PACROFI VI - Electronic Program

Contrasting Fluid Evolution in the Kolar Gold Field: Evidence From Fluid Inclusion Studies

B. Mishra and M.K. Panigrahi

Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721302 INDIA

The structurally controlled lode-type Au mineralization in the Kolar Gold Field occurs within the metabasic rocks in the Dharwar Craton, S. India. Two distinct modes of occurrence of the metal has been identified on the bases of field observation and microscopic studies. These are: (i) Au- quartz (+ calcite + scheelite) lodes constituting of the bulk of mineralization in the Champion Reef and (ii) Au-quartz-sulfide veins (Oriental lode), parallely disposed with respect to the general NNW-SSE trend. Detailed fluid inclusion studies in quartz from the two ore associations reveal some of the hitherto unreported gross differences in fluid chemistry vis-a-vis mechanism of transportation and deposition of Au.

The dominance of a carbonic-component in the fluid giving rise to the Au-quartz lodes is indicated by the presence of aqueous-carbonic and monophase-carbonic (CO2+CH4) inclusions, as against their total absence in the Au-bearing quartz-sulfide veins (Oriental lode). Though aqueous-biphase and halite-bearing polyphase inclusions are common to both the types, a paucity of the latter variety in the Au-quartz-sulfide lodes is clearly marked. The P-T conditions of evolution of the ore-fluid and the mineralization event have been worked out for the Champion Reef from appropriate inclusion assemblages. The FLINCOR software package (Brown, 1989) was used for the P-V-T-X properties of pertinent inclusion fluids. Pressure determined from aqueous-carbonic inclusions using the methods outlined by Brown and Lamb (1986, 1989) and Parry (1986), range from 3.6 to 3.1 Kb at a temperature range of 300 - 282oC. Intersecting isochores of coexisting aqueous(L+V) and carbonic inclusions furnished a pressure range of 1.9 - 0.65 Kb at temperatures of 260 - 200oC. These P-T values necessarily indicate a dominantly decompression phenomenon which brought about the unmixing of the parent H2O-CO2 fluid. This picture when collated with the fluid evolution (temperature-salinity) diagram for the Champion Reef helps in delimiting the P-T condition of ineralization. A clustering of points on the temperature-salinity plot within a temperature interval of 300 - <200oC for the high saline polyphase and some low-saline aqueous-biphase inclusions indicate that the bulk of Au (+quartz) precipitated in that interval in response to a pressure drop and concomitant unmixing of the parent fluid. However, there is no evidence of boiling in any stage of evolution of the ore-fluid in either of the two ore regimes, contrary to an earlier reported work (Santosh, 1986). Lack of appropriate inclusion assemblage rules out any P-T estimation for the Au- quartz-sulfide ores. The simple and reasonably linear trend obtained from temperature-salinity plots of mostly the aqueous-biphase inclusions therein, indicates mixing of two fluid components - (i) a high-temperature(>350oC) high-saline(up to 30 wt% NaCl eq.) and (ii) a low-temperature(< 150oC) low-saline(~ 4 wt% NaCl eq.) fluid. Though the origin of the first component is debatable, the second component is most likely to be of meteoric ancestry.

The deduced fluid evolution path in the Champion Reef indicates that the CO2 derived during the metamorphism of the regionally extensive mafic volcanics, has been the major carrier of Au in the form of carbonyl (CO) complexes. Lack of data does not allow speculation on the mode of transport of Au in the Au-quartz-sulfide regimes.