Geology and Geochemistry of the Jianshan Banded Iron Formation in Shanxi Province, China: Constraints on the Genesis

The Jianshan banded iron formation (BIF) is embedded in Paleoproterozoic strata. The Jianshan BIF can be divided into oxide and silicate facies. The former consists mainly of quartz and magnetite, and the latter contains hornblende and actinolite. Geochemical analyses of the Jianshan BIF show that Al2O3 and TiO2 are quite scarce with no significant correlation, and the contents of incompatible elements and transition metal elements are low, indicating negligible involvement of terrigenous clastics during the deposition. The rare earth element (REE) curves reveal depletions in light REE, enrichments in heavy REE, and positive La, Y, and Eu anomalies, suggesting that the material source was a mixture of submarine hydrothermal fluid and seawater. Although the Ce anomalies are unobvious, the decoupling of Th and U and below-crustal Th/U ratios resulting from the oxidation of U4+, as well as the enrichment of heavy Fe isotopes, imply a rapid rise of oxygen content shortly after the Great Oxidation Event. The Jianshan BIF might deposit in a redox-stratified ocean. It might be an important possibility for the Jianshan BIF that Si deposition is likely attributed to oversaturation caused by a temperature drop of Si-rich hydrothermal fluids and that Fe deposition is probably due to rapid oxidization in the upper oxygenated layer of the redox-stratified ocean.