Cd isotope constraints on metal sources of the Zhugongtang Zn-Pb deposit, NW Guizhou, China

The Sichuan-Yunnan-Guizhou metallogenic province (SYGMP) includes > 400 Zn - Pb deposits and prospects, eight of which are large-scale deposits with large reserves of critical metals such as Cd and Ge, including the Huize Zn - Pb - Cd - Ge and Daliangzi Zn-Pb-Ge-Cd deposits. The newly discovered Zhugongtang Zn - Pb deposit is a super-large deposit with Zn-Pb reserves of > 3 Mt. Its geochemical features are similar to those of the Huize deposit, with similar sulfide delta 34S values and concentrations of critical elements in sphalerite (e.g., Cd and Ge). However, the two deposits have different host strata, and it remains unclear as to whether they have similar oregenesis. In this study, delta 114/110Cd values and major- and trace-element compositions of sphalerites collected from a drill-core and tunnels of the Zhugongtang deposit were determined in an investigation of metal sources. Drill-core samples were impure and exhibited strong correlation (R2 = 0.89) between Zn and Cd contents. For samples from mining tunnels, the Cd and Fe contents of selected sphalerites were positively correlated, especially yellow sphalerites (R2 = 0.76). Cadmium is likely hosted in sphalerite by the substitution mechanism of (Fe2+, Cd2+) <-> Zn2+. The delta 114/110Cd values of all samples ranged from - 0.43 %o to 0.06 %o. Based on Zn/Cd ratios, and excluding geochemical processes that may have caused the variable Cd isotopic compositions, we suggest that the metal sources of the deposit were derived from the mixing of sedimentary and basement rocks. This model is supported by the strong relationship between the delta 114/110Cd and 1/Cd values of sphalerites from 11 typical Zn-Pb deposits in the SYGMP (R2 = 0.81). The quantification of metal contributions of source rocks indicates that deposits derived mainly from sedimentary rocks generally have relatively low sphalerite Ge contents and small Ge reserves, whereas those derived mainly from basement rocks have higher Ge contents and larger Ge reserves. This study provides a new model for explaining the enrichment of critical metals in Zn-Pb deposits of the SYGMP, thus extending the applications of Cd isotopes in hydrothermal systems.