Abstract

Strata-bound massive sulfide deposits of the central Virginia Piedmont occur in a volcanic-plutonic belt composed of Lower Cambrian(?) metamorphosed volcanic rocks and locally of pre-Upper Ordovician, low potassium granitoid rocks. The belt, interpreted as an ancient island arc, begins about 50 km south of Washington, D.C., and extends some 175 km south-westward. The volcanic rocks within the belt are located along the flanks of the Quantico-Columbia synclinorium and the Arvonia syncline. The Chopawamsic Formation, a western facies of the belt, consists of volcanic rocks that have the geochemical features (rare earth and immobile elements) of a tholeiitic island-arc suite; it also contains calc-alkaline components. Tholeiitic amphibolites of the Ta River Metamorphic Suite on the eastern side of the belt form an oceanward facies of the island arc, coeval with the Chopawamsic. This distribution suggests an underlying westward-dipping Cambrian(?) subduction zone. Contrasts in stratigraphy, geochemistry, and magnetic and gravity features indicate that the central Virginia volcanic-plutonic belt is not related to the Carolina slate belt as has been proposed in the past.Known sulfide bodies in the Chopawamsic and its coeval units total about 13.5 million tons. The Mineral district on the northwest flank of the Quantico-Columbia synclinorium has the largest deposits and contains about 10 million tons. Sulfide bodies in the belt consist largely of pyrite with minor sphalerite and chalcopyrite; Zn and Pb are important in a few of the smaller deposits. There are local concentrations of sphalerite and other base metal minerals within the major pyritic deposits (Zn = 1-12.5%; Cu: 1-2%; Pb as much as 5%). The dominantly sphaleritic deposits are found in and northward for about 15 km from the Mineral district. Sulfide mineralization is found in all types of volcanic rock, but the large sulfide bodies on the northwest flank of the Quantico-Columbia synclinorium are in felsite. Amphibolite, containing local beds of iron-formation, is abundant on the southeast flank of the Arvonia syncline; however, most sulfide mineralization there took place in felsitic to intermediate layers. Mineralization, attributed to submarine volcanic exhalation, took place during formation of the island arc.

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