Platinum-group elements in the Boulder Bed, western Bushveld Complex, South Africa

Abstract

Concentrations of platinum-group elements in samples from the Boulder Bed at five localities in the western Bushveld Complex range between 50 ppb and 70 ppm. Boulders thus have much more variable, and sometimes highly enriched, PGE contents relative to the other lithologies in the immediate foot-wall sequence of the Merensky Reef. The PGE enrichment can largely be modelled as a result of primary magmatic processes including collection of PGE by segregating sulphide melt and fractionation of mss. Other features of the Boulder Bed, such as the selvages of pure anorthosite and the chromitite stringers surrounding some of the boulders, bear evidence of recrystallisation. A model is proposed by which the Boulder Bed formed as a result of a combination of early and late magmatic processes. The PGEs were collected by magmatic sulphide melt which accumulated in a pyroxenite layer. The host rock to the pyroxenite was a thick package of norites which recrystallised in response to upward-migrating magmatic fluids. The fluids caused partial hydration melting of the norites adjacent to the pyroxenite, producing anorthosite. The boulders represent the broken-up remnants of the pyroxenite layer. The selvages of chromite and pure anorthosite around some of the boulders remain poorly understood, but may represent the latest recrystallisation event, in response to localised late-magmatic fluid overpressure upon cooling.