Ferruginous Quartzites in the Chopawamsic Terrane, Piedmont Province, Virginia: Evidence for an Ancient Back-Arc Hydrothermal System

Metamorphosed massive sulfide deposits in Virginia's Piedmont province are spatially associated with locally abundant narrow, undulating resistant lenses of ferruginous quartzite (and spilites, garnet-quartz-chlorite rock, and quartz-staurolite rock). The precursors to ferruginous quartzites, some of which are also manganiferous, are largely hydrothermal, with a negligible input from hydrogenous materials and a low, but detectable input from continental detritus with a composition similar to regional metavolcanic rocks. However, most ferruginous quartzites possess trace element characteristics, including relatively low Ge/Si and negative Eu anomalies, which make their source fluids distinct from typical modern hydrothermal fluids. Additionally, chondrite-normalized abundances of rare earth elements are among the highest yet reported for highly siliceous metamorphosed Fe-bearing exhalites. These features appear to be a consequence of ancient hydrothermal activity that operated at relatively low temperature and pH compared to modern hydrothermal environments. Such characteristics are common among hydrothermal fluids in back-arc basins, and this interpretation is consistent with the arc setting of regional metavolcanic rocks. The ferruginous quartzites, which are considered to be meta-exhalites, and associated massive sulfides were likely produced in an ancient arc that operated around the time the arc was accreted to the continental margin.