Anoxic to suboxic Mesoproterozoic ocean: Evidence from iron isotope and geochemistry of siderite in the Banded Iron Formations from North Qilian, NW China

Banded Iron Formations (BIFs) provide important constraints on geochemical cycling of Fe and ancient marine environment. Globally, the formation of BIFs peaked during late Archean and early Paleoproterozoic, with limited reappearance during Neoproterozoic. Here we investigate a rare example of Mesoproterozoic (ca. 1.3 Ga) BIFs from Jingtieshan in the North Qilian region of NW China. We present rare earth element and yttrium (REE + Y) compositions together with iron isotope features of siderite from the BIFs, and integrate these with our published data on carbon isotope compositions from the siderite-rich BIFs (including carbonate facies BIFs and mixed carbonate-oxide facies BIFs). Siderite from the BIFs shows REE + Y characteristics that are consistent with submarine hydrothermal fluids mixed with seawater, including positive Eu/Eu∗ anomalies and high Y/Ho ratios. The siderite also displays homogeneous iron isotope compositions (δ56Fe range from −0.71‰ to −0.41‰, one sample down to −1.62‰) and moderate negative carbon isotope compositions (δ13C values = −8.40‰ to −3.0‰). These findings argue against the traditional view that siderite associated with BIFs was formed through microbial dissimilatory iron reduction (DIR). Instead, siderite from Jingtieshan BIFs is in equilibrium with submarine hydrothermal fluids and seawater suggesting an inorganic origin, and was directly precipitated from the CO2 and Fe oversaturated water column, with only insignificant role for DIR. These Mesoproterozoic BIFs are also characterized by Ce anomalies, and the siderite falls within the pe-log [S]T and (PO2)-(PCO2) stability fields, suggesting that the BIFs were deposited in an anoxic to suboxic ocean. The results from our study suggest the presence of iron-rich Mesoproterozoic deep ocean in the North Qilian area.