Mechanisms of germanium enrichment in Zn-Pb deposits of the Sichuan-Yunnan-Guizhou metallogenic province: Constraints from Cd and S isotopes of the Wusihe deposit

In China, most Ge-rich Zn-Pb deposits are located in the Sichuan-Yunnan-Guizhou metallogenic province (SYGMP), with Ge reserves exceeding 5000 tons. Of these deposits, the Wusihe deposit exhibits distinct sedimentary exhalative mineralization (banded/nodular sphalerite) in the Dengying Formation (ca. 551–541 Ma) and unique Ge mineralization characterized by abundant Ge-independent minerals (e.g., argutite and ruizhongite) that have not been observed in other deposits. Thus, the deposit features well-constrained metallogenic ages and proximal spatial association with basement rocks. Based on the spatial and temporal variations of chemical compositions in banded/nodular sphalerite, we conclude the following: (1) Ge enrichment in sphalerite is primarily controlled by its concentration in the ore-forming fluid, rather than by substitution mechanisms. (2) Reduced sulfur originated from thermochemical sulfate reduction (TSR) of paleo-seawater sulfate. Fluid mixing triggered Rayleigh fractionation of sulfur isotopes during sphalerite precipitation, resulting in a systematic decrease in δ34SCDT values from core to rim. (3) Sphalerite exhibits Cd isotopic variations consistent with those of igneous rocks but distinct from carbonate rocks. This suggests that the Precambrian basement, primarily composed of igneous rocks, could be a significant source of metals (e.g., Ge, Cd, and Zn) for the formation of Zn-Pb deposits. This study provides robust evidence that the metal source of Ge-rich Zn-Pb deposits from the SYGMP is primarily derived from Precambrian basement rocks and establishes a “deep-source + shallow-precipitation” model to explain the different Ge enrichment levels in Zn-Pb deposits from the SYGMP and worldwide.