Explained by Douglas B. Stoeser
On the choice of deposit models
Although no hot-spring Au-Ag deposits are known in the State, this type of deposit is commonly related to volcanic activity, evidence for which is abundant in Utah.
On the delineation of permissive tracts
Three areas make up the permissive tract for hot-spring Au-Ag deposits. The two most important areas are defined by east-trending, dominantly calc-alkaline, Tertiary magmatic belts in the western part of the State which contain most of the significant base- and precious-metal deposits of Utah (Shawe and Stewart, 1976; Seedorff, 1991). Two adjacent belts, the Oquirrh-Uinta belt and the Tintic-Deep Creek belt, combine to form the north-central area. The southern area is collectively defined by the Pioche-Marysvale (or Wah Wah-Tushar) and Delamar-Iron Springs belts. The erosional level on the north-central area is deeper than the southern area, such that much of the once-extensive volcanic rocks have been removed, whereas to the south, extensive areas of volcanic rocks are still preserved. Thus, the southern area is more favorable for hot-spring Au-Ag deposits, which form near the paleosurface. In addition to the two areas identified above, a third area in the northwest is also recognized. The northwestern area is underlain by local volcanic and minor intrusive rocks, and is also permissive for hot-spring Au-Ag deposits. The three permissive terranes have approximately 70 percent alluvial cover less than 1 km thick.
Important examples of this type of deposit
There are no known hot-spring Au-Ag deposits in Utah, nor are there any significant prospects.
On the numerical estimates made
Although no hot-spring Au-Ag deposits are known from the area, the presence of felsic volcanic rocks suggests that they could have formed. Deposits may be concealed beneath younger volcanic units or beneath the extensive alluvial cover. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more deposits consistent with the grade and tonnage model for hot-spring Au-Ag deposits of 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 deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.
Seedorff, Eric, 1991, Magmatism, extension, and ore deposition of Eocene to Holocene age in the Great Basin—Mutual effects and preliminary proposed genetic relationships, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and ore deposits of the Great Basin—Symposium proceedings: Reno, Geological Society of Nevada, v. 1, April 1990, p. 133-178.
Shawe, D.R. and Stewart, J.H., 1976, Ore deposits as related to tectonics and magmatism, Nevada and Utah: Transactions of Society of Mining Engineers, v. 260, p. 225-231.