Rattlesnake Hills Alkali Intrusive Complex

Region West, Rocky Mountains
Mineral systems
Deposit types
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Established historical Au mining district in an Eocene subalkalic-alkalic extrusive and intrusive igneous complex with a hydrothermal alteration zone decreasing outwards in grade; depositional analog to Cripple Creek district, Colorado. Intersection with the North Granite Mountain fault. Current exploration Au and Ag exploration.
Identified resources Historical production of gold and silver.
Production Unknown.
Status Past mining. Current exploration for Au and Ag.
Estimated resources Unknown.
Geologic maps Hausel (1996a), scale 1:24,000; Sutherland and Hausel (2003), scale 1:100,000; Sutherland and Hausel (2005a), scale 1:24,000; Autenrieth (2012), scale 1:12,000; Sutherland and others (2012), scale 1:24,000; Sutherland and Worman (2013), scale 1:24,000; Lynds and others (2016), scale 1:24,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Alkaline intrusions; extrusive volcanic rocks; skarn.
Deposits Unknown.
Evidence from mineral occurrences No data.
Geochemical evidence A NI 43-101 technical report for GFG Resources (US), Inc. gives values for Au and Ag along with a few anomalously high values of W (Turner and others, 2016). Minor occurrences of bismuthinite, cobaltite, Co-rich pyrite, Ag+Bi sulfides, gersdorffite (Ni-As sulfide), molybdenite, and sphalerite reported by Ripple (2012).
Geophysical evidence No data.
Evidence from other sources Regional patterns: Rattlesnake Hills igneous rocks are similar in age and composition to northern Black Hills alkalic province (for example, Bear Lodge, Wyoming). The Cripple Creek district, Colorado, and Golden Sunlight, Montana may be mineral system analogs (Turner and others, 2016; Autenrieth, 2012).
Comments Explored heavily for Au, including significant aerial geophysics, but all data are proprietary. Very little data are in the public realm with none focusing on anything other than the Au system. Presence of As, Sb, fluorite, Te, and V inferred from the mineral system/deposit type table (Hofstra and Kreiner, 2020).
Cover thickness and description Thick Neogene sedimentary cover in places. Numerous intrusions through entire Phanerozoic section.
Authors Ranie M. Lynds, Rachel N. Toner, Benjamin J. Drenth, Joshua M. Rosera.
New data needs Geophysics, mapping, and lidar.
Geologic mapping and modeling needs Two additional 1:24,000 scale maps with infill more detailed outcrop mapping. Subsurface modeling from geophysical surveys crucial.
Geophysical survey and modeling needs Rank 1 aeromagnetic and aeroradiometric data.
Digital elevation data needs Lidar in progress.