Explained by Steve Ludington
On the choice of deposit models
Colorado contains numerous prospects that meet many of the criteria for porphyry copper deposits, but there is uncertainty about how well the overall characteristics of any of the individual mineralized systems fit the descriptive model for porphyry copper deposits. Only one of the prospects, the A-O in northern Colorado, has been formally described in the literature as a porphyry copper deposit (Karimpour and Atkinson, 1983). Nevertheless, there are many prospects, and in addition, many intrusions have a halo of polymetallic vein deposits that might reflect a concealed porphyry copper deposit associated with the intrusive bodies.
On the delineation of permissive tracts
The general rule for delineating permissive areas for porphyry copper deposits of middle Tertiary and younger age is to exclude all areas that show no evidence for intrusive activity of this age. In addition, areas where intrusions are overwhelmingly gabbroic or granitic, or strongly alkaline, are also excluded. Manifestation of intrusive activity may include exposed intrusive terranes, known polymetallic vein, skarn, and(or) replacement deposits, unexposed intrusions, inferred to exist by geophysical or other means, and any other inferred magmatic trends. In Colorado, middle Tertiary intrusive rocks are widespread (Mutschler and others, 1988).
Important examples of this type of deposit
No middle to late Tertiary porphyry copper deposits are known, but fifteen porphyry targets were identified in Colorado, although classification was not always certain. Some of the more prominent include: Timberline Lake, near Leadville, and Whitepine-Tomichi, east of Gunnison, which were partially explored in the 1970s; and Conundrum Creek and East Maroon Creek, southwest of Aspen.
On the numerical estimates made
We judged the area to be about 85 percent exposed and 15 percent covered. The assessment team evaluated the known Colorado prospects individually; then, considering the extent of exploration and the amount of covered area, the team extrapolated those judgments to the entire area. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 8 or more deposits consistent with the worldwide porphyry copper grade and tonnage model (Singer and others, 1986).
Karimpour, M.H., and Atkinson, W.W., Jr., 1983, Petrogenesis of the A.O. porphyry copper complex in Jackson and Grand Counties, northwestern Colorado: Global Tectonics and Metallogeny, v. 2, p. 15-27.
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.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.