The felsic volcanism associated BIF-like iron formations: Their origin and implication for BIFs

The Banded Iron Formations (BIFs) have been invaluable archives for the Precambrian eons, providing evidence for the paleoenvironment conditions, including the atmospheric oxygen levels. This study explores the geological origin and depositional mechanism of BIF-like Carboniferous Heiyingshan (HYS) and Mesoproterozoic Pilot Knob hematite (PKH) formations, characterized by hematite-rich (Fe-rich) layer and quartz-rich (Si-rich) jasper layer alternations within felsic volcanic rocks. Through comprehensive mineralogical analyses and geochemical modeling, we present evidence supporting their formation in intra-caldera hot spring lakes, where episodic inputs of Fe(H3SiO4)2-bearing fluids mixed with ambient lake water. Experimental simulations and PHREEQC modeling corroborate a mechanism where ferrihydrite and amorphous silica precursors sequentially precipitate, possibly driven by the oxidation and decomposition rates of Fe(H3SiO4)2 complexes. The sharp or gradational change in the Fe- and Si-rich layering transition can be caused by varying atmospheric oxygen levels. The Carboniferous HYS specimen exhibits a sharp FeSi boundary that is in accordance with modeling under high atmospheric O2 condition. In contrast, the Mesoproterozoic PKH sample contains both sharp and gradual change in the FeSi transition, implying the pO2 lies between 10−3 and 10−5. This chemical precipitation model of BIF-like iron formations could provide insights into the FeSi source and banding mechanism of BIFs, and Precambrian pO2 levels.