The geochemistry of the PGE Subzone in the Selukwe Subchamber, Great Dyke: an intraformational layer model for Platinum Group Element enrichment in layered intrusions

Abstract

¶The zone of enrichment of platinum group elements (PGE) in the pyroxenite hosted Main Sulphide Zone (MSZ) of the Great Dyke coincides with the stage at which the magma had become saturated in sulphide. No fundamental characteristics of the MSZ have previously been recognized other than the concentration of PGE at the base of the sulphide zone. This zone of enrichment, referred to here as the PGE Subzone, as well as the footwall and hangingwall sequences to the PGE Subzone, are recognized in this work as comprising a series of petrologically and chemically distinct rock layers. Characterization is on the basis of (a) distinct Pd:Pt ratios, (b) cyclicity in pyroxene compositions and, (c) cyclicity in the concentrations of incompatible elements in the whole rock. The incompatible element contents in whole rocks (eg. Zr) are closely linked with the TiO2 contents of the orthopyroxene. The strong interdependence of these parameters indicates an integrated primary process for the PGE mineralization and does not support an origin by which late stage metasomatising fluids transported PGE into the mineralized zone. It also does not support previous models for the MSZ based on progressive fractional segregation of sulphide as a continuous process extracting PGE.