Belt Basin

Region West, Northwest
Mineral systems
Deposit types
Other minerals

Information leading to the delineation of this focus area

Basis for focus area The focus area outlines Proterozoic sedimentary rocks deposited in the Belt-Yellowjacket intracratonic and Cu-Co occurrences; includes the Ravalli Group.
Identified resources Historical production of copper, gold, lead, and silver.
Production Troy mine (1981-1993): produced a total of 44.4 million oz Ag and 180,000 tons Cu; Snowstorm mine (1904-1943): produced 826,580 ore with 4,123,186 oz Ag, 55,170,447 lbs Cu, 3,283 oz Au; grades were 1.99 oz/ton Au and 3.34% Cu (Mitchell and Bennett, 1983).
Status Past mining; major mines are closed and reclaimed. Some mines in development.
Estimated resources Rock Creek (12/31/2020): Inferred resources 100 Mt of ore @ 0.7% Cu and 1.5 oz/t Ag; Montanore (12/31/2020): Inferred resources 112,185,000 t of ore @ 0.7% Cu and 1.6 opt Ag; Troy (closed 2015 and reclaimed; 1/31/2015): Reserves and resources of 70,107, 236 t of ore @ 0.543% Cu, 42 g/t Ag; J-F resources: 630 t Ag and 56,000 t Cu; Eagle Creek resources: 240 t Ag and 70,500 t Cu; Snowstorm resources: 138 t Ag and 26,000 t Cu.
Geologic maps Harrison and others (1992), scale 1:250,000; Lewis and others (1999), scale 1:100,000; Lonn and McFaddan (1999), scale 1:100,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Proterozoic Belt Supergroup rocks in northern Idaho and western Montana (including highly metamorphosed areas marginal to the Idaho batholith) as well as rocks mapped as Yellowjacket Formation and Lemhi Group in Idaho and Spokane Formation in Washington. Revett Formation in Montana.
Deposits Troy (Spar Lake) (MRDS dep_id: 10124835), J-F, Rock Creek, Rock Lake (Montanore), Eagle Creek, Snowstorm.
Evidence from mineral occurrences MRDS.
Geochemical evidence Tourmalinization, the alteration type most closely related to mineralization at Sullivan is identified in at least 30 locations in Proterozoic sedimentary rocks in the United States (Slack, 1993).
Geophysical evidence Unknown.
Evidence from other sources Unknown.
Comments Zinc-Pb mineralization reported near the Cu-Co deposit at Sheep Creek (Black Butte), Montana and in the Highland Mountains (Soap Gulch prospect), Montana. Areas favorable for Sullivan-type Pb-Zn mineralization were identified in both the Wallace (the Perma-Plains area) and Dillon (Highland Mountains) 1° x 2° mineral resource assessments (Pearson and others, 1990; Harrison, Cressman and others, 1986).
Cover thickness and description Exposed at the surface.
Authors Jane M. Hammarstrom.
New data needs Consider geochemistry on tailings for critical minerals. Geologic mapping with an emphasis on structural control or ore fluids.
Geologic mapping and modeling needs For Washington <25% mapped at a scale of 1:24,000; remaining mapped at intermediate scales. Need higher resolution mapping for much of the focus area.
Geophysical survey and modeling needs High resolution, Rank 1 aeromagnetic and aeroradiometric coverage.
Digital elevation data needs Need lidar for about 30% of the focus area; rest is covered or planned. For Washington ~70% coverage with QL3 lidar. Additional collections to cover data gaps recommended.