Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
In southwestern Alaska carbonate rocks and units containing carbonate rocks, which have been intruded by Late Cretaceous or Tertiary plutons, may host deposits that fit the Cu-(Au) skarn model of Cox and Theodore (1986), and Theodore and others (1991).
On the delineation of permissive tracts
The permissive tract for skarn deposits in southwestern Alaska encompasses one larger area in the eastern part of the region, and a smaller area in the southwestern part of the region. The larger area is delineated by three geologic settings: (1) Cambrian to Devonian carbonate rocks of the Farewell terrane (Decker and others, 1994) crop out in the McGrath, Sleetmute, Taylor Mountains, and Lime Hills quadrangles; Late Cretaceous and Tertiary plutons locally intrude the limestone (Bundtzen and others, 1987; Miller and others, 1989). (2) Carbonate roof pendants are scattered throughout a belt that also contains porphyry Cu-type plutons in the Lake Clark (Nelson and others, 1983) and Iliamna quadrangles (Detterman and Reed, 1980); the porphyry plutons locally intrude limestone pendants. (3) Triassic limestones of the Kamishak Formation also lie within the belt that contains porphyry Cu-type plutons in the Iliamna quadrangle (Detterman and Reed, 1980); the limestone may have been intruded locally. The smaller area, in the Goodnews Bay quadrangle, contains blocks of Ordovician, Devonian, and Permian limestone (Hoare and Coonrad, 1978), which are part of a structurally disrupted unit of mixed marine sedimentary and volcanic rock that forms the Nukluk subterrane of the Goodnews terrane (Decker and others, 1994). Late Cretaceous and early Tertiary granitic plutons occur within the Nukluk subterrane in this area, but none is known to intrude limestone .
Important examples of this type of deposit
There are numerous small Cu skarn occurrences in carbonate rocks in roof pendants in the northern Aleutian Range of southwestern Alaska. The Kasna Creek prospect in the Lake Clark quadrangle, for example, is a chalcopyrite- and hematite-bearing skarn that contains an estimated 9 million tons grading 1 percent Cu; it occurs in Upper Triassic dolomite and limestone near the contact with Jurassic tonalite (Wilson and others, 1987)
On the numerical estimates made
The presence of several Cu skarns within the tract led the team to conclude that the likelihood of undiscovered deposits is relatively high. The estimate of the minimum number of undiscovered Cu skarn deposits, consistent with the tonnage and grade model of Jones and Menzie (1986), is:
Percentile 90 50 10 5 1
Expected number of deposits 2 3 8 12 24
Bundtzen, T.K., Kline, J.T., Smith, T.E., and Albanese, M.D., 1987, Geologic map of the McGrath A-2 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 91, 20 p., scale 1:63,360.
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.
Decker, John, Bergman, S.C., Blodgett, R.B., Box S.E., Bundtzen, T.K., Clough, J.G., Coonrad, W.L., Gilbert. W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., and Wallace, W.K., 1994, Geology of southwestern Alaska, 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, p. 285-310.
Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p., scale 1:250,000.
Hoare, J.M., and Coonrad, W.L., 1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-file Report 78-9-B, 2 sheets, scale 1:250,000.
Jones, G.M., and Menzie, W.D., 1986, Grade and tonnage model of Cu skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.
Miller, M.L., Belkin, H.E., Blodgett, R.B., Bundtzen, T.K., Cady, J.W., Goldfarb, R.J., Gray, J.E., McGimsey, R.G., and Simpson, S.L., 1989, Pre-field study and mineral resource assessment of the Sleetmute quadrangle, southwestern Alaska: U.S. Geological Survey Open-File Report 89-363, 115 p., 3 plates, scale 1:250,000.
Nelson, W.H., Carlson, Christine, and Case, J.E., 1983, Geologic map of the Lake Clark quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1114-A, scale 1:250,000.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns, U.S. Geological Bulletin 1930, 61 p.
Wilson, F.H., Anderson, G.L., Bundtzen, T.K., and Nokleberg, W.J., 1987, Significant metalliferous lode deposits, Aleutian Islands and Alaska Peninsula, in Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, Donald, Robinson, MS., Smith, T.E., and Yeend, Warren, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, p. 41-46.