Biogenic wad in Iron Ore Group of rocks of Bonai-Keonjhar belt,Orissa |
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| Authors: | B K Mohapatra S Mishra P P Singh |
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| Institution: | (1) Oelbergstr. 10, 53343 Wachtberg b. Bonn, Germany; |
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| Abstract: | Outcrop of wad, about 3–5 m thick, associated with low to medium-grade manganese ore deposits in Iron Ore Group (IOG), is present in large quantum in Bonai-Keonjhar belt, Orissa. It is often inter-bedded with volcanic ash layers. Wad is powdery, fine grained, black to blackish-brown in colour, very soft, readily soils the fingers and its hardness on the Mohs’ hardness scale is 1–3. The wad zone is capped by a thin lateritic zone and overlies manganese ore beds of variable thickness in Dalki, Guruda and Dubna mines. Wad constitutes two mineral phases, viz. manganese oxides (δ-MnO2, manganite, romanechite with minor pyrolusite) and iron oxides (goethite/limonite and hematite) with minor clay and free quartz. Mixed limonite-clay and cryptomelane-limonite are commonly observed. Under microscope the ore appears oolitic, pisolitic, elipsoidal to globular in shape having small detritus of quartz, pyrolusite / romanechite and hematite at the core. The ore contains around 23% Mn and 28% Fe with ～7% of combined alumina and silica. Wad might have developed in a swampy region due to slow chemical precipitation of Fe-Mn-Co enriched fluid, nucleating over quartz/hematite grains. Influence of a marine environment is indicated from δ-MnO2 phase. Remnants of some microfossils, like algal filament, bacteria, foraminifera and diatomite are observed in wad sample under SEM. These microorganisms might have been responsible for the oxidation of dissolved Mn2+ and Fe2+ precipitates. These findings suggest biochemogenic origin of wad in Bonai-Keonjhar belt of Orissa. |
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