Fluid evolution in the Kolar Gold Field: evidence from fluid inclusion studies

Gold mineralization in the Kolar schist belt of the Dharwar craton occurs d ominantly in the form of a sulfide-poor Au-quartz lode (the Champion lode e xposed in the Mysore and other mines) and sulfide-rich auriferous lodes (fr om the Nundydroog mine). Fluid inclusion microthermometric experiments were conducted on primary inclusions in quartz intimately associated with Au-mi neralization. Homogenization studies on aqueous-biphase (L + V)! aqueous po lyphase (L + V + halite) and aqueous-carbonic (L-CO2 +/- V-CO2 + L-aq) incl usions from the Champion lode furnish a temperature range of 120 to 420 deg rees C. Freezing of aqueous biphase inclusions and dissolution of halite in the aqueous polyphase inclusions provide salinity of 5 to 50 wt.% NaCl equ ivalent. Fluid inclusion thermobarometry from the total homogenization of a queous-carbonic inclusions and from intersecting isochores of coeval pure-c arbonic (L-CO2 +/- V-CO2) and pure-aqueous inclusions constrain the P-T pat h of evolution of the fluid in the Champion lode. Gold precipitation was li kely to have been brought about in response to a sharp fall in pressure wit h attendant unmixing of liquid-CO2 from the parent H2O-CO2 fluid of possibl e metamorphic origin. This would imply transportation of gold by some press ure-sensitive complex such as the Au-carbonyl. Fluid characteristics are di fferent in the sulfide-rich auriferous lodes, as indicated by the virtual a bsence of the CO2-bearing and the halite-bearing inclusions. The fluid evol ution path, as evident from the crude positive colinearity of temperature a nd salinity, is due to mixing of a low (less than or equal to 200 degrees C ) temperature-low saline (less than or equal to 7 wt.% NaCl equivalent) flu id with a high temperature (greater than or equal to 400 degrees C)-high sa line (greater than or equal to 50 wt.% NaCl equivalent) fluid. The lack of CO2 and association of Au with sulfides indicate a different mode of gold t ransport, as chloride or bisulfide complexing, deposition of which was poss ibly brought about by fluid mixing.