Discriminating hydrothermal fluid sources using tourmaline boron isotopes: Example from Bailingshan Fe deposit in the Eastern Tianshan, NW China

Abstract

Tourmaline is an important B-bearing mineral in many types of hydrothermal deposits, and its chemical and boron isotopic signatures shed light on the nature and evolution of the metallogenic processes. Two generations of hydrothermal tourmaline in potassic-altered rocks (tourmaline I) and subsequent veinlets (tourmaline II) have been identified at the volcanic-hosted Bailingshan Fe deposit in the Chinese Eastern Tianshan, NW China. Both tourmaline I and II have low Ca contents and X-site occupancy, and belong to the alkali group. Tourmaline I grains plot in the schorl field or between the dravite and schorl fields, whereas tourmaline II grains are largely Mg-rich dravitic. Mineral textural and geochemical features suggest that the compositions of tourmaline I and II were mainly controlled by the host rock and hydrothermal fluid compositions, respectively.In-situ B-isotope analyses of tourmaline I yielded δ11B values of +1.7‰ to +4.1‰, whereas tourmaline II yielded slightly higher values of +3.5‰ to +6.6‰. Calculated B-isotope compositions (δ11BH2O) of the hydrothermal fluids in equilibrium with tourmaline I are +3.3‰ to +5.7‰ (at 550 °C), suggesting a mixed magmatic fluid (major) and basinal brine (minor) origin. Meanwhile, tourmaline II-forming fluids yielded δ11BH2O values of +6.2‰ to +9.3‰ (at 400 °C). Based on geological constraints and modal calculations, we suggest that the observed B-isotope variations from tourmaline I to II are mainly controlled by Rayleigh fractionation process, and indicate that they may have crystallized from similar fluids.The presence of basinal brine incursion into the K-rich hydrothermal fluids and geological constraints lead us to suggest that the Bailingshan deposit was likely formed in a Paleozoic Aqishan-Yamansu arc-related basin in the Eastern Tianshan.