Crucial roles of local hydrothermal activities in the generation of large-scale iron formations during Mesoproterozoic tectonic quiescence

Abstract

Iron formations (IFs), as extremely Fe-rich chemical deposits, play an important role in the global steel industry and the investigation of Precambrian ocean chemistry. However, the controlling factors of IF generation remain enigmatic, particularly during the Mesoproterozoic era when large-scale IFs are scarce. Here we report rare earth element (REE) results of a recently confirmed large-scale Mesoproterozoic-aged IF, the Jingtieshan IF in Northwest China, which could provide new insights into this issue. REE systematics is instrumental in understanding the origins of IFs and ancient ocean chemistry. However, interpretations derived from prior REE studies remain ambiguous due to the analytical challenges posed by the unusually high concentrations of Ba in the Jingtieshan IF. In this study, we present robust REE analyses using a novel method consisting of coprecipitation of REE with Fe, followed by chromatographic purification. This method enables accurate determination of REEs in samples rich in Fe and Ba matrices with ICP-MS. Our results reveal significant positive Eu anomalies (1.52–3.43) in the Jingtieshan IF, demonstrating substantial high-temperature hydrothermal contributions. Asynchronous Eu anomalies across three close IF sections formed during the same period imply that Fe was sourced from different local hydrothermal activities rather than a unified source of deep seawater upwelling. The variable Ce anomalies (0.76–1.62) and subchondritic Y/Ho ratios (18.1–28.4) indicate that the Jingtieshan IF was deposited in anoxic-suboxic waters between Mn and Fe chemoclines within a stratified ocean. Furthermore, global compilations of carbonate Ce anomalies and Fe isotope data in IFs indicate that redox conditions during the mid-Proterozoic allowed large-scale IF deposition under prevalent ferruginous oceans. Growing evidence also highlights strong hydrothermal signatures in mid-Proterozoic IFs worldwide. Collectively, these findings suggest that the scarcity of Mesoproterozoic IFs was the result of reduced hydrothermal Fe flux due to prolonged tectonic quiescence during the transition between the Columbia and Rodinia supercontinents, rather than shifts in ocean redox conditions. We emphasize that the local hydrothermal activities associated with regional rifting played a crucial role in the generation of Mesoproterozoic large-scale IFs.