English: Chromitite from the Precambrian of Montana, USA.

Southern Montana’s Beartooth Mountains have one of the few platinum mines in all of North America. There, platinum and palladium ores 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. Some Stillwater lithologies have been changed by metamorphism. Olivine is the most commonly altered component - it's usually been metamorphosed to serpentine.

The main platinum and palladium 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 Stillwater rock seen here is a chromitite, an intrusive igneous rock dominated by the mineral chromite (FeCr2O4 - iron chromium oxide). Chromite has a metallic luster, a blackish color, and is moderately heavy for its size. Chromitite rocks are typically masses of granular chromite. The Stillwater Complex has economic concentrations of chromitite, but Cr mining was only done in the mid-20th century. Today, the chromitites are not the targets of active mining. This chromitite is not from the Johns-Manville Reef that hosts platinum-palladium mineralization.

Locality: unrecorded/undisclosed site in the Beartooth Mountains, southern Montana, USA