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Abstract

The Platreef is predominantly a pyroxenitic PGE–Cu–Ni-bearing package which lies at the base of the northern limb of the Bushveld Complex. Geochemical data from two cores from the southern sector of the Platreef on the farm Turfspruit suggest that the Platreef here is a complex intrusive body comprising three or four feldspathic pyroxenite sills each with a distinctive chemistry. Each package is characterised by difference in Mg#, SiO₂:Al₂O₃ and CaO:Al₂O₃ ratios, trace element abundances and Pt:Pd and Ni:Cu ratios. Compositional breaks in major and trace element chemistry between the different pyroxenite packages are supported by changes in mineral chemistry. The more primitive feldspathic pyroxenite with the highest Cr content and Mg# occurs towards the top of the succession, whereas pyroxenites lower in the succession show varying degrees of interaction with floor rocks and hornfels xenoliths resulting in inhomogeneous textures, variability in composition even on a metre scale, increased incompatible elements and migration of sulphur from shale to magma.

The earliest intrusive pulse that contributed to the Platreef package appears to have reacted with the country rocks and cooled rapidly to form a grey micronoritic layer, equated with the Marginal Zone of the eastern and western limbs of the Bushveld Complex. Thick fine-to-medium grained peridotites ascribed to the Lower Zone are equated with the pyroxenites, harzburgites and serpentinites of the Grasvally area south of Mokopane. These olivine-bearing layers have subsequently been intensely serpentinised. As further pulses of magma followed, it is envisaged that metasedimentary material became detached from the floor and later intrusive pulses, which gave rise to feldspathic pyroxenites, flowed under or over these layers. The result is a succession of pyroxenite sills that are separated from each other by interlayers of serpentinite, clinopyroxenite, pegmatitic norite or cordierite spinel hornfels. The Main Zone differs mineralogically and geochemically from the Platreef. It hosts both xenoliths of hornfels and Platreef pyroxenite, especially towards the base, indicating that the Main Zone intruded post-Platreef. Thus, it is implied that shales or dolomites were originally the roof rocks to the Platreef that became incorporated into the Main Zone magma during emplacement.

Within the Platreef package, the number of pyroxenitic layers may vary but preliminary attempts have been made to correlate three pyroxenites in one core with four pyroxenites in a second core from boreholes 2 km apart. The most striking characteristics are that the highest Pt+Pd concentration and lowest Pt:Pd ratio occur in the basal pyroxenite in association with sulphides, with an upward increase in Pt:Pd ratio and Ni:Cu so that the top pyroxenite has a Pt:Pd ratio of 2 and Ni:Cu of > 3 whilst the bottom pyroxenite has a ratio of Pt:Pd ratio of 1 and a Ni:Cu of < 2. These increased ratios upward are not a simple fractionating sequence from the bottom to the top of the Platreef otherwise the top pyroxenite would be the most evolved, which is not the case as it has the most primitive Mg# and highest Cr content. The Platreef is, therefore, a complex zone of sill inter-fingered lithologies reflecting a multiphase emplacement.

Keywords

PLATREEF BUSHVELD COMPLEX PGE-CU-NI MULTIPHASE EMPLACEMENT

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Geochemical evidence for multiphase emplacement in the southern Platreef

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