National mineral assessment tract AK-WC20 (Polymetallic vein)

Tract AK-WC20
Geographic region West Central, AK
Tract area 11,026sq km
Deposit type Polymetallic vein
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
Polymetallic vein deposits are associated with hypabyssal intrusions of nearly any age and a wide range of compositions, in sedimentary and metamorphic terranes (Cox, 1986). Polymetallic veins may also be spatially related to a large number of other types of deposits, thus making their permissive area quite extensive. This model also serves as somewhat of a catch‑all for deposits of unknown type that, given minimal information, do not appear to fit into other similar categories.
On the delineation of permissive tracts
This tract includes contains several polymetallic vein deposits associated with three distinct suites of Late Cretaceous plutonic rocks (Till and others, 1986): (1) 99‑108 Ma alkaline, nepheline-bearing intrusions; (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. The tract also includes several types of polymetallic veins, and areas of Ag, Cu, Pb, Zn, Mo, Sn, As, Sb, and rare Au anomalies in stream sediments and heavy-mineral concentrates (Gamble, 1988a, 1988b).
Important examples of this type of deposit
Mineral deposits include the Independence Ag-Pb-Zn polymetallic vein, Omilak East and Foster Ag-Pb polymetallic veins(?), and Cu-rich polymetallic veins of the upper Niukluk River
On the numerical estimates made
Because of the limited exposure, uncertainty of classification of polymetallic vein varieties, and lack of information on controls of mineralization, estimating the expected number of undiscovered polymetallic vein deposits was not attempted.
References
Cox, D.P., 1986, Descriptive model of polymetallic veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 125.
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.

Geographic coverage

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