Massive sulfide (4.6 cm across) - a piece of Pt/Pd-rich massive sulfide from the Johns-Manville Reef, Lower Banded Series, Stillwater Complex (Neoarchean, 2.71 b.y.) in the Stillwater Mine, Beartooth Mountains, Montana, USA.

Platinum- and palladium-bearing pyrrhotite & chalcopyrite in the Stillwater Complex usually occur as intercumulate fills between crystals of plagioclase or pyroxene or olivine/serpentine. Occasionally, these sulfide minerals occur in a massive state. This is a fragment of massive sulfide from the Stillwater Complex’s J-M Reef. The yellowish-gold colored material is Pt/Pd-rich chalcopyrite, and the brownish-gold colored material is Pt/Pd-rich pyrrhotite. There are other minerals present, including bornite (Cu5FeS4) (on the back of the specimen), and small patches of some silvery-colored mineral (what?). Several rare sulfide and element and element-alloy minerals have been reported from the Stillwater, including hollingworthite ((Rh,Pt,Pd)AsS), gold (Au), tetraferroplatinum (PtFe), palladobismutharsenide (Pd2(Bi,As)), braggite ((Pt,Pd,Ni)S), keithconnite (Pd3-xTe), moncheite (Pt(Te,Bi)2), vysotskite ((Pd,Ni)S), etc.

Locality: 46W500 stope (4600’ elevation above sea level & 500’ west of shaft), Stillwater Mine, underground & west of the Stillwater River, southwestern Stillwater Coutny, Beartooth Mountains, southern Montana, USA.

Southern Montana’s Beartooth Mountains has one of only three platinum mines in North America. There, platinum and palladium are mined from the 2.71 billion-year-old Stillwater Complex, a classic example of an LLI (large, layered igneous province). LLIs are large intrusive bodies that display large-scale and small-scale layering, even including cross bedding, ripples, graded bedding, channelforms, and other sedimentary-like features. The Stillwater started out as a large subsurface mass of slowly cooling magma. As various minerals crystallized, they settled to the bottom of the magma chamber. This resulted in layering. Igneous rocks that formed this way have a cumulate texture. Currents in the still-liquid portions of the magma chamber produced the sedimentary structures mentioned above. Most of the Stillwater displays only large-scale layering.

The rocks in the Stillwater are ultramafic & mafic intrusive igneous rocks. Common lithologies include gabbros, norites, harzburgites, anorthosites, troctolites, chromitites, pyroxenites, and dunites. Portions of the Stillwater have been metamorphosed. Olivine is the most commonly altered component, usually metamorphosed to serpentine.