Tourmaline as a recorder of magmatic-hydrothermal evolution: In-situ elements and boron isotope analysis of tourmaline from the Qinghe pegmatite, NW China

We conducted systematic elements and boron isotope studies on the tourmalines from a single pegmatite vein of the Qinghe pegmatite field (NW China), aiming to reveal its magmatic-hydrothermal evolution and implications for Li mineralization. The pegmatite vein is barren, intruded into the Paleozoic mica schists, and texturally divided into a border zone, a transition zone and a core zone. Tourmalines occur in all these zones, and they commonly have late-stage hydrothermal tourmaline overgrowths. All tourmalines belong to the alkali group, but show varied Mg/(Mg+Fe) ratios (0.39-0.66), with the border and transition zone tourmalines belonging to schorl, while the core zone and hydrothermal tourmalines to dravite. Most tourmalines follow the (Na+Mg) (Al+Xvac)-1, FeMg-1 and MnMg-1 exchange vectors. The core zone tourmalines, however, show positively correlated FeOt and MgO, and negatively correlated FeOt and Al2O3, suggesting a Fe3+Al-1 substitution for them, which could be related to a rise of fO2. Thus, tourmaline FeAl-1 correlation could be reflective of the redox state of pegmatite system. The elevated fO2 can be linked to concentration of aqueous fluid during pegmatite evolution. Moreover, the core zone tourmalines differ from other tourmalines by their positive Eu anomalies, reflecting an increased polymerization of Eu2+-Al-Si complexing in the zone. The border zone tourmalines have δ11B = -14.0 ~ -12.7‰, similar to the transition zone (-13.9 ~ -12.1‰). The core zone and hydrothermal tourmalines show slightly lower δ11B values (-14.4‰ ~ -13.3‰). The commonly light B, along with low Li (<120 ppm), of the tourmalines from barren pegmatite are in contrast to the relatively heavy B (-6.0 ~ -9.0‰) and high Li (>6000 ppm) of tourmalines from global Li-mineralized pegmatite, which is significant in Li mineralization prospect.