Geochemistry and Si–O–Fe isotope constraints on the origin of banded iron formations of the Yuanjiacun Formation, Lvliang Group, Shanxi, China

Abstract

Banded iron formations (BIFs) within the Lvliang region of Shanxi Province, China, are hosted by sediments of the Yuanjiacun Formation, part of the Paleoproterozoic Lvliang Group. These BIFs are located in a zone where sedimentation changed from clastic to chemical deposition, indicating that these are Superior-type BIFs. Here, we present new major, trace, and rare earth element (REE) data, along with Fe, Si, and O isotope data for the BIFs in the Yuanjiacun within the Fe deposits at Yuanjiacun, Jianshan, and Hugushan. When compared with Post Archean Australian Shale (PAAS), these BIFs are dominated by iron oxides and quartz, contain low concentrations of Al2O3, TiO2, trace elements, and the REE, and are light rare earth element (LREE) depleted and heavy rare earth element (HREE) enriched. The BIFs also display positive La, Y, and Eu anomalies, high Y/Ho ratios, and contain 30Si depleted quartz, with high δ18O values that are similar to quartz within siliceous units formed during hydrothermal activity. These data indicate that the BIFs within the Yuanjiacun Formation were precipitated from submarine hydrothermal fluids, with only negligible detrital contribution. None of the BIF samples analyzed during this study have negative Ce anomalies, although a few have a positive Ce anomaly that may indicate that the BIFs within the Yuanjiacun Formation formed during the Great Oxidation Event (GOE) within a redox stratified ocean. The positive Ce anomalies associated with some of these BIFs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed Ho, LREE, and Ce4+; these deposits formed during reductive dissolution at the oxidation–reduction transition zone or in deeper-level reducing seawater. The loss of Ce, LREE, and Ho to seawater and the deposition of these elements with iron hydroxides caused the positive Ce anomalies observed in some of the BIF samples, although the limited oxidizing ability of surface seawater at this time meant that Y/Ho and LREE/HREE ratios were not substantially modified, unlike similar situations within stratified ocean water during the Late Paleoproterozoic. Magnetite and hematite within the BIFs in the study area contain heavy Fe isotopes (56Fe values of 0.24–1.27‰) resulting from the partial oxidation and precipitation of Fe2+ to Fe3+ in seawater. In addition, mass-independent fractionation of sulfur isotopes within pyrite indicates that these BIFs were deposited within an oxygen-deficient ocean associated with a similarly oxygen-deficient atmosphere, even though the BIFs within the Yuanjiacun Formation formed after initiation of the GOE.