Genesis of the Neoarchean Algoma-type banded iron formation: constraints from Fe isotope and element geochemistry of the Qian'an iron deposit, eastern North China craton

The Neoarchean Qian'an BIF deposit in eastern Hebei, eastern North China Craton has attracted extensive attention of study in the last decades, but the genesis, e.g., Fe sources, metallogenic mechanism, as well as tectonic attributes of the BIFs remains highly disputed. Based on field investigations, microscopic observations and geochemical analysis, this study tries to unravel the source characteristics of ore-forming materials in the Qian'an deposit. The results show that the chemical compositions of the BIF ore samples are mainly composed of TFe2O3 and SiO2, with minor Al2O3 and TiO2. The total trace element contents of the samples are relatively low. The PAAS-normalized REE distribution patterns of the ores show LREE depletion and HREE enrichment, with robust positive anomalies of Eu, Y and La. These characteristics indicate that the BIFs are attributed to dominant chemical precipitation originated from paleo-ocean with obvious volcanic hydrothermal contributions and minor clastic input. Their positive to no Ce anomalies and positive delta 56Femagnetite values unveil that the iron was precipitated at low oxygen fugacity. These results, in collaboration with evidence from previous lithological, structural geology, metamorphic P-T paths, geochemistry, geochronology and numerical modeling studies, support a mantle plume model to explain the complicated tectono-thermal processes, and the sources of BIFs in Eastern Hebei, eastern North China Craton.