Explained by Steve Ludington
On the choice of deposit models
Polymetallic replacement deposits consist of hydrothermal, epigenetic, Ag, Pb, Zn, and Cu sulfide minerals in massive lenses, pipes, and veins in limestone, dolomite, or other reactive rocks near contacts with intrusions. Colorado contains some of the classic examples of polymetallic replacement deposits.
On the delineation of permissive tracts
Polymetallic replacement deposits form where intermediate to felsic intrusive rocks, which are the principal sources of the metals, are emplaced into or near carbonate rocks, which, in Colorado, are principally of Paleozoic age. The permissive tract for Laramide polymetallic replacement deposits is that broad area where Laramide intrusive rocks are coincident with Paleozoic carbonate units. Data were taken from the State map (Tweto, 1979) and from Mutschler and others (1988). This tract generally coincides with the northeast-trending Colorado mineral belt.
Important examples of this type of deposit
Laramide polymetallic replacement deposits are relatively rare, perhaps due to the relatively great depth of erosion and small area of carbonate rock outcrops; many Laramide intrusions in Colorado are emplaced into Proterozoic basement. There is only one deposit, Aspen, which is a significant exception to the generalization, as it produced more than 3,000 metric tons of silver (Stegen and others, 1990). Other small mineralized areas exist at Fulford, Lenado, and La Plata.
On the numerical estimates made
The relative dearth of known deposits and prospects and the small area of permissive outcrops produced a relatively small assessment. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986).
Mosier, D.L., Morris, H.T., and Singer, D.A., 1986, Grade and tonnage model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 101-104.
Mutschler, R.E., Larson, E.E., and Bruce, R.M., 1988, Laramide and younger magmatism in Colorado — New petrologic and tectonic variations on old themes: Golden, Colorado School of Mines Quarterly, v. 82, p. 1-47.
Stegen, R.J., Beaty, D.W., and Thompson, T.B., 1990, The origin of the Ag-Pb-Zn-Ba deposits at Aspen, Colorado, based on geologic and geochemical studies of the Smuggler orebody, in Beaty, D.W., Landis, G.W., and Thompson, T.B., eds., Carbonate-hosted sulfide deposits of the central Colorado mineral belt: Economic Geology Monograph 7, p. 266-300.
Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Map, scale 1:500,000.