Stratigraphy, Depositional Setting, and SHRIMP U-Pb Geochronology of the Banded Iron Formation–Bearing Bailadila Group in the Bacheli Iron Ore Mining District, Bastar Craton, India

Abstract

The Bailadila Group of the Bastar Craton, India, is host to a 200-m-thick banded iron formation (BIF). We document the lithostratigraphic context for the BIF, informally referred to as the Bose iron formation, and provide radiometric constraints for its depositional age. Field evidence illustrates that the BIF was deposited on an inner-shelf succession with a quartz arenite that grades upward into the BIF through storm-dominated offshore shelf deposits. The quartz arenite to BIF transition records a relative sea level rise from transgressive to highstand systems tract when the BIFs were deposited in a starved outer continental shelf. U-Pb SHRIMP analyses of zircons from the basement of the Bailadila Group yielded mostly highly discordant U-Pb SHRIMP ages. However, the ages fall on well-defined discordia lines from which concordia intercept ages could be determined. These ages, in combination with the ages of a few zircons that are less than 6% discordant, indicate that the granitoid basement crystallized at 3500–3550 Ma. The maximum depositional age of the Bailadila Group is constrained from the weighted mean 207Pb/206Pb SHRIMP age of 2725±57 Ma from detrital zircons from the basal arenites. A well-constrained weighted mean 207Pb/206Pb SHRIMP age of 2733±53 Ma for zircons from a unit that unconformably overlies the Bailadila Group is within error of that age. Stratigraphic relationships suggest that the Bailadila succession is unconformably overlain by the ~2.5 Ga Kotri and Dongargarh Supergroups. The depositional age of the Bailadila Group is well constrained between ~2.7 and 2.5 Ga. In contrast to most other Archean Algoma-type iron formations of peninsular India, which are closely related to volcanic rocks in greenstone belts, the Bose iron formation is associated with siliciclastic shelf succession. It thus is considered a Superior-type iron formation that represents the oldest known one of its kind in India.