"Rock of the Month # 220, posted for October 2019" ---

Rustenburg is the world centre for the mining and refining of the six platinum group elements (PGE), chromium and related metals, including vanadium. South Africa has long been the chief producer of platinum-group elements, though Russia, with its Noril'sk mines, is a major producer of palladium. The PGE are produced from igneous rock units within the huge Bushveld igneous complex, the principal components of which are the kilometres-thick Rustenburg layered series (the world's largest layered intrusion), overlain by a sequence of granophyric and granitic intrusives. The complex is of early Proterozoic age, circa 2055 Ma. In economic terms, the key units in the Rustenburg series are the Merensky Reef, a thin but laterally extensive layer of pyroxenite with sparse disseminated base-metal sulphides, and underlying layers of chromitite, a rock composed mainly of the chromium-iron oxide chromite, ideal formula FeCr₂O₄ (a member of the spinel family, which typically contains appreciable impurities such as Al and Mg). The UG-2 chromitite is the most important of these. The Merensky Reef and the chromitites each have extensive lists of minerals of the platinum group.

Rustenburg lies roughly 100 km northwest of Johannesburg, in the Western Limb of the Bushveld complex. In addition to the main mining and metallurgical complex at Rustenburg there are additional PGE mines, such as Kroondal (on the UG-2, 12 km southeast of Rustenburg: Anon, 2000). Some mines, such as Horizon (Ntuane) are principally for their chromite, for ferrochrome production (Chadwick, 2002). Another is Purity Chrome (Richardson and Aylmer, 1993).

The chromite seams occur 560 to 3,150 feet (171 to 960 m) below the Merensky Reef, in a sequence of pyroxenite, norite and anorthosite. There are 3 groups, totalling at least 25 seams - the seams range from a fraction of an inch to about 6 feet thick, averaging some 18 to 40 inches (46 to 102 cm). Chrome ores can be divided into end uses, dictated largely by chromite composition: refractory, chemical and metallurgical ores (Fourie, 1959). Just 13 of the chromitite seams in the Bushveld have been numbered: UG 1 and 2, MG 1 to 4 and LG 1 to 7. The LG6 (Main seam or Magazine seam) is generally considered the most economically viable for chrome (Richardson and Aylmer, 1993). This mine has a mining width near 1.8 m, including an internal 40-cm waste band. The seam dips north at 12.5 degrees, comprising 30 cm of chromite, 40 cm of pyroxenite and 110 cm of chromite.

The UG-1 and UG-2 chromitite layers are exposed in the Brakspruit shaft. About 45 m above the UG-1 is the UG-2, the uppermost major chromitite in the Bushveld. The main layer, usually circa 75 cm thick, is underlain by pegmatoidal pyroxenite, and there are 3-4 thin chromitite leader layers in pyroxenite above the main layer. The main UG-2 layer contains substantial PGM (platinum-group mineral) content. The Cr₂O₃ content is low, usually circa 32%, hence the layer is not viable purely as a Cr ore (Minter, 1995).

The world-class PGE deposits and their genesis have been studied intensively (see, e.g., Barnes and Maier, 2002; Barnes et al., 2010), as has the extractive metallurgy of these complex ores (Cole and Ferron, 2002). The Bushveld complex is one of the principal districts in which the metallogeny of mafic magmas has been studied intensively, others being the layered intrusions of Stillwater (Montana) and Skaergaard (eastern Greenland), and the intrusions of the Noril'sk-Talnakh region of Siberia. Particular abundance of olivine and Cr in cumulates of the Lower and Lower Critical zones, in the northern sector of the Western Limb of the Bushveld, may be explained by proximity to the original feeder and/or the input of crystal-charged batches from a lower magma chamber (Eales, 2002). The cumulates become more evolved and leucocratic with distance from Union Section towards the northeast (Amandelbult) and southeast (Impala, Rustenburg and Brits areas), suggesting that there may have been a feeder zone near Union Section (Maier and Eales, 1994). It is indeed very likely that the Rustenburg series formed by multiple injections of one or more mantle-derived magmas, and by the partial melting and assimilation of crustal host rocks by the hot melts (e.g., Eales et al., 1993; Eales and Costin, 2012). Crystal settling and concentration of early-crystallizing chromite would add to the chrome content on the transient floor of a magma chamber, and it is possible that this happened also in an intermediate staging chamber at depth, further enriching the eventual chromitite ores. Osmium isotope studies also suggest that a small proportion of lower-crustal material, perhaps of the order of 1-5%, was assimilated by the mafic magma (McCandless and Ruiz, 1991; McCandless et al., 1999).

The UG-2 unit incorporates chromitite and pyroxenite separated by feldspathic cumulates, and may itself be a multiple unit, rather than the result of a single magma injection (Maier and Bowen, 1996). The UG-2 chromitite is 90-180 cm thick and may contain pyroxenite partings. The hangingwall of the UG-2 is usually a feldspathic pyroxenite 6-12 m thick, usually with 1 to 6 minor chromitite `leader' layers 1-2 cm thick. The UG-2 chromitite and pyroxenite may result from a buoyant plume of magma replenishing the magma chamber (Maier and Walters, 1994).

Fig. 3: UG-2 chromitite sample 1730 from the Breskop shaft at Rustenburg. From the centre of the UG-2 layer. Fine-grained (1 mm grain size) chromite rock with dark green clinopyroxene oikocrysts. Collected by Mark Rebagliati, November 1995.

Chromite mineral chemistry is surprisingly variable: although the spinel mineral group has simple ideal formulae of the form AB₂O₄ there is major substitution between the divalent and trivalent cations. Pure FeCr₂O₄ has the atomic proportions Fe:Cr:O of 1:2:4, and by weight contains 46.46 percent Cr, or (in oxide proportions) 67.90 percent Cr₂O₃ and 32.10 percent FeO. Speaking purely of chromium, then, these values represent theoretical maxima in a single crystal, or in all the crystals comprising a sample of chromitite.

Aliquots of the two samples shown here were submitted as possible geostandards, reference materials, for in situ analysis by electron microprobe. Polished samples were prepared and then analysed in 2011 by Dr Claudio Cermignani, using a Cameca SX-50 microprobe at the University of Toronto. Homogeneous grains would have been offered as standards by Astimex, a company established by Prof. John Rucklidge to supply well-characterized materials for analytical labs. 20 points on 20 or fewer grains were analysed in each sample, in wavelength-dispersive mode, and the results treated by simple statistics. In the end, the chromites were not judged sufficiently homogenous to be employed for this purpose. This is a common problem: micron-scale chemical variation, often reflecting chemical zonation during crystal growth. Such zonation is often concentric. A related problem is the frequency of small inclusions of other minerals, within the growing crystal. This is why many reference compounds are synthesized in the lab, in preference to using naturally-occurring crystals.

The key results are summarized below (Table 1) in oxide weight percent. There are also decreasing amounts of minor elements besides Ti, such as V₂O₃, MnO and NiO, at levels of 0.4-0.1 wt.%. Oxide totals are 97-98 wt.% (Fe is all calculated as FeO, so some of the shortfall is Fe as Fe₂O₃). Variability expressed as two standard deviations of the mean is 1-3% for major elements and 10% or more for Ti and the other minor elements. Note the major substitution of Cr by Al.

A detailed study of major-element and precious-metal contents of chromitites reveals two trends of chromite composition. For the precious metals, Os, Ir, Ru and Rh are likely concentrated into chromite and PGM inclusions therein, whereas Pd and Pt prefer sulphide carrier minerals. Chromitites now show very low S contents, ascribable to destruction of sulphide at high temperatures during crystallization. The more fractionated the melt in the magma chamber, the higher the sulphide content, segregating more Pt and Pd relative to lower units in the Rustenburg layered series (Naldrett et al., 2009).

References

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Barnes,S-J and Maier,WD (2002) Platinum-group element distributions in the Rustenburg layered suite of the Bushveld complex, South Africa. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 431-458.

Barnes,S-J, Maier,WD and Curl,EA (2010) Composition of the marginal rocks and sills of the Rustenburg layered suite, Bushveld complex, South Africa: implications for the formation of the platinum-group element deposits. Econ.Geol. 105, 1491-1511.

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