Li and B isotopic fractionation at the magmatic-hydrothermal transition of highly evolved granites

The isotopic compositions of Li and B have great potential for the tracing of fluid sources and for the characterization of ore-forming processes. The fractionation of Li and B isotopes during the magmatic-hydrothermal transition in evolved granites, however, is not well understood as these rocks commonly show multiple late hydrothermal overprints. Tourmaline is sensitive to changes in melt and fluid compositions during its growth and resistant to alteration by later fluids, which makes it a good recorder of Li and B isotopic signatures and fractionation at late evolutionary stages of magmatic systems. To characterize the evolution of a Sn-ore forming granitic intrusion, we analyzed magmatic, hydrothermal, and detrital-sedimentary tourmaline from the Yuanbaoshan granite, associated cassiterite-tourmaline-quartz veins, and disseminated tourmaline-bearing cassiterite-sulfide ore in the sedimentary wall rocks. The Li and B isotopic compositions of tourmaline from these rocks do not covary, largely due to mineral-controlled decoupling during magmatic and hydrothermal processes. Because of the dominant role of tourmaline on the B budget of the rock, magmatic and hydrothermal tourmalines show little variation in delta B-11 (-12.5 to -8.2 parts per thousand), a signature reflecting the metasedimentary protoliths of the melt. In contrast, several rock-forming minerals contribute to the budget of Li, which shows significant variation in delta Li-7 (-0.3 to +6.9 parts per thousand). The isotopic fractionation of Li in tourmaline from rocks of the Yuanbaoshan area is controlled by three major processes: (i) fractional crystallization results in magmatic tourmaline having progressively higher delta Li-7 values and higher Li contents, (ii) the magmatic-hydrothermal transition leads to higher delta Li-7 values at lower Li contents in hydrothermal tourmaline, and (iii) extensive fluid-rock interaction adds isotopically light Li to hydrothermal tourmaline in the wall rocks. Extensive fluid-rock interaction also leached HREE, Mg, isotopically light B, and probably Sn from the granites and/or wall rocks. Tourmaline seems to robustly record progressive changes in Li isotopes in evolving magmatic-hydrothermal systems and hydrothermal mineral deposits. (C) 2020 Elsevier B.V. All rights reserved.