ARTICLE

A. H. Wilson á C. A. Lee á R. T. Brown

Geochemistry of the Merensky reef, Rustenburg Section,Bushveld Complex: controls on the silicate frameworkand distribution of trace elements

Received: 7 October 1998 /Accepted: 5 March 1999

Abstract This whole rock and silicate mineral studyfocuses on the genesis of the Merensky reef sequence, aswell as the footwall and hanging wall norites at an areaof Rustenburg Platinum Mines in a demonstrably nor-mal (undisturbed) environment. Continuous samplingprovides major and trace element variations and mineralcompositions and allows an evaluation of the post-liquidus processes which a�ected the sequence. Follow-ing the formation of liquidus phases three stages areenvisaged to have modi®ed the rocks. These are (a)migration of ¯uid during early compaction of cumulates,(b) circulation of ¯uids within the crystal mush, and (c)reaction and solidi®cation of trapped liquid. Liquiduscompositions are nowhere preserved in the sequence. Astrong link is demonstrated between orthopyroxenecompositions (e.g. Mg # and TiO2) and the incompatibletrace element content of the whole rocks. The ®nalamount of trapped liquid is shown to have been variablebut never exceeded 10%. Calculated liquidus (pre-equilibration) orthopyroxene compositions show an up-sequence progression of evolving compositions from thefootwall norite to the hanging wall norite. Initial Srisotopic values do not support a simple magma mixingmodel by which radiogenic Main Zone magma mixeswith that of the Critical Zone at the level of theMerensky reef. There is evidence that the hanging wallnorite formed from a much more evolved magma. Theseconclusions have implications for the distribution andorigin of the PGE-enriched Merensky reef package.

Introduction

The Merensky reef is the world's major source of theplatinum-group elements (PGE) as well as being a majorresource for Au, Cu, Ni and Co. It can be tracedthroughout the Bushveld Complex, where it is mined(Fig. 1). The dip ranges from 9° to 27°, with small sec-tors as steep as 65°. Seismic surveys show that re¯ectorsthat correlate with the position of the Merensky reef canbe traced as far as 50 km down-dip of outcrop, and asdeep as 6 km below surface (DuPlessis and Kleywecht1987). Lithological and stratigraphic variations of thissequence are well documented (e.g. Vermaak 1976;Wagner 1929 has the best summary of the di�erent stylesof the reef and the relative positions of the higher PGEvalues and base metal sulphide distributions). The variedgeological environments of the Merensky reef have beenthe basis of many models for mineralisation in layeredma®c bodies, but in spite of the economic importanceand its place in the scienti®c literature, broad-basedconsensus as to its origin has never been achieved.

The various schools of thought relating to the originof the Merensky reef have revolved around (a) themagmatic model in which the silicate mineral assem-blage and sulphides carrying PGE were essentially liq-uidus components, and (b) the hydrothermal model bywhich ¯uids arising from the underlying crystal pile hada profound in¯uence on the rock and mineral compo-sitions and certain groups of elements including thePGE. Coupled with the wholly magmatic model is thehypothesis that magma mixing gave rise to the Meren-sky reef and its associated mineralization (Campbellet al. 1983).

The understanding of the role of interstitial liquidand postcumulus processes in the evolution of crystalmushes is central to interpreting the textures and mineralcompositions. On a theoretical basis it has been shownthat partially molten crystal mushes may become com-pacted at critical thicknesses (Sparks et al. 1985;McKenzie 1987) and that original textures may become

Mineralium Deposita (1999) 34: 657±672 Ó Springer-Verlag 1999

Editorial handling: J.L. Walshe

A.H. Wilson (&) á R.T. Brown1

Department of Geology and Applied Geology,University of Natal, Durban, 4041 South Africa

C.A. LeeAnglo American Platinum Corporation, PO. Box 62179,Marshalltown, 2107, South Africa

Present address: 1Hartley Platinum Mine,PO. Box CY 2288, Harare, Zimbabwe

modi®ed through recrystallisation and redistribution ofcomponents (Hunter 1987, 1996; Boudreau 1987).Campbell (1987), in contrast, suggested that primarycrystallisation processes are the dominant controls. Allof these points of view focus on the behaviour of inter-stitial liquid within the original framework of cumuluscrystals. The distribution and concentration of trace el-ements provides important evidence of the role of thelate ¯uid phase, with both mildly incompatible andstrongly incompatible elements giving di�erent infor-mation (see discussions by Wilson 1992; Mathez 1995;Cawthorn 1996a). The distribution of minor elements inpyroxene, particularly Ti and Cr, also provides infor-mation on re-equilibration processes.

The object of the study is to present detailed petro-logical and geochemical information on the Merenskyreef and its environment and to evaluate models for itsorigin and examine the role of late stage liquid includingthe trapped liquid component and migration of ¯uids.

Recent advances in understanding of the Merensky reef

Notwithstanding the many ideas concerning the Merensky reefthere is little geochemical detail available on the reef itself. Aspectsof the textures and compositions of the platinum-group minerals(PGM) were documented by Vermaak and Hendriks (1976),Mostert et al. (1982), Kinloch and Peyerl (1990), and Ballhaus andStump¯ (1986). Platinum-group element (PGE) geochemistry of thesuccession was described for whole rock samples (Lee 1983), andthe rare-earth element geochemistry of pyroxenes by Mathez(1995). Lee and Butcher (1990) and Kruger (1990, 1992, 1994) re-ported the Sr-isotope geochemistry of the east and west Bushveld

complex respectively. Other studies have been concerned with reefdisturbances such as potholes (Elliot et al. 1982; Ballhaus 1988;Carr et al. 1994). The compositions and textures of the Merenskyreef vary regionally around the Bushveld Complex (Brynard et al.1976; Mostert et al. 1982; Vermaak and Hendricks 1976).

The Merensky reef has been regarded as a consequence ofdisturbance of the pre-existing magmatic system by the in¯ux of amore primitive magma causing reversals in lithology (from noriteto pyroxenite) together with reversals in mineral compositions(Kruger and Marsh 1982; Eales et al. 1986). Campbell et al. (1983)suggested that ma®c magma ponded at the base of the chamberwith associated complicating factors such as mixing of magmas(which caused precipitation of sulphide) and thermal and physicalerosion of the substrate. The foundation of that model was thedramatic increase in 87Sr/86Sr reported by Kruger and Marsh(1982) and was developed by Naldrett et al. (1986). While such aprocess undeniably occurs in the magmatic environment, laterstudies have shown the picture to be far more complicated. Srisotope studies of the Atok facies of the reef (Lee and Butcher 1990)have shown that the generalisation of Kruger and Marsh was notcorrect and that at least in some occurrences there is no markedisotopic change at the Merensky reef itself, but the change occur-ring some distance above the reef.

Other studies of the Merensky reef have emphasised the im-portance of ¯uids and how petrographic evidence points to theinvolvement of volatiles (Boudreau et al. 1986; Ballhaus andStump¯ 1986) and possible recrystallisation of pre-existing ma®ccumulates. Nicholson and Mathez (1991) proposed that volatileswere introduced into melt-rich ma®c cumulates causing hydrationand isothermal melting of the proto-reef in the form of a migratinghydration/melting front. Mathez (1995) proposed that metaso-matism of the crystalline assemblage, associated with melt migra-tion, occurred in the Merensky reef resulting in pyroxenes strongly(but variably) enriched in rare-earth elements. Postcumulus py-roxenes in the norites also have elevated rare-earth element con-tents suggesting decoupling of the early stage mineralogy and thestrong in¯uence of a metasomatic ¯uid. Cawthorn (1996b) showedthat such enrichments can be explained by late-stage interstitial

Fig. 1 Geographic location ofRustenburg Platinum Mine(Rustenburg Section) in theBushveld Complex. The posi-tions of other mines referred toin the text are also shown

658

(postcumulus) phase growth as a result of reaction and re-equili-bration of primocryst phases with trapped liquid.

Much attention has in the past been directed to describing thepothole environments of the Merensky reef. This has led to animbalance of information between disturbed reef environments andwhat could be described as normal reef. The object of the presentstudy is to provide a detailed framework for comparison of mineraland rock compositions on a continuous sampling basis throughnormal Merensky reef (Brown 1994) and thereby shed further lighton the variety of suggested processes. The detailed study by Mathezet al. (1977) on the Merensky reef at Atok in the northeasternBushveld is in a pothole and therefore the overall geochemicalvariations reported need to be viewed with some caution.

Geological setting of the Merensky reef

Assigning the Merensky reef to a speci®c major stratigraphic unitof the Bushveld Complex is in itself a contentious issue. Kruger(1990) suggested on the basis of rock types and lithogeochemistrythat the base of the Merensky reef should be regarded as the base ofthe Main Zone. This is contrary to established views which placethe Merensky reef at the top of the Critical Zone. Unlike the MainSulphide Zone of the Great Dyke, but similar to the JM reef of theStillwater Complex, the Merensky reef is underlain by plagioclasecumulates. The signi®cance of the underlying cumulates is funda-mental to some of the proposed mechanisms for the formation ofsulphide mineralisation in layered intrusions (Campbell et al.1983). There is no doubt that the relationship of the reef to theunderlying sequences is of major genetic importance.

The Merensky reef on a large scale (kilometres to hundreds ofmetres) has the appearance of being conformable with the foot-wall plagioclase cumulates, but on a small scale (tens of metres tocentimetres) both conformable and transgressive relationshipsexist with the footwall (Fig. 2). There are signi®cant regionaldi�erences in the thickness of Merensky reef. In the west Bushveldcomplex the interval between the underlying UG2 chromitite is16 m to 150 m, whereas in the east Bushveld this interval is 100 mto 350 m. Whatever the stratigraphic level of the Merensky reef inrelation to the underlying cumulates a thin anorthosite layer isinevitably present but highly variable in thickness. A widespreadfeature of the lower contact of the Merensky reef is the dimplednature of the reef-footwall interface which is best developed in thevarieties of thinner (4 to 20 cm) reef. The anorthosite layer isconformable with the reef over large areas (Vermaak 1976), andeven parallels the dimples.

Base metal sulphides (BMS) are noted throughout the sequenceunderlying the Merensky reef, the amount dependent on thethickness of the reef; the thinner the reef the higher the proportionof contained sulphides and PGE grade. Sulphide grains are gen-erally small (£2 mm across), and are associated with pyroxene oi-

kocrysts in leuconorite and anorthosite. The sulphides are mainlypyrrhotite and chalcopyrite as complex bimineralic intergrowths.No sulphide is observed in the monomineralic plagioclase adcu-mulate although this rock type hosts above background amounts ofPGE. The overlying feldspathic pyroxenite has ®ner grained pyr-rhotite and chalcopyrite than the Merensky reef, but is otherwisesimilar. Sulphides in the Merensky reef analysed in this study rangein size from £0.005 mm to 7 cm as anhedral grains, blebs, and largemasses interstitial to pyroxene. Pyrrhotite and chalcopyrite domi-nate. The coarseness of both the sulphides and the silicates imposesconstraints on the sampling and in part accounts for the geo-chemical patterns observed.

In general the reef consists of a pegmatoidal feldspathicpyroxenite, partially pegmatoidal feldspathic pyroxenite, or fe-ldspathic pyroxenite. The rock is an orthocumulate consisting of aframework of very coarse-grained subhedral to euhedral orthopy-roxene constituting 70±90%, and up to 30% plagioclase as an in-tercumulus phase. Clinopyroxene oikocrysts up to 3 cm in diameteroccur throughout the rock. Phlogopite is a common accessorymineral in the Merensky reef together with quartz, apatite, zircon,amphibole and magnetite. This assemblage occurs in local pocketsand is interpreted as resulting from crystallisation of the ®nal stagesof trapped liquid. Two to four thin chromitite layers (each 1±2 cmin width) de®ne the upper and lower limits of the main economicmineralization. The underlying lithology to the Merensky reef iseither plagioclase cumulate or, less commonly, feldspathicpyroxenite or harzburgite. In the latter case the anorthosite layer isabsent. A centimetre-thick anorthosite usually occurs below thelower chromitite. Olivine occurs sporadically in the RustenburgSection. In the northwest at Union and Amandelbult mines the reefgenerally contains olivine, and olivine-rich rocks occur in thefootwall sequences. At the Atok mine the rock type underlying theMerensky reef is generally feldspathic pyroxenite, but clinopyrox-ene-norite occurs in parts of the mine (Lee and Butcher 1990). Theoverlying rocks in all these geographic areas grade upwardsthrough feldspathic pyroxenite, and norite to anorthosite, in turnfollowed by the pyroxenite-norite-anorthosite sequence of theBastard Unit. Base metal sulphide and PGE occur in the rocksoverlying the Merensky reef, which is re¯ected in the whole rockchemistry of the Merensky Unit (Lee 1983). In all facies the BMSmineralisation extends into the Merensky pyroxenite. In additionto the lithological variations the reef ranges in thickness fromseveral cms to around four metres.

Within this broad based description of normal Merensky reefseveral facies variations are observed in the Rustenburg area. Thisis largely based on the thickness of the Merensky reef. ThinMerensky reef (in this work called thin reef facies) comprises a reef4±20 cm in thickness with a thin chromitite layer marking the up-per and lower contacts with a pronounced development of under-lying anorthosite (10±35 cm). In places this anorthosite is very thinbut never completely absent.

Fig. 2 Transgressive form ofthe Merensky reef relativeto the layered footwall norite

659

In medium facies Merensky reef (20±40 cm), three chromititelayers are intermittently developed, two of them bounding thepegmatoidal pyroxenite, and the third enclosed either within thepegmatoidal layer or within the overlying Merensky pyroxenite.The basal anorthosite layer is highly variable but in this facies it isnoted that the thinner the layer the sharper and less undulatory thenature of the contact.

The present study reports on detailed variations of theMerensky reef facies which is 1±2 m thick, called thick reef facies.In this type up to three chromitite layers are developed althoughthe most common morphology (as sampled in detail in this study)has only two chromitite layers, located near the top of the layer,within a pegmatoidal framework of subhedral to euhedral coarse-grained orthopyroxene crystals with interstitial plagioclase andsubordinate interstitial clinopyroxene. Small amounts of olivineoccur particularly in the bottom 20 cm of the reef. In addition therock type contains interstitial phlogopite and base metal sulphideas well as minor disseminated chromite. Most BMS mineralisationoccurs towards the top of the reef especially in the vicinity of thechromitite layers, as interstitial blebs of variable size. The lowerboundary of the Merensky reef is a narrow anorthosite layer (1 cmto 1 mm thick) separating the underlying footwall norite, but hereit is quite commonly absent.

Sampling methodology and objectives of the current study

The Rustenburg mining area was chosen for this study because ofexcellent ®eld control and continuous mapping of reef types.Critical to the site selection was a knowledge of the distribution ofpotholes and the location of pothole edges, regions of crosscuttingmetasomatic replacement pegmatites and later structural distur-bance, all of which are considered for the present purposes asdisturbed (or abnormal) Merensky reef. Thickness and distributionof lithologies cannot only be taken as a basis for the recognition ofnormal or abnormal reef.

In order to permit contiguous sampling of the di�erent normalreef environments, four sites were selected at the mine (Fig. 3) so asto document:

1. The nature of the reef being mined in the various parts of themine;

2. To record the characteristics of these sites where they are de-monstrably distant to complications such as potholes, dykes,faults, replacement pegmatite, and abnormally high frequencyof joints.

The reef was sampled by means of upward drill holes located tointersect the complete reef succession ahead of mining. When thereef thus sampled was exposed by mining in the stope the area wasmapped in detail to provide a geological setting for the samples andto con®rm the reef was not a�ected by disturbances. Three closelyspaced cores were drilled at each site to provide an adequateamount of material for study. This provided a range of reef typesfrom thin (4±20 cm) high-grade reef to thick (1±2 m) lower-gradereef. The four sections studied are illustrated in Fig. 4. In the fol-lowing discussion regarding the drill hole intersections the noriticrocks underlying the Merensky reef pyroxenite succession are re-ferred to as the footwall norite (FWN) and those overlying the reefas the hanging wall norite (HWN). Each section was sampled on acontinuous basis from the footwall to the hanging wall, and ana-lysed for major and trace elements.

Pyroxene and plagioclase compositions were determined for thethickest intersection (R27). PGE were determined on all samplesand the results of this study will be presented separately. The thickreef variant of the Merensky reef was chosen for the most detailedstudy because it allowed the greatest number of samples to be takenthrough the reef section and thereby a comparison with thinner reefvariants and other types of Merensky reef. Fifty two samples wereselected on a continuous basis through 2.65 m of footwall leu-conorite (FWN) (16 samples), 1.82 m of Merensky reef (21 sam-ples), 1.00 m of Merensky pyroxenite (7 samples) and 1.58 m ofhanging wall norite (HWN) (8 samples). The width of the samples

was approximately 5 cm for the Merensky reef but increased toapproximately 10 cm above and below the Merensky reef.

Whole rocks were analysed for major and trace elements byXRFS and orthopyroxene and plagioclase compositions were de-termined on mineral separates to provide a representative compo-sition for each sample and also to obtain the highest precision byduplicate analysis. Selected trace elements (notably Zr and P) weredetermined at the highest precision with long counting times onduplicate samples. Strontium isotopes were determined on plagio-clase separates at CSIR (South Africa).

The question of sample heterogeneity must be discussed, espe-cially for the pegmatoidal phase of the Merensky reef. In the case ofmining, for which the reef is routinely sampled for the purposes ofgrade control, mine planning, and metallurgical control, samplesare composited to achieve an average PGE grade for a particulararea. Mine-wide experience has shown that on a sample by sampleanalysis the distribution of the PGE is lognormal, positivelyskewed. This re¯ects the variation in grain size, and, in the case of

Fig. 3 Map of part of Rustenburg Platinum Mines (RustenbergSection) showing localities of underground drill holes (R24, R25, R26and R27) used in this study. The stippled area shows the mined outarea as at September 1992

Fig. 4 Details of the Merensky reef, Merensky pyroxenite, footwalland hanging wall sequences at Rustenburg Platinum Mine studied.Sections R24, R25 and R26 are called thin reef facies and R27 thickreef facies. For location of drill holes see Fig. 3

660

the economic metals, the variation in abundance of base metalsulphides and platinum-group minerals. Thus, large compositesamples of the reef would appear to be more appropriate for ageochemical study. However, such samples would not reveal theinherent variability of the reef. The objective of the present study isto examine this smaller scale variability. No one petrological orgeochemical pro®le will be representative of the reef, and any onesection will be di�erent to an adjacent section, but this re¯ects thetrue character of the reef. If geochemical variability on a small scaleis a characteristic of the reef, then this too must be documented andunderstood. As will become evident, the overall geochemical pat-terns for R27 and for the thinner reef variants, for which there istrace element data, are similar.

Geochemical pro®les

Composite whole rock and mineral composition pro®lesthrough this succession are depicted in Figs. 5 and 6.The constant sampling interval is shown as a linear scalewith the stratigraphic interval for each of the rock unitsshown in metres. Representative data of mineral andwhole rock compositions, including Sr isotopes on pla-gioclase separates, are shown in Table 1.

Whole rock major element pro®les clearly re¯ect themineralogical distribution in the various rock types. Thesmooth transition from the norite to the Merensky reefand Merensky pyroxenite as monitored by FeO + MgOand Al2O3 (Fig. 5a) is clearly apparent for section R27.This transition for both contacts is over 20±30 cm.There is also an overall progressive increase in Al2O3

(and decrease in FeO + MgO) upwards in the pyroxe-nite.

Zr and P are generally regarded as incompatible forthe silicate and sulphide minerals with P being the moreincompatible of the two. The usefulness of these ele-ments is that they would largely re¯ect the behaviour ofsupernatant liquid in the vertical pro®le and it may bepossible to relate such behaviour to the distribution ofsulphides and PGE and silicate mineral compositions. Ifminor phases incorporating these elements do not ap-pear on the liquidus during the early stages of solidi®-cation, then these elements should show a strong mutualdependence. There is a wide range in the abundance ofthese elements in the sections studied (Fig. 5b and 5c),giving the distinct saw-tooth pattern, with some samplesin the Merensky reef and Merensky pyroxenite beinghighly enriched relative to the norite above and belowthe pyroxenite. The range for P in these lithologies variesby a factor of 500 even for adjacent samples. An im-portant observation for the thick reef facies (R27)(Fig. 5b) is that in the pyroxenite portions of the suc-cession (and in particular for the Merensky reef), Zr andP do not show corresponding behaviour. Decouplingbetween these two elements in the Merensky reef, likelyre¯ects minor mineral phases reaching saturation andundergoing physical separation in a ¯uid system. Bothzircon and apatite are observed in the Merensky reef. Itis important to establish whether the system is open orclosed and whether the contrasting behaviour of Zr andP results from decoupling on a local scale. The same

patterns are observed for the narrow reef sections (onesection, R26 in Fig. 5c, is shown for comparison).

Silicate mineral compositions (Fig. 5d) show distinc-tive behaviour. Plagioclase has relatively low Ca# [asrepresenting Ca/(Ca + Na)] in the Merensky reef andMerensky pyroxenite and re¯ects the interstitial status ofthe mineral in these lithologies compared to the noritewhere it is cumulus. The compositions of plagioclase inboth the footwall norite and hanging wall norite aree�ectively the same. In contrast Mg# [as representingMg/(Mg + Fe2+)] in orthopyroxene in the hanging wallnorite is signi®cantly lower than that in the footwallnorite indicating that the compositional controls onthese phases were not the same in the footwall andhanging wall norite. There is also a sharp decrease inMg# in passing from the footwall norite to the base ofthe Merensky reef but also remarkable consistency incomposition through this zone. As for the whole rockmajor elements there is a clear but rapidly changingtransitional zone between the di�erent lithologies. Thebetween-sample variation is signi®cantly greater inthe Merensky pyroxenite and Merensky reef than inthe norites.

The distribution of S (as re¯ecting the variation ofsulphide) shows much the same pattern for the di�erentreef types (Fig. 6). Lowest values occur in the footwallnorite rising to highest values in the Merensky reef andthen decreasing through the Merensky pyroxenite intothe base of the hanging wall norite. Sulphide content inthe base of the hanging wall norite is an order of mag-nitude greater than that at the top of footwall norite.Although highly variable from sample to sample, the Scontent increases upwards through the Merensky reef. Asmall reversal in sulphide distribution in the middle ofthe Merensky pyroxenite is observed in both the thickreef (Fig. 6a) and thin reef (Fig. 6b) facies.

A strong mutual dependence of the major componentoxides Al2O3 and FeO + MgO (Fig. 7a) re¯ects themodal gradation from pyroxenite to norite and anor-thosite. The linearity of these plots indicates that thedominant control was plagioclase and orthopyroxenewith clinopyroxene contributing little to the bulk com-positions. A plot of Sr versus MgO (Fig. 7b) showsstrong linearity with a clear distinction between therocks up to and including the Merensky reef and thehanging wall norite. A similar relationship was identi®edby Eales et al. (1986).

Inter-element behaviour

The interdependence of incompatible trace elements isexempli®ed in the plots of Zr versus P (Fig. 8). Forsection R27 all the data plotted show mutual depen-dence in all rocks except the Merensky reef in which theP/Zr ratio is lower and variable. In contrast there is agood correlation between Zr and P in the Merenskypyroxenite and combines with the samples of the foot-wall and hanging wall norite (Fig. 8a). The average for

661

the Merensky reef samples also lies on the Merenskypyroxenite linear array, consistent with P or Zr beingredistributed within the Merensky reef interval. Thesmall positive intercept on the Zr axis (3±5 ppm) isconsidered to re¯ect the slight compatibility of Zr inpyroxene compared with P.

The variations of Zr and P for the thin reef sections(R24, R25, R26) (Fig. 8b) are generally similar to R27.In these sections the hanging wall and footwall noriteplot in a close cluster or in a linear array and are in-distinguishable from those of R27. One di�erence is thatin the thin reef facies many samples of Merenskypyroxenite and Merensky reef show lower P/Zr ra-tios with a few samples having very high values forP. Average values for P and Zr in the Merensky reef andMerensky pyroxenite of R27 and the thin reef faciesR24, R25 and R26 lie on essentially the same trend asthe norites (Fig. 8c) suggesting that P/Zr is constant in

Fig. 5a±d Variation of selected major and trace elements for mineralsand whole rocks in the sections studied. a Variation of MgO + FeOand Al2O3 in whole rock for R27 ; b variation of Zr and P in wholerock for R27; c variation of Zr and P in whole rock for R26; dvariation of Mg# for orthopyroxene and Ca# for plagioclase in R27.Errors of determination are smaller than symbol size

662

Table

1Mineralandwhole

rock

compositionsforsectionR27

Rock

Type

HWN

HWN

HWN

MP

MP

MP

MR

MR

MR

MR

MR

MR

MR

FWN

FWN

FWN

Relative

Position

13

59

11

14

17

21

24

26

29

31

36

40

44

47

Orthopyroxene

%SiO

253.72

53.86

53.80

54.30

54.42

54.57

54.46

54.52

54.65

54.59

54.64

54.68

54.51

54.82

54.80

55.10

Al 2O

31.15

1.11

1.06

1.31

1.32

1.29

1.32

1.19

1.22

1.39

1.20

1.39

1.50

1.34

1.24

1.25

Fe 2O

31.84

1.85

1.79

1.50

1.47

1.44

1.48

1.51

1.46

1.45

1.49

1.44

1.39

1.38

1.36

1.37

FeO

14.93

14.85

14.49

12.12

11.89

11.67

11.95

12.20

11.83

11.74

12.06

11.68

11.25

11.19

11.08

11.08

MnO

0.33

0.34

0.32

0.27

0.26

0.26

0.26

0.26

0.26

0.26

0.27

0.25

0.25

0.27

0.26

0.26

MgO

25.76

25.98

26.02

27.85

27.92

28.25

28.07

28.26

28.42

28.09

28.04

28.10

28.22

28.65

28.65

28.62

CaO

1.64

1.61

1.60

1.68

1.66

1.56

1.65

1.37

1.42

1.68

1.47

1.81

1.76

1.60

1.63

1.65

Na2O

0.00

0.00

0.00

0.06

0.06

0.00

0.00

0.00

0.07

0.05

0.05

0.07

0.01

0.00

0.00

0.01

TiO

20.227

0.223

0.212

0.186

0.213

0.206

0.224

0.272

0.246

0.227

0.242

0.220

0.200

0.215

0.204

0.212

Cr 2O

30.333

0.328

0.345

0.447

0.440

0.449

0.426

0.384

0.444

0.427

0.396

0.429

0.455

0.430

0.450

0.451

NiO

0.084

0.087

0.093

0.107

0.128

0.118

0.134

0.130

0.122

0.127

0.111

0.108

0.101

0.083

0.079

0.080

Total

100.01

100.23

99.73

99.83

99.78

99.81

99.97

100.09

100.14

100.03

99.97

100.18

99.65

99.98

99.75

100.08

Mg#Opx

0.755

0.757

0.762

0.804

0.807

0.812

0.807

0.805

0.811

0.810

0.806

0.811

0.817

0.820

0.822

0.822

Plagioclase

Ca#Plag

0.789

0.786

0.776

0.714

0.655

0.675

0.633

0.678

0.789

0.786

0.776

0.714

0.655

0.675

0.633

0.678

Rb

0.7850

0.6335

3.064

2.118

2.298

15.78

0.9401

13.93

2.545

1.145

0.6235

0.7241

Sr

170.8

156.4

165.3

163.1

154.5

351.6

181.7

188.2

197.8

210.3

145.9

151.4

Srinit2060Ma

0.70648

0.70626

0.70616

0.70620

0.70637

0.70657

0.70660

0.70584

0.70654

0.70658

0.70644

0.70632

Whole

rock

%MgO

3.77

4.17

5.14

21.79

22.45

23.03

22.51

23.14

22.46

20.10

20.97

18.60

20.28

6.21

6.20

6.03

FeO

2.74

2.86

3.45

10.82

10.64

10.64

12.85

11.60

10.73

9.83

9.93

10.28

9.14

2.92

2.87

2.84

Al 2O

327.52

26.83

26.01

7.64

6.89

6.51

4.22

6.76

7.53

8.85

7.31

8.25

11.27

25.43

25.85

25.72

ppm

P24.0

17.9

22.0

38.1

766

80.3

899

53

29.2

675

82.6

86.9

72.9

31.1

29.9

31.2

Zr

7.3

6.3

6.3

8.4

19.6

12.4

26.6

18.9

14.5

22.0

71.0

29.9

16.4

7.6

6.5

7.6

Sr

405

398

385

104

96

91

47

90

109

122

98.6

118

175

381

383

386

Rb

1.4

0.9

0.9

2.3

4.0

4.3

7.7

5.0

1.3

23.2

23.7

9.3

4.3

1.2

1.1

1.3

Cu

60

74

105

558

741

863

2182

1314

916

1316

32

1685

31

15

16

7

S567

693

1156

4001

4957

4389

19537

7234

5291

4984

191

17255

438

119

51

67

663

any one zone, but that strong decoupling of Zr and Phas taken place on a sample to sample basis with the

wide range of P values giving rise to large standard de-viations. Other incompatible elements, such as Ba and K(not shown) also show evidence of decoupling but lesspronounced compared with P. Overall the Merenskyreef is enriched in incompatible elements relative to thefootwall and hanging wall norites in the thick reef faciesbut this di�erence is less clear in the thin reef facies.

Cu and S show a positive linear correlation over awide range of values. In section R27 samples of thehanging wall norite and most samples of the footwallnorite also lie within this grouping (Fig. 9) but with thelargest variation being shown for the Merenskypyroxenite in which S ranges from 100 ppm to20 000 ppm. In the thin reef facies the footwall norite isenriched in sulphide and has maximum values similar tothose of Merensky pyroxenite and Merensky reef. Incontrast, mineralisation in the footwall norite in thethick reef facies of R27 (inset to Fig. 9) is very low anddoes not overlap with the Merensky pyroxenite andMerensky reef.

Sr-isotopic variation

Sr-isotopic determinations were carried out on 12 pla-gioclase separates from all lithological units of drillsection R27 and currently represent the most detaileddata set for the Merensky reef and its immediate envi-ronment. Initial Sr ratios (Fig. 10a) are calculated to anage of 2060 Ma. Comparison of Ba in whole rocks withRb in plagioclase is shown in Fig. 10b. There is indica-tion that one sample (in the middle of the Merenskyreef) is depleted in Rb also with an anomalously lowvalue isotopic ratio and may have been disturbed bylater processes. The remaining four samples within theMerensky reef have an almost constant value of 0.7066which are elevated relative to samples in the footwallnorite and Merensky pyroxenite. Previous studies of

Fig. 6a, b Distribution of S in a thick reef facies of R27, and b thinreef facies of R26

Fig. 7a, b Interelement variation in R27 for a Al2O3 versus FeO +MgO in whole rocks, and b Sr versus MgO. The inset in the latter®gure is an expansion of part of the diagram to emphasise thedisplacement between the footwall and hanging wall norites. Errors ofdetermination are smaller than symbol size

664

isotopic variations of the Merensky Unit (e.g. Kruger1992; Lee and Butcher 1990) have not clearly identi®edwhere the increase takes place but the sparser data ofthese previous accounts are consistent with the presentstudy. While there is no suggestion of high isotopicvalues characteristic of the Main Zone these data do notnegate mixing of Critical Zone magma with the radio-genic magma of the Main Zone.

Interpretation and constraints for a petrogenetic model

An understanding of the subliquidus and subsolidushistory is essential to reconcile the mineralogical andgeochemical variations. The variability of trace andchalcophile elements in the Merensky reef and Merenskypyroxenite may be attributed to sample size and particledistribution. This nugget e�ect re¯ects the small-scalestructure of these rocks. Larger-scale sampling wouldsmooth out the variation but would also obscure thecontrols which were operating on a smallscale and whichprovide the vital clues to understanding these rocks. Onthe scale of 5±10 cm length core samples there is cleardecoupling of P and Zr, S, REE (the latter as describedby Mathez 1995) and also the PGE (Lee and Wilson inpreparation).

These variations can be related to the e�ect of theliquid within the early formed network of crystals andtheir modi®cation by the subliquidus processes arisinginitially from circulating ¯uids and ®nally by the trap-ped liquid when the system became a closed system. Therole of trapped liquid in modifying mineral composi-tions was highlighted by Barnes (1986) and also indi-cated by Wilson (1992) for the pyroxenites in the GreatDyke. There is evidence that displacement of liquid andeven its exclusion from the network by a maturation orannealing can occur as a result of compaction(McKenzie 1984). Convective circulation of liquidwithin the crystal pile is likely to have occurred (Taitand Jaupart 1992).

The observed reversal in %MgO and Mg# in severalsections (referenced in Naldrett et al. 1986; Cawthorn1996a) from the top of the footwall norite to the base ofthe Merensky reef is not observed in the RustenburgSection. In section R27 there is a small but sharp de-crease in pyroxene Mg# from the top of the footwallnorite to the base of the Merensky reef and also from thetop of the Merensky pyroxenite to the base of thehanging wall norite. Direct comparison cannot be madewith other published sections as no data are provided onreef thickness or types. Although none of those previousstudies have been carried out using continuous samplingit is also clear that the extent of the reported composi-tional reversals varies greatly at di�erent localities of theBushveld Complex. It is therefore inadvisable to attemptto incorporate all available data into a single unifyingmodel and each section should be interpreted on itsmerits. The range and variability also indicates that asimple mixing model to explain the lithologies, the py-roxene compositions and the mineralisation, as sug-gested by Naldrett et al. (1986) and Campbell et al.(1983) is unlikely and that much more complex controlsoperated.

Cawthorn (1996a) explained apparent variation andtrends of pyroxene compositions in the Merensky Unitand the footwall units to result from the re-equilibrationof pyroxene and trapped liquid. Cawthorn's (1996a)analysis for the Merensky reef suggested that the

Fig. 8a±c Interelement variation of Zr and P. a Section R27. Errorsof determination are smaller than symbol size; b element concentra-tions in Merensky pyroxenite and Merensky reef for sections R24,R25 and R26. Arrows indicate additional points outside range ofdiagram. Errors of determination are smaller than symbol size; cmeanvalues and standard deviations for the Merensky reef and Merenskypyroxenites for all sections. For R27 the means for the Merensky reefand Merensky pyroxenite are shown separately

665

amount of trapped liquid was essentially constant atabout 10%. Several factors were cited by Mathez et al.(1997) as supporting evidence for lack of fractionation-controlled mineral compositions in the hanging wallnorite. These are: (a) that the pyroxenes are interstitialand the most evolved compositions are those present invery low modal abundance, (b) there is evidence for raremore magnesian compositions in some grains, and (c)that variation of minor elements in pyroxene (e.g. Ti)does not follow the variation in major components(Mg# of pyroxene). An alternative explanation is thatthese features may have resulted from re-equilibration ofprimocrysts with trapped liquid causing decoupling ofminor and major components.

Signi®cance of the pyroxene compositions

Pyroxene compositions in the Merensky reef were in-terpreted by Mathez (1995) as having been strongly in-¯uenced by metasomatic ¯uids which could also explainthe enrichment in platinum group elements. It is shownin that work that pyroxenes with lowest Mg# are alsoenriched in REE. In contrast, Cawthorn (1996b) ex-plained the enrichment of mildly incompatible elementsby their concentration in trapped liquid and incorpora-tion into late stage pyroxenes growing from the trappedliquid. Cawthorn (1996b) also suggested that pyroxenesin the footwall section were essentially of uniformcomposition.

By utilising incompatible trace elements it is possibleto assess the in¯uence of late-stage liquid on ®nal py-roxene compositions. This has been tested in section R27using Zr as a monitor of trapped liquid. Phosphoruscould also have been used but, as noted previously, there

Fig. 9 Variation between S andCu for combined sections R24,R25 and R26 with the insetshowing the data set for R27

Fig. 10 a Initial Sr isotopic variation for plagioclase separatescalculated to 2060 Ma for section R27. One sample shown as anopen square has an anomalously low value. b Plot of Ba in whole rockversus Rb in plagioclase for samples considered in the isotopic study.Standard error bars shown

666

is evidence that in some parts of the sequence P mayhave been decoupled from the other trace elements on asample to sample scale, probably by local crystallisationand growth of apatite. Zirconium is not totally incom-patible with pyroxene and this is clearly demonstrated inplots of Zr against P which show a positive intercept of3±8 ppm Zr at zero P (see Fig. 8a). The intercept for thenorites is 3±4 ppm Zr. These values are consistent withdeterminations by ion probe (study in progress) and alsowith other studies i.e. Papike (1996). This represents thepartitioning of Zr into the primocryst phases andtherefore in the evaluation of Zr in the liquid the inter-cept values for Zr are subtracted from all whole rockanalyses (shown in geochemical diagrams as Zr*). Thisvalue would then represent the contribution by the in-terstitial liquid phase.

Mass balance calculations can be used to establishchanges in pyroxene compositions that would resultfrom re-equilibration and growth in a cumulate rockwhich contained trapped liquid (Barnes 1986; Cawthorn1996a). The shift in pyroxene composition is stronglydependent on the ®nal ratio of plagioclase to pyroxeneand such changes are shown for a Merensky reef - typepyroxene compositions (Fig. 11). Such changes arelikely to have been controlled by the ®nal stage of thesolidi®cation process.

Plots of Mg# of orthopyroxene versus Zr* show aseries of trends for speci®c stratigraphic intervals withrocks having highest Zr* also having lowest Mg#(Fig. 12a±c). This gives strong indication that late-stageliquid controlled the ®nal composition of orthopyrox-ene. In these diagrams the sequence of the plotted pointsdoes not conform to contiguous samples in the drillcores; this indicates inherent variability within the se-quence. The well-de®ned arrays of data also give anindication of pyroxene compositions before re-equili-bration with the trapped liquid. The zero intercepts forthese arrays correspond to a situation in where there waszero trapped liquid; ie. Zr* is zero in the whole rock.Modelling of the amount of liquid in the closed systemand the shift in pyroxene composition has been carriedout for the various sections detailed in this study usingthe intercept on the Mg# axis (at zero Zr*) as the ®rstapproximation of the initial composition before re-equilibration commenced. This initial composition orhighest Mg# for each array (expressed as Mg#°) is thenfurther re®ned by constraining the plot to ensure thatZr* is zero where trapped liquid is zero.

In the di�erent rock units each compositional array isdistinct and therefore indicates that the pyroxene com-positions prior to the last re-equilibration stage wereessentially constant for each of the intervals delineatedby the arrays. The thicker intervals can also be dividedinto at least two subintervals on the basis of the geo-chemical variation. The initial pyroxene compositionsfor each of the intervals is summarised in Table 2 (av-erages) and are designated Mg#°. There is a sequentialand consistent decrease in Mg#° for each unit upwardsin the succession from 0.830 in the lower footwall norite

to 0.815 in the Merensky pyroxenite. There is a suddendecrease to Mg# 0.780 in the hanging wall norite whichcannot be explained by normal fractionation but is acharacteristic of many PGE reefs including Great Dykeand Stillwater Complex (Barnes 1997 personal commu-

Fig. 11 Modelled variation of the response of pyroxene compositionby re-equilibration with liquid in a closed system. The initial liquidcomposition (Mg#°) is 0.825. The degree of change is stronglydependent on the ®nal percentage of pyroxene in the whole rock (ie.plagioclase±orthopyroxene ratio) and the amount of trapped liquid

Fig. 12a±c Variation of Mg# of pyroxene with the Zr in trappedliquid (expressed as Zr*) for section R27. a Footwall norite, bMerensky reef and Merensky pyroxenite, c hanging wall norite. Theaverage modelled pre-equilibration pyroxene composition for each ofthe arrays is noted asMg#° and represented by the range bar based onanalytical errors. Standard error bars for the analyses are shown

667

nication). This treatment agrees with the broad estimateof En 0.82 determined by Cawthorn (1996a) but is not inagreement that the entire section contained pyroxene ofa uniform composition as purported in that work. Theseresults are also at variance with the widely acceptedconcept that there always exists a compositional reversalat the base of the Merensky reef to higher Mg# andinstead places the Merensky reef in a sequence of downtemperature compositions from the footwall norite tothe Merensky pyroxenite.

Variation of TiO2 in orthopyroxene with Mg#(Fig. 13a) gives the same pattern as the relationship ofZr in the whole rock with Mg# of orthopyroxene, witheach layer showing a distinct chemical trend. This isfurther illustrated in the plot of Zr (in whole rock) vsTiO2 in orthopyroxene (Fig. 13b) and demonstrates thatthe minor incompatible element in pyroxene is stronglylinked with the distribution of incompatible elements inthe whole rock and is further evidence of the role of latestage liquids in in¯uencing the ®nal composition of or-thopyroxene.

The results of modelling the trapped liquid behaviourusing mass balance calculations are shown in Fig. 14.Pyroxenite samples of the Merensky reef and Merenskypyroxenite are shown to have had between 2% and 10%trapped liquid. This agrees with the analysis of Cawt-horn (1996a) of the trapped liquid content of Merenskyreef at Impala Platinum Mine but also shows that theassumption of a constant 10% trapped liquid is incor-rect. Footwall norites have ®nal trapped liquid betweenzero and 2% and lie on the same general trend as thepyroxenites. In contrast the hanging wall norites indicatea liquid strongly depleted in Zr relative to that of theMerensky reef and Merensky pyroxenite. As notedpreviously Sr is also depleted in the hanging wall norite(see Fig. 7b) and therefore the magma giving rise tothese rocks are indicated to have been generally depletedin incompatible trace elements (50 ppm Zr in liquid)compared with that which gave rise to the Merensky reefpyroxenite and the footwall succession (400 ppm Zr inthe liquid).

The behaviour of sulphide

Sulphide is interstitial to the cumulus silicate phases andtherefore is of similar status to the late stage liquidcomponent. It is likely to have separated as a relativelyhigh temperature phase but would have remained liquidover a wide temperature interval. The modal variationof sulphide must be seen in relation to the spatialcontrol on the late stage liquid. It is likely that migra-tion of dense, low viscosity immiscible sulphide liquidwith low `wettability' of silicate minerals would haveoccurred. The amount of sulphide tails o� downwardfrom the Merensky reef and Merensky pyroxenite but isin much smaller amounts in the footwall norite com-pared with the hanging wall norite. The decrease insulphide into the footwall norite in this section is over1.5 m in contrast with 5 m as reported by Cawthorn(1996a) for the Impala Platinum Mine. For the thickand medium-reef facies there is little evidence fordownward migration of sulphide in the RustenburgSection. In thin reef facies there is evidence that sul-phide has percolated down over a similar short distance(up to 1 m) into the footwall norite and concentrationsin the zone immediately below the Merensky reef are

Table 2 Pre-equilibration compositions of orthopyroxene in theMerensky reef and bounding lithologies at the ®nal closed systemstage

Lithounit Thickness(m)

Original Mg#�Opx

Range Mg#Opx

Hanging wallnorite (HWN)

1.60 0.782 0.754±0.778

Merenskypyroxenite (MP)

1.04 0.815 0.804±0.813

Upper Merenskyreef (UMR)

0.90 0.813 0.803±0.811

Lower Merenskyreef (LMR)

0.90 0.819 0.805±0.817

Upper footwallnorite (UFN)

1.60 0.826 0.820±0.825

Lower footwallnorite (LFN)

1.00 0.829 0.821±0.822

Fig. 13 a Variation of TiO2 in orthopyroxene with Mg#, and bvariation of Zr in whole rock with TiO2 in orthopyroxene. The boldvertical lines indicate the initial (pre-equilibration) compositions(Mg#°) for each of the rock groups. Errors of determination aresmaller than symbol size

668

much higher than for the footwall norite in the thickreef facies.

Variation of sulphide (indicated by the concentrationof S in the whole rock) reveals some important relationsto the amount of the liquid phase in the rock expressed

in terms of Zr (or Zr*). Plots of Zr* in whole rock versusS show remarkable consistency when considered fordistinct stratigraphic units. In the footwall norite ofsection R27 there exists a positive correlation of S withZr* (Fig. 15a) which indicates that either sulphide wascrystallising from trapped liquid (given the ratio of sul-phide to trapped melt this is not possible) or else sul-phide and silicate melt were associated as a slurry andtherefore exhibited a mutual variation in a spatially re-stricted environment. In the ®nal stages the sulphidemay also have interacted with circulating ¯uids in thecrystal pile. Essentially the sulphide and the silicate liq-uid are indicated to have been trapped in the crystalnetwork with low interconnectivity which retained theoriginal association of these two liquid phases. In thethin reef facies (R25) where the S content in the footwallnorite and anorthosite is signi®cantly higher comparedwith the same section in R27 there remains a positivecorrelation of S with Zr (Fig. 15b). These relationshipsfor the footwall norite contrast with those of theMerensky pyroxenite and Merensky reef in all sectionsby which there is a pronounced inverse dependence be-tween Zr* (or Zr for section R25 in Fig. 15b) and S.

The general observation is that the footwall noritecontrasts with the Merensky reef and Merenskypyroxenite where in the latter a decrease in the incom-patible element content is accompanied by an increase inthe amount of sulphide. This indicates that where sul-phide was a primary liquid phase in the ®nal stages ofsolidi®cation it displaced the less dense silicate liquid by

Fig. 14 Results of the modelling of the amount of trapped liquid foreach of the rock units in section R27 as a variation with Zr in trappedliquid (Zr*). This is based on the range of pyroxene Mg# for each ofthe arrays and on the ®nal proportion of pyroxene in the whole rockas shown in Fig. 11. The Merensky reef and Merensky pyroxenite areenriched in Zr because of the high trapped liquid content. The liquidgiving rise to the hanging wall norite is also shown to be highlydepleted in incompatible elements compared with that which formedthe Merensky reef and Merensky pyroxenite (50 ppm compared with400 ppm)

Fig. 15a±c The variation of Zr with S content in whole rockfor the Merensky reef package in a the footwall norite in R27,b Merensky pyroxenite and footwall units in R25, and c theMerensky reef in R27. These diagrams serve to illustrate themutual dependence and controls on the distribution of latestage silicate liquid and sulphide liquid. Standard error barsshown

669

competition for the available pore space. The originaldistribution of sulphide may therefore have been ex-tensively modi®ed in the Merensky reef and Merenskypyroxenite.

Mechanisms operating in the Merensky Unit

The geochemical evidence in this work is that late stageliquids within speci®c layers also played a major role incontrolling the ®nal compositions of primary phases.There is strong theoretical evidence to show that com-paction occurs in the magmatic environment. Theseprocesses will e�ect the redistribution of late stage meltand the timing and nature of silicate re-equilibration.Critical to the understanding of this rock system is toestablish at what stage in the cooling history did com-paction and ¯uid circulation cease to operate and closedsystem conditions become established. Only at this stagecan the trapped liquid impart a dominant in¯uence asthe observed geochemical signature.

The process of compaction involves densi®cation of acrystal±liquid mush by removal of interstitial melt bydeformation of the solid matrix. McKenzie (1984, 1987)shows that several important factors control the rate ofseparation of solid and melt by this process, these beingmelt viscosity, porosity and the density contrast betweensolid and melt. Sparks et al. (1985) show that the initialporosity at any stage during the formation of ma®ccumulates in large layered intrusions is likely to be lowand that compaction occurs rapidly relative to solidi®-cation. This militates strongly against long preservationtimes of thick sequences of cumulates with high porositybut active ¯uid circulation is likely to have taken placewithin the crystal mush on a short time scale (Tait andJaupart 1992). Compaction occurs in partially moltenrocks (Stolper et al. 1981) but this must also be recon-ciled against in situ crystallisation processes. Compac-tion rate can be variable for di�erent mineral - meltassociations and this can conceivably give rise to dif-ferent ®nal porosities (and therefore also permeabilitiesfor passage of ¯uids) in di�erent rock types.

Pyroxenite should, by virtue of its higher solid-meltdensity contrast have lower porosity and permeabilitythan anorthosite or leuconorite but this is not the casefor the Merensky reef sequence as demonstrated in thiswork with the footwall norite having less than 2% of®nal trapped liquid and the hanging wall norite less than6%, at the stage when the system became closed. It hasalso been demonstrated for the Great Dyke (Wilson andChaumba 1997) that the ®nal amount of trapped liquidin the norite is signi®cantly less than that for the un-derlying pyroxenite. The exclusion of liquid from theleuconorite resulted from either an inherently low initialporosity or by densi®cation resulting from crystalovergrowth. In addition to compaction it is also likelythat the porous structure of the rock is established at anearly stage and close to the crystallisation boundary.Geochemical evidence shows that the residual liquid

content (that stage at which the liquid±solid assemblagewas e�ectively a closed system) was variable and lessthan 10% and may have exerted the ®nal in¯uence onthe pyroxene compositions. Prior to the ®nal closedsystem stage the pyroxenes are indicated to have hadalmost constant composition within the individual rockunits and this can be explained by re-equilibrationhaving taken place by ¯uid interaction in a circulatingsystem.

The base of the hanging wall norite may have acted asa trap for upward migrating melt, thereby increasing theamount of trapped melt in the pyroxenite. The ®nalamount of trapped liquid in this norite is indicated to behigher than the footwall norite but it is also anomalousin the very low content of incompatible elements. Thesigni®cant decrease in the pre-equilibration compositionof pyroxene (Mg#°) in the hanging wall norite is indi-cative of mixing of a more evolved magma at this point.The isotopic data in this study show that in the lower-most section of the hanging wall norite there is a trend ofgradually increasing values to the highly radiogeniccompositions which characterise the upper part of theMerensky Unit (Kruger 1992, 1994).

An understanding of the pegmatoidal nature of theMerensky reef is also crucial to reconstructing the evo-lution of this package of rocks. Coarsening of ma®crocks is most easily explained by the presence of smallamounts of water-rich ¯uid which would cause a lower-ing of surface energy promoting recrystallisation andalso a depression of the liquidus temperature. Nicholsonand Mathez (1991) and Mathez et al. (1997) invoked thelatter process to cause hydration melting which resultedin the formation of chromitite layers and poikilitic clin-opyroxene from a pyroxenite protolith but the lattertexture occurs as clinopyroxene oikocrysts throughoutthe Critical Zone and therefore the situation is likely tobe even more complex. This fact is also realised by Ma-thez et al. (1997) who note that the observed geochemicalpro®le cannot be explained by di�erential compactionrates of di�erent lithologies and that the footwall noriteis presumed to have undergone compaction whereas thehanging wall norite was cemented rather than compact-ed. Circulation of ¯uids (containing water) through thepyroxenite mush would have facilitated the grain coars-ening observed in the Merensky reef.

The origin of the various magmas which gave rise tothe Critical and Main Zones of the Bushveld Complex,and therefore also for the Merensky reef, have beendebated at length (e.g. Kruger and Marsh 1982; Irvineet al. 1983; Naldrett et al. 1986). The implications ofthese possible liquids will not be reviewed here except toemphasise that the initial liquid (B1), as proposed byHarmer and Sharpe (1985), was a magnesian basalt withrelatively high Zr and REE contents and the later liquid(B2) was more iron-rich and depleted in incompatibleelements. This is consistent with the observed composi-tions as determined for the ®nal trapped liquid in thisstudy. Cawthorn (1996a reporting on Cawthorn andBiggar 1993) shows that the magma which gave rise to

670

the footwall succession of the Merensky reef had 5±6%MgO but even more importantly, there is no reversal inMgO content at the Merensky reef, at least in this sec-tion studied in detail.

There is clear evidence that the magma was becomingprogressively evolved and that the silicate minerals werestrongly in¯uenced by ¯uid circulation within each ofthe lithological layers. This resulted in e�ectively con-stant compositions of pyroxene by continuous re-equil-ibration after which the system became sealed and fromthat stage each rock package evolved and equilibratedseparately to establish the ®nal silicate compositions.This process would also have had a major in¯uence onthe distribution pattern of PGE the Merensky reef andMerensky pyroxenite (Lee et al. in preparation).

Summary and conclusions

This study shows that the Merensky reef, the Merenskypyroxenite and the bounding rock units are chemicallydistinct in terms of the mineral compositions and traceelement distribution. The chemical trends for each of therock units suggest interaction of the primary silicatephases with ¯uid at various stages during cooling bywhich these minerals were signi®cantly modi®ed fromtheir original liquidus compositions. Upward expulsionof liquid by compaction of the cumulus pile is likely tohave occurred at the earliest sub-liquidus stage but the®nal distribution of incompatible trace elements andmineral compositions was established during the laterstages. The ®nal preserved mineral and whole rockcompositions most strongly re¯ect these latter stages.The liquids which gave rise to the Merensky reef, and toa lesser extent the Merensky pyroxenite, were enrichedin incompatible trace elements (and also probably thePGE) by up to a factor of 10 compared with the hangingwall norite. There is a successive decrease in pre-equili-bration compositions of orthopyroxene (Mg#°) upwardsin the sequence with each rock unit consisting of or-thopyroxene of progressively lower Mg#° value. Theinitial pyroxene compositions were not constantthroughout this part of the sequence as reported byCawthorn (1996a). There is a very dramatic decrease inMg#o for orthopyroxene from the top of the Merenskypyroxenite into the hanging wall norite which cannot beexplained by simple fractionation of the main body ofmagma and a new magma in¯ux is envisaged at thispoint.

Further constraints on the processes for the origin ofthe Merensky reef unit arising from the present work areas follows:

1. Redistribution of P and other incompatible elementshas taken place in the Merensky reef and Merenskypyroxenite. There exists variability in this enrichmentbetween the thick reef and thin reef facies. Recrys-tallisation and coarsening of the pyroxenites has ta-ken place probably as a result of trapped circulating¯uids containing water.

2. The compositions of the liquidus pyroxenes havebeen extensively modi®ed by a series of processesduring the cooling history. A three stage process isenvisaged: (a) upward migration of ¯uid during earlycompaction of cumulates; (b) circulation of ¯uidswithin the crystal mush resulting in e�ectively con-stant (but further modi®ed) compositions; (c) ®nalcompositions established by reaction with trappedliquid and most readily identi®ed by the composi-tional relationships.

3. The trapped liquid component which contributedstrongly to the ®nal re-equilibration stage was notconstant but varied from less than 2% to a maximumof (but never exceeding) 10%. Constant porositywithin the crystal pile cannot be assumed.

4. The amount of trapped liquid in the footwall noriteand anorthosite was very low (<2 wt.%) whereas theMerensky pyroxenite and Merensky reef are indicat-ed to have contained 2±10% trapped liquid. Thehanging wall norite has a range of trapped liquidcontent of 2±10% and appears to have been derivedfrom a highly depleted magma.

5. A strong interdependence exists between orthopy-roxene compositional components (Mg# and TiO2)and incompatible element (P and Zr) content of thewhole rock. On this basis a cryptic layered structure isdiscernible on a scale of about 2 m. This type oflayering in the Great Dyke has been referred to ascrypto-rhythmic layering (Wilson 1992). TheMerensky reef and the footwall norite, within thesuccession studied at Rustenburg Platinum Mine,each comprise two distinct subzones.

6. The distribution of sulphide can also be related to thespatial controls of silicate and sulphide liquid withinthe network of cumulus crystals. The sulphide andthe PGE distribution will also have been in¯uencedby the various stages of down-temperature re-equili-bration processes.

7. Sr isotopic evidence does not support a model ofsimple magma mixing of Critical Zone and MainZone magmas at the level of the Merensky reef butthe onset of such mixing may be marked by thehanging wall norite.

Acknowledgements AHW thanks the Foundation for Researchand Development and the University of Natal Research Committeefor supporting this project. RTB is grateful for the support given bymanagement and sta� at the Rustenburg Section of RustenburgPlatinum Mines. Permission to publish the results of this work aregiven by Amplats management. RTB and CAL gratefully ac-knowledge Amplats for funding for analytical work. Steve Barnesis thanked for a thoughtful review of the manuscript. Nick Arndt isthanked for stimulating discussions on this topic.

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