Explained by David Frishman
On the choice of deposit models
Zinc-lead skarns occur in Montana, though none are known to exist in Wyoming. Favorable environments for their occurrence exist in both States because sedimentary carbonate rocks are present adjacent to granitic intrusions. Zinc-lead skarns are responsible for an insignificant proportion of the zinc and lead produced in Montana. Polymetallic vein and replacement deposits containing Zn and Pb also occur in close association with Cretaceous or Tertiary intrusive rocks in many mining districts such as the Barker, Elkhorn, Hecla, and Castle Mountain districts, and skarns could occur in these environments.
On the delineation of permissive tracts
We used the maps of Ross and others (1955) and Love and Christiansen (1985) to delineate areas where Cretaceous or Tertiary intrusive rocks were emplaced into or were in close proximity to carbonate rocks. Precambrian (Belt Supergroup) and Phanerozoic carbonate rocks were considered separately. The permissive tract for skarn Zn-Pb deposits is identical to the corresponding polymetallic replacement tract, i.e., within the porphyry copper permissive tract where carbonate rocks are present, either at the surface or at a shallow depth. Areas that make up the tract are located in northwestern, southwestern, and south-central Montana and another area extends southeastward from south of Livingston into northwestern Wyoming.
Important examples of this type of deposit
We know of no important skarn Zn-Pb deposits in either Montana or Wyoming. Very small skarn Pb deposits (the Mary and Edna mine and the McKinley mine) occur between the Hughsville stock and limestones of the Madison Group in the Barker (Hughsville) mining district in Cascade and Judith Basin Counties, Montana (Witkind, 1973). These deposits, probably between about 45 and 51 Ma in age, contained an insignificant proportion of the 20,000 metric tons of lead, 8,000 metric tons of zinc, and 80 metric tons of silver produced from the district; almost all the production from the district was from polymetallic vein deposits.
On the numerical estimates made
Probably the best prospective area for skarn Zn-Pb deposits is in carbonate terranes near the Pioneer batholith, which is similar to the Boulder Batholith, and is the host for, or genetically related to, many varied base-metal deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 2, 4, and 6 or more districts consistent with the grade and tonnage model for skarn Zn-Pb deposits (Mosier, 1986).
Cox, D.P., 1986, Descriptive model or Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90.
Love, J.D., and Christiansen, A.C., 1985, Geologic map of Wyoming: U.S. Geological Survey, scale 1:500,000.
Mosier, D.L., 1986, Grade and tonnage model of Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
Ross, C.P., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey . scale 1:500,000.
Witkind, I.J., 1973, Igneous rocks and related mineral deposits of the Barker quadrangle, Little Belt Mountains, Montana: U.S. Geological Survey Professional Paper 752, 58 p.