The Origin and Environmental Significance of Banded Iron Formations in the Baizhiyan Formation of Wutai Greenstone Belt, Shaanxi Province

The Wutai greenstone belt preserves abundant BIFs resources, which distributes in the formations of Jingangku, Wenxi and Baizhiyan. Among them, the BIFs of Baizhiyan Formation have the largest industrial scale and commercial value. In this study, the authors present detailed petrographic and geochemical analyses for twelve iron ores from eight mines or ore occurrences, and LA-ICP-MS U-Pb zircon dating for one metamorphic volcanic rock (quartz chlorite albite schist) interbedded with BIFs. The results show that the petrography of BIFs from the eastern to western part has changed, with the iron-bearing mineral switching from magnetite to iron carbonate (ankerite or siderite) gradually, corresponding to the increasing tendency of CaO content and FeO/Fe₂O₃ value. LA-ICP-MS U-Pb zircon dating result suggest that the protolith of meta-volcanic rock formed in the (2 526±14) Ma, which represents the deposition age of BIFs in the Baizhiyan Formation. High concentrations of Al₂O₃, TiO₂, HFSEs, TREY and their positive correlations indicate that there was a minor terrigenous input, and these detritus are probably crustal felsic rocks based on La/Sc, Th/Sc and Zr/Sc ratios. Even so, the majority of BIF samples display seawater-like REY profiles characterized by concordant positive La and Y anomalies, HREE enrichment relative to LREE and superchondritic Y/Ho ratios in PAAS-normalized REE diagrams. Consistently positive Eu anomalies are also observed, which suggests the participation of high-T hydrothermal fluids. According to the improved element mixing calculation model of Sm/Yb vs. Y/Ho and Eu/Sm vs. Sm/Yb, the source of BIFs in the Baizhiyan Formation are derived from mixtures of major seawater and minor high-T hydrothermal fluids (0.1%~1%), accompanied by minute contamination of crustal felsic clasts (<0.1%). Compared with carbonate-rich BIFs in the west, the magnetite-rich BIFs in the eastern and middle part received larger contribution from the terrigenous clast. In addition, based on the "two-step tectonic evolution model" for Neoarchean-paleoproterozoic Wutai greenstone belt, the BIFs in the Baizhiyan Formation was formed in the end of the first phase(2.56~2.52 Ga). Moreover, no Ce anomalies, positive Eu anomalies,low Th/U values and extremely high Fe/Mn ratios might indicate an anoxic condition for the contemporary sedimentary water. Furthermore, the water was inhomogeneous and showed subtle redox layering. The western carbonate-rich BIFs deposited in deeper reduced water with higher positive Eu anomalies, while the middle and eastern magnetite-rich BIFs deposited in relatively shallow weak-reduced water with lower positive Eu anomalies.