Magnetitic serpentinite from the Precambrian of Montana, USA. (~5.9 cm across at its widest)

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.

This serpentinite sample is from one of the Red Lodge District's metamorphosed ultramafic bodies. Serpentinite is a metamorphic rock that forms by alteration of peridotites in the presence of water. The dominant mineral is serpentine (Mg3Si2O5(OH)4 - magnesium hydroxy-silicate). Some samples may be relatively rich in magnetite (Fe3O4 - iron oxide), as in this sample. A magnet will stick to it.

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