Characteristics of Ore-forming Fluids of the Jing'erquan Granitic Pegmatite-type Lithium-Beryllium Deposit in the Eastern Tianshan

Crystal-rich fluid inclusions serve as a crucial carrier in the evolution of pegmatitic melts-fluids, providing the most direct evidence for unraveling the origin, migration, phase transitions, and associated element enrichment mechanisms of pegmatitic fluids. The Jing'erquan rare metal deposit is a typical granite-pegmatite type lithium-beryllium deposit in the East Tianshan, western Central Asian Orogenic Belt. However, the source and evolutionary characteristics of its ore-forming fluids remain unclear. This study conducted detailed petrographic observations, along with laser Raman spectroscopy, microthermometry, gas chromatography, and ion chromatography analyses on fluid inclusions from pegmatites in this deposit. The results indicate that the fluid inclusions at Jing'erquan are diverse in type. They are generally characterized by moderate-low temperatures (mainly ranging from 240 ℃ to 380 ℃) and salinities (predominantly between 1.0 wt% NaCl_eqv and 5.5 wt% NaCl_eqv), as well as low densities (0.5～0.9 g/cm³). The fluid is rich in CO₂, H₂O, and N₂, with minor amounts of C₂H₂ and CH₄, and is relatively enriched in F⁻, Cl⁻, and alkali metal cations. This study proposes that the ore-forming fluids for the Jing'erquan Li-Be deposit were primarily magmatic in origin, resulting from the high fractional crystallization of granitic magma in a relatively closed environment. Two distinct pegmatite veins, dominated by spodumene and beryl, respectively, were formed during different stages of granitic magma differentiation. During the mineralization process, the fluid properties transitioned from moderate-low temperature-low salinity-low density to low temperature-moderate-low salinity-low density. The primary reason for this shift is that CO₂ gradually reached saturation and separated from the ore-forming fluid, as the temperature, pressure, and solubility of CO₂ and NaCl-H₂O decreased. In this process, CO₂ acted as an important carrier, facilitating the transportation and enrichment of lithium. Furthermore, the relative enrichment of F⁻, Cl⁻, and alkali metal cations played a significant role. Specifically, F⁻ was conducive to the formation of complexes or compounds with rare metal cations such as Li⁺ and Be²⁺, aiding their migration and subsequent precipitation and enrichment to form ore.