Sphalerite Records Cd Isotopic Signatures of the Parent Rocks in Hydrothermal Systems: A Case Study From the Nayongzhi Zn–Pb Deposit, Southwest China

Abstract

Metal stable isotopes (e.g., Zn, Cd, and Cu) have been used to track metal sources in different types of hydrothermal systems. However, metal isotopic variations in sulphides could be triggered by various factors such as mineral precipitation and fluid mixing. Thus, tracking the metal sources of hydrothermal systems is still a big challenge for metal isotopes. In this study, we investigated the Cd isotopic systematics of sphalerite from the Nayongzhi Zn–Pb deposit, which is a Mississippi Valley‐type (MVT) deposit in the Sichuan–Yunnan–Guizhou mineralization province (SYGMP). We reinterpreted the published S isotope data for the SYGMP and found that the large S isotopic variations were controlled by Rayleigh fractionation between sulphide and reduced S. As such, a model that involves mixing of a metal‐rich fluid with a reduced S pool formed by thermochemical sulfate reduction (TSR) can explain the ore formation in the Nayongzhi deposit. Based on this model, no Cd isotopic fractionation was observed due to its low solubility in fluids during mixing, and thus the Cd isotopic variations of sphalerite were inherited from the source rocks. The large range of Zn/Cd ratios and uniform Cd isotopic compositions of the sulphides are similar to those of igneous rocks but different from those of sedimentary rocks, indicating that Zn and Cd were derived mainly from basement rocks (e.g., migmatite, gneiss, and granulite). Our results reaffirm that metal stable isotopes, particularly Cd isotope compositions of sphalerite, are powerful geochemical tracers for investigating the formation mechanisms of ore deposits.