Chemical and boron isotope compositions of tourmaline fingerprint petrogenesis and evolution of the Taerlang high-purity quartz pegmatite, Altay Orogenic Belt, Northwest China

Pegmatites serve as the primary host for high-purity quartz (HPQ); however, the behavior of impurity elements during pegmatite evolution remains unclear. In this study, we identified 14 pegmatites in the Taerlang area (northwestern China), nine of which show potential for HPQ production. We selected a representative pegmatite with exceptional purity (quartz with 0.2 ppm Li and <20 ppm total impurities) to explore the key controls governing such purity. Owing to the limited elemental and isotope information preserved in HPQ, we conducted systematic elemental and boron isotope analyses of tourmaline, which is intergrown with the quartz. Samples Tur 1, Tur 2, and Tur 3 were collected from the bottom, intermediate, and upper zones of the pegmatite, representing early-, middle-, and late-stage crystallization, respectively, whereas sample Tur 4 was collected from the wall rock (Kanas Group mica schist). All of the tourmalines belong to the alkali group and are classified as dravite, following the (NaMg)(AlXvac)−1 exchange vector (Xvac—X-site vacancy). Tur 1 (−14.3‰ to −13.1‰) and Tur 2 (−14.6‰ to −13.1‰) exhibit similar δ11B values, but Tur 3 has slightly heavier δ11B values (−13.6‰ to −12.6‰), with a narrower range that overlaps that from the mica schist (Tur 4, −16.0‰ to −9.3‰). A bookended wall-rock contribution was recorded: early assimilation evidenced by the Mg-rich characteristics of Tur 1 and late-stage mixing of an external fluid derived from the mica schist, as indicated by the geochemical similarities between Tur 3 and Tur 4. The main stage of crystallization was open-system crystallization. This is evidenced by results for Tur 2: (1) The Rayleigh fractionation model cannot account for the relatively homogeneous δ11B values, and (2) there is an abrupt increase in Ca, overlapping Na, Li, B, and F contents. Significantly higher Fe and Co/Ni ratios and lower V, Sc, and total rare earth element contents of Tur 2 reflect external fluids derived from granite. From a comparison of the Li contents of tourmaline in this study with those of other barren pegmatites globally, we suggest that an initial Li-poor magma is the common characteristic of HPQ pegmatites. Similar low Li contents were preserved, together with a decrease of Al, throughout the crystallization of the pegmatite, indicating that the influx of external fluids did not introduce additional impurities but rather induced melt dilution and a disruption of the internal differentiation trajectory. Our findings provide the novel insight that an initial Li-poor pegmatite magma, coupled with multi-stage, open-system infiltration, is essential for HPQ pegmatite formation.