Shear Zone Controlled Sulfide-gold Occurrences and their Fluid Inclusion Characteristics in the Northern part of Eastern Dharwar Craton, Southern India

Abstract

Abstract Brittle to semi-brittle shear zones near Gogalgatti, Mincheri and Yerjanti villages in the northern part of Eastern Dharwar craton show sporadic sulfide and gold occurrences. These shear zones are confined to a late Archaean (2.5 Ga) granitoid complex, consisting of TTG gneisses and granite plutons. Intense mylonitization, brecciation and fracturing are conspicuous in all the three shear zones. The mineralization is of disseminated type and the ore minerals are confined to fracture filling quartz veins. The veins are mainly parallel to the shear zones with minor veins showing cross- cutting relationship. The observed sulfide minerals in decreasing order of abundance are pyrite, chalcopyrite, arsenopyrite, tennantite and sphalerite with minor oxide minerals represented by specular hematite. Random chip sampling in Gogalgatti shear zone gave gold values from 0.52 to 0.65 ppm. However, similar sampling from Mincheri has not indicated any gold, though one of the polished sections of the samples from this area showed an exsolved gold globule within arsenopyrite. In the mineralized veins of these shear zones, primary inclusions of both type I (H2O) and type II (H2O-CO2) are more conspicuous. Secondary inclusions, though present, are mostly affected by deformation. Sulfide minerals are associated with moderate to high saline, type I fluids formed in the early stage of mineralization in all the three shear zones, whereas gold precipitated from high saline (28 -30 wt% NaCl eqvi. in Gogalgatti and ~29% wt% NaCl eqvi. in Mincheri) type II fluids in the later stages, in Gogalgatti and Mincheri shear zones. The temperature of fluid ascent has been determined to be 100 - 200° C in epithermal conditions of mineralization at a depth of 150 to 200 m. Granitoids, in which these shear zones are located, form the main source of hydrothermal fluids, which ascented and precipitated in the structural domains.