Compositional and Lithological Controls on the PGE-Bearing Sulphide Zones in the Selukwe Subchamber, Great Dyke: a Combined Equilibrium-Rayleigh Fractionation Model

The platinum group element (PGE)-bearing Main and Lower SulphideZones of the Selukwe Subchamber of the Great Dyke are made upof a series of subzones within which the ratios Pd:Pt are effectivelyconstant, whereas these ratios vary significantly between thesubzones. Fractionation of Pd with respect to Pt varies by afactor of 10 and cannot be modelled using a sulphide collectorphase and constant partition coefficients. The link with sulphideis indisputable and the control is likely to have been the degreeof oversaturation of PGE micro-nuggets in the magma. The apparentpartition coefficients for Pt and Pd between silicate and sulphideliquid are dependent on the degree of oversaturation and therebyexhibit spurious correlation with the PGE content of the sulphide.Modelling replicates the Pt and Pd distribution and ratios onlyby dramatically changing the effective partition coefficients.Pyroxene compositions (including TiO₂) are shown to be stronglydependent on the incompatible element content of the whole rock,and specific linear arrays relating these variables can be relatedto 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 subzonesis indicative of equilibrium crystallization. This layered structuremay have been derived from liquid layers in the magma chamber. KEY WORDS: platinum group elements; sulphide; Great Dyke; pyroxene compositions