Explained by Douglas B. Stoeser
On the choice of deposit models
Western Utah contains extensive areas of felsic volcanic rocks that contain known quartz-adularia epithermal deposits. Although production data are meager, the team's general knowledge of the deposits and prospects in the area suggests that they have a relatively small Ag:Au ratio and very low base metals, fitting more closely the characteristics of the Comstock (Mosier, Singer, and Berger, 1986) and Sado (Mosier, Berger, and Singer, 1986) epithermal vein deposits. As a result, the team compared the Utah quartz-adularia veins with a composite model of Comstock and Sado vein deposits (Mark3 index 25).
On the delineation of permissive tracts
Quartz-adularia gold deposits belong to the epithermal class of mineral deposits, and are typically localized within a few hundred meters of the Earth's surface. They occur mainly in or adjacent to volcanic vent areas. Permissive rocks for this type of deposit consist of intermediate to silicic volcanic fields which have undergone minimal erosion. These are present in three east-trending belts in western Utah and southernmost Idaho. In addition to the Miocene and older calc-alkaline belts which also have porphyry copper potential, the younger (20 Ma to present) extension-related, evolved, silicic systems are also prospective for epithermal gold deposits. Most of the silicic extensional magmatism also occurs within the three same areas that contain intrusion-related deposits (porphyry copper, polymetallic replacement, and skarn).
Important examples of this type of deposit
All significant quartz-adularia gold deposits occur within the southern area and include Gold Mountain (Marysvale District), Stateline, Escalante, and possibly Goldstrike (although the Goldstrike District is generally considered to be of the sediment-hosted gold deposit type). Prospects in the southern area include Gold Springs, Modena, Fortuna, Rob Roy, and the Belknap area of the Marysvale District, and in the north-central area, Erickson (Indian Canyon), and possibly the Desert Mountain (Coyote Knoll prospect) and Gold Hills districts.
On the numerical estimates made
The presence of a large permissive tract, coupled with the fact that about 70 percent of the area is covered by alluvium, led the team to believe that the chances for the existence of undiscovered deposits are fairly good. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 2, 4, 6, and 9 or more deposits consistent with the composite grade and tonnage model.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986, Grade and 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.
Mosier, D.L., Berger, B.R., and Singer, D.A., 1986, Grade and tonnage model of Sado epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 155-157.