Geochemistry and origin of the banded Iron formations (BIFs) from the Western Dharwar craton, southern India: Implications for evolving redox conditions of Archean oceans

Abstract

We present mineralogic and elemental data on the Banded Iron Formations (BIFs) from the volcano-sedimentary greenstone belts of the Western Dharwar Craton to address redox conditions of Archean Oceans and origin of BIFs. The studied BIFs are restricted to the uppermost stratigraphic levels in the Sargur Group and Dharwar Supergroup greenstone belts. The BIFs of the older Sargur greenstone sequence are characterized by thick chert layers, which are interspersed with thin Fe-oxide and silicate layers. Iron-rich amphibole grunerite indicates that BIFs from the Sargur Group underwent lower-amphibolite facies metamorphism. In contrast, oxide-silicate‑carbonate-sulphide facies BIFs characterize younger Dharwar Supergroup greenstone belts. Rare silicate assemblages (e.g., stilpnomelane orreibeckite) in BIFs of Dharwar Supergroup reveal greenschist facies metamorphism. Field, mineralogic and elemental characteristics of studied samples show affinity to Algoma-type BIFs. The studied BIFs show significant variation in SiO₂ (49.00–53.00 wt%) and Fe₂O₃ (27.00–51.5 wt%) content. The concentration of all other elements is typically <1 wt% except two samples of BIFs from the Chitradurga basin exhibit higher Al₂O₃ (5.42 wt%) and CaO (9.56 wt%) reflecting traces of terrigenous input. The low ΣREE content (<20 ppm) of studied BIFs from the Sargur Group Holenasirpur, Dharwar Supergroup Bababudan, and Chitradurga – Dharwar-Shimoga greenstone belts preclude continent derived terrigenous input whilst higher total REE (30–53 ppm) of two BIFs samples from the Chitradurga basin is in agreement with traces of terrigenous input. BIFs from the older Sargur Group and the younger Dharwar Supergroup exhibit positive Eu anomalies pointing to BIFs sourced from hydrothermal plumbing system. The trace element ratios (Y/Ho, Sm/Yb, and Eu/Sm) consistent with a dominant hydrothermal input in their origin. The mineralogical facies changes, combined with the redox-sensitive elemental signatures coupled with published redox sensitive isotope biomarkers suggest fluctuation in the redox conditions of ocean basins through Archean and oxygenation of ocean initiated at least two hundred million years before the Great Oxidation Event (GOE).