Explained by Douglas B. Stoeser
On the choice of deposit models
Several belts of Tertiary calc-alkaline igneous rocks, including plutons and associated volcanic rocks, with related porphyry copper, polymetallic vein and replacement, and skarn deposits are exposed in western Utah. The North American subset of the worldwide grade and tonnage model (Mark3 index 81) was used for the quantitative assessment.
On the delineation of permissive tracts
Two permissive tracts were recognized for porphyry copper deposits in Utah. The basis for delineating these tracts are the occurrence of Tertiary calc-alkaline intermediate to silicic intrusive rocks, associated volcanic rocks, and intrusive-related mineral deposits (porphyry copper, polymetallic vein and replacement, and skarn). The two tracts are defined by east-west-trending Tertiary dominantly calc-alkaline magmatic belts in the western part of the State, and they contain most of the significant base and precious metal deposits of Utah (Shawe and Stewart, 1976; Seedorff, 1991). The southern permissive tract is defined by the combination of the Pioche-Marysvale (or Wah Wah-Tushar) and Delamar-Iron Springs mineral belts. Ages of emplacement for the volcanic and intrusive rocks are mainly 34 to 18 Ma. The tract contains intermediate to silicic volcanic rocks, which are primarily associated with ash-flow calderas (Steven and Rowley, 1984; Best, 1989; Stoeser, 1993). The erosion level is such that extensive areas of volcanic rocks are still preserved, although more deeply eroded plutons are exposed near Milford. The southern permissive tract is more favorable for epithermal-type deposits, with possible associated intrusive-related systems at depths below 1 kilometer, the depth to which this assessment was conducted. It should be noted that the tract is approximately 70 percent covered by alluvium. Aeromagnetic surveys were used to define areas of high magnetics that might indicate intrusions at depth. Outside of the three permissive terranes defined above, a few minor base and precious metal occurrences associated with small intrusions are located in the nearby Colorado Plateau, but these areas were considered to have insignificant mineral potential for intrusive-related deposits and were not considered in the estimates for this tract.
Important examples of this type of deposit
Utah contains one of the largest and richest porphyry copper deposits in the world at Bingham Canyon. This, however, is the only porphyry copper deposit to have been exploited in the State. Three porphyry copper prospects are known in the southern area (Cactus, OK, and Rocky). It should be noted that the Cactus and OK prospects are pipe-like bodies and the classification of them as porphyry copper type is questionable.
On the numerical estimates made
Estimates of numbers of undiscovered deposits took into account the distribution of known deposits and prospects, beliefs about the extent and efficiency of past mineral exploration, and the large proportion of the permissive tract that is covered by alluvium. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 2, 3, and 4 deposits consistent with the grade and tonnage model for the North American subset of porphyry copper deposits.
Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., McKee, E.H., and Noble, D.C., 1989, Excursion 3A—Eocene through Miocene volcanism in the Great Basin of the western United States, in Chapin, C.E., and Zidek, Jiri, eds., Field excursion to volcanic terranes in the western United States, Volume II: Cascades and Intermountain West: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 91-133.
Seedorff, Eric, 1991, Magmatism, extension, and ore deposits 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: Reno, Symposium proceedings, Geological Society of Nevada, 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.
Steven, T.A., and Rowley, P.D., 1984, Calderas of the Marysvale volcanic field, west central Utah: Journal of Geophysical Research, v. 89, p. 8751-8764.
Stoeser, D.B., 1993, Tertiary calderas and regional extension of the east-central part of the Tintic-Deep Creek Mineral Belt, eastern Great Basin, in Scott, R.W., Jr., Detra, T.S., and Berger, B.R., eds., Advances related to United States and international mineral resources—Developing frameworks and exploration technologies: U.S. Geological Survey Bulletin 2039, Chap. A, p. 5-23.