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 their veins, alteration halos, and associated sulfides, and their host rocks to those of model 36a (Berger, 1986). 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.
One of the most laterally-extensive gold belt in the southeastern United States is the gold-pyrite belt of northern and central Virginia that averages about 30 km wide and is more than 144 km long. Most of the deposits are gold-bearing quartz-carbonate veins and lenses, or wide zones of disseminations or veinlets of quartz in the moderately- to highly-deformed, metavolcanic and metasedimentary rocks of the Cambrian central Virginia volcanic-plutonic belt (Pavlides, 1981). Several of these deposits contain veins as thick as 5 m; some mineralized zones containing disseminated gold are up to 20 m wide (Pardee and Park, 1948). This belt of rocks is distinctive because of a strong metamorphic gradient with mid-greenschist facies rocks grading into mid-amphibolite facies rocks in a west to east direction.
On the delineation of permissive tracts
This tract is comprised of the rocks of the Central Virginia Volcanic-Plutonic belt (Pavlides, 1981) and is extended northeastward to include the gold deposits near the Great Falls in southern Maryland. Three deposits in this tract, Franklin, Whitehall, and Vaucluse, are probably the largest in Virginia, although production records for most mines in Virginia are incomplete or not available. More than 150 other small gold prospects and occurrences are found in the northeast-trending belt, which also includes several dozen small- to moderate-size kuroko massive sulfide deposits. Other significant abandoned mines in this tract include the Moss-Tellurium, Melville, and Eagle mines
On the numerical estimates made
A mean predicted number of deposits for the permissive tract was calculated using its area multiplied by 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 the team believed to be too large. This large tract has a relatively small number of known deposits , even though it has been well explored for both gold veins and kuroko massive sulfide deposits. For this reason, the team used a reduced density factor of 0.0014 deposits per square kilometer. This number when multiplied by the area of the permissive tract gives a predicted mean number of 7.56 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 4.56. 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, 2, 5, and 8 or more low-sulfide Au-quartz vein deposits in the tract consistent with the grade and tonnage model of Bliss (1986).
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.
Pavlides, L., 1981, The central Virginia volcanic-plutonic belt—An island arc of Cambrian(?) age: U.S. Geological Survey Professional Paper 1231-A, 34 p.