| Basis for focus area |
The ~1.78 Ga Elkhorn Gabbro mafic intrusion south of Cheyenne Belt in Wyoming's Sierra Madres and Colorado's Park Range includes multiple pulses of gabbro (typically olivine gabbro, but also hornblende- and biotite-containing gabbro), diorite, amphibolite, peridotite, and basalt dikes (Snyder, 1980; Snyder and others, 1988), Similar reversely-zoned intrusions in Alaska and Russia are interpreted to be the source of placer PGE (Loucks and Glasscock, 1989). The Elkhorn peridotite may host chromite (Sutherland and Hausel, 2004). The Encampment district Puzzler Hill layered mafic-ultramafic massif north of the Cheyenne Belt has documented and theorized mineralization. The region in part conforms to the "Wyoming Platinum-Palladium-Nickel Province" of Hausel (2000). Metasedimentary and metavolcanic belts and associated dike and sill complexes also exist within the focus area. |
| Identified resources |
Historical production of copper, gold, and silver (in the Sierra Madres). |
| Production |
From Hausel (1986): "The amount of copper mined from the Encampment district is unknown; however, from 1899 to 1908, 24 million lbs of copper were produced in Wyoming, nearly all from the Encampment district. |
| Status |
Past exploration and mining for copper, gold, and silver throughout Encampment district. |
| Estimated resources |
From Hausel (2000); Charter Oak mine, "Ore reserves were estimated at 680,000 tons Co to a depth of 300 feet. |
| Geologic maps |
Houston and Graff (1995), scale 1:50,000; Sutherland and Hausel (2004), scale 1:100,000. |
| Geophysical data |
Inadequate aeromagnetic and aeroradiometric coverage, poor ground gravity coverage. |
| Favorable rocks and structures |
Layered mafic/ultramafic intrusive complexes with associated vein systems, gabbro, peridotite; related units (schist, quartzite, breccia, gneiss, and mylonite) found along the Mullen Creek-Nash Fork shear zone (Cheyenne Belt) and associated shear zones. |
| Deposits |
Portland and Hercules mine (MRDS dep_id: 10205227), Broadway mine (MRDS dep_id: 10048493), Creede prospect (MRDS dep_id: 10080886), Three Forks Group (MRDS dep_id: 10132774). |
| Evidence from mineral occurrences |
MRDS; locations in Hausel (1986). |
| Geochemical evidence |
From Hausel (1985): "It has been reported that the Charter Oak ores also carried significant Co (as much as 7 to 5% Co) (Armstrong, 1970)." From Hausel (1985): "Fresh [Puzzler Hill/Charter Oak] samples collected for whole rock analyses included...a pyroxenite, which yielded 19.36% MgO, and...an actinolite-chlorite schist, which yielded 16.43% MgO"; From Hausel (2000): "Mineralized samples from prospects and mines on Puzzler Hill yielded 0.01 to 4.43% Cu, 66 ppm to 3.72% Ni, 14 ppb to 0.29 opt Au, <5 to 828 ppb Pt, 5 ppb to 0.12 opt Pd, <0.1 ppm to 0.19 opt Ag, 21 to 831 ppm Co, 64 to 294 ppm Cr, and trace Pb and Zn (Hausel, 1985). |
| Geophysical evidence |
No data. |
| Evidence from other sources |
No data. |
| Comments |
Wyoming State Geological Survey emphasized that this focus area includes the Elkhorn gabbro. |
| Cover thickness and description |
Variable, mix of good exposure to abundant surficial cover and heavily forested areas. |
| Authors |
Rachel N. Toner, Patricia M. Webber, Benjamin J. Drenth, Michael L. Zientek, Robert W. Gregory, Kevin Mosser. |
| New data needs |
1:24,000 scale geologic mapping supported by high-quality geophysics and lidar, especially along trace of Cheyenne Belt; new reconnaissance geochemistry. |
| Geologic mapping and modeling needs |
1:24,000 scale geologic mapping of the igneous and metamorphic stratigraphy, structural and deformation features (especially along Cheyenne Belt), and mineralization trends with precise ground control is essential. Requires outcrop-level mapping. |
| Geophysical survey and modeling needs |
Rank 1 aeromagnetics and radiometrics, also a major ground gravity campaign. |
| Digital elevation data needs |
Lidar in progress with small areas complete. |
