Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
High-level felsic intrusive rocks, some of which host porphyry-type mineral occurrences similar to that described by Cox (1986), occur in a broad belt that encompasses the Alaska and Aleutian Ranges in the eastern part of southwestern Alaska. Most reported occurrences have some Mo or Au in addition to Cu, but too little is known to determine whether specific deposits are porphyry Cu, porphyry Cu-Au, or porphyry Cu-Mo types. The BC-Ak type porphyry Cu model (Menzie and Singer, 1993), is considered to be the best example of the expected grade and tonnage for undiscovered deposits.
On the delineation of permissive tracts
Porphyry copper deposits are permissive in a large area that encompasses parts of McGrath, Lime Hills, Lake Clark, Iliamna, and Dillingham quadrangles and connects with similar tracts to the east and south. Numerous examples of porphyry-type mineral occurrences occur within this region. The permissive area was extended into the Dillingham quadrangle on the basis of geophysical data that indicate the possible presence of buried plutons.
Important examples of this type of deposit
There are numerous occurrences that have been described as porphyry-type mineralization. The Pebble Copper deposit in the Iliamna quadrangle (St. George, 1991) is the largest known porphyry Cu-Au deposit in the tract. Pebble Copper is reported to have an identified resource of 454 million tonnes grading 0.35% copper, 0.4 g/tonne Au, and 0.03-0.04% Mo (Swainbank and others, 1993). A second example, the Kijik River deposit in the Lake Clark quadrangle was estimated to contain 91 million tonnes; grab samples yield up to 0.25% Cu and 0.17% Mo (Wilson and others, 1987). A prospect in the Crystal Creek area of the Lime Hills quadrangle (the Chill Group) is listed by MacKevett and Holloway (1977; p. 22, loc. 12) as having porphyry-copper-type mineralization.
On the numerical estimates made
We believe that additional deposits of the BC-Ak porphyry-type lie within the tract. The minimum number of undiscovered porphyry Cu deposits, consistent with the grade and tonnage curve of Menzie and Singer (1993), is estimated to be:
Percentile 90 50 10 5 1
Estimated no. of deposits 1 2 6 8 10
Cox, D.P., 1986, Descriptive model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 76.
MacKevett, E.M., Jr., and Holloway, C.D., 1977, Table describing metalliferous mineral deposits in the western part of southern Alaska: U.S. Geological Survey Open-file Report 77-169-F, 39 p., scale 1:1,000,000.
Menzie, W.D., and Singer, D.A., 1993, Grade and tonnage model of porphyry Cu deposits in British Columbia, Canada, and Alaska, U.S.A.: U.S. Geological Survey Open-file Report 93-275, 8 p.
St. George, Phillip, 1991, The Pebble porphyry copper-gold deposit [abs]: Paper presented at 15th Alaska Miners Association Annual Convention, Anchorage, Alaska.
Swainbank, R.C., Bundtzen, T.K., Clough, A.H., Hansen, E.W., and Nelson, M.G., 1993, Alaska’s mineral industry 1992: Alaska Division of Geological and Geophysical Surveys Special Report 47, 80 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, M.S., Smith, T.E., and Yeend, Warren, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, p. 41-46.