Platinum-group elements in silicate rocks of the Lower, Critical and Main Zones at union section, Western Bushveld Complex

Abstract

Concentration patterns of platinum-group elements (PGE) in the Lower, Critical and Main Zones of the Bushveld Complex are modelled by a process of metal collection by segregating sulphide melt followed by fractional crystallization of monosulphide solid solution (mss). Separation of Os-Ir-Ru-Rh-enriched mss cumulate ore and Pt-Pd-enriched residual sulphide melt is inferred to have occurred by upward percolation of interstitial silicate melt entraining sulphide melt, whereas mss crystals are entrapped by early crystallizing chromite. This process resulted in Os-Ir-Ru-Rh enrichment of the chromitites and an upward increasing trend of (Pt + Pd)/(Os + Ir + Ru). Crystallization of platinum-group minerals (PGM) from S-undersaturated sulphide melt or from coalescing noble metal clusters are alternative possibilities to explain the observed PGE patterns, but remain difficult to model at present. Remobilization of Pt and Pd by percolating late magmatic hydrous fluids could have resulted in metal enrichment of pre-existing magmatic sulphides, but this model is not supported by the metal budget of the 'reefs' and the field data. There is no evidence for solid substitution of PGE into oxides and silicates.