Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Kuroko massive sulfide deposits are stratabound accumulations of massive Fe-sulfides with subordinate (but economically significant) layers and lenses of Cu-, Zn-, and Pb-sulfides (Singer, 1986). These are deposited on the seafloor around hydrothermal vents associated with felsic and intermediate volcanic rocks in and around island arc volcanic complexes. Two kuroko deposits are known in Permian volcanic rocks of the Blue Mountains of Oregon.
On the delineation of permissive tracts
Island arc volcanic terranes in the Blue Mountains of northeastern Oregon are considered permissive environments for kuroko massive sulfide deposits. These include the Wallowa and Huntington terranes in Oregon, Idaho and southeasternmost Washington (Brooks and Vallier, 1978; Walker and MacLeod, 1991). The Baker melange terrane is also included because it incorporates some fragments of arc volcanic and plutonic rocks. Plutonic bodies that intrude these terranes were excluded from the permissive tracts.
Important examples of this type of deposit
In the Hells Canyon area of Oregon and Idaho, the Red Ledge (Fifarek and others, 1994) and Iron Dyke (Bussey and Anderson, 1994) deposits of Permian age have grade and tonnage characteristics that fit the kuroko grade and tonnage model of Singer and Mosier (1986). In addition, several more prospects of this type are known in the Hells Canyon area in Idaho and south of the Wallowa Mountains in Oregon.
On the numerical estimates made
Within the tract, only the Hells Canyon and south Wallowa Mountains areas are considered favorable for the presence of undiscovered deposits. Despite the presence of two deposits and several more prospects in the Hells Canyon area, the poor economics for any but the largest of these has led to an incomplete exploration effort. The tendency for these deposits to occur in clusters leads us to further optimism. The uncertainty, given the presence of only two known deposits in the area, is rather high. For the 90th, 50th, and 10th percentiles, the team estimated 1, 3, and 6 or more kuroko deposits consistent with the grade and tonnage model of Singer and Mosier (1986).
Brooks, H.C., and Vallier, T.L., 1978, Mesozoic rocks and tectonic evolution of eastern Oregon and western Idaho, in David G. Howell and Kristin A. McDougall, editors, Mesozoic paleogeography of the western United States: Pacific section, Society of Economic Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium 2, p. 133-146.
Bussey, S.D., and Anderson, P.J., 1994, Geology of the Iron Dyke mine and surrounding Permian Hunsaker Creek Formation in Vallier, T.L., and Brooks, H.C., eds., Geology of the Blue Mountains region of Oregon, Idaho, and Washington: U.S. Geological Survey Professional Paper 1439, p. 151-182.
Fifarek, R.H., Juhas, A.P., and Field, C.W., 1994, Geology, mineralization, and alteration of the Red Ledge volcanogenic massive sulfide deposit, western Idaho, in Vallier, T.L., and Brooks, H.C., eds., Geology of the Blue Mountains region of Oregon, Idaho, and Washington: U.S. Geological Survey Professional Paper 1439, p. 113-150.
Singer, D.A., 1986, Descriptive model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 131.
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.
Singer, D.A. and Mosier, D.L., 1986, Grade and tonnage model of kuroko deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.