Explained by D.P. Cox, Steve Ludington, B.R. Berger, M.G. Sherlock, and D.A. Singer, (USGS); and J.V. Tingley (Nevada Bureau of Mines and Geology).
On the delineation of permissive tracts
The tract permissive for copper skarn deposits is defined as an area extending 10 km outward from the outcrop of a pluton, or, in the case that the pluton has a geophysical expression as discussed by Grauch and others (1988), from the inferred subsurface boundary of the pluton, based on its geophysical expression. It also includes areas around plutons whose presence is inferred from geophysics or from the occurrence of skarn mineralization. The tract covers about 40 percent of the area of the State. Carbonate-bearing and calcareous sedimentary rocks are present in all sedimentary assemblages in Nevada (Stewart, 1980), hence, no areas were excluded from the permissive tract on the basis of age or composition of intruded country rock. Those parts of the tract that do contain assemblages rich in carbonate rocks, however, are considered favorable.
About 72 percent of the permissive tract is covered by 1 km or less of upper Tertiary and Quaternary rocks and sediments. Areas covered by more than 1 km (Blakely and Jachens, 1991) are excluded as are areas that are within a Tertiary caldera. In these latter areas, permissive pre-Tertiary host rocks are likely to be covered by more than 1 km of volcanic rock.
On the numerical estimates made
Our estimate was based on the belief that the number of concealed undiscovered deposits within 1 km of the surface is at least as large as the number of known deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 6, 10, 14, 16, and 18 or more undiscovered deposits in the delineated area that are consistent with the grade and tonnage model for copper skarn of Jones and Menzie (1986a).
Blakely, R.J., and Jachens, R.C., 1991, Concealed ore deposits in Nevada: insights from three-dimensional analysis of gravity and magnetic anomalies, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and ore deposits of the Great Basin—Symposium proceedings: Reno, Geological Society of Nevada, v. 1, April 1990, p. 185-192.
Grauch, V.J.S., Blakely, R.J., Blank, H.R., Oliver, H.W., Plouff, Donald, and Ponce, D.A., 1988, Geophysical delineation of granitic plutons in Nevada: U.S. Geological Survey Open-File Report 88-11, 7 p.
Jones, G.W., and Menzie, W.D., 1986, Grade-tonnage model of Cu skarns, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.
Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Special Pub. 4, 136 p.