Explained by T.L. Klein
On the choice of deposit models
The compositionally-bimodal, metamorphosed, late Precambrian to Cambrian volcanic terrain of the Central Carolina slate belt (CSB) contains many examples of epithermal precious metal deposits. Extensive accumulations of felsic volcanic rocks, ranging in composition from dacite to rhyolite, are associated with volcanic centers that are found in the area between the North Carolina-Virginia border southward to the north-central part of South Carolina, just north of Columbia, with the greatest felsic volcanic accumulations occurring in the area between the cities of Asheboro and Troy, North Carolina. The relationship of gold deposits to the waning stages of felsic volcanism was first suggested by Worthington and Kiff (1970) after they observed that many gold deposits in this region were found along the contact between a dominantly felsic volcanic unit (the Uwharrie Formation) and overlying mudstones. The application of the hot-spring model to explain the origin of some gold deposits in the CSB was proposed by Spence (1975); Spence and others (1980); and Worthington and others (1980); and has been supported by subsequent studies (Feiss, 1988; Butler and others, 1988; Klein, 1988; Koeppen and Klein, 1989).
Volcanic textures and the oxygen isotope signature of many of these felsic volcanic rocks and some of the gold deposits strongly suggest that they were deposited in a subaerial environment (Klein and Criss, 1988). Several of these deposits are thought to be deformed and metamorphosed stratabound gold-bearing zones related to paleo-hot-spring systems (e.g., Haile). Others may be discordant, fracture-controlled deposits originally emplaced at shallow depths below the surface in a paleo-hot-spring hydrothermal system (e.g., Hoover Hill) (Koeppen and Klein, 1989). Metamorphosed epithermal quartz-alunite gold deposits are also thought to occur in the CSB but are generally restricted to the more intermediate-composition volcanic rocks.
Three of the four producing or recently producing gold deposits and many abandoned mines and prospects in the CSB are thought to be epithermal in origin, and some were formed at shallow levels in hot-spring systems. Recent mineral exploration in the CSB has resulted in the renewed mining activity at the Haile mine and the discovery of a large, new deposit at Ridgeway in South Carolina. In addition, several abandoned mines and prospects have been recently explored in central North Carolina that contain substantial inferred reserves (e.g., Russell) and persistent ore zones that are only partially constrained by drilling. The most productive area has been in north-central South Carolina, where recent mining has exploited probable metamorphosed hot-spring deposits (Haile, Ridgeway).
Three of the known deposits(Haile, Ridgeway, Russell) are found near the contact of large accumulations of dominantly felsic, bimodal volcanic rocks with overlying epiclastic rocks, probably of marine origin. All are characterized by steeply dipping ore zones that consist of disseminated sulfide minerals (predominantly pyrite) that have been intensely silicified and sericitized and contain few, if any, well-defined mineralized quartz veins. All three deposits are slightly enriched in Cu, Zn, As, and Mo. Ore zones at the Haile mine also contain gold and silver telluride minerals. Gold at the Portis mine, in the eastern CSB, is concentrated in pyrite-quartz stockworks within a shallow-level granitoid sill and is tenatively classified as a hot-spring gold deposit.
The Haile, Ridgeway, and Russell mines are considered to be hot-spring gold deposits because of the subaerial character of some of their felsic volcanic host rocks, the similarities of their ore and alteration mineralogy and host rock textures with those of hot-spring deposits (Model 25a of Berger, 1986), and the apparent widespread involvment of meteoric water in the hydrothermal fluids. However, an alternative origin as shear zone-controlled deposits has been proposed by investigators at the Haile and Ridgeway deposits (see Tomkinson, 1985). Although these deposits are locally highly deformed and gold may be controlled by small-scale, shear-related structures, this control is probably due mainly to local remobilization during metamorphism. The overall character, including typical grades and size, of these CSB deposits is similar to hot-spring deposits, elsewhere, and not at all typical of low-sulfide Au-quartz veins.
Laminated mudstones, some of which contain marine fossils, are commonly associated with CBS deposits. This seems to be inconsistent with a simple, subaerial hot-spring origin, however, the evidence for meteoric water-dominated hydrothermal fluids from oxygen-isotope studies and the absence of substantial quantities of base-metals in these deposits suggest that marine waters were not the principal source of fluids. The apparent conflict can be rationalized if a subaerial hot-spring system, related to coastal volcanism,was subsequently covered by marine sediments or formed in subsiding coastal fresh water lakes that eventually became marine.
Other types of epithermal deposit (e.g., Creede, Comstock, and Sado deposits of Cox and Singer (1986) are found in similar environments but are formed at deeper levels within epithermal systems. Although deposits of this type are present in the CSB (see Klein, 1988), most of the ore in CSB deposits is disseminated rather than in veins and is not characteristically as silver-rich as the deeper epithermal vein deposits.
Grade and tonnages of the three mines for which information is available (Haile, 9.1 million metric tons, 3 g/t; Ridgeway, 51 million metric tons, 1 g/t; and Russell, 3.7 million metric tons, 2 g/t), plot between the 20th and 80th percentile for tonnage and between the 10th and 80th percentiles on the grade curves for hot-spring deposits (see Berger and Singer, 1992). In general, these three CSB deposits appear to be typical of hot-spring deposits in their grade and tonnage characteristics and lower in grade and of greater tonnage than other types of epithermal deposits. Even the smallest CSB deposit, the Russell mine, is larger than 70 percent of the Creede deposits and the Ridgeway deposit is larger than all but one example suggesting that these deposits do not belong to this deposit tonnage population. The results are even more convincing when the same comparison is made to the Comstock and Sado deposits. Not only are all three CSB deposits larger than 80 percent of the Comstock and all but one Sado deposit but all of the three CSB deposit gold grades are lower than approximately 90 percent those of both deposit types. When a similar comparison is made to low-sulfide gold deposits (Bliss, 1986) the three CSB deposit are larger than more than 95 percent of all low-sulfide gold deposits and their grades lower more than 99 percent of these deposits, indicating that they are not likely to belong, as has been proposed, to any known type of shear-zone controlled, low-sulfide gold deposits.
On the delineation of permissive tracts
The permissive tract encloses the exposed area of the CSB and eastern CSB and includes areas that contain evidence for significant amounts of felsic volcanism. This tract includes the Haile and the Ridgeway deposits. Even though much of the volcanism in the CSB was submarine, we cannot reliably discriminate, in many cases, between submarine and subaerial felsic volcanic centers using the present geologic framework. Therefore, we have included in this tract those parts of the slate belt that are known to contain bimodal (rhyolitic-basaltic) volcanism and where the depositional environment is incompletely known. As more information is made available concerning the distribution of subaerial versus submarine volcanic centers, a significant part of this tract where submarine volcanism dominates could be reclassified.
On the numerical estimates made
The following calculations, based on deposit densities observed in other hot-spring Au-Ag districts were used to estimate the number of undiscovered deposits in the favorable areas at the 50th percentile by using the product of the area of the favorable tracts and the deposit density:
District | Size, km | No.of deposits | Density (deposits/km2) | Reference
Surigoa del Norte, Philippines | 30 x 700 | 20 | 0.001 | Mitchell and Balce, 1990.
Paracales, Philippines | 10 x 20 | 20 | 0.10 | Mitchell and Balce, 1990.
Round Mountain, Nevada | 20 x 20 | 6 | 0.015 | Mills and others, 1988.
Coromandel peninsula, New Zealand | 10 x 30 | 10 | 0.033 | Irvine and Smith, 1990.
Humboldt Range, Nevada | 10 x 40 | 5 | 0.012 | Hastings and others, 1988.
The tract contains the two largest examples (Haile, Ridgeway) and has a large surface area. A low density of 0.001 deposits/km2 was used to calculate a first pass at the mean number of deposits in the tract because much of the geologic framework is not well known. This calculation yielded 18.66 deposits. Six hot-spring deposits consistent with the grade and tonnage model are known in this tract, leading to a net estimate of 12.66 mean undiscovered deposits. The team believed that the number of deposits calculated in this manner was too large by a factor of about two, and the final probabilistic estimate used a mean number of 6.3 as a guide. For the 90th, 50th, and 10th percentiles, the team estimated 3, 5, and 12 or more hot-spring Au-Ag deposits consistent with the grade and tonnage model of Berger and Singer (1992).
References
Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag , in Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p.143
Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23–25.
Bliss, J.D.,1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239–243.
Butler, J.R., Scheetz, J.W., Stonehouse, J.M., Taylor, D.R., and Callaway, R.Q., 1988, The Brewer gold mine, Chesterfield County, South Carolina: Chapel Hill, University of North Carolina, Guidebook for Field Trip, April 5, 1988, Southeastern Section, Geological Society of America, 22 p.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Feiss, P.G., 1988, Gold mineralization in the Southern Appalachian Piedmont—Pardee and Park redivivus, in Kisvarsanyi, G., and Grant, S.K., eds., Tectonic controls of ore deposits and the vertical and horizontal extent of ore systems, Proceedings volume: Rolla, University of Missouri, p. 491–500.
Hastings, J.S., Burkhart, T.H., and Richardson, R.E., 1988, Geology of the Florida Canyon gold deposit, Pershing County, Nevada, in Schafer, R.W, Cooper, J.J., and Vikre, P.G., eds., Bulk mineable precious metal deposits of the western United States, symposium proceedings: Geological Society of Nevada, p. 433–452.
Klein, T.L., 1988, Geochemical and alteration study of gold deposits in the central Carolina slate belt: Geological Society of America Abstracts with Programs, v. 20, no. 4, p. 274.
Klein, T.L., and Criss, R.E., 1988, An oxygen isotope and geochemical study of the meteoric-hydrothermal systems at Pilot Mountain and selected other localities, Carolina slate belt: Economic Geology, v. 83, no. 4, p. 801–821.
Koeppen, R.P., and Klein, T.L., 1989, Caraway-Back Creek volcanic center, central slate belt, North Carolina—Newly recognized thermal source for precious- and base-metal mineralization: in K.S. Schindler, K.S., ed., USGS research on mineral resources—1989 program and abstracts, fifth annual V.E. McKelvey Forum on Mineral and Energy Resources: U.S. Geological Survey Circular 1035, p. 40–41.
Mills, B.A., Boden, D.R., Sander, M.V., 1988, Alteration and precious metal mineralization associated with the Toquima caldera complex, Nye County, Nev., in Schafer, R.W., Cooper, J.J., and Vikre, P.G., eds., Bulk mineable precious metal deposits of the western United States, symposium proceedings: Geological Society of Nevada.
Mitchell, A.H.G., and Balce, G.R., 1990, Geological features of some epithermal gold systems, Philippines, in Hedenquist, J.W., White, N.C., and Siddeley, G., eds., Epithermal gold mineralization of the Circum-Pacific I: Association of Exploration Geochemists Special Publication No. 16a, p. 241–296.
Spence, W.H., 1975, A model for the origin of the pyrophyllite deposits in the Carolina slate belt: Geological Society of America Abstracts with Programs, v. 7, no. 4, p. 536–537.
Spence, W.H., Worthington, J.E., Jones, E.M., and Kiff, I.T., 1980, Origin of the gold mineralization at the Haile mine, Lancaster County, South Carolina: Mining Engineering, v. 32, no. 1, p. 70–73.
Tomkinson, M.J., 1988, Gold mineralization in phyllonites at the Haile mine, South Carolina: Economic Geology, v. 83, no. 7, p. 1392–1400.
Worthington, J.E., and Kiff, I.T., 1970, A suggested volcanogenic origin for certain gold deposits in the slate belt of the North Carolina Piedmont: Economic Geology, v. 65, no. 5, p. 529–537.
Worthington, J.E., Kiff, I.T., Jones, E.M., and Chapman, P.E., 1980, Applications of the hot springs or fumarolic model in prospecting for lode gold deposits: Mining Engineering, v. 32, no. 1, p. 73–79.
