Primary Iron Minerals of the Late Paleoproterozoic Xuanlong-Type Ironstone in the North China Craton Constrained by Paleomagnetic Methods

Iron formations (IFs) have long been considered to be the product of the interactions between the earth’s lithosphere, hydrosphere, atmosphere and biosphere, which made it an ideal research objective to uncover the evolution of the earth. Hematite is a common iron oxide in IFs and is always used as an indicator of an oxygenated environment. The Xuanlong-type ironstone is a late Paleoproterozoic (~1.64 Ga) hematite-dominated granular iron formation (GIF) in the North China Craton (NCC). However, recent geochemical researches indicated this GIF was precipitated in a weakly oxygenated environment. The conflicts between the mineral composition of the Xuanlong-type ironstone and its geochemical interpretations can be attributed to the potential occurrence of secondary hematite. This is because the age of the growth of secondary hematite could postdate that of the host stratigraphic unit. Therefore, we integrated paleomagnetic, rock magnetic measurements and in-situ techniques to determine the primary iron minerals of the Xuanlong-type ironstone by analyzing the mineralization age of the hematite and to further explore the paleoenvironmental information preserved by the Xuanlong-type ironstone. Stepwise systematic thermal demagnetization showed that the high-temperature characteristic remanent magnetizations, which passed a C-classification reversal test and a fold test, yielded a paleomagnetic pole position at 50°N, 193°E. This pole position is in close proximity to the coeval paleomagnetic pole obtained from the Cuizhuang Formation in the southern NCC. Scanning electron microscope-X-ray energy dispersive spectrometer (SEM-EDS) and rock magnetic results confirmed Al-substituted hematite in the Xuanlong-type ironstone. Taking all experimental results into account, the high-temperature component of the natural remanent magnetization is interpreted to be the primary remanent magnetization acquired through crystal growth of Al-substituted hematite during early diagenesis, marking Al-substituted hematite was the primary iron mineral in the Xuanlong-type ironstone. We analyzed the most possible mineral transformation from ferrihydrite to Al-substituted hematite and proposed that the different degrees of Al-substitution in alternating dense and porous laminae within the hematitic ooids of Xuanlong-type ironstones actually reflected two sources of water (hydrothermal fluids input and runoff input) that dominated differently in the pore water of sediments during the precipitation of Al-substituted hematite. In the end, it is hypothesized that these laminae are a result of periodic sea level fluctuations, considering that the ironstones were likely formed in a shallow water setting in the land-ocean transitional zone.