National mineral assessment tract AK-WC23 (Cu (Au) skarn)

Tract AK-WC23
Geographic region West Central, AK
Tract area 11,026sq km
Deposit type Cu (Au) skarn
Deposit age Cretaceous - Tertiary

Rationale

Explained by Bruce M. Gamble, Robert G. Eppinger, John W. Cady, Stanley E. Church, Byron R. Berger, Gregory T. Spanski
On the choice of deposit models
Felsic to intermediate plutons and carbonate rocks are widespread throughout the Seward Peninsula. A Fe-Cu skarn deposit is present on the flank of the Kugruk pluton. Presence of the felsic to intermediate plutonic rocks in the vicinity of carbonate rocks, plus the occurrence of related or similar deposits, indicate that this area is permissive for Cu-(Au) skarn deposits as described by Cox and Theodore (1986).
On the delineation of permissive tracts
This tract encompasses the Seward Peninsula plutons, exclusive of tin-granites. There are three distinct suites of plutonic rocks: (1) 99-108 Ma alkaline, nepheline-bearing intrusions (Till and others, 1986); (2) 90‑96 Ma sub-alkaline to alkaline plutons, typically monzonite, quartz monzonite, monzogranite, and granodiorite; and (3) 81‑83 Ma calc-alkaline plutons, typically quartz monzodiorite, granodiorite, and monzogranite. Stream-sediment and heavy-mineral-concentrate geochemistry include anomalously high concentrations of Ag, Cu, Pb, Zn, Mo, Sn, As, Sb, and rare Au (Gamble, 1988).
Important examples of this type of deposit
No Cu-Au skarns are known in the tract. Similar or related mineral deposits types include the Kugruk Fe-Cu skarn; Independence Ag-Pb polymetallic vein; Windy Creek porphyry Mo; Omilak East and Foster Ag-Pb polymetallic veins(?); Cu-rich polymetallic veins of the upper Niukluk River; and many deposits/occurrences, chiefly Ag-Pb-(Zn), of unknown type (Gamble and Till, 1988).
On the numerical estimates made
Insufficient data exist to support an estimate of the expected number of undiscovered Cu-(Au) skarn deposits.
References
Cox, D.P., and Theodore, T.G., 1986, Descriptive model of Cu skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86.
Gamble, B.M., 1988, Non-placer mineral occurrences in the Solomon, Bendeleben, and parts of the Kotzebue quadrangles, western Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF 1838-B, 1 sheet, scale 1:250,000, 13 p.
Gamble, B.M., and Till, A.B., 1993, Maps showing metallic mineral resources of the Bendeleben and Solomon quadrangles, western Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF 1838-D, 3 sheets, scale 1:250,000, 22 p.
Till, A.B., Dumoulin, J.A., Gamble, B.M., Kaufman, D.S., and Carroll, P.I., 1986, Preliminary geologic maps and fossil data, Solomon, Bendeleben, and southern Kotzebue quadrangles, Seward Peninsula, Alaska: U.S. Geological Survey Open-File Report 86‑276, 62 p., scale 1:250,000.

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