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 Cretaceous and Tertiary age occur throughout this broad tract.
On the delineation of permissive tracts
The tract includes most of the Eocene and older rocks of northeastern Washington and Idaho north of the Snake River plain in which intermediate plutons of Cretaceous and Tertiary age are widespread (Stoffel and others, 1991; Bond, 1978). Since the deposit model requires shallow (<5 km) emplacement depths for the ore-related plutons, the deeply emplaced western part of the Cretaceous Idaho batholith and the Tertiary core complexes (i.e., Okanagan, Kettle, Priest River-Spokane, and Pioneer core complexes) are excluded, except where intruded by shallow Eocene plutons. Areas north of the Idaho batholith, where no plutonic rocks are known at the surface and geophysical evidence for plutons at depth is not known, were also excluded from the tract.
Important examples of this type of deposit
No copper deposits of this type are known from this tract. Several molybdenum porphyry deposits, some with significant copper resources, do occur in the tract, but are characterized by a separate deposit model (Theodore, 1986; Menzie and Theodore, 1986).
On the numerical estimates made
For the assessment, a North American subset of the porphyry copper tonnage and grade model of Singer and others (1986) was used (Hammarstrom and others, 1993; Mark3 index 81). The lack of known deposits or prospects of this deposit type in the tract, combined with relatively thorough exploration in the 1960s and 1970s led the team to give a very low estimate for the number of undiscovered deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the grade and tonnage model.
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
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.
Hammarstrom, J.M., Zientek, M.L., and Elliott, J.E., eds., 1993, Mineral resource assessment of the Absaroka-Beartooth study area, Custer and Gallatin National Forests, Montana: U.S. Geological Survey Open-File Report 93-207, 296 p., 19 plates.
Menzie, W.D., and Theodore, T.G., 1986, Descriptive model of porphyry Mo, low-F, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 120.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.
Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, S.W., Korosec, M.A., and Bunning, B.B., 1991, Geologic map of Washington-Northeast quadrant: Washington Division of Geology and Earth Resources, Geologic Map GM-39, scale 1:250,000.
Theodore, T.G., 1986, Grade and tonnage model of porphyry Mo, low-F, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 120-122.