Explained by Alan R. Wallace, Virginia McLemore, and Steve Ludington
On the choice of deposit models
The alkaline Au-Te model was chosen because of the presence of middle Tertiary alkaline igneous rocks with associated Au-Ag-(Te) vein deposits. The descriptive model of Cox and Bagby (1986) and the grade-tonnage and target area model of Bliss and others (1992) were deemed to be the most appropriate models available for this class of deposits. The model is roughly comparable to the deposits termed “alkaline rock-related, gold-only epithermal systems” by Mutschler and Mooney (1994), although we note that many of these deposits are not epithermal using classic definitions. These deposits are also described by McLemore (1991).
On the delineation of permissive tracts
Some of the well-studied examples (Cripple Creek and Boulder County, Colorado and Vatukoula, Fiji) are characterized by great abundance of precious-metal tellurides and the presence of vanadium-bearing mica (roscoelite) in the alteration assemblage. These features are sufficient to characterize deposits, but, because they are easily destroyed by oxidation and often not noted or overlooked when mining is by bulk methods, they were not useful in tract delineation.
The assessment team decided that the most useful general rule for delineating permissive areas for alkaline Au-Te deposits is the same as that developed for porphyry copper-gold deposits in the southern and central Rocky Mountains; areas that are the site of middle Tertiary alkaline igneous activity, evidenced by mesozonal and epizonal intrusions and minor volcanic rocks, are permissive for the occurrence of alkaline Au-Te deposits. Alkaline rocks of this age are part of a sinuous north-trending belt that extends from the Big Bend area of Texas into central Colorado, in the easternmost part of the Rocky Mountains, along the margin of the Great Plains. The tract includes large areas between known alkaline igneous centers that show no direct evidence of concealed intrusions. If these areas were excluded from the tract, the apparent density of undiscovered deposits would be substantially higher.
Important examples of this type of deposit
There are at least 5 known deposits: Ortiz (Maynard and others, 1990), Carache Canyon, Lukas Canyon, White Oaks, and the Nogal district (Fulp and Woodward, 1991), which had formerly been called Creede-type epithermal (Mosier, Sato, and Singer, 1986; Mosier and others, 1986). In total, there are eighteen areas that bear characteristics of alkaline gold deposits. They are Folsom, Laughlin, Cimarroncito, Elizabethtown-Baldy, Cerrillos, Old Placer, New Placer, Gallinas, Tecolote, Jicarilla, White Oaks, Veracruz, Nogal, Schelerville, Capitan, Jarilla (Orogrande), Organ, and Cornudas.
On the numerical estimates made
The team evaluated eighteen alkaline igneous centers in New Mexico individually, and 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. A relatively small amount of erosion has left many areas in this tract as attractive targets. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2, 5, 8, 12, and 20 or more districts consistent with the grade and tonnage model of Bliss and others (1992).
Bliss, J.D., Sutphin, D.M., Mosier, D.L., and Allen, M.S., 1992, Grade-tonnage and target-area models of Au-Ag-Te veins associated with alkaline rocks: U.S. Geological Survey Open-File Report 92-208, 15 p.
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.
McLemore, V.T., 1991, Base- and precious-metal deposits in Lincoln and Otero counties, 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. 305–309.
Maynard, S.R., Nelson, C.J., Martin, K.W., and Schultz, J.L., 1990, Geology and gold mineralization of the Ortiz Mountains, Santa Fe County, New Mexico: Mining Engineering, v. 42, no. 8, p. 1007-1011.
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.
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.
Mutschler, F.E., and Mooney, T.C., in press, Precious metal deposits related to alkaline igneous rocks—provisional classification, grade-tonnage data, and exploration frontiers: IAGOD symposium.