Regulatory mechanisms of fluid exsolution in LCT pegmatite mineralization: evidence from the Murong and Jiajika No. 134 Pegmatite, Songpan-Ganze, SW China

As the primary global source of lithium, LCT-type pegmatites warrant detailed study of their mineralization in LCT-type pegmatite is of paramount importance for their exploration and development. This study investigates the Murong pegmatite (Asia’s largest single-vein LCT-type pegmatite) through a systematic geochemical analysis of columbite-group minerals (CGMs), tourmaline, and apatite. In addition, the fluid exsolution was investigated by comparing the geochemical characteristics of apatite from the No. 134 pegmatite, a typical LCT-type deposit in the Jiajika of the Songpan-Ganzé orogenic belt (SGOB). The CGMs U-Pb dating shows that the Murong Pegmatite was intruded between 193.2 and 199.2 Ma, which is consistent with the end of the Late Triassic Barrovian-Buchan metamorphism in the SGOB. The substitution between Li and Mg in tourmaline reveals the limited efficiency of B-rich fluids in transporting Li into the surrounding hornfels. A comparative analysis of apatite between the Murong and the Jiajika No. 134 pegmatites reveals a single fluid exsolution at Murong, contrasting with the multi-stages fluid exsolution at No.134. Fluid exsolution shifted the melt toward a slightly alkaline, low alkali-to-lithium ratio (low (Na + K)/Li ratios), which triggered spodumene crystallization. We conclude that fluid exsolution plays a regulatory role in the melt environment during pegmatite formation. Furthermore, the escape of a large volume of volatiles caused the melt liquidus temperature to rise, leading to fast crystallization and limiting Li diffusion into the wall rocks, thus facilitating the formation of single, super-large Li deposits with pervasive mineralization throughout the entire vein.