Explained by Donald Grybeck, Thomas D. Light, William J. Keith, Dennis P. Cox, Gregory K. Lee, and Jeffrey D. Phillips.
On the choice of deposit models
Epithermal vein deposit models in the conterminous US have been divided into several types based largely on the underlying basement rocks (Cox and Singer, 1986). For the most part, information about Alaska is inadequate to classify individual epithermal gold vein types. Therefore, we have combined deposit models for Creede epithermal veins (Mosier and others, 1986b), Comstock epithermal veins (Mosier and others, 1986c), Sado epithermal veins (Mosier and others, 1986a), and epithermal quartz-alunite gold (Berger, 1986a) into a generic epithermal vein model. We evaluated the permissive area for undiscovered deposits of all types of epithermal veins based on this generic model, which includes gold and/or silver veins associated with intermediate to felsic volcanic rocks similar to those described by Panteleyev (1987).
This tract is permissive for the occurrence of epithermal Au veins based on the known occurrence at the Democrat deposit in the Big Delta quadrangle.
On the delineation of permissive tracts
The tract is defined as the part of the Yukon-Tanana terrane containing the epithermal Au-Ag Democrat deposit, gold placers in Tenderfoot and Banner Creeks, and nearby high-level residual gold placers. Until the discovery of the Democrat deposit, epithermal gold veins were not expected in this area. There is almost certainly potential for additional such deposits beyond the tract as defined, but it is difficult to establish the limits of this larger permissive area. Much of the Yukon-Tanana terrane to the north, east, and west is mantled by a thick cover of loess (up to several hundred feet thick) and surficial material that makes exploration difficult and could well hide additional such deposits. The small size of the epithermal deposits further reduces their likelihood of discovery. Probably the best regional guide to such deposits are placer gold deposits, especially those without a clear lode source. One such placer is at Caribou Creek, a tributary to the Salcha River about 15 miles northeast of the tract, which contains large nuggets of native bismuth. Small placer occurrences also are sprinkled throughout Yukon-Tanana terrane between the Yukon and Tanana Rivers which suggest that the permissive area may include most of the Yukon-Tanana Upland. However, epithermal vein deposits need not necessarily form placers and a better guide to the extent of such deposits may be the distribution of late Cretaceous plutons, which are widely distributed in the Yukon-Tanana Upland.
Important examples of this type of deposit
The Democrat Au-Ag deposit is tentatively classified as an epithermal deposit. Native gold, acanthite, and a variety of silver sulfosalts typical of epithermal deposits are associated with a 86.9 Ma. felsic dike variously called a granite porphyry, porphyry, or rhyolite porphyry. The deposit is associated with a relatively small alluvial placer below the deposit. In addition, there are several high-level placers in the district, notable for their angular gold, that are associated with northwest-trending lineaments and/or extensions of the rhyolite porphyry dike at the Democrat deposit or with other similar dikes. These are probably residual placers (oral communication, Don May, 1993).
There was some production from the Democrat deposit in the late 1980’s; a test shipment was made to a mill in Fairbanks and a small open pit was developed that recovered gold with a simple washing plant. There was also some drilling but more work is necessary to fully define the property; its extension at depth is particularly uncertain and some have speculated that it represents the top of a porphyry gold system (Thomas Bundtzen, Alaska Division of Geological and Geophysical Surveys, oral communication, 1992).
The placers in the area have been known since before World War I but most of the work on the Democrat lode deposit began in the mid-1970’s. Regional exploration is greatly hindered by a thick loess cover that mantles the hillsides.
On the numerical estimates made
Insufficient data precludes estimation of the number of undiscovered epithermal gold veins in this tract.
Berger, B.R., 1986a, Descriptive model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 158.
Bundtzen, T. K., and Reger, R. D., 1977, The Richardson lineament, a structural control for gold deposits in the Richardson mining district, Interior Alaska, in Short Notes on Alaskan Geology-1977: Alaska Division of Geological and Geophysical Surveys Geologic Report 55, p. 29-34.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 130.
Miller, T. P., 1994, Pre-Cenozoic plutonic rocks in mainland Alaska, in Plafker, George, and Berg, H. C., The Geology of Alaska: The Geology of North America, volume, G-1, Geological Society of America, p. 535-554.
Mosier, D.L., Berger, B.R., and Singer, D.A., 1986a, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 154.
Mosier, D.L., Sato, Takeo, Page, N.J., Singer, D.A., and Berger, B.R., 1986b, Descriptive model of Creede epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 145.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986c, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 150.
Nokleberg, et. al, 1993, Metallogenesis of mainland Alaska and the Russian Northeast: U. S. Geological Survey Open-File Report 93-339, 222 p.
Panteleyev, Andrejs, 1987, A Canadian cordilleran model for epithermal gold-silver deposits, in Roberts, R.G., and Sheahan, P.A., Ore Deposit Models: Geoscience Canada Reprint Series 3, p. 31-43.