National mineral assessment tract NR47 (Low-sulfide Au-quartz vein, Archean)

Tract NR47
Geographic region Northern Rocky Mountains
Tract area 2,390sq km
Deposit type Low-sulfide Au-quartz vein, Archean
Deposit age Archean

Deposit model

Model code 36b
Model type descriptive
Title Descriptive model of Homestake Au
Authors Byron R. Berger


Confidence Number of
90% 0
50% 0
10% 1
5% 4
1% 5

P(none): 0.9

Estimators: Day, Ludington, Hausel


Explained by Warren Day
On the choice of deposit models
The criteria used to evaluate the potential for undiscovered Archean lode gold deposits in Wyoming include: (1) the presence of supracrustal (volcanic or sedimentary) and granitoid host rocks of Archean age; (2) medium-grade (greenschist to lower-amphibolite facies) regional metamorphism; (3) the presence of gold deposits, prospects, and occurrences; (4) evidence of large-scale shearing and faulting, and; (5) alteration mineral assemblages within and adjacent to faults and shear zones that contain quartz and carbonate veining, phyllosicilate minerals (sericite, chlorite, and/or biotite), and sulfide minerals (pyrite, arsenopyrite, and/or pyrrhotite). On a worldwide basis, Archean low-sulfide Au-quartz deposits are larger in tonnage than younger low-sulfide Au-quartz vein deposits, but are lower grade; worldwide, they account for the third largest amount of gold production of any major deposit type after the Witwatersrand-type paleoplacer deposits of South Africa and porphyry Cu deposits. Therefore, deposits of this type are potentially a significant mineral resource for Wyoming. The model used in this assessment reflects the revisions made by Klein and Day (1994), who present new grade and tonnage data for Archean deposits from Precambrian shield terranes worldwide. The reader is referred to Eckstrand (1984), Colvine (1989), and Groves and Foster (1991) for excellent reviews of this deposit type.
On the delineation of permissive tracts
In Wyoming, areas that meet the basic criteria are exposed in the cores of several crustal blocks uplifted in Laramide and Tertiary time. To delineate the permissive tract, we used the distribution of Archean supracrustal rocks of appropriate metamorphic grade, and known occurrences of gold that exhibit appropriate alteration mineral assemblages. In Wyoming, no significant gold deposits is known in Archean granitoid intrusions, and these intrusions were excluded from the permissive tract.
Important examples of this type of deposit
Archean supracrustal sequences of Wyoming contain deposits, occurrences, and prospects typical of worldwide Archean lode gold deposits. The deposits of the South Pass-Atlantic City mining district have produced the most gold. Although total gold production from the district is unknown because accurate records were not kept prior to 1933, Hausel (1991) estimated that production may have been as much as 10 metric tons of gold. The South Pass-Atlantic City area contains several known deposits, such as the Carissa, Duncan, Mary Ellen, Atlantic City, and Miners Delight mines in the western part of the permissive tract and the Burr, Goldhope, Bullion, Lone Pine in the eastern part, as well as several gold placer deposits developed from nearby bedrock sources.
On the numerical estimates made
We identified fourteen areas in Wyoming that could host an undiscovered Archean lode gold deposit, which together, make up the permissive tract. The South Pass-Atlantic City area, which includes the Lewiston mining district is viewed as the most favorable area, and we estimated that there is a 30 percent chance that it hosts an undiscovered deposit. For the entire tract, for the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 4, and 5 or more deposits consistent with the grade and tonnage curve of Klein and Day (1994).
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Colvine, A.C., 1989, Empirical model for Archean gold deposits, in Keays, R.R., Ramsay, W.R.H., and Groves, D.I., eds., The geology of gold deposits—The perspective of 1988: Economic Geology Monograph 6, Economic Geology Publishing Company, p. 37-53.
Eckstrand, O.R., 1984, Canadian mineral deposit types—A geological synopsis: Geological Survey of Canada Economic Geology Report 36, 86 p.
Groves, D.I., and Foster, R.P., 1991, Archean lode gold deposits, in Foster, R.P., ed., Gold metallogeny and exploration: London, Blackie and Sons, p. 63-103.
Hausel, W.D., 1991, Economic geology of the South Pass granite-greenstone belt, southern Wind River Range, western Wyoming: Geological Survey of Wyoming Report of Investigations No. 44, 129 p.
Klein, T.L., and Day, W.C., 1994, Descriptive and grade-tonnage models of Archean low-sulfide Au-quartz and a revised grade-tonnage model of Homestake Au: U.S. Geological Survey Open-File Report 94-250.

Geographic coverage

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