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.
Several small districts in the eastern Alabama Piedmont have produced small amounts of gold from low-sulfide Au-quartz deposits, perhaps as much as 2 metric tons (Dean, 1989). The districts include the Cragford, Devil's Backbone, Eagle Creek, Goldville, Gold Ridge, Hog Mountain, and Riddles Mill. Host rocks are typically amphibolite-grade metasedimentary rocks with minor metavolcanic rocks and metamorphosed tonalite intrusions (Guthrie and Lesher, 1989). The Devils Backbone and Eagle Creek districts are located along the Brevard zone, where amphibolite-grade metasedimentary rocks have been retrograded to greenschist-grade. Most deposits are found in discontinous, folded, quartz veins and pods. Production at Hog Mountain, the largest gold mine in Alabama, was estimated by Pardee and Park (1948) to have been 0.75 metric tons Au. Calculations by Paris (1985) based on data of Pardee and Park (1948) suggest that up to 7.5 metric tons of gold may be present to moderate depths.
On the delineation of permissive tracts
Three separate small areas in Alabama were evaluated together as one permissive tract. The southernmost area includes the Eagle Creek and Devils Backbone districts in the Brevard zone and a thin belt of rocks located between them. The Gold Ridge, Cragford, Goldville, Hog Mountain districts and the rocks located between them, in the Ashland-Wedowee belt comprise the middle area of the three. An area containing small gold deposits in the Riddle's Mill district in the rocks of the Talladega Group, along the Talladega fault defines the small, northern most area in Alabama. The Hog Mountain mine is located within 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 mean number of undiscovered deposits of 3.63. The number of known deposits in the tract with grade and tonnage consistent with the deposit model is 1. This was subtracted from the predicted number to obtain a net of 2.63. 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, 3, and 4 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.
Dean, L.S., 1989, A review of gold mining and exploration in Alabama, in Lesher, C.M., Cook, R.B., and Dean, L.S., eds., Gold deposits of Alabama: Geological Survey of Alabama Bulletin 136, p. 1–10.
Guthrie, G.M., and Lesher, C.M., 1989, Geologic setting of lode gold deposits in the northern Piedmont and Brevard zone, Alabama, in Lesher, C.M., Cook, R.B., and Dean, L.S., eds., Gold deposits of Alabama: Geological Survey of Alabama Bulletin 136, p. 11–32.
Pardee, J.T., and Park, C.F., Jr., 1948, Gold deposits of the southern Piedmont: U.S. Geological Survey Professional Paper 213, 156 p.
Paris, T,A, 1985, The Goldville project—Results from an exploration project for gold in the northern Alabama Piedmont, in Misra, K.C., ed., Volcanogenic sulfide and precious metal mineralization in the southern Appalachians: University of Tennessee, Studies in Geology 16, p. 182–205.