Geochemistry, zircon U-Pb chronology and Hf isotope composition of the Heishan’gou iron deposit in the Bikou Terrane, central China: Implication for the genesis of the Yudongzi banded iron formations

Abstract

The Yudongzi banded iron formations (BIFs) are widely spread in the Neoarchean Yudongzi Group of the Bikou Terrane, Central China. Their sedimentary and metamorphic ages, however, remain controversial, either their forming or reworking processes. In this contribution, we conducted an integrated study of zircon U-Pb-Hf isotope geochemistry, as well as mineral and bulk-rock geochemistry on the Heishan’gou iron deposit, the largest iron deposit of the Yudongzi BIFs, to constrain their formation and metamorphism ages, source of ore-forming material and tectonic setting. Mineral chemistry analysis indicates that the amphibolite, host-rock of the Heishan’gou deposit suffered amphibolite facies metamorphism. LA-ICP-MS U-Pb dating of zircon grains from orthometamorphic host-rocks suggests that the Heishan’gou deposit was formed at the early Neoarchean (2781 ± 43 Ma for amphibolite and 2776 ± 43 Ma for plagioclase-hornblende schist), and metamorphosed at ca. 2480 ± 26 Ma and 1818 ± 44 Ma, respectively. The low contents of aluminum, titanium and high field strength elements in BIFs suggest that they are detritus-free marine chemical precipitates. The rare earth elements (REE) and Y patterns of BIFs display seawater-like signatures with light REE depletion, and positive La and Y anomalies. Meanwhile, their positive Eu anomalies suggest that submarine hydrothermal fluid might also be involved in the precipitation of BIFs. Combined with zircon Hf isotope data and geochemical compositions of amphibolites, it is proposed that the Yudongzi BIFs should be formed at back-arc basin setting. Compared with the representative Archean continent nucleus of the Yangtze Block (Kongling Complex), the Yudongzi Complex experienced distinct crustal evolutionary history. It indicates that the Yudongzi Complex might represent the accretionary block combined to the Yangtze continental nucleus later than the Paleoproterozoic.