MINERALOGICAL AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF KYANITE QUARTZITES IN THE KINGS MOUNTAIN TERRANE, NORTH CAROLINA AND SOUTH CAROLINA

Aluminosilicate deposits containing kyanite, sillimanite, or andalusite are abundant in the southeastern U.S. The origins of these deposits have been debated, but recent interpretations favor alteration of the protoliths via leaching by highly acidic hydrothermal fluids. Although this interpretation seems widely accepted, geochemical data that could be used to evaluate it in more detail have not been reported for most occurrences. Kyanite quartzites in the Kings Mountain Terrane, North Carolina and South Carolina have been interpreted as originating either from clay-rich sands or silts or hydrothermally altered volcanic rocks. The purpose of this study is to evaluate the origin of the Kings Mountain deposits using the high-sulfidation alteration model of Owens and Pasek (2007, Econ. Geol., v. 102, p. 495-509) for occurrences in Virginia (e.g., Willis Mt.) as a guide. Evidence for this model includes the widespread presence of rutile, bulk compositions dominated by SiO₂ and Al₂O₃ (with negligible MgO, CaO, and alkalis), strong depletion of Ga relative to Al, and unusual U-shaped chondrite-normalized REE patterns.

We investigated kyanite quartzites from three areas in the Cambrian Battleground Formation: Crowders Mountain, the Pinnacle, and Henry Knob. Rocks from all areas are mineralogically similar, dominated by quartz and kyanite, with smaller and variable amounts of muscovite + rutile ± pyrite. Bulk compositions of nine samples (three from each site) are rich in SiO₂ (60-70 wt%) and Al₂O₃ (15-27 wt%), as expected. Concentrations of Fe₂O₃(T) are typically low but variable (2.6-7.2 wt%), reflecting either the amount of pyrite or intensity of Fe-staining (which is locally pervasive). The amount of TiO₂ is typically ~0.6 wt%. All other oxides are present only in trace amounts. Gallium is depleted relative to Al, but not to the same extent as in the Virginia occurrences, which could reflect alteration of the protoliths by less acidic fluids (or lower pSO₄). All samples show the unique, U-shaped REE patterns displayed by the Virginia rocks, which is observed for hydrothermally altered rocks elsewhere. Thus, the Kings Mountain kyanite quartzites display all of the mineralogical and geochemical characteristics of the Virginia rocks, and we conclude that the same high-sulfidation alteration model can be applied here.