Changes of Ge/Si, REE + Y and Sm[sbnd]Nd isotopes in alternating Fe- and Si-rich mesobands reveal source heterogeneity of the ~ 2.54 Ga Sijiaying banded iron formation in Eastern Hebei, China

Abstract

The North China Craton (NCC) is one of the most important regions hosting abundant banded iron formations (BIFs). The ~2.54Ga Sijiaying BIF, the best-preserved and most extensive deposit in Eastern Hebei, is intercalated and closely associated with meta-volcanic rocks of the Luanxian Group. In this context, major and trace element and SmNd isotopic analyses of individual mesobands of a Sijiaying BIF specimen were conducted to characterize the source and depositional environment over a transient period.Low Al2O3, TiO2 and high field strength elements (HFSEs) concentrations show that the BIF is relatively detritus-free. Shale-normalized rare earth and yttrium distributions (REE+Y) of individual BIF mesobands exhibit positive La anomalies, enrichment in HREE relative to LREE and MREE and suprachondritic Y/Ho ratios, which are typical features of marine waters throughout the Archean and Proterozoic. The presence of consistently positive Eu anomalies indicates a significant high-T hydrothermal input, while the absence of true Ce anomalies suggests deposition from an anoxic water column. By comparison with other typical BIFs (e.g., the Isua BIF from Greenland; the Kuruman BIF from South Africa), the Sijiaying BIF is depleted in HREE, and appears to record variations in solute fluxes related to changing intensities of hydrothermal activity. These features, coupled with SmNd isotopic relations and Ge/Si distributional patterns, point to two decoupled sources controlling the depositional environment of the BIF and thus reveal source heterogeneity for silica and iron of the Sijiaying BIF. High Fe fluxes were associated with seafloor-vented hydrothermal fluids, which received their SmNd isotopic signature from alteration of depleted oceanic crust; whereas significant amounts of silica were associated with ambient seawater whose REE signature was controlled by solutes derived from weathering of nearby Mesoarchean continental landmasses. The old (up to ~3.0Ga) Nd (TDM) model ages of Si-rich mesobands of the BIF support such a scenario.