Explained by James J. Rytuba
On the choice of deposit models
Miocene volcanic centers and associated subvolcanic intrusive rocks of intermediate to felsic composition are present in the Mojave part of the southern Basin and Range province. These volcanic centers consist of dacite to rhyolite dome, flow, and ash-flow fields. The volcanic centers host several Comstock-type deposits as well as several epithermal gold deposits and prospects of other types with their associated areas of alteration. Most of the deposits and prospects are hosted in volcanic rocks but some ore occurs in the crystalline basement rocks and sedimentary rocks deposited in middle Tertiary basins. Not enough is known about the geology of the basement beneath the volcanic rocks to clearly support a Comstock classification following Mosier and others (1986a). The model was chosen because of the high silver content of the known deposits. Associated mineral deposits include hot-spring and quartz-alunite gold deposits and barite vein deposits.
On the delineation of permissive tracts
The permissive tract for Comstock deposits was delineated on the basis of areas of known silver, gold, and barite prospects and deposits, Miocene volcanic centers, and areas of shallow cover where buried extensions of volcanic centers may occur.
Important examples of this type of deposit
The Calico mining district located in this tract is an example of this deposit type. The very large silver resource in the Waterloo and Langtry deposits located in the western part of the Calico mining district are Comstock type deposits that are hosted by middle Tertiary lacustrine sedimentary deposits and water-laid tuffs (Fletcher, 1986).
On the numerical estimates made
Three factors contributed to the numerical estimate for undiscovered resources. One is the presence of several prospects with alteration and mineralization characteristics of this deposit type. The second factor is the presence of Comstock type deposits, and the third factor is the presence of large volcanic centers, parts of which are buried beneath a thin veneer of alluvium or sedimentary rocks that covers parts of the permissive tract. Based upon these factors, the team estimated for the 90th, 50th, and 10th percentiles, respectively, 0, 1, and 2 or more deposits consistent with the grade and tonnage model of (Mosier and others, 1986).
Fletcher, D.I., 1986, Geology and genesis of the Waterloo and Langtry silver-barite deposits, California: Stanford University Ph.D. dissertation, 202 p.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 150.
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.