Beryllium mineralisation in high-sulphur-fugacity fluids in the Dawan Be (Mo) deposit, Southeast China

Beryllium mobilisation, transfer, and enrichment in volcanogenic Be deposits are presumably controlled by F-bearing hydrothermal fluids. However, the stability of Be-bearing species, evolution of Be-bearing minerals, nature of Be deposits, and controls of sulphur fugacity (ƒS2) and alkalinity in such hydrothermal systems are not fully understood. We conducted a detailed petrographic, mineralogical, and geochemical study of the Dawan Be (Mo) deposit in Xiapu County, Fujian Province, China, to investigate Be mineralisation from hydrothermal fluids with high S fugacity and low Al concentrations. The Dawan Be (Mo) deposit occurs in greisen and pyritized zones, hornfels, and quartz veins, which are typically hosted by volcanic rocks (i.e., tuffs). The main Be ore minerals are helvine ((Mn)4Be3(SiO4)3S) and beryl. Helvine occurs mainly in proximal quartz veins, while beryl is found in distal quartz veins. The crystallisation of helvine indicates high-ƒS2 and low-ƒO2 conditions, but danalite is absent, suggesting the hydrothermal fluids had a relatively high ƒS2 and ƒO2. The change in crystallisation from helvine to beryl was mainly due to variations in ƒS2 and ƒO2 in the ore-forming fluids. The abrupt decrease in ƒS2 associated with the formation of sulphides (e.g., pyrite, helvine, and molybdenite) led to the crystallisation of beryl in the distal quartz veins. Bertrandite is one of the alteration products of helvine. Sulfidation, along with redox conditions, SiAl activity, P-T conditions, and fluid F contents, controlled the transport, enrichment, and crystallisation of Be in the Dawan Be (Mo) deposit. In situ U-Th-Pb dating of monazite indicates that the Dawan Be (Mo) deposit formed at between 92‐ 94 Ma, which is similar to the intrusion age of the associated granite. The geological features and similar Pb isotope ratios of pyrite and the granite suggest that Be in the Dawan Be (Mo) deposit was derived from the granite, but the surrounding tuff may have been another source of Be. The mineralogical and geochemical features of the Dawan Be (Mo) deposit reveal that mobilisation, transfer, and enrichment of Be occur in high-ƒS2 fluids, and further exploration of potential Be deposits containing helvine is warranted.