Fluid evolution in the Kolar Gold Field: evidence from fluid inclusion studies |
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| Authors: | B Mishra M K Panigrahi |
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| Institution: | (1) Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India e-mail: bmgg@gg.iitkgp.ernet.in, IN |
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| Abstract: | Gold mineralization in the Kolar schist belt of the Dharwar craton occurs dominantly in the form of a sulfide-poor Au-quartz
lode (the Champion lode exposed in the Mysore and other mines) and sulfide-rich auriferous lodes (from the Nundydroog mine).
Fluid inclusion microthermometric experiments were conducted on primary inclusions in quartz intimately associated with Au-mineralization.
Homogenization studies on aqueous-biphase (L + V), aqueous polyphase (L + V+ halite) and aqueous-carbonic (LCO2± VCO2 + Laq) inclusions from the Champion lode furnish a temperature range of 120 to 420 °C. Freezing of aqueous biphase inclusions and
dissolution of halite in the aqueous polyphase inclusions provide salinity of 5 to 50 wt.% NaCl equivalent. Fluid inclusion
thermobarometry from the total homogenization of aqueous-carbonic inclusions and from intersecting isochores of coeval pure-carbonic
(LCO2± VCO2) and pure-aqueous inclusions constrain the P-T path of evolution of the fluid in the Champion lode. Gold precipitation was likely to have been brought about in response
to a sharp fall in pressure with attendant unmixing of liquid-CO2 from the parent H2O-CO2 fluid of possible metamorphic origin. This would imply transportation of gold by some pressure-sensitive complex such as
the Au-carbonyl. Fluid characteristics are different in the sulfide-rich auriferous lodes, as indicated by the virtual absence
of the CO2-bearing and the halite-bearing inclusions. The fluid evolution path, as evident from the crude positive colinearity of temperature
and salinity, is due to mixing of a low (≤200 °C) temperature-low saline (≤7 wt.% NaCl equivalent) fluid with a high temperature
(≥400 °C)-high saline (≥50 wt.% NaCl equivalent) fluid. The lack of CO2 and association of Au with sulfides indicate a different mode of gold transport, as chloride or bisulfide complexing, deposition
of which was possibly brought about by fluid mixing.
Received: 17 April 1997 / Accepted: 30 June 1998 |
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