Germanium and gallium enrichment and ore genesis of the Aozigang zinc deposit, Hubei Province, South China: Constraints from ore mineralogy, in situ analysis of sphalerite compositions and S isotopes

• The sphalerite from the Aozigang deposit contains high concentrations of critical metals (e.g., Ge and Ga) in the Yangtze Block. • Organic matter plays a crucial role in Zn mineralization and contributes to Ge enrichment. • The sulfur of the Aozigang Zn deposit was derived from the thermochemical reduction of sulfate. The Aozigang Zn deposit is situated within the Neoproterozoic Dengying Formation on the eastern flank of the Huangling anticline, which is part of the western Hunan-Hubei metallogenic region in the eastern section of the Yangtze Block. This deposit contains more than 10 Mt of zinc metal. A complex paragenesis is defined by three generations of sphalerite (Sp) deposition: Stage I greenish Sp(a) formed in dolomite breccias and was later cemented by carbonate minerals. In Stage II, light brown Sp(b) precipitated with modest amounts of bitumen, galena, pyrite, calcite, and dolomite. In Stage III, orange Sp(c) is cemented by smithsonite and white sparry dolomite. Detailed petrographic observations, along with in situ trace element and sulfur isotope analyses of multiple sphalerite generations, were conducted to elucidate the source and evolution of the Zn mineralization, as well as the mechanisms behind gallium (Ga) and germanium (Ge) enrichment at Aozigang. LA-ICP-MS trace element analyses show that the green Sp(a) contains relatively high levels of Ag (22–139 ppm) and Fe (537–1693 ppm), whereas the light brown Sp(b) has relatively high contents of Cu (517–2457 ppm), Ga (12–647 ppm), and Ge (243–1432 ppm). Orange Sp(c) has high contents of Cd (789–1400 ppm). The strong positive correlation between Ge and Fe in Sp(a), as well as between Ge and Cu in Sp(b) and Sp(c), suggests coupled substitutions of Ge for Zn via Ge 4+ +2Fe 2+ +□↔4Zn 2+ , 2Ge 4+ +2Cu + +□↔5Zn 2+ , and Ge 4+ +2Cu + ↔3Zn 2+ , respectively. The in situ δ 34 S values of the three types of sphalerites narrowly range from + 22.5 to + 29.4 ‰. These positive δ 34 S values suggest that seawater sulfates served as the primary source of sulfur, while organic matter may have contributed reduced sulfur through thermochemical sulfate reduction (TSR). Ge and Ga are concentrated at relatively low temperatures. Furthermore, organic matter plays a crucial role in Zn mineralization and contributes to Ge enrichment observed in Sp(b) and Sp(c) at Aozigang Zn deposit and other carbonate-hosted Zn-(Pb) deposits within the Yangtze Block.