Alteration mineralogy, fluid inclusions and stable isotope studies from Chigargunta and Bisanatham gold deposits, South Kolar Greenstone Belt, Dharwar Craton, India: Implications on genesis of gold mineralization

Abstract

There are many Neoarchaean orogenic gold hosting greenstone belts in the Dharwar Craton (DC), India; among them the southern extension of the Kolar greenstone belt (SKGB) contain promising gold deposits such as Chigargunta and Bisanatham. Auriferous laminated quartz-calcite veins are hosted mainly within the sheared and altered Champion gneiss and amphibolite in Chigargunta and Bisanatham respectively. The hydrothermal alteration mineral assemblages show three main alteration zones at Chigargunta. However, only two alteration zones were identified at Bisanatham. The hydrothermal mineral assemblage at Chigargunta and Bisanatham indicate a similar mineralogy that consists of tourmaline, muscovite, biotite, chlorite, epidote, sericite, calcite and quartz along with sulfide and gold. Sulfides associated with ore are predominantly pyrrhotite, pyrite, arsenopyrite, chalcopyrite, ±molybdenite in both the places. Native gold is mainly associated with pyrrhotite in the inner and proximal zone of both the deposits. Fluid inclusion petrography and microthermometry from quartz-calcite veins within the mineralized zone at Chigargunta reveal a low to medium salinity (0.5 to 13.3 wt% NaCl equiv.) H2O-NaCl-CO2-CH4 ± N2 bearing fluid. However, at Bisanatham only H2O-NaCl fluid with low salinity (1.56 to 6.44 wt% NaCl equiv.) was observed. P-T conditions of the gold mineralization at the Chigargunta and Bisanatham deposits range between 1.7 and 3.5 kbar pressure/285 °C to 378 °C temperature and 0.8 to 1.2 kbar pressure/365 °C to 405 °C temperature respectively. Alteration mineralogy, ore mineral assemblages, fluid compositions and P-T conditions from the SKGB confirm that the near neutral to slightly alkaline mineralizing fluid transported gold as Au(HS)2− complexes with low ƒO2 conditions. The observed drastic pressure and temperature fluctuation suggests that fault-valve mechanism/pressure cycling occurred at the time of gold precipitation.Carbon (δ13CCO2) isotopic composition of ore fluid deduced from calcite varies from −2.4 to +3.3‰ (average: −0.3‰) and −2.1 to +1.4‰ (average: −0. 8‰) for Chigargunta and Bisanatham respectively. Oxygen isotopic (δ18OH2O) values for both the deposits vary from +7.2 to +14.6‰ (average: +9.3) and +7.3 to +8.9‰ (average: +8.3‰) respectively. Ore-forming fluid could have possibly been derived from the decarbonation of marine carbonates during the metamorphic devolatilization of greenstones. The slight intra-deposit variation of the carbon isotopic value implies the Rayleigh fractionation with progressive consumption of the CO2 as a result of interaction between hydrothermal fluid and wall rock as well as immiscibility of CH4 and CO2 fluid. Ore fluid sulfur (δ34SH2S) isotopic composition, derived from δ34S values of sulfides, varies from −0.42 to +2.43‰ and +0.25 to +2.34‰ for Chigargunta and Bisanatham respectively. Both the sulfur and carbon isotopic values, from these deposits are comparable with other orogenic gold deposits in the DC and elsewhere in the world. Although carbon isotopic values possibly indicate the metamorphic fluid source, the sulfur isotopes signify average crustal sulfur composition that is from crustally derived/modified fluid. Based on the field-geological, mineralogical, fluid, and isotopic compositions, the possible fluid source could be from the metamorphic devolatilization process of the thick greenstone sequence in the SKGB. However, a magmatic source could be further tested with geochronological studies on intrusives in the area.