Explained by T.L. Klein
On the choice of deposit models
Many of the gold deposits in 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 their veins, alteration halos, and associated sulfides, and their host rocks to those of model 36a (Berger, 1986). A cluster of low-sulfide Au-quartz vein deposits that incudes the Rudisil mine occurs in the Charlotte district (Pardee and Park, 1948) just west of the southern end of the Gold Hill fault. The cluster is associated with mafic and granitic intrusions. Elsewhere in the Carolina slate belt, the largest mine is the Iola, located near Candor, North Carolina. The Iola, and two smaller deposits nearby, discovered in the early 1900's, are the most recently discovered gold deposits in North Carolina. They have produced approximately 1.5 metric tons of gold (Pardee and Park, 1948). These deposits are in northeast-trending quartz-carbonate veins in a poorly exposed area of intermediate volcanic rocks. Their structural setting is poorly known.
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.
On the delineation of permissive tracts
Areas of the Charlotte belt, Carolina slate belt, eastern Carolina slate belt, the Belair belt and low-grade metamorphic rocks buried beneath the Cretaceous and Eocene Coastal Plain sedimentary rocks that have not been otherwise included in other favorable areas are included in this permissive tract. Several inactive mines, the Iola and Rudisil mines and the Capps-McGinn, Carter-Star, and Barringer mines are found in this tract.
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 number of deposits that is unreasonably large. Because the tract included a large amount of unexplored, covered bedrock that may include large areas of nonpermissive rocks and structures, the assessment team used a deposit density factor reduced to 0.0001. This factor gives a predicted mean number of undiscovered deposits of 9.2. The number of known deposits in the tract with grade and tonnage consistent with the deposit model is 2. This was subtracted from the predicted number to obtain a net of 7.2. Using this number as a guide, a geologically reasonable distribution of the number of deposits was selected: at the 90th, 50th, 10th, and 5th percentiles, respectively, 4, 7, 11, and 13 or more low-sulfide Au-quartz vein deposits in the tract consistent with the grade and tonnage model of Bliss (1986).
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.
Pardee, J.T., and Park, C.F., Jr., 1948, Gold deposits of the southern Piedmont: U.S. Geological Survey Professional Paper 213, 156 p.