Efflorescent sulfate and borate mineralization in a low-temperature fumarole from the Cerritos Colorados geothermal field (Mexico)

Efflorescent minerals commonly form in acidic near-surface environments through gas–rock–water interactions, yet their mineralogical assemblages and formation processes in fumarolic settings remain incompletely documented in volcanic regions of Mexico and Central America. This study presents a detailed mineralogical and geochemical characterization of efflorescent deposits associated with La Azufrera, a low-temperature fumarole (≈90 °C) located in the Cerritos Colorados area, western Mexico. Mineral precipitates collected at and around the fumarole vent were analyzed using VNIR–SWIR spectroscopy, X-ray diffraction with quantitative Rietveld refinement, and scanning electron microscopy coupled with EDS. In addition, two fumarolic condensates were analyzed for major ions and selected elements to provide qualitative constraints on the chemical conditions associated with mineral formation. The efflorescent assemblage is dominated by sulfate and borate minerals, including alunogen (Al 2 (SO 4 ) 3 ·17H 2 O) and tincalconite (Na 2 [B 4 O 5 (OH) 4 ]·3H 2 O), accompanied by native sulfur (S°), gypsum (CaSO 4 ·2H 2 O), anhydrite (CaSO 4 ), halite (NaCl), tridymite (SiO 2 ), and opaline silica (SiO 2 ·2H 2 O). Mineral distribution reveals a clear spatial zonation, with sulfur-rich deposits proximal to the vent and aluminum- and boron-bearing efflorescences in distal zones, reflecting variations in oxidation, hydration, and evaporation. Condensate chemistry indicates acidic, sulfate- and chloride-rich conditions consistent with steam-heated environments and provides qualitative insights into element mobilization and efflorescent mineral precipitation. This study documents the first occurrence of tincalconite in a fumarolic environment in Mexico and highlights the relevance of fumarolic alteration processes for understanding near-surface geochemical evolution in volcanic regions of southern North America. • Alunogen and tincalconite were identified as efflorescent minerals in a low-temperature fumarole at La Azufrera, Mexico. • These minerals record advanced argillic alteration under acidic, steam-heated conditions. • Gas–rock interactions followed by cooling and evaporation during gas–air mixing control mineral precipitation.