Explained by Steve Ludington
On the choice of deposit models
The alkaline Au-Te model (Cox and Bagby, 1986; Bliss and others, 1992) was chosen because of the presence of Laramide 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. Deposits of Laramide age are distinguished from middle Tertiary deposits of this type. By the term “Laramide”, we include deposits as young as 45 Ma.
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 Rocky Mountains; areas that are the site of Laramide alkaline igneous activity, evidenced by mesozonal and epizonal intrusions and minor volcanic rocks, are permissive for the occurrence of Laramide alkaline Au-Te deposits. Alkaline rocks of Laramide age evidence permissive areas at the northeastern and southwestern ends of the Colorado mineral belt.
Important examples of this type of deposit
The northeastern area is the site of many important districts that produced large amounts of gold from veins related to Laramide-age alkaline intrusions. The most important of these are Gold Hill-Sugarloaf, Jamestown, Magnolia, and Ward, all in Boulder County (Saunders, 1991), and part of Central City (Wallace, 1989), in Clear Creek County. To the southwest, precious-metal deposits in the La Plata Mountains are closely associated with a Laramide alkaline igneous complex (Werle and others, 1984).
On the numerical estimates made
The permissive area is relatively deeply eroded, well exposed, and well explored, and the team thought it unlikely that there are any undiscovered deposits, although important amounts of metal remain to be discovered in known districts. However, the presence of Laramide alkaline igneous rocks and related deposits and prospects makes the area permissive. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 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 alkalic rocks: U.S. Geological Survey Open-File Report 92-208, 15 p.
Cox, D.C., 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.
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.
Saunders, J.A., 1991, Gold deposits of the Boulder County gold district, Colorado: U.S. Geological Survey Bulletin 1857-H, p. 37-48.
Wallace, A.R., 1989, Gold in the Central City mining district, Colorado, in Shawe, D.R., Ashley, R.P., and Carter, L.M.H., eds., Geology and resources of gold in the United States: U.S. Geological Survey Bulletin 1857-C, p. C38-C47.
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.