KYANITE QUARTZITES IN THE PIEDMONT PROVINCE OF VIRGINIA: EVIDENCE FOR PROTOLITH PRODUCTION IN A HIGH-SULFIDATION (ADVANCED ARGILLIC) HYDROTHERMAL SYSTEM

The world's largest kyanite mine is hosted within kyanite quartzite at Willis Mountain, Virginia, and numerous smaller bodies occur throughout this portion of the Piedmont Province.  Most occur in the Ordovician Chopawamsic or Milton Terranes, which consist primarily of arc-derived metavolcanic rocks, volcanogenic metasediments, and felsic plutonic rocks.  Earlier investigators viewed these rocks as metamorphosed aluminous sandstones, but there is compelling evidence that the protoliths were hydrothermally altered igneous rocks.  All rocks contain accessory white mica (locally green Cr-muscovite) and rutile, and locally abundant pyrite.  Topaz, lazulite, and other rare minerals occur in some samples.  In a previous contribution (SE GSA abstract, 2002), we reported whole-rock data for 23 samples from 9 separate localities, and noted:  1) low to negligible concentrations of K, Na, Ca, Mg, and many trace elements;  and 2) a strong depletion in Ga relative to Al.  We report new REE data here for 4 samples, all of which are LREE-enriched (La_N = 9-339;  Lu_N = 1-7), and show curious ladle-like chondrite-normalized patterns with relative depletions from Gd through Tm. New reconnaissance oxygen isotopic data on quartz separates from 8 samples show a range in  del ¹⁸O values from 3.5 to 11.6 per mil, indicating regionally variable (but locally homogeneous) fluid compositions of -5.4 to 2.7 per mil at 250°C, implying a significant meteoric water (or seawater?) component.  We suggest that these rocks were produced by severe leaching of many elements in a high-sulfidation alteration system, leading to protoliths dominated by quartz and kaolinite (or pyrophyllite).  Such an interpretation is consistent with most mineralogical and compositional features, and with the regional geologic setting, which includes numerous small, but historically productive deposits of Cu sulfides and Au, as well as sporadic occurrences of ferruginous quartzite (metamorphosed silica-iron exhalites). In addition, similar distinctive REE patterns have been reported from other advanced argillic systems, as have similar del ¹⁸O values for quartz. Finally, thermodynamic calculations suggest that S-bearing acids (H₂S or H₂SO₄) are the only type capable of fractionating Ga from Al, consistent with the chalcophile nature of Ga.