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. The three known districts in the State contain deposits of three distinct ages: Yerington, Jurassic; Robinson, Cretaceous; and Copper Canyon, Tertiary. The favorable area for Tertiary deposits is very broad because many the small epizonal plutons of Tertiary age scattered across northern and eastern Nevada are associated with small base- and precious-metal deposits and are favorable for porphyry copper systems. In addition, Tertiary plutons accompanied by areas of widespread alunite alteration (Wallace, 1979; Hudson, 1983) within the western andesite belt are also favorable for porphyry copper deposits. It is possible, however, that some alunite alteration areas are associated with porphyry molybdenum or porphyry gold systems. Because of a lack of equivocal knowledge of the age of all Nevada plutons, the tract for all three ages has the same physical boundaries.
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. Where the pluton is Tertiary in age, however, the enclosing Tertiary volcanic rocks are delineated as permissive.
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 1970s, 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 for Tertiary deposits was partly influenced by the large number of alunite alteration areas in western Nevada. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 3, 4, 6, and 8 or more Tertiary 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, in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States, Rubey Volume VII: New Jersey, Prentice Hall, p. 110–178.
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. PhD. dissertation, 229 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.
Theodore, T.G., and Blake, D.W., 1975, Geology and geochemistry of the west orebody and associated skarns, Copper Canyon porphyry copper deposits, Lander County, Nevada, with a section on Electron microprobe analysis of andradite and diopside by N.G. Banks: U.S. Geological Survey Professional Paper, 798-C, 85 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.