Cadmium isotope fractionation in the Fule Mississippi Valley-type deposit, Southwest China

High-precision cadmium (Cd) isotope compositions are reported for sphalerite, galena, and smithsonite from the Fule Zn–Pb–Cd deposit, a typical Mississippi Valley-type deposit in Southwest China. Dark sphalerite has lighter δ 114/110 Cd values (0.06 to 0.46 ‰) than light sphalerite (0.43 to 0.70 ‰), and the Cd in galena is primarily in the form of sphalerite micro-inclusions with δ 114/110 Cd of −0.35 to 0.39 ‰. From early to late stages, δ 114/110 Cd values of smithsonite regularly increase from 0.19 to 0.42 ‰, whereas Cd/Zn ratios decrease from 252 to 136; the δ 114/110 Cd variation pattern of supergene smithsonite reflects kinetic Rayleigh fractionation during low-temperature processes. From the bottom to the top of the orebody, the dark sphalerite has different patterns in δ 114/110 Cd values, Cd/Zn ratios, δ 34 S values, and Fe concentrations compared to the light sphalerite, indicating that dark and light sphalerite formed by different processes. The varying patterns of δ 144/110 Cd values and Cd/Zn ratios within light sphalerite are similar to those of layered smithsonite, and the δ 144/110 Cd values have a positive correlation with δ 34 S values, indicating that Cd isotope fractionation in the light sphalerite was due to kinetic Rayleigh fractionation. Instead, in dark sphalerite, the δ 144/110 Cd values have a negative correlation with δ 34 S values and a positive correlation with the Cd/Zn ratio. Thus, it can be concluded that dark sphalerite could be modeled in terms of two-component mixing (basement fluid and host-rock fluid), which is in agreement with previous explanations for the negative correlation between δ 66 Zn and δ 34 S in some typical Zn–Pb deposits. We propose that the significant variation in Cd isotope composition observed in the Fule Zn–Pb–Cd deposit confirms that Cd isotopes can be used for tracing fluid evolution and ore formation.