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 gold 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.
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.
Meinert (1989) suggested that host rocks for gold skarns are typically carbonate-bearing sequences with an important clastic or volcaniclastic component. These rocks commonly represent parts of accreted terranes. Because the known examples in Nevada are related to Tertiary plutons, a favorable area for gold skarns might be based on the intersection of Tertiary plutons and accreted terranes of the Black Rock, Paradise, Pine Nut, Golconda, and Roberts Mountains assemblages and the overlying Mesozoic carbonate assemblage. Undiscovered deposits are believed to exist mainly in concealed parts of the permissive tract.
On the numerical estimates made
About 2.5 times as much of the permissive tract is concealed as is exposed, and we estimated an expected value of undiscovered deposits of about three to four. Because the grade and tonnage model of Theodore and others (1991) includes some very small deposits, we felt we could estimate more accurately using a model that is truncated to include only those deposits with more than 25,000 metric tons of mineralized rock. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 3, 6, 8, and 12 or more deposits that are comparable in grade and tonnage to the gold skarn grade and tonnage model (truncated) of Theodore and others (1991).
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.
Meinert, L.D., 1989, Gold skarn deposits—Geology and exploration criteria, in Keays, R.R., Ramsay, W.R.H., and Groves D.I., eds., The geology of gold deposits—The perspective in 1988: Economic Geology Monograph 6, p. 537-552.
Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Special Pub. 4, 136 p.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns: U.S. Geological Survey Bulletin 1930, 61 p.