The Rattlesnake Hills of Central Wyoming lie along the north-eastern edge of the Granite Mountains located within the Archean Wyoming geological province. The Rattlesnake Hills are the result of erosion of the northwest plunging Rattlesnake anticline.
Upwards of 42 Eocene trachyte, phonolite and quartz monzodiorite stocks, domes, dykes and plugs that have intruded Precambrian greenstone rocks have been mapped throughout the property, which collectively comprise the Rattlesnake Alkaline Intrusive (RAI) complex (Autenrieth, 2012). Cross cutting relationships indicate the quartz monzodiorite was emplaced first and may be genetically related to the latite and latite porphyry supracrustal rocks at North Stock. Volcaniclastic rocks of the Wagon Bed Formation, interpreted to be coeval with the emplacement of the RAI complex, are preserved within the North Stock Structural Basin (Norby, 1995). A series of northeast and northwest trending dykes parallel structure in the North Stock area and sills intrude the Archean stratigraphy throughout the region.
During the Eocene the Archean rocks in the Rattlesnake Hills were intruded by the RAI complex. The RAI covers an area of approximately 125 km2 and is analogous to gold-bearing alkalic systems in Montana (Golden Sunlight), South Dakota (Wharf) and Colorado (Cripple Creek) (Koehler, 2012). The RAI complex intruded along the intersection of three prominent regional structures:
- North Granite Mountain (NGM) Fault: east – west trending fault which bounds the Sweetwater Arch to the north;
- Belle Fourche Lineament (BFL): north east trending lineament which links the RAI complex to alkali intrusive complexes in southwestern and northeastern Wyoming (Leucite Hills and Bear Lodge Mountain respectively);
- Rattlesnake Hills Anticline.
Hoch and Frost (1993) divided the RAI complex into three groups (the Eastern Felsic Group – EFG, the Western Felsic Group – WFG and the Central Alkaline Group – CAG) based largely on location and lithology. The EFG intrusions are located along the northeast limb of the Rattlesnake anticline and comprise quartz latites and rhyolites. The WFG, which makes up the southwest portion of the RAI complex, is mineralogically and chemically similar to the EFG only differing texturally (Koehler, 2012). The WFG straddles the North Granite Mountain fault. The EFG and WFG consist of large, up to 1,800 m in diameter, domes. The bulk of the precious metal mineralization identified to date in the Project area is hosted within the CAG. The CAG comprises phonolite, trachyte and latite domes of less than 500 m in diameter located proximal to the axis of the Rattlesnake anticline (Pekarek, 1977). The three groups broadly lie along the Belle Fourche Lineament (BFL) which links the RAI complex to other alkalic complexes regionally.
To date four significant zones of precious metal mineralization have been identified at the Rattlesnake Hills Project (North Stock, Antelope Basin, South Stock and Black Jack). Gold mineralization was drilled by American Copper and Nickel Company (ACNC) in the early 1980’s after they acquired a land position based on reports by A.H. Pekarek (1971 and 1978), and D. Hausel (1982). Hausel identified the first publicly reliable anomalous gold of 7.55 grams per tonne (g/t) gold (Au) in a chip sample from Precambrian sulphide rich chert. Mineralization at that time was broken into two categories: stratabound (within the Archean rocks) and disseminated. Subsequently epithermal gold associated with the RAI complex was identified along zones of highly fractured and altered metasediments as well as within the intrusives themselves. Shortly thereafter, ACNC intersected the first anomalous gold mineralization in drill holes in 1986 at what today is the Antelope Basin mineralized zone. Six distinct styles of mineralization are currently recognized on the Property.
Extensive widespread alteration footprints have been mapped throughout the Rattlesnake Hills Property. In total, ten distinct alteration assemblages, four major and six minor, have been identified. The major alteration types in decreasing order of abundance are: carbonate, potassic, clay and Fe/Mn oxide-hydroxide (FEOH). The minor alteration assemblages include late silica/chalcedony, sericitization, actinolite-riebeckite-magnetite, roscoelite, talc, epidote-hematite and phlogopite. The extensive and complex nature of the hydrothermal alteration mapped throughout the property is indicative of a large prolonged or multiphase Tertiary hydrothermal event affecting the Archean lithologies throughout the project area.
Based upon the type, intensity and distribution of the alteration and mineralization observed by the primary author on the Property, and described by previous workers, the primary exploration target at Rattlesnake Hills is potentially bulk mineable Alkalic Intrusion Associated Gold – Silver deposits. Mesothermal, porphyry and low sulphidation epithermal gold mineralization may also be present.
To view a the most recent NI 43-101 Technical Report on the Rattlesnake Hills Gold Project, click here.