Petrographic and geochemical characteristics of postmagmatic hydrothermal alteration and mineralization in the J-M Reef, Stillwater Complex, Montana

The mineralogy, composition and texture of magmatic silicates, alteration minerals, base-metal sulfides (BMS) and platinum-group minerals (PGM) were determined for seven borehole samples from the J-M Reef and its footwall in the Stillwater Complex. Montana. We used optical microscopy. scanning electron microscopy and electron-microprobe analysis. The BMS identified are pyrrhotite. chalcopyrite, pentlandite and minor pyrite. Thirty-six grains of PGM were found in four of six polished thin sections. The samples record evidence of a hydrothermal event or events that have locally altered magmatic silicates, recrystallized BMS aggregates and remobilized sulfides and platinum-group elements (PGE). The alteration assemblage consists dominantly of chlorite, clinozoisite, serpentine, calcite, talc, white mica, and magnetite, with traces of tremolite, Cl-rich ferropargasite and quartz. The intensity of alteration ranges from 90% altered in the footwall sample to 10% altered in the least-altered reef sample. In the weakly altered samples of the reef, BMS occur in aggregates interstitial to essentially unaltered plagioclase and are probably of primary origin. Some of these aggregates contain inclusions of PGM from 80 to 130 mum across at the BMS-silicate contact. In strongly altered samples, the BMS are strongly recrystallized and intergrown with hydrous minerals in aggregates; they also replace primary silicates and appear in veinlets up to 2 mm wide. The PGM in this association occur with or without BMS as fine grains (1-30 mum) intergrown with alteration minerals in replacements of silicates and in veinlets. The intergrowth of secondary sulfides with chlorite, clinozoisite, and Cl-rich ferropargasite indicates an alteration assemblage that probably formed between 230 to over 350degreesC. Hydrothermal alteration and mineralization are associated with thin dilational fractures that provided channelways for hydrothermal fluids. As the assay data for the four borehole cores indicate, there is an inverse relationship between intensity of hydrothermal alteration and PGE grade that Could explain some of the variability in the grade and thickness of the J-M Reef.