Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Porphyry copper deposits consist of copper-bearing minerals in disseminated grains and in stockwork quartz veinlets in hydrothermally altered, intermediate to felsic porphyritic intrusions and adjacent country rocks (Cox, 1986). Porphyry copper deposits are generally found in magmatic belts associated with convergent plate margins, and are associated with plutonic rocks of a wide variety of igneous compositions, ranging from diorite to granite. However, gabbros and high-silica granites are seldom associated with porphyry copper deposits. Associated mineral deposits include polymetallic vein, base metal skarn, and (or) base metal replacement deposits (Cox, 1986). Compositionally appropriate granitic plutons of Jurassic, Cretaceous, and Tertiary age intrude accreted Mesozoic terranes in northeast Oregon and adjacent parts of west-central Idaho and southeasternmost Washington. Porphyry copper deposits in Mesozoic accreted terranes in British Columbia and Alaska are somewhat smaller than the well-known Arizona deposits, and have been characterized by a separate grade and tonnage model used here (Menzie and Singer, 1993). Choice of this model reflects the opinion of the team that deposits in this tract would be more like those of British Columbia than the deposits in southwestern U.S. that have higher copper grades.
On the delineation of permissive tracts
All areas with shallowly emplaced (<5 km) convergent-margin plutonic rocks of any age are considered permissive for porphyry copper deposits. Factors that exclude areas from such permissive terranes would include location outside of known plutonic belts, the presence of gabbroic or high-silica granites only, or the presence of only deeply emplaced plutonic rocks. Both large and small bodies of Jurassic and Cretaceous granitic rocks occur throughout the Wallowa, Baker, and Olds Ferry terranes of eastern Oregon and adjacent parts of west-central Idaho and southeasternmost Washington (Walker and MacLeod, 1991). Areas of Eocene and Oligocene(?) volcanic and volcaniclastic rocks (Clarno Formation) are included since they are typically intruded by coeval hypabyssal dikes, sills, and plugs of intermediate to felsic composition. The boundaries of the tract are generally delimited where the cover of younger rocks is greater than 1 km. A favorable area within the tract consists of a belt of associated copper skarns in the Wallowa-Seven Devils Mountains region that straddles the Oregon-Idaho border.
Important examples of this type of deposit
No porphyry Cu deposits are known within the tract, but the area contains several small skarn Cu deposits in a belt from the Seven Devils Mountains to the Wallowa batholith. Five or more areas with acid sulfate alteration are known in the Mineral district of western Idaho. These areas might be related to porphyry Cu systems at depth.
On the numerical estimates made
Although no porphyry Cu deposits are known, the estimate was influenced by the presence of the small Cu skarns around the Wallowa batholith, and in the Mineral district. Some major copper companies have had exploration programs in the area, so the lack of even a single known deposit is an indication for relatively low likelihood for undiscovered deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 5 or more deposits consistent with the grade and tonnage model of Menzie and Singer (1993).
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.
Menzie, W.D., and Singer, D.A., 1993, Grade and tonnage model of porphyry Cu deposits in British Columbia, Canada, and Alaska, USA: U.S. Geological Survey Open-File Report 93-275, 8 p.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.