Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and Warren Yeend.
On the choice of deposit models
This tract covers parts of the Talkeetna Mountains. It is permissive for low-sulfide Au-quartz vein deposits (Berger, 1986) because of known mines, gold placer deposits, favorable bedrock geologic units, favorable regional greenschist facies metamorphism, and geochemical anomalies (Ag, As, Au).
On the delineation of permissive tracts
The favorable geologic units that define the tract are the extensive Lower Jurassic Talkeetna Formation, local Upper Jurassic and Lower Cretaceous flysch, and Jurassic and Cretaceous granitic rocks where deformed and metamorphosed to lower greenschist facies.
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 Singer and others (1977, 1980), Nokleberg and others (1990), and Madden-McGuire and Winkler (1993).
Important examples of this type of deposit
The lode gold mines (Gold Cord, Independence, Thope) (Ray, 1954; Burleigh, 1987; Madden-McGuire and others, 1988) and associated placer mines of the Willow Creek district (Capps, 1915; Jasper, 1967; Cobb, 1973) in the Anchorage quadrangle are examples of low-sulfide Au-quartz veins in this tract. The Independence mine produced about 90% of the ore in the tract. Two significant smaller mines are at Gold Bullion and Lucky Shot. More than ten smaller mines occur in the district.
On the numerical estimates made
The Willow Creek district is a relatively small area that may have a possible extension to the north. The numerous low-sulfide Au-quartz veins known in the tract suggest that it probably contains at least one additional undiscovered deposit. The estimated minimum number of undiscovered deposits, consistent with the grade and tonnage model of Bliss (1986), is:
Percentile 90 50 10 5 1
Estimated number of deposits 1 5 7 7 7
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
Burleigh, R.E., 1987, A stable isotope, fluid inclusion and ore petrographic study of gold-quartz veins in the Willow Creek mining district, Alaska: Unpublished M.Sc. thesis, University of Alaska, Fairbanks, 246 p.
Capps, S.R., 1915, The Willow Creek district, Alaska: U.S. Geological Survey Bulletin 607, 86 p.
Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1324, 213 p.
Jasper, M.W., 1967, Geochemical investigations, Willow Creek southerly to Kenai Lake region, south central Alaska: Alaska Division of Mines and Minerals Geochemical Report 14, 47 p.
Madden-McGuire, D.J., Silberman, M.L., and Church, S.E., 1988, Geologic relationships, K-Ar ages, and isotopic data from the Willow Creek gold mining district, Alaska, in Keays, R.R., Ramsay, W.R.H., and Groves, D.I., eds., The geology of gold deposits: Economic Geology Monograph 6, p. 242-251.
Madden-McGuire, D.J., and Winkler, G.R., 1993, Mineral resource maps of the Anchorage quadrangle, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Map MI-2393, scale 1:250,000, 3 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., Lange, I.M., Singer, D.A., Curtin, G.C., Tripp, R.B., Campbell, D.L., and Yeend, Warren, 1990, Mineral resource assessment maps of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1996-A, 4 sheets, scale 1:250,000, 22 p.
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.
Ray, R.G., 1954, Geology and ore deposits of the Willow Creek mining district, Alaska: U.S. Geological Survey Bulletin 1004, 86 p.
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., Curtin, G.C., and Foster, H.L., 1977, Mineral resources map of the Tanacross quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-767-E, 1 sheet, scale l:250,000.