Explained by Steve Ludington
On the choice of deposit models
In assessing this deposit type in New Mexico, we knew that the copper prospects about which we had information were somewhat different, especially in igneous rock chemistry, than those defined by the general porphyry copper model. 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. 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 in New Mexico. There there are several known porphyry copper prospects related to alkaline rocks, but their gold grades are not known.
On the delineation of permissive tracts
The permissive area in New Mexico is based on the presence of an alkaline rock province that stretches north from Trans-Pecos Texas, through eastern New Mexico, and into southern Colorado. This tract contains numerous prospects that contain evidence for middle Tertiary alkaline copper-gold porphyry deposits.
Important examples of this type of deposit
New Mexico has eighteen districts that bear characteristics of porphyry Cu-Au deposits. They are Folsom, Laughlin, Cimmaroncito, Elizabethtown-Baldy, Cerrillos, Old Placer, New Placer, Gallinas, Tecolote, Jicarilla, White Oaks, Veracruz, Nogal, Schelerville, Capitan, Jarilla (Orogrande), Organ, and Cornudas. In southern New Mexico, in addition to the SOL prospect (Seager, 1981), evidence for middle Tertiary alkaline copper-gold porphyry deposits is present
at the Vera Cruz mine and at the Mudpuppy-Waterdog prospect, both in the Nogal district (Ryberg, 1991; Fulp and Woodward, 1991). Gold- and copper-bearing skarns and disseminated copper, as well as potassic alteration in the Orogrande district, are indicative of possible porphyry copper deposits (Beane and others, 1975). Further north in New Mexico, the deposit described by Wargo (1964) at Cerrillos matches the models of both those who emphasize alkaline magma chemistry and those who emphasize shallow emplacement to explain gold-rich porphyry copper deposits.
On the numerical estimates made
Although there are no demonstrable known deposits, and the overall level of exploration is judged to be relatively low, considerable information about mineralized districts is available, including that several have been drilled with encouraging results, although details of the exploration have never been made public. The team evaluated eighteen districts individually, then extrapolated those judgments to the entire permissive area, allowing for the intensity of exploration and the amount of area covered by thin surficial deposits, which was judged to be about 20 percent. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 3, 5, 9, 15, and 20 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.
Beane, R.E., Jaramillo C., L.E., and Bloom, M.S., 1975, Geology and base metal mineralization of the southern Jarilla Mountains, Otero County, New Mexico, in Seager, W.R., Clemons, R.E., and Callender, J.F., eds., Guidebook to the Las Cruces country: New Mexico Geological Society, Twenty-second Annual Field Conference, p. 151-156.
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.
Fulp, M.S., and Woodward, L.A., 1991, The Mudpuppy-Waterdog prospect, an alkalic copper-gold porphyry system in the Nogal-Bonito mining district, Lincoln County, New Mexico, in Barker, J.M., Kues, B.S., Austin, G.S., and Lucas, S.G., eds., Geology of the Sierra Blanca, Sacramento and Capitan Ranges, New Mexico: Guidebook of the New Mexico Geological Society, Forty-second Annual Field Conference, October 9-12, 1991, p. 327-328.
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.
Ryberg, G.E., 1991, Geology of the Vera Cruz mine and breccia pipe, Lincoln County, New Mexico, in Barker, J.M., Kues, B.S., Austin, G.S., and Lucas, S.G., eds., Geology of the Sierra Blanca, Sacramento and Capitan Ranges, New Mexico: Guidebook of the New Mexico Geological Society, Forty-second Annual Field Conference, October 9-12, 1991, p. 329-332.
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 Report 1989–22, 86 p.
Seager, W.R., 1981, Geology of Organ Mountains and southern San Andres Mountains, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Memoir 36, 97 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.
Wargo, J.G., 1964, Geology of a disseminated copper deposit near Cerrillos, New Mexico: Economic Geology, v. 59, no. 8, p. 1525-1538.