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 ideal 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 few 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 Allard stock (Werle and others, 1984).
In southwestern Colorado, the alkaline Allard stock in the La Plata Mountains hosts an incompletely-explored precious metal- and platinum group-rich porphyry copper deposit (Werle and others, 1984). At the northeastern end of the Colorado mineral belt, a field of Laramide-age alkaline intrusions is associated with productive precious-metal deposits, but none of the deposits and prospects closely resemble porphyry copper 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
Based upon the sufficient, but not necessary evidence of the occurrence of alkaline rocks, there are two permissive areas, both subsets of the Laramide general porphyry copper permissive tract. At the northeastern end of the Colorado mineral belt, Laramide-age alkaline intrusions define an area permissive for the occurrence of porphyry copper-gold deposits, and in southwestern Colorado, the deposits in the La Plata Mountains suggest another. Together, they make up the permissive tract.
Important examples of this type of deposit
The La Plata district in southwestern Colorado is the best example of an alkaline porphyry copper deposit; its gold content is not well known, though it does contain anomalous PGE.
On the numerical estimates made
The amount of covered area is roughly equal to that exposed. In general, the level of erosion is relatively deep, and possible porphyry copper-gold systems that may have been present have a high probability of having been eroded away, especially in the northeastern area. Exploration in the La Plata district has revealed a deposit that is similar in size and copper grade to others in the model. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 1, and 2 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: Reprint Series 3, Geoscience Canada, 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.