National mineral assessment tract AK-SC03 (Besshi massive sulfide)

Tract AK-SC03
Geographic region South Central, AK
Tract area 9,684sq km
Deposit type Besshi massive sulfide
Deposit age Late Triassic

Estimates

Confidence Number of
deposits
90% 1
50% 2
10% 4
5% 4
1% 4

Estimators: Nokleberg, Goldfarb, Cox, Campbell, Koch, Yeend

Rationale

Explained by Warren J. Nokleberg
On the choice of deposit models
Parts of the eastern Alaska Range and Wrangell Mountains are permissive for Late Triassic Besshi massive sulfide deposits (Cox, 1986), based on favorable bedrock units and the presence of known deposits.
On the delineation of permissive tracts
The favorable geologic units for the tract are the lower, marine, (meta)basalt member of the Upper Triassic Nikolai Greenstone of the Wrangellia terrane, and the Cottonwood Bay greenstone of the Peninsular terrane.
Important examples of this type of deposit
The Denali deposit (Seraphim, 1975) was classified as basaltic copper by Cox (1986), and alternately as a Besshi massive sulfide by Nokleberg and others (1987).
On the numerical estimates made
The estimated minimum number of undiscovered Besshi massive sulfide deposits, consistent with the grade and tonnage model of Singer (1986), is:
Percentile 90 50 10 5 1
Estimated number of deposits 1 2 4 4 4
References
Cox, D.P., 1986, Descriptive model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136.
Cox, D.P., Light, T.D., Csejtey, Béla, and Campbell, D.L., 1989, Mineral resource assessment map of the Healy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2058-A, scale 1:250,000.
Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, Donald, Robinson, M.S., Smith, T.E., and Yeend, Warren, 1987. Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, 104 p.
Seraphim, R.H., 1975, Denali—A nonmetamorphosed stratiform sulfide deposit: Economic Geology, v. 70, p. 949-959.
Singer, D.A., 1986, Grade and tonnage model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136-138.

Geographic coverage

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