Explained by Douglas B. Stoeser
On the choice of deposit models
The model used here follows Einaudi and others (1981); Cox (1986); and Cox and Theodore (1986) who distinguish two types of copper skarns, those related to intrusions that host porphyry copper deposits and those related to other intrusions. The latter class of copper skarns is relatively small, typically containing 1–50 million metric tons of ore, whereas, porphyry-related copper skarns commonly contain 50–600 million metric tons of ore (Einaudi and others, 1981). Many of the world's largest porphyry-related copper skarns occur in the southwestern United States. Porphyry-related copper skarns are associated with Mesozoic to Tertiary calc-alkaline granodiorite to granitic porphyry-textured hypabyssal stocks emplaced in continental-margin orogenic belts (Einaudi and others, 1981).
On the delineation of permissive tracts
Because western Utah contains calc-alkaline magmatic belts superimposed over the continental marginal Sevier orogenic belt (Armstrong, 1968; Shawe and Stewart, 1976), the region is favorable for the occurrence of copper skarns. The primary criteria for the occurrence of porphyry-related copper skarn are the presence of porphyry copper deposits, and carbonate host rocks found proximal to the porphyry copper intrusions. Thus, the permissive terranes for porphyry copper, skarn-related deposits are defined where the porphyry copper permissive terranes are underlain by carbonate sedimentary rocks. Because the detailed distribution of carbonate rocks beneath the approximately seventy percent alluvial cover in the area is not known, we used the same permissive tract for skarn-related porphyry copper deposits as for general porphyry copper deposits.
Important examples of this type of deposit
In Utah, there is one significant deposit of this type, the Carr Fork copper skarn associated with the Bingham porphyry copper deposit (Atkinson and Einaudi, 1978; Tooker, 1990; Reid, 1991), and a few more in the Bingham district and in other areas.
On the numerical estimates made
Einaudi and others (1981, p. 341) state that for the southwestern United States: "Large copper skarn deposits are associated with all porphyry copper plutons emplaced in carbonate rocks". Therefore, the estimated number of porphyry-related copper skarns should be approximately the same as for copper porphyry deposits, with perhaps a somewhat lower number allowing for roughly 10–20 percent non-carbonate rocks underlying the permissive terranes. For the 90the, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 2, 5, 6, and 8 or more deposits consistent with the grade and tonnage curve for skarn-related porphyry copper deposits of Cox (1986).
Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: Geological Society of America Bulletin, v. 79, p. 429-458.
Atkinson, W.W., Jr., and Einaudi, M.T., 1978, Skarn formation and mineralization in the contact aureole at Carr Fork, Bingham, Utah: Economic Geology, v. 73, no. 7, p. 1326–1365.
Cox, D.P., 1986, Descriptive model of Cu skarn Deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 82-85.
Cox, D.P., and Theodore, T.G., 1986, Descriptive model of Cu skarn Deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.
Einaudi, M.T., Meinert, L.D., and R.J. Newberry, 1981, Skarn deposits, in Skinner, B.J., ed., Economic Geology Seventy-Fifth Anniversary Volume, 1905–1980: Lancaster, Pennsylvania, Economic Geology Publishing Company, p. 317-392.
Reid, J.E., 1991, Skarn occurrences in Utah and the potential for associated gold mineralization: Utah Geological Survey Contract Report 91-13, 49 p.
Shawe, D.R., and Stewart, J.H., 1976, Ore deposits as related to tectonics and magmatism, Nevada and Utah: Las Vegas, American Institute Mining Engineers Transactions, 1977 Annual Meeting, v. 260, p. 225-232.
Tooker, E.W., 1990, Gold in the Bingham District, Utah, in Shawe, D.R., Ashley, R.P., and Carter, L.M.H., eds., Gold in porphyry copper systems: U.S. Geological Survey Bulletin 1857-E, p. E1-E-16.