National mineral assessment tract AK-SC29 (Polymetallic vein)

Tract AK-SC29
Geographic region South Central, AK
Tract area 16,669sq km
Deposit type Polymetallic vein
Deposit age Cenozoic

Rationale

Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
Parts of the Wrangell Mountains are permissive for polymetallic vein deposits because of favorable Tertiary volcanic rocks and sparse hypabyssal granitic plutonic rocks, sparse geochemical anomalies (Ba, Pb, Zn, Sn, and Mo), local chalcopyrite and pyrite in heavy-mineral concentrates, and local aeromagnetic anomalies. The tract is not considered permissive for epithermal vein deposits because exploration geochemical data indicate undiscovered polymetallic rather than epithermal vein deposits, because of the lack of known epithermal vein deposits, and because of a lack of As, Sb, Ag, and Au geochemical anomalies.
On the delineation of permissive tracts
The favorable rock units defining the tract are the felsic volcanic and hypabyssal granitic plutonic rocks of the middle and late Cenozoic Wrangell Lava, a constructional Andean-arc-type volcanic complex. High volcanoes, mud volcanoes, and hot springs are characteristic of parts of the tract. Portions of the tract exhibit areas of bleaching and silicification; heavy mineral concentrates locally contain abundant chalcopyrite and pyrite, and exhibit anomalous Ba, Pb, Zn, Sn, and Mo. On the aeromagnetic map of Alaska (Godson, 1994), aeromagnetic anomalies indicate that the late Tertiary and Quaternary volcanic rocks that define the tract may continue beneath areas of late Quaternary and Recent sedimentary rocks. An annular magnetic high anomaly along the northern edge of the tract might reflect a concealed and related porphyry granitic pluton.
The distribution of the geologic units defining the tract is adapted from Nokleberg and others (1994). The distribution of this belt of mineral deposits is adapted from MacKevett and others (1978) and Nokleberg and others (1987, 1993). Mineral resource assessments of 250,000-scale quadrangles within the tract were done by Richter and others (1975), MacKevett (1976), and Singer and MacKevett (1977), Singer and others (1980).
Important examples of this type of deposit
No polymetallic veins are known in the area, but the geologic setting is permissive for deposits of this type.
On the numerical estimates made
The tract is consists mostly of high late Tertiary to currently active volcanoes and associated volcanic fields. Parts of the tract are covered by extensive snow and ice fields and glaciers. Access is poor. No estimate of undiscovered deposits is attempted because no polymetallic sulfide veins are known.
References
Godson, Richard H., 1994, Composite magnetic anomaly map of Alaska and adjacent offshore area, in Plafker, G. and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America: The Geology of North America, v. G1, plate 10, scale 1:2,500,000.
MacKevett, E.M., 1976, Mineral deposits and occurrences in the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-773B, scale 1:250,000, 2 sheets.
MacKevett, E.M., Jr., Singer, D.A., and Holloway, C.D., 1978, Maps and tables describing metalliferous mineral resource potential of southern Alaska: U.S. Geological Survey Open-File Report 78-1E, 2 sheets, scale 1:1,000,000, 45 p.
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.
Nokleberg, W.J., Bundtzen, T.K., Grybeck, Donald, Koch, R.D., Eremin, R.A., Rozenblum, I.S., Sidorov, A.A., Byalobzhesky, S.G., Sosunov, G.M., Shpikerman, V.I., and Gorodinsky, M.E., 1993, Metallogenesis of mainland Alaska and the Russian Northeast: Mineral deposit maps, models, and tables, metallogenic belt maps and interpretation, and references cited: U.S. Geological Survey Open-File Report 93-339, 222 pages, 1 map, scale 1:4,000,000; 5 maps, scale 1:10,000,000.
Nokleberg, W.J., Moll-Stalcup, E.J., Miller, T.P., Brew, D.A., Grantz, Arthur, Reed, J.C., Jr., Plafker, George, Moore, T.E., Silva, S.R., and Patton, William W., Jr., 1994, Tectonostratigraphic terrane and overlap assemblage map of Alaska: U.S. Geological Survey Open-File Report 94-194, 53 p., scale 1:2,500,000.
Richter, D.H., Singer, D.A., and Cox, D.P., 1975, Mineral resources map of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-655-K, 1 sheet, scale l:250,000.
Singer, D. A., Csejtey, Bèla, Jr., and Miller, R.J., 1980, Map and discussion of the metalliferous and selected non-metalliferous mineral resources of Talkeetna Mountains quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-558-Q, 34 p., 1 sheet, scale 1:250,000.
Singer, D.A., and MacKevett, E.M., Jr., 1977, Mineral resources map of the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-773-C, l sheet, scale l:250,000.

Geographic coverage

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