Zn isotopic variations in ore-related granite and ore minerals in a magmatic-hydrothermal deposit

Metal isotopes are powerful tools for investigating the migration and distribution of ore-forming elements. Zinc (Zn) is predominantly transported as chloride complexes in fluids, and its valence state remains relatively unaffected by redox conditions, thus Zn isotopes can provide valuable insights into the formation of magmatic-hydrothermal ore deposits. However, significant uncertainties persist regarding Zn isotopic fractionation in magmatic-hydrothermal systems during exsolution of hydrothermal fluids from silicic melts and crystallization of Zn-bearing minerals. The Lengshuikeng Ag–Pb–Zn deposit, one of the largest magmatic-hydrothermal Ag deposits in South China, was formed associated with the Mesozoic granitoid magmatism, and thus serves as an ideal natural laboratory for investigating Zn isotopic variations during the granite magmatism and hydrothermal mineralization. Systematic Zn isotope analyses were conducted on hydrothermal Zn-bearing ores, gangue minerals, and ore-related and unrelated felsic rocks. Sphalerite is the main ore mineral, which can be divided into two generations based on the “chalcopyrite disease” texture and Fe content variations. The early-formed sphalerite displays lighter Zn isotopic compositions (δ66Zn = 0.05 ‰ to 0.30 ‰) and higher Fe contents (6 wt% – 10 wt%) than the late-formed ones (δ66Zn = 0.30 ‰ to 0.60 ‰, Fe < 6 wt%), which probably resulted from Rayleigh-type Zn isotopic fractionation between sphalerite and ore-forming fluid. The ore-forming fluid was derived from large-scale exsolution of volatiles under a cumulative mode. Fe-Mn carbonate minerals are the main Zn-bearing gangue minerals, and they coexist with, and have similar δ66Zn values (0.24 ‰–0.41 ‰) to those of the late sphalerite. The estimated δ66Zn value of the initial ore-forming fluid is around 0.26 ‰, which is lower than those of the ore-related and unrelated felsic rocks (0.23 ‰–0.69 ‰), further supporting the role of fluid exsolution during the Zn isotopic fractionation. Based on the early experimental data and our theoretical modeling, a new Zn-isotope fractionation model has been reconstructed during the processes from Zn-bearing fluids released from the felsic melt to Zn-bearing mineralization. Our findings reveal that hydrothermal fluids are richer in Zn and have lighter Zn isotopic compositions than their felsic residual melts (i.e., solidified granites), and the Zn isotopic fractionation is influenced by amounts of the released fluids and volatile components (i.e., Cl- and HS-) in magmatic-hydrothermal systems. Therefore, Zn isotopes serve as powerful and sensitive indicators for tracing mineralization processes.