Mechanisms of cadmium isotopic fractionation during the substitution of cadmium for zinc in sphalerite (ZnS)

The chalcophile character of Cd controls its hyper-enrichment in Zn–Pb ore deposits via substitution for Zn in sphalerite (up to 13.2 wt%) and, as such, these deposits form Earth’s richest Cd reservoirs. Cadmium isotopes can provide insights into the origins of these deposits and the behavior of Cd in hydrothermal systems. However, Cd isotopic variations in sulfides are affected by mineral precipitation and the initial composition of the fluid, which remain poorly constrained for sphalerite precipitation. This hinders the application of Cd isotopes to ore-forming systems. In this study, we investigated Cd isotopic fractionation during absorption into sphalerite under varying pH, ionic strength, initial Cd2+ content, and mineral crystallinity. Using extended X-ray absorption fine structure (EXAFS) spectroscopy and spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM), we demonstrate that Cd is incorporated into the ZnS lattice rather than being adsorbed onto its surface. Isotopic equilibrium is achieved after 18 h in pure water and 10 % NaCl at pH 5.5, with lighter Cd isotopes preferentially partitioning into the solid phase. The magnitude of isotopic fractionation (Δ114/110Cdsolid–solution ≈ –0.5 ‰) is largely unaffected by the initial Cd2+ concentration, mineral crystallinity, and reaction pH in 10 % NaCl, but is higher than that in pure water (–0.7 ‰ to –0.9 ‰). This is probably due to Cl– complexation favoring the lighter Cd isotopes, leaving isotopically heavier Cd2+ to be absorbed into the sphalerite. Monte Carlo simulations using the obtained fractionation factors indicate the composition of the ore-forming fluid is the primary control on the Cd isotopic variations in sphalerite. These results advance our understanding of Cd isotopic systematics in hydrothermal systems and during supergene sulfide weathering, and provide a framework for interpreting Cd isotopic data in natural environments.