STAGES IN THE TROPICAL WEATHERING OF …minersoc.org/pages/Archive-CM/Volume_6/6-4-351.pdf · 352 P. E. Fairbairn and R. H. S. Robertson been so intense that no surface outcrops of

Clay Minerals (1966) 6. 351,

S T A G E S I N T H E T R O P I C A L W E A T H E R I N G

O F K I M B E R L I T E

P. E. F A I R B A I R N AND R. H. S. R O B E R T S O N

Selection Trust Ltd, Mason's Avenue, Coleman Street, London, E.C.2, and Dunmore, PitIochry, Scotland

(Received 3 January 1966)

A B S T R A CT : Numerous exposures of highly altered kimberlites in exploratory trenches and pits have afforded an opportunity of studying the very great changes which take place chemically and mineralogically under conditions of tropical weathering in Sierra Leone.

Diamondiferous gravels in the Kono and Kenema District of Sierra Leone, West Africa, have been worked continuously by Sierra Leone Selection Trust Limited since 1933 and constitute one of the world's major sources of gem and industrial diamonds: see map, Fig. 1. The only previous description of the Sierra Leone kimberlites has been given by Grantham & Allen (1960) whose paper contains a concise account of the original discovery of kimberlite in 1948, describes its petro- graphy and that of its xenoliths in detail with photographs of thin sections, and includes a map of the Yengema-Koidu area showing the distribution of the dykes as then known. During the past 4 years considerable further work has been achieved in tracing these dykes along strike by excavating lines of pits to bedrock and panning the overlying residual/alluvial gravels for 'kimberlitic minerals' (mainly ilmenite with subordinate pyrope garnet); a shaft is currently being sunk in order to obtain a depth bulk sample of a small but high-grade pipe near Koidu. Grantham & Allen brieflly mention the discovery of a second kimberlite zone on the Tongo lease area, 30 miles south of Yengema : during the past 2 years a surface prospecting programme in this area has resulted in the identification of three dykes similar to those already exposed at Yengema. The economic significance of kimberlite dykes is two-fold: if of sufficiently high grade they may constitute economic orebodies in their own right and in addition they serve as guides to the presence of three- dimensional pipes or 'dyke enlargements', which form more attractive mining targets than dykes owing to their greater tonnage for every foot of depth and the possibility of using cheaper larger-scale mining methods.

Grantham & Allen's petrological study of kimberlites was based exclusively on diamond drill cores; as they point out, tropical weathering in Sierra Leone has

352 P. E. Fairbairn and R. H. S. Robertson

been so intense that no surface outcrops of kimberlite are known. Webb (1959) began the study of kimberlite weathering here by measuring the cation exchange capacity of residual soil capping the Koidu No. 1 Pipe.

+" '- ; ~ F 7 i / - ! I

+ ~.+ ) / z , i'-,

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Po.+ l_~ko ,I I / / ' t ~ - ~ - - EMA c~+ r ~ c~, / / ' ~ LEASE A~2EA t

~ . , , j 7 ] l d

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15LANO ~ ~ /l

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$ H O W I N G P O S I T I O N OF ! i

__ ,i+.+ I + ~ " , . _ _ _ _ :

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FIc. 1. Map of diamond fields in Sierra Leone.

F I E L D W O R K

Prospecting for kimberlite dykes The kimberlite can normally be distinguished from country rock by its content

os residual ilmenite, but in a number of cases, particularly at Tongo, this diagnostic feature is absent, and it may be necessary to excavate trenches to a depth of 12 ft

Weathering of kimberlite 353

or more before reaching a horizon at which it is possible to distinguish kimberlite from the Archaean (mainly granitic) country rock. Considerable experience is necessary before a new field engineer can be confident of reliably identifying kimberlite in the zone of weathering.

Climate, topography, and vegetation Sierra Leone lies wholly within a tropical belt of high rainfall, the south-west

monsoons giving a wet season which lasts from April until the end of November. Records taken at Yengema and Tongo for the years 1959-63 inclusive provide the following average figures:

Average annual rainfall tin.) Average daily maximum shade temperature (~ Average daily minimum shade temperature (~ Average humidity---09.00 hours (%)

o/ Average humidity--15.00 hours (~,~)

Yengema Tongo 98" 1 96"9 91 "6 93 "4 67-2 69"3 90"2 86"7 91-4 81"1

The Yengema area is shown on Government 1 : 62,500 sheets 55 (Kayima S.E.) of 1929 and 56 (Sefadu S.W.) of 1930 and the Tongo area is on sheet 78 (Panguma S.E.) of 1932. The Yengema area is higher and shows more relief, ranging from roughly 1100 to 1400 ft A.M.S.L. with numerous isolated hills above 1500 ft and a few rising above 2000 ft. To the south of the area the Nimini Hills rise to 2400 ft and to the west the Bail and Sewa Rivers run in valleys deepened below 1100 ft. At Tongo relief is gentler, ranging from roughly 600 to 900 ft A.M.S.L. with only a few rounded hills above 1000 ft and with valley sides flatter than at Yengema.

Vegetation consists of tropical rain forest, which has been locally cleared to permit growing of rice, cassava, and cacao.

Sampling Since fresh kimberlites thousands of miles apart vary so little in composition,

it was not thought essential to obtain samples from a vertical profile. The purpose of the present study was to trace the stages of progressive weathering of kimberlite by determining the mineralogical composition of samples taken at various horizons in relation to the water-table. The original kimberlite might therefore be considered as a constant and samples could be taken from places where different stages of weathering could be most favourably represented. As long as granite xenoliths could be recognized they were avoided but in the more intensely weathered samples the material had become homogeneous and the analysed samples could therefore contain elements derived from xenoliths.

Six samples of kimberlite have been studied. Grantham & Allen's sample has been included as No. 1�89

1. Koidu Shaft (S 3602) The kimberlite dyke (Dyke Zone A) from which the two weathered samples,

Nos. 5 and 6, were taken was intersected at 300 ft depth by a crosscut from a shaft at Koidu. It is fresh-looking, hard, medium dark-grey rock.


1�89 Pipe No. 1, Koidu (Grantham & Alien's sample)

2. Pipe No. 1, Koidu (S 3431) The material was taken from the floor of opencast workings, 24 ft below the

surface where the rock is only slightly weathered.

3. River Mavehun, Tongo (S 3353) The main (Lando) dyke here was encountered at a point where it was exposed

on the right bank of the River Mavehun; it appears to have been well below the water-table.

4. Mine Cut, Upper Gaiya (S 3432) A sample of weathered kimberlite from a 2-ft dyke exposed in the floor of an

alluvial flat which had recently been mined. It is thought to have come from above the dry- and below the wet-weather water-table.

5. Trench No. 3 (S 3434) The weathered kimberlite came from a point 15 to 20 ft above the valley ttoor

and about 5 to 10 ft above the water-table.

6. Trench No. 2 (S 3433) The weathered kimberlite here came from a higher level than No. 5, at an

elevation some 40 ft above the neighbouring valley floor and about 20 ft above the water-table.

Macroscopic appearance Only the fresh rock No. 1 was hard; No. 2 can be scratched with the finger nail;

the others were cheese-soft though No. 2 contained some harder, less weathered lumps. The colour of the moist samples was compared with the 'Munsell Rock-Color Chart ' (1963).* The Munsell colour names and numerical descriptions are given in Table 1.

TABLE 1. Munsell colour descriptions

Sample No. Colour name Numerical description

I. S 3603 Medium dark grey N4 1�89 Blue green 2. S 3431 Greenish grey with dark spots 5 GY 6/1 with N4 spots 3. S 3353 Greyish olive 10 Y 4/2 4. S 3432 Dark yellowish brown 10 YR 4/2 5. S 3434 Moderate to light brown 5 YR 4�89 6. S 3433 Moderate brown 5 YR 4/4

L A B O R A T O R Y S T U D I E S

Microscopic Examination of the Jresh kimberlite Dyke Zone A in the unweathered state (Sample No. 1) is a massive hard

* Geological Society of America. distributed by Bailey Bros & Swinfen Ltd, London.


medium-dark grey to bluish porphyritic rock containing numerous xenocrysts of fresh olivine, ilmenite, phlogopite, and less abundant pyrope garnet. The xenocrysts have rounded outlines and resemble flattened ellipsoids. The largest diameter of the olivine and phlogopite is c. 4 cm, while the ilmenite and garnet are smaller, c. 1 cm, but much larger nodules of ilmenite occur at some places. The rock also contains many rounded or subangular fragments of granite (the principal country rock), dolerite and some amphibolite. There is often a selvage of talc around these inclusions. Ultrabasic inclusions of deep-seated origin are rare.

Thin sections show numerous perfectly fresh crystals of olivine, embedded in a groundmass of very fine-grained phlogopite, olivine, serpentine, and calcite. Scattered small (0.1 0.2 ram) black cubes of magnetite and ilmenite and brownish black equant crystals of perovskite are also present. The olivine occurs in three grain sizes, i.e. : (a) 1-4 cm, (b) 1-2 mm, and (c) from 0"1 mm downwards. The largest variety is unaltered, except for some slight black alteration along cracks, and is xenomorphic. The smaller ones are more idiomorphic and much altered to serpentine, although complete replacement does not occur. This obvious difference in alteration suggests that the large olivines are true xenocrysts of accidental origin, while the smaller ones are first and second generation crystals. Where measured, both types are biaxial positive, that is forsterite. The phlogopite is present as small length-slow laths with a very faint pleochroism in light brown and olive-brown. The laths show generally ragged and bent margins. Rare pseudomorphs of inter- growths of serpentine and magnetite may represent previous orthopyroxene.

Grantham & Allen's material, which came from No. 1 Pipe and not from a dyke, was blue-green, indicating incipient weathering, and contained only pseudomorphs of serpentine after olivine, large flakes of phlogopite, fragments of black ilmenite, and sporadic grains of garnet. The pseudomorphs often showed abundant exsolved magnetite though others were clear and suggested a variation in the iron content of the original olivine. The phlogopite frequently showed some degree of replacement by bright green chlorite, length-slow and anisotropic in ultra-blue, which extended along the cleavage planes into the mica.

A N A L Y T I C A L W O R K

Table 2 shows chemical analyses of samples 1-6, together with that of Grantham & Allen's sample '1�89 The analysis of Nos. 1 and 2 was made by Dr J. A. Clements of the Robertson Research Co. Ltd, Abergele, North Wales, and the other new analyses were made by W. H. Herdsman, Glasgow. The samples were equilibrated at a relative humidity of 56% for over 4 days before the estimation of H20-- .

The moisture content as received (in moisture-proof wrapping) and the cation exchange capacity are also given in Table 2.

X-ray examination of the weathered samples was carried out by three operators (I, II, III), thermal analysis by two operators (see Table 3) and electron-microscopy by one; their results, given in the description of each sample, established semi- quantitatively the phases that were present. These values were taken into account


when making a final mineralogical calculation from the chemical analyses. We have attempted to account for decaying mineral species as well as for newly-formed ones. Calculations were made to two or three places of decimals so as to enable them to be checked or improved upon.

TAnL[ 2. Chemical analyses of Sierra Leone Kimberlites fresh and weathered

1 1�89 2 3 4 5 6 S 3602 - - S 3431 S 3353 S 3432 S 3434 S 3433

Below Between 6-10 ft 20 ft Depth Depth 24 ft below water wet and dry above above 300 ft 260 ft pit floor table water-table water-table water-table

SiO2 28.3 33.25 43 -5 43.06 34-35 26.77 29 "60 TiO~ 1.65 2.09 1.13 4'33 6.44 2.83 2'41 ZrO~ 0.01

A1203 2.0 1-75 6.80 15.29 6.69 23'38 20.85 Sc2Os 0.005 N.D. 0-003

Fe~O~ 4.40 7 -99 2.90 13 -69 25.33 28.10 27-38 FeO 5'68 2"95 3.78 1.58 2.58 2.01 2"01 MnO 0.17 0"16 0.09 0.20 0.27 0-17 0-32 CoO (6 ppm)

NiO 0.15 0.18 0.09 CuO 0.01 (75 ppm) N.D. Cr203 0" 18 0 '23 0"08

MgO 31 '0 31 '40 20'0 2'46 3.98 1 '08 0-79 CaO 10.50 5.22 5.70 0'90 2-49 nil 0"11 SrO 0-20 0.03 0.20 BaO 0.20 0"07 0-20 PbO 0.001 N.D. 0-003 LizO 0.003 N.D. 0"01 Na20 0.02 0'10 1"71 0.06 0-09 0.06 0'07 K20 1.85 0'88 1.86 nil nil 0.21 0.48 Rb20 0"0 J N.D. 0"008

C N.D. N.D. N.D. 0"14 0"12 0 ' 2 6 0 ' 4 3

CO.., 8 -67 2"82 2"52 nil 0.06 0'24 0.36 F 0-15 0"12 0.10 P~O5 1.40 0.53 1-14 V205 0-017 S 0.10 0"13 0.01

H20+ 3.06 9.66 2-94 8.41 6.56 10.66 11-26 H 2 0 - 0.30 0.63 5-46 9.65 10.84 4-00 3"82

100.00,j 1 0 0 - 2 1 7 100.23~ 99.77 99-80 99.77 99-89

Moisture as received (~ ) - - 49.90 57.78 36.33 34.78 Cation exchange capacity

(m-eq/100 g dry clay) 51 29.7 37.5 8.1 7'5

Weathering of kimberlite

TAaLE 3. Differential thermal analyses and thermal gray,metric curves

357

3 4 5 6 S 3353 S 3432 S 3434 S 3433

Differential thermal analysis Operator I Endo 124' C Endo 142 ~ C

Endo 554 ~ C Endo 544 ~ C Exo 920 ~ C Exo 866 ~ C

Operator 1l Endo 129 ~ C Endo 140 ~ 154 ~ C Endo 545 ~ C Endo 450 ~ 505 ~ C Exo 800 and 900 ~ C Exo 850 ~ C

Inference 'smectite* 'smectite (little nontronite?) ' (more nontronite)'

Endo 120 ~ C Endo 130 ~ C Endo 302 ~ C Endo 304 ~ C Endo 554 ~ C Endo 560 ~ C Exo 958 ~ C Exo 950 ~ C

Endo 100 ~ C Endo 100 ~ C Endo 300 ~ C Endo 300 ~ C Endo 550 ~ C Endo 555 ~ C Exo 850 ~ C Exo 850 ~ C

'40 ~ goethite, '40 ~ goethite, 30 ~ kaolinite' 30 ~ kaolinite'

Thermal gravimetric curve 10.7)o to 110 ~

1'0~ 110-270 ~ C 1 .0~ 270-415 ~ C 3.7)~ 415-570 ~ C 0"4~ 570-1000 ~ C

1 3 . 1 5 ~ t o 1 1 5 ~ 4 - 2 ~ to 85~ 3 . 7 ~ t o l 0 0 ~ 2 ' 5 ~ 115-385 ~ C 3 . 0 ~ 8 5 - 2 7 0 ~ 1"8~o 100-243 ~ C 2 . 3 ~ 385-550 ~ C 2'0~o 270-325 ~ C 2-65~ 243-316 ~ 0 .05~ 550-1000~ 2 .25~ 325-435~ 1-65~o 316-441~

1.55 ~ 435-478 ~ C 3.95 ~ 441-566 ~ C 2 . 3 ~ 478-525~ 0 ' 5 5 ~ 566-680~ 0 . 9 ~ 525-543 ~ C 0.95 ~o 543 -1000~ C

* Montmorillonite-saponite group.

TABLE 4. Phlogopite in sample 1

K Na Rb Li I zl

SiO2 28.3 7-07 0"116 0"061 0"038 7"283 21"017 A1203 2'0 2"0 (0"033) (0'017) (0'01o) 2"0 - - Fe208 4.40 0-09~ 0'094 4"308 MgO 31.0 4.75 0.078 0"041 0"024 4.89~ 26"107 LiO2 0.003 0 . 0 0 3 0 . 0 0 3 - -

NazO 0.02 0 . 0 2 0'02 - - Rb20 0.01 0 . 0 1 0.01 - - K20 1.85 1 .85 1 "8 ~ - -

F 0-15 0"02~ 0.025 0"125 H~O+ 3'06 0'70 0'012 0.006 0'004 0"722 2'338

16-900

It is our convention to italicize the oxide or element which forms the basis of each mineral

calculation.


0

~ l l l l l l l l


Mineralogical calculations Sample No. 1, the fresh kimberlite. This calculation has been made after a

number of preliminary attempts at allocating the oxides and elements analysed. It has been assumed that P205 is in fluorapatite and that the remaining F is in phlogopite; all the alkalies have been allocated to this mica and, as there is not enough AI:O~, this has been made up with Fe203 (Table 4).

For the calculation of the accessory minerals it has been assumed that some of the CaO is in perovskite--indeed the same amount as in Grantham & Allen's analysis on sample '1�89 (Table 5).

At this stage olivine can be considered; H 2 0 + is allocated to serpentine and FeO and MgO to olivine, using up all the remaining silica (Table 6).

The O ~ F in this analysis is 0-024 if our allocations are correct and O = S is 0'043%. The total is thus reduced from 100-01 to 99"934% leaving 0"066 unaccounted for. It is reasonable to assume that BaO is present as barite; this requires only 0"1% sulphate. Thus we may have 0"30% barite, 0"28% SrCO~,and 13.64% CaCO.~; the remaining CO2 is allocated to 4-952% MgCO3; and a small quantity of MgO is left over, equivalent to 0-20,% brucite.

The calculated minerals have been summarized in Table 13.

TABLE 6. Olivine and serpentine in sample 1

Olivine Serpentine Z A

Fa Fo

sio2 21-077 7.792 1.494 11.73~ 21.017 - - MgO 26"107 7.84~ 15.75~ 23-59n 2-51, FeO 3"574 3"574 3"574 - - H20+ 2-33s 2"338

17.97~ 5-06s 27-482 32"55o

Calculation o[ the mineral constituent o] sample '1�89 Grantham & Allen calculated the composition of their sample (1�89 from painstaking microscope measure- ments. We have used the subtractive method to interpret their chemical analysis (by R. Pickup, Overseas Geological Surveys), see Table 7.

The accessory minerals were first calculated as for sample 1; to these we can add zircon and calcite (CaO 3.57 + CO2 2"80 = 6"37, not 9"1 as in the optical determination). The remaining CO2 (0"02 only) is allocated to SrO (0"03) to make SrCO8 (0"05); and BaO (0-7) is assumed to be in barite (0"105) although the SO~ (0"035) was not estimated.


The remaining oxides are then ready for calculation of the phlogopite, chlorite, and serpentine observed by Grantham & Allen.

TABLE 7. Phlogopite, chlorite and serpentine in sample 1�89 (Grantham & Allen)

Phlogopite Magnetite in serpentine

Na K, OH H Chlorite Serpen- Quartz A a b c Fe Mg tine

SiO2 33'24 0"67 3'37 2"16 2.56 AI~O3 1"75 0"19 0"95 1"61 Fe203 5"65 2.91 2.87 0.04 FeO 1 "29 1.29 0.01

I 0'45 2"26 1 "45 2.94 MgO 31.40

/ 0.09 0.45 0.29 Na20 0.10 0.11 K20 0'88 0"88 H20+ 9"66 0.33 0.31 H~O- 0.63 F 0"07 0"07

23"30 1 "18

23 "46

0.09 6-99 + 0.94

1.64 8.24 4.82 9.50 4.01 53.75 1.18 0.94 J ,r

24 "20

Notes: (a) There is enough F to account for 1-64 ~ of fresh phlogopite. This mica must already have suffered some degree of degradation, not to vermiculite, for there is not enough A12Oa for that, but apparently to chlorite. Owing to the extreme shortage of Al~Oz we have taken Fe~Oa for calculating the chlorite content, but it is to be understood that the sesquioxides would occur in both the phlogopite and the chlorite.

(b) Here the potash determines the mica and OH replaces F. (c) The rest of the alumina has been allocated to a weathered phlogopite in which K~O and F

have both been replaced by OH. By these three devices we can account for only 14'7~ of mica. The (ferriferous) chlorite has been calculated to make this 'constituent' 24.2 ~ as measured optically.

The remaining FeO almost uses up all the Fe~O3. We can bring in 0"0l MgO to complete the magnetite.

The remaining MgO 23"46 makes 53-75 serpentine; and leaves 1-18 quartz and 0.94 H20 + unaccounted for. This is perhaps rather high for adsorbed water. An alternative suggestion is to say that the serpentine itself is beginning to decay and that part of the MgO is, temporarily, in brucite; and that there is also more free quartz. We feel that this is a more likely picture of the rock at this state of alteration. The finish of the calculation then becomes:


T~LE 8. Altered serpentine in sample '1�89

361

9 ~, brucite Serpentine Free si l ica AH~O+

SiOz 24 '48 17-11 6.37 MgO 23 '46 6.23 17.23 HzO 7-93 2'77 5.13 0-03

55.87 9.00 39.57 6-37 0.03

The estimated mineral composition is given in Table 13. We agree with Grantham & Allen that at this stage the olivine is wholly weathered to serpentine and even beyond. The mica is more altered than the petrological examination suggested.

Calculation oJ the mineralogical composition o[ sample No. 2. The chemical analysis shows a far higher sodium content than any of the other analyses. If it were in albite it would surely have been detected by X-rays. If some of the Na is in phlogopite, as postulated for sample 1, the rest may very well be as exchangeable ions. The calculation to begin with follows that of sample 1, with the exception that the apatite requires all of the F, leaving none for the phlogopite.

Phlogopite thus calculated amounts to 17"13~%, and the accessories amount to 15"382%. The sulphides may have already oxidized, for there is not enough sulphur to satisfy even the nickel. There is no chalcopyrite or pyrite in this sample. The free nickel may be acting temporarily as an exchangeable ion.

Over 32% of the analysis has now been accounted for or 38% if moisture is included. The remaining 62% will consist of hydrated magnesium clay minerals-- vermiculite, chlorite or saponite and some free silica. The cation exchange capacity of 51 m-eq/100 g dry sample is the equivalent of 48"2 m-eq in the analysis. This suggests a high proportion of a smectite.

A powder pattern of sample 2 shows a mineral with a 10 A spacing making up, it was thought, about a third of the sample and a mineral with a 14-4 A spacing making up the remainder, except for about 5% of feldspar. The 10 A diffraction peak was sharp, suggesting a good mica, not illite. The 14-4 A peak is also quite sharp.

An oriented slide shows a broad diffraction band from about 10 to 14"4 A which shifts to about 17 A on glycol treatment. On heating to about 360 ~ C the expandable component completely collapses to about 10 .&. It is difficult to classify this expandable component but Grim (operator I) would call it a member of the montmorillonite group because it expands with glycol. 'It appears that the 14.4 A mineral has been degraded beyond vermiculite to montmorillonite' or what might be called a smectite, because another operator (III) established that the chief clay mineral of sample 2 was a trioctahedral smectite and that the l0 A material (illite or chlorite) was present in only a small amount. Sample 2 contained a little 8 A amphibole in the coarse material, according to operator III .


Since the X-ray diffractogram indicated a line at 14.4 A the choice for the remaining constituent lies between vermiculite and chlorite. The colour of the specimen could be due to chlorite or to ferrous saponite. Final choice fell upon vermiculite because even as little as 5 % of chlorite requires about 0"5 % of H20 +, which is too high a proportion to allocate thus.

In preliminary calculations it was found that 32"74% of vermiculite and 5-32% of Fe 2§ chlorite left one with 17"10% stevensite; 26-55% vermiculite left room for 19"97% ferrous saponite and 10-90% stevensite; 10% vermiculite left room for 45"10% ferrous saponite and 0"78% stevensite; and 8% vermiculite virtually eliminates the stevensite (see Table 9).

TABLE 9. Clay minerals in sample 2

Saponite Vermic- Steven- Free

ulite A Fe z+ Mg site silica A

Sit8 36'115 3"91a 32"197 2'648 22"490 0"107 A12Os 4"79 0"950 3"84o 0"404 3"436 FeO 2-59x 2'591 2"591 MgO 15"035 2-620 12"407 12-35o 0"054 Cat 0-205 0.205 NasO 1 "69 1-69 1.69 H~O+ 2.20 0"504 1-69e 0-216 1"838 0-008 H~O- 5-46 (1"344) (4-11e) (0"864) (7-352) (0-032)

6"942

-0"366

8.000 5-859 40.807 0.190 6.942 J

~r

47"835

We have assumed that Na + (54.5 m-eq) is the exchangeable ion here. The measured value was 48"2 m-eq. I t seems probable that sodium ions would accumulate at this level, just below the water table.

The 'hydroxyl error' (H20 + calculated -- HzO-t- observed) of --0-366 would normally be considered an objection to the validity of the calculation, but where there are degraded clay minerals one may suppose that some of the hydroxyls are loosely bound and are released below 105 ~ C. The hydroxyls of the newly formed saponite come off at a much higher temperature, as the d.t.a, shows.

The mineral composition is shown in Table 13. Mineralogical composition of sample 3. X-ray examination showed: (I) poorly

crystallized montmorillonite and some mixed-layer material, (II) montmorillonite and a little mica in large crystals, and (III) mostly a smectite with some quartz. Electron diffraction (II) showed thirty-four lines agreeing with montmorillonite in MacEwan (1961, p. 191); the ring diagram was almost complete--seldom so perfectly seen. Electron micrographs show montmorillonite in the characteristic cloud-

PLATE 1

~ i: :!i!ii!!i �84

i

d ,:iii!i~iii!!i~ii!!Jiiii!

Electron micrographs showing: (a) smectite and colloform particles in sample 3, (h) nontronite laths in sample 4, (c) sheaves of young kaolinite in sample 6, (d) young

kaolinite in sample 6. (Facing p. 362)


like form. It often surrounds tabular crystals. A large corroded plate may be mica. There is much colloform material of minute particle size, probably silica gel and allophane (Plate l a). Thermal analysis suggests smectite (some nontronite).

To calculate the mineralogical composit ion of this sample one must take into account the cation exchange capacity of 26"9 m-eq/100 g. If it is assumed that the smectite in it has a cation exchange capacity (c.e.c.) of 100 and the nontronite a c.e.c, of 60 m-eq/100 g (for it is often lower than montmorillonite), we could have 24"75% nontronite and 14-85% montmorillonite, say 25% and 15% respec- tively. The rest of the calculation is explained in notes.

TABLE 10. Mineral composition of sample 3

Smectites Halloy-

Carbon- 25~ 157o Chlorite site or Ana- Free aceous non- mont- allo- tase silica matter tronite moril- MgO FeO Goeth- phane

lonite ire

SiO2 43 "06 TiO2 4"33 A1~O3 15-29 Fe2Oa 13-69 FeO 1 "58 MnO 0.20 MgO 2.46 CaO 0.90 Na~O 0.06 C 0"14 H20 + 8.41 H20 - 9.65

0"14 0"10

10"77 6'79 1 "79 0"77 8'29

1 "65 4-75 1 "30 0-56 7"03 7"78 5"91

1 "58

0-45 0"38 0 "47 0.06

2"08

0'87 0-51 0'77 0'33 0"67 2'48 "/ Y 3"48 2"04 2-48

4"33 14.65

99-77 0.24 25~00 15.00 5.94 3.24 6-58 20.28 4.33 14.65

9.18

The rest of the MgO, after using some in the montmorillonite to account for the c.e.c., is allocated to chlorite by the convention of Brough & Robertson (1959), as is the FeO. This means that we have assumed that the magnetite has by now been oxidized and hydrated. The perovskite too seems to have decayed away; hence TiO2 is allocated to anatase.

The Fe203 has been allocated to goethite because the colour of the clay has a yellowish component. The goethite content has perhaps been a little exaggerated, since it is conceivable that some Fe203 may be in a chloritic component, although the low alumina precludes a large proportion of this mineral. The remaining alumina has been allocated to allophane or hydrated halloysite, partly because the colloform particles seen in the electron micrographs suggest the former, and partly because

364 P. E . Fa i rba i rn a n d R . H . S . R o b e r t s o n

there is enough H20 + and H20- - left over to accommodate the extra H~O required. Moisture in Table 13 is calculated on the assumption that hydrated halloysite is present.

Mineralogical composition o] sample 4. The basal spacing of 14.7 A increases to 16 A by glycol treatment (1); there is some 7 A kaolinite or chlorite. Mont- morillonite may be formed from the chlorite. A second operator (III) showed that the sample contained mostly smectite and vermiculite with some coarse magnetite. The 060 line at 1.52-1-53 A appears to suggest a trioctahedral smectite though it may indicate nontronite.

Electron diffraction (II) showed twenty-four lines, mostly agreeing with MacEwan's montmorillonite, but some might be referred to illite. In electron micrographs, montmorillonite is recognizable, but is accompanied by ribbons like nontronite (Plate I b). Tabular forms are also present, some with holes, suggesting solution.

Thermal analysis suggests less smectite than in sample 3 and more nontronite. Preliminary calculation showed that a c.e.c, of 37"5 m-eq/100 g is equivalent

to about 62-5% nontronite. As alumina has decreased and Fe203 has increased one gets the impression that the smectite here must be mostly nontronite. The magnesia may occur in chlorite, but not perhaps the FeO in these very oxidizing conditions. The high TiO2 content suggests that ilmenite or perovskite may have persisted. The remaining Fe203 is allocated to goethite, There is some free silica left over, though silica gel is not apparent electronoptically.

TABLE 11. Mineral composition of sample 4

Carbon- 62"5 % aceous nontr- Mg- Ilmen- Perov- matter Calcite onite chlorite ite skite

Goeth- Free Anatase ite MnOOH silica

SiO2 34"35 TiO~ 6"44 A1203 6"69 Fe203 25.33 FeO 2"58 MnO 0.27 MgO 3 -98 CaO 2-49 Na20 0'09 K~O nil CO,., 0"06 C 0.12 H~O+ 6.56 H,O- 10-84

0"12 0"09

0"08

0 "06

26 "82 3 "46

4"14 2"51 19"45

1 '05 0'09

2"19 8 "76

3'98

1 '48

2"87 1 "94 1 "63

2"58

1 "36

5 "88

0"66

0"27

0-07

4.07

0.21 0"14 62.50 11.43 5.45 3.30 1-63 6-54 0"34 L--,

6-88

4"07 J

tg5 ~a f~

c- O

8

~q

<

b-


O e , ' ~

I

b,.

r

e~

7"

v

~D

t'q oo

0 0

0

C7~

e~ 6

~ 6

C

r,,"

e-q

365


T~O3LE 13. Summary of mineralogical analyses

Sample

1 1�89 2 3 4 5 6

Olivine 32.55 Serpentine 17.97 39.57 Brucite 0-21 9.00 Free silica 6.37 6.94 14.64 4.07 3.64

Phlogopite 16.90 t4.70 17.14 Chlorite 9-50 9-14 11.43 3.11 4.83

Vermiculite 8-00 Saponite, ferroan 47.84 Nontronite 25.00 62.50 6.25 3-13 Montmorillonite 15.00

Allophane or halloysite 20.28

Kaolinite 46.53 40.85 Boehmite 3.08 lllite 3.24 7.43

Magnetite 6-24 7.49 4.00 Goethite 8.66 6-54 29.10 29-39 MnOOH 0.34 0.21 0.40 lhnenite 0.46 1.29 5.45 3.42 3.00 Perovskite 2.40 2.40 1-92 3-30 Anatase 4.33 1.63 1.03 0-83

Apatite 3.40 1.24 2.74 Chromite 0.27 0.30 0-12 Sulphides 0-30 0.33 0-13 Zircon 0.02 Barite 0.30 0-11 0.30 Celestine 0.35 Carbonates, Ca 13.64 6.37 5.73 0.14

Mg 4-95 Sr 0.29 0.05 Fe 0.63 0.95

Carbonaceous matter 0.24 0-21 0.45 0.71

Moisture (calculated) 0.30 0.63 5.09 4.33 4.15 2.77 4.62

We thus have 2"09% H,~O+ and 0 .04% AI~O:~ u n a c c o u n t e d for. T h e f o r m e r

m a y be assoc ia ted wi th the non t ron i t e and the la t te r m a y be n e g l e c t e d . T h e M n O

can conven ien t ly be inc luded with the goethi te .

Mineralogical composition o[ sample 5. X- r ay e x a m i n a t i o n (I) showed a 7 A

minera l (kao l in i te o r i ron-r ich chlor i te) and a b o u t 30% of deg raded chlor i te , poss ib ly

as a m ixed - l aye r minera l . A n o t h e r d e t e r m i n a t i o n ( I I I ) showed mos t ly d i so rde red

kao l in i te wi th a smal l a m o u n t of i l l i te; goe th i te was also present.


Thermal analysis shows about 40,% goethite and 30% kaolinite. Since this sample has been very severely weathered we may assume that FeO

occurs in a little resistant ilmenite and not in a smectite or chlorite. The COs is assumed to have been derived from rotting vegetation and to have formed a little chalybite. The rest of the FeO is allocated to ilmenite, the MgO to chlorite and the potash to illite. The c.e.c, of the analysed clay is 7'8 m-eq; If the contributions to c.e.c, made by illite, kaolinite, chlorite, and organic matter are deducted, about 4 m-eq may be ascribed to residual nontronite--in amount about a tenth of that in sample 4. (A smectite is still to be seen in electron-micrographs of sample 0, so it is assumed to be present also in sample 5.) The remaining silica is allocated to kaolinite; the remaining alumina to boehmite and the remaining Fe203 to goethite, as shown in Table 12.

Mineralogical composition o/sample 6. X-ray examination (I) showed no difference from sample 5; but a spacing at 9'82 A (II) suggested that illite or mica is present; operator IlI found mostly disordered kaolinite with a trace of illite; magnetite and goethite are also present.

Electron diffraction showed twenty lines easily indexed to a mica lattice. Electron- micrographs still show some montmorillonite. Tabular particles have a sharp periphery like mica.

Most of the sample consists of sheaves of elongated particles (Plate lc), looking like hydrated halloysite tending to become kaolinite. The particles, though elongated, are mostly flat and appear to be joining up to form parallel oriented aggregates (Plate ld). Goethite is present in the form of a very fine pepper.

Thermal analysis suggests 40% goethite and 30% kaolinite. Since samples 5 and 6 come from similar terrain the same procedure for calcula-

tion was used, including the assumption that a little chalybite may be present. The final calculation is given in Table 13.

D I S C U S S I O N

The weathering of kimberlite in the Russian province of Yakutia has been studied by a number of research workers. The original kimberlite assemblage was considered by Litinskii (1961) to have been olivine, phlogopite, picroilmenite (ilmenite rich in Mg), magnetite, pyrope, perovskite, apatite, chrome-diopside, and chromite. The fresh rock is now mainly a phlogopite-calcite-serpentine assemblage. Davidson (1964) is inclined to think that the carbonates were an original constituent of kimberlites, which have thus a relationship with carbonatites. Our present results confirm that carbonates are present in greatest amount in the deepest and freshest rock.

Rozhkov (1963) has written extensively on the weathering of kimberlite breccias in this province. His samples always contained a proportion of minerals unaltered by hypergene processes. Olivine is completely replaced by serpentine, as in our sample 1�89 or by carbonates and serpentine. Phlogopite is replaced by chlorite. Pyrope


is replaced by aggregates of serpentine, chlorite, carbonates, and limonite. Ilmenite is altering to leucoxene.

When the kimberlite has altered to a dirty green colour, near perhaps to our samples 1�89 or 2, unaltered or slightly altered minerals still persist, such as serpentine, carbonates, chlorite, ilmenite, pyrope, magnetite, chrome-diopside, leucoxene, along with iron minerals and clay minerals.

In the Udachnaya-Zapadnaya pipe the most characteristic clay mineral is a smectite with N~ = 1-530-1"531; and with d.t.a, peaks at 70-170~ (several), 580-640 ~ C and 770-820 ~ C. The smectite is a pseudomorph after the mica whose charactristic sharp outlines may still be seen in electron-micrographs. (In a sample from the ' Iksovaya' pipe X-ray analysis still showed some distinct lines of hydromica along with lines attributable to sepiolite.)

In almost all samples from the zone of weathering of the 'Udachnaya' and 'Ikso- vaya' pipes one finds chlorite, calcite, hydrated iron oxide and sepiolite, as well as an admixture of montmorillonite and hydromica, quite easily detected by d.t.a, and electron microscopy. The endothermic peaks are identified thus: 106-126 ~ C, adsorbed water; 320-342 ~ C, hydrated iron oxide and possibly some organic matter; 572 612 ~ C (dehydroxylation of the silicates); 794-848 ~ C, breakdown of montmorillonite and calcite; 1025 ~ C, breakdown of hydromica.

Perhaps the most advanced weathering of kimberlite observed in Siberia is in the so-called 'yellow-ground', which may perhaps be compared with our sample 4, at least in so far as 'montmorillonite' is the predominant clay mineral. In the 'Mir" pipe the clay fraction is said by Rozhkov to consist basically of a mixture of montmorillonite and .-cerolite, to which Ginzburg & Rukavishnikova (1950) gave the formula, 3MgO, 3SiO~, 3H20. They said that its X-ray spectrum was similar to chrysotile and serpophite (a variety of serpentine), and that its structural formula was analogous to attapulgite, namely Mgz(Si4010)(OH)z, 0"4Mg(OH)2. Rozhkov admits that the .-cerolite of the weathered kimberlite is probably a mixture of sepiolite and minerals of the antigorite group. It is important to note that in the climate of Siberia magnesium ions are retained and used for forming new minerals. The d.t.a, curves of the 'Mir ' yellow earth have endotherms at 150 ~ C and 675 ~ C and an exotherm at 825-840 ~ C. The curves correspond well with those of a-cerolite, though the refractive index N,~ = 1"570 with low birefringence is a little higher than that of the type material. Rozhkov however admits that the X-ray lines of ,~-cerolite and sepiolite are rather hard to tell apart.

C O N C L U S I O N S

Although there are some large gaps in the story of the weathering of kimberlite in Sierra Leone, the general direction of changes can now be seen. The original kimberlite contained almost fresh olivine and phlogopite, and at 300 ft in depth the rock is substantially unaltered. Magnetite, ilmenite, pyrope, and perovskite are common, and calcite and dolomite of igneous origin may make up nearly 19% of the rock.


The conversion of olivine to serpentine is complete in a sample taken from 260 ft in depth, and the phlogopite, though fresh-looking under the microscope, has already lost most of its fluorine and is presumably substantially hydrated. Some of the material of this mica may already have changed to a hydrated magnesium silicate such as chlorite, low in alumina.

Somewhat higher up the profile, sample 2 shows remarkable changes; phlogopite, though it still preserves sufficient structure to give identifiable X-ray lines, is probably much altered. The serpentine which we saw in sample '1�89 had become partly decomposed to brucite and free silica, seems to have recrystallized as vermiculite; but although this mineral has enough organization still to give X-ray lines it has really become mostly altered to a ferroan saponite. Sodium ions have apparently accumulated here.

This unstable assemblage catastrophically loses most of its magnesia as we trace the profile upwards, so that just below the water-table even the smectites are different, for we find nontronite and montmorillonite repacing saponite. The other major constituent here appears to be ailophane; and goethite has risen to over

o / 8%. Free silica is highest at this stage at over 14/o. The removal of magnesia from the silicate and of the carbonates has resulted

in a loss of 40-50% of the original rock substance. Both the serpentine and the phlogopite have been reduced to hydrated silica

residues, and the iron minerals have been converted mainly to goethite. There may have been a little loss of iron at this stage. Alumina, however, has been astonishingly acquired and has increased by a factor of five, even allowing for the 40-50% loss of substance. We suspect that it has reacted with silica residues to form allophane, and that the remaining silica is opaline or amorphous.

The hydrated silica next reacts with more ferric oxide, further supplies of which have risen to a level above the dry season water-table from below, and a highly nontronitic rock is formed. Some chloritic interlayers may be present.

This very unstable state of affairs doubtless occurs in a narrow band between the wet and dry season water-tables. When the kimberlite dyke is traced to near the surface, where it shows the familiar characteristics of 'lateritic weathering', we find high goethite contents, growth of disordered kaolinite to a major constituent lhrcugh further acquisition of alumina, and rather surprisingly, a late increase of illite, though some residual nontronite persists. Can it be that the potash from below is drawn to the surface and caught during the degradation of the nontronite? There is a suggestion that this is a transitory phenomenon and that the illite itself will soon be degraded in the gradual transition towards a typical latosol consisting of kaolinite minerals, goethite, and hydrated alumina compounds.

The changes are characterized also first by an increase in silica, due to the extrac- tion of the alkaline earth oxides and potash; and then, especially above the water- table, to a gradual decline. Sample 5 appears to be slightly peraluminous, but in the neighbourhood we may expect the kimberlite to become, in suitable situations, even more highly aluminous. The alumina, it should be added, comes from the


weathering of the granitic rocks which surround the kimberlite dykes, and also from the granitic xenocrysts.

A C K N O W L E D G M E N T S

We wish to thank Professor Ralph E. Grim and Professor O. E. Radczewski for their X-ray and electron optical work, Dr T. G. Carruthers for thermal analysis, Dr A. I. A. Janse for petrological observations, Mr K. E. S. Applin of the Prospecting Department of Sierra Leone Selection Trust Limited for critical reading of the paper in draft, and Sierra Leone Selection Trust Limited for defraying the cost of the work and for permission to publish.

R E F E R E N C E S

BROUGH J. • ROBERTSON R.H.S. (1959) Clay Miner. Bull. 3, 221. DAVIDSON C.F. (1964) Econ. Geol. 59, 1368. GINZBURG l.I. & RUKAVISHNIKOVA I.A. (1950) Zap. imp. miner. Obshch. s6r. 2, 79, 33. (Mineralog.

A bstr. 11-405). GRANTHAM D.R. & ALLEN J.B. (1960) Overseas Geol. Miner. Resour. 8, 5. LrnysKn V.A. (1961) Geochemistry, Ann Arbor, 9, 813, translated from: Geokhimiya, 742.

(Mineralog. A bstr. 16-274). MAcEWAN D.M.C. (1961) The X-ray Identification and Crystal Structure of Clay Minerals

(G. Brown, editor), Chap. IV, p. 191. Mineralogical Society, London. ROZHKOV I.S. (1936) Kora Vyvetriv. 6, 203. WEBB J.S. (1959) int. Geol. Congr. XXth Session, Symposium on Geochemical Prospecting,

1, 143.

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