Features and genesis of vein-type magnesite deposit in the Doddakanya area of Karnataka, India

Vein-type magnesite deposit of Doddakanya area is situated in the southern part of the Archaean Dharwar craton. Magnesite is confined to a c.3.3 Ga old ultramafic body which constitutes a tectonically dismembered part of a layered ultramafic-mafic complex emplaced into supracrustal rocks of the Sargur Group. The magnesite-bearing ultramafic body is composed of dunite with minor harzburgite and exhibits varying degrees of serpentinization and weathering. About 50 vol.% of the ultramafic body is serpentinized and the intensity of serpentinization decreases with depth. The serpentine minerals are represented essentially by lizardite/chrysotile exhibiting pseudomorphic mesh-texture. Serpentinization of the ultramafic body took place within 6 km of the Earth's surface at low temperatures (<200°C) and essentially involved volume expansion (isochemical) process. During weathering, the relict olivine of the serpentinized ultramafic body was pseudomorphously replaced by a mixture of amorphous material (with variable amounts of Fe, Mg, Si and volatiles) and minor ferric oxyhydroxides and secondary silica. The Mg mobilized during weathering participated in the formation of magnesite in tensional fractures developed during the late- to post- serpentinization event. Oxygen isotope data (δ¹⁸O = av. 30.6‰ SMOW) indicate the precipitation of magnesite at ambient temperatures and carbon isotope values (δ¹³C = av. -2.6‰ PDB) suggest the derivation of carbon from a biogenic soil source dominated by C4 plants. The latter constrains the timing of the magnesite formation to a period not older than 8 - 7 Ma.