) 6Pt-Fe alloys (plates) bull(1) 4PtCu (coats plates) bull(2) bull(1) bull(3) 10RuS2 ndash OsS2 bull(2) bull(1) 4IrAsS bull(1) 1Cu(PtRhIr)2S4 bull(1) bull(1) bull(1) 3PtSndashPdS bull(1) 5Au-(PdAg) bull(1) 1Ag-(FeCu) bull(1) 1

(a)Number of nuggets on which these associated PGM and native metals were found(b)Total number of grains located (approx)Attached to ilmenite grains

S229

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

S234

J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The Alako and the Big Water drain entirelydifferent sequences of the layered rocks yet thealluvial PGM are similar The Alako runs for 2 kmalong the west side of Mateki Ridge in a small well

defined watershed that does not include knownPGM-bearing horizons Drilling in this area(Barber 1962) was not followed up There is noevidence of river capture or change of course that

TABLE 2 Results from pitting in the Big Water river terraces

Samplenumber

Depth(m) Lithology

Weight ofblack sand

(kg) Pt (gt)

Repeat assaysBank grade(mg Ptm3)Pt (gt) Pt (gt) Pt (gt)

Pits on the east bankBW 6A 0ndash05 10 cm soil + fine gravel 191 075 13BW 6B 05ndash10 Fine gravel 315 063 18BW 6C 10ndash15 Gravel with boulders 703 nd 0BW 6D 15ndash20 Gravel with big boulders at base 597 157 83BW 6E 20ndash22 Orange-red weathered bedrock 154 5830 1040 1176BW 8A 0ndash05 Dark brown soil 048 964 1630 1720 2590 75BW 8B 05ndash10 Red to orange brown soil 155 072 10BW 8C 10ndash15 Red to orange brown soil 224 035 07BW 8D 15ndash20 White clay amp gravel boulders

in pit wall 20 cm gravel atbase

242 021 05

BW 8E 20ndash215 White clay 037 286 31Pits on the west bankBW 9A 0ndash05 10 cm soil + orange soil below 059 5120 1030 769 1350 480BW 9B 05ndash10 Kharki-brown soil 043 051 02BW 9C 10ndash15 Auger sample below water

table030 011 003

BW 11A 0ndash05 Brown soil 081 015 01BW 11B 05ndash10 Orange soil with some boulders 121 3950 195 223BW 11C 10ndash15 Orange soil many big boulders 137 029 04BW 13A 0ndash05 20 cm soil gravel + boulders

below382 267 91

BW 13B 05ndash10 Gravel + abundant boulders 480 1930 823

The samples studied are shown in bold type

TABLE 1 Panning of river gravels using a lsquoheadpanrsquo

Sample numberWeight of

black sand (kg)

Assay results Bank grademg (Pt+Pd+Au)

per m3Pt(gt) Pd (gt) Au (gt) PtPd PtAu

AlakoA 1 220 2950 nd 008 gt1475 369 130

Big WaterBW 2 1220 1490 018 003 83 496 370

nd ndash less than detection limit (002 gt)The bank grade is an estimate of the in situ grade calculated using a lsquoheadpanrsquo volume of 12 litres and a factor of 138 toallow for the difference between the loose gravel in the pan and the undisturbed gravel It is an overestimate as boulderswere avoided when the gravel was collected

S228

J F W BOWLES ETAL

could have brought alluvial PGM from elsewhereThe west slope of Mateki Ridge above the Alako isthe dip slope of the layers above the PGM-bearinghorizons B C and D that outcrop on the east slopeof Mateki ridge There are no tributaries or dryvalleys on the west side of the ridge indicatingsufficient erosion to expose the PGE-enrichedlayers Therefore simple erosion of the PGE-enriched layers should only release PGM into theBig Water and cannot explain the occurrence ofPGM in the AlakoBig Water rises 5 km above the sampling points

(Fig 1) and flows along the eastern side of MatekiRidge with several dry valleys cutting into theridge The main stream comes from the eastcrossing the strike of the layered rocks including

the PGM-bearing HorizonMwhich has an As- Sb-PGE-bearing mineralogy (Bowles et al 2013)There could be some PGM input fromHorizonM atthe sampling sites but the nuggets do not showsignificant mineralogical differences between westbank stream bed and east bank (Table 3) and thereis no sign of As- or Sb-bearing phases It seemslikely that the majority of the PGM in BigWater arederived from the PGE-bearing horizons that followMateki Ridge

Mineralogy of the black sands

The black sands are mainly composed of magnet-ite ilmenite and hematite grains about 200 to

TABLE 3 Alluvial PGM in the Big Water and Alako

Big WaterAlakoRiver

West bank terraceRiver

East bank terrace

Sample BW13B BW9A BW2C BW8A BW6E Alako1 TotalsPit Pit Panned Pit Pit Panned

NuggetsNumber 1 5 13 1 12 4 36Size (microm) 263times318 80ndash1540 134ndash635 249times228 119ndash822 167ndash1030Shape

Euhedral 1 1 1 1 111Subrounded 1 2 3 1 194Rounded 2 2 111Elongate 2 3 3 1 250Irregular 2 5 1 3 1 333

Composition Pt3Fe 2 11 5 3 583Pt3ndashxFe (x = 016ndash038) 3 2 6 306Pt2FeCu 1 28(PtPd)S 1 1 56Cu0 1 28

Associated PGM No Nuggets(a) Grains(b)

OsplusmnIr-Ru alloy bull(3) bull(2) bull(1) 6Pt-Fe alloys (plates) bull(1) 4PtCu (coats plates) bull(2) bull(1) bull(3) 10RuS2 ndash OsS2 bull(2) bull(1) 4IrAsS bull(1) 1Cu(PtRhIr)2S4 bull(1) bull(1) bull(1) 3PtSndashPdS bull(1) 5Au-(PdAg) bull(1) 1Ag-(FeCu) bull(1) 1

(a)Number of nuggets on which these associated PGM and native metals were found(b)Total number of grains located (approx)Attached to ilmenite grains

S229

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

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coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

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J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

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J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

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J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

could have brought alluvial PGM from elsewhereThe west slope of Mateki Ridge above the Alako isthe dip slope of the layers above the PGM-bearinghorizons B C and D that outcrop on the east slopeof Mateki ridge There are no tributaries or dryvalleys on the west side of the ridge indicatingsufficient erosion to expose the PGE-enrichedlayers Therefore simple erosion of the PGE-enriched layers should only release PGM into theBig Water and cannot explain the occurrence ofPGM in the AlakoBig Water rises 5 km above the sampling points

(Fig 1) and flows along the eastern side of MatekiRidge with several dry valleys cutting into theridge The main stream comes from the eastcrossing the strike of the layered rocks including

the PGM-bearing HorizonMwhich has an As- Sb-PGE-bearing mineralogy (Bowles et al 2013)There could be some PGM input fromHorizonM atthe sampling sites but the nuggets do not showsignificant mineralogical differences between westbank stream bed and east bank (Table 3) and thereis no sign of As- or Sb-bearing phases It seemslikely that the majority of the PGM in BigWater arederived from the PGE-bearing horizons that followMateki Ridge

Mineralogy of the black sands

The black sands are mainly composed of magnet-ite ilmenite and hematite grains about 200 to

TABLE 3 Alluvial PGM in the Big Water and Alako

Big WaterAlakoRiver

West bank terraceRiver

East bank terrace

Sample BW13B BW9A BW2C BW8A BW6E Alako1 TotalsPit Pit Panned Pit Pit Panned

NuggetsNumber 1 5 13 1 12 4 36Size (microm) 263times318 80ndash1540 134ndash635 249times228 119ndash822 167ndash1030Shape

Euhedral 1 1 1 1 111Subrounded 1 2 3 1 194Rounded 2 2 111Elongate 2 3 3 1 250Irregular 2 5 1 3 1 333

Composition Pt3Fe 2 11 5 3 583Pt3ndashxFe (x = 016ndash038) 3 2 6 306Pt2FeCu 1 28(PtPd)S 1 1 56Cu0 1 28

Associated PGM No Nuggets(a) Grains(b)

OsplusmnIr-Ru alloy bull(3) bull(2) bull(1) 6Pt-Fe alloys (plates) bull(1) 4PtCu (coats plates) bull(2) bull(1) bull(3) 10RuS2 ndash OsS2 bull(2) bull(1) 4IrAsS bull(1) 1Cu(PtRhIr)2S4 bull(1) bull(1) bull(1) 3PtSndashPdS bull(1) 5Au-(PdAg) bull(1) 1Ag-(FeCu) bull(1) 1

(a)Number of nuggets on which these associated PGM and native metals were found(b)Total number of grains located (approx)Attached to ilmenite grains

S229

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

S234

J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

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J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 2 Scanning electron microscope images of alluvial PGM nuggets from the Alako and Big Water York areaFreetown (a) Abraded Pt3ndashxFe with remnants of (111) faces (bndashc) euhedral isoferroplatinum (Pt3Fe) and Pd-bearingcooperite (PtS) nuggets (dndashe) subhedral Pt3Fe with pits and furrows on surface ( f ) a more rounded Pt3ndashxFe (g)subrounded Pt3Fe with Os-Ru-Ir and hongshiite (PtCu) plates disposed parallel to the surface (hndashi) rounded Pt3ndashxFe andPt3Fe nuggets showing three sets of surface linear features ( j ) rusty Pt3ndashxFe bearing an irregular plate of Os-Ir-Ru alloy(k) elongate Pt3Fe with fine striations covering the whole nugget (l ) another elongate Pt3Fe with two sets of surfacestructures at right angles (m) elongate and irregular single nugget of Pt3ndashxFe (n) elongate Pt3Fe nugget with aheterogeneous surface showing aggregates of more euhedral to irregular crystals of Pt-Fe alloys (o) tulameenite(Pt2FeCu) ( pndashs) irregular Pt3Fe and Pt3ndashxFe nuggets with large PGM attached including erlichmanite (OsS2)cuproiridsite (CuIr2S4) Os-Ir-Ru alloys irarsite (IrAsS) osmian laurite (RuS2) and cuprorhodsite (CuRh2S4) (t)irregular Pt3Fe nugget with several regular shaped depressions on the surface a large pyritohedron of laurite is attached(u) irregular composite nugget of Pt3ndashxFe and PtCu displaying a lobate and irregular contact between them where (u1)Pt-Cu-bearing oxides and platelets of Pt-Fe and Pt-Cu lie parallel to the surface of the nugget (v) irregular and ratherporous Pt3ndashxFe nugget (w) nugget of native Cu with cuprite (Cu2O) on the surface (x) irregular (PdPt)S bearing PtCuplatelets on surface along with large masses of Fe-Al-oxides (yndashz) two examples of ilmenite (Ilm) that have smallclusters of PGM (cooperite and probable malanite‒CuPt2S4) Au and Ag grains all of them lt5 microm in size attached to

their surface

S230

J F W BOWLES ETAL

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

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J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

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J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

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J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

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J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

600 microm across Titanomagnetite sometimes dis-plays oxidation textures with ilmenite lamellae butgenerally magnetite is replaced by hematite andstriking martitization textures are frequentlyobserved In other cases magnetite shows theshrinkage cracks that are typically caused bymaghemitization Ilmenite grains occasionallyshow hematite exsolution and less frequently thehematite is replaced by rutile Some ilmenite grainsenclose elongate Al-spinels near their rimsIlmenite has generally been replaced by secondaryTi-bearing oxides at the edges and along fissures oris altered to Al-bearing maghemite at the edgesAccessory phases are rutile zircon garnet and

baddeleyite with rare galena and sphalerite Thereis a lighter fraction of rounded quartz grains withrare relicts of feldspar pyroxene andmica Goethiteand gibbsite are present as rounded aggregatesenclosing minute fragments of the resistateminerals

PGM nuggets

Thirty-six PGM nuggets have been recoveredfrom four pit samples from Big Water and fromthe gravels panned from the Big Water and theAlako (Table 3) The majority (89) are brass

FIG 2 Continued

S231

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

S234

J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

FIG 3 Scanning electron microscope images of the polished sections of some of the alluvial PGM shown in Fig 2(a) Euhedral Pt3Fe nugget (Fig 2b) showing ragged edges and regular voids (b) a well-developed Os-Ir lamellaextending to and eroded at the edge of a Pt3Fe nugget (c) PtCu replacing Pt3Fe at the edge (Fig 2g) (d ) cluster of Os-Ru-Ir alloy inclusions (bright) with Ru-Ir-Os alloy inclusions (dark) in a Pt3Fe nugget (Fig 2i) The inclusions followpreferred orientations which are intersecting at 55‒60deg (e) elongate Pt3ndashxFe (Fig 2m) full of base-metal bearing Rh-Sinclusions One of the edges (top right) encloses PGE-sulfides (cooperite and braggite) ( f ) a nugget of tulameenite(Fig 2o) showing porous and corroded edges in section (g) thin crystal of an Os-Ru-Ir alloy that may correspond to abroken or corroded Os platelet extending to the edge of the host Pt3Fe (h) erlichmanite (OsS2) and cuproiridsite(CuIr2S4) crystals at the edges of Pt3Fe (Fig 2p) (i) a large platelet of Os-Ru-Ir alloy attached to Pt3ndashxFe (Fig 2q) ( j )irregular elongate Pt3Fe grain (Fig 2s) full of varied polygonal crystals of PGM (k) Pt3Fewith the large crystal of lauriteshown in Fig 2t (l ) section of a composite Pt3ndashxFe-PtCu nugget (Fig 2u u1) showing the irregular contact between thephases suggesting replacement by the PtCu (m) porous Pt3ndashxFe nugget (Fig 2v) hosting various Pt-Pd-alloys (n)

irregular Pt3ndashxFe nugget with a large crystal and lamellae of Or-Ru-Ir alloys

S232

J F W BOWLES ETAL

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

S234

J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

coloured Pt-Fe alloys occasionally forming com-posite grains with minor hongshiite (PtCu) Thereis also one nugget of tulameenite (Pt2FeCu) andtwo nuggets of PGE-sulfides belonging to thecooperitendashbraggitendashvysotskite series (PtSndashPdS)One nugget of native Cu has been recovered Thenuggets are typically between 200 and 800 micromacross and the largest from the panned streamconcentrates are up to 15 mm times 14 mm in sizeThe shape of the nuggets is variable as shown in

Figs 2 and 3 Four euhedral nuggets were located(Fig 2andashc) The nugget shown in Fig 2b is veryclearly a cube with well preserved corners that havenot suffered abrasion The nugget in Fig 2a has(100) faces and five of them can be distinguished in

the image The region between the (100) faces hasbeen abraded but there appear to be the remnants of(111) faces present Subrounded to rounded shapes(n = 11 grains Fig 2dndashj ) or elongate shapes (n = 9Fig 2kndasho) are more common Some nuggets occuras irregular grains often with multiple isolatedplanar surfaces (n = 12 Fig 2pndashx)Several grains show pits and etched sub-parallel

furrows on the surface (Fig 2e k) There are alsosurface linear features that either intersect at sim60degor 90deg indicating a crystallographic control (Fig 2hi l ) The pattern of intersecting features maintainsthe same orientation across the whole surface of thenuggets suggesting that the nuggets are probablysingle crystals

FIG 3 Continued

S233

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

S234

J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

The nuggets are often partially coated by Fe- andAl-oxides or altered silicates commonly on irregu-lar grains that are also often covered by smallerPGM grains (Fig 2j p r s u x)There are many smaller PGM associated with the

Pt nuggets (Table 3) they occur singly or as clusterson the nugget surface or within the nugget They arealso seen as free grains Those at the nugget surfaceare generally larger (01 to 04 times the diameter of thenugget) whilst those within the nuggets are micro-metre sized but exhibit the same mineral assem-blage The surface PGM form irregular platelets upto 80 microm across consisting of Os plusmn Ir-Ru-(Rh) alloys(osmium according to the nomenclature in Harrisand Cabri 1991) as shown in Figs 2g j r 3i Thereare smaller irregular platelets up to 20 microm across ofPt-Fe and Pt-Cu alloys (Fig 2u u1) nonwater worneuhedral pyritohedra of Os-bearing laurite (sim130microm in diameter Figs 2t 3k) and crystals of laurite(RuS2) erlichmanite ((OsRu)S2) irarsite ((IrRh)AsS) cuproiridsite and cuprorhodsite ((CuFe) (IrPtRh)2 S4) that are sim50 microm across (Figs 2p s 3h)The osmium platelets can be set at a steep angle

or lie parallel to the surface of the Pt-Fe host (Figs2j r 3b g i also Bowles 1981 fig 1 Bowles1995 figs 8 9 10) The lauritendasherlichmanitecrystals project from the surface of the Pt-Fe alloyhost and are frequently euhedral (Figs 2p t 3h kalso Bowles 1981 fig 2 Bowles 1995 figs 1 7

Cabri 1981 figs 79AndashC and 726A) Cuproiridsiteand cuprorhodsite (Figs 2p s 3h) may also showthis relationship In polished section the contactbetween the host and the associated mineral is astraight line (Fig 3g h i) Examples (Fig 3k andBowles 1995 fig 10) demonstrate that this contactis between planar crystal faces of two crystals thatgrew at the same time In some cases the hostappears to have grown outwards to surround theassociatedmineral (Fig 2g j and Bowles 1995 fig10) A totally different relationship is seen (Fig 3cl ) where hongshiite is situated along the rim of a Pt-Fe alloy Here the contact between the two mineralsis curved or lobate and within the boundary of thewhole grain These textures suggest replacement ofPt-Fe by PtCu A cartoon diagram to illustrate theserelationships is shown in Fig 4

PGM on the surface of ilmenite grains

In a separate association very small (5 microm) PGMand native Au and Ag grains have been foundattached to ilmenite grains recovered from the pits(Table 3) In one case a cluster of cooperite andmalanite grains occurs on the surface of an ilmenitegrain (Fig 2y y1) and euhedral grains of Pd-Ag-bearing gold are also present A grain approximat-ing to native silver was also observed on the surfaceof another ilmenite grain (Fig 2z)

FIG 4 Cartoon to assemble various textures of the nuggets in one diagram The host Pt-Fe alloy (grey) may have asculpted or euhedral outline Laurite-erlichmanite (green) and Os-rich alloys (blue) exterior to the Pt-Fe alloy arenormally euhedral Osmium-rich alloys within the nugget are often euhedral Alteration of the Pt-Fe alloy to hongshiite(PtCu in orange) shows cuspate textures This sketch is drawn from examples shown in Figs 2 and 3 and from Bowles(1981 1995) Inclusions from the present work (Fig 3g k) show broken surfaces where they protrude from the hostObservations from previous work of the probable original form of these crystals are drawn with lighter tones of blue and

green

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J F W BOWLES ETAL

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

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J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

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J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

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MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

Composition of the PGM nuggets

Pt-Fe alloys

The 32 Pt-Fe nuggets recovered have a compositionranging from Pt3Fe to Pt26Fe14 Themajority of thenuggets (70) have a composition close toisoferroplatinum with a general formula of Pt3ndashxFewith x = 0ndash038 Compositions are plotted in Fig 5and representative analyses are presented in Table 4There is substitution of Pt by other PGE (mainly Pdwith Rh and rarely Ir andRu) and Fe byCu and to alesser extent Ni Palladium and Rh have beenrecorded in sim20 of the Pt-Fe nuggets recovered andthe concentration of either is usually lt8 at butrarely nuggets can contain as much as 25 at Pdand these can contain Pd-bearing inclusions

Hongshiite (PtCu)

Hongshiite (Fig 2g u u1) forms both irregular rimsto six of the Pt-Fe alloys (Fig 3c l ) and plates lyingparallel to the nugget surface (Fig 2u1) The

hongshiite has uniform Pt and Cu contents withoccasional minor Pd (av 18 at) Fe (18 at)and Rh (05 at) (Table 4)

Tulameenite

One elongate nugget of tulameenite was found(BW6 Table 3) It is 600 microm across and largelycoated with patches of Al-Si-(Fe)-oxides (Fig 2o)In section the nugget appears slightly cracked andhas very irregular and porous edges (Fig 3f ) Thecomposition is consistent throughout ((PtPdRh)2Cu125Fe075) having higher Cu than idealtulameenite and uniform Pd and Rh contentstotalling le40 at (Fig 5 Table 4) Unlike thetulameenite from Guma Water (Bowles 1981) notraces of Os and Ru were detected in this nugget

Pt-Pd sulfides

Two nuggets belonging to the cooperitendashbraggitendashvysotskite series were located during pitting beside

FIG 5 The compositions of the eluvial and alluvial PGM from the Mateki area plotted on a triangular (Pt+Pd)ndashCundash(Fe+Ni)diagramSomeanalysesof the small eluvial PGMarequalitativeAdditional data from the literature is included forcomparison

S235

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

BigWater (Table 3)One nugget is euhedral cooperite300 microm in diameter (Fig 2c) containing an averageof 42 at Pt and 10 at Pd along with low Ni andCu contents totalling 15 at The other nugget is anirregular grain of braggitendashvysotskite 250 microm indiameter (Fig 2x) that is partially coated by Al-oxides and has small plates of hongshiite 22 micromacross on its surface This nugget has an average of342 at Pd 143 at Pt and minor amounts of Fe(14 at) Ni (06 at) and Co (03 at)

Ru-Os sulfides

Members of the laurite (RuS2)ndasherlichmanite (OsS2)series occurattached toPt3Fe nuggets fromBigWaterand nonewere found as discrete grains (Table 4) One

example is an irregular grain of erlichmanite (Fig 2p)whilst the other grains are laurite with variable Oscontent the largest being a 50 microm diameterpyritohedron (Fig 2t 3k) with Os (av 119 at)Rh (23 at) and minor As (018 at)

Rh Ir Pt base metal sulfides

Two grains of the cuprorhodsitendashcuproiridsitendashmalanite series were found both attached to Pt3Fenuggets from Big Water (Table 3) One grain isabout 40 microm in diameter (Figs 2p 3h) with acomposition close to the cuproiridsite (CuFe) (IrRh)2 S4 and sim20 at Ir and 8 at Rh The othergrain is 30microm across (Fig 2s) and has acomposition closer to cuprorhodsite (CuFe) (Rh

TABLE 4 Representative compositions of the Pt-nuggets and large associated PGM from energy-dispersivespectrometry

Single nuggets Associated PGM

Pt3Fe Pt3Fe Pt3Fe Pt3ndashxFe Pt3ndashxFe Pt3ndashxFe Pt2FeCu PtCu PtCu OsS2 RuS2 Os-RuSample Alako1 BW6 BW2 BW6 BW6 BW9 BW6 BW2 BW6 BW6 BW2 BW6Figs 2b 3a 2e 2s 3j 2q 3i 2u 3l 2m 3e 2o 3f 2g 3c 2u 3l 2p 3h 2t 3k 3i

wtPt 9030 8951 8892 8812 8589 6143 7268 7498 7453 ndash ndash 155Pd ndash ndash 084 ndash 150 2319 142 ndash 040 ndash ndash ndashRh ndash 105 ndash 110 108 ndash 447 058 ndash ndash 374 108Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 1523 3282 2200Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 1409Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 5319 2900 6094Cu 071 035 037 198 084 271 1253 2507 2504 ndash ndash ndashNi ndash ndash ndash ndash ndash 057 ndash ndash ndash ndash ndash ndashFe 938 934 916 968 1025 1143 893 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 112 ‒S ndash ndash ndash ndash ndash ndash ndash ndash ndash 3067 3330 ndashTotal 10039 10025 9929 10088 9956 9933 10003 10063 9997 9909 9998 9966

Atoms per formula unitPt 288 286 288 271 266 159 189 098 098 ndash ndash 001Pd ndash ndash 005 ndash 009 110 007 ndash 001 ndash ndash ndashRh ndash 006 ndash 006 006 ndash 022 001 ndash ndash 007 002Ru ndash ndash ndash ndash ndash ndash ndash ndash ndash 033 062 035Ir ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 012Os ndash ndash ndash ndash ndash ndash ndash ndash ndash 061 029 051Cu 007 003 004 019 008 022 100 101 101 ndash ndash ndashNi ndash ndash ndash ndash ndash 005 ndash ndash ndash ndash ndash ndashFe 105 104 104 104 111 104 081 ndash ndash ndash ndash ndashAs ndash ndash ndash ndash ndash ndash ndash ndash ndash ndash 003 ndashS ndash ndash ndash ndash ndash ndash ndash ndash ndash 207 199 ndashTotal 40 40 40 40 40 40 40 20 20 30 30 10

ndash not detected

S236

J F W BOWLES ETAL

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

Pt Ir)2 S4 with about 12 at Rh 105 at Pt and73 at Ir A platinum-rich thiospinel resemblingmalanite (CuFe)(PtRh)2S4 containing Pt (av 172at) and Rh (103 at) was found on an ilmenitesurface (Fig 2y y1)

Osmium alloys

Alloys of Os-RuplusmnIr form part of many of thecomposite nuggets found in the Big Water and theAlako (Table 3) They occur as thin platelets lyingeither parallel to the host surface (Figs 2g j 3i) orat a steep angle to the surface (Fig 2q r) Some ofthe steeply set platelets are partially containedwithin the nugget and extend outwards from thenugget surface where damage can occur (Fig 3g)Inclusions of Os-RuplusmnIr alloys are seen in crosssections and in Big Water both Os-Ru-Ir alloyinclusions and Ru-Ir-(Os-Pt Rh) alloys occur in thesame host (Fig 3d )The osmium-rich alloys contain variable Ru (0ndash

37 at) and Ir (6ndash21 at) and minor Rh Pt andAs totalling up to 48 at About 70 of the Os-alloys analysed contain higher Ru than Ir regardlessof the relation to their hosts Similar variablecompositions were reported for the hexagonalplatelets of Os on the surface of the Guma Waternuggets (Bowles 1981)

Discussion

Contrasting explanations have been offered for theoccurrence of alluvial PGM nuggets (1) Nuggetsderived from the erosion of igneous rocks can travelwith soil that is washed down slope and into riversto be trapped in patches of gravel that form a naturalriffle The PGM are unaltered except for surfaceabrasion (Cabri and Harris 1975 Cabri andLaflamme 1997 Cabri 1981 Cabri et al 19811996 Malitch and Thalhammer 2002 Weiser2002) (2) Weathering of the rocks aided byorganic acids in the soil breaks down the primaryPGM releasing the PGE which move through thesoil in solution Soil close to a river is a locationwhere the contrast between the Eh and pH of thesoil and the river is highest A large change fromacidic to neutral conditions accompanied by areduction in Eh can cause precipitation of the PGEallowing the growth of nuggets This growth maybe aided by bacterial activity (Reith et al 2016)Areas of tropical weathering with high rainfall andabundant organic material seem to be whereneoformation can occur The distinctive mineralogy

(mineral assemblage size composition and surfacefeatures) of alluvial PGM led to proposals of aneoform origin for such PGM This view has beentaken during studies in Minas Gerais Brazil(Hussak 1906 Cassedanne and Alves 1992Cabral et al 2007) Yubdo Ethiopia (Ottemannand Augustithis 1967 Augustithis 1998) theWitwatersrand reefs and the Bushveld IntrusionSouth Africa (Cousins 1973 Cousins and Kinloch1976) and for the Freetown Peninsula in SierraLeone (Bowles 1981 1986 1988 1995)The alteration of primary PGM may involve

several stages during both serpentinization andweathering (Prichard et al 1994 Wang et al2008 Aiglsperger et al 2017) leading to theformation of oxidized PGM The Great DykeZimbabwe (Locmelis et al 2010) and theAguablanca deposit Spain (Suaacuterez et al 2010)offer notable examples The oxidized PGM are thenlost during the transition from PGM in soils orlaterites close to the source forming a Pt-alloydominated placer PGM as demonstrated byOberthuumlr and Melcher (2005) for the oxidizedMain Sulfide Zone and placer PGM in the GreatDyke The study of PGM in fresh rocks and closelyassociated saprolite in the Freetown area (Bowleset al 2017) has shown that the primary PGM havebeen destroyed by weathering This has released thePGE which have moved separately through thesaproliteThe processes postulated for PGM neoformation

include (1) long term weathering in an area thathas or has had a warm climate and high rainfall(2) destruction of the primary PGM and transport ofthe PGE in solution (3) organic compounds suchas humic or fulvic acids that are abundant in tropicalrain forest soils appear likely to be involved insolution and transport of the PGE (4) differentialmovement of the PGE in solution (5) concentrationof the least mobile PGE in soils close to the riversthe more mobile PGE (especially Pd) being carriedaway in solution by the rivers (6) accretion of theremaining PGE to form a new PGM assemblagepossibly where there is a change in Eh and pHconditions favourable for deposition and (7)bacterial action may assist or be responsible forthe PGM growthThe main evidence that PGM grow in situ as

neoforms is that they comprise a different mineralassemblage from the PGM in the source rocks Thedifferences are (1) PGE arsenides tellurides andsulfides present in the host rocks become lessabundant or disappear completely from the alluvialsuite Pt-Fe alloys including tulameenite Os-Ir

S237

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

alloys and lauritendasherlichmanite become moreabundant (2) oxidized PGM appear in weatheredor oxidized rocks Cloudy porous or filamentousaltered PGMmay also be present (3) there is a lossof the more soluble Pd which is present in the hostrocks but much less abundant in an alluvial mineralsuite (4) there is a considerable difference in size(typically three orders of magnitude) betweenmicrometre-sized PGM in the host rocks andmillimetre-sized PGM in the alluvial suite(5) some PGM in an alluvial suite show delicatecrystal features that would not survive mechanicaltransport these features include dendritic growthsperfect crystal faces and perfect edges and cornersbetween crystal faces The corners are the mostsusceptible to mechanical damage and the presenceof undamaged corners provides convincing evi-dence for the absence of abrasion The dendritic

PGM are not known to occur in the host rocks(6) There is over plating of mineral faces collo-form cyclic and fibrous textures not characteristicof PGM in the host rocksExamples of alluvial PGM showing these

textures have been described by Oberthuumlr et al(2003 2013) Zaccarini et al (2013) Bowles(198119861995) Cabral et al (2007 2008) andBurgath (1988)Some workers have pointed out that the question

of the smaller grain size in the host rocks comparedto the grain size of the alluvial PGM could be due tosampling For a given grade small grains are morenumerous and more likely to be encountered bychance in a polished surface than large grains It hasbeen suggested therefore that the large grains arepresent in the rocks but are not detected IndeedOberthuumlr et al (2003) have shown that larger PGM

FIG 6 Pie charts showing the changes in the Freetown PGM from the fresh source rocks to the laterite (Bowles et al2013 2017) and to the alluvial deposits in the Alako and Big Water streams BMS = base metal sulfides n = number of

PGM grains and their size range

S238

J F W BOWLES ETAL

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

are difficult to locate but do occur in the MainSulfide Zone (MSZ) of the Great Dyke ZimbabweLarge Pt-Fe aggregates do occur in the rocks of theNizhni Tagil complex and the VetnayandashVyvenka

Belt in Russia and the Alto Condoto complex inColombia (Duparc and Tikonowitch 1920 Cabriand Genkin 1991 Salinas et al 1992 Tistl 1994Zaitsev et al 1998) Where such large nuggets

FIG 7 Themineralogical evolution in the PGMassemblage from rock through theweathering profile to alluvial deposits(a) The variation in PtPd ratio and grain size for the Freetown examples described here and comparable PGM from theGreat Dyke Zimbabwe (Oberthuumlr et al 2003) (b) The possible processes involved in the alluvial concentration of PGMin the rivers (c) Freetown mineral assemblage from the PGE-bearing primary rocks through weathered rocks soils toalluvial deposits in the rivers The evolution of the PGM from the Great Dyke Zimbabwe (Oberthuumlret al 2013) is shownfor comparison BMS = base metal sulfides MSZ = main sulfide zone PGM shown by number of occurrences found

S239

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

occur in the host rocks they can unquestionably bereleased by weathering It has also been suggestedthat perfectly crystallized or delicate alluvial grainscould have been transported within a protectingrock matrix from which they have recently beenreleased (eg Hagen et al 1990 Weiser andBachmann 1999 Cabri et al 1996) Inclusions ofPGM and other minerals found in the alluvialnuggets such as chromite silicates sulfides orcomposite glass-silicate inclusions (eg Nixonet al 1990 Johan et al 1990 Augeacute andLegendre 1992) are taken to indicate a primaryorigin for the inclusions and therefore the nuggetsAlthough growth of placer PGM appears to be

favoured by a warm wet climate and a long period ofweathering the dissolution of igneous PGM theirtransport in solution and growth of the PGM in theplacer could be relatively rapid Vehicle catalyticconverters have been in use for lt40 years butparticulate Pt and Pd catalytic converter fragmentsoccur in road dust (Prichard and Fisher 2012) and areconcentrated in nearby acidic stream sediments(Prichard et al 2009) In the water courses the PGEcan become bioavailable after reaction with artificialadditives (phosphates from fertilisers NaCl fromroad de-icing) or natural organic compounds (espe-cially citrates and fulvic acids Pawlak et al 2014)Bowles et al (1994b 1995) examined the relevanceof the organic acids present in soils to the solutiontransport and deposition of the PGE Experimentsshowed that organic acids can take Pt into solutionfrom fine-grained PtS in as little as 393 days Thelaboratory experiments on bacterial growth of PGMby Reith et al (2016) were conducted over 112 daysThis is particularly significant because purelyinorganic growth at low temperatures is likely to bevery slow The evidence indicates that the dissolutionof igneous PGM their transport in solution andgrowth of the PGM in the placer could be rapidRecent studies have very reasonably shown that

alluvial PGM assemblages contain componentsderived by both mechanical means and by PGMalteration (eg Goodnews Bay Alaska Mardockand Barker 1991 Eastern Bushveld South AfricaMelcher et al 2005 Great Dyke ZimbabweOberthuumlr et al 2002 2003 2013 Uktus CentralUrals Zaccarini et al 2013)

Evolution of the PGM in the York areaFreetown

Lithogeochemical and geomagnetic surveys asso-ciatedwith thepresent studyhavenot encountered any

anomalies in the rocks or soils indicating PGM-bearing horizons within the catchment areas of thestreamsexamined inaddition to thosealready reported(Bowles 2000b Bowles et al 2013) Repeatedassays of samples of splits from those samples and ofrepeated samples from the same outcrops showconsistent results This shows that the Freetownprimary PGM grain size in the rocks is clearly smallwith no indication in the rocks of a lsquonugget effectrsquoThe PGM assemblage shows a development

from the source rock through the saprolite to thealluvial deposits (Figs 6 7) with the source rocksdominated by Pt-Fe alloys (Pt3Fe PtFe) andcooperite (PtS) with minor Pt-Rh-Ir base metalsulfides Pd-Cu minerals laurite tulameenite Pd-Pt alloys and scarce Pt-arsenides and Pt-antimonides The weathered rocks contain oxidizedPGM (Bowles et al 2013 2017) which marks thefirst stage of weathering In more weathered rocksthe PGM assemblage is dominated by Pt3FendashPtFeand Pt2FeCundashPt(FeCu) compositions and PtS Thealloys show a wider range of compositions thanthose in the source rocks Sulfides rich in Pt-Rh andosmian laurite remain as accessory minerals butPd-PGM are absent suggesting that both Pd and Cuare dispersed Within the saprolite only a few Pt3ndashxFendashPtFe3 alloys seem to survive (Fig 6)Weatheringof the PGM would enable PGE-bearing solutionsto drain towards both Big Water and the Alakoresulting in a similar alluvial PGM assemblage inboth rivers (Figs 6 7)The alluvial PGM nuggets are predominantly Pt-

Fe alloys mainly of Pt3Fe disordered Pt3ndashxFe is alsopresent with minor tulameenite and there is aninfrequent occurrence of native copper The PtFe intheweathered rocks does not appear in the alluviumThe Pt-Fe nuggets are often partly replaced byhongshiite (PtCu) This may be part of a relation-ship between hongshiite and the Pt-Fe alloys thathas been recorded fromBrazil where hongshiite hasbeen further altered to native platinum (Kwitkoet al 2002 Cabral et al 2008) Members of thecooperitendashvysotskite series (PtSndashPdS) laurite anderlichmannite also appear as large single grains inthe alluvium Less abundant phases (lt16 of thetotal number of PGM located) are the PGM andnative metal grains that occur enclosed in the Pt-nuggets or more rarely on the surface of ilmenite(Fig 2y z Fig 3) A comparable story is told by theweathered rocks of the Great Dyke Zimbabwewhere PGE bismuthotellurides and sulfarsenides inthe MSZ in fresh rock underground and theoxidized PGM in the oxidized ores are all missingin the alluvium Alloys of Pt-Fe that form a minor

S240

J F W BOWLES ETAL

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

proportion of the MSZ and the oxidized MSZ aremuch more abundant in the alluvial nuggetstogether with sperrylite cooperite braggitelaurite rare Pd-Sb-As compounds and Os-Ir-Rualloys (Oberthuumlr et al 2003 2013)The Freetownmineral assemblage reflects both Pt

and Pt Ir Os and Ru enrichment down slope asobserved in placer PGM deposits worldwide(Weiser 2002) It is the loss of Pd that is the mostnotable especially in the alluvial deposits In thepanned samples it is evident that the PtPd ratio ishighest in those samples that show the highest Pt Insamples rich in Pt where Pd was below detection thePtPd ratios can be even higher (Table 1) Thesesamples appear to be themost evolved and are foundin natural traps in the stream bedwhere the course ofthe stream is level and the water is moving slowly

Size

The alluvial nuggets in BigWater and Alako streamsare in the same size range as the colluvial PGM inGumaWater (maximumnugget size 2 cm) There is asharp increase in size from a maximum of 7 microm forthe PGM in the rocks and the laterite to the placersexamined in this study The nuggets reported herehave a maximum size of 15 mm but larger nuggets(up to 2 cm) have been reported by earlier workers(Pollet 1931) For example laurite is lt1micromacross inthe rock but occurs as 1 mm sized euhedral crystalsdownslope TheOs alloys not found in the rocks aretypically 50 microm across in the alluvium withoccasional examples up to 80 microm across The threeorders of magnitude difference between the PGM inthe alluvium and the source rocks contributes to thecontrast between the mineral assemblagesIn the Big Water small PGM (06ndash5 microm) have

also been found during this study with grains ofgold and silver forming clusters on ilmeniteIlmenite-rich black sand concentrates from theartisanal workings were assayed to give 240 gt Ptmetal and the slimes after titanium pigmentpreparation gave about 400 gt Pt (Pollet 1931)which suggests that these small PGM may be quitecommon in the stream black sands They are thesame size as the PGM in the igneous rocks andinclude cooperite and malanite so they may beprimary PGM but they may equally well representthe early stage of nugget growth

Shape

The Big Water and Alako are rejuvenated streamswith extensive stretches of water moving quickly

among rocks an environment where abrasion androunding of nuggets would be expected Thepresence of euhedral nuggets and nuggets withwell-preserved crystal faces indicates either that theirsource is very close or that they have grown in situSome nuggets display clear hexagonal and triangular-shaped depressions (Fig 2i p q t Fig 3b)suggesting a recent loss of attached euhedral grainsOther surface inclusions have survived abrasion forexample a large pyritohedron of laurite (Fig 3k) andan Os platelet (Fig 3i have been damaged) Theirregular surface of some PtFe alloys also suggestssome erosion has occurred as illustrated in Fig 4

Pt-Fe alloy compositions

Pt-Fe alloys with compositions approximating toPt2Fe have been reported previously especiallyfrom placer deposits Some examples are the Chocoacuteregion Colombia (Cabri et al 1996) the AikoraRiver Area Papua New Guinea (Weiser andBachmann 1999) the Great Dyke (Oberthuumlret al 2003 2013) British Columbia Canada(Barkov et al 2005) the Urals (Malitch andThalhammer 2002 Evstigneeva 2009 Zaccariniet al 2013) or Yubdo (Evstigneeva 2009) Malitchand Thalhammer (2002) and Evstigneeva (2009)discuss in detail whether a composition near Pt2Ferepresents a new mineral phase a fine intergrowthof PtFe and Pt3Fe (as suggested by Zhernovskyet al 1985) or a disordered structure that is stableunder particular conditions The majority of thegrains examined using X-ray diffraction consistedof a disordered cubic phase but the studies alsoconfirmed a micrometric intergrowth in two grainsfrom the Nizhni Tagil placersIt seems probable that the Freetown Pt-Fe alloy

with a composition near to Pt2Fe is cubic and issimilar to cubic tetraferroplatinum (Pt3Fe) butcontaining more Fe with some degree of disorder-ing of the structure It should probably be describedas Pt3ndashxFe where x = 016ndash038 in the alluvialnuggets There is a larger range of disordering inthe weathered Pt-Fe alloy where x = 002ndash058(Bowles et al 2017)The existence of the disordered phases with a

range of compositions is relevant to consideration ofthe genesis of these PGM At high temperature withgreater mobility of the metals there is a preferencetowards forming minerals with stoichiometriccompositions having a restricted compositionalrange and an ordered structure The inference thatcan be made from the presence of an almost

S241

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

continuous range of compositions leading to theformation of disordered phases is that formationoccurred under conditions of restricted mobility asimplied by a lower temperatureThe compositions of Pt-Fe alloys from similar

localities are plotted in Fig 5 Some of the Uraliannuggets described by Zaccarini et al (2013) haveisoferroplatinum-type solid cores (Pt3ndashxFe) sur-rounded by porous PtFe The PtFe in turn includesminute particles with a transitional Pt3ndashx (FeCu)composition (x le 035) that contain a higher base-metal content than either the alluvial grains fromFreetown or from the Great Dyke (Oberthuumlr et al2013) A minority of grains examined in the Uktuscomplex andGreatDyke have PtBM ratios close to 1that represent solid solutions of PtFe and Pt2FeCuThese all fall in the same field as the Pt-Fe-Cu alloysin the Freetown laterite (Fig 5) In other areas such asBorneo and Madagascar (Burgath 1988 Augeacute andLegendre 1992) the alloy compositions occupy thePt-rich part of the system

Conclusions

The suite of alluvial PGM identified in this studycomprises mainly Pt3Fe to Pt3ndashxFe and a lesserproportion of tulameenite and PtSndashPdS nuggetsLarge crystals associated with the nuggets areOs-Ir-Ru alloys lauritendasherlichmanite and PGEbase-metal sulfides (eg cuprorhodsite and cupro-iridsite) The presence of tulameenite and nativecopper as individual nuggets and hongshiitealteration of the rims of Pt-Fe alloy is an aspect ofthe involvement of Cu in the development of thenuggets A late generation of Pt-enriched isoferro-platinum is observed on a few nugget surfacesSome alluvial PGM such as tulameenite or Pt-sulfides can contain higher Pd than their counter-parts in the rocks This denotes changes in themineralogy and geochemistry between the primaryand alluvial PGM in FreetownThe significant points are

AssemblageThere is a distinctive difference between the PGMassemblage in the fresh rocks and the alluviumCooperite abundant in the fresh rocks is lessabundant in the streams whereas Pt-Fe alloys form amuch larger proportion of the alluvial mineral suiteHongshiite appears as an alteration to the alluvialPt-Fe alloys and tulameenite is more abundantPalladium minerals in the fresh rocks are notencountered in the alluvium

OxidesOxidized PGM in the weathered rocks (Bowleset al 2013 2017) are a transitional stage in thealteration of the primary PGM and the formation ofthe placer nuggets Their fragile textures indicatethat they are the result of alteration of primary PGMand mobility of PGE

DisorderingThe Pt-Fe alloys in the weathered rocks show analmost continuous range of composition from Pt3Feto PtFe and nearly PtFe3 indicating varying degreesof disordering due to low-temperature alterationThe alluvial Pt-Fe nuggets are closer to Pt3Fe

PalladiumThere is a significant loss of Pd relative to Pt Ratiosof PtPd of lt75 in the host rocks rise to 100ndash500 inthe alluvial deposits demonstrating the greatermobility of Pd in solution

CopperSome Pt-Fe alloys in the placers contain a smallproportion of Cu (03ndash27 wt) that increases thecompositional variability and degree of disorderTulameenite hongshiite and native Cu are presentand appear to be a part of the involvement of Cuwith some Pt-Fe nuggets being replaced byhongshiite at the edges Apart from the presenceof smaller grains of tulameenite these features arenot encountered in the fresh rocks

SizeThere is a three-orders of magnitude difference insize between the PGM seen in the fresh rockscompared with the larger alluvial mineral suite

Delicate crystalsThe delicate and well-preserved crystal features ofsome nuggets suggest a local origin damagednuggets have had a longer residence time in therivers

Organic coatingThe coating of organic material on the nuggets isevidence of an association between the nuggets andorganic material and may indicate involvement oforganic material or bacterial action during forma-tion of the nuggets

PGM on ilmeniteFine-grained PGM and grains of gold and silveroccurring on the surface of ilmenite correspond to

S242

J F W BOWLES ETAL

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

historic reports of an association of PGE withilmenite in the rivers andmay reveal initial stages ofnugget growth

DrainageThe presence of PGE-enriched horizons onlycropping out in the Big Water drainage basinsuggests that the PGE required to form the nuggetsin both the Big Water and Alako streams werederived from PGE-bearing solutions draining bothsides of the Mateki ridge rather than by mechanicalerosionTaken together these points clearly indicate

neoformation of the nuggets within the saproliteand alluvium after leaching of the PGE from thesource rocks and transport of the PGE in solutionThey are not indicative of derivation of all of thenuggets by erosion and mechanical transport

Acknowledgements

This work has been made possible by the technicalsupport of Drs J Rodriacuteguez RS Garciacutea J SanguumlesaM Saacutenchez-Lorda (SGIker UPVEHU) and generousfunding for SS from the Department of EducationUniversities and Research of the Basque Government(Refs BFI-2011-254 IT762-13) Our manuscript wasimproved by the helpful advice of two reviewers whowish to remain anonymous The panning and pittingwere done by the people of York Sierra Leone whowethank for their cheerful efforts

References

Aiglsperger T Proenza JA Zaccarini F Lewis JFGaruti G Labrador M and Longo F (2015)Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit Loma Caribe peridotite (DominicanRepublic) Mineralium Deposita 50 105ndash123

Aiglsperger T Proenza JA Font-Bardia M Baurier-Aymat S Galiacute S Lewis JF and Longo F (2016)Supergene neoformation of Pt-Ir-Fe-Ni alloys multistagegrains explain nugget formation in Ni-lateritesMineralium Deposita Published on-line 12 November2016 httpsdoiorg101007s00126-016-0692-6

Aiglsperger T Proenza JA Galiacute S Rius J Longo Fand Domenech C (2017) The supergene origin ofruthenian hexaferrum in Ni-laterites Terra Nova 29106ndash116 Published on-line 12 March 2017 httpsdoiorg101111ter12254

Augeacute T and Legendre O (1992) Pt-Fe nuggets fromalluvial deposits in Eastern Madagascar TheCanadian Mineralogist 30 983ndash1004

Augustithis S (1998) Platinum nuggets formation inlaterites Pp 221ndash224 in International Platinum (PNLaverov and VV Distler editors) TheophrastusPublications St Petersburg Athens

Barber MJ (1962) A Report on the Prospecting for GoldPlatinum andMolybdenum Carried Out During 1961-62 on Behalf of the Sierra Leone Government SierraLeone State Development Co Ltd Freetown 39 pp

Barkov AY Fleet ME Nixon GT and Levson V(2005) Platinum-group minerals from five placerdeposits in British Columbia Canada The CanadianMineralogist 43 1687ndash1710

Belete KH Mogessie A Hoinkes G and Ettinger K(2000) Platinum-group minerals and chrome-spinelsin the Yubdo ultramafic rocks Western EthiopiaJournal of African Earth Science 30 10ndash11

Belete KH Mogessie A and Bowles JFW (2002)Platinum-group minerals in the Alaskan type mafic-ultramafic intrusion of the Yubdo area WesternEthiopia Abstract 9th International PlatinumSymposium 3p httpsnicholasdukeedupeoplefacultyboudreau9thPtSymposiumBelete_Abstractpdf

Bowles JFW (1981) The distinctive suite of platinum-group minerals from Guma Water Sierra LeoneBulletin de Mineacuteralogie 104 478ndash483

Bowles JFW (1986) The development of platinum-group minerals in laterites Economic Geology 811278ndash1285

Bowles JFW (1988) Further studies of the developmentof platinum-group minerals in the laterites of theFreetown Layered Complex Sierra Leone Pp273ndash280 in Proceedings of the Symposium Geo-Platinum 87 (HM Prichard PJ Potts JFW Bowlesand SJ Cribb editors) Elsevier Applied ScienceLondon

Bowles JFW (1995) The development of platinum-groupminerals (PGM) in laterites mineral morphologyChronique de la Recherche Miniere 520 55ndash63

Bowles JFW (2000a) Prassoite vysotskite and keith-connite from the Freetown Layered Complex SierraLeone Mineralogy and Petrology 68 75ndash84

Bowles JFW (2000b) A primary platinum occurrence inthe Freetown Layered Intrusion Sierra LeoneMineralium Deposita 35 583ndash586

Bowles JFW Atkin D Lambert JLM Deans T andPhillips R (1983) The chemistry reflectance and cell-size of the erlichmanite (OsS2)-laurite (RuS2) seriesMineralogical Magazine 47 465ndash471

Bowles JFW Giże AP and Cowden A (1994a) Themobility of the platinum-group elements in the soils ofthe Freetown Peninsula Sierra Leone The CanadianMineralogist 32 957ndash967

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1994b) The development of platinum-group mineralsin laterites an initial comparison of the organic andinorganic controls Transactions of the Institute of

S243

MINERALOGY GEOCHEMISTRYAND GENESIS OF ALLUVIAL PLATINUM-GROUP MINERALS

Mining and Metallurgy (Section B Applied EarthScience) 103 B53ndashB565

Bowles JFW Giże AP Vaughan DJ and Norris SJ(1995) Organic controls on platinum-group element(PGE) solubility in soils initial data Chronique de laRecherche Miniere 520 65ndash73

Bowles JFW Lyon IC Saxton JM and Vaughan DJ (2000) The origin of platinum group minerals fromthe Freetown Intrusion Sierra Leone inferred fromosmium isotope systematics Economic Geology 95539ndash548

Bowles JFW Prichard HM Suaacuterez S and Fisher PC(2013) The first report of platinum-group minerals inmagnetite-bearing gabbro Freetown LayeredComplex Sierra Leone occurrences and genesisThe Canadian Mineralogist 51 455ndash473

Bowles JFW Suaacuterez S Prichard HM and Fisher PC(2017) Weathering of PGE-sulfides and Pt-Fe alloysin the Freetown Layered Complex Sierra LeoneMineralium Deposita Published on-line 20 January2017 httpsdoiorg101007s00126-016-0706-4

Burgath K-P (1988) Platinum-groupminerals in ophioliticchromitites and alluvial placer deposits Metatus-Bobaris Area Southeast Kalimantan Pp 343ndash403 inProceedings of the SymposiumGeo-Platinum87 (HMPrichard PJ Potts JFW Bowles and SJ Cribbeditors) Elsevier Applied Science London

Cabral AR Beaudoin G Choquette M Lehmann Band Polocircnia JC (2007) Supergene leaching andformation of platinum in alluvium evidence fromSerro Minas Gerais Brazil Mineralogy andPetrology 90 141ndash150

Cabral AR Galbiatti HF Kwitko-Ribeiro R andLehmann B (2008) Platinum enrichment at lowtemperatures and relatedmicrostructures with examplesof hongshiite (PtCu) and empirical lsquoPt2HgSe3rsquo fromItabira Minas Gerais Brazil Terra Nova 20 32ndash37

Cabri LJ (editor) (1981) Platinum-Group ElementsMineralogy Geology Recovery The CanadianInstitute of Mining and Metallurgy Special Volume23 267 pp

Cabri LJ and Genkin AD (1991) Re-examination of Ptalloys from lode and placer deposits Urals TheCanadian Mineralogist 29 419ndash425

Cabri LJ and Harris DC (1975) Zoning in Os-Ir alloysand the relation of the geological and tectonicenvironment of the source rocks to the bulk PtPt+Ir+Os ratio for placers The Canadian Mineralogist 13266ndash274

Cabri LJ and Laflamme JHG (1997) Platinum-groupminerals from the Konder Massif Russian Far EastThe Mineralogical Record 28 97ndash106

Cabri LJ Criddle AJ Laflamme JHG Bearne GSand Harris DC (1981) Mineralogical study ofcomplex Pt-Fe nuggets from Ethiopia Bulletin deMineacuteralogie 104 508ndash525

Cabri JL Harris DC and Weiser TW (1996)Mineralogy and distribution of platinum-groupmineral (PGM) placer deposits of the worldExploration and Mining Geology 5 73ndash167

Cassedanne JP and Alves JN (1992) Palladium andplatinum from Coacuterrego Bom Sucesso Minas GeraisBrazil The Mineralogical Record 23 471ndash474

Cousins CA (1973) Platinoids in the Witwatersrandsystem Journal of the South African Institute ofMining Metallurgy 73 184ndash199

Cousins CA and Kinloch ED (1976) Some observa-tions on textures and inclusions in alluvial platinoidsEconomic Geology 71 1377ndash1398

Duparc L and Tikonowitch M-N (1920) Le platine etles giacutetes platiniferes de lrsquoOural et du monde GeneacuteveSocieacuteteacute anonyme de eacuteditions Sonor 542 pp

Evstigneeva TL (2009) Phases in the Pt-Fe systemVestnik Otdelenia nauk o Zemle RAN 1 1ndash2

Forde W (2011) The Story of Mining in Sierra LeoneXlibris LLC Dartford UK pp 78

Fuchs WA and Rose AW (1974) The geochemicalbehavior of platinum and palladium in the weatheringcycle in the Stillwater Complex Montana EconomicGeology 69 332ndash346

Gregory S (1962) The raised beaches of the Peninsulararea of Sierra LeoneTransactions andPapers Instituteof British Geographers Publication No 31 15ndash22

Hagen D Weiser TH and Than H (1990) Platinum-group minerals in Quaternary gold placers in the upperChindwin area of Northern Burma Mineralogy andPetrology 42 265ndash286

Harris DC and Cabri LJ (1991) Nomenclature ofplatinum-group-element alloys review and revisionThe Canadian Mineralogist 29 231ndash237

Hattori KH Cabri LJ and Hart SR (1991) Osmiumisotope ratios of PGM grains associated with theFreetown Layered Complex Sierra Leone and theirorigin Contributions to Mineralogy and Petrology109 10ndash18

Hattori KH Chalokwu CI and Hart SR (1996) Fluidintroduction of 187Os into a solidified magmachamber Evidence from the Freetown LayeredComplex Sierra Leone EOS Transactions of theAmerican Geophysical Union 77 S-277

Hattori KH Takahashi Y and Augeacute T (2010)Mineralogy and origin of oxygen-bearing platinum-iron grains based on an X-ray absorption spectroscopystudy American Mineralogist 95 622ndash630

Hughes RH and Hughes JS (1992) A Directory ofAfrican Wetlands IUCN Gland Switzerland andCambridge UKUNEP Nairobi KenyaWCMCCambridge UK 820 pp

Hussak E (1906) O palladio e a platina do Brasil AnnaesEscola de Minas de Ouro Preto 8 75ndash188

Johan Z Ohnenstetter M Fischer M and Amosseacute J(1990) Platinum-group minerals from the Durance

S244

J F W BOWLES ETAL

river alluvium FranceMineralogy and Petrology 42287ndash306

Junner NR (1929) Report of the Geological Departmentfor part of the year 1927 and for the year 1928Government printer Freetown 17 pp

Junner NR (1930) Geology and mineral resources ofSierra Leone Mineralogical Magazine 42 73ndash82

Kwitko R Cabral AR Lehmann B Laflamme JHG Cabri LJ Criddle AJ and Galbiatti HF (2002)Hongshiite PtCu from Itabirite-hosted AundashPdndashPtmineralization (Jacutinga) Itabira District MinasGerais Brazil The Canadian Mineralogist 40711ndash723

Locmelis M Melcher F and Oberthuumlr T (2010)Platinum-group element distribution in the oxidizedMain Sulfide Zone Great Dyke ZimbabweMineralium Deposita 45 93ndash109

Malitch KN and Thalhammer OAR (2002) PtndashFenuggets derived from clinopyroxenite-dunite massifsRussia A structural compositional and osmium-isotope study The Canadian Mineralogist 40395ndash418

Mardock CL and Barker JC (1991) Theories on thetransport and deposition of gold and PGM minerals inoffshore placers near Goodnews Bay Alaska OreGeology Reviews 6 211ndash227

Melcher F Oberthuumlr T and Lodziak J (2005)Modifications of detrital platinum-group mineralsfrom the Eastern Bushveld Complex South AfricaThe Canadian Mineralogist 43 1711ndash1734

Ndjigui P-D and Bilong P (2010) Platinum-groupelements in the serpentinite lateritic mantles of theKongo-Nkamouna ultramafic massif (Lomieacute regionSouth-East Cameroon) Journal of GeochemicalExploration 107 63ndash76

Nixon GT Cabri JL and Laflamme JHG (1990)Platinum-group-element mineralization in lode andplacer deposits associated with the Tulameen Alaskan-type complex British Columbia The CanadianMineralogist 28 503ndash535

Oberthuumlr T andMelcher F (2005) Behaviour of PGE andPGM in the supergene environment a case study ofpersistence and redistribution in the Main SulfideZone of the Great Dyke Zimbabwe Pp 97ndash111 inExploration for Platinum-Group Element Deposits (JE Mungall editor) Mineralogical Association ofCanada Short Course 5 35 Mineralogical Associationof Canada Quebec Canada

Oberthuumlr T Weiser TW Gast L Schoenberg R andDavis DW (2002) Platinum-group minerals andother detrital components in the Karoo-ageSomabula gravels Gweru Zimbabwe The CanadianMineralogist 40 435ndash456

Oberthuumlr T Weiser TW and Gast L (2003)Geochemistry and mineralogy of platinum-groupelements at Hartley Platinum Mine Zimbabwe

Part 2 Supergene redistribution in the oxidized MainSulfide Zone of the Great Dyke and alluvial platinum-group minerals Mineralium Deposita 38 344ndash355

Oberthuumlr T Weiser TW Melcher F Gast L andWoumlhrl C (2013) Detrital platinum-group minerals inrivers draining the Great Dyke Zimbabwe TheCanadian Mineralogist 51 197ndash222

Ottemann J and Augustithis SS (1967) Geochemistryand origin of ldquoplatinum-nuggetsrdquo in lateritic coversfrom ultrabasic rocks and birbirites of W EthiopiaMineralium Deposita 1 269ndash277

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Prichard HM Jackson M and Sampson J (2009)Dispersal and accumulation of Pt Pd and Rh derivedfrom a roundabout in Sheffield (UK) from stream totidal estuary Science of the Total Environment 40190ndash99

Reith F Zammit CM Shar SS Etschmann B BottrillR Southam G Ta C KilburnM Oberthuumlr T BailAS and Brugger J (2016) Biological role in thetransformation of platinum-group mineral grainsNature Geoscience 9 294ndash298 Published on-line21 March 2016 httpsdoiorg 101038NGEO2679

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Tistl M (1994) Geochemistry of platinum-group ele-ments of the zoned ultramafic Alto Condoto complexnorthwest Colombia Economic Geology 89158ndash167

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Weiser TW and Bachmann HG (1999) Platinum-groupminerals from the Aikora River area Papua

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Zaccarini F Pushkarev E Garuti G Krause J DvornikGP Stanley C and Bindi L (2013) Platinum groupminerals (PGM) nuggets from alluvial-eluvial placerdeposits in the Uktus concentrically zoned mafic-ultramafic complex (Central Urals Russia) EuropeanJournal of Mineralogy 25 519ndash531

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J F W BOWLES ETAL