Zinc, cadmium and sulfur isotope fractionation in a supergiant MVT deposit with bacteria

Abstract

Zinc (Zn) and cadmium (Cd) are two important biological-sensitive elements, and their isotopes are strongly fractionated during biological processes. To develop the potential application of Zn and Cd isotopes for tracing the contribution of bacteria to metal mineralization, in this study we investigate for the first time the ranges and causes of isotope fractionation of Zn and Cd in a large hydrothermal system with bacteria from the world-class Jinding Zn-Pb deposit, southwest China. Extremely negative δ34SCDT values of sulfides (−48.6‰ to +7.7‰) obtained in this study and literature reveal the dominant biogenic sulfur associated with bacterial sulfate reduction. Both Zn and Cd isotope ratios are highly variable and significantly lighter compared with most of Zn-Pb ore deposits worldwide. The extremely negative δ66ZnJMC 3-0749L (down to −0.69‰) exceeds the range of Rayleigh fractionation during inorganic precipitation of ZnS. Zinc isotope ratios display a well-defined positive correlation with Zn/Cd ratios and the samples with lightest δ66Zn have lowest Zn/Cd ratios. This reflects that sphalerites formed in the early stages of mineralization already have extremely light Zn isotopic compositions. There is a negative relationship between δ34SCDT and δ66Zn, indicating that lighter Zn isotope ratios are related to stronger bacterial sulfate reduction, which results in heavier sulfur isotope ratios. Since organic compounds (e.g., Zn-carboxylate) in fluids are strongly enriched in heavy isotopes relative to free Zn2+ and Zn-sulfide, sulfides formed in the bacteria-rich hydrothermal systems would acquire light δ66Zn. Therefore, the extremely negative δ34SCDT and δ66Zn ratios demonstrate for the first time that bacterial metabolisms not only induced sulfate reduction, but also were directly involved in the formation of metals (e.g., Zn) in a large hydrothermal system. A similar process may have also partly contributed to the substantial fractionation of cadmium isotopes in the hydrothermal system. Overall, our studies show that zinc and cadmium isotopes may act as an effective tool in deciphering the specific role of organisms in formation of sulfides in hydrothermal systems and other settings.