Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Skarn Au deposits are a subset of a spectrum of skarn 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 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
No significant deposits of this type are known from the tract.
On the numerical estimates made
The lack of known deposits of this type and the lack of known porphyry copper deposits in this tract lead the team to give a very low estimate. Because the grade and tonnage model of Theodore and others (1991) includes some very small deposits, we felt we could estimate more accurately using a model that is truncated to include only those deposits with more than 15,000 metric tons of mineralized rock. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more gold skarn deposits consistent with the grade and tonnage model (truncated) of Theodore and others (1991).
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
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.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns: U.S. Geological Survey Bulletin 1930, 61 p.