Fluid inclusion and multiple isotope geochemical constraints on the hydrothermal evolution and metal sources of the Qiaomaishan Cu–W skarn deposit, eastern China

Abstract

The Xuancheng ore district of eastern China is a newly discovered district within the Middle and Lower Yangtze River Metallogenic Belt (MLYB) that hosts several magmato-hydrothermal related mineral deposits. The Qiaomaishan deposit is a representative example of the skarn deposits within this district and is also the only deposit within the district that hosts tungsten mineralization. However, the source of metals and the hydrothermal evolution of the mineralizing system that formed this deposit remain controversial. This study addresses these issues using new fluid inclusion microthermometric and isotopic data, including fluid inclusion H, quartz O, sulfide S, and pyrite Pb data, which constrain the evolution of the hydrothermal fluids within the system and the source of metal within the Qiaomaishan deposit. This study identified three main stages of hydrothermal evolution and paragenesis within the deposit, namely pre-ore (stage 1), syn-ore (stages 2 and 3), and post-ore formation stages. Stage 1 garnet (andradite)-hosted fluid inclusions homogenize between 390 °C and 590 °C and have salinities of 9–20 wt% NaCl equivalent whereas stage 2 quartz-hosted fluid inclusions homogenize between 200 °C and 460 °C and have salinities of 5–18 wt% NaCl equivalent. Finally, stage 3 quartz- and calcite-hosted fluid inclusions homogenize at temperatures of 120 °C–240 °C and 3–15 wt% NaCl equivalent salinities. The stage 2 quartz has oxygen and hydrogen isotopic compositions (δD_fluid from −99 ‰ to −57 ‰; δ¹⁸O_fluid from 4.0 ‰ to 6.1 ‰) that are close to those expected for magmatic fluids, whereas stage 3 quartz (δD_fluid from −105 ‰ to −86 ‰; δ¹⁸O_fluid from −1.7 to −0.6 ‰) records the mixing of meteoric and hydrothermal magmatic fluids. These fluid inclusion data suggest that cooling was the main mineralizing process involved in stages 1 and 2, and this process may favor the enrichment of tungsten and copper in the residual hydrothermal fluids. In contrast, fluid mixing became increasingly important between stages 2 and 3, leading to a reduction in salinity and temperature as well as changes in fluid isotopic compositions. Water–rock (W/R) interaction is also likely to have been an important process during deposit formation. The δ³⁴S (2.7 ‰–5.7 ‰ with a mean of 4.3 ‰) of sulfides from the Qiaomaishan deposit also provide evidence that the sulfur within the deposit has a magmatic origin. The homogeneous pyrite Pb isotopic data (^206/204Pb = 18.158–18.518, ^207/204Pb = 15.592–15.650, and ^208/204Pb = 36.179–38.634) for samples from the Qiaomaishan deposit further indicates that the metals within the deposit were derived from both mantle and crustal sources. In summary, the Qiaomaishan deposit formed from hydrothermal fluids derived from a magmatic source that subsequently cooled, mixed with meteoric water, and underwent W/R interaction, causing sulfide precipitation. The large-scale folding present in this area may also have been a key focus for mineralizing fluids and this a potential vector for use in mineral exploration as these structures focused both magmatism and fluid flow, promoting the formation of the skarn mineralization in this region.