Cadmium-Fe-Mn coupling in sphalerite from MVT Pb-Zn deposits: Implications for critical metal Cd enrichment

Cadmium (Cd) has been increasingly recognized for its critical role in modern high-technology applications. Accordingly, investigations into its natural enrichment mechanisms have garnered heightened scientific interest. The Pb-Zn deposit, recognized as the principal industrial source of Cd, predominantly hosts this element within sphalerite, with only minor occurrences in independent Cd minerals such as greenockite (CdS). Current studies have demonstrated that the distribution of Cd in sphalerite is significantly correlated with Fe and Mn, particularly through the substitutions of Cd, Fe, and Mn, such as Fe²⁺ + Mn²⁺ + Cd²⁺ → 3Zn²⁺ and Fe²⁺ + Cd²⁺ → 2Zn²⁺. However, the coupling mechanism of these elements within sphalerite have not been thoroughly investigated, and the implications of the Cd enrichment process remains poorly understood. In this study, we analyzed sphalerite samples of varying colors from two major Mississippi Valley-type (MVT) Pb-Zn deposits (Huize and Maoping) in the SYG (Sichuan-Yunnan-Guizhou) area, Southwest China. We employed micro-X-ray fluorescence (μ-XRF), laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS), synchrotron X-ray fluorescence (SXRF), and X-ray absorption near-edge structure (XANES) spectroscopy techniques to qualitatively and quantitatively investigate the presence of Cd, Fe and Mn in sphalerite of different colors. The results demonstrate that Fe in our sphalerite samples exists in a divalent state (Fe²⁺), with concentrations ranging from 1.2 to 10.8 wt%. In Fe-rich sphalerite, Cd and Mn incorporate into the lattice alongside Fe at varying ratios via the substitutions Cd²⁺ + nFe²⁺ → (n + 1)Zn²⁺ and Mn²⁺ + nFe²⁺ → (n + 1)Zn²⁺, respectively. Notably, as Fe content increases further, both the Cd/Fe and Mn/Fe ratios decrease progressively. Cd concentrations drop sharply when Fe exceeds 6.7 wt%. In Fe-poor sphalerite, Cd and Mn can directly substitute for Zn within the crystal structure (Cd²⁺ → Zn²⁺ and Mn²⁺ → Zn²⁺), and their concentrations exhibit an inverse correlation. This study indicates that the enrichment of Cd within sphalerite is predominantly influenced by temperature, while its correlation with sulfur fugacity appears to be insignificant. Cadmium enrichment can occur in both high-temperature and low-temperature sphalerite. Consequently, the Pb-Zn deposits formed under higher temperature conditions may facilitate two distinct stages of Cd enrichment in sphalerite, thereby enhancing overall Cd concentration.