Explained by Roger P. Ashley
On the choice of deposit models
Both Comstock-type epithermal deposits and quartz-alunite gold deposits are shallow-level products of hydrothermal systems associated with volcanism. Systems that produce quartz-alunite Au-Cu deposits are distinguished from those that produce Comstock deposits by higher thermal gradients and abundant, oxidized sulfur. Quartz-alunite deposits show an even stronger preference for calc-alkaline volcanic centers than do Comstock deposits, because shallow sub-volcanic intrusions are required to supply heat and sulfur to the near-surface environment. Calc-alkaline centers of Tertiary age are widespread in the western Great Basin; most are related to the ancestral Cascade volcanic arc. Some of these centers host quartz-alunite-type epithermal deposits, here and to the east in western Nevada.
On the delineation of permissive tracts
The geologic environment for quartz-alunite deposits is volcanic rocks with associated subvolcanic intrusions. The existence of subvolcanic intrusions can be inferred using geologic criteria (presence of hydrothermal alteration, intermediate to silicic domes) or geophysical criteria (magnetic patterns or anomalies). Because published geologic and geophysical maps do not provide the information necessary to apply these criteria everywhere, the permissive tract consists of all areas of volcanic rocks in the area. Also, since these deposits are limited in vertical extent (a few hundred meters), they could be present within the 1-km depth limit, yet have little surface manifestation.
The tract boundaries are the same as for Comstock-type epithermal deposits. They are drawn to include all areas underlain predominantly by Cenozoic volcanic rocks between the Sierra Nevada front and the Nevada border, to the southern edge of the Great Basin. Where volcanic areas are covered by non-volcanic basin deposits of Tertiary or Quaternary age, the boundary is delineated where the cover is inferred to be 1 km thick, as derived from gravity models of the intermontane basins.
Important examples of this type of deposit
The most important known deposit in the tract is the Masonic district, with production of just over 1 metric ton of gold (Mosier and others, 1986). Other deposits of this type occur in similar volcanic host rocks in western Nevada; the most prominent example is Goldfield, having a production of more than 120 metric tons of gold (Ashley, 1990).
On the numerical estimates made
The individual prospects considered likely to yield new quartz-alunite type deposits are not the same as those expected to yield new Comstock-type deposits, but the rationale for the two deposit types is similar in the Great Basin of California. Bedrock areas in the tract are generally well exposed, and many parts of the tract are covered by detailed geologic maps. Geophysical and geochemical data are also extensive. The estimators felt that continued exploration of the known prospects could yield one, or less likely two, deposits of size and tonnage appropriate to the quartz-alunite model. For the 90th, 50th, and 10th percentiles, the team estimated 0, 1, and 2 or more districts consistent with the grade and tonnage models of Mosier and Menzie (1986) (Mark3 index 38).
The probability of large numbers of deposits is considered extremely low because the area has been relatively thoroughly explored, therefore the 5th and 1st percentile estimates show no additional deposits.
References
Ashley, R.P., 1990, The Goldfield gold district, Esmeralda and Nye Counties, Nevada, 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.
Mosier, D.L., Menzie, W.D., and Kleinhampl, F.J., 1986, Geologic and grade-tonnage information on Tertiary epithermal precious- and base-metal vein districts associated with volcanic rocks: U.S. Geological Survey Bulletin 1666, 39 p.
