Stillwater Complex

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

Information leading to the delineation of this focus area

Basis for focus area LIP-related magmatic mineral deposits associated with ultramafic to mafic layered intrusions, such as Reef-type PGE and contact-type Cu-Ni-PGE.
Identified resources Historical production of chromium, nickel, PGE (palladium, platinum).
Production J-M Reef (1986 to 2018): 592,608 oz Pd+Pt (
Status Past and active mining on the J-M Reef at the East Boulder and Stillwater Mines. An expansion project is underway at the far eastern end of the complex.
Estimated resources Stillwater (in 2020): Measured 12.4 Mt @15.1 g/t PGM; Indicated 17.1 Mt @ 14.6 g/t PGM; Inferred 96.2 Mt @ 15.5 g/t PGM (
Geologic maps Berg and others (1999), scale 1:100,000; Berg and others (2000), scale 1:100,000; Lopez (2000, 2001), scale 1:48,000; Geraghty (2013), scale 1:48,000; Segerstrom and Carlson (1982), scale 1:24,000; Page and Nokleberg (1974), scale 1:12,000.
Geophysical data Rank 1 aeromagnetic coverage (Parks and others, 2019); also see Finn and others (2020) for geophysical modeling.
Favorable rocks and structures 5.5 km of layered ultramafic and mafic cumulates; cyclically layered in parts of the section; evidence for open magmatic system-many injections of magma of varying composition; lateral continuity of units up to the exposed length of the complex-~40 km. Basal contact exposed with concentrations of Fe-Ni-Cu sulfide minerals.
Deposits Reef-type: J-M Reef (East Boulder and Stillwater mines) and A/B chromitites in the West Fork area; Contact-type deposits: Chrome Lake, Benbow, Nye Basin, Mouat, Iron Mountain.
Evidence from mineral occurrences See information summarized in Zientek (1993); Zientek and others (2002); Zientek and Parks (2014).
Geochemical evidence Pt and Pd soil geochemistry effective in locating lode occurrences of PGE (Zientek, Corson and West, 2005).
Geophysical evidence Magnetic data do not find magmatic sulfides in reef-type deposits but can be used to delineate the rock packages that host mineralization. No airborne EM anomalies for reef-type deposits. Contact-type deposits characterized by mag and EM anomalies.
Evidence from other sources Unknown.
Comments Ongoing exploration of deposits in the region by GroupTen Metals has produced encouraging results.
Cover thickness and description Stillwater Complex is exposed on northern flank of Beartooth Mountains. Some parts well exposed; some have alluvial, talus/scree, and glacial cover. Structural cover (back thrust) more of an issue.
Authors Michael L. Zientek, Jay A. Gunderson, Kaleb C. Scarberry.
New data needs Geophysical surveys that can be used to see through structural cover. Very high resolution airborne surveys where magmatic unconformities are suspected. Compilation and integration of data from lidar, airborne geophysics, outcrop maps, and underground workings.
Geologic mapping and modeling needs Geologic mapping adequate.
Geophysical survey and modeling needs High resolution geophysical coverage adequate.
Digital elevation data needs Lidar inadequate.