National mineral assessment tract GB26 (Epithermal vein, quartz-alunite)

Tract GB26
Geographic region Great Basin
Tract area 161,600sq km
Deposit type Epithermal vein, quartz-alunite
Deposit age Tertiary

Deposit model

Model code 25e
Model type descriptive
Title Descriptive model of epithermal quartz-alunite Au
Authors Byron R. Berger


Confidence Number of
90% 2
50% 5
10% 9
5% 12
1% 15

Estimators: DCox, Singer, Berger, Ludington, Tingley


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
The quartz-alunite deposit type requires the presence of extensive hypogene acid-sulfate alteration. In addition to the Goldfield district, at least six additional occurrences of hypogene alunite are known in western Nevada, mainly in rocks of the western andesite assemblage. Alunite alteration locally affects pre-Miocene rocks but can be shown to be genetically related to western andesite assemblage volcanism (D. John, written comm., 1993).
On the delineation of permissive tracts
The known epithermal 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; Seedorff, 1991; Cox and others, 1991; Ludington and others, in press). This distribution of volcanic-hosted epithermal 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, Comstock 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). 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 epithermal precious-metal deposits in this part of Nevada. A strong negative correlation exists between the magnetic quiet zone and the distribution of volcanic-hosted epithermal deposits. This is especially clear in the southern arm of the crescent where a gap exists between the deposits in the Walker Lane and the Atlanta and Stateline districts to the east in Lincoln County.
The permissive tract for quartz-alunite districts 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 about 55 percent of Nevada; about 47 percent of the tract is covered by superficial deposits younger than the mineralized rocks. Because some epithermal deposits occur in sedimentary rocks close to volcanic centers (Willard, Atlanta, Florida Canyon), sedimentary rocks within and between the volcanic rock areas are included in the tract.
The area considered favorable for undiscovered quartz-alunite districts includes exposed and covered areas of western andesite assemblage rocks in the Walker Lane belt, because nearly all examples of alunite alteration in Nevada are in rocks of this assemblage.
Important examples of this type of deposit
The Goldfield district is the premier example in Nevada, and one of the most important districts in the world (Ransome, 1909; Ashley, 1990).
On the numerical estimates made
In our estimate of undiscovered districts, we considered that the favorable area of andesitic rocks is small relative to the permissive area, and that quartz-alunite gold deposits form under special conditions of intense sulfidation and are thus inherently less abundant than hot-spring and quartz-adularia deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2, 5, 9, 12, and 15 districts consistent with the epithermal quartz-alunite grade-tonnage model (Mosier and Menzie, 1986).
Ashley, R.P., 1990, The Goldfield gold district, Esmeralda and Nye Counties, in Shawe, D.R., and Ashley, R.P., eds., Geology and resources of gold in the United States, Epithermal gold deposits—Part 1: U.S. Geological Survey Bulletin 1857-H, p. H1–H7.
Mosier, D.L., and Menzie, W.D., 1986, Grade-tonnage model of epithermal quartz-alunite veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 159-161.
Ransome, F.L., 1909, Geology and ore deposits of Goldfield, Nevada: U.S. Geological Survey Professional Paper 66, 258 p.

Geographic coverage

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