Explained by Roger P. Ashley
On the choice of deposit models
Calc-alkaline volcanic centers of Tertiary age are widespread in the western Great Basin. Most are part of the ancestral Cascade volcanic arc. Several Comstock-type deposits occur in these centers, and many more occur immediately to the east in the same geologic environment in western Nevada. In addition, alkaline volcanic rocks belonging to the bimodal suite of the Basin and Range Province occur in this area.
On the delineation of permissive tracts
All areas of volcanic rocks in this tract are considered permissive for Comstock-type deposits. Because these deposits usually have limited vertical extent (about 1000 m maximum), they could be present within the 1-km depth limit and have little surface manifestation.
The tract boundaries 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 prominent example is the Bodie district, which produced about 46 metric tons of gold and 31 metric tons of silver (Chesterman and others, 1986; Mosier, Menzie, and Kleinhampl, 1986). The Comstock and Aurora districts lie immediately adjacent to the tract, in western Nevada.
On the numerical estimates made
The volcanic centers in this tract have been relatively well explored for precious metal deposits, and some Comstock-type prospects are known. Bedrock areas in the tract are generally well exposed, and many parts of the tract are covered by detailed geologic mapping. 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 Comstock 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, Sato, and Singer (1986) (Mark3 index 16).
The probability of large numbers of deposits is considered extremely low because the area has been relatively thoroughly explored, so no completely new prospects are likely to be discovered. Therefore the 5th and 1st percentile estimates show no additional deposits.
Chesterman, C.W., Chapman, R.H., and Gray, C.H., Jr., 1986, Geology and ore deposits of the Bodie mining district, Mono County, California: California Division of Mines and Geology Bulletin 206, 36 p.
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.
Mosier, D.L., Sato, Takeo, and Singer, D.A., 1986, Grade-tonnage model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 151-153.