ORIGIN OF GRUNERITE SCHIST-HOSTED MAGNESITE MINERALIZATION IN COPPER MOUNTAIN AREA, SANDUR SCHIST BELT, KARNATAKA

Magnesite mineralization in the Copper Mountain area of the Sandur sch ist belt occurs scattered in a 5km long and 50 to 200 m wide grunerite schist unit of Late Archaean age. Magnesite occurs mostly as less tha n a cm to about 50 cm thick parallel to subparallel veins. The veins a re confined to gently dipping joints and tensional fractures transecti ng the steeply dipping bedding and schistosity of the host rock. Magne site is cryptocrystalline and is associated with minor amounts of dolo mite/magnesian calcite, calcite, cryptocrystalline silica and iron oxy hydroxides. Magnesite veins are essentially of cavity filling origin. Field setting, rock and mineral chemistry and stable isotope data indi cate that magnesite was precipitated at ambient temperatures from desc ending meteoric waters enriched in biogenic and atmospheric CO2 and Mg 2+, the latter derived from the upper weathered portion of the host gr unerite schist.