The Kabirdham iron deposit, a new discovery by the Geological Survey of India in the northern margin of the Bastar Craton, is associated with banded iron formations (BIFs) below a dolomite horizon in the lower part of the Chilpi Group (Mishra and Mohanty, 2021). The presence of primary chamosite, greenalite and siderite associated with early magnetite and late hematite has been identified during petrographic, XRD and EPMA investigations. The geochemical analysis reveals dominant composition of SiO2 + Fe2O3total (average value 85.86 wt %) of BIF bands suggest their formation predominantly through chemical precipitation. Major oxides such as Al2O3 (0.41 to 19.15 wt %) and K2O (0.004 to 3.56 wt %), and trace elements like Rb (0.35 to 40.2 ppm), Sr (7.75 to 208.96 ppm), Zr (4.2 to 189.4 ppm), Hf (0.09 to 3.84 ppm), Cr (6.64 to 29.86 ppm), Co (2.68 to 35.96 ppm) and Ni (13.09 to 30.2 ppm) show wide variations in the BIF bands. The PAAS normalized REE-Y pattern shows positive La anomaly, positive Gd anomaly, and super chondritic Y/Ho ratio (average 32.15), indicating the preservation of seawater like signatures in the basin of deposition of the BIF. Involvement of hydrothermal fluids is negated from Eu/Sm vs. Sm/Yb, and Eu/Sm vs. Y/Ho contents. The extremely low MnO/Fe2O3total ratio (average 0.001) and redox-sensitive trace element ratios indicate a dysoxic to suboxic-anoxic state of the seawater at the time of deposition. The low positive to transitional negative Ce anomaly, presence of oolitic textures and occurrence of chamosite further strengthen the arguments for the shallow water and anoxic condition in the depositional basin of the Kabirdham BIF. We infer a low atmospheric oxygen content, similar to the reported value from the Archean, during the deposition of these iron-rich rocks. Significance of such a depressed oxygen content in the Paleoproterozoic atmosphere needs further investigation to understand a billion year delay in biological evolution after the deposition of these rocks.

Reference(s)

Mishra, PK and Mohanty, SP 2021, Geochemistry of carbonate rocks of the Chilpi Group, Bastar Ccraton, India: implications on ocean paleoredox conditions at the late Paleoproterozoic era. Precambrian Research, 353,106023. doi:10.1016/j.precamres.2020.106023.