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Journal of Petrology Volume 42 Number 7 Pages 1321-1347 2001
© Oxford University Press 2001
Crustal Contamination and Fluid–Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
Received March 20, 2000; Revised typescript accepted December 5, 2000
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe Platreef is the main platinum group element (PGE)-bearing facies of the northern limb of the Bushveld complex, but unlike the Merensky Reef of the eastern and western limbs, it is in direct contact with the country rock. Mineral separate
18O values for samples from the Upper Zone and Main Zone of the northern limb indicate crystallization from a well-mixed, already contaminated, magma having a 18O value of 7·5
. Pyroxenes from the pyroxenites facies of the Platreef at Sandsloot Mine have 18O values that are up to 2·4 higher than pyroxenes from the Upper and Main Zones. These differences can be explained by additional assimilation of up to 18% dolomite, which is in contact with the intrusion at this locality. Samples from the Platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted with fluids during slow cooling. Quartz veins with granophyric margins have 18O values between 10·1 and 12·2, which suggest that the fluid that passed through the cooling Platreef had a 18O value of 7–9. These data, together with hydrogen isotope data from minerals and whole-rock samples (D of biotite -60 to -88) of the Platreef suggest interaction with magmatic fluid at low water/rock ratios. Interaction of this fluid with the calcsilicate footwall rocks lowered their 18O values. KEY WORDS: Bushveld complex; Platreef; stable isotopes; crustal contamination; fluid–rock interaction
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INTRODUCTION |
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The mafic component of the 2050 Ma Bushveld complex of South Africa is the largest igneous intrusion on Earth and contains some of the most important magmatic ore deposits yet discovered. The intrusion covers an area of roughly 65 000 km2 (e.g. Tankard et al., 1982
) and lies almost entirely within the bounds of the Transvaal Basin (Fig. 1). The layered igneous rocks (the Rustenburg Layered Suite, SACS, 1980) can be divided into eastern and western limbs of approximately the same size, and a smaller northern limb. The northern limb differs in several important aspects from the more well-known eastern and western limbs (e.g. Van der Merwe, 1976; Buchanan et al., 1981; Cawthorn et al., 1985; Eales & Cawthorn, 1996). First, the layered rocks show a pronounced transgression across the floor rocks from a level above the Magaliesberg Quartzite Formation south of the town of Potgietersrus, across the whole Pretoria Group, and the banded iron formations and dolomites of the older Chuniespoort Group, onto Archaean granitic rocks (Fig. 1). Second, the igneous layering shows an apparent transgressive relationship with respect to the contact, such that ultramafic rocks of the Lower Zone abut against the contact south of Potgietersrus, whereas to the north, rocks forming the base of the intrusion are progressively higher in the stratigraphic succession of the intrusion. Third, although the major platinum group element (PGE) mineralized horizon of the northern limb (the Platreef) bears some petrographic resemblance to the Merensky Reef at the top of the Critical Zone of the eastern and western limbs, it is situated at the base of the layered rocks and the mineralized zone is much thicker (Gain & Mostert, 1982).
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The Platreef has been described in detail by Gain & Mostert (1982) and Lee (1996), who considered it to be part of the Critical Zone, which is only poorly developed in the northern limb to the south of Potgietersrus. The Platreef consists of a complex series of medium- to coarse-grained pyroxenites and norites, which contains xenoliths of the floor rocks. Mineralization of Ni, Cu and PGE is sporadic but occurs over a zone up to 200 m in width (Gain & Mostert, 1982). The Platreef north of Potgietersrus has been mined for platinum (and other base metals) intermittently since 1926 (Wagner, 1925; Buchanan et al., 1981). In 1993 mining operations using open pit methods started at the Sandsloot Mine operated by Potgietersrus Platinum Mines Limited, 
30 km NW of the town of Potgietersrus.
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PURPOSE OF STUDY |
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The mineralization of the Merensky Reef of the main Bushveld intrusion is generally thought to be of magmatic origin with some evidence for fluid involvement but little or no evidence for significant effects of fluid on grade of mineralization (e.g. Barnes & Campbell, 1988
; Schiffries & Rye, 1989, 1990; Reid et al., 1993). The Platreef, however, shows considerably more visual evidence for post-magmatic fluid interaction. In addition, the situation of the Platreef, in direct contact with the country rock, means that wall-rock assimilation has been suggested as being especially important in the origin and mineralization of the basal part of the northern limb (Buchanan et al., 1981; Cawthorn et al., 1985; Barton et al., 1986).
One of the most interesting geochemical features of the Rustenburg Layered Suite in the eastern and western limbs is the isotope evidence for contamination (e.g. Harmer & Sharpe, 1985; Schiffries & Rye, 1989). Initial Sr-isotope ratios vary with stratigraphic height and change suddenly at certain stratigraphic horizons (e.g. the Merensky Reef; Kruger & Marsh, 1982). These variations have been interpreted by various workers as being caused by the sudden influx of magma of distinctly different isotope composition. The 18O values of Bushveld magmas estimated from mineral 18O values are typically 1 higher than expected for a mantle-derived basaltic magma (Schiffries & Rye, 1989). In contrast to the behaviour of Sr-isotopes, the oxygen isotope data show no systematic change with stratigraphic height, which is consistent with the parental magmas assimilating a significant amount of crust before emplacement.
The Platreef is important as a source of platinum group elements (e.g. Lee, 1996). There has been much debate in the recent literature on the origin of platiniferous ‘reefs’ (e.g. Ballhaus & Stumpfl, 1986; Barnes & Campbell, 1988; Boudreau & McCallum, 1992), in particular concerning the role of fluids. The present paper has two major aims. The first is to compare the oxygen isotope composition of the northern limb of the Bushveld with previously published data (Schiffries & Rye, 1989) from the eastern and western limbs of the intrusion. It is important to establish whether or not the northern limb shares the unusual O-isotope character of the main intrusion. To make the comparison, we have analysed samples from a 2500 m core recovered from drilling through the northern limb on the farm Bellevue (Fig. 1). The second aim is to determine the role of crustal contamination in the formation of the Platreef, and to assess the subsequent role of fluid–rock interaction. Our study of the Platreef is based on samples collected at Sandsloot (Fig. 1), the site of the largest platinum mine on the Platreef. The main analytical tool employed has been stable (O, H and C) and Sr isotopes, which are particularly useful in understanding the processes of crustal contamination and fluid–rock interaction (e.g. Gregory & Taylor, 1981; Gregory & Criss, 1986; Taylor & Sheppard, 1986; Gregory et al., 1989). A study of the mineralization of the Platreef is beyond the scope of this paper and will be presented elsewhere.
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GEOLOGY |
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The geology of the Bellevue core has been described by Knoper & von Gruenewaldt (1992)
and is not described in detail here. The boundary between the Main Zone and the Upper Zone was intersected at 1580 m, but the core does not extend as far as the Platreef. The samples are the same as those analysed by Ashwal & Hart (1995) and were taken from plagioclase-rich horizons. The geology of the opencast mine at Sandsloot at the time of sampling, in May 1995, is shown in Fig. 2. The mine then consisted of two pits, the Main Pit and the Satellite Pit. The width of the mineralized zone (the Platreef) in the Main Pit thinned to the north and also downwards. Three sample traverses were made across the Platreef at levels (‘bench’) 14, 16 and 20. The detailed geology of the bench 20 traverse is shown in Fig. 2, and the essential features of the geology are the same in the other two traverses.
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The Platreef reaches a maximum thickness of 250 m over the northern limb as a whole, and consists of an assemblage of pyroxenites, melanorites, norites, xenoliths and fragments of dolomite (Van der Merwe, 1976; Gain & Mostert, 1982). At Sandsloot Mine, the Platreef is in contact with the dolomite and forms a unit up to 65 m thick. The part of the Platreef in contact with the dolomite is termed ‘parapyroxenite’ by the mine geologists and denotes a highly altered rock, of variable thickness, which is possibly a mechanical mixture of calcsilicate material (metamorphosed and metasomatized dolomite), pyroxene-rich igneous rocks, and minor serpentinite. This is followed by a ‘pyroxenite’ unit, which on account of its generally >10% plagioclase content should properly be termed a pyroxene-rich gabbro (e.g. Le Maitre, 1989). This is followed by the norites of the Main Zone of the intrusion. In this paper we will use the nomenclature in common usage at the mine, namely, norite, pyroxenite and parapyroxenite. The mineralized zone (the Platreef) is generally confined to the pyroxenite and parapyroxenite units (Fig. 2). The boundary between the parapyroxenite and the country rock is impossible to locate; in this paper the term calcsilicate is used only for samples that demonstrably come from the country rock.
The youngest igneous rock at Sandsloot is a ferrogabbro dyke exposed in the wall of the pit about 50 m to the west of the bench 20 traverse (Fig. 2). This dyke might be related to the subsequent intrusion of the Upper Zone (C. A. Lee, personal communication, 1999). Also present are a number of veins up to 1 m in thickness. These consist of massive quartz interiors with 20–30 cm thick granophyric margins which presumably indicate relatively high temperatures of formation. The core of one vein (PP10) contains minor amounts of calcite. Three such veins cut the bench 20 traverse (Fig. 2).
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PETROGRAPHY |
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The petrography and mineral chemistry of samples analysed from the Bellevue core (Fig. 1) have already been described by Knoper & von Gruenewaldt (1992)
and Ashwal & Hart (1995). The following is a generalized description of the changes in petrography passing from the hanging-wall norite, through the mineralized Platreef to the contact with the calcsilicates and encompasses all the petrographic features of the three sample traverses at Sandsloot (Fig. 2).
The norites are coarse grained with maximum grain size of 5 mm and contain primocrysts of plagioclase and orthopyroxene. The freshest samples contain 30–40 modal % pyroxene and 60–70 modal % plagioclase. The orthopyroxene crystals show fine exsolution lamellae of clinopyroxene. Postcumulus phases in the norites are clinopyroxene, quartz, biotite and opaque minerals. The postcumulus clinopyroxene crystals are commonly <1 mm in size, occurring in spaces between plagioclase cumulates and to a lesser extent between the orthopyroxene crystals. Biotite and opaque minerals also occur in the interstices between plagioclase crystals, and in most cases the biotite is partially altered to chlorite. The size of these biotite crystals is typically <1 mm. Amphibole is present in some samples as a postcumulus mineral, but its modal proportion in norites does not exceed 3%. A common feature of the norites is the presence of veinlets of white mica, typically <0·5 mm, which cut across the plagioclase and orthopyroxene crystals. In most samples of norite, however, this alteration is of minor volumetric significance.
The pyroxenites are more coarse grained than the norites (typically, grains are up to 10 mm or more). Locally, pegmatoidal patches are developed with pyroxenes up to 6 cm in diameter. The pyroxenites usually contain about 90% pyroxene and 10% plagioclase by volume, but the exact mode is often obscured by the greater amounts of alteration minerals that are present compared with the norites. As in the norites, orthopyroxene is dominant over clinopyroxene. Plagioclase occurs as a postcumulus phase, enclosing the pyroxenes or filling the interstices between them, and most grains have a cloudy appearance caused by alteration to white mica. Late-stage biotite can sometimes reach 15 modal %, and in most cases is partially altered to chlorite, and amphibole, which in some thin sections approaches 13 modal %.
The parapyroxenites are petrographically varied rocks that seem to be a mélange of pyroxenites (variably recrystallized and altered) and calcsilicate rocks all of which are highly altered. In the investigated area, the amount of alteration increases towards the contact with the dolomite (or calcsilicate as it is near the contact). In thin section, clinopyroxene is found to be the dominant mineral and in places shows recrystallization with 120° grain boundaries. Serpentine with relict cores of fresh olivine, biotite, calcite, chlorite and amphibole is also present. The grain size ranges from fine to medium grained. The fine-grained facies present in the bench 20 traverse consists almost entirely of clinopyroxene (e.g. sample PP17; see Table 2, below) and appears to be a reaction rim around a calcsilicate xenolith. Given that clinopyroxene and olivine are common constituents of the parapyroxenite, but are not found in the overlying pyroxenite, it seems likely that the parapyroxenites consist mostly of transformed dolomite floor rocks.
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The ferrogabbro, which intrudes the norite, west of the bench 20 traverse (Fig. 2), is one of the freshest rock-types exposed at Sandsloot and contains 50% pyroxene (dominantly orthopyroxene), 40–45% plagioclase and 5–10% opaque oxide. This rock is finer grained than the main gabbro types, with an average grain size of
1–2 mm. The granophyric margins of the quartz veins that cut the Platreef consist almost entirely of intergrown albite and quartz. Biotite, which is partially replaced by chlorite, is the only other mineral present (<5 modal %).
The sulphide minerals were not studied in detail during this work, but appear to be similar to those identified in the Platreef 10 km north of Sandsloot by Gain & Mostert (1982). The major sulphide minerals present are pyrrhotite, pentlandite, chalcopyrite and pyrite, which are heterogeneously distributed as crystals of up to 1 cm diameter throughout the mineralized zone (Fig. 2). The maximum sulphur content in the analysed samples was 9308 ppm, which is equivalent to 2·7 wt % sulphide.
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ANALYTICAL METHODS |
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All data were obtained at the University of Cape Town (UCT). Mineral chemical data were determined using a wavelength-dispersive Cameca microprobe and whole-rock chemical analyses were made using standard X-ray fluorescence (XRF) methods (e.g. Duncan et al., 1984
). Major elements, with the exception of Na, were determined on fusion discs, and trace elements and Na were determined on 6 g powder briquettes.
All stable isotope ratios were measured using a Finnigan MAT 252 mass spectrometer and are reported in the familiar notation where = (Rsample/Rstandard - 1) x 1000 and R = 18O/16O, D/H or 13C/12C. Oxygen isotope data for whole rocks and silicate minerals were obtained by conventional methods using ClF3 as the reagent (Borthwick & Harmon, 1982). Further details of the extraction methods for oxygen from silicates employed at UCT have been given by Vennemann & Smith (1990) and Harris & Erlank (1992). The quartz standard NBS-28 was analysed in duplicate along with eight samples and data were normalized to the SMOW scale using the value of 9·64 recommended by Coplen et al. (1983). The average mean deviation of 12 duplicate NBS-28 quartz standard analyses was 0·10. Hydrogen was produced from about 30 mg of biotite and 100 mg of whole rock by the method of Venneman & O’Neil (1993), with ‘Indiana’ Zn being used to reduce the water to H2. The amount of Zn was about five times that normally required to reduce the water to prevent ‘poisoning’ of the Zn by any fluorine that might be liberated from the minerals. For hydrogen, an internal water standard (CTMP, D = -9) was used to calibrate the data to the SMOW scale and the data were normalized so that SLAP gave a value of -428 on the SMOW scale, as recommended by Coplen et al. (1983). Water contents of minerals and whole rocks were estimated from the voltage measured on the mass two collector on the mass spectrometer, as described by Venneman & O’Neil (1993). The carbon and oxygen isotope ratios of carbonate minerals were determined on CO2 produced by reaction of whole-rock powders with ‘103%’ phosphoric acid (McCrea, 1950). Samples were reacted overnight at 50°C and analyses are of the total carbonate present. The dominant carbonate mineral present was calcite, hence the calcite–phosphoric acid fractionation factor (1·009) was used in the correction procedure. Data were normalized to the SMOW and PDB scales, respectively, using an internal standard calibrated against NBS-19 (18O 28·64, 13C 1·95). Graphite was analysed using an on-line continuous flow device.
The Sr-isotope data were obtained using a VG Sector multi-collector mass spectrometer in dynamic mode, following conventional ion-exchange separation. To correct for mass fractionation effects, measured 87Sr/86Sr ratios were normalized such that 86Sr/88Sr = 0·1194. Three analyses of NBS-987 made during the course of this work gave 87Sr/86Sr ratios of 0·710239(10), 0·710279(11) and 0·710239(9), where the figures in parentheses are the standard errors for repeated measurements of the isotope ratio and relate to the least significant digit(s).
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MINERAL CHEMISTRY |
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Pyroxene analyses from selected samples of norite (PP3), pyroxenite (PP8) and several samples of parapyroxenite (PP17, PP20, PP33 and PP49) are presented in Table 1 and plotted on the pyroxene quadrilateral in Fig. 3. It should be noted that the norite contains both clino- and orthopyroxene whereas the pyroxenite contains only orthopyroxene. The orthopyroxene in the pyroxenite is slightly less magnesian than that of the norite. The parapyroxenites contain only clinopyroxene, but these are distinctly more calcic than clinopyroxene in the norite. Although some analyses plot above the Di–Hd join (Fig. 3), this is due to the presence of Al in octahedral sites and Ca does not exceed one cation per six oxygens (Table 1). Clinopyroxenes in parapyroxenite sample PP20 are notable for their high Al content (up to 8·75 wt % or 0·38 p.f.u., Table 1). The higher Ca content of parapyroxenite clinopyroxenes compared with those of the norites and pyroxenites suggests that the former are of metamorphic origin [also recognized by Buchanan et al. (1981)
]. The ferrogabbro that cuts the norite near the traverse of bench 20 (Fig. 3) contains pyroxenes that are considerably more iron rich (typical mg-number for opx 0·51, for cpx 0·40) than those of the norite (mg-number for opx 0·80–0·83), consistent with a relative of the Upper Zone.
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Olivine analyses were made only for sample PP20 (a parapyroxenite) where olivine is present as cores to serpentine. Olivine in this rock shows a restricted range in composition, averaging Fo84 (Fig. 3; Table 1), which is consistent with either a magmatic or metamorphic origin. Given that olivine is absent in the pyroxenites, it is more likely that olivine in the parapyroxenites is of metamorphic origin.
The norite sample (PP3) contains cumulus plagioclase of fairly restricted composition (An70–66), which is slightly more calcic than the intercumulus plagioclase (An65–62) in the pyroxenite sample (PP8). The ferrogabbro dyke contains rather more sodic plagioclase (An64–45). The plagioclase in both the norites and pyroxenites is more calcic than in samples of Platreef in contact with Archaean granite (An30–60; Cawthorn et al., 1985).
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WHOLE-ROCK GEOCHEMISTRY |
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Interelement variations
Chemical analyses of selected rocks are given in Table 2, and selected major element oxides vs SiO2 and MgO are shown in Fig. 4. The most important feature to note is that the norites, pyroxenites and the ferrogabbro (i.e. the magmatic rocks) show coherent variations. Because these rocks are cumulates the chemical variation is controlled largely by modal mineralogy. In the oxide vs MgO and SiO2 plots (Fig. 4), the norite and pyroxenites plot between typical plagioclase and orthopyroxene compositions, which indicates that varying proportions of these two minerals are the dominant control on whole-rock composition.
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Data for parapyroxenites and footwall calcsilicates are also plotted in Fig. 4. They show considerably less coherent variation than the pyroxenites and norites (Fig. 4), having lower SiO2 (<50 wt %), FeO and Na2O contents, and higher CaO contents. By comparison with the parapyroxenites, the calcsilicates have lower SiO2 (<40 wt %), lower FeO/MgO and higher CaO/MgO. The high CaO and the low sodium content of the parapyroxenites are consistent with a metamorphic origin. The calcsilicates have lower SiO2 and higher CaO/MgO than the parapyroxenites, which suggests that they contain a higher component of dolomite. The high (up to 9 wt %) Al2O3 content of two of the calcsilicate samples appears to be due to a high content of sheet silicate alteration minerals, and the high Al2O3 content of the clinopyroxenes (Table 1).
Because the pyroxenites and norites are cumulate rocks, they have very low abundances of the immobile incompatible trace elements such as Zr, Nb and Y (e.g. <20 ppm Zr, Table 2). In Fig. 5, various trace elements are plotted against SiO2 and MgO. The norites show a systematic decrease in Cr and Ni with decreasing MgO. The pyroxenites show a systematic decrease in Cr with decreasing MgO, but Ni shows no systematic variation with MgO. Both the norites and the pyroxenites show a systematic increase in Ba and Sr content with increasing SiO2. These variations are qualitatively consistent with variation in pyroxene and plagioclase content.
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The parapyroxenites and calcsilicates show no systematic variations in trace element content with SiO2 and MgO (Fig. 5). Both the parapyroxenites and calcsilicates have consistently low Ba and Sr content and, apart from one sample, low Cr contents (<2000 ppm). This is consistent with a non-magmatic origin for the abundant clinopyroxene in the parapyroxenites. The Ni content of the parapyroxenites is much higher than that of the calcsilicates, but both the parapyroxenites and calcsilicates have much higher Cu, Ni and S contents than the norites. The variation of Cu, Ni, Pb and Zn with S is shown in Fig. 6. In the norites and pyroxenites, Cu and Ni correlate positively with S, whereas Zn and Pb do not. In the parapyroxenites and calcsilicates there is no correlation between any of the elements and S. These data suggest that the high Ni and Cu content of the norites and pyroxenites is related to sulphide mineralization.
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Variations in chemistry across the Platreef
The variation in selected oxides and trace elements with distance along the bench 20 traverse through the Platreef is shown in Fig. 7. The most obvious features of these traverses are abrupt changes in composition at the norite–pyroxenite boundary, and the transition from coherent variations in the norites and pyroxenites to much greater variability in the parapyroxenites. The norites have lower FeO and MgO but higher CaO than the pyroxenites, which is consistent with higher plagioclase and lower orthopyroxene in the norites. In the parapyroxenites, the CaO content increases and the MgO and Cr content decreases with increasing distance away from the contact with the pyroxenites. This is consistent with a general decrease in the magmatic component of the mechanical mixture forming the parapyroxenites with distance away from the contact with the pyroxenites. The loss on ignition (LOI) is generally low (<3 wt %) in the magmatic rocks but is highest in the parapyroxenites closest to the contact with the pyroxenites. The high LOI is due mainly to high water content because these rocks do not now contain high levels of carbonate (see Table 6, below).
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Sulphur is generally higher in the pyroxenites than in the norites, with variable levels in the parapyroxenites (up to nearly 1 wt %). There appears to a general enrichment of S at the boundary between the pyroxenites and the parapyroxenites (Fig. 7), and similar enrichments are seen in Cu and Ni.
The sudden change in texture and mineralogical and chemical composition at the pyroxenite–norite contact suggests that the pyroxenites and norites crystallized from two separate magmas. However, we found no field evidence such as veins of norite within the pyroxenite (or the converse) to support this view. The coarse grain size of the pyroxenite is consistent with it crystallizing before intrusion of the norites, as is the greater degree of visible alteration seen in the pyroxenites and the lack of dolomite xenoliths in the norites.
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STABLE ISOTOPES |
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The Bellevue core
The
18O values of plagioclase and pyroxene samples from the Bellevue core are given in Table 3. Both plagioclase and pyroxene show a restricted range in 18O values, from 7·0 to 8·3 (mean 7·9; n = 18) and 6·1–7·6 (mean 6·7; n = 11), respectively. The only exceptions are two samples that have plagioclase with much higher 18O values (10·4 and 13·1). Figure 8 shows the 18O value of plagioclase vs that of pyroxene for the Bellevue samples, and most form a tight cluster very close to the data obtained by Schiffries & Rye (1989) for Rustenburg Layered Suite rocks, including the Merensky Reef, from the eastern Bushveld. Exceptions are one sample (B1745) with a very large 
plagioclase–pyroxene and two samples with a slightly negative value of plagioclase–pyroxene. These samples may have been affected by alteration.
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The oxygen isotope stratigraphy of the Bellevue core is shown in Fig. 9, with data for the eastern Bushveld (Schiffries & Rye, 1989) also plotted, using the Main Zone–Upper Zone boundary as a reference. The two sets of data correspond well. The pyroxene 18O values in the Bellevue core decrease slightly with increasing stratigraphic height and plagioclase–pyroxene increases with height. The increase in plagioclase–pyroxene with increasing stratigraphic height is also seen in the data of Schiffries & Rye (1989) through a much larger stratigraphic thickness of the eastern limb of the Bushveld. The value of plagioclase–pyroxene would be expected to increase with stratigraphic height for two reasons: (1) in the Bellevue core, the plagioclase becomes more sodic with height (Ashwal & Hart, 1995; Table 3) and plagioclase–pyroxene increases with decreasing anorthite content in the plagioclase (Chiba et al., 1989); (2) decreasing crystallization temperature would have resulted in an increase in plagioclase–pyroxene with stratigraphic height.
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A combination of both of the above effects would result in an increase in plagioclase–pyroxene [calculated from data of Chiba et al. (1989)] from 0·50 (An76 at 1150°C) to 0·75 (An47 at 1050°C). An additional effect, which is probably more important than (2), is that of closure to oxygen diffusion (e.g. Giletti, 1986) where oxygen continues to equilibrate between minerals after crystallization to a temperature of 550°C. The final 18O values of minerals in the rock depend on modal mineralogy. Gabbros that contain mainly plagioclase (such as these) can show shifts in pyroxene 18O value of up to 1·16. This would account for the slight decrease in pyroxene 18O value with stratigraphic height shown in the Bellevue core. The most important conclusion from the above discussion is that the 18O value of the magma did not change significantly during crystallization of the Main and Upper Zones of the northern limb.
Platreef oxygen
Unlike the samples from the Bellevue core, the Platreef samples show considerable variation in plagioclase and pyroxene 18O values (see Table 4). The two most important features of Fig. 8 to note are that (1) plagioclase and pyroxene in the Platreef have generally higher 18O values than those of the Bellevue core, and (2) there are a significant number of samples with plagioclase and pyroxene that are not in oxygen isotope equilibrium at magmatic temperatures. The latter feature is generally considered to indicate water–rock interaction (e.g. Gregory & Criss, 1986) whereas the former is usually ascribed to crustal contamination (e.g. Taylor, 1980; James, 1981). However, before any reliable conclusions can be reached concerning the extent of crustal contamination, the effects of post-magmatic alteration must be taken into account.
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Most of the norites (and the ferrogabbro sample) plot slightly above the 550°C isotherm with average plagioclase–pyroxene values of 2·0. By comparison, the Bellevue core samples have smaller values of plagioclase–pyroxene (0·6) as does sample PP27, a norite collected from the Main Zone midway between Sandsloot and Potgietersrus (Fig. 1). One Platreef norite (PP54) has significantly lower plagioclase–pyroxene than the others. The pyroxeneite samples can be divided into two groups; those that plot within the 550 and 1150°C isotherms and appear to be in oxygen isotope equilibrium at magmatic temperatures, and those samples where plagioclase–pyroxene indicates disequilibrium. The latter group appears to form an array that shows a negative correlation between plagioclase and pyroxene 18O values. The implications for these data for sub-solidus fluid–rock interaction will be discussed below.
The 18O values of plagioclase and pyroxene show no systematic variation with major element oxides such as CaO, SiO2 and MgO (Fig. 10). By contrast, whole-rock 18O values of the parapyroxenites (which contain no magmatic pyroxene and/or plagioclase) show a crude positive correlation with SiO2 and CaO, and a negative correlation with MgO and LOI. Petrographic examination of the samples shows that those with low 18O values are rich in serpentine, and those with high 18O values are rich in diopside, and this is consistent with the 18O–oxide correlations. A possible interpretation is that diopside was relatively resistant to fluid–rock interaction whereas forsterite was not, being transformed to serpentine with significantly lower 18O value.
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The variation in 18O value in the bench 20 and bench 14 traverse across the Platreef is shown in Fig. 11. In the bench 14 traverse, plagioclase and pyroxene show a slight increase towards the pyroxenite–parapyroxenite boundary, with diopside in the parapyroxenite being 2·5 higher than in the pyroxenites. In the bench 20 traverse, where a greater thickness of parapyroxenite was sampled, the parapyroxenites increase in 18O value away from the contact with the pyroxenite. Another feature of the bench 20 traverse is that plagioclase–pyroxene in the pyroxenites increases towards the contact with the parapyroxenites.
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Two quartz-granophyre veins that intersect the bench 20 traverse (Fig. 2) have quartz 18O values of 10·1 and 12·2, whereas quartz in the calcite-bearing quartz-granophyre vein that cuts the pyroxenite has a 18O value of 6·9. As these veins are likely to be related to fluid–rock interaction, their 18O values are important for reconstructing the oxygen isotope composition of the alteration fluid.
Platreef hydrogen
Hydrogen isotope ratios were determined for biotite from six samples where the mineral appears to be a primary magmatic mineral. The Platreef rocks also contain a variety of secondary hydrous minerals and whole-rock D values were determined on 11 other samples. These data are reported in Table 5, along with the water content and a list of the hydrous minerals present. The biotite D values vary from -60 in a norite sample (PP30) to -88 in a pyroxenite sample (PP60). The high water content of biotite in PP30 (5·58 wt %) indicates that it is altered to more water-rich minerals such as chlorite. The whole-rock D values show a similar range to those of the biotites, from -64 to -99. Figure 12 shows that the D values of biotite and whole rock from the Platreef show a vague positive correlation with H2O+, an effect that is generally ascribed to magma degassing (e.g. Taylor, 1986). There is no correlation between biotite and whole-rock D values and plagioclase–pyroxene. The latter gives a crude indication of the amount of alteration given that plagioclase is more susceptible to alteration than pyroxene (e.g. Taylor, 1968). There is no correlation between hydrogen isotope data and major or trace element content.
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The D values of the biotite and the whole-rock samples overlap considerably, which is consistent with formation of biotite and other hydrous minerals during the same fluid–rock interaction. Although the hydrous minerals must have formed over a range of temperatures (biotite close to 700°C, chlorite close to 300°C), the mineral–water fractionation factor for hydrogen of biotite at 700°C is similar to that of chlorite at 300°C (mineral-water -30, O’Neil, 1986). If fluid D values are calculated from the biotite and whole-rock data using the same per mil fractionation (–30), values similar to typical ‘primary magmatic water’ (that is, water that had equilibrated with the magma regardless of the origin of the water that was transported with the magma from its deep-seated source; Sheppard, 1986) are obtained (Fig. 12). Mathez et al. (1994), obtained very similar results for the Merensky Reef and related rocks in the Atok mine in the eastern limb of the Bushveld (Fig. 12). Schiffries & Rye (1990) obtained D values between -34 and -71 for quartz vein inclusion water from the eastern lobe, which are also similar to the data obtained for the Platreef (allowing for mineral–water fractionations of the order of -30).
Carbonates
Carbonate minerals, calcite and dolomite, are present in small amounts in many of the Platreef rocks. The 13C and 18O values of the total carbonate present in a variety of whole-rock samples are given in Table 6. Both the 13C and 18O values show a very wide variation (Fig. 13). The majority of samples analysed have 13C values between 0 and -5 and 18O values <15. Three calcsilicates have relatively higher 18O values but lower 13C values. One parapyroxenite sample has a low 18O and 13C value. None of the Platreef samples contain carbonate with an isotope composition at all similar to unaltered dolomite in the area (Fig. 13).
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Decarbonation during metamorphism causes changes in both 13C and 18O value in the remaining rock (e.g. Valley, 1986), and two Rayleigh decarbonation paths (using different fractionation factors) are shown in Fig. 13, starting from an arbitrary value appropriate for unaltered limestone. The samples with high 18O values (>18) and low 13C values are consistent with formation during closed-system decarbonation. By contrast, those samples with low 18O could not have formed during closed-system decarbonation. In the case of the parapyroxenites, the lowering of carbonate 18O values could have occurred during diffusional equilibration with the igneous silicate rocks. Low 18O values in the calcsilicate carbonates must, however, reflect interaction with an externally derived fluid. The carbonate in the norite and in the vein represent reprecipitated carbonate derived from relatively undecarbonated country rock. Graphite nodules, a few centimetres in diameter, from the calcsilicate rocks have an average 13C value of -26·9 (Table 6), which is considerably more negative than the 13C values of the host calcsilicate rocks (average -4·9, n = 13). The 13C value of the graphite presumably reflects the original sedimentary value and indicates a lack of equilibration of carbon isotopes between graphite and carbonate during metamorphism because calcite–graphite is <6 above 500°C (Chacko et al., 1991).
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STRONTIUM ISOTOPES |
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Strontium isotope data for samples from the bench 20 traverse at Sandsloot are reported in Table 7. Measured 87Sr/86Sr ratios of both the norites and pyroxenites, and the parapyroxenites show a good correlation with Rb/Sr (Fig. 14) and the line of best fit has a gradient close to the accepted age of intrusion (2050 Ma, Walraven et al., 1990
). Taken as a whole, the data indicate an average initial ratio of 0·7093, which is higher than the generally accepted values for the Main Zone of the Bushveld intrusion elsewhere (0·7075–0·7090; Hamilton, 1977; Harmer & Sharpe, 1985). However, six of the seven norite and pyroxenite samples form an array with a line of best fit approximately parallel to the 2050 Ma reference line but with lower initial ratio (0·7080), and eight out of nine of the parapyroxenite and calcsilicate samples form an array with similar apparent ‘age’ but with a higher initial ratio (0·7100).
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Initial Sr-isotope ratios (at 2050 Ma) are plotted against distance across the Platreef in Fig. 11 and it is evident that the non-magmatic rocks generally have significantly higher initial Sr-isotope ratios, and that there is a sharp change in initial Sr-isotope ratio, except for samples taken very close to the contact. The similarity in apparent ‘age’ between the magmatic and country rocks indicates that the Sr-isotope ratios of the latter were reset at the time of intrusion, but the low Sr concentration and higher initial ratio of the immediate country rocks suggest that diffusion of Sr from the magmatic rocks into the footwall of the Platreef was limited.
The correlation between initial Sr-isotope ratio and MgO, CaO and SiO2 is poor (as expected given the rocks are cumulates) (Fig. 15), but in general samples with high MgO and CaO, and low SiO2 have higher initial Sr-isotope ratios. There is no apparent correlation between 18O and initial Sr-isotope ratio (Fig. 15), but this is understandable given the low Sr content of the non-magmatic potential contaminants.
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OXYGEN ISOTOPE COMPOSITION OF THE PARENT MAGMA |
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The norites and pyroxenites of the Platreef and the rocks of the Bellevue core do not represent quenched liquid compositions. It is, therefore, necessary to estimate the magma
18O value from mineral data using appropriate fractionation factors and assumptions about cooling history. It was shown above (Fig. 8) that most of the Bellevue core samples, and some Platreef pyroxenite samples, have plagioclase and pyroxene 18O values that are consistent with oxygen isotope equilibrium at magmatic temperatures. Samples that plot significantly outside the closure and crystallization isotherms (Fig. 8) have probably been affected by alteration, and the nature of this process is discussed below. Pyroxene is less susceptible to exchange with fluids than is plagioclase (e.g. Gregory & Taylor, 1981), and the pyroxene 18O values should, therefore, provide the best indication of changes in the magma 18O value. Changes in pyroxene 18O value resulting from slow cooling between the temperatures of crystallization and closure to oxygen diffusion are dependent on factors such as mineral mode, grain size and cooling rate (see inset to Fig. 9).
The shift in pyroxene 18O value caused by closure effects during slow cooling is comparatively small in magnitude, but is difficult to quantify because the rate of cooling is not known. The pyroxene 18O values will decrease during slow cooling because feldspar–pyroxene becomes larger with decreasing temperature. Let us consider the case of a biminerallic gabbro with 50% plagioclase and 50% pyroxene. At the moment of crystallization from a mantle-derived basaltic magma with a 18O value of 5·7, this rock will have plagioclase with a 18O value of 5·90, pyroxene with a 18O value of 5·40, and a whole-rock 18O value of 5·65, assuming plagioclase–pyroxene = 0·5, appropriate for plagioclase (An70) and pyroxene at 1150°C (Chiba et al., 1989) and that plagioclase–melt = +0·2 (Kyser et al., 1981). If cooled slowly to 750°C, a plausible closure temperature for pyroxene in a medium-grained igneous rock, plagioclase–pyroxene= 1 (Chiba et al., 1989) and the 18O values of the coexisting plagioclase and pyroxene will therefore be 6·15 and 5·15, respectively. It should be noted that the magnitude of the change in pyroxene 18O value is the same as that of plagioclase because its modal abundance is the same. Thus the pyroxene 18O value is now 0·55 less than the 18O value of the original magma as opposed to the original melt–pyroxene value of -0·3. For these reasons we will assume that the original magmas that produced the Bellevue and Sandsloot cumulates had 18O values 0·55 higher than the measured 18O values of the pyroxene
The majority of the Bellevue samples have pyroxene 18O values between 6·1 and 6·6 (mean 6·4, n = 9). These data indicate that the original magma had a 18O value of 7·0, similar to the estimate of Schiffries & Rye (1989) for the eastern limb of the Bushveld. The highest 18O value obtained for pyroxene from the Platreef was for pyroxenite sample PP45, which has pyroxene and plagioclase in apparent oxygen isotope equilibrium at magmatic temperatures (Fig. 9). The pyroxene 18O value of 8·9 indicates crystallization from a magma having a 18O value of 9·5. These values indicate that the Platreef magmatic rocks formed from magma(s) that were significantly more contaminated than is generally the case for the Bushveld.
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CRUSTAL CONTAMINATION |
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The oxygen isotope constraints on the amount of crustal contamination in the eastern limb of the Bushveld and in the Merensky Reef have been discussed in detail by Schiffries & Rye (1989)
and Reid et al. (1993), respectively. As discussed above, the Bellevue samples have very similar oxygen isotope compositions to the rest of the Bushveld complex. The magma(s) from which these rocks crystallized had a 18O value estimated to be 7·0, which is significantly higher than expected from an uncontaminated mantle-derived magma (5·7, Ito et al., 1987). Schiffries & Rye (1989) suggested that partial melting of an 18O-enriched mantle was an unlikely explanation for the high 18O values in the Bushveld magmas. This explanation has become even less tenable since that time because mantle xenoliths from the Kaapvaal craton show no evidence for elevated 18O values (Mattey et al., 1994). The other important constraint is that the magma 18O values in the eastern, western or northern limb do not appear to have changed during crystallization. This suggests that the magmas became contaminated before emplacement and were well mixed at the time of intrusion (Schiffries & Rye, 1989).
Schiffries & Rye (1989) concluded that the rocks of the Transvaal Supergroup were the likely contaminants because of their availability and relatively high 18O values (9–15), and that simple mixing models indicate between 10 and 29% contamination. The available 18O values for Archaean granitic rocks of the Kaapvaal craton suggest typical values of around 6–8 (Taylor, 1968; Barker et al., 1976; Faure & Harris, 1991), and Archaean granite 3 km from Sandsloot Mine (PP67, Table 4) has a whole-rock 18O value of 8·3. The oxygen isotope composition of lower crust in the region is not known. However, the average 18O value of lower-crustal rocks exposed in the Vredefort impact structure 100 km to the south (La Grange et al., 2000) is 8·67 (n = 35). It would require 44% contamination by material having a 18O value of 8·67 to raise the 18O value of the Bushveld magma from 5·7 to 7·0, and greater degrees of contamination (49%) by the adjacent Archaean granite (PP67). Amounts of contamination as high as this are unrealistic, and we therefore concur with Schiffries & Rye (1989) that the Transvaal Supergroup sedimentary rocks are the most viable contaminants. It should be emphasized that these estimates of amount of contamination are minimum values because they assume simple mixing, rather than assimilation accompanied by crystal fractionation (AFC, DePaolo, 1981). If the contamination process took place in a magma chamber at a lower level in the crust before emplacement, it could have been via an AFC process, in which case the integrated amount of contamination would have had to have been larger than for simple mixing.
Pyroxenes in the pyroxenite rocks of the Platreef at Sandsloot have 18O values that are up to 2·4 higher than pyroxenes in the Bellevue norites. Although differences in grain size and mode might have slightly accentuated this difference (e.g. inset to Fig. 9), it is clear that the Platreef magmas were affected by greater degrees of contamination than the Bushveld in general. The country rock in the area is dolomite, and unaltered dolomite in the area typically has a 18O value of 19–24 (Buchanan et al., 1981; Table 6). Dolomite is by far the most likely contaminant, not only because of its proximity and the presence of calcsilicate xenoliths, but also because its high 18O value produces a much greater shift in 18O per unit assimilated than the other potential contaminants. The dolomite has very low Sr content and dolomite contamination would not, therefore, affect the Sr-isotope composition of the magma.
Because of the thermal, and possibly hydrothermal, effect of the intrusion on the dolomite, the nature of the contaminant could vary from unaltered dolomite through decarbonated dolomite to material with the same chemical and isotope composition as the calcsilicate material in the parapyroxenites. The calcsilicate material in the parapyroxenites has 18O values that are too low (<13) to be realistic contaminants but this may reflect 18O depletion as a result of fluid–rock interaction and/or diffusional equilibration with the magmatic rocks during slow cooling. Dolomite that became decarbonated in the absence of significant volumes of external fluid might be expected to have 18O values only a few per mil lower than the unaltered dolomite (e.g. Valley, 1986, p. 453) and is, therefore a realistic contaminant in terms of its high 18O value. An alternative for contamination mechanism is that the magma absorbed appreciable quantities of 18O-enriched CO2 produced by decarbonation of the dolomite. Physical evidence for this process in the form of primary CO2 inclusions in minerals is, however, lacking.
Simple mixing calculations indicate that assimilation of 18% unaltered dolomite by the Bellevue norite magma would raise the magma 18O value by 2·4 (assuming that the magma contained 1·3 times the oxygen than the dolomite). The effect of dolomite contamination on the phase relations of the crystallizing magma is considered in a highly simplified manner with respect to the ternary diopside–forsterite–silica phase diagram (Fig. 16). The composition of the Platreef magma is poorly constrained but possible candidates are the B2 magma of Sharpe (1981) proposed as parental to the Critical Zone, or a more fractionated version of the parental magma proposed for the Lower Zone (Davies et al., 1980). The only cumulus mineral observed in rocks of the Platreef is orthopyroxene, which suggests a parent liquid that plots in the liquid + enstatite field. Provided the material assimilated plots close to the diopside apex, with moderate amounts of contamination, the resultant composition will not plot outside [fractionated B2 magma, or Davies et al. (1980) magma] the liquid + enstatite field and rocks containing only cumulus orthopyroxene would still be the first to form. The dolomite in contact with the Platreef at Sandsloot is from the Malmani subgroup. Unmetamorphosed Malmani rocks consist of chert-rich and chert-poor units, whereas metamorphosed carbonate rocks in the area are dominated by diopside with only minor forsterite (Nell, 1985). Dolomite assimilation is not, therefore, inconsistent with the observed cumulus mineralogy in the Platreef.
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, various parental magma compositions proposed for Bushveld. D, Davies et al. (1980)
, calcsilicate rocks.
At Sandsloot, the norite with the lowest pyroxene 18O value (PP26) was collected from the west wall of the open pit (Fig. 2) and is not associated with the Platreef. It and the nearby ferrogabbro (PP24) have pyroxene 18O values that are within the range shown by the Bellevue samples. Norite sample PP2 has the pyroxene with the highest 18O value (7·8), which is 1·8 higher than PP26. This difference in pyroxene 18O value is consistent with 13% contamination with dolomite. If, as discussed above, the norites crystallized from a second magma, which intruded the pyroxenites, the pyroxenites would have insulated the norite magma from direct contact with the dolomite. Thus the high 18O values of the norites adjacent to the pyroxenites could reflect either assimilation of already contaminated pyroxenites or mixing of norite magma with contaminated supernatant magma above the pyroxenite layer. Petrographic evidence for either process is, however, lacking.
Cawthorn et al. (1985) investigated the interaction of the Platreef with the country rock at Overysel, 5 km north of Sandsloot, where the Platreef is in contact with Archaean granite. The initial Sr-isotope ratios of the Platreef magmatic rocks at Overysel are generally higher than at Sandsloot (0·7079–0·7227, mean 0·7148). Cawthorn et al. (1985) pointed out that although the high initial Sr-isotope ratios in the Platreef could be explained by 60% contamination with Archaean granite, such high degrees of contamination are not supported by variations in the major elements. Cawthorn et al. (1985) therefore proposed that the contamination occurred via a partial melt or fluid phase derived from the granite. Cawthorn et al. (1985) rejected the possibility of contamination by dolomite because the 87Sr/86Sr ratios of the dolomite at 2050 Ma were not high enough to have produced the variation in initial Sr-isotope ratios in the Platreef at Overysel. At Sandsloot, contamination by dolomite is consistent with both the stable and radiogenic isotope data. The differences observed in the Platreef at these two locations thus seem to be a function of the nature of the immediate country rock.
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FLUID–ROCK INTERACTION |
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The following lines of evidence suggest that the Platreef was affected by fluid–rock interaction to a much greater degree than is generally the case in the Bushveld and, specifically, in the Merensky Reef:
- the per mil difference between plagioclase and pyroxene (2–3·6) in some of the pyroxenites is not consistent with formation at magmatic temperatures;
- the per mil difference between plagioclase and pyroxene (2) in the norites and the ferrogabbro in the hanging wall of the Platreef is higher than that observed in similar rock types at higher levels of the northern limb;
- hydrous minerals are relatively abundant in the Platreef;
- calcsilicate rocks within and just below the Platreef shown evidence for 18O depletion, which must have occurred during interaction with fluids;
- sulphide mineralization in the Platreef encompasses both the pyroxenites and the dominantly metacarbonate parapyroxenites, which suggests that the sulphides were, at least in part, transported by fluids.
Although these features could result from a number of different periods of fluid–rock interaction, the lack of disturbance to the Sr-isotope system suggests that the alteration event(s) took place soon after intrusion.
Origin of the fluid
At the time of intrusion, the Bushveld complex was situated at relatively low latitude (about 20°; Hattingh, 1983) and meteoric water at that time would not be expected to have been much different from present-day ground water. Water collected from a spring in the lowest part of the mine at Sandsloot (24°S and 500 km from the Indian Ocean) has a D value of -22 (Table 5). The D values of biotites and whole rock, and 18O values of plagioclase and whole rock, have been used to reconstruct the isotope composition of the fluid that interacted with the Platreef (Fig. 12). The calculated composition of the Platreef fluids is typical of magmatic fluid (e.g. Taylor, 1977) and distinct from meteoric water and meteoric fluid in equilibrium with dolomite.
Although it cannot be proved, it seems likely that the quartz-granophyre veins that cut the Platreef were associated with the fluid–rock interaction. Given that these veins have granophyric margins, they must have formed at high temperatures (>500°C) during slow cooling of Platreef and overlying norites. Quartz from the three veins that intersect the bench 20 traverse (Fig. 3) has (18O values between 10·1 and 12·2. At relatively high temperatures, the mineral–water fractionation factors are low (quartz–water is +3 at 550°C, Clayton et al., 1972), which means that fluid in equilibrium with the quartz would have had a 18O value of 7–9). At temperatures between 500 and 700°C, calcite–water changes from +2 to 0. At these temperatures, interaction between a magmatic fluid (18O 7–9), and the dolomite would decrease the 18O value of the dolomite (or calcsilicate) towards the 18O value of the fluid. Low 18O values of carbonate in the calcsilicates (18O as low as 12·2) are consistent with interaction with a magmatic fluid at high temperature. The ultimate origin of the fluid is uncertain; it is probable that the fluid is a mixture of water from the source region of the magma and water derived during the contamination process(es).
Hydrothermal interaction in gabbroic rocks normally produces steep arrays on a plagioclase vs pyroxene – diagram such as Fig. 8 (e.g. Gregory & Taylor, 1981; Gregory & Criss, 1986). Values of plagioclase–pyroxene for the Bellevue core samples (and PP27, a norite collected 10 km SE of Sandsloot) are consistent with equilibrium at magmatic temperatures. The norites collected from the hanging wall of the Platreef (with the exception of one sample, PP59) plot on an isotherm (Fig. 8) with a slightly larger value of plagioclase–pyroxene (average 2·0), which corresponds to a temperature of 512°C [using the equation for An55–Di of Chiba et al. (1989)]. These data are consistent with oxygen isotope equilibrium between plagioclase and pyroxene persisting to slightly lower temperatures than is normal in the Bushveld. The most likely explanation for this is the presence of fluid during slow cooling. The relatively large difference in bulk 18O value of these norites shows that oxygen isotopes were not homogenized during this process, which is consistent with a relatively low water/rock ratio. It should be noted also that the ferrogabbro sample also has a plagioclase–pyroxenevalue close to 2·0, which is consistent with the alteration event occurring at some later time.
A significant number of pyroxenite samples have plagioclase and pyroxene 18O values that are not consistent with oxygen isotope equilibrium at magmatic temperatures. These pyroxenites form an array that cuts across the isotherms of Fig. 8. As the amount of alteration observed petrographically increases, the 18O value of plagioclase increases and the 18O value of pyroxene decreases. This is contrary to the normal situation in hydrothermally altered layered intrusions (e.g. Taylor & Forester, 1979; Gregory & Criss, 1986; Taylor, 1987). There are two possible explanations for the Platreef pyroxenite data:
- the pyroxenites crystallized with a range of 18O values, and the subsequent fluid–rock interaction resulted in shifts to higher 18O value in the plagioclase whereas the pyroxene 18O values remained constant.
- Equilibrium was maintained between plagioclase and pyroxene (and their alteration products) during fluid–rock interaction. The value of 3·6 for plagioclase–pyroxenein sample PP11 would be consistent with a temperature of equilibration between plagioclase and pyroxene of 300°C (Chiba et al., 1989). However, at these temperatures in the presence of fluid, the pyroxene and plagioclase would have, at least partially, converted to low-temperature hydrous silicates such as chlorite and clay minerals
The montmorillonite–chlorite fractionation factor for oxygen [calculated by combining the chlorite–water and montmorillonite–water equations of Wenner & Taylor (1973) and Sheppard & Gilg (1996)] should approximate the expected fractionation between highly altered plagioclase and pyroxene, and ranges from 0·6 at 400°C to 3·3 at 200°C. Thus, even at relatively low temperatures, the observed per mil difference between ‘plagioclase’ and ‘pyroxene’ cannot be produced by their alteration to montmorillonite and chlorite, respectively. Explanation (1) is therefore more likely to be correct, with the pyroxenites having a range of pre-alteration 18O values presumably produced by crustal contamination. The modal abundance of plagioclase in these rocks is much less than that of pyroxene and as a result its 18O values would have been much more sensitive to fluid–rock interaction than those of the pyroxene. Again, this suggests that the alteration process is one of low water/rock ratio. Unfortunately, the data do not allow a distinction to be made between the single or multiple intrusion hypotheses for the origin of the norites and pyroxenites.
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CONCLUSIONS |
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- Apart from the Platreef, the northern limb has the same oxygen isotope characteristics as the Eastern Limb of the Bushveld. The original magma(s) had 18O values that are 1–1·5 higher than expected in an uncontaminated mantle-derived magma. Archaean granite and lower-crustal granulites do not have high enough 18O values for them to be realistic potential contaminants. We therefore agree with Schiffries & Rye (1989) that contamination by Transvaal Supergroup sedimentary rocks is the most likely cause of these high 18O values. There is no evidence for changes in magma 18O value over a 2500 m section through the upper part of the northern limb. This suggests that intrusion was filled with well-mixed, already contaminated magma(s). The fact that the Transvaal Supergroup sedimentary rocks are the only realistic contaminants implies that the contamination process took place in a magma chamber within the crust.
- The Platreef norites and pyroxenites were affected by additional crustal contamination. The most likely contaminant was the country rock dolomite, which forms the footwall at Sandsloot. Simple mixing calculations suggest that the Platreef pyroxenites with the highest 18O values crystallized from magmas that had assimilated 18% dolomite in addition to the earlier contamination by Transvaal Supergroup rocks. Dolomite contamination is unlikely to have resulted in diopside or olivine crystallizing instead of orthopyroxene, or to have affected the trace element or Sr-isotope composition of the Platreef.
- It is clear is that the rocks within and just above the Platreef were affected by fluid–rock interaction to a much greater degree than in the rest of the intrusion. The Platreef shows both petrographic and oxygen isotope evidence for interaction with an external fluid. This fluid appears to have been magmatic in origin, on the basis of its hydrogen isotope composition and that of apparently associated quartz veins. The fluid both affected the magmatic component of the Platreef and lowered the 18O values of the dolomite within the Platreef and in the immediate footwall. The fluid–rock interaction appears to have taken place at low water/rock ratios.
- The lack of disturbance of the Sr-isotope system suggests that the fluid–rock interaction took place soon after intrusion. Hence the alteration of the Platreef did not occur as a response to long-term or much younger fluid movement along the contact between the intrusion and the country rock.
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ACKNOWLEDGEMENTS |
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Financial support for this work was provided by the National Research Foundation. We are indebted to Fayrooza Rawoot, Fran Pocock, Shireen Govender and Steve Richardson for help with the analytical work. AMPLATS provided access to Sandsloot Mine and gave permission for this paper to be published. The help of the staff of the Geology Department at the Potgietersrus Platinum Mine is gratefully acknowledged. We also thank Chris Lee of AMPLATS for help in getting the project off the ground, Lew Ashwal for providing material from the Bellevue core, and Marian Tredoux for reading a preliminary version of this manuscript. We are particularly grateful to Grant Cawthorn and Christian Tegner for their many constructive comments on the manuscript. This paper was written while the first author was Professeur Invité at Université Jean Monnet, St-Etienne.
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FOOTNOTES |
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*Corresponding author. Present address: Département de Géologie, Université Jean Monnet, 23 rue Paul Michelon, F-42023 Saint Etienne Cédex 2, France. E-mail: chris.harris@univ-st-etienne.fr
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