Explained by Richard M. Tosdal

On the choice of deposit models

The southeast corner of California contains widely distributed volcanic rocks that unconformably overlie metamorphic and igneous rocks; subvolcanic and epizonal calc-alkaline dikes and stocks intrude the crystalline rocks. These environments are permissive for hot-spring Au-Ag deposits. In addition, numerous epithermal gold prospects and deposits are present. Some of these deposits have been classified as hot-spring Au-Ag type deposits. Most deposits or prospect occur in crystalline rocks that underlie the volcanic rocks.

On the delineation of permissive tracts

Included in this permissive tract for hot-spring Au-Ag deposits are areas of known gold prospects, Tertiary volcanic rocks that might conceal one of these deposits, and areas of shallow cover on the extensive pediments on the flanks of the ranges. The two known deposits, currently being mined, and two prospects are included within the tract.

Important examples of this type of deposit

There are two examples of this deposit type within the permissive tract. One is the Mesquite deposit, a major producer of gold in California (Burnett, 1990; Willis and Tosdal, 1992), and the second is the Picacho deposit, which is also actively being mined. Both are north of Yuma, Arizona. Another example of this type of deposit in the Great Basin part of southern California is the Hart deposit, near the Nevada-California border.

On the numerical estimates made

Two factors are especially important to the numerical estimate for undiscovered deposits. One is the presence of two developed mines of this type, as well as the two other prospects that have been explored but are of too low grade or tonnage to be mined at present. Second is the wide expanse of pediment buried beneath thin veneer of alluvium or sedimentary rock that underlies most of the permissive tract. Based upon these two factors, for the 90th, 50th, and 10th percentiles, the team estimated 1, 3, and 5 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.

Willis, G.F., and Tosdal, R.M., 1992, Formation of gold veins and breccias during dextral strike-slip faulting in the Mesquite mining district, southwestern California: Economic Geology, v. 87, no. 8, p. 2002-2022.