Mode of Occurrence and Origin of Magnesite Deposits in Parts of Mysore District, Karnataka

Magnesite occurs as a network of veins and bands within serpentinised peridotite and cap-rock (birbirite). Three stages by which the original peridotite is altered to give rise to magnesite and birbirite are recognised. Geochemical studies indicate that by the action of weakly acidic (with dissolved CO₂) water vapour, the original ferromagnesian minerals in peridotite broke down releasing Mg²⁺ and to a minor extent Si⁴⁺ to be precipitated as magnesite and quartz in favourable locales. Ca²⁺, Al and Fe²⁺ were retained in the silicate matrix. Al and Fe²⁺ precipitated as hydroxides while Ca²⁺ formed CaCO₃(calcite and aragonite).

The approximate P-T conditions at which magnesite crystallised is suggested to be between 500 to 1000 bars and 150 to 250°C respectively with XCO₂ in the fluid phase around 0.05. Magnesite formation is related to late-stage hydrothermal activity associated with ultramafic-mafic magmatism in the area.