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

Highlights

• The first record of B-rich fluid inclusions associated with fumarolic fluids is presented.

• Sassolite, barite, sulfur, and anhydrite were found around a 258 °C fumarolic discharge.

• Mineral-forming processes are discussed based on microscopic and spectroscopic methods.

• Fluid sources of magmatic and hydrothermal origin were identified.

• Fluid evolution patterns are discussed according to microthermometric data.

Abstract

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, SO_2, 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.

Introduction

Boron is a lithophile and incompatible behavior element, with an average concentration in the continental crust of 17 ppm (Rudnick and Gao, 2003). High boron concentrations (up to ~500 ppm) are occasionally found in pelagic sedimentary or granitic rocks (e.g., Gao et al., 1998; Peretyazhko et al., 2000; Thomas, 2002; Sazontova et al., 2003; Grew, 2017), whilst exceptionally high boron concentrations (up to ~1000 ppm) have also been found in thermal waters (e.g., Ngawha, New Zealand, Sheppard, 1987; Humeros, Mexico, Bernard et al., 2011). Limited studies have described the occurrence of boron in fumarolic emissions, which is usually inferred from the occurrence of boron-bearing crystalline phases such as sassolite, chubarovite, barberiite, or knasibfite, among others (e.g., Garavelli and Vurro, 1994; Campostrini et al., 2011; Pekov et al., 2015; Shablinskii et al., 2020). In addition, significant boron concentrations have been found (up to 65 ppm) in volcanic gas condensate (e.g., Goff and McMurtry, 2000), suggesting that it is a common constituent of volcanic gases.

The dehydration of a subducted slab is the primary boron source in subduction zones (Palmer, 2017), which supplies boron-rich fluids to the arc-related volcanic and geothermal areas. In volcanic systems, boron-rich fluids are emitted by quiescently degassing volcanoes or sporadic volcanic eruptions (e.g., Zhu and Wang, 2007). The most common evidence of boron-rich fluids is the occurrence of sassolite in fumarolic deposits, which has been previously documented at Vesuvius (Italy; Pelloux, 1927), Vulcano (Italy; Campostrini et al., 2011), Momotombo (Nicaragua; Quisefit et al., 1989), Satsuwa-Iwojima (Japan; Okano, 1962; Kanzaki et al., 1979), and Guallatiri and Lastarria (Chile; Inostroza et al., 2020a) volcanoes.

The Central Volcanic Zone (CVZ) of the Andes represents an ideal location for the formation of borate deposits, primarily due to the Tertiary-Quaternary volcanic activity, geomorphology, and climate conditions (e.g., Chong et al., 2010; Helvaci, 2017; Garcés-Millas, 2020). High‑boron concentrations in thermal waters have also been found in Surire, El Tatio, and Tuja geothermal areas (e.g., Tassi et al., 2010), as well as sassolite reported at El Tatio (e.g., Garcia-Valles et al., 2008). Recent studies demonstrate the presence of boron in actively degassing volcanoes of the CVZ in trace concentrations: (i) in the volcanic plume of Lascar volcano (e.g., Menard et al., 2014), (ii) in the volcanic gas condensates at Lastarria volcano (Aguilera et al., 2016), and (iii) as sassolite around medium-low temperature fumaroles at Guallatiri and Lastarria volcanoes (Inostroza et al., 2020a).

Fluid inclusions are small fractions of fluids trapped within a natural crystal during its crystallization processes. They preserve direct evidence from the parental fluid, tracing temperature, pressure, density, and chemical composition of ancient mineralizing fluids (e.g., Roedder, 2002). Fluid inclusions with high‑boron concentrations and boron-bearing aqueous and crystalline phases have been previously recognized in mineral phases associated with granitic pegmatites (e.g., Williams and Taylor, 1996; Smirnov et al., 1999, Smirnov et al., 2000; Peretyazhko et al., 2000; Sazontova et al., 2003; Rickers et al., 2006; Thomas et al., 2012; Huong et al., 2017) and hydrothermal ore deposits (e.g., Prokofev et al., 2002). Those works centered on improving analytical techniques associated with the identification and quantification of boron compounds in fluid inclusions and determining physicochemical properties of boron-rich solutions and their role during crystallization processes. However, according to our research, boron-rich aqueous solutions within fluid inclusions associated with fumarolic emissions have not been studied.

Fluid inclusions hosted in fumarolic minerals pose an exceptional opportunity to investigate mineral forming processes in a remote volcano under extremely acidic and low atmospheric pressure conditions. This work presents detailed mineralogical, chemical, and fluid inclusion data from a sample taken from the summit fumarolic field at the Guallatiri volcano (northern Chile) to determine the mineral phases and obtain insights into the physicochemical conditions associated with its formation and deposition. The method for the calculation of the boric acid concentration in boron-rich fluid inclusions was used (Peretyazhko et al., 2000; Bakker and Schilli, 2016). In addition, the petrology of boron-rich aqueous solutions within fluid inclusions related to fumarolic deposits, the evolution of hydrothermal fluids associated with an active volcanic system, and mineral-forming processes are discussed.