Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Copper skarns are an end-member of a spectrum of skarn deposit types that are variously copper-, zinc-lead-, or iron-rich. Skarns are metallic sulfide and oxide replacement deposits that occur in carbonate host lithologies adjacent to plutonic bodies with metal-bearing hydrothermal systems. The deposits are associated with shallow intermediate plutons, commonly those that are host to porphyry-style mineralization. The carbonate bodies that host the mineralization need not be regionally extensive, but can be small local bodies that are widely scattered essentially throughout the map area.
On the delineation of permissive tracts
The tract was delineated by excluding areas without significant carbonate rocks from the corresponding porphyry Cu tract. Permissive sedimentary packages include the Paleozoic sequence of northeastern Washington, the middle Belt carbonate units of the Belt Supergroup in northern Idaho, and the Paleozoic sequence of south-central Idaho (Bond, 1978; Stoffel and others, 1991).
Important examples of this type of deposit
The Empire mine near Mackay, Idaho is the only copper skarn deposit known in the tract. This deposit is large and rich, compared with the grade and tonnage distributions of Jones and Menzie (1986). Significant gold grades (1.6 g/metric ton) occur in the Empire deposit. The deposit is localized in a Pennsylvanian limestone around an Eocene granodiorite stock on the east side of the Idaho batholith. A small skarn Zn-Pb deposit and a small skarn Fe deposit occur on the opposite side of the stock from the Empire deposit.
On the numerical estimates made
The occurrence of only one known deposit of this type and the lack of known porphyry copper deposits in this tract lead the team to give a very low estimate. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more copper skarn deposits consistent with the grade and tonnage model of Jones and Menzie (1986).
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
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.
Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, S.W., Korosec, M.A., and Bunning, B.B., 1991, Geologic map of Washington-Northeast quadrant: Washington Division of Geology and Earth Resources, Geologic Map GM-39, scale 1:250,000.