DETERMINING THE PEAK METAMORPHIC CONDITIONS OF THE WILLIS MOUNTAIN KYANITE DEPOSIT: A GEOTHERMOBAROMETRIC APPROACH

Willis Mountain, Virginia, is the largest known deposit of kyanite in the United States, mined and processed by Kyanite Mining Corporation. The rocks comprising the deposit are a mix of kyanite quartzites and kyanite schists, containing up to 55% kyanite plus varying amounts of quartz, muscovite, rutile, pyrite and iron hydroxides. The deposit occurs within an overturned anticline and the surrounding rocks are kyanite-bearing garnet mica schists. Prior work indicates that the protolith had a metasedimentary origin and underwent considerable alteration within a back-arc hydrothermal system (Owens & Peters, 2018). However, more limited work has been published on the peak metamorphic conditions under which the kyanite developed, with most authors referring to amphibolite facies metamorphism.

This work presents a detailed petrographic and geothermobarometric study of kyanite quartzite (1 sample), kyanite muscovite schist (2 samples) and kyanite-bearing garnet mica schist (1 sample) to determine the peak metamorphic conditions at Willis Mt. This study applied Ti-in-Quartz, Zr-in-Rutile and Fe-Mg in garnet-biotite thermometers and the GASP (Garnet-Aluminosilicate-Quartz-Plagioclase) barometer. The calculated range of P-T conditions from these methods are 420 – 681ºC and 4.8 – 6.0 kBar. The upper range of temperatures, which includes data from both Ti-in-Quartz and Zr-in-Rutile thermometry, would place the rocks within the sillimanite stability field. As sillimanite is absent from these rocks and kyanite is in equilibrium, this indicates that the use of trace element thermometers in these rocks should be carefully evaluated.

Owens, B.E. and Peters, B.J., 2018, Ferruginous Quartzites in the Chopawamsic Terrane, Piedmont Province, Virginia: Evidence for an Ancient Back-Arc Hydrothermal System. Economic Geology, 113 (2), 421-438; doi: 10.5382/econgeo.2018.4556