National mineral assessment tract AK-BR14 (Placer Au)

Tract AK-BR14
Geographic region Brooks Range, AK
Tract area 94,199sq km
Deposit type Placer Au
Deposit age Quaternary

Rationale

Explained by Jeanine M. Schmidt, Karen D. Kelley, Richard I. Grauch, Byron R. Berger, John W. Cady, Gregory T. Spanski, James E. Kilburn.
On the choice of deposit models
Placer gold (Yeend, 1986) can be expected to occur in unconsolidated deposits downstream from any of the gold‑bearing lode deposit types. Therefore the placer gold permissive area is quite broad.
Placer gold has accounted about 24 million ounces, or more than 70%, of the gold production from Alaska (Bundtzen and others, 1996). Placer gold (Yeend, 1986) can be expected to occur in unconsolidated deposits downstream from any of the gold‑bearing lode deposit types. Therefore the placer gold permissive area is quite broad.
On the delineation of permissive tracts
This tract is defined entirely by anomalously high Au values in lake sediments of the North Slope and Arctic coastal plain, as reported by the NURE program (Kelley and Sutley, 1993; Karen Kelley, USGS, written commun. 1994). These anomalous values suggest that the area is permissive for high-volume, low-grade, placer deposits containing very sparsely disseminated fine-grained gold in Quaternary and Tertiary(?) lake sediments.
This composite tract comprises areas of unconsolidated deposits downstream from areas permissive for gold-bearing lode deposits.
Important examples of this type of deposit
No deposits are known; the tract is defined by the anomalously high gold values.
There are numerous important examples of placer gold deposits in Alaska, including deposits in the Fairbanks district, which produced more than 8 million ounces of gold, the Nome and Council districts, which together produced about 6 million ounces of gold, and the Iditarod and Circle districts, which each produced over a million ounces of gold.
On the numerical estimates made
No estimate of the number of undiscovered deposits has been attempted.
Because of the long period of intense exploration and the geographically broad classification of placer districts, no new major districts are expected to be discovered. It is expected that by appliying new technology and ideas, new deposits within the districts will be discovered, and that the districts will be extended. Berg and others (1964) assessed the placer gold resources of Alaska. They reported that 20 million ounces of gold had been produced from placer deposits in the state, and estimated that the remaining resources were “at least equal in quantity and grade to those that have been mined.” We basically accepted their assessment and tried to express values for the different confidence levels. The undiscovered placer gold resources, in million ounces, at various confidence levels is estimated to be:
Percentile 90 50 10 5 1
Million ounces estimated 10 15 20 25 30
References
Kelley, K.D., and Sutley, S.J., 1993, Maps showing the geochemistry of sediment samples from the northern part of the Chandler Lake quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2144D, scale 1:250,000
Yeend, Warren, 1986, Descriptive model of placer Au-PGE, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 261.
AK03 Placer Gold Alaska
Berg, H.C., Eberlein, G.D., and MacKevett, Jr., E.M., 1964, Metallic mineral resources, in Mineral and water resources of Alaska, Report prepared by the United States Geological Survey in cooperation with State of Alaska Department of Natural Resources at the request of Senator Ernest Gruening of Alaska of the Committee on Interior and Insular Affairs United States Senate, U.S. Government Printing Office, Washington, D.C., p. 95-125.
Bundtzen, T.K., Swainbank, R.C., Clough, A.H., Henning, M.W., and Charlie, K.M., 1996, Alaska’s Mineral Industry, 1995, Alaska Division of Geological and Geophysical Surveys Special Report 50, 72 p.
Yeend, Warren, 1986, Descriptive model of placer Au-PGE, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 261.

Geographic coverage

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