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 delineates an area of Eocene and older rocks surrounded by Tertiary volcanic and sedimentary rocks. The geology is a southwest continuation of the Idaho batholith and its country rock across the Neogene Snake River plain to the northeast.
Important examples of this type of deposit
No porphyry Cu deposits or prospects are known from this tract.
On the numerical estimates made
The lack of known deposits or prospects of this deposit type in the tract, combined with its small size and the apparent relatively thorough exploration led the team to give a very low estimate for undiscovered porphyry copper deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more porphyry-copper deposits consistent with the grade and tonnage model of Singer and others (1986).
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.
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.