Lateral variations in the platinum-group element content and mineralogy of the UG2 chromitite layer, Bushveld Complex

Abstract

Investigation of a number of samples of UG2 Chromitite from the Brits-Marikana area revealed significant mineralogical variations. Most of the deviations from typical UG2 can be attributed to interaction with fluids at intermediate to low temperatures, sometimes related to local disturbances such as the presence o~ replacement pegmatoid, potholes, or faulting, and in other cases resulting from less well-defined hydrothermal activity. In some areas, primary silicates have been replaced by low-temperature assemblages consisting of quartz, albite, sphene, and hydrous silicates (talc, prehnite, chlorite, epidote, and pumpellyite). In these areas, the base-metal sulphide assemblage consists of millerite + pyrite + chalcopyrite, instead of the pentlandite + chalcopyrite + pyrrhotite ± pyrite assemblage found in normal UG2. Such changes can be attributed to iron and sulphur loss resulting from sulphide-chromite equilibration and interaction with fluids. Expulsion of platinum-group elements (PGE) from base-metal sulphides during cooling resulted in the presence of platinumgroup minerals, predominantly PGE-sulphides, enclosed in, or at the grain edges of, base-metal sulphides. Corrosion of sulphides by fluids leads to the isolation of these platinum-group minerals in hydrous silicates. Elsewhere, interaction with iron-rich metasomatic fluids resulted in a platinum-group mineral assemblage characterized by the presence of abundant Pt-Fe alloys, PGE sulpharsenides, PGE-bismuthtellurides, and other non-sulphide phases.