Deciphering the source of banded iron formations in the North China Craton

Deciphering the source of banded iron formations (BIFs) is crucial to well understand effects of the dynamic interplay between chemical weathering, tectonism, and volcanism on early ocean composition. So far, it still remains controversial with regard to sources and pathways of iron (Fe) and silicon (Si) within BIFs. In China, BIFs occur throughout mainly within the late Archean supracrustal successions of the North China Craton (NCC). In this regard, we present a thorough compilation of geochemical and Nd-Fe-Si isotopic data of the NCC BIFs in order to offer a unique glimpse into source characteristics of both Fe and Si. Major oxide contents and patterns and specific anomalies of rare earth elements (REEs) confirm that the NCC BIFs are marine chemical sediments free of significant syn- and post-depositional alterations and that they precipitated from hydrothermally influenced seawater. This hydrothermal influence would have provided the dissolved Fe necessary for the formation of BIFs, as further corroborated by their Sm-Nd isotope systematics. Seafloor-vented hydrothermal fluids obtained their REE features via alteration of either juvenile (i.e., more depleted mantle-like) or evolved (i.e., enriched continent-like) underlying mafic rocks and periodically mixed with ambient seawaters that possibly received a significant amount of the benthic REE and Fe inventory through recycling Fe on the continental margin, as indicated by coupled Fe and Nd isotope compositions. In marked contrast, hyperbolic relationships between germanium (Ge) versus Si ratios and Si contents suggest that the Si within the NCC BIFs is dominated by terrigenous weathering Si input. This seems to be in conflict with light Si isotope compositions of the NCC BIFs, generally proposed to imply a predominantly hydrothermal Si source. However, a number of different factors other than the relative proportions of continental versus hydrothermal Si contributions, such as precipitation mechanisms and diagenetic processes, limits the current ability to form a unified interpretation of the Si isotope variation of the BIFs. Therefore, it is concluded that there is a potential source heterogeneity for Si and Fe components of the NCC BIFs, and by extension, contemporaneous seawater, most likely the consequence of the crustal growth and continental evolution of the NCC.