Abstract

Approximately 80 percent of the platinum-group mineral grains in the Precambrian Witwatersrand conglomerate reefs are (Ir, Os, Ru) alloys. Osmium, iridosmine, osmiridium, and iridium are most abundant; rutheniridosmine, ruthenosmiridium, and ruthenian iridium are less abundant. Zoned grains are common, with Os-rich cores enclosed by (Ir, Pt)-rich outer zones filled with exsolution lamellae of (Pt, Fe) alloy. Discrete grains of this alloy proved to be isoferroplatinum of a composition close to Pt 3 Fe. In addition, grains ranging in composition between Pt 3 Fe and platiniridium are present.Sperrylite commonly occurs as an outer coating on all the above grain types. Sperrylite and isoferroplatinum make up 15 to 20 percent of the platinum-group mineral grains and account for nearly all the Pt present. Hollingworthite, iridarsenite, and ruthenarsenite occur in variable quantities (1-5%) as coatings on alloy grains, which in turn may be coated with sperrylite.Euhedral and subhedral inclusions in the common platinum-group mineral grains are michenerite, moncheite, laurite, geversite, and two unnamed minerals: rhodium sulfide and (Pt, Rh, Ru) alloy. Free grains of geversite, moncheite, sudburyite, and stibiopalladinite are very rare. The last two may be intergrown.Occasionally, partial or complete alteration of (Ir, Os, Ru) alloys has resulted in mixtures of erlichmanite, irarsite, osarsite, and phases that could not be identified.The evidence suggests that the (Ir, Os, Ru) and (platinum-group elements, Fe) alloys and their inclusions underwent little or no alteration during weathering, transportation, and burial, or even during later metamorphism of the reefs when sperrylite, holling-worthite, and related phases probably formed from less stable platinum-group minerals, possibly tellurides and antimonides.

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