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 copper, gold, and silver.
Production Unknown.
Status Past mining; current exploration for Au near boundary of focus area in Eocene rocks of the Rattlesnake Hills (see Rattlesnake Hills focus area).
Estimated resources Unknown.
Geologic maps Harris and others (1985), scale 1:500,000; Love and Christiansen (1985), scale 1:500,000; Jones and others (2011), scale 1:100,000; Hausel (1994), scale 1:24,000; numerous 1:24,000 scale maps cover the majority of Granite Mountains, Sage Hen Rocks, Gas Hills, Rattlesnake Hills quadrangles.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Ultramafic schists; serpentinites; mafic/ultramafic intrusions, flows, sills, and dikes; komatiites; pillow lavas; metagraywackes, pelitic schists, and iron-stained graphitic schists; banded iron-formation; stratiform mica schist-hosted gossan; quartz veins/pegmatites and epidote-chlorite-carbonate altered zones (fractures and shear zones) within previously-listed rock types; units are Sunday Morning Creek metavolcanic rocks, Bradley Peak metavolcanics, Seminoe metavolcanics; low-grade disseminated gold possible within large zones surrounding altered sequences. Tin Cup mining district: quartz veins in micaceous and carbonate schists, iron-formation, and granitic gneisses, also associated with albite-orthoclase-microcline-muscovite pegmatites that is often jade-bearing.
Deposits Spanish mines (MRDS dep_id: 10279045), Shirley Mountains prospect (MRDS dep_id: 10096571), Charlie's Glory Hole (MRDS dep_id: 10080794), Sunday Morning mine (MRDS dep_id: 10080793), Sunday Morning prospect (MRDS dep_id: 10157766), Junk Creek prospect and mine (MRDS dep_id: 10205696), Apex mine (MRDS dep_id: 10400300), Deserted Treasure No. 2 (MRDS dep_id: 10400302), King mine ( MRDS dep_id: 10400299), Penn mines (MRDS dep_id: 10400298), Cherry Creek prospect (MRDS dep_id: 10081156), King Solomon claim (MRDS dep_id: 10080976), Anderson mine (MRDS dep_id: 10133116), Red Boy, Tin Cup, Sutherland, Lone Tree, Cedar Ridge mines and prospects.
Evidence from mineral occurrences MRDS.
Geochemical evidence Spanish mines: From Hausel (1997) underground muck pile grab samples contains 4.1 ppm Ag and 1330 ppm As; results of 100 samples in other nearby prospects (Neubert, 1985) have ranges of Au (to 0.19 opt), Ag (to 4.3 opt), As (maximum 28%), Cu (maximum 4.8%), Pb (maximum 2.8%), Zn (maximum 2.0%). Junk Creek mine: grab sample of Cu-stained quartz contained 1.2% Cu, 0.05 ppm Au, 1.4 ppm Ag, 6 ppm Ga. Chip sample contained 0.78% Cu, 1.7 ppm Au, <0.05 ppm Au. Sunday Morning mine: From Hausel (1994); grab Cu-stained quartz: 26.9 ppm Ag, 2.1 ppm Au, 5.8% Cu, 1,970 ppm Pb, 140 ppm Zn. Tin Cup mining district: From Hausel (1982d); sheared sulfides in iron formation, and massive sulfides assayed up to 0.92% Cu, 0.05 opt Au, 0.01% Co. From Hausel (1996b); select sample near Sutherland Shaft yielded 1.71% Cu, 3.6 ppm Ag, 7 ppb Au, 74 ppm Zn and 10 ppm Pb; sample of schist with jasper yielded 0.13% Cu, 694 ppm Zn, 43 ppm Pb, 11 ppm Mo and 86 ppm As.
Geophysical evidence Reynolds and Neubert (1988).
Evidence from other sources See maps and assays on the Seminoe Mountain Greenstone belt from Klein (1981), and data on the mineral resources of the Ferris Mountains Wilderness (Reynolds and Neubert, 1988).
Comments From Hausel (1996a): Gold mineralization is largely hosted in gold-bearing quartz veins, determined to be part of a low-sulfidation epithermal system; these veins occur in dilational zones as well as shear zones, coupled with chlorite-carbonate-epidote alteration. It is hypothesized that most metals were derived from mafic to ultramafic sequences, but some were introduced with fluids. There are multiple overlapping systems, which have not been clearly delineated; assay data from literature is not sufficient to where certain elements like W are hosted. Rather than an Orogenic system/Gold deposit type, this focus area could also be considered to fit into a Meteoric convection system/Low-sulfidation epithermal gold-silver deposit type category.
Cover thickness and description Variable, mix of good exposure to thick surficial cover. Significant landslide cover in Seminoe Mountains.
Authors Patricia M. Webber, Rachel N. Toner.
New data needs New Rank 1 geophysics-supported and modern geochronology-supported mapping could be a game-changer. Could use new reconnaissance geochemistry.
Geologic mapping and modeling needs Variable, but new, modern 1:24,000 scale mapping generally needed, supported by Rank 1 geophysics and modern geochronology.
Geophysical survey and modeling needs Rank 1 aeromagnetics and radiometrics, also additional gravity stations; the greenstone belts might be a good place for airborne gravity gradient survey (i.e., small area, rugged terrain that may preclude ground gravity); reconnaissance geochemistry.
Digital elevation data needs Lidar complete in Wyoming.