Fractionation of cadmium isotope caused by vapour-liquid partitioning in hydrothermal ore-forming system: A case study of the Zhaxikang Sb-Pb-Zn-Ag deposit in Southern Tibet

Here, we conducted a case study of the Zhaxikang Sb-Pb-Zn-Ag deposit to explore Cd isotopic fractionation mechanisms in hydrothermal ore-forming system. The delta Cd-114/110 values of sphalerite systematically decrease from ore mineralization stage 1 (-0.30 parts per thousand to 1.01 parts per thousand; average value=0.17 parts per thousand; n=4) through stage 2 (-0.51 parts per thousand to -0.09 parts per thousand; average value=-0.23 parts per thousand; n=3) to stage 3 (-0.34 parts per thousand to-0.23 parts per thousand; average value=-0.285 parts per thousand; n=2). A simple Rayleigh distillation models this temporally decreasing trend resulted from Cd isotopic fractionation that is most likely related to vapor-liquid dynamics of ore-forming fluid. This mechanism for Cd isotopic fractionation is further augmented by the general geochemical characteristics and fluid inclusion data of sulfides and cogenetic gangue minerals. Firstly, the sphalerite has low Cd concentrations (1183-2199 ppm), correspondingly high Zn/Cd ratios (248-421), and large Cd isotopic variation range with relatively lower delta Cd-114/110 values (-0.51 parts per thousand to 1.01 parts per thousand). Commonly, sphalerite from sedimentary exhalative systems possess these characteristics caused by vapor-liquid interactions. Secondly, fluid inclusion data from the Pb-Zn sulfides and cogenetic carbonate alteration minerals indicates that vapor-liquid two-phase inclusions are in dominance (more than 90 parts per thousand). Meanwhile, the vapor-liquid ratios (20-50 parts per thousand) of these two-phase inclusions are consistent with those of the established Cd (25.48-55.07%) and Zn (26.70-48.70 parts per thousand) isotopic Rayleigh distillation model. To conclude, vapor-liquid partitioning is main cause for observed Cd isotopic variations in Zhaxikang deposit, and is demonstrated as an important Cd isotopic fractionation mechanism in hydrothermal ore-forming system.