Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
Several areas in southwest Alaska are considered permissive for Besshi massive sulfide deposits (Cox, 1986) because they contain outcrops of mixed sedimentary and tuffaceous rocks of marine origin (mainly volcanic sandstone and andesite tuff).
On the delineation of permissive tracts
This tract is delineated on the basis of geology considered permissive for Besshi massive sulfide deposits. No Besshi-type deposits are known in the area, but the lack of geological and geochemical data to constrain the model leads to delineation of large permissive areas. The permissive rock types are concentrated in two areas of southwestern Alaska. (1) In the north, permissive mixed sedimentary and tuffaceous rocks are found in parts of the Koyukuk, Ruby, and Innoko terranes (Patton and others, 1994). Rocks belonging to the Koyukuk terrane crop out in the Kwiguk, Marshall, Holy Cross, and Russian Mission quadrangles and include Lower Cretaceous andesitic tuffs and volcanic sandstones (Csejtey, 1992). Mafic schists and metasedimentary rocks of the Ruby terrane are in the north-central part of the Iditarod quadrangle (Miller and Bundtzen, 1994). Triassic to Mississippian chert, andesitic tuff, and metasandstone of the Innoko terrane also crop out in north-central Iditarod quadrangle (Miller and Bundtzen, 1994). (2) In the southwestern part of southwest Alaska, permissive mixed sedimentary and tuffaceous rocks are found in the Bethel, Goodnews Bay, Hagemeister Island, Dillingham, Taylor Mountains, and Sleetmute quadrangles. This extensive area includes all of the Goodnews terrane and most of the Togiak terrane (Decker and others, 1994). The heterogeneous Goodnews terrane locally includes tuff and graywacke (Hoare and Coonrad, 1978; Box and others, 1993). 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) includes tuffs, tuffaceous sedimentary rocks, graywackes, and siltstone (Hoare and Coonrad, 1978; Miller and others, 1989). Zn and Cu anomalies are found locally in the Togiak terrane (Kilburn and others, 1993).
Important examples of this type of deposit
No examples of Besshi massive sulfide deposits are known in southwestern Alaska.
On the numerical estimates made
No estimate of undiscovered resources was attempted.
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.
Cox, D.P., 1986, Descriptive model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136.
Csejtey, Bela, Jr., compiler, 1992, Reconnaissance geologic compilation map of the Holy Cross quadrangle, Alaska, plate 1 in, Csejtey, Bela, Jr., and Keith, W.J., eds., Preassessment report on the Holy Cross Quadrangle, Alaska: Menlo Park, California, U.S. Geological Survey unpublished administrative report, 52 p., 3 sheets, scale 1:250,000.
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.
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.
Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska, showing potassium-argon, major-oxide, trace-element, fossil, paleocurrent, and archaeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 p., 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, v. G-1 of Geology of North America: Boulder, Colorado, Geological Society of America, Decade of North American Geology series, p. 241-269.