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 Desert part of the southern Basin and Range province. These consist of andesite to rhyolite dome fields, intermediate lava flow and ash-flow fields, and one large caldera structure. Active geothermal systems are present in the Coso Range. These environments are permissive for hot spring Au-Ag deposits (Berger, 1986). Two hot-spring deposits have been mined recently, but are now inactive. In the East Mojave National Scenic Area, one large hot-spring Au-Ag deposit is being mined and epithermal gold prospects and associated alteration are present in several of the volcanic centers. Several epithermal gold prospects and deposits and large areas of alteration are present in all of 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 epithermal quartz-alunite and quartz-adularia deposits.

On the delineation of permissive tracts

The permissive tract for hot spring Au-Ag deposits was delineated on the basis of areas of known gold prospects and deposits, Miocene volcanic centers, areas of shallow cover where buried extensions of volcanic centers may occur, and areas of active geothermal systems. Included within the tract are known deposits, one of which, the Castle Mountain deposit, in the Hart district, is currently being mined, and several epithermal gold prospects and quartz-alunite and quartz-adularia gold deposits which can be associated with hot spring Au-Ag deposits.

Important examples of this type of deposit

The Standard gold deposit in the Soledad Mountain volcanic center, the Shumake and Cactus Queen deposits in the Middle Buttes volcanic center, and deposits in the Randsburg district have all been major producer of gold and silver in California. The Castle Mountain gold deposit in the Hart district near the Nevada-California border is hosted in a Miocene dome and flow field. It is a major producer of gold in California (Burnett, 1990), and recent exploration has defined a large new reserve amenable to open-pit mining methods.

On the numerical estimates made

Four factors contributed to the numerical estimate for undiscovered resources. One is the presence of five 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-alunite and quartz-adularia type deposits and prospects which are associated with hot spring Au-Ag deposits. Third is the presence of several large volcanic centers, parts of which are buried beneath a thin veneer of alluvium or sedimentary rock that covers parts of the permissive tract. Finally, there is an active geothermal system in 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, and 5th percentiles, the team estimated 0, 2, 4, 6, and 8 or more deposits consistent with the worldwide hot spring Au-Ag grade and tonnage model (Berger and Singer, 1992).

References

Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.

Burnett, J.L., 1990, 1989 California mining review: California Geology, v. 43, no. 10, p. 219-224.

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.

Schantz, Radford, Wetzel, Nicholas, Adams, Robert, and Raney, R.G., 1990, Minerals in the East Mojave National Scenic Area, California—An economic analysis, volume II: U.S. Bureau of Mines Open-File Report MLA 6-90, v. 2, 252 p.

U.S. Bureau of Mines, 1990, Minerals in the East Mojave National Scenic Area, California—A minerals investigation, volume I: U.S. Bureau of Mines Open-File Report MLA 6-90, v. 1, 356 p.

U.S. Bureau of Mines, 1992, Economic analysis of the minerals potential of the East Mojave National Scenic Area, California, executive summary: U.S. Bureau of Mines Open-File Report 55-92, 12 p.

U.S. Bureau of Mines, 1992, Economic analysis of the minerals potential of the East Mojave National Scenic Area, California: U.S. Bureau of Mines Open-File Report 56-92, 43 p. plus attachments.

U.S. Geological Survey, 1991, Evaluation of metallic mineral resources and their geologic controls in the East Mojave National Scenic Area, San Bernardino County, California: U.S. Geological Survey Open-File Report 91-0427, 278 p., 6 plates.

U.S. Geological Survey, 1992, Permissive terranes for metallic and selected non-metallic mineral resources, West Mojave Management Area, southern California: U.S. Geological Survey Administrative Report, 21 p., 1 plate.

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.