Platinum-Group Elements in Sulphide Minerals, Platinum-Group Minerals, and Whole-Rocks of the Merensky Reef (Bushveld Complex, South Africa): Implications for the Formation of the Reef

Abstract

The concentrations of platinum-group elements (PGE), Co, Re, Au and Ag have been determined in the base-metal sulphide (BMS) of a section of the Merensky Reef. In addition we performed detailed image analysis of the platinum-group minerals (PGM). The aims of the study were to establish: (1) whether the BMS are the principal host of these elements; (2) whether individual elements preferentially partition into a specific BMS; (3) whether the concentration of the elements varies with stratigraphy or lithology; (4) what is the proportion of PGE hosted by PGM; (5) whether the PGM and the PGE found in BMS could account for the complete PGE budget of the whole-rocks. In all lithologies, most of the PGE (∼65 up to ∼85%) are hosted by PGM (essentially Pt–Fe alloy, Pt–Pd sulphide, Pt–Pd bismuthotelluride). Lesser amounts of PGE occur in solid solution within the BMS. In most cases, the PGM occur at the contact between the BMS and silicates or oxides, or are included within the BMS. Pentlandite is the principal BMS host of all of the PGE, except Pt, and contains up to 600 ppm combined PGE. It is preferentially enriched in Pd, Rh and Co. Pyrrhotite contains, Rh, Os, Ir and Ru, but excludes both Pt and Pd. Chalcopyrite contains very little of the PGE, but does concentrate Ag and Cd. Platinum and Au do not partition into any of the BMS. Instead, they occur in the form of PGM and electrum. In the chromitite layers the whole-rock concentrations of all the PGE except Pd are enriched by a factor of five relative to S, Ni, Cu and Au. This enrichment could be attributed to BMS in these layers being richer in PGE than the BMS in the silicate layers. However, the PGE content in the BMS varies only slightly as a function of the stratigraphy. The BMS in the chromitites contain twice as much PGE as the BMS in the silicate rocks, but this is not sufficient to explain the strong enrichment of PGE in the chromitites. In the light of our results, we propose that the collection of the PGE occurred in two steps in the chromitites: some PGM formed before sulphide saturation during chromitite layer formation. The remaining PGE were collected by an immiscible sulphide liquid that percolated downward until it encountered the chromitite layers. In the silicate rocks, PGE were collected by only the sulphide liquid.