GENESIS OF MAGNESITE DEPOSITS OF ARCHEAN SARGUR SUPRACRUSTAL BELT, KARNATAKA, INDIA

Vein-type magnesite deposits are common in the ultramafic rocks of the Sargur supracrustal belt of Dharwar craton, southern India. The ultramafic rocks predominated by serpentinized dunites and peridotites contribute to the magnesium in the deposits while the source of CO₂ has always been more speculative. As the ultramafic rocks are exposed well in the weathering zones it is easy to assume that the sources could be CO₂-laden meteoric waters. The other possibility is the CO₂-rich fluids that might have risen up through the fractures from deeper regions. The deep-seated sources could either be devolatilizing carbonate sediments or magmatic CO₂. A carbon and oxygen isotope study of 25 magnesite samples collected depth-wise from several working and abandoned magnesite mines of Karnataka lays constraints on the genesis of the magnesite and the possible sources for their deposition. Stable isotope compositions obtained from the Sargur magnesites give δ¹³C_PDB values ranging from -6.72‰ to -1.21‰ and δ¹⁸O_SMOW values ranging from 30.26‰ to 31.68‰, which are suggestive of a near-surface epithermal environment. The positive δ¹⁸O may indicate contribution to the oxygen isotopic composition from atmospheric sources as a consequence of plant transpiration. But the tight clustering and non-variability of stable isotope values of δ¹⁸O values with depth may be indicative of deeper source or a mixture of sources, deeper and meteoric.

The C and O isotope compositions of Sargur magnesites are comparable to the Archean Budd magnesite deposits of the Barberton greenstone belt, South Africa. The stable isotope compositions of the Sargur magnesites are suggestive of their precipitation in low-temperature epithermal environment and were probably deposited during Archean times. The sources for CO₂ could be devolatalizing carbonate sediments undergoing metamorphism as evidenced from the marble occurrences near Bettadabidu, close to Nanjugud area, Karnataka. The heat source could be the charnockitization process evidenced from the granulites present in the vicinity. This may also explain the absence of significant magnesite deposits in the Sargur Group ultramafic rocks that occur about 100 km north of the Sargur type area and as well rule out a singular meteoric source for the origin of these magnesite deposits.