Cobalt-Bearing Grunerite in the Metamorphosed Banded Iron Formation in Orissa, India

Banded iron formation (BIF) comprising high grade iron ore are exposed in Gorumahisani-Sulaipat-Badampahar belt in the east of North Orissa Craton, India. The ores are multiply deformed and metamorphosed to amphibolite facies. The mineral assemblage in the BIF comprises grunerite, magnetite/martite/ goethite and quartz. Relict carbonate phases are sometimes noticed within thick iron mesobands. Grunerite crystals exhibit needles to fibrous lamellae and platy form or often sheaf-like aggregates in linear and radial arrangement. Accicular grunerite also occur within intergranular space of magnetite/martite. Grunerite needles/accicules show higher reflectivity in chert mesoband and matching reflectance with that of adjacent magnetite/martite in iron mesoband. Some grunerite lamellae sinter into micron size magnetite platelets. This grunerite has high ferrous oxide and cobalt oxide content but is low in Mg- and Mn-oxide compared to the ones, reported from BIFs, of Western Australia, Nigeria, France, USA and Quebec. The protolith of this BIF is considered to be carbonate containing sediments, with high concentrations of Fe and Si but lower contents of cobalt and chromium +/- Mg, Mn and Ni. During submarine weathering quartz, sheet silicate (greenalite) and Fe-Co-Cr (Mg-Mn-Ni)-carbonate solid solution were formed. At the outset of the regional metamorphic episode grunerite, euhedral magnetite and recrystalized quartz were developed. Magnetite was grown at the expense of carbonate and later martitized under post-metamorphic conditions. With the increasing grade of metamorphism greenalite transformed to grunerite.