English: Bronzite pyroxene from the Precambrian of Montana, USA. (~20.6 centimeters across at its widest)

Southern Montana’s Beartooth Mountains has one of the few 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 and 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.

The main platinum-palladium occurrence is in the Johns-Manville Reef (J-M Reef), an interval in the lower part of the Lower Banded Series. In the J-M Reef, Pt and Pd occur in intercumulate sulfides, typically pyrrhotite (Fe1-xS) and chalcopyrite (CuFeS2). Platinum ores in the J-M Reef are principally sulfidic anorthosites, but other lithologies also occur. The J-M Reef is the highest grade deposit known for platinum-group elements (PGEs).

Seen here is a large cleavage fragment of a single crystal of bronzite, an orthopyroxene mineral, (Mg,Fe)2(SiO3)2 - magnesium iron silicate. The original crystal was larger and left in the wall of the Stillwater Mine.

Stratigraphy: pegmatitic, non-Pd/Pt mineralized area of the Troctolite-Anorthosite I zone (very near the Johns-Manville Reef), Lower Banded Series, Stillwater Complex, Neoarchean, 2.71 Ga

Locality: 40W4800 stope of the Stillwater Mine (= 4000' elevation above sea level & 4800' west of shaft), southwest of the town of Nye, southwestern Stillwater County, Beartooth Mountains, southern Montana, USA