The kyanite quartzites of the Willis Mountain and East Ridge deposits in Buckingham County, Virginia hosts the world's largest kyanite mine and constitute the largest known reserves of kyanite. The stratiform kyanite quartzite is overlain and underlain by quartz-muscovite schists which are in turn enveloped by biotite-amphibole gneiss of the Cambrian Chopawamsic formation, metamorphosed volcanic and volcaniclastic rocks. The entire package of rocks has been metamorphosed to amphibolite grade peak conditions of 6.5 kb and 600°C. The kyanite quartzite is very similar in bulk composition and accessory mineralogy to high-alumina rocks of probable hydrothermal origin in North Carolina, South Carolina, and Georgia, and to active stratiform hydrothermal alteration zones in Tatun, Taiwan and Otake, Japan. The major difference among these deposits is the type of aluminosilicate phase present, a reflection of the grade of metamorphism, with kaolinite and pyrophyllite appearing in unmetamorphosed deposits, pyrophyllite and andalusite appearing in zones of greenschist grade, and kyanite appearing in zones of amphibolite grade. Data from active alteration zones and mineral equilibria suggest that the kyanite quartzite was formed from intermediate volcanic rocks by hydrothermal action of acidic fluids (pH 2 to 4) at a temperature between 100 and 200°C under mildly oxidizing conditions. Alumina and silica were residually enriched as alkali and alkaline earth cations were extracted from the rock by hydrolysis. The surrounding quartz-muscovite schist was formed in the same manner by less acidic, cooler fluids. The adjacent biotite-amphibole gneiss also shows signs of partial hydrothermal alteration, such as residual enrichment of alumina represented by a kyanite and/or sillimanite-staurolite-gedrite assemblage.