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been replaced by low-temperature assemblages consisting of quartz, albite, sphene, and hydrous silicates (talc, prehnite,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.
Description:
Lonrho's Platinum Division is gratefully acknowledged for
supplying the samples and permission to publish results.
Many colleagues at Mintek, in particular Dr. E.J. Oosthuyzen,
Mr. E.A. Viljoen, Ms. L.A. Andrews, Ms. J.e. Mostert, Dr.
J.P.R. de Villiers, and Dr. J. Nell, as well as Prof. S.A. de
Waal of the University of Pretoria also contributed to this
investigation, for which we thank them.
