Tobacco Root chromite

Region West, Northwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Focus area is based on a permissive area for chromite defined in the Butte CUSMAP study (Elliott and others, 1992); area includes 2 historical chromite mines.
Identified resources Historical production of chromium (chromite).
Production 1940-1944: 200 tons of chromite ore produced daily.
Status Past mining; no current mining or exploration.
Estimated resources Unknown.
Geologic maps McDonald and others (2012), scale 1:100,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Alpine-type" chromite deposits in Precambrian serpentinized ultramafic bodies in hornblende gneiss. Chromite forms irregular pods and lenses in sill-like and/or faulted bodies of serpentinite.
Deposits Silver Star chromite (MRDS dep_id: 10148673), Mohawk mine (MRDS dep_id: 10108480).
Evidence from mineral occurrences MRDS.
Geochemical evidence Chromite from the Silver Star mine had ore samples that ranged from 37.6 to 44.8% Cr2O3 (James, 1943).
Geophysical evidence Unknown.
Evidence from other sources Chromite boulders in the area were a guide for prospecting.
Comments Small chrome mine operated during WWII, employing about 20 men who mined 200 tons of chromite ore daily. Ore was processed in the Iron Rod mill on Highway 41 near Silver Star, Montana. The operation ran from 1940 until 1944 since the government classed chrome as vital to the war effort (Trauerman and Reyner, 1950). Known deposits are small. In today’s global economy, Montana chromite deposits such as these cannot compete with large reserves of high-purity chromite from overseas mining operations (Gammons and others, 2020).
Cover thickness and description Exposed at the surface.
Authors Jane M. Hammarstrom, Jay A. Gunderson.
New data needs High resolution geophysics, geologic mapping, lidar.
Geologic mapping and modeling needs 1:12,000 scale mapping of known plutons and related structures and surface sampling of plutons for trace element analysis. A geophysical survey to determine if plutons continue at depth.
Geophysical survey and modeling needs A geophysical survey consisting of aeromagnetics and gravity to determine if plutons continue at depth and help identify covered plutons.
Digital elevation data needs Lidar inadequate.