Sources and thermo-chemical sulfate reduction for reduced sulfur in the hydrothermal fluids, southeastern SYG Pb-Zn Metallogenic Province, SW China

Abstract

Located on the western Yangtze Block, the Sichuan (四川)-Yunnan(云南)-Guizhou(贵州) (SYG) Pb-Zn metallogenic province has been a major source of base metals for China. In the southeastern SYG province, structures are well developed and strictly control about 100 Pb-Zn deposits. Almost all the deposits are hosted in Devonian to Permian carbonate rocks. Lead-zinc ores occur either as veinlets or disseminations in dolomitic rocks with massive and disseminated textures. Ore minerals are composed of pyrite, sphalerite and galena, and gangue minerals are calcite and dolomite. Sulfide minerals from four typical Pb-Zn deposits are analyzed for sulfur isotope compositions to trace the origin and evolution of hydrothermal fluids. The results show that δ34S values of sulfide minerals range from +3.50‰ to +20.26‰, with a broad peak in +10‰ to +16‰, unlike mantle-derived sulfur (0±‰). However, the mean δ34Ssulfide and δ34SΣS-fluids values are similar to that of sulfate-bearing evaporites in the host rocks (gypsum: ⊃+15‰ and barite: +22‰ to +28‰) and Cambrian to Permian seawater sulfate (+15‰ to +35‰). This suggests that reduced sulfur in hydrothermal fluids was likely derived from evaporates in the host rocks by thermo-chemical sulfate reduction (TSR). Calculated δ34SΣS-fluids values of the Shanshulin(杉树林), Qingshan(青山), Shaojiwan(筲箕湾) and Tianqiao(天桥) Pb-Zn deposits are +21.59‰, +18.33‰, +11.43‰ and +10.62‰, respectively, indicating sulfur-bearing hydrothermal fluids may be evolved from the Shanshulin to Qingshan and then the Shaojiwan to Tianqiao deposition sites along the Yadu(垭都)-Ziyun(紫云) lithospheric fracture in the southeastern SYG province.