Abstract

The different modes of occurrence, compositional variations, and possible paragenetic positions of the platinum-group minerals (PGM) in samples of Merensky Reef pyroxenite from exposures of unoxidized and undisturbed normal reef facies in the central section of the Rustenburg mine are described and discussed. The study indicates that the main platinum-group minerals are braggite (38 vol %), cooperite (17 vol %), moncheite (13 vol %), laurite (8 vol %), kotulskite (8 vol %), merenskyite (4 vol %), Pt-Fe alloy (3 vol %), palladian electrum (1 vol %), and sperrylite (1 vol %). The remainder (less than 1 vol %) include atokite, paolovite, rustenburgite, mertieite II, a (PdHg)(BiTe) phase, and a palladium arsenide. Electron microprobe analyses of these platinum-group minerals are tabulated and graphically presented to display compositional ranges and substitutional trends. The reflected light properties of cooperite and braggite, about which a confusion still exists in the literature, are described, and reflectance and hardness data are presented. The platinum-group minerals are an intimate part of the crystallization history of the base metal sulfides, and their paragenetic sequence, as determined from a textural interpretation, conforms to the known geochemical character of the platinum-group elements, especially with regard to the fractionation of palladium and platinum. Thus, the earliest Pt-Pd minerals to crystallize from the sulfide liquid are platinum rich, with the replacement of cooperite by braggite, and moncheite and merenskyite by kotulskite as evidence of a later palladium metasomatism. Subsolidus diffusion of siderophilic platinum in the monosulfide solid solution during decomposition is favored for the formation of myrmekitic Pt-Fe alloy in pyrrhotite, with the relatively more chalcophilic palladium initially concentrating in intermediate solid solution and finally forming tellurides and stannides in chalcopyrite. Some palladium would be retained in solid solution in pentlandite. Fractionation of platinum earlier than palladium during chalcopyrite genesis resulted in the overgrowth and replacement of early formed platinum-rich tellurides and stannides from intermediate solid solution by palladium-rich ones.

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