Open-File Report 2006-1259

Published 2006
Online Only

General Geology

The Carolina slate belt, in the southeastern Piedmont, is characterized by mostly low-grade metamorphic rocks of Neoproterozoic to early Paleozoic age. It extends 375 mi (600 km) from northeastern Georgia to southern Virginia (fig. 1). Although typically 25 to 50 mi (40–80 km) wide, it has a maximum width of 88 mi (140 km) in central North Carolina. Its geology and the results of earlier studies were summarized by Butler and Secor (1991).

In central and northern North Carolina, the Carolina slate belt is underlain by tuffs, abundant volcaniclastic rocks, lesser flow rocks and epiclastic strata, and, in parts of the belt, intrusions having a range of compositions and ages. Southwestward, parts of the belt are underlain by mostly fine epiclastic sedimentary and volcaniclastic rocks. Near the study area, the Carolina slate belt is bounded on the northwest by granitic rocks, gneisses, and mica schists of the Charlotte belt, and on the southeast by rocks of the Wadesboro-Deep River-Durham basin, a partly fault-bounded trough of sedimentary and volcanic rocks of Mesozoic age. A variety of magnetite-bearing gabbroic rocks of Mesozoic age form narrow crosscutting dikes in slate belt rocks of all ages. The dikes formed only small discontinuous outcrops that were too small to map for this study.

Rocks in the area are mostly metamorphosed, generally at lower greenschist facies, but in places the degree ranges widely within the greenschist facies and locally into the lower amphibolite facies. Some plutons and subvolcanic bodies have recognizable contact metamorphic aureoles independent of the regional metamorphism. Most intrusions within or adjacent to the Carolina slate belt fall into one of two general age brackets, 595 to 520 million years ago (Ma) and about 300 Ma (Fullagar, 1971; Fullagar and Butler, 1979; and Speer and others, 1980). The older plutons and the volcanic rocks probably overlapped in time of formation, and some of the plutons were probably cogenetic with nearby volcanic rocks.

The structural geology is perhaps the least understood aspect of the areal geology of this area because field studies were greatly inhibited by the general lack of good outcrops and detailed stratigraphic data. A consensus is lacking as to the time of folding and faulting, and structures are probably much more complex than interpreted.

The Carolina slate belt is characterized by multiple segments of divergent ages, structural styles, and metamorphic grades, leading to it being interpreted as an accreted exotic terrane (Rodgers, 1972; Secor and others, 1983; and Butler and Secor, 1991). Harris and Glover (1985) proposed several stratigraphic sections for different areas from Virgilina, Va. (northeasternmost slate belt), to Ramseur, N.C. They divided the stratigraphic sequence, from oldest to youngest, into the Hyco Formation, Aaron Formation, and Virgilina Formation, overlain unconformably southwest of Ramseur by the Uwharrie Formation. All units are dominated by volcanic rocks. Mueller and others (1996) assigned an age of 650 to 600 Ma for the Virgilina sequence volcanism and 570 to 530 Ma for the Uwharrie Formation volcanism. Harris and Glover (1985) thought that the intermediate-composition volcanic rocks mapped by Wilkinson (1978) correlated with the Hyco Formation of northern North Carolina and, therefore, implied a correlation with at least some of the volcanic rocks in the Snow Camp-Saxapahaw area. Although our mapping in the Snow Camp-Saxapahaw area did not allow the assignment of any of the volcanic strata to a specific formation of the Virgilina sequence, a correlation of some of our intermediate to felsic volcanic rocks with that sequence seems reasonable. If so, the unconformity at the base of the Reedy Branch Tuff is probably equivalent to the unconformity at the base of the Uwharrie Formation, described by Glover (1974) and Harris and Glover (1985, 1988). These rocks were folded with no apparent penetrative deformation during the Neoproterozoic Virgilina deformation, named by Glover and Sinha (1973), and without recognizable metamorphism or formation of foliation (Harris and Glover, 1985, 1988). Later deformation, during the Taconic (Mid- to early Late) orogeny resulted in tight folds and penetrative deformation to broad open folds with no distinct foliation. This deformation was accompanied by a metamorphic event (Kish and others, 1979; Noel and others, 1988; and Offield and others, 1990).

A major structural study of the Carolina slate belt by Offield (1994) led to somewhat different interpretations of the deformational history. He found little evidence for a “Virgilina deformation,” instead he thought that folding, cleavage development, and greenschist-facies metamorphism all took place during the Taconic orogeny. He thought that a major thrust fault separates the Uwharrie Formation from older units. We think, however, that this contact is an unconformity in the Snow Camp-Saxapahaw area.

The Harris and Glover (1985, 1988) interpretation of the regional deformational history seems more compatible with our conclusions in the Snow Camp-Saxapahaw area where we found good evidence for two periods of major folding. A strongly folded layered sequence, limited to the Neoproterozoic intermediate to felsic volcanic complex, is truncated by the high-angle unconformity at the base of the Reedy Branch Tuff. Subsequently the Reedy Branch Tuff, arguably correlative with the Neoproterozoic Uwharrie Formation of the Asheboro region, was deformed into several broad open folds. Metamorphism in the study area certainly postdated deposition of the Reedy Branch Tuff and may well have been Taconic.

The mineral deposits in the Carolina slate belt have been described by Carpenter (1976), Feiss (1982), and Feiss and Slack (1989). Mining in the belt has produced significant gold from relatively small high-grade deposits, as well as lesser silver, copper, lead, iron, and zinc. Only recently has the Carolina slate belt been considered to contain large low-grade gold deposits. Production from the Ridgeway gold mine near Ridgeway, S.C., was expected to exceed 1 million ounces of gold bullion by the end of 1995, and further to yield an ultimate production of 1.5 million ounces by 2000 (Gillon and others, 1995).

Large, siliceous, high-sulfidation alteration systems are common in the slate belt; some of which have been major sources of pyrophyllite and andalusite (Espenshade and Potter, 1960), and several are hosts for gold deposits (Carpenter, 1976; Carpenter and Allard, 1982; Feiss, 1982; Schmidt, 1982, 1985a; and Klein and Schmidt, 1985). The linear distribution of these systems along a relatively narrow trend for 112 mi (180 km) was noted by Hughes (1987) and may be related to a major fault at depth.

 
 

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