Geochemistry of Late Archaean shaly BIF formed by oxic exogenic processes: an example from Ramagiri schist belt, Dharwar Craton, India

Abstract

The central block of the auriferous Ramagiri schist belt, in the Eastern Dharwar Craton, India consists of bimodal volcanics (mafic-felsic), shaly BIF and metasedimentary rocks. Geochemical studies of the associated shaly BIF have indicated the enrichment of the major and trace elements such as SiO2, Al2O3, TiO2, K2O, MgO, Fe2O3(T), Zr, Y, Cr, Ni, alkali and alkaline earth elements indicates that the clastic component of the shaly BIF had their contribution from the contemporaneous bimodal volcanics. The concave chondrite normalized REE patterns share ubiquitously anomalous positive cerium anomaly, absence of positive europium anomaly and the overall HREE enrichment. The REE patterns resemble those from the modern day sea water, except for positive Ce anomaly. The data suggests that arc related bimodal volcanism had been the plausible source of Fe, silica, REE and other trace elements. The coherent behaviour of Fe, Ti, Mn and P with the REEs indicates that they got incorporated from Fe–Ti–Mn bearing primary minerals and secondary products like clays. The variability of REE patterns in the BIF formation samples probably results from the differences in scavenging efficiency. The BIF bears signatures of mixing of the contemporaneous clastic and chemical processes, as well as the changes accompanying diagenesis and metamorphism. The precipitation of Fe did not stop during the sedimentation in an island arc related tectonic setting. The BIF strongly lacks the signatures from hydrothermal input. The presence of positive cerium anomalies and the absence of positive europium anomalies in the shaly banded iron-formations imply that iron oxidation during BIF deposition took place in shallow waters rather than at depth, at oxic-anoxic boundary.