Explained by 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 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. 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.
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. The tract also contains large areas that might conceal alkaline intrusive rocks, but that do not display any direct evidence of concealed alkaline centers. 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.
Important examples of this type of deposit
The permissive area includes the giant Cripple Creek deposit, hosted in Oligocene mafic alkaline volcanic and intrusive rocks, that is a continuing exploration target where reserves continue to be discovered (Thompson and others, 1985). Other alkaline centers, like those in the Rosita Hills and at Spanish Peaks, have associated precious metal prospects and deposits, but have been poorly explored.
On the numerical estimates made
Although the exposed alkaline rocks are not extensive, the general lack of exploration in some of them increases the possibility of finding an undiscovered deposit. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more districts consistent with the grade and tonnage model of Bliss and others (1992) (Mark3 index 80).
In addition, three small mineralized areas in the San Juan Mountains — Beartown, Cave Basin, and early mineralization at Lake City — are reported to contain telluride minerals (Van Loenen and others, in press). The team judged that there was too little information about the associated igneous rocks and tectonic setting to delineate a permissive area or make an estimate of numbers of deposits.
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 alkalic rocks: U.S. Geological Survey Open-File Report 92-208, 15 p.
Cox, D.P., and Bagby, W.C., 1986, Descriptive model of Au-Ag-Te veins in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 124.
Van Loenen, R.E., Nash, J.T., Foley, N. K., and Gibbons, A.B., in press, Locatable mineral resources,in Van Loenen, R.E. and Gibbons, A.B., eds., Mineral resource potential and geology of the San Juan National Forest, Colorado: U.S. Geological Survey Bulletin, p. ?.
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.
Thompson, T.B., Trippel, A.D., and Dwelley, P.C., 1985, Mineralized veins and breccias of the Cripple Creek district, Colorado: Economic Geology, v. 80, no.6, p. 1669-1688.