Chemical and isotopic study of thermal springs and gas discharges from Sierra de Chiapas, Mexico

Thermal water and gas discharges south-east of El Chichon volcano, Mexico are associated mainly with NW-SE oriented fault systems. Spring discharges include i) waters with Na-Cl composition and TDS>3000 mg/L; ii) waters with Ca-SO4 composition and TDS values between 1400 and 2300 mg/L; iii) waters with Na-Cl composition and TDS of 800 to 2400 mg/L and sulphate content up to 650 mg/L and iv) waters with Ca-HCO3 composition and low salinity (TDS <250mg/L). Most of these waters are associated with free-gas discharges of N-2 (up to 93 % by vol.), CO2 (2.4 to 31.2 % by vol.) and Ar (up to 1.25 % by vol.) with a predominant meteoric origin. H2S is present only in gas samples collected at El Azufre (up to 1.1 % by vol.). The delta C-13 CO2 values are always below -9.7% (PDB) and suggest a partially biogenic origin for CO2. Chemical and isotopic features of spring discharges indicate that fluid circulation in the Sierra de Chiapas is mainly regulated by meteoric waters that tend to infiltrate the upper and middle-Cretaceous carbonate units up to the lower Cretaceous-upper Jurassic evaporitic formations (by Lopez-Ramos, 1982). The latter provide the main source of the species in solution. No evidence for high-to-medium enthalpy systems at depth beneath the Sierra de Chiapas has been found.