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 the Quesnellia terrane in northeastern 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
The tract encompasses Triassic, Jurassic, and (or) Cretaceous intermediate composition plutons that intrude the Quesnellia terrane (Stoffel and others, 1991). Since some of these plutons could be yet unexposed by erosion, the entire Quesnellia terrane is considered permissive.
Important examples of this type of deposit
The Kelsey deposit (Derkey and others, 1990), along the Washington-British Columbia border, is the southernmost known (and the only U.S. representative) in the British Columbia porphyry copper belt, a 1,300-km-long belt of Late Triassic and Early Jurassic deposits associated with mildly alkaline intermediate plutons. The deposit has a tonnage greater than about 60 percent of those shown on the tonnage distribution of Menzie and Singer (1993), but has a copper grade (0.286 percent) lower than 90 percent of those shown on the grade distribution. The grade of the Kelsey deposit is about half of the median of these deposits, the tonnage is about twice the median, and the contained metal is about equal to the median size of these deposits.
On the numerical estimates made
Polymetallic veins occur in two clusters south of the Kelsey deposit, and a gold skarn associated with a separate Mesozoic pluton occurs to the east. The relatively small favorable area and the knowledge of extensive exploration for these deposits in the region in the 1960s and 1970s limits our estimate of the number of undiscovered deposits. For the 90th, 50th, 10th, and 5th percentiles, the team estimated 0, 0, 2, 3, and 4 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.
Derkey, R.E., Joseph, N.L., and Lasmanis, Raymond, 1990, Metal mines of Washington—Preliminary report: Washington Division of Geology and Mineral Resources Open-File Report 90-18, 577 p.
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.
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.