Explained by Richard M. Tosdal
On the choice of deposit models
The Salton Sea basin is the site of recent volcanism, hot-spring activity, and geothermal activity principally in the southern and eastern parts of the basin. Fossil hot-spring systems are known along the western margin of the basin.
On the delineation of permissive tracts
Hot-spring deposits form near shallow heat sources and represent fossil geothermal systems. In the Salton Sea basin, recent rhyolitic volcanism and active geothermal systems are localized in the southern and eastern parts of the basin, and a gold-bearing hot-spring prospect, the Modoc prospect, is known along the western shore of the Salton Sea (Hillemeyer and others, 1991). These two observations justify the delineation of this permissive tract. In addition, the extensive young alluvial sedimentation, particularly along the western margin of the basin may conceal hot-spring deposits.
Important examples of this type of deposit
The nearby Mesquite and Picacho deposits in the Chocolate Mountains are the closest examples of hot-spring Au-Ag deposits to this permissive tract.
On the numerical estimates made
Several lines of evidence lead to the numerical estimate. One is the presence of one known hot-spring prospect, which suggests the potential for additional fossil geothermal systems. Second is the presence of volcanism and high heat flow centered on the Salton Sea basin, indicative of magmatism and the potential to drive hydrothermal circulation. Third is the presence of active geothermal systems, some of which are known to be depositing precious and base metals from brine at depth. Finally, the large expanse of area covered by unconsolidated alluvial deposits may conceal hot-spring Au-Ag deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 1, 3, 5, and 5 or more deposits consistent with the worldwide grade and tonnage model (Berger and Singer, 1992).
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.
Hillemeyer, F.L., Johnson, M.D., and Kern, R.R., 1991, Geology, alteration and mineralization of the Modoc hot springs gold prospect, Imperial County, California, in McKibben, M.A., ed., The diversity of mineral & energy resources of southern California: Society of Economic Geologists Guidebook Series, v. 12, p. 139-156.