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 west Mojave part of the southern Basin and Range province. These consist of dacite to rhyolite dome fields, and lava flow and ash-flow fields of similar composition. These environments are permissive for quartz-alunite gold deposits (Berger, 1986). Four quartz-alunite-type deposits have been mined but are now inactive. Several epithermal gold prospects and deposits of other types, and large areas of alteration are present in the volcanic centers. Most of the deposits and prospects occur in the volcanic centers but some mineralization occurs in the crystalline basement rocks. Associated mineral deposits include hot-spring and quartz-adularia gold deposits.
On the delineation of permissive tracts
The permissive tract for quartz-alunite deposits was delineated on the basis of areas of known gold prospects and deposits, Miocene volcanic centers, and areas of shallow cover where buried extensions of volcanic centers may occur. Included within the terrane are four deposits at the Middle Buttes volcanic center which had a pre-mining reserve of about 2.4 metric tons of gold (Blaske and others, 1991) and several epithermal gold prospects and quartz-adularia type gold deposits which can be associated with quartz-alunite deposits.
Important examples of this type of deposit
There are several examples of this type of deposit within the Mojave mining district located in the permissive tract. These include four deposits in the Middle Buttes volcanic center, and the Tropico deposit in the Soledad Mountain volcanic center, all of which have been major producers of gold in California (Blaske and others, 1991).
On the numerical estimates made
Three factors contributed to the numerical estimate for undiscovered resources. One is the presence of four mines developed for this deposit type and the presence of several prospects with alteration and mineralization characteristics of this deposit type. The second factor is the presence of quartz-adularia type deposits and prospects which are associated with quartz-alunite deposits. The third factor is the presence of large volcanic centers with pervasive alteration, parts of which are buried beneath a thin veneer of alluvium or sedimentary rock that covers parts of the permissive tract. Estimates were guided by results of two previous assessments in this area: the East Mojave National Scenic Area (Hodges and Ludington, 1991), and the West Mojave Management Area (U.S. Geological Survey, 1992) For the 90th, 50th, 10th, 5th and 1st percentiles, the team estimated 0, 0, 2, 3, and 4 or more epithermal quartz-alunite Au deposits consistent with the grade and tonnage model of Mosier and Menzie (1986).
Berger, B.R., 1986, Descriptive model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 158.
Blaske, A.R., Bornhorst, T.J., Brady, J.M., Marsh, T.M., and McKitrick, S.A., 1991, The Shumake volcanic dome-hosted epithermal precious metal deposit, western Mojave Desert, California: Economic Geology, v. 86, no. 8, p. 1646-1656.
Hodges, C.A., and Ludington, Steve, 1991, Quantitative assessment of undiscovered metallic mineral resources in the East Mojave National Scenic Area, southern California: U.S. Geological Survey Open-File Report 91-0551, 18 p.
Mosier, D.L., and Menzie, W.D., 1986, Grade and tonnage model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 159.
U.S. Geological Survey, 1992, Evaluation of selected metallic and nonmetallic mineral resources, West Mojave Management Area, southern California: U.S. Geological Survey Open-File Report 92-595, 89 p.