National mineral assessment tract CR12 (Porphyry Cu-Au)

Tract CR12
Geographic region Central and Southern Rocky Mountains
Tract area 12,900sq km
Deposit type Porphyry Cu-Au
Deposit age Middle Tertiary

Deposit model

Model code 20c
Model type descriptive
Title Descriptive model of porphyry Cu-Au
Authors Dennis P. Cox


Confidence Number of
90% 0
50% 0
10% 0
5% 0
1% 1

P(none): 0.99

Estimators: Ludington, Wallace, Nash, Berger, Spanski


Explained by Steve Ludington
On the choice of deposit models
Singer and Cox (1986) presented a grade and tonnage model for porphyry copper-gold deposits, and Cox (1986) published an accompanying descriptive model that emphasized coeval volcanic rocks, an oxidized ore fluid, and the presence of magnetite in alteration assemblages. In a later paper (Cox and Singer, 1992) that further analyzes a subset of the deposits included in the grade and tonnage model, a relatively shallow depth of emplacement is postulated as the factor primarily responsible for geochemical differences between the gold-rich deposits and other porphyry copper deposits. Ratios among Cu, Mo, and Au remained the basis for classification.
Barr and others (1976) apparently originated the idea that porphyry copper deposits associated with alkaline rocks are different. Their data suggest gold enrichment, but they took no note of it. Later, Mutschler and others (1985, 1991) speculated that porphyry copper deposits associated with alkaline rocks would be rich in precious metals, but presented little numerical data. They, like Cox and Singer (1992), also emphasized the role of an oxidized ore fluid. McMillan and Pantaleyev (1988) acknowledge alkaline as a subdivision of their volcanic type of porphyry copper deposit. Lowell (1989) states his opinion that gold content of porphyry copper deposits can be grouped according to their whole-rock geochemistry.
While Singer and Cox find both calc-alkaline and alkaline magma chemistries in their class of gold-rich porphyry copper deposits, all 6 of the deposits that they classified as alkaline, without exception, displayed high Au:Mo ratios. This observation leads us to believe that porphyry deposits associated with alkaline rocks can be appropriately represented by the grade and tonnage model published by Singer and Cox (1986). Grade and tonnage information in Schroeter and others (1989) seems to confirm that, whereas not all gold-rich porphyry copper deposits are alkaline, all alkaline porphyry copper deposits seem to be gold-rich. An important exception may be the Laramide Allard stock, in southwestern Colorado (Werle and others, 1984).
A belt of alkaline rocks, which contains known porphyry Cu-Au deposits and prospects in New Mexico, extends north into Colorado. Although no porphyry copper deposits or prospects of this type and age are known in Colorado, the team judged it necessary to consider the potential for undiscovered deposits. We note in passing that the Canadian examples of alkaline porphyry copper deposits that are described in the literature formed in island arcs, and not on the continent, so some caution is exercised in application of this model to Colorado. There are several known porphyry copper prospects related to alkaline rocks in nearby New Mexico, but their gold grades are not known.
On the delineation of permissive tracts
The permissive tract is based on the sufficient, but not necessary evidence of a belt of middle Tertiary alkaline rocks in southern Colorado that encompasses the Cripple Creek and Rosita Hills areas. The tract is contiguous with a belt of alkaline rocks in New Mexico that contains known porphyry Cu-Au deposits and prospects.
Important examples of this type of deposit
No deposits or prospects of this deposit type are known.
On the numerical estimates made
Although gold-rich deposits are known in this tract, the team knew of little evidence for copper-rich deposits associated with alkaline rocks. Therefore, the estimate is based largely on the presence of alkaline igneous rocks similar to those in New Mexico that have associated copper-gold deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the porphyry copper-gold grade and tonnage model (Singer and Cox, 1986).
Barr, D.A., Fox, P.E., Northcote, K.E., and Preto, V.A., 1976, The alkaline suite porphyry deposits—A summary: Special volume of the Canadian Institution for Mining and Metallurgy, v. 15, p. 359–367.
Cox, D.P., 1986, Descriptive model of porphyry Cu-Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 110.
Cox, D.P., and Singer, D.A., 1992, Gold—Distribution of gold in porphyry copper deposits, in DeYoung, J.H., Jr., and Hammarstrom, J.M., eds., Contributions to commodity geology research: U.S. Geological Survey Bulletin 1877, p. C1–C14.
Lowell, J.D., 1989, Gold mineralization in porphyry copper deposits discussed: Mining Engineering, v. 41, no. 4, p. 227–231.
McMillan, W.J., and Pantaleyev, A., 1988, Porphyry copper deposits, in Roberts, R.G., and Sheahan, P.A., eds., Ore deposit models: Geoscience Canada Reprint Series 3, p. 45–58.
Mutschler, F.E., Griffen, M.E., Stevens, D.S., and Shannon, S.S. Jr., 1985, Precious metal deposits related to alkaline rocks in the North American Cordillera; an interpretive review: Transactions of the Geological Society of South Africa, v. 88, p. 355–377.
Mutschler, F.E., Mooney, T.C., and Johnson, D.C., 1991, Precious metal deposits related to alkaline igneous rocks—A space-time trip through the Cordillera: Mining Engineering, v. 43, no. 3, p. 304–309.
Schroeter, T.G., Lund, C., and Carter, G., 1989, Gold production and reserves in British Columbia: B.C. Ministry of Energy, Mines and Petroleum Resources, Open File 1989–22, 86 p.
Singer, D.A., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu-Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 110-114.
Werle, J.L., Ikramuddin, M., and Mutschler, F.E., 1984, Allard stock, La Plata Mountains, Colorado — An alkaline rock-hosted porphyry copper-precious metal deposit: Canadian Journal of Earth Sciences, v. 21, p. 630-641.

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