Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
This deposit type consists of massive quartz-gold veins that typically are low in sulfide minerals (<5 percent), are vertically and horizontally persistent, and are deformed into pinch-and-swell structures due to compressive deformation (Berger, 1986). These mesothermal veins occur in belts of regionally metamorphosed, low- to moderate-grade, marine sedimentary and volcanic rocks, penetratively deformed, and cut by high-angle regional-scale faults . Pre-Cenozoic rocks of northeastern Oregon and adjacent west-central Idaho and southeasternmost Washington are the appropriate lithologies and metamorphic grade for this deposit type.
On the delineation of permissive tracts
Pre-Cenozoic rocks of northeastern Oregon and adjacent west-central Idaho and southeasternmost Washington consist of andesitic and basaltic volcanic rocks, marine clastic and pelagic sedimentary rocks, dismembered ophiolitic sequences, and cross-cutting granitic rocks (Walker and MacLeod, 1991; Bond, 1978). Except for the granitic rocks, these rocks were moderately to strongly deformed and metamorphosed to greenschist during Triassic and Jurassic time. Amphibolite-facies metamorphism of Early Cretaceous age is localized along Salmon River suture, which constitutes the northeast margin of the tract (Lund and Snee, 1988). The area of these rocks in west-central Idaho is currently under study for its mineral resource potential (Bruce Johnson, U.S. Geological Survey, personal commun., 1994), and this tract includes all the pre-Cenozoic rocks of the Wallowa, Baker, and Olds Ferry terranes east of the western rim of the Snake River canyon (Brooks and Vallier, 1978). Where large enough, cross-cutting, post-metamorphic Jurassic and Cretaceous plutons are excluded from the tract.
Important examples of this type of deposit
Although several prospects of this deposit type occur within the tract, no significant deposits are known. The Black Lake deposit in the Seven Devils mining district produced about 11,000 metric tons of ore averaging 15 g/metric ton of Au (Livingston and Laney, 1920). This is smaller than 85 percent of the deposits used to construct the grade-tonnage model for low-sulfide Au-quartz veins by Bliss (1986), but the grade is near the median for such deposits. In addition four deposits are known in the generally similar rocks in the tract immediately to the west.
On the numerical estimates made
Thorough exploration for placer and lode gold in this region in the late 1800s and early 1900s probably led to the discovery of almost all deposits that are exposed at the surface. However some of the target rock units are covered by younger rocks along the margin of the tract and locally within it. Possibly some of the known prospects, upon more thorough testing, might be found to be deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 2, 4, and 7 or more undiscovered deposits consistent with the grade and tonnage model of Bliss (1986).
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
Brooks, H.C., and Vallier, T.L., 1978, Mesozoic rocks and tectonic evolution of eastern Oregon and western Idaho, in Howell, D.G., and McDougall, K.A., eds., Mesozoic paleogeography of the western United States: Pacific section, Society of Economic Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium 2, p. 133-146
Livingston, D.C., and Laney, F.B., 1920, The copper deposits of the Seven Devils and adjacent districts: Idaho Bureau of Mines and Geology Bulletin No. 1, 105 p.
Lund, Karen, and Snee, L.W., 1988, Metamorphism, structural development, and age of the continent-island arc juncture in west-central Idaho,in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States, Rubey volume VII: New Jersey, Prentice-Hall, p. 296-331.