Fluid inclusion, H–O–S isotope and rare earth element constraints on the mineralization of the Dong’an Sb deposit, South China

Abstract

Systematic geochemical study on the Cambrian strata-hosted Dong’an Sb deposit in the Central Hunan Sb belt would provide key information on the origin of the low-temperature mineralization in South China. In this study, microthermometric, H–O isotopic and fluid composition analyses on ore-bearing quartz, S isotopic analyses on sulfides, and rare earth element (REE) analyses on wall-rocks and monominerals (stibnite, pyrite and quartz) in ores were conducted to constrain the genesis of the Dong’an Sb deposit. Two major stages of mineralization were identified in the Dong’an deposit: (1) an early pyrite-stibnite-quartz stage mineralization with intense wall-rock alteration occurring at depth of the deposit, and (2) a late quartz-stibnite stage mineralization occurring at shallow depths. Microthermometric analyses reveal a low temperature (150 to 230 °C), low salinity (4.5–7.0 wt% NaCl equiv.), low density (0.88–0.96 g/cm3), low pressure (4.5 MPa) and weakly acid (pH = 5.74) fluid for the Sb mineralization. Compositional analyses for fluid inclusions show that the main ion compositions are Na+, K+, Mg2+, Ca2+, F−, Cl− and SO42−, while the gas compositions are dominated by H2O and CO2. This suggests that the ore-forming fluids were enriched in alkali metals, sulfur and halogen. The H–O isotopic compositions (δ18OH2O = 2.52‰ to 7.13‰, δ2HH2O = −88.6‰ to −58.3‰) of fluid inclusions in quartz suggest a mixture of magmatic water, metamorphic rocks-derived brine and meteoric water. The δ34S values of stibnite (−1.32‰ to +2.86‰) and pyrite (+17.4‰ to +23.6‰) are very different, suggesting multi-sulfur sources involving buried magmatic rocks, ore-bearing meta-sedimentary rocks and deep basement rocks. The decreasing of REE contents from early to late mineralization stages reflects a decrease of temperature, reduction of fluid-rock interaction and input of meteoric water along with the stibnite precipitation. We propose that Precambrian strata that underwent regional metamorphism during Early Paleozoic acted as a Sb source bed, whereas strong tectono-magmatism in the early Mesozoic provided thermal driving force for the Sb mineralization occurring under a tensile stress in secondary faults at the shallow crust. This study highlights the use of multi-geochemical methods to constrain the genesis of large-scale Sb mineralization in South China.