Journal of Petrology Advance Access originally published online on September 26, 2006
Journal of Petrology 2006 47(12):2369-2403; doi:10.1093/petrology/egl048
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Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa
SCHOOL OF GEOLOGICAL SCIENCES, UNIVERSITY OF KWAZULU-NATAL DURBAN, SOUTH AFRICA 4041
RECEIVED MAY 20, 2005; ACCEPTED AUGUST 16, 2006
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe Merensky Reef of the Bushveld Complex is one of the world's largest resources of platinum group elements (PGE); however, mechanisms for its formation remain poorly understood, and many contradictory theories have been proposed. We present precise compositional data [major elements, trace elements, and platinum group elements (PGE)] for 370 samples from four borehole core sections of the Merensky Reef in one area of the western Bushveld Complex. Trace element patterns (incompatible elements and rare earth elements) exhibit systematic variations, including small-scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas. Ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas (Critical Zone and Main Zone types) that gave rise to the Bushveld Complex. Mingling, but not complete mixing of different magmas is suggested to have occurred during the formation of the Merensky Reef. The trace element patterns are indicative of transient associations between distinct magma layers. The porosity of the cumulates is shown to affect significantly the distribution of sulphides and PGE. A genetic link is made between the thickness of the Merensky pyroxenite, the total PGE and sulphide content, petrological and textural features, and the trace element signatures in the sections studied. The rare earth elements reveal the important role of plagioclase in the formation of the Merensky pyroxenite, and the distribution of sulphide.
KEY WORDS: Merensky Reef; platinum group elements; trace elements
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INTRODUCTION |
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The Bushveld Complex (Fig. 1a and b) is the world's largest resource of platinum group elements (Pt, Pd, Ru, Rh, Os and Ir; referred to as PGE), located in three main occurrences—the Merensky Reef, UG-2 chromitite and Platreef. The UG-2 chromitite and the Merensky Reef are associated with a series of pyroxenite layers close in the stratigraphy to where ultramafic rocks of the Critical Zone give way to extensive development of norite in the Main Zone. This study focuses on the Merensky Reef in the western Bushveld Complex (Fig. 1c). Platreef mineralization is associated with the lower marginal contact in the northern limb of the Bushveld Complex. The Merensky pyroxenite is part of the Merensky Cyclic Unit and is located within the Upper Critical Zone of the Rustenburg Layered Suite (Fig. 1d).
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The term ‘Reef’ refers to the economically important zone contained largely within a medium- to coarse-grained plagioclase-pyroxenite and is specifically the mining zone of payable metal values. The nature and location of the economic zone is highly variable both in thickness and in exact location (Viljoen & Hieber, 1986
; Viljoen & Schurmann, 1998; Viljoen, 1999) and is usually, but not always, completely enclosed within the plagioclase-pyroxenite. Despite being mined for nearly 80 years, and the current focus of intense exploration and development, there is little consensus on the origin of the Merensky pyroxenite and the PGE mineralization. The aim of this work is to examine the variability of the host Merensky pyroxenite in one area of the western Bushveld Complex where distinct facies changes occur, based on aspects of the textures, compositions, PGE and trace element contents of both the highly mineralized and less-mineralized portions of the pyroxenite.
Several important questions on the Merensky Reef remain to be answered. These include the link between sulphide distribution and PGE mineralization in terms of the enclosing rock-types and silicate mineral associations, evidence for the possible involvement of hydrous fluids in the mineralization event, the extent to which fractional segregation of sulphide controlled the PGE distribution, and the role and nature of different magmas that may have interacted in this zone. The highly incompatible trace elements (Nb, Ta, Th, U, Zr, Hf) contained in the pyroxenite largely reflect the trapped liquid in the cumulates and, therefore, have the potential to provide important information on liquid compositions and the origin of the parent magmas. Apart from one specific type of narrow reef facies documented by Barnes & Maier (2002), there is scant information on the detailed distributions of these trace elements in the Merensky Reef, and none that consider a variety of reef types in a single area.
The limitation in most studies to date has been the restricted scope of the investigations, with few comparative data within local areas. Previous studies have focused on mine-wide occurrences of the Merensky Reef (Wilson et al., 1999) or have been restricted to single borehole intersections (Barnes & Maier, 2002). In the present study, four borehole sections of undisturbed or ‘normal’ reef of contrasting types were investigated in one area in the western Bushveld Complex (Fig. 2). A total of 370 closely spaced samples were analysed, using high-precision analytical methods to determine major and trace element pattern types and the distribution of PGE.
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THE BUSHVELD COMPLEX |
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Structure and stratigraphy
The Bushveld Complex is a large lopolithic intrusion of mafic and ultramafic cumulate rocks up to 10 km thick, formed from magmas that intruded the early Proterozoic Transvaal Supergroup at 2·05 Ga (Walraven et al., 1990
; Buick et al., 2001). There are five main limbs of the intrusion developed on a regional basis (Fig. 1a) and each (possibly apart from the Far Western Limb) has essentially the same stratigraphic succession, comprising (from the base upwards) the Marginal, Lower, Critical, Main and Upper Zones. These make up the succession of mafic and ultramafic rocks called the Rustenburg Layered Suite (SACS, 1980). The Lower Zone and the lower part of the Critical Zone comprise ultramafic rocks (harzburgites and pyroxenite) and the Upper Critical Zone is made up of pyroxenite layers, norite and anorthosite. The Main and Upper Zones are norite and gabbronorite with magnetitite layers in the latter. The Merensky Cyclic Unit, containing the Merensky pyroxenite, is located close to the boundary of the Upper Critical Zone and Main Zone. It is overlain by the Bastard Cyclic Unit, similar in form and thickness to the Merensky Cyclic Unit, but only sparsely mineralized. In general, the pyroxenite layer of the Merensky Cyclic Unit has a sharp lower contact, commonly with a narrow basal chromitite overlying pyroxene anorthosite, and an upper contact grading into norite. It ranges in thickness from 1 to 10 m and the mineralized zone varies in thickness from 10 to 200 cm. The PGE mineralization is associated with sulphide in the vicinity of, and generally overlying, one or more narrow chromitite layers, located approximately in the centre of the pyroxenite unit. These chromitites separate the pyroxenite into lower and upper portions. In some localities the economically important zone straddles the lower chromite layer and extends into the underlying norite. The texture is mainly cumulus orthopyroxene crystals enclosed within plagioclase with sparse large oikocrysts of clinopyroxene. Olivine is also present in some localities. In many occurrences parts of the lower pyroxenite may be pegmatoidal.
Terminology based strictly on modal mineral abundances (Le Maitre, 1989) classifies the ‘pyroxenite’ unit as a range of rock-types from true pyroxenite to melanorite; however, textures (relating mainly to the interstitial nature of the plagioclase) also need to be considered in the nomenclature (Gillespie & Styles, 1999; Brown, 2004). Recent classification of the Merensky pyroxenite based on whole-rock compositions, cumulus status of minerals and mineral proportions (Gillespie & Styles, 1999; Brown, 2004; Wilson et al., 2005) recommended that these rocks be termed pyroxenite or plagioclase pyroxenite, depending on the amount of interstitial plagioclase present. In some sections, norite layers occur where plagioclase is cumulus; however, they are different from the footwall and hanging-wall norites, in which the plagioclase is strongly zoned.
Magma compositions of the Bushveld Complex
No chilled magma compositions that may represent the parental magmas to the Bushveld Complex have been found. Instead, different rock-types in the Marginal Zone and fine-grained sills in the sedimentary floor rocks have led to the recognition of several magma types (Cawthorn et al., 1981; Sharpe, 1981; Davies & Tredoux, 1985; Harmer & Sharpe, 1985). However, it is debatable to what extent these rock compositions represent true liquids, are crystal-enriched liquids (Eales & Cawthorn, 1996), or indeed even reflect the range of Bushveld magmas (Eales & Cawthorn, 1996; Eales, 2002; Kruger, 2005). Ultramafic sills with spinifex pyroxene textures and quenched olivine are likely to represent closely the parent magmas (Davies et al., 1980). The crystallization sequences of some of these reproduce that seen in the Bushveld Complex (Cawthorn & Davies, 1983). There is also evidence of considerable crustal assimilation (Cawthorn et al., 1981). Compelling evidence for the emplacement of a different magma type that gave rise to the Main Zone compared with the Critical Zone is the marked overall, but erratic, increase in initial 87Sr/86Sr ratio close to the boundary of these rock units (Kruger, 1994). This change was initially shown to take place at the level of the Merensky pyroxenite (Kruger & Marsh, 1982) but subsequent studies have shown that no clear pattern exists (Lee & Butcher, 1990) and in some cases virtually no change in isotopic signature is observed at this level (Wilson et al., 1999).
There is general acceptance that two main parental magmas gave rise to the Rustenburg Layered Suite (e.g. Eales, 2002). A high-magnesium andesite magma (somewhat akin to a boninite on major element grounds but lacking distinctive trace element characteristics), termed magma-type B1, gave rise to the Lower Zone and lower Critical Zones. Low-magnesium, low-Ti-tholeiitic basalts, termed magma-types B2 and B3, were the principal components of the Main Zone and Upper Zone (Sharpe, 1981; Irvine & Sharpe, 1983; Harmer & Sharpe, 1985; Sharpe & Hulbert, 1985; Curl, 2001). There is no agreement on the precise compositions of these proposed parental magmas (Eales & Cawthorn, 1996; Eales, 2002), and Kruger (2005) provided interpretative evidence to suggest that up to five distinct magma types were involved in the formation of the Rustenburg Layered Suite. Therefore, the two broad magma types that gave rise to the Critical Zone and Main Zone will be referred to in this study in more general terms as Critical Zone (CZ) and Main Zone (MZ) magmas, respectively.
The CZ and MZ parental magmas are differentiates of more primitive magmas with relatively steep rare earth element (REE; normalized La/Yb = 5–12) patterns and with Nd isotope characteristics indicative of crustal contamination with a change in 
Nd (t = 2·05 Ga) from +5·5 to +7·0. This change occurs in the transition zone from the Upper Critical Zone to the Main Zone at the approximate level of the Merensky Reef (Maier et al., 2000). The Re–Os isotopic compositions of the Merensky Reef sulphides (Schoenberg et al., 1999) are highly radiogenic and also highly variable (187Os/188Os 0·168–0·181), further confirming a continental crustal signature. The CZ magma has a higher concentration of incompatible trace elements and a steeper REE pattern than the MZ magma.
An important feature of the Bushveld Complex is that PGE are present in variable but low levels (10–200 ppb) in most rocks (including chromitites and pyroxenite) of the Lower Zone and Lower Critical Zone, and it is likely that the parental magma that gave rise to these rocks was also relatively enriched in PGE (Lee, 1996). Recent estimates of the Pd content of the CZ magma are as high as 100 ppb (R. Keays, personal communication, 2005).
Precise trace element data for the most primitive magma compositions of the Bushveld Complex remain sparse. X-ray fluorescence (XRF) data have been obtained for marginal sills that occur in the country rock peripheral to both the eastern and western Bushveld Complex (Davies et al., 1980; Cawthorn et al., 1981). A recent study (Curl, 2001) reported precise trace element data for marginal sills in the eastern Bushveld Complex, of both quenched and fine-grained granular varieties, and these data are used in this study for comparative purposes. Two broad compositional types of sills were identified corresponding to B1 (or Critical Zone-type) and B2/B3 (or Main Zone-type) magmas.
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GEOLOGICAL SETTING OF THE STUDY AREA |
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The Merensky Reef sequence is highly variable throughout the Bushveld Complex, both in thickness and lithology. Within the area of interest there is a recognizable facies change that includes significant variation in thickness of the Merensky pyroxenite (Fig. 2). The economic zone (the Merensky Reef) is identified by the development of abundant (2–5 vol. %), net textured to disseminated sulphide comprising pyrrhotite, pentlandite and chalcopyrite. This zone is typically about 1 m thick but lesser amounts of mineralization are developed above and below the economic zone.
The objective of this study is to investigate the Merensky Reef in a localized area encompassing a variety of undisturbed reef types (as opposed to disturbed pothole reef), where thickness of the pyroxenite is observed to vary. Four drill cores (GC1, SD45, SD22 and SD46) through the sequence were selected for detailed study (Fig. 2). In drill hole SD22, two deflections (two sub-parallel drill cores branching off the mother hole at depth) designated D3 and D5, separated spatially by about 1·5 m at 1500 m depth, were analysed for trace elements to investigate short range differences. The cores were logged to establish lithologies and textural types.
The four sections cover a lateral distance of about 5 km and range from the narrow Rustenburg-type ‘thin-reef’ facies (Wilson et al., 1999) of GC1 to thick-reef types (also colloquially called ‘Swartklip facies’ type; Viljoen & Schurmann, 1998) of SD22 and SD46. In GC1 the pyroxenite and the associated mineralization occur over a narrow vertical interval of 60 cm compared with SD46 where the pyroxenite is 10 m thick. A geographically restricted zone characterized by an intermediate facies separates these two distinct types and is represented by SD45. The position of GC1 is closer to the margin of the RLS, whereas the intermediate and thick-reef types are progressively further from the margin.
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SAMPLING AND ANALYSIS |
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A major limitation in studies of the Merensky Reef to date has been the selected nature of the samples and the variable types of data because of the different sampling and analytical techniques used. In this study an attempt is made to compare an internally consistent dataset for samples that have been prepared and analysed in the same manner. A problem with sampling the Merensky Reef using drill core intersections has been the extent to which the samples are sufficiently representative of the vertical interval to elucidate small-scale variations (Wilson et al., 1999
). In this study, wide diameter whole-core samples 60 mm in diameter (as opposed to commonly used half or quarter core) were sampled at between 5 and 10 cm intervals depending on the thickness of the pyroxenite unit. The narrow-type facies (GC1) was drilled to yield core 105 mm in diameter allowing the pyroxenite to be sampled over 1 cm intervals, each sample yielding about 100 g of rock. All cores were sampled on a continuous basis between the footwall of the Merensky pyroxenite into the norite hanging wall, giving a total number of 370 samples. Trace element and PGE concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) on separate whole-rock fractions derived from the same sample using an Elan Sciex® 6100 system. High-temperature (260°C)–high-pressure (75 bars) dissolution in HF–HNO3 was essential to dissolve the highly refractory mineral constituents that largely control the incompatible trace element budget. Closed Teflon® beaker dissolutions yielded erratic data 5–50% lower than the high-pressure dissolution method, because of the inability to dissolve refractory minor mineral components such as zircon, badelleyite and zirconolite. After drying down, the final dissolution was made up to 50 ml in 10% nitric acid that also contained the internal standards Rh, In, Re and Bi (a correction was made for those ore samples that contained Rh). Analysis was carried out against certified standard solutions from Specpure® and Perkin Elmer®. PGE pre-concentration was by means of the standard Ni–S fire assay method and the analysis was completed by ICP-MS. A further factor that was studied was the influence of particle size in the milled material and sample homogenization. It was ascertained that reproducible results for trace elements determined by ICP-MS could not be obtained on sample material where the median grain size exceeded 20 µm. In this exercise, samples were milled to <20 µm, with the median close to 10 µm. Milling was carried out using a high-purity C-steel swing mill. Size distributions were determined using a Malvern laser particle analyser.
Calibrations were against certified analytical solutions and international certified reference materials (BHVO-1, BCR-1 and BR-1), which were analysed with every run. All samples were analysed in duplicate. A further laboratory control (SD22 4691) was analysed with every sample batch. International reference materials SARM6 and SARM7 were analysed to ensure the accuracy of the PGE data. The accuracy and precision of the trace element data were typically about 5%. Detection limits for incompatible elements were determined using high-purity olivine separates (Fo94) from the Great Dyke which for Nb and Ta were both <1 ppb. Detection limits for PGE were better than 2 ppb for all elements. Whole-rock element analyses were carried out on fusion discs using a Philips PW1404 XRF spectrometer. S was determined by XRF using pressed pellets and matrix-matched standards, and a Cr-target X-ray tube.
Total Fe was analysed and Fe3+/Fe2+ was calculated on the basis of 10% Fe3+ of total Fe. Normative minerals were calculated using the standard method (Johannsen, 1931). Plagioclase components were grouped together and normative En, Fs and Wo components were combined to give diopside and orthopyroxene components.
Analyses of selected samples from all drill cores representing the range of major and trace element contents are given in Table 1. The complete dataset used in the study is given in the Electronic Appendix (http://www.petrology.oxfordjournals.org).
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ROCK-TYPES, TEXTURES AND MAJOR ELEMENT COMPOSITIONS |
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Rock-types
The Merensky Reef exhibits considerable variation petrographically and texturally; detailed descriptions have been given for several sections in the eastern (Seabrook et al., 2005
) and western Bushveld Complex (Leeb-du Toit, 1986; Wilson et al., 1999; Barnes & Maier, 2002). Different descriptive techniques and analytical strategies allow only limited correlation between these studies. The rock-types observed in cores SD45, SD46, GC1 and SD22, from base to top, are as follows (Fig. 3): leuconorite or pyroxene anorthosite footwall; a narrow basal chromitite (BC) 2–10 mm thick with sharp contacts; lower pyroxenite unit, which on the basis of its modal mineralogy ranges from pyroxenite and olivine pyroxenite to plagioclase pyroxenite; narrow top chromitite (TC); upper plagioclase pyroxenite; hanging-wall norite with gradational contact. The upper portions of the lower pyroxenite are commonly pegmatoidal.
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In the Merensky pyroxenite, orthopyroxene as a cumulus phase is the most abundant mineral, followed by plagioclase with minor clinopyroxene. The crystal size of the clinopyroxene is highly variable, ranging from 2 to 20 mm. Plagioclase occurs as large optically continuous oikocrysts 20–60 mm in diameter that enclose orthopyroxene. These oikocrysts commonly have an unzoned core and are strongly zoned towards the margins. In some cases, small (1–2 mm), strongly compositionally zoned plagioclase laths as cumulus crystals are present in specific layers. In the footwall and hanging-wall norite, plagioclase occurs as well-developed laths with minor compositional zoning. Olivine occurs in some sections as cumulus, oikocrystic, or skeletal crystals, or in reaction relationship with orthopyroxene.
Major element compositions and sulphide distributions
Because of the relatively coarse-grained nature of the pyroxenite, modal mineralogy estimates based on whole-rock normative compositions most readily represent the mineralogical variations in the four sections (Fig. 3). Figure 3 also shows the variation of S concentration, which reflects the distribution of sulphide. In all sections the lower pyroxenite unit exhibits pronounced small-scale mineralogical variations whereas the upper pyroxenite is relatively homogeneous. The distribution of normative orthopyroxene content is opposite to that of plagioclase. Variation in the lower pyroxenite is more marked in the transitional (SD45) and marginal (GC1) reef types. The thin facies-type GC1 has a series of pyroxenite and norite layers above the upper pyroxenite.
The distribution of sulphide (Fig. 3) is broadly similar in the transitional (SD45) and thick-reef sections (SD22 and SD46). There is a marked increase in S content at the base of the lower pyroxenite to relatively low levels of 100–1000 ppm S. A small S peak generally occurs immediately above the basal chromitite (BC). Small-scale variations are present, which in most cases coincide with increases in orthopyroxene. At the top of the lower pyroxenite, S content increases markedly through a series of well-defined layers. In all cases the highest S peak is located immediately above the top chromitite (TC). In the thin reef-type (GC1) S distribution is markedly different from the thicker reef-types. The thin pyroxenite of this facies-type cannot be clearly divided into a lower and upper pyroxenite. S attains relatively high concentrations in the norite below the BC and rises to a maximum to coincide with this chromitite. There is a marked decrease of S content in the centre of the pyroxenite, rising again higher in the unit.
Textural associations
Drill core SD22 illustrates the ranges of textures observed in the pyroxenite unit (Fig. 4). These textures relate mainly to the association of orthopyroxene and plagioclase, with orthopyroxene occurring as coarse-grained, irregular-shaped crystals with interstitial, or irregular-shaped plagioclase laths (Fig. 4a–d), to varieties with elongate orthopyroxene (Fig. 4e and f), and close-packed orthopyroxene (Figs. 4g and h). Part of SD45 (Fig. 4i) is a very coarse-grained (pegmatoidal) olivine pyroxenite with ‘harrisitic’ textures in which olivine (and more rarely orthopyroxene) occurs as elongate, hollow and hooked-shaped grains. These textures are typical of crystals that have grown rapidly in a high thermal gradient (Donaldson, 1974). Sulphide consists of an intergrowth of pyrrhotite (40–50%), pentlandite (20–40%), and chalcopyrite (15–25%). There commonly exists a well-developed zonal relationship, with the chalcopyrite forming the outer rim to the Fe–Ni sulphides.
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Small pockets of primary hydrous phases (phlogopite and amphibole) (Fig. 4c and d), and magnetite, are locally developed together with alteration of the primary silicates (fibrous amphibole). These pockets commonly include late-stage accessory phases such as zircon, badelleyite, apatite, zirconolite, rutile and davidite as extremely small crystals (Ohnenstetter et al., 1998
). These mineral associations are interpreted as resulting from the concentration of late-stage (hydrous) fluids as the last vestiges of the solidification process. There is no evidence of segregation or settling of dense minerals that could have caused decoupling of trace elements. |
TRACE ELEMENT COMPOSITIONS |
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Distributions of incompatible trace elements in the Merensky Reef
Ratios of incompatible trace elements provide a broad basis for establishing a link between the mafic cumulates and possible parental magmas. Ratios involving Nb, Th, U and La (Fig. 5a and b) delineate distinct fields for the two broad magma types recognized in the marginal sills and considered to be representative of Bushveld parental magmas. Ratios for different pyroxenites of the Merensky unit overlap, but are largely intermediate between the main magma-type fields, plausibly supporting mixing or mingling (a situation where mixing is incomplete) of the two magmas. There is also correspondence between the plagioclase content (expressed as normative wt %) and incompatible element ratios. However, the relationship is not linear for all samples in the pyroxenite section, but rather defines a series of sub-parallel arrays for sets of contiguous samples (shown for SD22 in Fig. 5c). The arrays, therefore, define narrow intervals in the sequence where variations in Th/Nb show inherent changes with normative plagioclase content. Values of Th/Nb for the Merensky pyroxenites are intermediate between the two inferred parent magma compositions, but there is also a tendency for the sections higher in the sequence to have higher Th/Nb values, or greater affinity with the Critical Zone (CZ) parent magma.
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standard deviations for Nb/U and U/Th for the lower pyroxenite (L. pyrox, open symbols and dashed lines) and upper pyroxenite (U. pyrox, closed symbols and continuous lines) in the four sections studied in relation to the fields for parental magma compositions. (b) Average values and 2Systematic variations in the abundances of trace elements through the four reef sections studied show broadly similar patterns, but in detail are different for each core. Typical patterns are illustrated for Ta, U, and Zr (Fig. 6). The overall concentrations of these elements are highly variable and are characterized by regular oscillations on a scale of 20–50 cm, resulting in a series of peaks and troughs. This pattern is most obvious in the lower pyroxenite, within which there is also a general tendency for the concentrations to increase upwards, particularly in core sections SD45 and SD46. There is excellent correspondence for the variations of Ta, U, Th, and Nb (not shown), but there is no direct lithostratigraphic correlation in pattern form between the different borehole sections. In the upper pyroxenite there is a general trend of an initial increase followed by a decrease upwards in the sections. Zr exhibits similar patterns to Ta, Nb, U and Th, but has a lower degree of correspondence compared with these highly incompatible trace elements.
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The two deflections of SD22 (D3 and D5) show similar distributions for all elements and broadly the same structure of peaks and troughs, although the fine detail is different. The patterns of peaks and troughs are displaced downwards in D5 relative to D3 by about 0·5 m relative to the datum of the top chromitite, indicating the lateral scale on which such variations occur.
Average cumulative trace element contents of the Merensky Reef
Average cumulative trace element contents provide a comparison of the bulk trace element compositions in the four sections studied (Fig. 7). Cumulative values help overcome the possible problem of small-scale decoupling of the trace elements as a result of crystallization of late-stage minerals, and are representative of the entire rock units. The value for each element is determined by sequentially adding, from the base of the pyroxenite upwards, the thickness of the core sample (m) multiplied by the element concentration (as µg/g or ppm), and dividing by the total thickness of the section analysed (m). Inter-element plots of the highly incompatible trace elements Nb, Ta, Th and U are close to linear with a small deviation (Fig. 7), and the regression lines in these plots (except for Zr) intersect close to the origin. Plots of Zr (Fig. 7a) and Hf (not shown) against the highly incompatible elements are also approximately linear but do not intersect at the origin because of incorporation of Zr and Hf in early formed cumulus orthopyroxene, as these elements are not highly incompatible. This intercept allows an estimate of the amount of Zr and Hf incorporated into cumulus phases, mainly orthopyroxene. It is therefore not advisable to use uncorrected whole-rock Zr contents to define parental magma signatures (e.g. Eales, 2002).
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The degree of correlation indicates consistent bulk trace element compositions between the four sections and also suggests that selective decoupling and partial removal of these elements from the cumulate pile did not occur. In Table 2 these ratios are compared with crustal and mantle values, and for the various proposed Bushveld liquids.
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The relative average cumulative incompatible element abundances reflect either the effects of fractionation by which these elements became concentrated in the magma, or the relative amounts of trapped liquid in the four sections. Section GC1 has the lowest incompatible trace element contents, and is used as a comparison for the other sections (Table 3). Section SD46 has approximately five times the concentration of incompatible trace elements compared with GC1; SD22 and SD45 are intermediate between SD46 and GC1. If fractionation of the magma was responsible for the differences in incompatible element concentration then this should be reflected in the average Mg-number for the whole-rocks [calculated as molecular MgO/(MgO + FeO); Table 3]. Mg-number does not indicate marked differences that may reflect significantly different degrees of fractionation in the four sections. Indeed, it shows the opposite effect of higher incompatible element contents correlating with higher Mg-number. The determination of average Mg-number excluded samples of the underlying pyroxene anorthosite, which are in all cases less than 0·75.
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The role of plagioclase
Plagioclase in the Merensky Reef is typically interstitial, or more rarely, present as strongly zoned laths. However, that does not necessarily mean that it crystallized entirely from trapped liquid in the pyroxene mush. A plot of Eu* (the Eu anomaly calculated to be the difference between normalized measured Eu concentration and that extrapolated from the abundances of the adjacent REE) vs Sr (Fig. 8a) shows a strong positive correlation where Eu* is positive reflecting the oikocrystic growth of plagioclase, or the early development of cumulus cores. For strongly negative Eu* (< –2), Sr increases slightly with increasingly negative Eu*, and therefore is probably related to the dominant effect of the liquid composition, as opposed to oikocrystic or cumulus plagioclase. Each drill core section exhibits a different trend in Fig. 8a, with SD22 having a trend that is distinctly different from the other sections.
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The degree of dispersion of highly mobile elements (such as Rb and K) relative to immobile incompatible elements could reflect the role of hydrous fluids. A plot of Rb vs (Nb + Ta + Th + U) for all sections (Fig. 8b) reveals a strong linear correlation with minor dispersion occurring only at very low Rb concentrations (0·5–2 ppm). Overall, the concentration of Rb in SD22 is consistently slightly higher than in the other sections and is probably indicative of relatively higher amounts of oikocrystic or cumulus plagioclase (also resulting in the more positive Eu* values) in this section.
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CHARACTERIZATION OF RARE EARTH ELEMENT PATTERNS IN THE MERENSKY REEF |
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All bulk-rock samples were characterized in terms of their chondrite-normalized REE pattern shape and Eu anomalies (Eu*). Three types of patterns were defined on the basis of Eu*: Type 1—samples exhibiting strong positive Eu*; Type 2—small or no Eu anomaly; Type 3—strong negative Eu*. Within these groups further subdivisions can be made based on the shape of the REE patterns [defined by steep light rare earth element (LREE)-enriched, flat, and U-shaped patterns; subdivided as types 1/1–1/3, 2/1–2/3, etc.]. Overall, 12 distinct patterns were observed (Fig. 9), some of which may be regarded as transitional from one type to another.
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The range of REE pattern types are related to the borehole sections in Fig. 10 and arranged in order of first appearance of a particular pattern type from the base of the section. Each section has a unique signature in terms of the distribution and proportions of pattern types. All sections commence with Type 1/1 in the underlying norite with steep REE patterns and a strong positive Eu*. This is followed in most sections, at the base of the pyroxenite, by Type 1/3, which has a steep REE pattern and small positive Eu*. In SD22 this zone has a relatively flat pattern of Type 2/2 or 2/4. Upwards from this lower section the sequence changes to negative Eu* values with flat REE patterns or to alternating layers with positive and negative Eu*. The characteristic pattern, for the thicker pyroxenite in particular, is the alternation of layers with positive and negative Eu*.
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In SD46, repetitions of positive and negative Eu* occur on a scale of 20–60 cm. In a few cases the cycles start with a flat pattern and no Eu anomaly. Alternating patterns of positive and negative Eu* are also observed for SD22 but these tend to be flatter or U-shaped compared with SD46. In SD45, over the first metre of the lower pyroxenite the REE patterns are generally steep with moderate to no Eu anomaly, although some are U-shaped and close to symmetrical. The upper part of the lower pyroxenite has slightly flatter REE patterns with strong negative Eu*. The upper pyroxenite has U-shaped patterns with strong negative Eu*. In GC1 the pattern types are similar to SD22 with several repeated sequences. The general trend is one of steep REE patterns and strongly positive Eu* migrating upwards into flatter patterns (some also La depleted) with negative Eu*. Higher in the sequence, the patterns tend to be U-shaped with no, or small negative Eu anomaly. Towards the top of the lower pyroxenite in GC1, the patterns are relatively flat, with some samples exhibiting depletion in La and Ce, and strong negative Eu*.
Rare earth modelling for the Merensky Reef sections
Modelling of the REE patterns allows constraints to be placed on the cumulus processes, and on the possible interactions between different magma compositions. Most reliable estimates of the mineral–melt partition coefficients (D) for orthopyroxene pertain to medium- or high-pressure conditions and compositions appropriate for mantle melting (E. Hauri, personal communication, 2000; McDade et al., 2003). These D values are a factor of two or three times lower than those suggested as being appropriate for basaltic melts at moderate pressures (Rollinson, 1993). The effect of liquid composition is well documented (Blundy & Wood, 2003; Nielsen, 2004) and there is a tendency for the ratio of DLa/DYb to decrease in more silica-rich magma compositions (Green & Pearson, 1985; Green, 1994), which would be appropriate for Bushveld-type magmas. The orthopyroxene–melt partition coefficients used in this study are based on estimates for mid-ocean ridge basalts (MORB; Asimow & Langmuir, 2003; McDade et al., 2003), but increased by 10% to account for the higher silica content in the Bushveld magmas. The most consistent plagioclase–melt partition coefficients are those reported by McKenzie & O'Nions (1991), but these are generally somewhat higher than measured values. Therefore, the mean has been taken of those obtained from theoretical considerations (McKenzie & O'Nions, 1991) and those reported in the MORB project database (http://www.petdb.org). The partition coefficients used in this study are given in Table 4.
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The observed REE patterns were modelled using combinations of the CZ and MZ magma compositions. Modelling involved fractionation, representing early stage open-system growth of the liquidus mineral assemblage, and the final combination of orthopyroxene, liquidus plagioclase and trapped liquid. Similar constraints on liquid compositions have been modelled for the cumulates of the Bjerkreim–Sokndal intrusion, Southern Norway (Charlier et al., 2005
). The steep, Eu-enriched REE patterns observed in the footwall pyroxene anorthosite or norite can be modelled using appropriate proportions of pyroxene (3–10%) and plagioclase (90–97%) and small amounts (<2%) of trapped liquid. The REE patterns of the pyroxenite are modelled using a fractionated mixture of the CZ and MZ magmas in equilibrium with plagioclase and orthopyroxene. In some cases better agreement is obtained by initially using one of the parent liquid derivatives, and then mixing with the second liquid. Some degree of fractionation (5–30%) is required to significantly change the slopes of the patterns, indicating that the system must have been open during the initial stages of cumulate formation. Where the proportion of the trapped liquid exceeds 3–5%, it has the major influence on the REE pattern and bulk-rock REE concentrations. Where the trapped liquid component is low (<3%), the REE patterns are mainly controlled by the abundances of cumulus mineral phases.
Fractionation of plagioclase within the cumulate pile is required to produce the strongly negative Eu anomalies in the residual liquids, whereas accumulation of plagioclase as oikocrysts results in positive anomalies. This indicates migration of liquid on at least the scale of the REE pattern-types shown in Fig. 10. Flat or U-shaped patterns are consistent with low trapped liquid contents and are controlled mainly by orthopyroxene, which tends to produce heavy rare earth element (HREE) enrichment. Accumulation of plagioclase as oikocrysts or cumulus crystals results in LREE enrichment. The combination of these controls results in U-shaped patterns.
The final mixture before complete solidification of the rock comprises cumulus orthopyroxene, plagioclase oikocrysts (or in some cases cumulus plagioclase) and differentiated trapped melt. The results of the modelling representing the various pattern types are illustrated in Fig. 11 and Table 4. The least satisfactory models are those for the footwall pyroxene anorthosite. In these rocks the modelled patterns (Type 1/1) are generally steeper, and the positive Eu* is larger than actually observed. The modelling does not provide unique solutions because of the number of variables involved. However, the following observations are robust. (1) Most patterns require a combination of Critical Zone and Main Zone-type magmas in differing ratios ranging from dominantly CZ (for some even 100%), to 30% CZ and 70% MZ. For most samples no one magma type can produce the observed range of compositions. (2) Plagioclase crystallized as an early stage phase in some sections with extensive postcumulus overgrowth, or formed entirely from the interstitial liquid. (3) Some degree of fractionation (usually 5–30%) of the interstitial liquid is required to produce the observed steeper REE patterns.
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REE patterns and magma compositions
A well-defined relationship exists between abundances of the highly incompatible elements (Nb, Ta, Th, U) and the total REE (Fig. 12a). However, the non-constant ratios between these two groups of elements result from the relatively high degrees of incorporation of REE in early formed silicate minerals, compared with the incompatible trace elements. Increasing amounts of trapped liquid dominate over the cumulus minerals in controlling the distribution and concentrations of REE. The Eu anomaly becomes more negative with increasing Nb + Ta + Th + U content (Fig. 12b). This indicates that the trapped liquid dominating the REE patterns had a small to moderate negative Eu anomaly resulting from plagioclase fractionation.
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The slopes of the REE patterns (expressed as [La/Yb]N) are shown in relation to the total incompatible element (Nb + Ta + Th + U) content in Fig. 13. In the pyroxenites the REE patterns become steeper ([La/Yb]N >1) as the total incompatible element content increases. Flat REE patterns ([La/Yb]N
1) are the result of the dominating influence of cumulus orthopyroxene crystallizing from a relatively evolved liquid. The footwall norite has higher [La/Yb]N as a result of increased cumulus plagioclase content. Some groups of pyroxenite (mainly from SD46) exhibit opposite behaviour in which [La/Yb]N decreases (i.e. the REE patterns become flatter) with increasing incompatible element content. The compositions of the Bushveld marginal sills overlap with the high [La/Yb]N end of the Merensky Reef dataset (inset to Fig. 13). Although some of these compositions are undoubtedly crystal enriched, there is a strong compositional similarity with the Merensky Reef data.
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Variation of Eu* and sulphide content
Sulphide, and for the most part, plagioclase, are interstitial to the network of cumulus pyroxenes in the pyroxenite layers. As noted in the previous section, plagioclase may variably have crystallized either from late-stage trapped liquid, or as early formed oikocrysts, or more rarely as cumulus crystals. Eu* ranges overall from +6 to –5 reflecting the contrasting influence of early formed plagioclase oikocrysts (giving positive Eu*) or the trapped liquid (with negative Eu* signature).
The variation of S and Eu* with stratigraphic height for all sections indicates broad-scale contrasting patterns (Fig. 14a and c). Although a simple linear relationship is not observed between S and Eu* for the complete dataset, individual arrays for sets of contiguous samples indicate a strong dependence between Eu* and sulphide content (shown for SD46 and GC1 in Fig. 14b and d) for specific stratigraphic sections. From these relationships plagioclase is indicated to have influenced the distribution of immiscible sulphide liquid.
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DISTRIBUTION OF PLATINUM GROUP ELEMENTS AND GOLD |
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In mafic layered intrusions, the PGE and Au are trace elements that are generally regarded as having being derived from outside the immediate layers that host them. Sulphide is likely to have formed as a dense, immiscible liquid that accumulated with the silicate cumulates (Naldrett et al., 1987
, 1990). Fluids may also have been responsible for transport or remobilization of PGE (Boudreau & McCullum, 1992; Boudreau & Meurer, 1999). At present, for the Merensky Reef, there is little information on the specific links between the PGE distributions and other trace elements arising from the silicate framework of the mineralized zone.
Concentration profiles and element ratios
Concentration profiles are shown for Pt, Pd, Au and S in the four sections studied (Fig. 15). In each section the distributions of these elements are similar, indicating the dominant control by sulphide, but not necessarily the only control. Concentrations of these elements increase overall upwards in the sections through a series of peaks and troughs and then decrease markedly at the base of the upper pyroxenite. The drop-off is, in all cases, significantly greater for Pt and Pd than it is for Au. S also decreases sharply at this point but then rises again in the upper pyroxenite layer, which is largely devoid of PGE. In all sections (including the thin reef section GC1), smooth distributions are observed in the upper pyroxenite layer and there is almost perfect correspondence between Au and S, underlying the strong control by sulphide on the Au content. The sulphide and PGE trends in the upper pyroxenite are similar to other PGE-enriched disseminated sulphide zones (such as the MSZ of the Great Dyke, Zimbabwe; Naldrett et al., 1987; Prendergast & Keays, 1989; Wilson & Tredoux, 1990), in which removal of the PGE from the magma is considered to have occurred by precipitation of sulphide at the base of the upper pyroxenite layer.