Explained by T.L. Klein

On the choice of deposit models

Many of the gold deposits of the southeastern United States are thought to be low-sulfide Au-quartz vein deposits because of their similarity in ore controls (i.e., veins controlled by regional fault systems or folds), the mineralogy of ther veins, alteration halos, and associated sulfides, and their host rocks to those of model 36a (Berger, 1986). Although low-sulfide gold deposits are typically found in low-grade metamorphic terrains, we feel that the metamorphic grade is probably a less important criteria than the character of the protolith and alteration assemblage and the presence of appropriate regional structures. Therefore, the deposits in the Alabama, Dahlonega, and South Mountain districts are classified as low-sulfide Au-quartz veins deposits (model 36a, Berger, 1986), even though they are found in high-grade metamorphic rocks.

Grade and tonnage of low-sulfide Au-quartz vein deposits for which information is available are as follows: Creighton, Ga., 140,000 metric tons, 10 g/t; Franklin, Va., 88,000 metric tons, 20.5 g/t; Gold Hill, N.C., 1.3 million metric tons, 4.1 g/t; Hog Mountain, Ala., 2.7 million metric tons, 2.7 g/t; Howie, N.C., 310,000 metric tons, 10 g/t; Iola, N.C., 220,000 metric tons, 7 g/t; Rudisil, N.C., 140,000 metric tons, 14 g/t; Vaucluse, Va., 160,000 metric tons, 14 g/t. These eight deposits are larger in tonnage than 60 percent of the low-sulfide Au-quartz vein deposits that make up the grade and tonnage model (Bliss, 1986), plotting between the 10th and 40th percentile . Grades for these deposits are lower than 30 percent of low-sulfide Au-quartz vein deposits in the model, plotting between the 30th and 90th percentiles. In general, the low-sulfide gold deposits in the southeastern United States, for which we have grade and tonnage information, appear somewhat larger but of similar grade when compared with low-sulfide gold deposits elsewhere.

The Dahlonega gold belt and its southwestward extension, the Carroll County gold belt, extend for more than 160 km. The Carroll County belt is separated from the Dahlonega belt by a gap of a few miles that contains no known gold deposits or occurrences. The belt contains metamorphosed clastic and chemical sedimentary, volcanic, and intrusive rocks. Retrogressive metamorphism of sillimanite-grade mineral assemblages to lower-grade kyanite- and staurolite-bearing assemblages is typical in this belt where kinematic indicators in this high strain zone show an overall dextral sense of movement. Gold deposits are typically in well developed quartz-carbonate veins or zones of quartz-calcite pods (Albino, 1990). Many kuroko massive sulfide deposits (Model 28a) are found in the volcanic-rich parts of this belt. Much of the production was from hundreds small deposits many of which were alluvial placers or hydraulically-mined saprolite deposits derived from weathering of quartz vein deposits.

On the delineation of permissive tracts

The Dahlonega gold belt and the Carroll County gold belt at its southwestern end define this permissive tract. This highly-deformed, 1–2 km-wide, rock package was retrograded to the greenschist facies from the staurolite metamorphic grade, and was last deformed in a transpressive structural setting between the high-grade metamorphic rocks of the Richard Russell and Tallulah Falls thrust sheets (Albino, 1989). The few moderate-sized and hundreds of small deposits from this area have accounted for an estimated 19 metric tons of the 27 metric tons of gold produced from Georgia (Koschmann and Bergendahl, 1968; Albino, 1989). The tract includes the Creighton mine. Other mines with significant past production in this tract are the Battle Branch, Barlow, Cherokee, Findley Ridge, Sixes, Royal-Vindicator, and the Dahlonega-Consolidated.

On the numerical estimates made

A mean predicted number of deposits for the permissive tract was calculated using its area, times a deposit density factor derived from four well-characterized, major low-sulfide Au-quartz vein regions (i.e., the Meguma area, Nova Scotia; the central Victoria area, Australia; Klamath Mountains, Oregon; the Sierra Nevada foothills, California) (Bliss and others, 1987). The deposit densities for these four regions are within 12 percent of their mean of 0.0048 deposits per square kilometer. This mean density when multiplied by the area of the permissive tract gives a predicted mean number of 5.34 undiscovered deposits. The number of known deposits in the tract with grade and tonnage consistent with the deposit model is 3. This was subtracted from the predicted number to obtain a net of 2.34. Using this number as a guide, a geologically reasonable distribution of the number of deposits was selected: at the 90th, 50th, and 10th percentiles, respectively, 1, 2, and 4 or more low-sulfide Au-quartz vein deposits in the tract consistent with the grade and tonnage model of Bliss (1986).

References

Albino, G.V., 1989, Gold deposits of the Dahlonega belt, northeastern Georgia, in Cook, R.B., ed., Economic mineral deposits of the southeast: metallic ore deposits: Georgia Geologic Survey Bulletin 117, p. 85–120.

Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds. Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.

Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins,in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239–243.

Bliss, J.D., Menzie, W.D., Orris, G.J., and Page, N.J, 1987, Mineral deposit density—A useful tool for mineral-resource assessment [abs.], in Sachs, J.S., ed., USGS research on mineral resources, 1987 program and abstracts, third annual V.E. McKelvey Forum on Mineral and Energy Resources: U.S. Geological Survey Circular 995, p. 6.

German, J.M., 1989, Geologic setting and genesis of gold deposits of the Dahlonega and Carroll County gold belts, Georgia: Economic Geology, v. 84, no. 4, p. 903–923.

Koschmann, A.H., and Bergendahl, M.H., 1968, Principal gold-producing districts of the United States: U.S. Geological Survey Professional Paper 610, 283 p.