Explained by D.P. Cox, S. D. Ludington, B.R. Berger, M.G. Sherlock, and D.A. Singer, (USGS); and J.V. Tingley (Nevada Bureau of Mines and Geology)
On the choice of deposit models
Nevada contains 50 known sediment-hosted gold deposits, which comprise the bulk of deposits on which the grade and tonnage model of Mosier and others (1992) is based.
On the delineation of permissive tracts
Delineation of tracts permissive for sediment-hosted gold deposits is difficult because: (1) the deposits occur in a wide variety of types and ages of host rock; (2) structures believed to control the distribution of deposits are too subtle to be shown on published geologic maps; (3) direct determination of mineralization age by isotopic analysis is restricted by the lack of suitable minerals in the deposits; and (4) the genetic association of mineralization with igneous rocks is uncertain, primarily because of the lack of age constraints.
During the last two decades of intensive exploration for gold, sediment-hosted gold deposits have not been found in significant numbers outside of the Great Basin. If these deposits are unique to the Great Basin, then, as pointed out by Seedorff (1991), their origin must be related to some unique feature of the region. Speed and others (1988) showed that this region is distinctive among other forelands in its great width, lack of extensive uplift and unroofing, and absence, on the oceanward side, of a colliding basement-terrane. Berger and Henley (1989) point to the overthickening of the crust by thrust faulting as an important characteristic of the part of the Great Basin occupied by known sediment-hosted gold deposits, adding that tectonic stacking of marine sedimentary deposits could have created large reservoirs of connate water. Existence of such a fluid reservoir is suggested (Rose and Kuehn, 1987; Hofstra and others, 1988) by the geochemistry and isotopic composition of ore-stage fluid inclusions (elevated δ18O, CO2, H2S, and Cl) from sediment-hosted gold deposits.
Large parts of the crust in Nevada have been overthickened by crustal shortening along the active western margin of North America from during the late Paleozoic and Mesozoic (Speed and others, 1988). The Roberts Mountains and Golconda assemblages were thrust eastward during the late Paleozoic Antler and Triassic Sonoma orogenies, respectively. In the Cretaceous, several allochthons within the Paradise volcanic assemblage were thrust southeastward and stacked east of the Pine Nut fault and behind and atop the Golconda assemblage; thrust faulting resulted in thickening of Triassic rocks of the Jungo assemblage; eastern assemblage rocks were thickened by east-directed thrusting during the Elko and Sevier orogenies in much of northeastern Nevada (Thorman and others, 1991).
Based on the assumed relationship between crustal thickening and sediment-hosted gold deposits, all sedimentary and metasedimentary rocks within the area of tectonically thickened crust are delineated as permissive for sediment-hosted gold deposits. Tract delineation was based on distribution of the allochthons mentioned above, and, in the Sevier orogenic belt in eastern Nevada by patterns of folding and thrust faulting shown on the geologic map of Stewart and Carlson (1978). The permissive tract encompasses 62 percent of the area of Nevada; 77 percent of the tract is covered by Tertiary and Quaternary rocks and sedimentary deposits less than 1 km thick.
Important examples of this type of deposit
Sediment-hosted gold deposits are distributed in three groups which reflect differences in host rocks, structures, and possibly, age of formation. The central group is represented by deposits in the Jerritt Canyon (Burns Basin) district; the Carlin trend; Marigold; deposits around Cortez; Tonkin Springs; and Northumberland. Many of these districts are situated on or close to the Roberts Mountains Thrust. Ages of some of these gold deposits have been indirectly estimated at between 36 and 39 Ma (Bonham, 1989; Berger and Bagby, 1991). The western group of sediment-hosted gold deposits is represented by deposits in the Getchell trend, Standard, and Fondaway Canyon. The Getchell trend deposits are hosted by siliceous shale and phyllite of Cambrian age. The eastern group of deposits is represented by the Bald Mountain, Golden Butte, Alligator Ridge, Illipah, Night Hawk, and Green Springs, mainly hosted in Cambrian, Devonian, and Mississippian carbonate rocks and shales.
Rationale for Numerical Estimates
Our estimate was influenced mainly by the high density of known deposits in the exposed part of the permissive tract and the large proportion of the tract that is concealed by less than 1 km of cover. Significant recent discoveries have been made beneath varying depths of cover. Also, recent exploration of known deposits has shown that their vertical extent can be great, suggesting that undiscovered deposits can exist over a wide range of depths within the permissive tract. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 15, 21, 27, 30, and 33 deposits that are consistent with the grade and tonnage model of Mosier and others (1992).
Berger, B.R., and Henley, R.W., 1989, Advances in the understanding of epithermal gold-silver deposits, with special reference to the western United States, 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. 405-423.
Berger, B.R., and Bagby, W.C., 1991, The geology and origin of Carlin type deposits, in Foster, R.P., ed., Gold exploration and metallogeny: London, Blackie and Sons, p. 210-248.
Hofstra, A.H., Landis, G.P., Leventhal, J.S., Northrop, H.R., Rye, R.O., Doe, T.C., and Dahl, A.R., 1991, Genesis of sediment-hosted, disseminated gold deposits by fluid mixing and sulfidation of iron in the host rocks—Chemical reaction path modeling of ore depositional processes at Jerritt Canyon, 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. 1, April 1990, p. 235-238.
Mosier, D.L., Singer D.A., Bagby, W.C., and Menzie, W. D., 1992, Grade and tonnage model of sediment-hosted Au, in Bliss, J.D., ed., Developments in deposit modeling: U.S. Geological Survey Bulletin 2004, p. 26-28.
Rose, A.W., and Kuehn, C.A., 1987, Ore deposition from highly acidic CO2-enriched solutions at the Carlin gold deposit, Eureka County, Nevada [abs.]: Geological Society of America, Abstracts with Programs, v. 19, no. 7, p. 824.
Seedorff, Eric, 1991, Magmatism, extension, and ore deposition of Eocene to Holocene age in the Great Basin—Mutual effects and preliminary proposed genetic relationships, 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. 133-178.
Speed, R.C., Elison, M.W., and Heck, F.R., 1988, Phanerozoic tectonic evolution of the Great Basin, in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States (Rubey Volume VII): Englewood Cliffs, New Jersey, Prentice Hall, p. 572-605.
Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: U.S. Geological Survey, scale 1:500,000.
Thorman, C.H., Ketner, K.B., Brooks, W.E., Snee, L.W., and Zimmermann, R.A., 1991, Late Mesozoic-Cenozoic tectonics in northeastern 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. 1, April 1990, p. 25–46.