Ore fluids associated with the Wynad gold mineralization, southern India: evidence from fluid inclusion microthermometry and gas analysis

Abstract

In southern India, major gold deposits occur in the Archean granite–greenstone terrain of Kolar, Hutti, Ramagiri, and within Proterozoic granulite terrain in Wynad. The Wynad Gold Field (WGF) which is located within the Moyar–Bhavani lineament—a Proterozoic transcrustal shear zone hosts gold within quartz veins, which are emplaced along the mesoscopic faults and shears in the region. The quartz veins cut the regional fabric of the country rocks, suggesting that the veins were formed in post-tectonic and post-metamorphic episode of crustal shearing. Gold as visible specks or interstitial grains and veinlets are seen within quartz and associated sulfide minerals, indicative of multiple mineralizing events. Fluid inclusion studies carried out on gold–quartz veins showed the frequent occurrence of bi-phase primary inclusions of H 2O–CO 2, pseudosecondary inclusions of H 2O–CO 2 and aqueous secondary inclusions. Microthermometric studies of different samples revealed a near pure CO 2 composition, associated with the aqueous phase, with melting temperatures close to −56.6 °C. Clathrate melting temperatures of primary and pseudosecondary inclusions show that the fluids were low to moderately saline (2–14 wt% NaCl equiv.). Homogenization temperatures of primary and pseudosecondary inclusions range between 250–330 °C and 230–300 °C, respectively. The secondary aqueous inclusions have much lower homogenization temperatures between 130 and 220 °C. Fluid densities calculated from the microthermometric data were between 0.8 and 1.0 g/cm 3. The isochores pertaining to these fluid densities intercept, the trapping temperatures, within pressure ranges of 0.5 and 2.9 kbars. Fluid inclusion gas analyses were carried out on selected samples using a quadrupole mass spectrometer. The analysis shows that the fluid inclusions were mainly of H 2O and CO 2 with minor quantities of N 2 and CH 4, which was in close correlation with our visual analytical data. We envisage that the fluids responsible for the Wynad gold mineralization evolved from an H 2O–CO 2 fluid and precipitated gold at temperatures of 300–350 °C, due to the sudden pressure and temperature decrease as the fluids entered the shear zone.