Explained by Dennis P. Cox
On the delineation of permissive tracts
The permissive tract is defined to be that part of the area that is underlain by Proterozoic and Paleozoic carbonate rocks, and that is near Mesozoic plutons. Small areas of basin fill more than 1 km in depth are excluded.
On the numerical estimates made
For the 90th, 50th, 10th, and 5th percentiles, the team estimated 0, 3, 5, and 8 or more districts consistent with the combined grade and tonnage model for zinc-lead skarn and polymetallic replacement districts of Singer (Mark3 index 92). This estimate was based on the belief that approximately three known districts have grades and tonnages close to the median of the combined model. An equal number of undiscovered districts probably exist under cover, or in old districts that are, as presently known, too small to fit the tonnage model distribution.
Carlisle, Donald, Davis, D.L., Kildale, M.B., and Stewart, R.M., 1954, Base metal and iron deposits of southern California: California Division of Mines and Geology Bulletin 170, Chap. 8, p. 41-49.
Mosier, D.L., 1986, Grade-tonnage model of Zn-Pb skarns, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
Mosier, D.L., Morris, H.T., and Singer, D.A., 1986, Grade-tonnage model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 101-104.
Hall, W.E., and MacKevett, E.M., Jr., 1962, Geology and ore deposits of the Darwin quadrangle, Inyo County California: U.S. Geological Survey Professional Paper 368, 87 p.
MacKevett, E.M., Jr., 1953 Geology of the Santa Rosa lead mine, Inyo County California: California Division of Mines and Geology Special Report 34, 9 p.
McAllister, J.F., 1955, Geology and mineral deposits of the Ubehebe Peak quadrangle: California Division of Mines and Geology Special Report 42, 63 p.
Newberry, R.J., Einaudi, M.T., and Eastman, H.S., 1991, Zoning and genesis of the Darwin Pb-Zn-Ag skarn deposit, California—A reinterpretation based on new data: Economic Geology, v. 86, no. 5, p. 960–982.