Petrography and geochemistry of the iron-rich rocks in the banded iron formation of the Chilpi Group, Central India: Implications on the level of oxygen in the Paleoproterozoic atmosphere before the “Proterozoic iron ore gap”

The Chilpi Group (2050–1850 Ma) in the Bastar Craton contains Banded Iron Formation (BIF) deposited immediately after the Great Oxidation Event but before the “Proterozoic iron ore gap”. Baseline geochemical data generated from this crucial period of Earth's history representing the transition from an initial high productivity period followed by productivity collapse, thereby delaying the evolution of biota, are interpreted in terms of the redox state of the ocean and atmospheric oxygen content. Presence of chamosite, greenalite and siderite in the ironstones of the Chilpi Group, identified during present analysis, provide valuable information regarding the redox state of the shallow sea. Geochemical analyses reveal high concentrations of Fe2O3total and SiO2 (average ~ 87.85 wt%) in iron-rich bands. Trace elements commonly enriched in detrital phases (e.g. Sc, Hf, Nb, Th and Zr) show good correlation with total REE concentration, but these have concentration below the cut-off limit defined for detrital sediments (except 3 samples). Higher concentration of these elements in greenalite/chamosite-rich samples indicates accumulation of these elements in greenalite/chamosite during primary precipitation. Most of the samples have low concentrations of Al2O3 (