Explained by Steve Ludington
On the choice of deposit models
Although Colorado contains the namesake for Creede-type epithermal vein deposits, and many of the examples (Mosier and others, 1986a), recent reexamination of these Colorado deposits suggests that most (though not all) of them may be best considered to be polymetallic vein deposits (Cox, 1986) emplaced in a volcanic host. Many deposits in the western San Juan Mountains (Animas, Eureka, Ophir, Red Mountain, Sneffels, and Telluride districts) were formed 5–15 m.y. after the cessation of volcanism, were emplaced at paleodepths as great as 2 kilometers or more, contain abundant base metals and minor amounts of W, Bi, and Mo, and are associated at depth with skarn mineralization. As with other Tertiary vein deposits in Colorado, these volcanic-hosted districts do not conform to the polymetallic vein grade and tonnage models of Cox (1986) and accordingly were not assessed, despite a feeling on the part of the team that there is significant potential for undiscovered deposits of this type in Colorado.
Many other volcanic-hosted quartz-adularia epithermal vein deposits, prospects, and occurrences are known in Colorado. Although production data for these deposits are largely unavailable, the team's general knowledge of them suggests that the deposits have a relatively lower Ag:Au ratio and few base metals, fitting more closely the characteristics of the Comstock (Mosier, Singer, and Berger, 1986) and Sado (Mosier, Berger, and Singer, 1986) epithermal vein deposits. As a result, the team assessed the Colorado tract with a composite model of Comstock and Sado vein deposits (Mark3 index 25).
On the delineation of permissive tracts
The permissive tract for quartz-adularia deposits in Colorado encompasses volcanic fields of felsic to intermediate composition. The largest such terrane is the San Juan volcanic field, which hosts most of the known volcanic-hosted quartz-adularia vein deposits in the State. Virtually any part of a volcanic field such as the San Juan Mountains is permissive for the occurrence of this type of deposit, so long as there was concurrent magmatic activity to provide metals and drive the hydrothermal systems and extensional faulting to provide a path for the fluids.
Important examples of this type of deposit
Creede is the largest deposit in the tract, although it contains proportionately more base metals than other deposits in the tract. Lake City (Slack, 1980) and Crystal Hill (Pansze, 1987) are other significant deposits. Other known prospects include Equity, Bondholder, San Luis Hills, Platoro, Stunner, Axtell, Crater Creek, Lincoln Gulch, and Independence.
On the numerical estimates made
The assessment team made separate assessments for nine districts in the State that contain known quartz-adularia deposits, prospects, and occurrences, and the total expected value obtained was about 1.3 deposits consistent with the combined Comstock-Sado epithermal vein grade and tonnage model (Mark3 index 25). The team also believed that large volcanic areas without known prospects within the San Juan volcanic field could host deposits, due to the interlayered piles of volcanic rocks that could conceal older deposits. The general feeling of the team was that there was a high degree of certainty that there is at least one more deposit out there somewhere. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 3, 5, 7, and 10 or more districts consistent with the combined Comstock-Sado epithermal vein grade and tonnage model.
Cox, D.P., 1986, Descriptive model of polymetallic veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 125.
Mosier, D.L., Sato, T., and Singer, D.A., 1986, Grade and tonnage model of Creede epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 146-149.
Mosier, D.L., Singer, D.A., Sato, T., and Page N.J, 1986, Relationship of grade, tonnage, and basement lithology in volcanic-hosted epithermal precious- and base-metal quartz-adularia-type districts: Mining Geology, v. 36, p. 245-264.
Pansze, Arthur J., 1987, Geologic sketch of the Crystal Hill breccia pipe, Saguache County, Colorado: DREGS field trip guide.
Slack, J.F., 1980, Multistage vein ores of the Lake City district, western San Juan Mountains, Colorado: Economic Geology, v. 75, no. 7, p. 963–991.