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 zinc-lead skarns and polymetallic replacement 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.
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
There are at least ten deposits known in Nevada, six of which belong to the grade-tonnage model (Cox and Singer, 1992). Because they are low in metallic mineral content and thus difficult to detect by traditional prospecting methods, we believe that the number of undiscovered deposits is approximately equal to the number of known deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 6, 10, 14, 15, and 17 deposits in the permissive tract that are comparable in grade and tonnage to the distal disseminated silver-gold model of Cox and Singer (1992).
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.
Cox, D.P., and Singer, D.A., 1992, Descriptive and grade and tonnage model of distal-disseminated Ag-Au, in Bliss, J.D., ed., Developments in deposit modeling: U.S. Geological Survey Bulletin 2004, p. 20-22.
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.
Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Special Pub. 4, 136 p.