Confirmation and global significance of a large-scale early Neoproterozoic banded iron formation on Hainan Island, China

Abstract

The Shilu iron ore deposit on Hainan Island, China, contains > 460 Mt of Fe ore reserves and formed during the early Neoproterozoic (1.0–0.8 Ga). However, the genesis of the deposit and whether the deposit actually represents a banded iron formation remain controversial. Here, we present new petrographic, whole-rock geochemical, and Fe isotopic data for Fe ores from the Shilu deposit. These samples have variable but strongly positive Fe isotopic compositions (−0.29‰ to 1.62‰ for δ56FeIRMM-014), contain alternating Si-rich and Fe-rich bands at millimetre to centimetre scales that are identical to those typically found in banded iron formations, and contain widespread fine-grained jasper. They have whole-rock compositions that contain elevated concentrations of Fe and Si, and have post-Archean Australian shale (PAAS)-normalised rare earth element (REE) patterns that are similar to the compositions of chemical sediments. These observations strongly suggest that the Shilu Fe ores were originally a banded iron formation.It is also likely that the Shilu iron formation of Hainan Island correlates with the Aok iron formations in Canada, as evidenced by paleogeographic reconstructions and stratigraphic correlations, indicating that these units represent an early Neoproterozoic iron-formation depositional event related to tectono-hydrothermal activity. The presence of these early Neoproterozoic iron formations that are not associated with glacial deposits indicates that global glaciation is not a requirement for the generation of Neoproterozoic iron formations. Therefore, tectono-hydrothermal activity, rather than seawater redox conditions, is the main controlling factor related to the deposition of banded iron formations.