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 porphyry copper 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.
Porphyry copper deposits tend to form in and around epizonal plutons rather than deep-seated batholiths, but, because we have only very general paleodepth information for many plutons in Nevada (Barton and others, 1988), no part of the tract could be excluded on this basis. Because of a lack of unequivocal knowledge of the age of all Nevada plutons, the tract for all three ages has the same physical boundaries. The most favorable area for deposits of Early Cretaceous age is a belt including the Eureka, White Pine, and Robinson districts. An area of Late Cretaceous plutons and associated copper skarn deposits near Luning is also favorable for porphyry copper deposits.
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
In our estimate of undiscovered deposits, we were guided by the fact that the area of concealed permissive bedrock that is unexplored, is about 2.5 times larger than the area of exposed permissive rock. On the negative side, we noted that during the period of intensive exploration for porphyry copper deposits in the 1960s and 70s, only a small number of deposits were found in Nevada, and that most of these were in the Yerington area.
We made separate estimates for undiscovered deposits of different ages because of the differences in their favorable areas and because we believe that their probabilities of occurrence are different. The estimate given for Cretaceous deposits is relatively low because we believe that, although Cretaceous plutons are numerous in Nevada, those of Early Cretaceous age are rare, and consequently, undiscovered systems like Robinson district (110 Ma) are less likely to exist. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 2, 2, 3, and 5 or more Cretaceous deposits consistent with the grade and tonnage model of Singer and others (1986).
Barton, M.D., Battles, D.A., Debout, G.E., Capo, R.C, Christensen, J.N., Davis, S.R., Hanson, R.B., Michelsen, C.J., and Trim, H.E., 1988, Mesozoic contact metamorphism in the western United States, p. 110-178, in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States, Rubey Volume VII: Prentice Hall, New Jersey, 1153 p.
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., Ludington, Steve, Sherlock, M.G., Singer, D.A., Berger, B.R., and Tingley, J.V., 1991, Mineralization patterns in time and space in the Great basin of Nevada 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. 2, April 1990, p. 193-198.
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.
Hudson, D.M., 1983, Alteration and geochemical characteristics of the upper parts of selected porphyry systems, western Nevada: Reno, University of Nevada, unpub. Ph.D. dissertation, 229 p.
Joralemon, I.B., 1973, Copper, the encompassing story of mankind’s first metal: Berkeley, California, Howell-North Books, 407 p.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade-tonnage model of porphyry copper, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.
Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Special Pub. 4, 136 p.
Wallace, A.B. , 1979, Possible signatures of buried porphyry copper deposits in middle to late Tertiary volcanic rocks of Western Nevada in Ridge, J.D., ed., Proceedings of the Fifth Quadrennial IAGOD Symposium: Reno, University of Nevada, Mackay School of Mines, v. 2, p. 69-76.