Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
Zn-Pb skarns (Cox, 1986) are permissive in southwestern Alaska where carbonate rock units are locally intruded by small felsic plutons.
On the delineation of permissive tracts
The permissive tract for skarn deposits in southwestern Alaska consists of a large area in the eastern part of the region, and a smaller area in the southwestern part of the region. The larger area includes three geologic settings: (1) Cambrian to Devonian carbonate rocks of the Farewell terrane (Decker and others, 1994) that crop out in the McGrath, Sleetmute, Taylor Mountains, and Lime Hills quadrangles and are locally intruded by Late Cretaceous and Tertiary plutons (Bundtzen and others, 1987; Miller and others, 1989); (2) Scattered carbonate roof pendants in proximity to porphyry Cu-type plutons in the Lake Clark (Nelson and others, 1983) and Iliamna quadrangles (Detterman and Reed, 1980); and (3) Triassic limestones of the Kamishak Formation that may be intruded by porphyry Cu-type plutons in the Iliamna quadrangle (Detterman and Reed, 1980). A small 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). Although none is known to intrude limestone, Late Cretaceous and early Tertiary granitic plutons lie within the Nukluk subterrane in this area.
Important examples of this type of deposit
The Bowser Creek skarn deposit in the McGrath quadrangle (Bundtzen and others, 1987) is estimated to contain 272,000 tonnes grading 20% Pb and Zn, and 100 g/t Ag (Nokleberg and others, 1987).
On the numerical estimates made
The presence of several examples of Zn-Pb skarns within the tract led the assessment team to conclude the likelihood of undiscovered deposits is relatively high. The estimated minimum number of undiscovered Zn-Pb skarn deposits, consistent with the tonnage and grade model of Mosier (1986) is:
Percentile 90 50 10 5 1
Expected number of deposits 1 2 4 6 12
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., 1986, Descriptive model of Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90.
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.
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.
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.