Mineralogical and geochemical characteristics of banded iron formations of the Dalli-Rajhara area, Central Bastar Craton: Implications for the origin and depositional environment

Abstract

An integrated study involving field, mineralogy and whole-rock geochemistry is presented for a Mesoarchean (∼2.9–2.7 Ga) banded iron formations (BIF) from the Dalli-Rajhara area, central Bastar Craton, India. BIF bands preserve alternate Fe-rich magnetite bands and Fe-poor bands comprising dominantly chert with minor Fe-oxide minerals. The mineral chemistry of magnetite phases reveal FeO and Fe₂O₃ contents of 29.16–30.67 wt% and 66.53–68 wt%, respectively, whereas Al₂O₃ (0–0.03 wt%) and TiO₂ (0–0.08 wt%) contents are observed to be typical of BIF. Bulk rock geochemical analyses of the BIF show an average value of 97.19 wt% SiO₂ + Fe₂O₃^T, low Al and Mg, which indicate chemical precipitation as the process of formation. The absence of Al-rich mineral phases and a poor correlation between Al₂O₃ and TiO₂ suggest insignificant contamination by terrigenous clastic materials. The Post Archean Australian Shale (PAAS) normalized REE pattern shows positive La, Gd and Y anomalies and a superchondritic Y/Ho ratio (avg. 43), indicating the preservation of seawater-like signatures. Al/(Al + Fe + Mn) vs. Fe/Ti, ΣREE vs. Cu + Co + Ni, Eu/Sm vs. Sm/Yb and Eu/Sm vs. Y/Ho ratio plots suggest the participation of hydrothermal fluids during deposition. From the Eu anomaly, we infer anoxic condition and intense hydrothermal activity in the basin during the deposition of BIF. This study reveals that the Dalli-Rajhara BIF shares strong similarities with Archean Algoma-type BIFs, regarding (a) stratigraphic linkages with amphibolites (meta-volcanics) and (b) a higher Eu anomaly (avg. 5.1). Based on the field observations and geochemical signatures, we suggest that the Dalli-Rajhara BIF was deposited in a submarine volcanic environment mixed with seawater similar to those of Archean Algoma-type BIFs.