Compositional and lithological controls on the PGE-bearing sulphide zones in the Selukwe Subchamber, Great Dyke: A combined equilibrium-Rayleigh fractionation model

Abstract

The platinum group element (PGE)-bearing Main and Lower Sulphide Zones of the Selukwe Subchamber of the Great Dyke are made up of a series of subzones within which the ratios Pd:Pt are effectively constant, whereas these ratios vary significantly between the subzones. Fractionation of Pd with respect to Pt varies by a factor of 10 and cannot be modelled using a sulphide collector phase and constant partition coefficients. The link with sulphide is indisputable and the control is likely to have been the degree of oversaturation of PGE micro-nuggets in the magma. The apparent partition coefficients for Pt and Pd between silicate and sulphide liquid are dependent on the degree of oversaturation and thereby exhibit spurious correlation with the PGE content of the sulphide. Modelling replicates the Pt and Pd distribution and ratios only by dramatically changing the effective partition coefficients. Pyroxene compositions (including TiO2) are shown to be strongly dependent on the incompatible element content of the whole rock, and specific linear arrays relating these variables can be related to the PGE subzones. The overall control is Rayleigh fractionation, but constancy of the ratio Pd:Pt and the initial pyroxene composition (before re-equilibration with trapped liquid) within the subzones is indicative of equilibrium crystallization. This layered structure may have been derived from liquid layers in the magma chamber.