Explained by Steve Ludington
On the choice of deposit models
In Colorado, middle Tertiary felsic to intermediate intrusive rocks are emplaced into Phanerozoic carbonate rocks, and, thus, provide a permissive environment for skarn Zn-Pb deposits. The descriptive model of Cox (1986) and the grade and tonnage model of Mosier (1986) were used for the assessment.
On the delineation of permissive tracts
Skarn deposits may form where intermediate to felsic intrusive rocks, which are the principal sources of the metals, are emplaced into carbonate rocks. Therefore, the permissive tract for middle Tertiary zinc-lead skarn deposits is that broad area where middle Tertiary intrusive rocks are emplaced into Paleozoic carbonate units. Middle Tertiary intrusions are found throughout the mountainous part of Colorado.
Important examples of this type of deposit
No deposits or prospects of this deposit type are known, although polymetallic replacement deposits are relatively common, and skarn minerals have been described from the deep parts of polymetallic vein deposits in the western San Juan Mountains (Mayor and Fisher, 1972).
On the numerical estimates made
In Colorado, skarns are extremely uncommon; the team failed to identify any known deposits or prospects. Nevertheless, largely because middle Tertiary igneous rocks are widespread, there is some possibility for skarn deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the grade and tonnage model of Mosier (1986).
Cox, D.P., 1986, Descriptive 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.
Mayor, J.N., and Fisher, F.S., 1972, Middle Tertiary replacement ore bodies and associated veins in the northwest San Juan Mountains, Colorado: Economic Geology, v. 67, no. 2, p. 214–230.
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.