Sweetwater Arch

Region West, Rocky Mountains
Mineral systems
Deposit types
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Precambrian mafic/ultramafic metasedimentary and metavolcanic belts, associated dike and sill complexes, and derived placers. Ophiolite/komatiitic greenstone belt suites and associated Cr-schists at Seminoe Mountains. Graphitic schists and BIFS common throughout metasedimentary sequences. Gold-bearing quartz veins associated with dilational structural features and epidote-chlorite-carbonate alteration. Uranium-bearing phases present within Neoproterozoic granites, with several REE showings in greater focus area. Lithium pegmatites present at Black Mountains. Focus area is bisected by theorized Archean suture zone (Oregon Trail structural belt), and bound to the north and south by the North Granite Mountains Fault and South Granite Mountains Fault; focus area captures greenstone belts on both sides of the proposed Archean suture, which are intruded by the Wyoming/Sweetwater batholith and covered with extensive Cenozoic sedimentary rocks. Multiple periods of reactivation along significant surface-exposed fault systems.
Identified resources Historical production of uranium.
Production Unknown.
Status Past mining.
Estimated resources Unknown.
Geologic maps Jones and others (2011), scale 1:100,000; Reynolds (1968), scale 1:24,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Several dikes with REE enrichment, none currently known to be economic; granite and associated phases of Wyoming Batholith intruded into complex country rock that hosts several greenstone belts and metasedimentary provinces. The Wyoming batholith potentially partially obscures a Neoarchean suture that has been reactivated several times.
Deposits Little Man mine (MRDS dep_id: 10096604).
Evidence from mineral occurrences MRDS.
Geochemical evidence Wyoming State Geological Survey assays from spodumene-tourmaline-bearing quartzite contain 2,420 to 7,000 ppm Li (Taboga and others, 2015).
Geophysical evidence No data.
Evidence from other sources Sutherland and Cola (2015); Osterwald and others (1966); USMIN shows Li pegmatite deposit (Black Mountain) near this trend-possible this is related to reduced-intrusion system but no further information available about host and other geochemistry.
Comments Granite at Little Man mine hosts uraninite, uranophane, kasolite, with molybdenite, pyrite, chalcopyrite, galena; granite intruded into graphitic schist and quartzite. Origin of U-bearing phases unclear (Osterwald and others, 1966).
Cover thickness and description Variable, mix of good exposure to thick surficial cover. Significant landslide cover in Seminoe Mountains.
Authors Benjamin J. Drenth, Kevin Mosser, Patricia M. Webber.
New data needs Geologic mapping, geophysics, geochronology.
Geologic mapping and modeling needs Modern 1:24,000 scale mapping generally needed, supported by Rank 1 geophysics and modern geochronology.
Geophysical survey and modeling needs Rank 1 aeromagnetic and radiometric data.
Digital elevation data needs Lidar complete in Wyoming.