Chromitite band in chromitic serpentinite from the Precambrian of Montana's Beartooth Mountains, USA. (6.5 cm across along the base)

The Red Lodge Mining District is located near the southeastern termination of the Beartooth Mountains, southwest of the town of Red Lodge in southern Montana, USA. Active mining took place during the 1940s. Most of the old, small mines occur on the high plateaus or just below the eroded plateau edges (for example, along the northwestern margin of Rock Creek Canyon).

Mines in the Red Lodge District targeted chromium ores in the form of chromitite rocks hosted in serpentinite. Lenses, or pods, or fault-chopped stratiform bodies of serpentinite occur among high-grade metamorphic rocks (quartzites, gneisses, migmatites, amphibolites, etc.).

The rocks in the Beartooth Mountains are very old. They are principally Archean in age (>2.5 billion years old). The chromite-bearing serpentinite bodies of the Red Lodge Mining District were originally chromite-bearing peridotites (dunites) of early Neoarchean age, or older. Granulite-grade to upper amphibolite-grade regional metamorphism at about 2.75 Ga altered the original chromitic dunites to serpentinites. The rocks of the area were also altered by greenschist-grade metamorphism between 1.6 to 1.8 billion years ago, during the late Paleoproterozoic.

Chromitite is a crystalline-textured, intrusive igneous rock exclusively or near exclusively composed of the mineral chromite (FeCr2O4 - iron chromium oxide). The black, granular part of the rock shown above is chromitite, which forms a band running through greenish-colored serpentinite. Serpentinite is a metamorphic rock, the result of alteration of olivine-rich peridotites (dunites) in the presence of water. The serpentinite itself contains scattered black chromite crystals.

Age: Archean (early Neoarchean metamorphism at 2.75 Ga, plus late Paleoproterozoic metamorphism at 1.6 to 1.8 Ga).