The effect of the Vaalkop replacement pegmatoid on the sulphide mineralogy at Western Platinum mine in the Mooinooi district

Abstract

Western Platinum mine is situated 30 km due east of Rustenburg in the Western Bushveld Complex. Replacement pegmatoid areas occur sporadically at the mine and range in diameter from a few metres to approximately 250 m. The effect of the replacement pegmatoid was investigated in six profiles through the chromitite. The replacement pegmatoid footwall consists predominantly of amphibole with minor pyroxene, talc, clay-minerals, serpentine, olivine, and magnetite. A characteristic feature of the alteration in the UG-2 chromitite by replacement pegmatoid is the sintering of the bottom 5-10 cm. The sintered area consists of magnetite and Fe-Ti-oxides. Footwalls of “unaltered" UG-2 chromitite are anorthosite or pyroxenite. Pentlandite is the major sulphide in altered UG-2 chromitite. Chalcopyrite, chalcocite-group minerals, bomite, violarite, mackinawite, haezlewoodite, cubanite and pyrrhotite are minor constituents in altered UG-2. Trace amounts of sphalerite and millerite occur. The base metals sulphides in a reference profile of unaltered UG-2 consist primarily of pentlandite with minor chalcopyrite, pyrrhotite, and pyrite. The platinum group mineral assemblage of the altered UG-2 chromitite contains a considerable amount (36%-55%) of alloys, Bi-containing, As-containing and Te-containing platinum group minerals. The amount of arsenides increases with increasing alteration and is 2% (reference profile), 28% (average of altered UG-2 profiles 1, 3, 5, and 2) and 64% for the most intensely altered UG-2 chromitite (profile 8) respectively. The proportion of platinum group sulphides decreases from 96% to 18% with increasing alteration. Platinum group element association with sulphides decreases gradually from 87% for the reference profile to 20% for the most intensely altered UG-2 chromitite. The sulphide mineral assemblages, platinum group element assemblages, and the presence of hydrous silicates in the profiles of altered UG-2 chromitite is an indication of the presence of a late-stage hydrothermal fluid modifying a primary magmatic assemblage. This is further supported by the sintering of the oxides, the presence of Co-rich pentlandite in the sintered area, and the textural and chemical overprinting of the chromites.