Chromitite (field of view ~5.3 cm across) from the Stillwater Complex (Neoarchean, 2.71 b.y.) in the Beartooth Mountains, Montana, USA.

Chromitites are intrusive igneous rocks dominated by the mineral chromite (FeCr2O4 - iron chromium oxide). The mineral 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. The chromitite shown here is not from the Johns-Manville Reef that hosts platinum-palladium mineralization.

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 & 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.