DETRITAL PLATINUM-GROUP MINERALS IN RIVERS DRAINING THE EASTERN BUSHVELD COMPLEX, SOUTH AFRICA

Abstract

During field work in the eastern Bushveld, more than 6000 grains of platinum-group minerals (PGM) were recovered from alluvial sediments by panning on the farm Maandagshoek and its environs. The grains were investigated by reflected-light microscopy, scanning electron microscopy and electron-microprobe analysis. More than 40 different species of PGM were found. Monophase grains of Pt–Fe alloy are the most common, followed by sperrylite, cooperite, braggite, isomertieite or mertieite I, stibiopalladinite or mertieite II, and Ru–Os–Ir–Pt alloy. Polyphase grains comprise various combinations of one or two of the aforementioned PGM and laurite, sulfides and arsenides of Rh and Pd, and a wide variety of Pd-dominant compounds with As, Sb, Te, Fe, Sn, Pb and Hg, some of which are previously unreported from the Bushveld Complex or even unnamed species. The individual PGM grains show various degrees of attrition. They are mostly unaltered, but the assemblage also contains partly to severely altered grains. The different states of preservation probably point to long-lived, continuous accumulation of the PGM by sedimentological processes. The large grain-sizes of the PGM in the placers (40 μm to 1.6 mm) compared to those of PGM in the Merensky Reef and the UG–2 (generally