Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
The lack of limiting geologic and geochemical data for southwestern Alaska leads to delineation of large areas permissive for kuroko massive sulfide deposits as described by Singer (1986). The permissive areas are underlain by mixed assemblages that include basaltic to andesitic marine volcanic rocks and local felsic volcanic units.
On the delineation of permissive tracts
This tract is delineated on the basis of geology considered permissive for kuroko massive sulfide deposits; no mineral occurrences of this type are known in the region. The permissive rock types are concentrated in three areas of southwest Alaska. (1) In the north, marine volcanic assemblages of the Koyukuk terrane (Patton and others, 1994) crop out in the Kwiguk, Marshall, Holy Cross, and Russian Mission quadrangles. These rocks constitute a mixed assemblage largely composed of Upper Jurassic(?) and Lower Cretaceous andesitic volcanic and volcaniclastic rocks, but which locally includes dacitic lava flows (T.K. Bundtzen, written commun., 1992). (2) In the southwestern part of southwest Alaska, permissive marine volcanic rocks occur in the Bethel, Goodnews Bay, Hagemeister Island, and Dillingham quadrangles. This extensive area includes nearly all of the Goodnews terrane, parts of the Togiak terrane, and all of the Kilbuck terrane (as defined by Decker and others, 1994). Here the Goodnews terrane includes mixed pillow basalts, intermediate to mafic flows, volcaniclastic rocks, and sedimentary rocks (Hoare and Coonrad, 1978; Box and others, 1993); felsic volcanic rocks occur locally. Stream-sediment samples from the southwestern part of the Goodnews terrane are anomalous in Cu, Zn. Co, Cr, and Au (Kilburn and others, 1993). The Hagemeister subterrane of the Togiak terrane (Decker and others, 1994) is also included as permissive for kuroko type deposits. These rocks consist largely of marine volcanic and volcaniclastic strata, which include mafic, andesitic, and trachytic compositions (Hoare and Coonrad, 1978). Zn and Cu anomalies occur locally in the Togiak terrane (Kilburn and others, 1993). The Proterozoic Kilbuck terrane (Box and others, 1990) is also included as permissive, because it locally contains metamorphosed felsic (unpublished mapping by M.L. Miller and T.K. Bundtzen, 1992) and mafic volcanic rocks (D.L. Turner and others, written commun., 1982). (3) The third area considered permissive for kuroko type deposits lies in the Lake Clark quadrangle where there are scattered roof pendants consisting in part of metavolcanic rocks (Nelson and others, 1983).
Important examples of this type of deposit
No examples of kuroko massive sulfide deposits are known in southwest Alaska.
On the numerical estimates made
Felsic volcanic rocks are a criterion for kuroko massive sulfide deposits. The volume of felsic volcanic rocks within this tract is probably quite small; therefore, the probability of undiscovered kuroko-type deposits is correspondingly low. The estimated minimum number of undiscovered deposits, consistent with the grade and tonnage model of Singer and Mosier (1986) is:
Percentile 90 50 10 5 1 0.5
Estimated number of deposits 0 0 0 1 3 20
Box, S.E., Moll-Stalcup, E.J., Frost, T.P., and Murphy, J.M., 1993, Preliminary geologic map of the Bethel and southern Russian Mission quadrangles, southwestern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2226-A, 20 p., scale 1:250,000.
Box, S.E., Moll-Stalcup, E.J., Wooden, J.L., and Bradshaw, J.Y., 1990, Kilbuck terrane: Oldest known rocks in Alaska: Geology, v. 18, p. 1219-1222.
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.
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.
Kilburn, J.E., Goldfarb, R.J., Griscom, Andrew, and Box, S.E., 1993, Map showing metallic mineral resource potential in the Goodnews Bay, Hagemeister Island, and Nushagak Bay 1o x 3o quadrangles, southwest Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2228, 4 sheets, 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.
Patton, W.W., Jr., Box, S.E., Moll-Stalcup, E.J., and Miller, T.P., 1994, Geology of west-central Alaska, chap. 7, in Plafker, George, and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-1, p. 241-269.
Singer, D.A., 1986, Descriptive model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189-190.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.