PETROGRAPHIC OBSERVATIONS AND IMPLICATIONS FOR THE PICKET PIN PGE DEPOSIT, STILLWATER COMPLEX, MONTANA

The processes of transport and deposition of platinum-group-element (PGE) deposits remain enigmatic although numerous models have been proposed. The Stillwater Complex, located in south-central Montana, contains reef-type, PGE-enriched, sulfide intervals including the JM Reef and the Picket Pin deposit. The Picket Pin deposit is located approximately 3000 m above the JM Reef within the 570 m thick anorthosite (AN II) of the Middle Banded Series. The PGE-enriched sulfide-bearing intervals occur from approximately 10 m below up to the upper contact of the anorthosite. Samples were taken on a 2 m grid pattern with a dimension of 10 X 20 m across five sulfide intervals. The samples were analyzed using the following techniques: petrographic microscope, SEM, EDS, electron microprobe, and whole-rock geochemistry.

Hydrothermal alteration envelops the PGE-bearing sulfide intervals. Patches and stringers of clinozoisite-epidote-quartz lead up section to the PGE-enriched sulfide intervals. Assay values of these clinozoisite patches away from obvious sulfide mineralization are up to 600 ppb combined PGE, whereas the surrounding rocks are barren and below the detection limit for PGE (5 ppb). The alteration mineralogy is zoned from epidote-magnetite-albite +/- quartz away from the sulfides to clinozoisite-quartz-albite in contact with the sulfides. The sulfides are intergrown by euhedral clinozoisite, albite, and quartz, which suggest the sulfides and alteration mineralogy may be contemporaneous. If contemporaneous, then a temperature range of 500 to 550° C can be interpreted using the quartz-clinozoisite system. The sulfide grains (pyrrhotite, pentlandite, braggite, and chalcopyrite) completely replace the intercumulate augite grains and in some areas relict pyroxene lamellae can be seen. Using both SEM and EDS, it is apparent that the PGE’s are exclusive to the outer boundaries of the sulfide grains. This may suggest that the PGE’s and sulfides precipitate from separate phases. Petrographic observations strongly suggest a hypersaline hydrothermal fluid at an approximate temperature of 500-550° C was responsible for the PGE-enriched sulfide intervals of the Picket Pin deposit.