Explained by T. L. Klein
On the choice of deposit models
Quartz-pyrophyllite or quartz-andalusite-pyrophyllite deposits that are the result of extreme leaching by acidic, high-sulfur hydrothermal fluids are common in the metamorphosed, dominantly andesitic volcanic rocks of the northern and eastern parts of the Carolina slate belt (CSB) (Feiss, 1982; Klein and Criss, 1988; Schmidt, 1985; Sykes and Moody, 1981). These deposits of aluminous minerals, which provide nearly all the pyrophyllite and andalusite produced in the United States, are thought to be metamorphosed analogs of acid-sulfate or quartz-alunite alteration zones (see Hayba and others, 1985; Berger, 1986). More than 50 of these aluminous alteration zones are known in the CSB. Gold deposits or prospects are associated with more than 10 percent of these zones and enrichments of copper sulfide minerals (chalcopyrite, enargite), and geochemically anomalous Mo, Sn, and Te are common. Minor amounts of alunite have been preserved in several deposits (see Lu and others, 1993; Schreyer, 1987). The oxygen isotope signature of many of the andesitic volcanic host rocks and minerals from many of the gold deposits strongly suggest that they were deposited in a subaerial environment (Klein and Criss, 1988; Feiss and others, 1993). The high-alumina alteration is equivalent to that in quartz-alunite gold deposits (see Berger, 1986) because of the character of the alteration (e.g., intense acid-leaching), the widespread involvement of high-temperature meteoric water in alteration, the character of their host rocks, and their distinctive chemistry. These deposits are thought to have formed over a wide range of paleodepths from subvolcanic to near-surface (Klein and Criss, 1988; Powers, 1988; Schmidt, 1985; Scheetz and others, 1991). Some are stratabound gold-bearing zones formed in the near-surface hot-spring environment whereas others may be discordant, linear or pipe-like fracture-controlled deposits originally emplaced at shallow depths below the surface of these paleo-hot-spring systems. .
The best-studied example of this type is the Brewer mine in South Carolina (Butler and others, 1985; Lu and others, 1993). It along with several other small abandoned deposits of this type (e.g., Nesbit mine, N.C.; Brassington mine, S.C.) and several hot-spring deposits (e.g., Haile mine) are clustered in the CSB near the North Carolina-South Carolina border. The gold deposit at the Brewer mine is localized in felsic volcaniclastic rocks along a crescent-shaped breccia zone that has a highly siliceous matrix containing abundant enargite, pyrite and free gold. The ore zone is located within an advanced argillic (pyrophyllite, andalusite, sericite, topaz) alteration zone that is surrounded by a zone of sericitic alteration (Butler and others, 1988; Lu and others, 1993). Geochemical enrichments of Cu, Mo, As, Hg, Sn, and Te coincide roughly with the mineralized zone. The system is overlain by laminated mudstones.
Other examples, such as the abandoned gold deposits in the Robbins district, N.C. and the Nesbit mine, S.C., are associated with massive and (or) foliated pyrophyllite bodies, devoid of the high-temperature aluminosilicate minerals (andalusite and topaz), and their surrounding sericite-rich alteration zones. Gold is associated with disseminated pyrite and quartz or pyrite veinlets. Most have geochemically anomalous Mo, As, or Sn. Several are interpreted to have formed from hydrothermal fluids dominated by meteoric water. These tend to occur along linear zones several kilometers in length, perhaps reflecting the fault control.
Several prospects in central North Carolina, including Pilot Mountain and Snow Camp are somewhat ellipsoidal in shape and may be related to alteration caused by shallow-level, porphyritic dacitic intrusions (Klein and Criss, 1988). The area of the alteration systems associated with these prospects is thought to extend over several tens of square kilometers. Eight major alteration systems have been described in the northern CSB, including the Brewer mine, South Carolina and, the Robbins-Glendon, Pilot-Fox Mountain, Staley, Montgomery County, Saxapahaw-Snow Camp, and the Oxford-Stovall areas of North Carolina (Espenshade and Potter, 1960; Stuckey, 1967; Schmidt, 1985; and McKee and Butler, 1985).
We consider the Brewer deposit and the other cited examples to be quartz-alunite gold deposits because of the subaerial character of many of the intermediate and felsic volcanic host rocks, the similarities of their ore and alteration mineralogy and geochemistry, associated metals, and rock textures with those typical of quartz-alunite gold deposits. The apparent widespread involvement of meteoric water in the hydrothermal fluids also is consistent with a subaerial origin. Extreme leaching by acidic, high-temperature hydrothermal fluids produces the relatively unique mineral assemblage found in Tertiary or younger quartz-alunite deposits elsewhere and in the CSB analogs. These assemblages are not typical of other types of epithermal or mesothermal gold deposits.
On the delineation of permissive tracts
This permissive tract consist of the late Precambrian to Cambrian volcanic and sedimentary rocks of the CSB, eastern CSB, Charlotte belt, and Kings Mountain belt and their equivalents that extend toward the southwest. No numerical estimate was made because of the rare occurrence of deposits of this type in the tract and the poor understanding of the depositional environment of the metavolcanic and metasedimentary rocks.
References
Berger, B.R., 1986, Descriptive model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 158.
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.
Espenshade, G.H., and Potter, D.B., 1960, Kyanite, sillimanite, and andalusite deposits of the southeastern States: U.S. Geological Survey Professional Paper 336, 121 p.
Feiss, P.G., 1982, Ore deposits of the northern parts of the Carolina slate belt, North Carolina: Geological Society of America Special Paper 191, p. 153–164.
Feiss, P.G., Vance, R.K., and Wesolowski, D.J., 1993, Volcanic rock-hosted gold and base-metal mineralization associated with Neoproterozoic-Early Paleozoic back-arc extension in the Carolina terrane, southern Appalachian Piedmont: Geology, v. 21, p. 439–442.
Hayba, D. O., Bethke, P.M., Heald, P., and Foley, N.K., 1985, Geologic, mineralogic, and geochemical characteristics of volcanic-hosted epithermal precious-metal deposits, in Berger, B.R., and Bethke, P.M., eds., Geology and geochemistry of epithermal systems: Society of Economic Geologists, Reviews in Economic Geology, v. 2, p. 129–167.
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.
Lu, C., Misra, K.C., Stonehouse, J.M., and Zwaschka, M.R., 1993, Geochemical signature of alteration at the Brewer gold mine, Jefferson, South Carolina: South Carolina Geology, v. 35, p. 37–54.
McKee, L.H. and Butler, J.R., 1985, Hydrothermal alteration and mineralization at four gold mines in southern Union County, North Carolina, in Misra, K.C., ed., Volcanogenic sulfide and precious metal mineralization in the southern Appalachians: Knoxville, University of Tennessee, Studies in Geology 16, p. 113–123.
Powers, J.A., 1988, Gold mineralization and high alumina alteration in the Robbins district of the Carolina slate belt, Moore County, North Carolina: Geological Society of America Abstracts with Programs, v. 20, p. 309.
Scheetz, J.W., Stonehouse, J.M., and Zwaschka, M.R., 1991, Geology of the Brewer gold mine in South Carolina: Mining Engineering, v. 43, no. 1, p. 38–42.
Schmidt, R.G., 1985, High-alumina hydrothermal systems in volcanic rocks and their significance to mineral prospecting in the Carolina slate belt: U.S. Geological Survey Bulletin 1562, 59 p.
Stuckey, J.L., 1967, Pyrophyllite deposits in North Carolina: North Carolina Division of Mineral Resources Bulletin 80, 38 p.
Sykes, M.L., and Moody, J.B., 1981, Pyrophyllite and metamorphism in the Carolina slate belt: American Mineralogist, v. 63, p. 96–108.
