Tourmaline geochemical and boron isotopic compositions of the Bailongshan rare-metal pegmatite deposit: Implications for magmatic-hydrothermal evolution of the West Kunlun Orogen (NW China)

Abstract

The Bailongshan in the West Kunlun Orogen (northern Tibet) is a newly-discovered super-large rare-metal (Li-Rb(-Be-Ta-Nb)) pegmatite deposit. Tourmaline occurs widely in muscovite granites and pegmatites at Bailongshan, but comprehensive study is yet to be done on the geochemical evolution of tourmaline from the magmatic to the hydrothermal stages. Here, we present a systematic study of in-situ major, trace elemental and boron isotopic variations of tourmaline from Bailongshan deposit, and discuss the tourmaline genesis and how boron isotopes change during the magmatic to hydrothermal stage. Three types of tourmaline have been identified at Bailongshan deposit, including disseminated tourmaline in the muscovite granite, medium-coarse-grained euhedral tourmaline in the barren pegmatite, and multicolored medium-coarse-grained tourmaline in the ore-bearing pegmatite. EPMA and LA-ICP-MS geochemical analyses reveal that the tourmaline samples belong mainly to alkali group, with the tourmalines from the muscovite granite and the barren pegmatite falling into the schorl solid-solution series and the ore-bearing pegmatite falling into elbaite. There is a major chemical change from Fe-rich compositions with moderate Al in the Y-sites (muscovite granite and barren pegmatite) to Li-rich compositions with high YAl and relatively high F (ore-bearing pegmatite). Most trace element contents vary over several orders of magnitude with median concentrations of 0.1–10 μg/g. High concentrations are found for Ga, Zn, Be, Sc, Sn and Li (several tens to thousands of μg/g), whereas low concentrations (<0.1 μg/g) are found for V, Rb, U, Th, W, and Hf. The tourmalines from muscovite granite and barren pegmatite yielded δ11B = -8.9 to −7.5 ‰ and −8.7 to −6.0 ‰, respectively, whilst the tourmaline in the ore-bearing pegmatite show a wider δ11B range (-8.8 to −5.2 ‰). The B isotopic values indicate a source of the continental crust in a post-collisional setting. The estimated δ11B value of parental granitic magma is −10.0 ‰. The overlap in δ11B values between muscovite granite and barren pegmatite indicates a little isotopic fractionation between tourmaline and melt. Rayleigh fractionation results in the observed δ11B decrease from zone I (-6.9 ‰) to zone V (-8.0 ‰). After the barren-pegmatite-hosted tourmaline formation, the δ11B value of boron-rich magma would have been about −11.5 ‰. The intragrain δ11B variation of tourmaline in the ore-bearing pegmatite is 2.7 ‰, much larger than that from the other zones, indicating the rim of the tourmaline crystallized from fluid exsolved. We calculated the boron-isotope fractionation (between fluid and melt) to be 4.2–5.7 ‰ at 600–450 °C. Chemical variations in tourmaline are continuous in the muscovite granite and barren pegmatite, consistent with fractional crystallization, whereas the tourmaline from the ore-bearing pegmatite may have formed during the magmatic-hydrothermal transition. The boron isotopic variations could be attributed to fractionation between melt-tourmaline, melt-fluid and tourmaline-fluid and Rayleigh fractionation.