National mineral assessment tract GB28 (Hot-spring Au-Ag)

Tract GB28
Geographic region Great Basin
Tract area 161,500sq km
Deposit type Hot-spring Au-Ag
Deposit age Tertiary

Deposit model

Model code 25a
Model type descriptive
Title Descriptive model of hot-spring Au-Ag
Authors Byron R Berger
URL https://pubs.usgs.gov/bul/b1693/html/bull0g4n.htm
Source https://pubs.er.usgs.gov/publication/b1693

Estimates

Confidence Number of
deposits
90% 13
50% 18
10% 21
5% 25
1% 29

Estimators: DCox, Singer, Berger, Ludington, Tingley

Rationale

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 choice of deposit models
Hot-spring Au-Ag deposits are disseminated or stockwork deposits that form near a paleosurface in volcanic rocks and, less commonly, in sedimentary rocks and alluvial sediments (Berger, 1985). They are closely related to quartz-adularia and quartz-alunite gold deposits but we have tried to classify them separately because they are distinctly larger in tonnage and lower in grade. Because they grade downward into fissure veins, they are, in some cases, difficult to distinguish from the other types of epithermal gold deposits. Moreover, some epithermal deposits, mined by open pit methods, are more appropriately placed in the hot-spring category because of their reported tonnage and grade, even though evidence for a paleosurface is lacking.
On the delineation of permissive tracts
In Nevada, known hot-spring gold-silver deposits are distributed in a crescent-shaped area, concave to the east, that corresponds poorly with the overall distribution of Tertiary volcanic rocks (Silberman and others, 1976; Stewart and others, 1977; Cox and others, 1991; Seedorff, 1991; Cox and others, 1991; Ludington and others, in press). This distribution of hot-spring gold deposits cannot be explained by the absence of volcanic rocks inward from the crescent. On the contrary, eastern Nevada contains extensive outcrops of older interior andesite-rhyolite assemblage rocks (older than 27 Ma) in which epithermal vein deposits are virtually unknown.
In addition to active volcanism, faulting and fracture permeability are important in controlling the distribution of epithermal deposits. The crescent-shaped area described above corresponds closely to those areas which were undergoing faulting in an extensional tectonic regime during active volcanism. The synvolcanic deformation is important because it provides fracture permeability at the same time that hydrothermal systems related to volcanism are active and circulating, thus facilitating the formation of veins and stockworks. Where Miocene volcanic rocks are relatively unfaulted, for example in the Sierra Nevada of California and in the Cascade Range of Oregon and Washington, epithermal mineral deposits are rare or absent.
The Walker Lane area contains well-developed normal faults, and is probably the best studied region of epithermal mineralization in Nevada (Stewart, 1988). Northwest-striking high-angle faults that predominate in this area have been shown by John and others (1989) to be at least as old as the earliest volcanic activity (22 Ma) in the Paradise Range suggesting that faulting and volcanism were synchronous throughout the period of andesite volcanism.
This region is shown by Blakely (1988) to be characterized by a northwest-trending grain in the pattern of magnetic anomalies that can be recognized about 50 km to the northeast of traditional boundaries of the Walker Lane that are based on topography and structure. This expanded area of characteristic magnetic fabric encompasses all of the volcanic-hosted epithermal districts in southwestern Nevada. The northeastern boundary of this magnetic anomaly pattern coincides with a line separating calderas younger and older than 27 Ma (Best and others, 1989), the eastern boundary is the magnetic quiet zone (Blakely, 1988). We believe that the Walker Lane deformation began locally at 27 Ma, and continued during succeeding volcanic episodes until the beginning of Basin and Range deformation at about 11 Ma, thus controlling the distribution of hot-spring precious-metal deposits in this part of Nevada.
Two northwest-striking linear permissive areas in central Nevada were drawn to enclose basalt flows, dike swarms, and linear magnetic anomalies associated with the northern Nevada rift (Blakely, 1988). The northern segment of this area contains the Fire Creek and Buckhorn hot-spring gold deposits. Basalt outcrops and magnetic anomalies die out at the southern end of this segment near Eureka, but shallow magnetic sources indicate a southern continuation that is slightly offset, but parallel to the northern one.
The permissive tract is based on the distribution of volcanic rocks, of epithermal mineral deposits, prospects, and occurrences, on the distribution of synvolcanic faults, and on the magnetic anomaly patterns described above. This tract covers 55 percent of Nevada; 47 percent of the tract is covered by superficial deposits younger than the mineralized rocks. The high-level environments permissive for hot-spring deposits are difficult to separate from those for other epithermal deposits using published geologic data.
Important examples of this type of deposit
Of the 15 deposits classified as hot-spring gold-silver deposits in Nevada, Round Mountain, the largest, is related to the interior andesite-rhyolite volcanic assemblage, and lies near the inner, eastern edge of the expanded Walker Lane belt. Borealis and Paradise Peak are associated with western andesite assemblage rocks, and the rest are in rocks of the bimodal basalt-rhyolite assemblage (e.g., Sleeper and Hog Ranch) and nearby sedimentary rocks or sedimentary deposits (e.g., Lewis). Seven deposits are located within 5 km of a rhyolite intrusion close in age to the time of mineralization. Buckhorn and Fire Creek are hosted in basaltic andesite and presumably lie above mafic dikes related to the Northern Nevada magnetic anomaly described by Blakely (1988).
On the numerical estimates made
Our estimate of the number of undiscovered deposits in these areas was influenced by the following considerations:
(1) About 15 deposits are known and prospecting was in progress in at 7 or more additional localities at the time of preparing the estimate (1989-92).
(2) MRDS records contain 19 additional occurrences that have descriptions suggestive of hot-spring gold-silver mineralization.
(3) There is a common association of gold and hot-spring mercury deposits. Roughly 75 hot-spring mercury deposits and occurrences are described in the MRDS records for Nevada.
(4) Most of these deposits and occurrences are in areas of exposed permissive rock. Additional permissive area, roughly equal to the exposed area, is covered by younger sediments, and could conceal undiscovered deposits.
(5) Because of their low grade and fine grain size of contained gold, hot-spring gold deposits are more difficult to detect by traditional exploration methods than vein deposits and most of the known deposits have been discovered since 1960. There has been no exploration during this period in the large part of the permissive area within the Nevada Test Site.
For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 13, 18, 21, 25, and 29 deposits comparable in grade and tonnage to the hot-spring gold grade and tonnage model (Berger and Singer, 1992).
References
Berger, B.R., 1985, Geologic-geochemical features of hot-spring precious metal deposits, in Tooker, E.W., Ed., Geologic characteristics of sediment- and volcanic-hosted disseminated gold deposits—Search for an occurrence model: U.S. Geological Survey Bulletin 1646, p. 47-48.
Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., ed., Developments in deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.
Best, M.G., Christiansen, E.H., Deino, A.L., Grommé, C.S., McKee, E. H., and Noble, D.C. 1989, Excursion 3A—Eocene through Miocene volcanism in the Great Basin of the western United States: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 91-133.
Blakely, R. J., 1988, Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada, Journal of Geophysical Research, v. 93, p. 11,817-11,832.
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.
John, D.A., Thomason, R.E., and McKee, E.H., 1989, Geology and geochronology of the Paradise Peak mine and the relationship of pre-Basin and Range extension to early Miocene precious metal mineralization in west-central Nevada: Economic Geology, v. 84, no. 3, p. 631-649.
Ludington, Steve, Cox, D.P., Sherlock, M.G., Singer, D.A., Berger, B.R., and Tingley, Joe, in press, Spatial and Temporal analysis of precious-metal deposits for a mineral resource assessment of Nevada: Ottawa, Canada, Transactions IAGOD/IUGS Symposium, 15 p.
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.
Silberman, M.L., Stewart, J.H., and McKee, E.H., 1976, Igneous activity, tectonics and precious metal mineralization in the Great Basin during Cenozoic time: Society of Mining Engineers, AIME, Transactions, v. 260, p. 253-263.
Stewart, J.H., 1988, Tectonics of the Walker lane belt, western Great Basin—Mesozoic and Cenozoic deformation in a shear zone, in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States (Rubey Volume VII): Englewood Cliffs, New Jersey, Prentice Hall, p. 683-713.
Stewart, J.H., Moore, W.J., and Zietz, Isidore, 1977, East-west patterns of Cenozoic igneous rocks, aeromagnetic anomalies, and mineral deposits, Nevada and Utah: Geological Society of America Bulletin, v. 88, p. 67-77.

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