Alteration Assemblages and P-T during the Second Phase of Hydrothermal Mineralization in the Hutti Gold Deposit, Raichur District, Karnataka

Abstract

Chlorite is observed along with albite-epidote-calcite±quartz veins and opaque minerals in the distal and proximal zones of the Hutti underground exposures as well as with the albite-epidote-calcite-sphene±k-feldspar±quartz (“inner chlorite”) assemblage in the quartz reefs of Hutti. The undeformed nature of albite-epidote-calcite-chlorite ±quartz-opaque veinlets in the distal metabasalts and in the proximal alteration zone close to the main reefs at Hutti indicate that these veinlets are a late hydrothermal phase that was deposited in brittle fractures of the host-rock after the phase of ductile deformation. Estimates of temperature based on the chlorite geothermometry yield values of 330±40°C for chlorite in the coarse veins with albite-epidote-calcite-k-feldspar±quartz in the proximal alteration zone. The estimates of temperature of the assemblage in the “inner chlorite” assemblage within the quartz reef give values of 290±60°C. The study of fluids entrapped in calcite belonging to this assemblage show that they can be modeled by using the H2O-CO2-NaCl system with a high mole-percent of CO2 and low mole-percent of H2O and NaCl. Estimates of pressure obtained from isochores constructed for the fluid inclusions in calcite from veins in the proximal alteration zone and independent estimates of temperature by chlorite geothermometry give an estimate of ∼2 kbars for fluid pressure at the time of entrapment of the fluid inclusions. Since ore-mineralization associated with this assemblage is limited, it is inferred that this phase of mineralization may have resulted from re-mobilization of pre-existing ores.