Explained by Steve Ludington
On the choice of deposit models
In Colorado, middle Tertiary felsic to intermediate intrusive rocks are emplaced into Paleozoic carbonate rocks. The model used here follows Einaudi and others (1981) and Cox and Singer (1986) that distinguish two type of copper skarns, those related to porphyry copper deposits and those not. Porphyry copper deposits are all but absent from Colorado, and we assess here for the latter model.
On the delineation of permissive tracts
Skarn deposits form where intermediate to felsic intrusive rocks, which are the principal sources of the metals, are emplaced into carbonate rocks, which, in Colorado, are principally of Paleozoic age. Therefore, the permissive tract for middle Tertiary copper skarn deposits is that broad area where middle Tertiary intrusive rocks are coincident with Paleozoic carbonate units, as defined by the State map (Tweto, 1979).
Important examples of this type of deposit
No deposits or prospects of this type are known.
On the numerical estimates made
In Colorado, skarns are extremely uncommon, but middle Tertiary igneous rocks and Paleozoic carbonate rocks are widespread and coincident. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the copper skarn model of Jones and Menzie (1986).
Cox, D.P., and Theodore, T.G., 1986, Descriptive 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.
Einaudi, M.T., Meinert, L.D., and Newberry, R.J., 1981, Skarn deposits, in Skinner, B.J., ed., Economic Geology Seventy-Fifth Anniversary Volume, 1905–1980: Lancaster, Pennsylvania, Economic Geology Publishing Company, p. 317-392.
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.
Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Map, scale 1:500,000.