Microscale sulfur isotopic compositions of sulfide minerals from the Jinding Zn–Pb deposit, Yunnan Province, Southwest China

Abstract

The Jinding Zn–Pb deposit, located in the Lanping basin in Northwest Yunnan Province, is the largest Zn–Pb deposit in China, and also probably the youngest sediment-hosted super giant Zn–Pb deposit in the world. Its genesis differs from the well-known major types of sediment-hosted Zn–Pb deposits. Based on mineral paragenesis and textures, there are two stages of mineralization: stage 1 that is typically characterized by fine-grained sulfide minerals (galena, sphalerite, pyrite and marcasite) disseminated in sandstones of the Lower Cretaceous Jingxing Formation (K1j), and massive sulfides in limestone breccias of the Paleocene Yunlong Formation (E1y); and stage 2 which mainly occurs as coarse-grained galena veins crosscutting stage 1 sulfides, and minor amounts of colloform sphalerite intergrown with galena. In situ sulfur isotopic analyses of galena, sphalerite and pyrite were determined by secondary ion mass spectrometry (SIMS), and showed highly variable δ34S values (−42.1‰–7.7‰) of different ore types. Stage 1 mineralization has δ34S values from −42.1‰ to −10.2‰ with the majority ranging from −26‰ to −14‰. Stage 2 mineralization has higher δ34S values (−8.3‰–7.7‰). Combined with the geological settings and mineral paragenesis, the sulfur isotopic data presented here suggest multiple sulfur sources (biogenic sulfur+evaporites) and formation mechanisms for reduced sulfur (H2S). H2S responsible for stage 1 sulfide precipitation was associated with bacterial sulfate reduction (BSR). However, H2S of stage 2 was likely derived from thermochemical sulfate reduction (TSR). The most reasonable scenario for the stage 1 mineralization is a metal-bearing brine mixing with an H2S-rich fluid, thereby causing rapid sulfide precipitation. Till the stage 2, the ore-forming fluid shifted to the meteoric water that infiltrated and reacted with evaporitic rocks, leached metals and transported them as sulfate- or sulfite-complexes to the Jinding dome where the oxidized sulfur was reduced by organic matters to H2S, leading to precipitation of metal sulfides. In contrast to other sulfide deposits in the Lanping basin, biogenic sulfur might have played a key role in the mineralization process, especially during the early stage of formation of the Jinding Zn–Pb deposit.