Explained by Leslie J. Cox
On the choice of deposit models
Copper skarns form where carbonate rocks are intruded by felsic plutons. The model considered here is the simple skarn model as exemplified by Oracle Ridge-Marble Peak. Many copper skarns in Arizona are closely related to large porphyry copper systems and are assigned to a different deposit model (porphyry Cu, skarn-related). Skarn deposits associated with intrusions with Cu grades lower than 0.2 percent were referred to this model (Jones and Menzie, 1986; Mark3 index 8).
On the delineation of permissive tracts
The permissive tract was delineated by modifying the tracts for porphyry Cu deposits, and including Late Cretaceous to early Tertiary muscovite-garnet-bearing peraluminous granite and associated pegmatite (unit TKgm, Reynolds, 1988). We then outlined areas within that modified tract that are known to contain limestone and other carbonate rocks. The tract also includes all pertinent known mineral districts.
Important examples of this type of deposit
Oracle Ridge (Marble Peak), in the Santa Catalina Mountains, is the most important copper skarn deposit in Arizona. Other deposits are in the Coyote district in the Baboquivari Mountains, the Johnson Camp district (the Mammoth, Burro, and Republic-Cyprus deposits) in the Little Dragoon Mountains, the Helvetia district (Elgin and Copper Chief deposits) of the northern Santa Rita Mountains and the Middle Pass district.
On the numerical estimates made
In the Middle Pass district alone, there are many small Cu skarn ore bodies, but few would individually make as much as a half million metric tons of ore. We would expect a similar distribution of deposits at depth, however, unexposed deposits of this type are very difficult to explore for. Some estimators felt that 90 percent of the Cu skarns in exposed rocks had been discovered, but at 500 m depth, only 10 percent have been discovered. We arrived at individual probabilities for districts that contain one or more skarn deposits as follows: Middle Pass (0.50), Empire, (0.10), the Turquoise area drilled by Newmont (0.90), the remainder of Turquoise, (0.33), Pima, Redington, Growler, and Apache Pass (0.01 each), and Papago (0.002). These individual probabilities sum to an expected value of about 2 undiscovered deposits in known districts which was used to constrain the lower limit for the 90th percentile estimate. Many more deposits were estimated to occur in the permissive tract at depths between 500 m and 1000 m. For the 90th, 50th, 10th, and 5th percentiles, the team estimated 3, 30, 50, and 60 or more deposits consistent with the grade and tonnage model of Jones and Menzie (1986).
Brant, A.A., 1966, Geophysics in the exploration for Arizona porphyry coppers, in Titley, S.R., and Hicks, C.L., eds., Geology of the porphyry copper deposits: Tucson, University of Arizona Press, p. 189-203.
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.
Einaudi, M.T., Meinert, L.D., and Newberry, R.J., 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.
Jones, G.M., and Menzie, W.D., 1986, Grade and tonnage 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.
Reynolds, S.J., 1988, Geologic map of Arizona: Arizona Geological Survey Map 26, scale 1:1,000,000.
Titley, S.R., and Anthony, E.Y., 1989, Laramide mineral deposits in Arizona in Jenney, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona: Tucson, Arizona Geological Society Digest 17, p. 485-514.