Mt. Emmons-Redwell Basin

Region West, Rocky Mountains
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area The focus area encompasses the Mt. Emmons and Redwell Basin Mo porphyry resources. These deposits constitute a Climax-type porphyry cluster. Since Climax-type porphyry systems commonly occur in clusters, a 15 km buffer was applied to these deposits to encompass undiscovered occurrences in adjacent areas.
Identified resources Identified resources of molybdenum. Historical production of lead, silver, and zinc.
Production Unknown.
Status Freeport-McMoRan currently owns the property and is performing environmental clean-up activities since 2016 as of 2019.
Estimated resources Reserves at Mt. Emmons are 155 million tons having an average grade of 0.44% MoS2, using a 0.2% MoS2, cutoff (Ganster and others, 1981; as referenced in Dowsett and others, 1981). 344,000 tons Mo as reported by Ludington and Plumlee (2009). No grade and tonnage figures for Redwell Basin stockwork Mo deposit have been announced, but the deposit is of comparable size to the Mt. Emmons deposit, though of lower grade.
Geologic maps Gaskill and Godwin (1966), scale 1:24,000; Gaskill and others (1967), scale 1:24,000; Gaskill and others (1987), scale 1:24,000; Gaskill and others (1991), scale 1:24:000; Gaskill and others (1986), scale 1:24,000.
Geophysical data Inadequate aeromagnetic coverage, partly (~50%) adequate aeroradiometric coverage.
Favorable rocks and structures At Mt, Emmons an inverted-cup-shaped zone of molybdenite ore is draped over the top of a multi-phase intrusive plug emanating from a stock of rhyolite-granite porphyry beneath Mt. Emmons. The molybdenite ore extends well out into altered and metamorphosed Cretaceous clastic wall rocks. The molybdenite zone is the only known economic resource in the deposit. At its shallowest point the top of the molybdenite zone is 885 ft (270 m) below the surface of the Red Lady Basin. The ore zone is between 245 ft (75 m) and 395 ft (120 m) thick with a cross-sectional dimension averaging 2,100 ft (650 m). Ore consists of stockwork veinlets of molybdenite; fine-grained quartz and fluorite are developed in both the Mount Emmons intrusive plug and metamorphosed sedimentary rocks adjacent to the intrusive. Host rocks include Late Tertiary rhyolite-granite porphyry and contact metamorphic rocks in the Cretaceous Mancos and Mesverde Formations (Streufert and others, 1999).
Deposits Mt. Emmons/Crested Butte (MRDS dep_id: 10215357); Redwell basin; Ruby mining district (Standard, Painter Boy mines); Elk Mountain mining district (to the north).
Evidence from mineral occurrences MRDS; known drilled deposit (AMAX Exploration discovered in the mid-1970s).
Geochemical evidence Tungsten and Zn haloes form in potassic to phyllic alteration haloes around granitic intrusions (concentrations up to ~ 100 ppm W and 9,000 ppm Zn). Hübnerite occurs in vein assemblages associated with quartz-fluorite mineralization (Thomas and Galey, 1982).
Geophysical evidence Vein-hosted magnetite mineralization is associated with a low-amplitude magnetic anomaly in the region. Gravity survey conducted during exploration in the 1970s were inconclusive (Thomas and Galey, 1982).
Evidence from other sources Known drilled deposit. State-wide trend of similar occurrences.
Comments Mineralization is associated with irregular roof of a silicic intrusive complex beneath Redwell and Red Lady basins. Complex is inferred to dip to the north. Historical production associated with nearby base/precious metal deposits (other mines in area have produced Pb, Zn, and Ag). Potential for W, Sn, REE but no production of those. These deposit types tend to form in clusters, so the 15 km buffer around known deposits should encompass undiscovered occurrences. Molybdenum is the principal commodity, with Sn, W, Re, REE, and Be as potential byproducts. Nearby skarn and polymetallic replacement deposits contain Au, Ag, Pb, and Zn, with Cu, Bi, Ga, Ge, As, Sb are potential byproducts. The focus area includes the Ruby mining district that includes the Standard and Painter Boy Mines (unknown production), also minor production between 1880 and 1890 (Streufert and others, 1999).
Cover thickness and description Sedimentary cover ranges from ~300 to 1000 m (Thomas and Galey, 1982).
Authors Ryan D. Taylor, Albert H. Hofstra, Joshua M. Rosera.
New data needs 1:24,000 scale geological mapping, more geochemistry data on REE, PGE, Nb-Ta, W, and Sn, and geophysics.
Geologic mapping and modeling needs Need 1:24,000 scale geological mapping on the Anthracite Range quadrangle.
Geophysical survey and modeling needs High resolution Rank 1 aeromagnetic and aeroradiometric surveys.
Digital elevation data needs Lidar adequate.