THE BIF-LIKE HEMATITE-JASPILITE FORMATIONS: THEIR ORIGINS AND IMPLICATIONS FOR BIFS

The banded iron formations (BIF)-like Carboniferous Heiyingshan (HYS) and Mesoproterozoic Pilot Knob hematite-jaspilite formations are characterized by alternating hematite-rich (Fe-rich) and jasper (Si-rich) layers and associated with felsic volcanic rocks. However, the geological setting and formation mechanism of BIF-like formations remain controversial as previous studies hold various hypotheses on the genesis: submarine intermittent volcanic eruptions, continental hydrothermal events, or volcanic-hot spring sedimentary origin. To explore the nature of the BIF-like hematite-jaspilite formations, a mineralogical study using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and (scanning) transmission electron microscopy was carried out to properly evaluate the rock. The Carboniferous HYS formation is mainly comprised of nano-size and relatively pure hematite and quartz, suggesting a low-temperature precipitation environment but displays lamination distortion caused by late-stage Hercynian orogeny. The Pilot Knob formation was never deeply buried nor went through orogenic events. Lab experiments and PHREEQC modeling are used to simulate the initial stage in the formation of oscillating Fe-Si layers, which is compared with a modern analogue from Chocolate Pots, Yellowstone National Park. These results suggest that the hematite-jaspilite formations are chemical sedimentary rocks deposited in an intra-caldera hot spring after Fe(H₃SiO₄)₂-bearing spring fluid mixed with ambient lake water during felsic volcanic intervals. After aqueous complex Fe(H₃SiO₄)₂ decomposition, the faster Fe²⁺ oxidation and precipitation rate led to hematite-rich layer preceding jasper layer with siliceous ferrihydrite as the precursor to the hematite. Episodic Fe(H₃SiO₄)₂ input resulted in successive cycles of layering. The Carboniferous HYS specimen exhibits a sharp Fe-Si boundary that is in accordance with modeling under high atmospheric O₂ condition (pO₂ = 0.35). Meanwhile, the Mesoproterozoic Pilot Knob sample contains both sharp and gradual change on the Fe-Si transition, the latter implies the pO₂ can be lower than 10^-5 based on PHREEQC modeling. This abiotic iron mineral and silica precipitation model could provide insights into the banding mechanism in the BIFs.