Elemental Substitution and Compositional Evolution of Tourmaline and Dumortierite Formation in Muscovite-schist Rock of Sakoli Area in Bhandara District, Central India

Abstract

Paleoproterozoic metapelitic schistose rocks are closely associated with the tourmaline deposit in the Girola area of the Sakoli Group of rocks of the Central India Tectonic Zone (CITZ). In this area, shear zones have effectively played a significant role in the formation and distribution of tourmaline along with hosts of minerals like garnet, sillimanite, dumortierite, etc. Local tourmaline concentrations can be found within quartz mylonites, granite mylonites, phyllonites, etc. wherein they are present as solitary crystal clusters, or discontinuous tourmaline bands or pockets. Mineralogical and chemical changes have been documented within these rocks in Girola Hill. The preliminary study had indicated the presence of tourmaline varieties, namely- schorl, dravite, foitite, elbite, liddicoatite, dumortierite, along with sillimanite, muscovite, sericite, rutile, topaz, and corundum. The presence of apatite, Sr-phosphate, and fluorite was also noticed. Within the assemblage zone, variations in boron metasomatism and potash leaching appear to be regulating variations in mineral assemblages and overall rock chemistry. Magmatic-metasomatic fluids have partially altered the original sedimentary patterns, according to the elemental analysis of the materials. The alteration could be due to interactions of schists with B-, F-carrying fluid. Tourmaline-rich assemblages develop when the fluid’s boron reacts with the nearby schistose rocks, leading to sheet silicate to cease functioning as a trap for Na, K, and Ba. Further metasomatic activity results in the formation of dumortierite from the tourmaline assemblages.