Abstracts

Abstract

The processes involved in the petrogenesis of the sulphidehosted platinum-group-element (PGE) deposits of the Stillwater Complex are still controversial, with theories ranging from orthomagmatic to hydromagmatic. To further constrain these models, we have been examining the trace phase assemblages in rocks away from the ore zones.High-temperature carbonates have been observed in association with sulphides below the platiniferous J-M Reef of the Stillwater Complex. The carbonate assemblage consists of dolomite with exsolved calcite and is found in contact with sulphide minerals: chalcopyrite and pyrrhotite in the Peridotite Zone; and pyrrhotite with pentlandite, pyrite and chalcopyrite in Gabbronorite I of the Lower Banded Series.The minimal silicate alteration and the lack of greenschist minerals in association with the mineral assemblage are consistent with a high-temperature origin for the carbonates. The calcite-dolomite geothermometer (Anovitz and Essene, 1987) yields a minimum formation temperature of 920uC for the unmixed assemblage. Carbonate Fe and Mn contents range from 0?40 to 2?25 wt-%FeO and 0?07 to 0?32 wt-%MnO and are consistent with literature data supporting a minimum formation temperature of y900uC.The carbonate assemblages are consistent with the fluid inclusion studies of Hanley et al. (2008), who showed that fluids comprising coexisting Cl-rich brine and carbonic fluid were trapped in pegmatitic quartz at 700-715uC