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 middle Tertiary polymetallic replacement deposits is that broad area where middle Tertiary intrusive rocks are coincident with Paleozoic carbonate units. Data were taken from the State map (Tweto, 1979), and from Mutschler and others (1988). The area assigned to the middle Tertiary (older than about 24 Ma) is the largest of the three polymetallic replacement tracts in Colorado.
Important examples of this type of deposit
Some of the largest mineral deposits in Colorado are polymetallic replacement deposits that formed during middle Tertiary time. Included in this group are the world-class deposits at Leadville and Gilman (Beaty and others, 1990), somewhat smaller but nevertheless significant deposits in the Alma, Tenmile, Tarryall Creek, and the Monarch-Garfield districts, and numerous prospected areas that contain polymetallic replacement occurrences. Leadville has produced ore nearly continuously for more than 130 years, and significant resources surely remain. Several districts also contain related polymetallic vein deposits that were mined along with the replacement deposits.
On the numerical estimates made
Much of Colorado has been heavily prospected for more than 100 years, and the chances of there being numerous exposed undiscovered districts are judged to be negligible. The most favorable place to explore for further resources is near the known districts and prospects. However, we identified eight areas that could, with further exploration, become districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986). In addition to individual evaluation of those eight areas, and consideration of covered areas, the team was guided by our belief that there is at least a 50 percent chance that there is at least one more undiscovered deposit in Colorado. The deposit at Gilman was cited as an example of locally fortuitous erosion that exposed an otherwise blind ore body. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 1, 2, 3, and 4 or more districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986).
Beaty, D.W., Landis, G.P., and Thompson, T.B., eds., 1990, Carbonate-hosted sulfide deposits of the central Colorado mineral belt: Economic Geology Monograph 7, 424 p.
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: Colorado School of Mines Quarterly, v. 82, p. 1-47.
Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Map, scale 1:500,000.