Genetic implications of fluid-deposited disordered graphite and methane-rich inclusions in the Jonnagiri granodiorite-hosted gold deposit, Eastern Dharwar Craton, India

Abstract

Laminated quartz veins, occurring within sheared granodiorite in the Jonnagiri greenstone belt, host lode gold mineralization. The greenstones were metamorphosed to lower amphibolite facies condition. Hydrothermal alteration produced inner zone of auriferous laminated quartz vein and contiguous proximal zone within the sheared granodiorite. Detailed fluid inclusion studies by Saravanan et al. (2009) reveal presence of three types of fluid inclusions. These are type-I aqueous-gaseous, type-II gaseous and type-III low saline aqueous inclusions. Raman spectroscopic studies performed in this study, on type-II inclusions in quartz veins from the inner and proximal zones reveal presence of disordered graphite, occurring as thin films within fluid inclusions, apart from variable proportions of CO2, CH4 and H2O. Perple_X -aided phase diagram computations were carried out in the C-O-H system at 300°C and 2kbar to examine the chemical evolution of the C-O-H fluids. The calculated logfO2 values on the graphite saturation surface range from −35.71 to −32.65 when XO varies from 0.1 to 1.0. However, log fO2 decreases significantly to −49.31 at XO=0.1E–07 that pertains to the stability of almost pure CH4. Nearly similar logfO2 values were reproduced from the calculations using the COH spread sheet. Decrease in fO2, as demonstrated by the occurrence of gaseous fluid inclusions with disordered graphite and varying proportions of CO2-CH4 in the gaseous part, played a dominant role in decreasing gold solubility during fluid-rock interaction and precipitating gold at Jonnagiri.