Explained by Steve Ludington

On the choice of deposit models

The Colorado mineral belt (Tweto and Sims, 1963) contains numerous Laramide calc-alkaline intrusive rocks. Some of these intrusive rocks have a halo of polymetallic vein deposits that might reflect a concealed porphyry copper deposit associated with the intrusive bodies. The two known prospects exhibit high Cu:Mo ratios, which distinguishes them from stockwork molybdenum deposits and suggests the possibility for porphyry Cu deposits.

On the delineation of permissive tracts

The general rule for delineating permissive areas for porphyry copper deposits of Laramide age is to exclude all areas that show no evidence for Laramide intrusive activity. 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. Laramide-age calc-alkaline rocks were identified using maps by Mutschler and others (1988). Most Laramide-age intrusions in Colorado are found in the Colorado mineral belt.

Important examples of this type of deposit

Colorado contains no known Laramide porphyry copper deposits. Two possible prospects were identified, The Blowout, in the San Juan Mountains near Ouray, and Blodgett Lake, west of Homestake Reservoir in the northern Sawatch Range.

On the numerical estimates made

Erosion exposes many of the intrusive systems in the Colorado mineral belt at a level below which porphyry copper deposits might have been emplaced; there are few known Laramide intrusions. The team judged, however, that it would be misleading to indicate that the presence of such deposits here is impossible. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the worldwide porphyry copper grade and tonnage model (Singer and others, 1986).

References

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.

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.

Tweto, Ogden, and Sims, P.K., 1963, Precambrian ancestry of the Colorado mineral belt: Geological Society of America Bulletin, v. 74, p. 991-1014.