National mineral assessment tract AK-SW20 (Epithermal vein, hot spring Au)

Tract AK-SW20
Geographic region Southwestern, AK
Tract area 17,898sq km
Deposit type Epithermal vein, hot spring Au
Deposit age Cretaceous - Tertiary


Confidence Number of
90% 0
50% 1
10% 2
5% 3
1% 6

Estimators: Miller, Bundtzen, Cox, Gray, Phillips


Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
This tract is underlain by Late Cretaceous and early Tertiary subaerial volcanic rocks that are permissive for Hot Spring Au (+ Hg) epithermal deposits. One prospect in the tract is thought to be an epithermal Hot Spring Au occurrence similar to those described by Berger (1986).
On the delineation of permissive tracts
This tract comprises several areas underlain by Late Cretaceous and early Tertiary subaerial volcanic rocks, predominately andesites, rhyolites, and minor basalts. The volcanic rocks of the Yetna River area in the western Iditarod (Miller and Bundtzen, 1994) and eastern Holy Cross quadrangles locally show chalcedonic silicification and elevated Au and Hg values. Similar subaerial volcanic rocks in the Russian Mission and Bethel quadrangles host the only identified Hot Spring Au type occurrence in the tract; this belt of permissive rocks was extended to the southwest by geophysical data. Subaerial volcanic rocks and a caldera complex in the central Holy Cross quadrangle, and a possible caldera complex inferred from geophysical data in the Russian Mission quadrangle, are all permissive for hot-spring gold deposits. Hg and Au anomalies in the McGrath and Lime Hills quadrangles are associated with hot springs in Tertiary subaerial volcanic rocks. St. Matthew Island is in part underlain by calc-alkaline felsic and intermediate volcanic rocks (Patton and others, 1975), but no mineral occurrences are known on the island.
Important examples of this type of deposit
The Firebear prospect (informal name) near the Poison Creek volcanic field in the Russian Mission quadrangle is thought to be a Hot Spring Au type deposit (Miller and Bundtzen, 1997).
On the numerical estimates made
The median deposit size is expected to be 13 million tonnes at 1.6 g/tonne Ag (Berger and Singer, 1992). Even though the geology in this region is known only at reconnaissance scale, the team was confident enough in the model designation, supported locally by geochemical data, to provide a numerical estimate. The estimated minimum number of undiscovered deposits expected to occur in southwestern Alaska is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 1 2 3 6
Berger, B.R., 1986, 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.
Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.
Bundtzen, T.K., and Miller, M.L., 1997, Precious metals associated with Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska, Economic Geology Monograph 9, p. 242-286.
Miller, M.L., and Bundtzen, T.K., 1994 Generalized geologic map of the Iditarod quadrangle, Alaska, showing potassium-argon, major-oxide, trace-element, fossil, paleocurrent, and archaeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 p., scale 1:250,000.
Patton, W.W., Jr., Miller, T.P., Berg, H.C., Gryc, George, Hoare, J.M., and Ovenshine, A.T., 1975, Reconnaissance geologic map of St. Matthew Island, Bering Sea, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-642, scale 1:125,000.

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