File:Sulfidic serpentintite (platinum-palladium ore) (Johns-Manville Reef, Stillwater Complex, Neoarchean, 2.71 Ga; Stillwater Mine, Beartooth Mountains, Montana, USA) 12.jpg

English: 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 occurs in intercumulate sulfides, typically pale brassy-colored pyrrhotite (Fe1-xS) and yellow brassy-colored 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 rock seen here is an altered pegmatitic dunite richly infused with intercumulate Pt/Pd-rich chalcopyrite and pyrrhotite. Dunites are 90%+ olivine peridotites, which are ultramafic, intrusive igneous rocks. During metamorphism in the presence of water, olivine converts to serpentine. The resulting rock is serpentinite. In this sample, the blackish areas are serpentine masses (formerly large olivine crystals). Some magnetite is also mixed in with the serpentine.

This ore sample grades to about 2.5 ounces of Pd-Pt per ton of rock, with a Pd-Pt ratio of about 3:1 (= highest grade platinum group metals deposit in the world).

Stratigraphy: Johns-Manville Reef, Troctolite-Anorthosite I zone, Lower Banded Series, Stillwater Complex, Neoarchean, 2.71 Ga

Locality: 55W15100 D1 area (5500’ elevation above sea level & 15,100’ west of shaft), Stillwater Mine, underground & west of the Stillwater River, southwest of the town of Nye, southwestern Stillwater County, Beartooth Moutains, southern Montana, USA