Geochemical constraints on the genesis of the Algoma-type banded iron formation (BIF) in Yishui County, western Shandong Province, North China Craton

Abstract

The Yishui iron deposit is a typical Algoma-type banded iron formation (BIF) located in Yishui County, Taishan Group, Shandong Province in the Eastern Block (EB) of the North China Craton (NCC). The BIF is interlayered with amphibolite, migmatitic granite, gneiss, and schist. Integrated petrological, mineralogical, and geochemical interpretations of the iron ores were used to reconstruct the genesis of the Yishui BIFs and their tectonic evolution. The iron ore samples were collected from surface outcrops. Banding textures are not always present in the iron ores because the BIF experienced complex high-grade metamorphism after precipitation. Interpretations of the major elements and rare earth elements plus yttrium (REY) data of the iron ores suggests that the ore-forming materials of the Yishui BIF precipitated from a mixture of seawater and high-temperature hydrothermal fluids (<0.1%) in a submarine environment with volcanic activity under relatively low oxygen conditions. Large amounts of iron and silica were produced under these conditions. The main compositions of SiO2 and Fe2O3T (SiO2+Fe2O3T=85.8–95.8wt%) are consistent with the major minerals, such as quartz and iron oxide, which represent chemical precipitates. Chlorite, amphibole, plagioclase, biotite, and calcite are present as minor minerals. The low contents of TiO2 (0.01–0.09wt%), Al2O3 (0.42–1.18wt%), and high field strength elements (HFSEs) indicate little to no detrital assimilation, which implies that the iron ores preserved their original geochemical signatures. Based on geochemical interpretations, we conclude that the Yishui BIF can be explained by a mantle plume model that describes not only the tectonic evolution but also the sources of iron and silica of the Yishui BIF.