Platinum-group element mineralization in the Platreef and calc-silicate footwall at Sandsloot, Potgietersrus District, South Africa

Abstract

The Platreef is a platinum-group element (PGE) deposit in the form of a mafic-ultramafic, tabular body at the base of the northern (Potgietersrus) limb of the 2050 m.y. Bushveld Igneous Complex. The reef transgresses sedimentary floor rocks (footwall) of the 2600-2200 m.y. Transvaal Supergroup and the Archaean granite basement. The roof rocks (hanging-wall) of the reef are PGE-free Main Zone gabbronorites of the Rustenburg Layered Suite. At the Sandsloot open-pit mine the Platreef consists of coarse to pegmatoidal pyroxenites and gabbros with accessory phlogopite, base-metal sulphides and oxides. Thermal metamorphism of siliceous dolomites that form the footwall has produced clinopyroxenites and calcsilicate hornfelses with a variety of skarn assemblages. These were subjected to later hydrothermal alteration and serpentinization that also affected parts of the Platreef.

The link between sulphides and PGE in the Platreef has led previous authors to consider the mineralization as an orthomagmatic sulphide deposit, where sulphide separation collected PGE from a large volume of melt. In the reef and footwall, however, the development of extensive alteration zones with high concentrations of PGE- and semi-metal (Te, Sb, Se, Bi and Ge)-bearing platinum-group minerals that are typical of many low-temperature PGE deposits suggests syn- to post-magmatic crystallization or redistribution of PGE by hydrothermal fluids. The results obtained to date in a new study suggest that the Platreef at Sandsloot is a complex PGE deposit that has been subject to a number of different processes during its development.