Genesis of Gold Mineralization in the South Kolar and Gadag Greenstone Belts, Dharwar Craton: Constraints from Hydrothermal Alteration, Tourmaline Chemistry, Fluid Inclusion and Stable Isotope Studies

Abstract

South Kolar greenstone belt (SKGB) in the Eastern Dharwar Craton (EDC) and Gadag greenstone belt (GGB) in the Western Dharwar Craton (WDC) are well known for Neoarchean orogenic gold deposits in India. Chigargunta (CG) and Bisanatham (BN) deposits with contrasting host rocks such as Champion gneiss and metabasalt respectively within the SKGB reflect almost similar steeply dipping structural attitudes (0–10/74W– 87E) that controls the emplacements of auriferous lodes. On the other hand, third generation deformations along the NW-SE were the key structural control for major gold mineralization in turbidite hosted Gadag gold field (GGF) in the GGB. Hydrothermal alteration mineral assemblages i.e., quartz + carbonate + muscovite + chlorite + sericite + tourmaline (± biotite) are common in both the SKGB and GGF deposits irrespective of their host rock compositions, deformation settings and P-T conditions of alteration. Although, mineralogically they are similar, alteration mineral chemistry and substantial mobility of elements during alteration of two contrasting lithounits from the CG (Champion gneiss) and BN (metabasalt) typically fingerprint the host rock chemistry. Abridged activity-activity [(aMg2+/aH+) vs. (aK+/aH+) and (aNa+/aH+) vs. (aK+/aH+)] diagrams corroborate the observed alteration-induced mineralogical changes, in accordance with the isocon plot and constrain the possible fluid composition. Occurrences of native gold in association with sulfides are more common in the SKGB while both invisible lattice bound refractory as well as native gold are observed in the GGF. Hydrothermally precipitated tourmalines intimately associated with/without sulfides, in the alteration zones, from the gold deposits of the CG, BN and GGF belong to dravite or oxy-dravite group. A significant fluctuation in chemical compositions (XFe, Mg, Ca) from proximal to inner zone and strong chemical zoning of tourmaline grains without changes in Na content reflect no changes in fluid salinity in the CG and suggest ore fluid evolution with multiple pulses in a cyclic fluid flow event. Such notable change in fluid chemistry is attributed to the result of fluctuation of fluid pressure during seismic fracture propagation accompanying gold mineralization event. The intra-deposit chemical fluctuation within tourmaline in the BN and GGF are insignificant. The low salinity and reduced nature of the ore fluid are consistent throughout all the deposits inferred from low to medium Na, medium to high X-site vacancy and low Fe3+/Fe2+ ratio. Detailed fluid inclusion study from the mineralized quartz-carbonate veins reveals low to medium saline (CG: 0.5–13.3 wt% NaCl equiv.; BN: 1. 6–6.4 wt% NaCl equiv; GGF: 0.04–9.6 wt% NaCl equiv.) H2O-NaCl-CO2±CH4±N2 primary fluid. Estimated P-T conditions (CG: 1.7–3.5 kbar/285–378 ℃; BN: 0.8–1.2 kbar/365405 ℃; GGF: 1.62.9 kbar/296333 ℃) by combining fluid inclusion, chlorite and arsenopyrite thermometry reflect greenschist facies conditions of alteration and mineralization at the SKGB and GGF. Alteration mineral assemblages, tourmaline chemistry and fluid inclusion study confirm that the low saline, reduced fluid transported gold as Au(HS)2 − complex and precipitated gold as a consequence of pressure drop induced phase separation as well as wall rock interaction processes rather than fluid mixing. Sulfur isotopic compositions of the ore fluid (34SH2S) (CG: –0.4 to +2.4‰, BN: +0.3 to +2.3‰ and GGF: +1.0 to +3.4‰) are indicative of average crustal sulfur source. The 34S (+1.5 to +4.5‰) values of mineralized sulfides overlap with host-rock early pyrites (–1.0 to +7.5‰) in the GGF. Thus, it can be inferred that the sulfur in the mineralizing fluid most likely have derived either by desulfidation and/or dissolution of early pyrites during the continuous fluid flux along the shear zone. Carbon (δ13CCO2) isotopic compositions of ore fluid deduced from δ13C of carbonates furnish a range from –2.4 to +3.3‰ in the CG, – 2.1 to +1.4‰ in the BN and –5.9 to +1.6‰ in the GGF. Such inferred narrow ranges signify that the carbonates in ore forming fluid could have possibly been derived by decarbonation or dissolution of marine carbonates during the metamorphic devolatilization of the greenstone belts. Hence, the metamorphic source of ore-forming fluid is postulated for the gold mineralization at the SKGB and GGF and it is comparable with other orogenic gold hosting greenstone belts in the Dharwar Craton and elsewhere in the world.