PETROGRAPHIC OBSERVATIONS AND IMPLICATIONS FOR THE PICKET PIN PGE DEPOSIT, STILLWATER COMPLEX, MONTANA
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 PGEs are exclusive to the outer boundaries of the sulfide grains. This may suggest that the PGEs 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.