Geochemical characteristics of tourmaline-muscovite granite and tourmaline from the Gabo lithium deposit, Xizang: Implications for rare metal metallogenesis in the post-collision extensional setting of the Himalayas

The Gabo lithium deposit in Xizang Autonomous Region is the first verified pegmatite-type lithium deposit within the Himalayan metallogenic belt, showing significant potential for large-scale exploration. The pegmatite-type lithium bodies are primarily hosted in the metamorphic rock series of the Paleozoic Guzi Formation, with their distribution controlled by extensional detachment structures. They have a close spatial relationship and genetic link with the underlying tourmaline-muscovite granite. However, further study on the magmatic properties and metallogenic potential of this granite remain lacking. To address this, this study conducted geochemical, mineralogical, and isotopic analyses on the tourmaline-muscovite granite. The results reveal that the granite is characterized by high silica content (SiO₂: 73.34%–74.46%), elevated alumina levels (Al₂O₃: 14.52%–14.94%), low CaO content (0.92%–1.06%), enrichment in large ion lithophile elements, and depletion in high field strength elements. It exhibits notably high Rb concentrations (605×10^－6–633×10^－6) coupled with high Rb/Sr ratios (8.01–8.59), indicating that the granite was derived from the dehydration melting of phyllitic muscovite and underwent extensive differentiation. The boron isotopic compositions (δ¹¹B) of tourmaline within the granite range from －11.61±0.38 to －10.47±0.35, suggesting a relatively homogeneous crustal source. In-situ trace element analyses of tourmaline show remarkably high contents of Li (900×10^－6–1821 ×10^－6), Zn (1835.39×10^－6–2871.81×10^－6), and Sn (31.56×10^－6–87.83×10^－6), with a gradual increase from the core to the rim. This indicates that magmatic differentiation and evolution drove the enrichment of rare metals, with tourmaline acting as a mineral indicator for rare metal concentration. Comprehensive research demonstrates that the enrichment of rare metals in the Gabo pegmatite-type lithium deposit is controlled by magmatic differentiation and evolution, while its occurrence is governed by extensional detachment structures. This makes it a typical example of rare metal mineralization in the post-collisional extensional setting of the Himalayan metallogenic belt.