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. Favorable areas are considered to be those which have known deposits or prospects of this type of deposit.
On the delineation of permissive tracts
The tract was delineated by excluding areas without significant carbonate rocks from the corresponding porphyry Cu tract, and consists of the Wallowa, Baker, and Huntington terranes of northeastern Oregon, west-central Idaho, and southeasternmost Washington (Brooks and Vallier, 1978; Walker and MacLeod, 1991). Plutonic rocks of Jurassic and Cretaceous(?) age are widely scattered through these terranes, and where large enough, these were excluded from the permissive tract. Small, irregular carbonate horizons occur throughout all three structurally complex terranes such that carbonate rocks could be present in the subsurface of any part of these terranes. The Izee terrane of north-central Oregon lacks carbonate rocks and was excluded from the tract.
Important examples of this type of deposit
No significant deposits are known from this 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 deposits that are comparable in grade and tonnage to the gold skarn grade and tonnage model (truncated) of Theodore and others (1991).
Brooks, H.C., and Vallier, T.L., 1978, Mesozoic rocks and tectonic evolution of eastern Oregon and western Idaho, in Howell, D.G., and McDougall, K.A., eds., Mesozoic paleogeography of the western United States: Pacific section, Society of Economic Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium 2, p. 133-146.
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.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.