Boron-metasomatism and fluid evolution revealed by the chemical and boron isotopic compositions of tourmalines from the Lhozhag area, Eastern Himalaya

Abstract

The interaction between boron-rich (B-rich) fluid and metamorphic rocks, along with the induced tourmalinization, has been widely observed in metamorphic and magmatic-hydrothermal processes. This interaction may significantly influence the formation of ore deposits and petrophysical properties of crustal rocks. However, chemical exchange and element transformation during the boron-metasomatism process are still not clear. In this study, we present a detailed textural and geochemical study of tourmalines and other minerals from tourmaline-rich veins and their host rocks in the Lhozhag area, Eastern Himalaya. Two zones with different mineral assemblages were recognized in the tourmaline-rich veins: the Pl-Qtz zone in the core and the Tur-Ms zone in the rim. Petrographic evidence shows that the tourmaline-rich veins were produced by reactions between the host rocks (i.e. biotite schist) and B-rich fluids. Tourmaline grains show compositional zonation, and the core, mantle, and rim have distinctive major and trace elemental, and boron isotopic compositions. The core of tourmaline (Tur-C) is characterized by the highest Mg/(Mg + Fe) ratio and Sc, V, and Cr concentrations, as well as the lowest Ca, Ti, Mn, Li, Be, and Zn concentrations. The mantle of tourmaline (Tur-M) shows a continuous transition from Tur-C in both colour and composition, which might result from the metasomatic system evolving from wall-rock-controlled to fluid-controlled. The high Li, Zn, and Mn concentrations in Tur-M indicate that the involved fluids (first-stage fluid) were likely magmatic in origin. The rim of tourmaline (Tur-R) is texturally and compositionally homogeneous, generally with intermediate Mg/(Mg + Fe) ratios and Ca, Ti, Li, Sc, V, and Zn concentrations, as well as the lowest Sc, Cr, Sn, and Ba concentrations relative to both Tur-C and Tur-M. The sharp compositional contrast between Tur-M and Tur-R at their boundary suggests an injection of external metamorphic fluid (second-stage fluid). Ti-in-quartz thermometry results show that the temperature of fluid-rock interaction decreases from 700 °C for the formation of Tur-C to 500 °C for the formation of Tur-R. Based on fluid-tourmaline trace element partitioning coefficients at the above temperatures and measured tourmaline compositions, we reconstructed the compositions of both the first-stage magmatic and the second-stage metamorphic fluids. The extremely high Li concentrations (up to 1700 ppm) in the first-stage magmatic fluids reflect the enrichment of Li in the Lhozhag leucogranite magma, which is consistent with the occurrence of spodumene-bearing pegmatite in the Lhozhag area. The relatively high Li concentrations in both the second-stage metamorphic fluid (up to 1000 ppm) and biotite (ca. 2000 ppm) from wall rocks suggest that the biotite-rich metapelite could supply abundant Li to fluid during fluid-rock interaction. The boron isotopic composition of tourmaline also shows a systematic change, with the δ¹¹B values gradually decreasing from Tur-C (−7 to −9 ‰) to Tur-M (−9 to −10 ‰. However, there is a slight rebound in the δ¹¹B values for Tur-R (−8 to −9 ‰). Quantitative modeling suggests that cooling and changes in fluid-to-rock ratio may explain the alternating rise and fall in the δ¹¹B values from Tur-C through Tur-M to Tur-R. This work demonstrates that biotite from the metasedimentary rock is a potential Li reservoir in the Himalaya, and the boron-rich fluid could effectively extract Li from biotite.