THE OCCURRENCE OF Pb Cl (OH) AND Pt Sn S COMPOUNDS IN THE MERENSKY REEF, BUSHVELD LAYERED COMPLEX, SOUTH AFRICA

Abstract

An unusual phase rich in Cl (78.4 wt. % Pb, 19.2 % Cl) and close in composition to penfieldite [Pb2Cl3(OH)] was found as a ~5 m inclusion in chalcopyrite, in a spatial association with platinum-group minerals, in a sulfide-poor (≤5 vol. % of base-metal sulfides) enstatite orthocumulate of the Merensky Reef, Bushveld layered complex, South Africa. This seems to be the first reported occurrence of a Pb–Cl–(OH) compound in mafic-ultramafic rocks. The associated platinum-group minerals are members of the braggite series, cooperite (which forms large intergrowths with braggite: up to 0.5 mm in the longest dimension), members of the rustenburgite–atokite and merenskyite–moncheite series, zoned laurite, and an unknown stannosulfide of Pt, the likely chemical formula of which is PtSnS. The stannosulfide probably formed at a hydrothermal stage from microvolumes of a late-stage fluid or liquid. The Cl-rich phase precipitated from a late-stage solution rich in Cl, or formed as a result of replacement of a precursor mineral (probably galena) by an aqueous hydrochloric solution at a very low temperature, at the final stage of hydrothermal alteration.