The composition and genesis of the Mesoarchean Dagushan banded iron formation (BIF) in the Anshan area of the North China Craton

Abstract

The Dagushan BIF-hosted iron deposit in the Anshan–Benxi area of the North China Craton (NCC) has two types of iron ore: quartz–magnetite BIF (Fe2O3T<57wt.%) and high-grade iron ore (Fe2O3T>90wt.%). Chlorite-quartz schist and amphogneiss border the iron orebodies and are locally present as interlayers with BIFs; chlorite-quartz schist and BIFs are enclosed by amphogneiss in some locations. The quartz–magnetite BIFs are enriched in HREEs (heavy rare earth elements) with positive La, Eu and Y anomalies, indicating their precipitation from marine seawater with a high-temperature hydrothermal component. Moreover, these BIFs have low concentrations of Al2O3, TiO2 and HFSEs (high field strength elements, e.g., Zr, Hf and Ta), suggesting that terrigenous detrital materials contributed insignificantly to the chemical precipitation. The high-grade iron ores exhibit similar geochemical signatures to the quartz–magnetite BIFs (e.g., REE patterns and Y/Ho ratios), implying that they have identical sources of iron. However, these ores have different REE (rare earth element) contents and Eu/Eu* values, and the magnetites contained within them exhibit diverse REE contents and trace element concentrations, indicating that the ores underwent differing formation conditions, and the high-grade ores are most likely the reformed product of the original BIFs.The chlorite-quartz schist and amphogneiss are characterized by high SiO2 and Al2O3 contents and exhibit variable abundances of REEs, enrichment in LREEs (light rare earth elements), negative anomalies in HFSEs (e.g., Nb, Ta, P and Ti) and positive anomalies in LILEs (large ion lithophile elements, e.g., Rb, Ba, U and K). A protolith reconstruction indicates that the protoliths of the chlorite-quartz schist are felsic volcanic rocks. SIMS and LA-ICP-MS zircon U–Pb dating indicate that this schist formed at approximately 3110 to 3101Ma, which could represent the maximum deposition age of the Dagushan BIF. However, two groups of zircons from the amphogneiss are identified: 3104 to 3089Ma zircons that are most likely derived from the chlorite-quartz schist and 2997 to 2995Ma zircons, which are interpreted to represent the time of protolith crystallization. Thus, the Dagushan BIF most likely formed before 2997 to 2995Ma. The ~3.1Ga zircons yield εHf(t) values of −8.07 to 5.46, whereas the ~3.0Ga zircons yield εHf(t) values of −3.96 to 2.09. These geochemical features suggest that the primitive magmas were derived from the depleted mantle with significant contributions of ancient crust.