Generation of reactive oxygen species on pyrite surfaces: A likely oxidation mechanism for near-vent, hydrothermal fluid-dominated BIFs

Abstract The key to understand the origin of banded iron-formations (BIFs) formed in 3.8–2.5 Ga is to identify the oxidation mechanism of Fe2+ in anoxic oceans. In this study, we proposed a likely pyrite-induced Fe2+ oxidation mechanism for near-vent, hydrothermal fluid-dominated BIFs. A series of important environmental conditions including light intensity, wavelength and pH were considered and monitored to check their effects on the production of reactive oxygen species (ROS) in anaerobic pyrite suspensions. By quantifying the concentration of OH and H2O2, we observed a stable ROS production as induced by pyrite in both dark and light under a wide pH range. Interestingly, light significantly promoted the producing rate of ROS, which increased with the light intensity, but was poorly correlated with the light wavelength. In addition, the producing rate of ROS remarkably increased as the pH decreased from 6.0 to 3.0, suggesting the acidic conditions were more beneficial for ROS generation. The production of ROS could be attributed to the semiconducting photocatalysis of pyrite, and the sulfur-deficient defect on its surface. Based on the sulfur content of volatiles in modern submarine basalts and black smokers, we assumed that the concentration of H2S in Archean hydrothermal fluids ranged from 0.5 to 55 mmol/kg. Combined with the experimental results of H2O2 producing rates under 150 mW/cm2 irradiation at pH 3.0 and under dark at pH 6.0, the H2O2 flux of 6.9 × 105–1.1 × 109 mol/yr could be estimated in a thermal flow. This value covered the amount of H2O2 required for Fe2+ oxidation in Archean oceans to form the near-vent, hydrothermal fluid-dominated BIFs (1.7 × 106–1.3 × 107 mol/yr). The reaction kinetics revealed that the conversion of amorphous FeS into pyrite under hydrothermal conditions was very quick, while the consumption of H2O2 during the back-reaction with pyrite was quite limited. This view can explain why this type of BIFs typically occurs as intercalated layers in volcanic rocks and is associated with volcanogenic massive sulfide ore (VMS)-type deposits.