Evidence of boron-rich aqueous and crystalline phases associated with fumarolic emissions at Guallatiri volcano, northern Chile

This work presents the first record of boron-rich aqueous solutions in barite-, anhydrite-, and sassolite-hosted fluid inclusions found in the fumarolic deposits of the Guallatiri volcano, an active and quiescently degassing volcano located in northern Chile. These findings offer a unique opportunity to investigate the mineral-forming and depositional processes in fumarolic environments. Chemical and mineral characterization was performed using XRD and SEM-EDS techniques, whereas petrologic and microthermometry methods, in addition to Raman and FTIR spectroscopy, were conducted to the physicochemical characterization of fluid inclusions. Characterizations of these fluid inclusions show high‑boron concentrations associated with crystalline phases of sassolite and boron-rich aqueous solutions in fluid inclusions. Analyses based on fluid inclusions reveal two potentially fluid sources: (i) a deep source with higher temperatures (up to 220 °C) and saline fluids (>8 wt% NaCl equivalent), in the presence of CO2, H2S, SO2, and CH4; and (ii) a shallower source with lower temperatures (between 113 and 163 °C) and salinities (<6 wt% NaCl equivalent). Those fluids from the shallower source were affected by dilution processes within the hydrothermal system of the Guallatiri volcano. Concentrations up to 30 wt% B(OH)3 were found in fluid inclusions from the deeper source. The mineralogical and petrological evidence suggests that sassolite was formed by direct deposition of the gas phase. In contrast, sulfate minerals were formed from condensation, leaching, and precipitation processes before being deposited at the surface. These results are a significant contribution to the knowledge of boron-rich fluid inclusions associated with active magmatic-hydrothermal systems.