Redox state of Archean surface environments: Insights from the Banded Iron Formations (BIFs) of the Western Dharwar Craton, Southern India

The interplay of geological, chemical and biological processes that drive the oxygenation of the oceans-atmosphere of the early earth are spatially linked to the emergence of biosphere. Banded Iron Formations (BIFs) from the Archean greenstone belts form important archives for understanding the redox conditions of Archean surface environments. The Archean Dharwar craton preserves BIFs in the volcano-sedimentary greenstone belts of two distinct stratigraphic units (older Sargur Group and younger Dharwar Supergroup) corresponding to a time span of 3300-2600 Ma.  These BIFs are confined to the highest stratigraphic levels forming summits of greenstone belts.  They show alternate layers of chert and iron oxides, and petrographic data reveal diverse mineralogy including oxides, carbonate, sulphide and silicate facies. The occurrence of riebeckite and stilpnomelane in BIFs of younger Dharwar Supergroup indicates recrystallization under low-grade metamorphism. Slightly higher abundances of CaO and Al₂O₃ reveal significant influence of crustal source and precipitation of CaCO₃ during BIFs formation. Mesoscopic layers of chert and iron oxide with variable thickness suggest fluctuating redox state of surface environments. The higher enrichment of Ni (6-26 ppm) than the Cr content (3-19 ppm) with variable Sr concentrations may be attributed to feldspar breakdown during hydrothermal fluid acceleration. Trace element ratios (Y/Ho, Sm/Yb, Eu/Sm) coupled with positive Eu anomalies of the BIFs from both older Sargur Group and younger Dharwar Supergroup BIFs reveal dominant hydrothermal input in BIFs origin. The PAAS normalized REE data preclude major continental input in the origin of BIFs. The variable negative Ce anomalies imply periodic fluctuating surface environments (oxic to anoxic) at the dawn of the Great Oxidation Event close to 2340 Ma. This is consistent with the published Fe, N, and S isotope data on the BIFs of the Western Dharwar craton.