Metamorphosed Banded Iron Formation of Gorumahisani-Sulaipat-Badampahar belt, in Precambrian Iron Ore Group, Singhbhum Craton, India: ore petrography, micro-chemical and genetic studies

Abstract

The metamorphosed Banded Iron Formation (BIF) of Gorumahisani-Sulaipat-Badampahar (GSB) belt in Precambrian Iron Ore Group, Singhbhum Craton, is well known in the mineral map of India. Detailed characteristics of iron ore and iron-containing rocks in BIF have been investigated at different mine profiles and results are interpreted in terms of their genetic evolution. The BIF constitutes four different facies such as carbonate, silicate, oxide and sulphide in variable proportion. The carbonate facies (Fe-Mn-Mg-Ca) appear as relicts within the oxide facies. The silicate facies includes Co-bearing cummingtonite-grunerite solid solution and quartz. The oxide facies is dominated by magnetite but markedly martitised/goethitised. The protolith of these facies of BIF ore envisioned as carbonate-containing mud originated from a submarine exhalative source, with high concentrations of Fe and Si and lower contents of Mn, Co, Mg and Ca which coprecipitated in an oxidised mixed hydrothermal-marine environment. Subsequently, this has undergone both submarine weathering and metamorphic changes in their evolution. During submarine weathering, carbonates (in Fe-Mn-Mg-Ca solid solution), sheet silicates (greenalite) and quartz were formed. At the outset of regional metamorphic episode, cummingtonite-grunerite and magnetite were developed. Magnetite formed at the expense of carbonate due to decarbonation and was later martitised to haematite and goethitised under post-metamorphic and supergene conditions respectively. With an increasing grade of metamorphism, greenalite transformed to grunerite-cummingtonite solid solution while quartz showed recrystallisation and grain size enlargement.