File:Bronzite in sulfidic serpentintite (platinum-palladium ore) (Johns-Manville Reef, Stillwater Complex, Neoarchean, 2.71 Ga; Stillwater Mine, Beartooth Mountains, Montana, USA) 1.jpg

English: Bronzite in sulfidic serpentinite from the Precambrian of 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 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. There, the Pt-Pd occur in intercumulate sulfides, typically pyrrhotite (Fe1-xS) (= dull brassy colored) and chalcopyrite (CuFeS2) (= yellow brassy colored). 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).

This Stillwater rock is principally a sulfidic serpentinite. The highly lustrous areas are cleavage planes of bronzite, a type of orthopyroxene.

Stratigraphy: "footwall rocks" beneath the main olivine-bearing zone associated with the Johns-Manville Reef, Troctolite-Anorthosite I zone, Lower Banded Series, Stillwater Complex, Neoarchean, 2.71 Ga

Locality: 50W141 D7 West in the Stillwater Mine (= western side of the D7 level, ~98’ below the 5000’ elevation level, 141’ west of shaft), underground and west of the Stillwater River, southwest of the town of Nye, southwestern Stillwater County, Beartooth Mountains, southern Montana, USA