Petrogenetic Study Of A Platiniferous Zone Of The Stillwater Complex, Montana

Abstract

Intrinsic oxygen fugacity values (fO2) were measured for the Pt-rich zone (described by Conn, 1979), and its enclosing units, of the Stillwater Complex, Montana, to approximate the early redox conditions under which the ore zone had formed. The oxidized values of fO2 measured for the Pt-rich horizon (10^-10.5 at 1100 C) are thought to have been the result of the autometamorphic breakdown of olivine to serpentine. The fO2 values measured for the plagioclase cumulate footwall units are between WI and WM buffers, and those values also agreed with published value for the Bushveld Complex, and the Skaergaard Complex. Graphite is found in both the Stillwater Pt-rich zone and the platiniferous Merensky Reef of the Bushveld Complex. The very reduced values measured for the Basal Chromite of the Merensky Reef (10^-16 at 1100 C) suggests graphite buffering conditions. Graphite in the Pt-rich zone is associated with sepentine minerals, but various lines of evidence indicate that graphite is not an equilibrium assemblage with serpentine. It is concluded that serpentinization, and other metamorphic reactions, oxidized both the silicate minerals and the iron-platinum alloys of the Stillwater Pt zone. The fO2 values measured for the Stillwater Pt-rich zone do not represent the actual redox conditions under which it was formed.