A STUDY OF THE SULFIDE ASSEMBLAGES BELOW THE J-M REEF, STILLWATER COMPLEX, MONTANA

Although layered intrusions such as the Stillwater Complex, Montana, USA, and the Bushveld Complex, South Africa, contain some of the richest platinum-group element (PGE) deposits in the world, the processes involved in PGE-enrichment are still debated. As the PGE are largely associated with base-metal sulfides, we are characterizing the sulfide assemblages in the "sulfur-poor" region below the J-M reef of the Stillwater Complex in an attempt to elucidate their formation mechanisms.

Fresh sulfide assemblages in Gabbronorite Zone I (GN-I) generally comprise large (~200μm), multiphase sulfides composed of pyrrhotite, chalcopyrite, pentlandite and occasional pyrite, sometimes hosting primary magnetite, quartz and calcite. When hosted by orthopyroxene, sulfides are often rimmed by Ca-rich pyroxene. Some sulfides are found in intimate association with carbonates comprising calcite exsolved from dolomite.

Fresh assemblages in the Bronzitite (BZ) and Peridotite (PZ) Zones range from small (<50μm) multiphase blebs of pentlandite, pyrrhotite and magnetite to larger multiphase sulfides comprising pentlandite, chalcopyrite, pyrite and pyrrhotite, with minor magnetite and calcite. Sulfides in the BZ are hosted by orthopyroxene with a Ca-rich pyroxene rim and with occasional proximal biotite. In the PZ sulfides are commonly hosted by olivine, sometimes in association with chromite.

Localized, heavily altered assemblages primarily include greenschist minerals such as clinozoisite, chlorite and disseminated calcite and quartz. These sulfides have not been included in this study.

Preliminary trends in sulfide data show that the total sulfide content below the reef is usually less than 0.005%, but increases in GN-I to 0.014%. In general, modal pentlandite decreases from 72% of the total sulfides in the PZ to <40% in the middle BZ, but increases again to 75% in upper GN-I. With the exception of one sample, chalcopyrite and pyrrhotite are both sparse (<10%) through the ultramafic series, but increase in upper GN-I to 13% and 55% respectively. Likewise, pyrite is uncommon in the ultramafic series (<8%) but increases upwards, reaching 100% in one GN-I sample.

The preliminary results of this study show that sulfides are not present in cotectic proportions, and thus support the ore element remobilization from the footwall.