Geology And Gold in the Rattlesnake Hills And Barlow Gap Areas

The Rattlesnake Hills lie along the northern margin of the Barlow Gap 1:24,000 scale quadrangle in central Wyoming. The Barlow Gap quadrangle was mapped by Wayne Sutherland and assisted by W. Dan Hausel in 1999. The Rattlesnake Hills district was mapped by W. Dan Hausel of the Metals and Precious Stones Section of the Wyoming State Geological Survey (WSGS) in 1995 (Geological Map of the Rattlesnake Hills). The results of the Rattlesnake Hills appeared in WSGS Report of Investigations 52. These mapping projects are part of an ongoing Precambrian geology and economic geology research project by the WSGS. Mapping of the Barlow Gap Quadrangle was partially funded by the US Geological Survey STATEMAP program. The Rattlesnake Hills project was funded entirely by the WSGS budget. This region of the northern Granite Mountains contains one of the best gold prospects in the State for open-pitable gold mineralization.

Barlow Gap lies within the ancient Archean craton known as the Wyoming Province, and includes geologic units ranging in age from oldest Precambrian to Quaternary. The southern two thirds of the quadrangle lie within the Granite Mountains, which are separated from the Rattlesnake Hills to the north by the North Granite Mountains fault. Precambrian rocks comprise an Archean metamorphic complex of gneiss, schist, and fragments of supracrustal rocks, which have undergone complex ductile folding , and were metamorphosed to amphibolite grade. These were intruded by medium- to coarse-grained granites and subsequently by diabase dikes. The metamorphic complex is similar to supracrustal rocks mapped in the Rattlesnake Hills greenstone terrain to the north (see Hausel, 1996).

Precambrian rocks represent about 30% of the outcrop area . Paleozoic sedimentary units occur only in limited outcrops west of the Rattlesnake Hills. Roughly 60% of the Barlow Gap quadrangle is underlain by Tertiary and Quaternary rocks including the Eocene Wagon Bed Formation to the north of the North Granite Mountains fault, and the Miocene Split Rock Formation to the south. Laramide folding produced the northwest-trending Rattlesnake Hills anticline which was cut off at its southern end by the late Eocene rise (and subsequent Pliocene subsidence) of the Granite Mountains along the North Granite Mountains fault (Love, 1970).

During Middle to late Eocene time, numerous felsic and alkalic igneous rocks intruded both sides of the North Granite Mountains fault. These volcanics are considered potential sources for large-tonnage, low-grade gold mineralization. Several other gold anomalies have been identified in the area, and are associated with exhalites, breccias, iron formation , veins, and stockworks (Hausel, 1996, 1998). Many of these gold targets in the region are relatively unexplored.

Past exploration in the Rattlesnake Hills to the north of Barlow Gap resulted in the discovery of several significant gold anomalies by Wyoming State Geological Survey geologist W. Dan Hausel in 1981. Follow-up exploration by various mining companies and consultants resulted in the identification of a low-grade disseminated gold deposit with a resource of 250,000 ounces. Recent review of the drill data by Dave Miller suggests the resource is more on the order of >1,000,000 ounces and is open in every direction (Hausel and others, 2000). This area is also known for agates and jaspers derived from extensive jasperoids along the trend of the North Granite Mountains fault.

References Cited

Hausel, W.D., 1996, Economic geology of the Rattlesnake Hills supracrustal belt, Natrona County, Wyoming: Geological Survey of Wyoming Report of Investigations 52, 28 p.

Hausel, W.D., 1998, The Rattlesnake Hills - Wyoming’s little known gold district: International California Mining Journal, v. 68, no. 4, p. 44-46.

Hausel, W.D., Miller, D.R., and Sutherland, W.M., 2000, Economic diversification through mineral resources: Classical Geology in the New Millennium, Wyoming Geological Association Field Conference Guidebook, p. 209-225.

Love, J.D., 1970, Cenozoic geology of the Granite Mountains area, central Wyoming: US Geological Survey Professional Paper 495C, 154 p.

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