MINERALOGICAL AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF FERRUGINOUS QUARTZITES IN THE CHOPAWAMSIC TERRANE, PIEDMONT PROVINCE, VIRGINIA

More than two dozen lenses of ferruginous quartzite occur within predominantly metavolcanic rocks of the Ordovician Chopawamsic Formation in the Andersonville and Willis Mt. 7.5’ quadrangles in central Virginia. Such rocks are locally associated with gossan zones that overlie small massive sulfide deposits, and thus have been interpreted to represent metamorphosed chemical sediments (Si-Fe exhalites). The quartzites occur as thin (<100 m wide), sinuous, locally resistant ridges that can be traced laterally for up to several km. The rocks range texturally from banded to massive varieties. All occurrences are dominated by quartz, magnetite, and specular hematite; magnetite is variably altered to hematite (martite). A few samples contain additional Fe- and Mn-rich silicates (garnet, amphibole, pyroxenoid), and one contains additional pyrolusite. We obtained whole-rock major and trace element analyses of ten samples from four lenses, with the goal of evaluating various inputs (hydrothermal, hydrogenous, detrital) to their protoliths. Major element compositions are dominated by SiO₂ (40-87 wt%) and Fe₂O₃(T) (9-60 wt%), with the exception of one MnO-rich sample (16.5 wt%). Minor amounts of Al₂O₃ (0.3-1.7 wt%) occur in all samples. Chondrite-normalized REE patterns (La_N = 0.7-30), vary from relatively flat to light REE-enriched and most samples show negative Eu anomalies (Eu/Eu* = 0.63-1.2). PAAS-normalized patterns are LREE-depleted, typically with slight positive Eu anomalies. Most samples show no Ce anomaly, regardless of normalization scheme. Numerous geochemical indicators are consistent with a dominantly hydrothermal source for the protoliths of these rocks, including major elements (Fe-Mn-Al), several trace elements (e.g., very low Co, Cu, Ni), and certain ratios (e.g., high Ge/Si up to 75 x 10^-6, typical of hydrothermal solutions). Nonetheless, the REE patterns of these rocks are atypical compared to those of many seafloor hydrothermal deposits, which commonly show positive Eu and negative Ce anomalies. These patterns probably result from mixing with a detrital component, an interpretation supported by positive correlations between Al and Ti, Zr, and Th. The nature of this detrital component is unclear at this point, but could possibly include felsic detritus from Chopawamsic volcanism.