Determination of Boron Content in Hydrothermal Vapor by Ion Selective Electrode Method: Insights into the Gaseous Transport of Boron

Geological phenomena of volcanism and metallogenesis indicate that boron can be transported significantly in aqueous fluids and gases. In this study, the Ion Selective Electrode Method was used to determine the solubility of boron in hydrothermal vapor in order to determine the ability of gaseous transport of boron. The electric potentials of boric acid standard solutions and boron-bearing condensates of water vapor after conversion to tetrafluoroborate ion with hydrofluoric acid were determined using a fluoroborate selective electrode. When the concentrations of boron change within the main range of geological fluids (0.52-524.50 mg/L), the calibration curve reveals a good linear Nernstian relationship between the potential and the logarithm of boron content with high sensitivity, good stability and good reproducibility. The detection limit of this method is 0.13 mg/L of boron, within uncertainties, which is lower than the upper limit of boron content standard for potable waters. This method is suitable for boron analysis of hydrothermal vapor. The experimental results of boric acid volatilization show that boron content in water vapor reaches 0.65%-0.72% at 150℃ and 0.37 MPa, which is consistent with boron-rich volcanic fumarole gases, demonstrating that boron can be remarkably transported in vapor phase and exists mainly in gaseous molecule species H₃BO₃ under high-temperature hydrothermal conditions. The geological characteristics of ore deposits and available experimental evidence indicate that boron transport in vapor and subsequent tourmalinization is closely related to the mineralization of rare metal type pegmatites and the formation of hydrothermal-pneumatolytic type Sn-W and Mo deposits and porphyry Cu and Au deposits.