Genesis of the Mahuaping Be-W-F Deposit in Sanjiang Region, SW China: Constraints from Rb-Sr Age of Muscovite and Geochemical Compositions of Beryl

The Mahuaping deposit is the largest Be-W-F deposit in the Jinshajiang–Ailaoshan metallogenic belt, Sanjiang region, SW China, with more than 72,700 t WO3, 41700 t BeO and 2.3 Mt CaF2. Despite recent studies, the ore-forming process of the Mahuaping deposit remains poorly understood, limiting further insight into its genesis. In this study, a new muscovite Rb-Sr age and elemental compositions of beryl have been reported to constrain the mineralization age and evolution of ore-forming fluids. Muscovite Rb-Sr isochron dating reveals the mineralization age of the Mahuaping Be-W-F deposit is 28.0 ± 1.5 Ma, indicating the formation of the Mahuaping deposit is probably related to the magmatism caused by the sinistral shearing of crust in the Oligocene. LA-ICP-MS elemental mapping and spot analysis suggest the mechanisms for the incorporation of trace elements into the beryl lattice primarily involve two substitution types: Be2+ ↔ Li+ + Na+/Cs+ in the crystal core, and Al3+ ↔ (Fe2+/Mg2+) + (Na+/Cs+/Rb+) occurring in both the core and rim. The enrichment of Fe2+ is responsible for the blue coloration observed in beryl. The compositional variation from core to rim in beryl crystal indicates the initial ore-forming fluid of the Mahuaping deposit is reducing and acidic, and dominantly originated from magmatic fluids derived from the highly evolved magma. During the evolution, in addition to the continuous mixing of meteoric water, due to pulsating exsolution, the magmatic fluids were also replenished into the ore-forming fluid, enhancing water/rock interaction.