Iron Enrichment Mechanism in Granular Iron Formation (GIF) of the Wuzhiling Formation, Songshan Group, North China Craton: Implications for Metallogeny

The extensive Paleoproterozoic iron formations (IFs) distributed across the North China Craton serve as ideal proxies for understanding the redox variations in paleo-oceans before and after the Great Oxidation Event (GOE). Here, we present a comprehensive analysis of the lithofacies, mineralogy, and geochemical characteristics of the granular iron formation (GIF) within the Paleoproterozic Wuzhiling Formation of the Songshan Group in the southern North China Craton. Our findings reveal that the GIF represents a chemical sedimentary rock, precipitated within a water column characterized by layered oxygen-depleted to suboxic conditions. The protolith sediments of the GIF in the Wuzhiling Formation are hypothesized to have comprised quartz-coated hematite microparticles, along with precursor deposits of aragonite and hexagonal iron oxides, commonly referred to as green rust. Mineralogically, the GIF is predominantly composed of quartz and hematite, with the iron content attributed to the weathering processes of continental crustal materials, the influence of low-temperature hydrothermal fluids, and the incorporation of iron from seawater. During the GOE, a pivotal transition in Earth's history, the (Pr/Yb) ratios normalized to PAAS (Post-Archean Australian Shale) standards exhibit increased dispersion, accompanied by a gradual decrease in the Y/Ho ratios. These geochemical shifts suggest a pronounced alteration in the ancient marine environment, transitioning from a monolithic reducing state to a more complex redox stratification within the seawater after the GOE. In summary, this study contributes to the reconstruction of paleo-ocean redox conditions around the GOE, providing evidence for the re-establishment of Precambrian paleoenvironmental conditions.