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 INTRODUCTION Sulphide Zones of the Selukwe Subchamber of the Great Dyke are An increasing number of mafic-layered intrusions are made up of a series of subzones within which the ratios Pd:Pt are reported to have distributions of platinum group elements effectively constant, whereas these ratios vary significantly between (PGE), Au and base metals characterized by vertically the subzones. Fractionation of Pd with respect to Pt varies by a offset profiles. Offset profiles involve the vertical sepfactor of 10 and cannot be modelled using a sulphide collector aration of metal peaks and were first reported in the phase and constant partition coefficients. The link with sulphide is Great Dyke (Prendergast & Keays, 1989; Wilson et al., indisputable and the control is likely to have been the degree of 1989; Naldrett & Wilson, 1990; Wilson & Tredoux, oversaturation of PGE micro-nuggets in the magma. The apparent 1990). The phenomenon has since been recognized in partition coefficients for Pt and Pd between silicate and sulphide the Munni Munni intrusion of Western Australia (Barnes liquid are dependent on the degree of oversaturation and thereby et al., 1990), the Skaergaard intrusion (Bird et al., 1991) exhibit spurious correlation with the PGE content of the sulphide. and the Rincon del Tigre Complex of eastern Bolivia Modelling replicates the Pt and Pd distribution and ratios only by (Prendergast, 2000). PGE are also postulated to be transdramatically changing the effective partition coefficients. Pyroxene ported in fluids emanating from expulsion of intercumulus compositions (including TiO2) are shown to be strongly dependent liquid from the crystal pile, and there is strong evidence on the incompatible element content of the whole rock, and specific to support such processes in several gabbroic sections linear arrays relating these variables can be related to the PGE of layered intrusions such as the Skaergaard intrusion subzones. The overall control is Rayleigh fractionation, but constancy (Andersen et al., 1998) and in the Stillwater Complex of the ratio Pd:Pt and the initial pyroxene composition (before re(Boudreau & McCullum, 1986, 1992). In contrast, the equilibration with trapped liquid) within the subzones is indicative mineralized zones in the Great Dyke and Munni Munni of equilibrium crystallization. This layered structure may have been intrusion are located in pyroxenite, and primary magderived from liquid layers in the magma chamber. matic processes have been invoked to explain the min-