National mineral assessment tract AK-AP04 (Epithermal vein, generic)

Tract AK-AP04
Geographic region AK Peninsula & Aleutians
Tract area 27,515sq km
Deposit type Epithermal vein, generic
Deposit age Cenozoic

Estimates

Confidence Number of
deposits
90% 5
50% 10
10% 27
5% 27
1% 27

Estimators: Wilson, Church, Saltus, Drew, Menzie

Rationale

Explained by Frederic H. Wilson, Stanley E. Church, Richard W. Saltus, Lawrence J. Drew, and W. David Menzie.
On the choice of deposit models
Epithermal vein deposit models in the conterminous US have been divided into several types based largely on the underlying basement rocks (Cox and Singer, 1986). For the most part, information about Alaska is inadequate to classify individual epithermal gold vein types. Therefore, we have combined deposit models for Creede epithermal veins (Mosier and others, 1986b), Comstock epithermal veins (Mosier and others, 1986c), Sado epithermal veins (Mosier and others, 1986a), and epithermal quartz-alunite gold (Berger, 1986a) into a generic epithermal vein model. We evaluated the permissive area for undiscovered deposits of all types of epithermal veins based on this generic model, which includes gold and/or silver veins associated with intermediate to felsic volcanic rocks similar to those described by Panteleyev (1987).
Occurrences known on the Alaska Peninsula and in the Aleutian Islands may or may not display structural control, can occur in rocks ranging from basalt to dacite or in sedimentary rocks adjacent to felsic intrusive igneous rocks, and may contain variable, although typically small, amounts of base metal sulfides. Gold grade tends to be higher than is typical for Sado districts and individual deposits on the Alaska Peninsula tend to be the median size of districts under the Sado model.
On the delineation of permissive tracts
This tract is defined by occurrences of epithermal gold-veins in Cenozoic dacitic to basaltic volcanic rocks on the Alaska Peninsula and adjacent islands (Detterman and others, 1981, 1987; Wilson and others, 1992, in press). Most of these occurrences are in the southern part of the tract; however, similar volcanic rocks extend the length of the tract, northward into adjacent areas of southwestern Alaska, and southward to the Aleutian Islands. Areas adjacent to (within about 5 km) sub-volcanic felsic plutons intruding the volcanic and sedimentary rocks of the Chignik subterrane on the Alaska Peninsula (Wilson and others, 1985) are geologically and geochemically permissive for epithermal gold vein deposits.
Important examples of this type of deposit
The Alaska-Apollo and Sitka mines and the Shumagin prospect in the Port Moller 1:250,000-scale quadrangle are examples of generic epithermal vein deposits (see Wilson and others, 1988).
On the numerical estimates made
There is no tonnage and grade model for generic epithermal vein deposits. Additional undiscovered epithermal vein deposits that are expected to occur in this tract would probably be about the same size as the known deposits (i.e., Apollo and Shumagin); that is, as much as 500,000 tons of ore containing 3-5 million grams of gold (90,000 to 160,000 ounces of gold). These estimates are for deposits, not districts.
The estimated minimum number of undiscovered deposits is:
Percentile 90 50 10 5 1
Estimated number of deposits 5 10 27 27 27
The large tracts of volcanic rocks and suggestive geochemical anomalies combined with the small target area for these deposits led to the assessment team to conclude there is a high probability for multiple additional epithermal vein deposits within this tract.
References
Berger, B.R., 1986a, Descriptive model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 158.
Berger, B.R., 1986b, Descriptive model of hot-spring Au-Ag, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 130.
Detterman, R.L., Miller, T.P., Yount, M.E., and Wilson, F.H., 1981, Geologic map of the Chignik and Sutwik Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1229, scale 1:250,000.
Detterman, R.L., Case, J.E., Wilson, F.H., and Yount, M.E., 1987, Geologic map of the Ugashik, Bristol Bay, and western part of Karluk quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1685, scale 1:250,000.
Mosier, D.L., Berger, B.R., and Singer, D.A., 1986a, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 154.
Mosier, D.L., Sato, Takeo, Page, N.J., Singer, D.A., and Berger, B.R., 1986b, Descriptive model of Creede epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 145.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986c, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 150.
Panteleyev, Andrejs, 1987, A Canadian cordilleran model for epithermal gold-silver deposits, in Roberts, R.G., and Sheahan, P.A., Ore Deposit Models: Geoscience Canada Reprint Series 3, p. 31-43.
Wilson, F.H., Detterman, R.L., and Case, J.E., 1985, The Alaska Peninsula terrane: A definition: U.S. Geological Survey Open-File Report 85-450, 19 p.
Wilson, F.H., Detterman, R.L., Miller, J.W., and Case, J.E., 1995, Geologic map of the Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2272, scale 1:250,000, 2 sheets.
Wilson, F.H., Miller, T.P., and Detterman, R.L., 1992, Preliminary geologic map of the Cold Bay and False Pass quadrangles, Alaska Peninsula: U.S. Geological Survey Open-File Report 92-545, scale 1:250,000.
Wilson, F.H., White, W.H., and DuBois, G.D., 1988, Brief descriptions of mines, prospects, and mineral occurrences in the Port Moller and Stepovak Bay quadrangles, Alaska Peninsula: U.S. Geological Survey Open‑File Report 88‑666, 128 p., scale 1:250,000, 1 map sheet.

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