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Boron isotopes, Hydrothermal ore systems, Boron sources, Fluid-tourmaline fractionation, Fluid-rock interaction, Fluid mixing
Abstract:
Tourmaline group minerals are typically the predominant host of boron in hydrothermal mineral deposits. Boron is a fluid-mobile element whose isotopic composition reflects many factors that are relevant to understanding mineralizing processes, including fluid source(s), fluid-rock interaction, and formational temperature. A new compilation of 2622 published δ11B values for tourmaline from diverse types of hydrothermal ore deposits is presented here, with the focus (2215 analyses) on seven main types: porphyry Cu-Mo-Au deposits, granite-related Sn-W deposits, IOCG deposits, orogenic Au deposits, stratabound VMS and SEDEX deposits, and sediment-hosted U deposits. The total range of δ11B values for the seven types is −26.8 to +35.0‰. Four (granite Sn-W, orogenic Au, stratabound VMS and SEDEX) have median δ 11B values close to the continental crustal average of about −10‰. The median values for IOCG and porphyry Cu-Mo-Au deposits are higher (−3.9‰ and −2.1‰, respectively), whereas sediment-hosted U deposits have distinctly high δ 11B (median = +25.3‰). Importantly, a considerable range of δ11B values exists for tourmaline within each deposit type, the smallest (17.8‰) for granite Sn-W deposits and the largest (48.0‰) for IOCG deposits.
The boron isotope variations in tourmaline from different deposits are suggested to reflect three levels of controlling factors and how these factors operated is illustrated with a selected number of case studies. The primary factor is the composition of the boron source; secondary effects relate to fluid-tourmaline fractionation (equilibrium or Rayleigh). There are commonly also tertiary factors that depend on evolution of the specific deposit. These include fluid mixing, changing water–rock ratio and/or depositional temperature, influences of other boron-bearing minerals, and where relevant, post-ore metamorphism. Separating the effects of these factors is rarely possible from boron isotopes alone. However, the growth of multi-isotope studies of tourmaline and coexisting phases such as mica, as well as developments in modelling/experimentation of boron isotopes and element partitioning, suggest that this limitation will be overcome.
