Kyanite Quartzites in the Piedmont Province of Virginia: Evidence for a Possible High-Sulfidation System

摘要

The world's largest kyanite mine is hosted within kyanite quartzite at Willis Mountain in the Piedmont Province of Virginia, and numerous smaller bodies occur throughout this region. Most occur in the Ordovician Chopawamsic or Milton terranes, which consist primarily of arc-derived metavolcanic rocks, volcanogenic metasedimentary rocks, and felsic plutonic rocks. Earlier investigators viewed these rocks as metamorphosed aluminous sandstones, but we report mineralogical and geochemical evidence that the protoliths were hydrothermally altered igneous rocks. All rocks contain accessory rutile, white mica (locally green Cr muscovite), and locally abundant pyrite. Topaz, lazulite, and other rare minerals occur in some samples. Whole-rock analyses for 22 samples from nine separate localities show very low to negligible concentrations of K, Na, Ca, Mg, and many trace elements; and a strong depletion in Ga relative to Al. Analyses of the rare earth elements (REE) in four samples show that all are light REE enriched, with ladlelike chondrite-normalized patterns (relative depletions from Gd through Ho). Reconnaissance oxygen isotope data on quartz separates show a range in delta~(18)O values from 3.5 to 11.6 per mil and probably reflect hydrothermal fluids with variable meteoric and magmatic signatures. We suggest that these rocks originated by severe leaching in a high-sulfidation (advanced argillic) alteration system, resulting in protoliths dominated by quartz and kaolinite. Such an interpretation is consistent with mineralogical and compositional features and with the regional geologic setting, which includes numerous small but historically productive deposits of Fe, Au, Cu, Zn, and S. In addition, thermodynamic calculations suggest that sulfuric acid-bearing fluids are the only type capable of significantly fractionating Ga from Al.