Abstract

Platinum-group minerals are present as inclusions in both disseminated and massive chromite but are modally more abundant in massive chromite, in the ultramafic zone of the Stillwater Complex. They are small, generally less than 20 mu in diameter and characterized by euhedral and subhedral habits. Their distribution is random (not crystallographically controlled) within the chromite host. The platinum-group mineral inclusions may occur with silicate and base metal sulfide inclusions in an unfractured chromite host. All identified inclusions in unfractured chromite are laurite, (Ru, Os, Ir)S 2 . The Ru/(Ru + Os + Ir)(= Ruranges from 0.92 to 0.83 with Ru <--> Os substitution more extensive than Ru <--> Ir. Both Pd and Rh abundances in laurite are less than 2 wt percent. No Pt was detected. At present, the chemical data do not indicate a correlation between stratigraphic height and the Ru ratio.Several Pt-, Pd-, and Rh-bearing interstitial minerals and a single unusual inclusion phase have been identified in the A chromite seam at West Fork and the B seam at Mountain View. The interstitial platinum-group minerals are sperrylite and isoferroplatinum, whereas the unusual inclusion is a polyphase intergrowth of Pt-Pd-Ni arsenides with chalcopyrite and pent-landite.Whole-rock platinum-group element analyses of the chromite seams range from a few ppb to nearly 16 ppm with Pd > Pt > Rh > Ru > Ir (Os was not analyzed). This is in opposition to the data for the inclusions which show high Ru, Ir, and Os, low Rh and Pd, and no detectable Pt.Thus, in the Stillwater magma chamber, Ru-, Ir-, and Os-bearing minerals appear to have precipitated early and have been included in chromite. In contrast, Pd, Pt, and Rh apparently remained in solution in the magma and are in phases that precipitated late and are interstitial to chromite.

First Page Preview

First page PDF preview
You do not currently have access to this article.