Discovery of a petalite-type pegmatite in the eastern Himalaya and implications for rare-metal mineralization

The Himalayan Orogen contains many leucogranitic plutons associated with Li mineralization, including those at Pusila, Gyirong, Lhozhag, and Kuqu. Pegmatites associated with these plutonic bodies typically host Li-bearing minerals, including spodumene, petalite, lepidolite, and elbaite. Lithium enrichment has also been identified in the Gaowu pegmatite, eastern Himalaya. Two distinct types of pegmatite (the dominant tourmaline pegmatite [Tur Peg] and minor garnet pegmatite [Grt Peg]) are observed intruding both the Gaowu pluton and the surrounding country rocks (Rouqiecun Group in Tethyan Himalayan sedimentary sequence). Unlike the Li minerals reported in Himalayan leucogranitic systems, petalite was discovered in the Gaowu Tur Peg and is the only primary Li-bearing mineral, with zinnwaldite and Li-bearing muscovite being secondary Li-bearing minerals. In this study, we investigated the petrogenesis of the pegmatites and related Li mineralization using the chemical compositions of petalite, micas, tourmaline, garnet, beryl, oxide minerals, and zircon, B isotope data for tourmaline, and U–(Th)–Pb ages of monazite and columbite-group minerals in the Gaowu pegmatites. The pegmatites and mineralization yield ages of ca. 23 Ma, coeval with the earliest Gaowu leucogranite and tectonic activity of the South Tibetan Detachment System (STDS). The narrow range of B isotopic compositions of leucogranite and the pegmatite (mean values of −12.32 ‰ ± 0.08 ‰ and − 12.58 ‰ ± 0.069 ‰, respectively) show they had the same source and underwent similar processes. The decreasing tourmaline and increasing garnet and beryl abundances from the leucogranite, through Tur Peg, to Grt Peg reflect an increasing degree of fractionation, and are associated with increasing zircon Hf content (up to 18 wt% HfO₂), garnet spessartite component (i.e. close to endmember composition), and Mn^# values (Mn/[Mn + Fe]) of columbite-group minerals; i.e. close to manganocolumbite endmember composition. The various occurrences of petalite correspond to crystallization during the magmatic-hydrothermal processes. Based on the P-T-controlled crystallization of Li-alumosilicate minerals (spodumene, petalite, and eucryptite etc.), the Gaowu petalite-bearing Tur Peg is inferred to have crystallized in a low pressure and may have been intruded at a shallow level in association with tectonic activity of STDS. The presence of zinnwaldite and Li-bearing muscovite indicates a Li-F-rich fluid during the late stage of the magmatism. In addition, beryl Li contents (up to ~3800 ppm) could be a useful indicator of potential Li mineralization in the Himalaya, given that the beryl is the most common accessory mineral in the Himalayan pegmatite.