Formation age, geochemical characteristics and geological significance of the Zhalanzhangzi BIF in eastern Hebei Province

Eastern Hebei Province is a main BIF concentration area in China. However, previous studies of BIF in this area which had undergone low-grade metamorphism were relatively weak. Zhalanzhangzi BIF occurs in the Zhuzhangzi Group which has generally suffered from green schist to low amphibolite facies metamorphism. The iron ore predominantly consists of magnetite and quartz, with minor tremolite and biotite. The main chemical constituents of iron ores are SiO2, Fe2O3 and FeO. Low content of Al2O3 and extremely low content of TiO2 as well as High Field Strength Element (HFSE) indicate that the contribution of continental detritus was insignificant. The concentration of REE in iron ores is low, and Post Archean Australia Shale (PAAS)-normalized rare earth elements profiles for the iron ores display depletion of light REE relative to high REE. All samples have distinct positive anomalies of Eu and slightly positive anomalies of Y, the ratio of Y/Ho in iron ores is high, and the typical characteristics of the REY(REE+Y) profiles resemble those of the mixture of high-temperature hydrothermal fluid and seawater, suggesting that the ore material might have been derived from submarine high temperature hydrothermal fluids and sea water. SHRIMP zircon U-Pb dating was conducted for the biotite-plagioclase granulites interlayered with orebodies in the Zhalanzhangzi BIF which are mainly prismatic grains with visible internal zonation. The mean Th/U ratio of 17 zircon samples is 1.02, indicating that they are magmatic zircons. Ten concordantly distributed data with a weighted mean 207Pb/206Pb age of 2572±8 Ma (MSWD=5.8) indicates the formation age of the Zhalanzhangzi BIF. A comparison with previous studies of medium or high-grade metamorphic BIFs shows that BIF underwent different grades of metamorphism that originated from the same sources and formed nearly simultaneously in eastern Hebei Province, and metamorphism of BIF was probably connected with mantle plume event in Eastern Block of the North China Craton at 2500Ma.