KYANITE HYDROTHERMAL DEPOSITS OF THE BARRA DO SALINAS REGION (CORONEL MURTA, MINAS GERAIS)

Kyanite hydrothermal deposits from the Barra do Salinas region (Coronel Murta, Minas Gerais)

Abstract

In the region located west of Araçuaí town, Minas Gerais, blue kyanite deposits that produce top quality material for the world mineral collection market have long been known. However, their exact location and mode of occurrence were never reported. Four primary similar deposits were studied in the vicinity of Barra do Salinas village, Coronel Murta county. The mineralized pockets occur along structural lineaments with N20-40°E direction, related to the foliation of the Neoproterozoic (620-570 Ma) biotite schists of the Salinas Formation. Veins are thin and connect meter-size pockets from where the kyanite is extracted. In addition to the kyanite megacrystals in a quartzose matrix, the mineralogical assemblage includes staurolite, rutile, schorl and a complex mixture of muscovite + paragonite + microcline. Other Al₂SiO₅ polymorphs, as andalusite and sillimanite, are absent. The schist that hosts the kyanite deposits contains quartz, biotite and kyanite in the foliation, with garnet and staurolite as porphyroblasts; retrograde textures indicate that biotite and garnet are metastable. We propose that kyanite deposits were formed by the action of hidrothermal fluids during late evolution of the Araçuaí-Ribeira orogenic system.