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. Whereas, the permissive tract for middle Tertiary polymetallic replacement deposits is a broad area where middle Tertiary intrusive rocks are coincident with Paleozoic carbonate units. The area delineated for the late Tertiary (younger than about 24 Ma) is more restricted, as igneous rocks this young generally do not occur to the east of the Rio Grande rift. Data were taken from the State map (Tweto, 1979) and from Mutschler and others (1988).
Important examples of this type of deposit
No unequivocal late Tertiary replacement deposits are known, although replacement ores are present at Redwell Basin, related to the Mt. Emmons-Redwell Climax Mo system, and at least part of the significant Rico district may consist of replacement ores related to Pliocene magmatism.
On the numerical estimates made
The potential for late Tertiary polymetallic replacement deposits is related primarily to suspected replacement ores associated with and below the large polymetallic vein districts in the western San Juan Mountains, such as Telluride and Red Mountain-Sneffels (Fisher, 1990). In addition, replacement ores could be associated with late Tertiary Climax-type molybdenite deposits. The group evaluated seven target areas individually, and then gave consideration to covered areas. Although the absolute number of late Tertiary magmatic systems is fewer than those with middle Tertiary ages, they are less likely to be eroded deeply enough to have destroyed related replacement deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 4 or more districts consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986).
Fisher, F.S., 1990, Gold deposits in the Sneffels-Telluride and Camp Bird mining districts, San Juan Mountains, Colorado, in Shawe, D.R., and Ashley, R.P., eds., Geology and resources of gold in the United States: U.S. Geological Survey Bulletin 1857, Chap. F, p. F12-F18.
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.
Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Map, scale 1:500,000.