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.
Most major zinc-lead skarn and polymetallic replacement districts tend to be associated with plutons of Cretaceous age, but a few major districts and a large number of occurrences are
known around Tertiary intrusive centers. A few occurrences are associated with Jurassic plutons in northeast and southwest Nevada. Most of the deposits and occurrences are situated in the part of Nevada underlain by Precambrian continental crust, and the host-rocks for most deposits belong to the lower Paleozoic carbonate assemblage. The first carbonate beds above or within the thick Precambrian and Lower Cambrian quartzite sequences have long been known to be the most productive (Woodward, 1972; Ivosevic, 1978). Some deposits are known in upper Paleozoic rocks and a few are found in the Luning Formation of Triassic age.
On the numerical estimates made
Zinc-lead skarn and polymetallic replacement districts are abundant in Nevada; at least eight districts are large enough to be on the grade-tonnage distribution. Many of the 25 smaller occurrences may be insufficiently explored, and little or no exploration has been carried out in covered areas. Of the eight known districts, four have tonnages distributed around the median (two above and two below) and are situated in the exposed part of the permissive tract. The covered part of the tract is 2.5 times greater in size, so the expected value of undiscovered deposits should about 10. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 9, 14, 18, 20, and 22 or more undiscovered districts in the delineated area that are comparable in grade and tonnage to the combined zinc-lead skarn and polymetallic replacement grade-tonnage model of Singer.
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.
Ivosevic, S.W., 1978, Johnnie gold district, Nevada, and implications on regional stratigraphic controls: Economic Geology, v. 73, no. 1, p. 100-105.
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.
Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Special Pub. 4, 136 p.
Woodward, L.A., 1972, Upper Precambrian strata of the eastern Great Basin as potential host rocks for mineralization: Economic Geology, v. 67, no. 5, p. 677-681.