REPLACEMENT OF MAGMATIC SULFIDES BY HYDROUS SILICATES AND CARBONATES IN THE BUSHVELD AND OTHER LAYERED INTRUSIONS: EVIDENCE FOR SULFUR AND METAL REDISTRIBUTION

Models for the genesis of reef-type PGE-bearing sulfide mineralization in large layered intrusions such as the Bushveld, the Stillwater and the Great Dyke range from segregation of immiscible sulfide liquid from magma to a reaction front of interstitial melt or fluid infiltrating upward through a crystal pile. All of these models are essentially magmatic (from orthomagmatic to hydromagmatic). Whole rock concentrations of S, Ni, Cu, Au and PGE have been selectively used to support these models, but the effects of secondary alteration on the current rock compositions are rarely considered. Detailed observations using back-scattered electron imaging from this study reveal that the primary sulfide aggregates (pyrrhotite + pentlandite + chalcopyrite +/- pyrite) in the PGE-bearing reefs in the layered intrusions are commonly replaced by actinolite, epidote and calcite or ankerite. Platinum-group minerals such as Ru, Pt, and Pd sulfides, Pt and Pd tellurides, Pd arsenides and Fe-Pt alloys occur within the base metal sulfides and in their replacement aureoles, indicating that the PGM are less soluble in the fluids responsible for alteration than base metal sulfides. Liberation of S, Ni and Cu, and local precipitation of chalcopyrite during the replacement are clearly indicated. Decoupling of Pt and Pd, at least on cm scales, may have also taken placed during the replacement. Thus, the effects of variable degrees of replacement on the compositions of S, Cu, Ni and PGE in individual samples must be taken into account when using present whole rock analyses to model primary processes for the elemental distributions observed in many reef-type PGE deposits in layered intrusions.