English: 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-years-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 originated 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 mafic and ultramafic, intrusive igneous rocks. Common lithologies include gabbro, norite, harzburgite, anorthosite, troctolite, chromitite, pyroxenite, and dunite. Portions of the Stillwater have been metamorphosed. Olivine is the most commonly altered component - it's usually been metamorphosed to serpentine.

The main platinum (Pt) and palladium (Pd) occurrence is in the Johns-Manville Reef (J-M Reef), an interval in the lower part of the Lower Banded Series. The 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).

The outcrop seen here does not have platinum ore. Rocks in this area mostly consist of inch-scale layered plagioclase feldspar and pyroxene. The layering was originally horizontal; Stillwater Complex rocks have been structurally tilted by mountain building.

Stratigraphy: Stillwater Complex, Neoarchean, 2.71 Ga

Locality: roadcut along West Fork Stillwater Road (= Forest Service Road 2846), near & above the Stillwater Mine, Beartooth Mountains, southern Montana, USA