Fluid inclusions and oxygen isotope constrain Be mineralization mechanism in the Dawan Be(Mo) deposit (Southeastern China)

• Reveal the homogenization temperatures and salinities of the fluid inclusions in quartz and helvite crystals from the Dawan Be(Mo) deposit. • Report chemical composition of individual fluid inclusions in quartz in the different alteration stages of the granite • Reveal Be mineralization mechanism in the Dawan Be(Mo) deposit. • Provide an example of the exploration of Be resources in volcanic-intrusive complexes along the southeast coast of China. The physicochemical signatures of fluid inclusions provide essential indicators for understanding fluid evolution and ore deposit formation. Although Be mineralization mainly occurred during hydrothermal processes, the origin of Be and metallogenic mechanisms in these hydrothermal Be deposits remain poorly understood. The Dawan Be(Mo) deposit (Southeastern China), hosted by Nanyuan Formation volcanic tuffs, is one of typical helvite-type Be deposits, and the Be mineralization is related to the hydrothermal alterations between the Nanyuan Formation tuffs and their underlying alkali feldspar granite. In this study, we conduct a comprehensive investigation of fluid inclusions in quartz and helvite from the Dawan Be(Mo) deposit by means of SEM-CL imaging, microthermometry, Raman spectroscopy, LA-ICP-MS microanalysis of individual fluid inclusion, and in situ SIMS oxygen isotope analysis. Microthermometric analyses of fluid inclusion assemblages (FIAs) in quartz from different alteration zones reveal that the crystallization temperatures of the granite range from 343.2 to 453.6 °C, 311.2 to 422.6 °C in greisenization zone, 341 to 435 °C in pyritization zone, and 311.2 to 391.2 °C in the silicification zone, accompanied by the decreasing salinities. Helvite crystallized under about 245 to 425 °C at an estimated depth of around 5 km. LA-ICP-MS analysis of FIAs in quartz illustrate that the concentrations of Li, Be, and Mo reach up to 672 ppm, 362 ppm, and 147 ppm in the ore-forming fluids during the precipitation of Be at different hydrothermal alteration stage, respectively. The Rb/Na and K/Na ratios in fluid inclusions, the decreasing trend δ 18 O fluid values (−3.42 and 3.25‰), fluid temperature, salinity, and Th/U ratios, as well as ThO 2 -CaO contents in monazite suggest predominantly magmatic origin of Be and mixing of magmatic fluids with multiple episodes of hydrothermal fluids. The associated spessartite crystals contain up to 2901 ppm S and near 3 wt% F, suggesting that the ore-formation fluids likely belong to an S-Be-F system. The Be mineralization mechanism of the Dawan deposit involve initial concentration of Be in granitic magma, further enrichment of Be through magmatic-hydrothermal evolution, and final precipitation of Be during the greisenization, pyritization and silicification hydrothermal stages. Fluid boiling, mixing and cooling likely triggered the crystallization of helvite-group minerals and beryl under varying conditions of ƒO 2 , ƒS 2 and the activities of SiO 2 , Al 2 O 3 and F.