Deciphering origins and pathways of low-enthalpy geothermal waters in the unconventional geothermal system of Juchipila graben (Central Mexico).

• The Juchipila graben is an unconventional geothermal system located in central Mexico. • Thermal water has anomalous concentrations of F, B, Li, and As. • δ2H and δ18O indicate a common meteoric source but different evaporation processes. • Mantle helium is transported into the system through deep-rooted faults. This work presents hydrochemical results for groundwater and dissolved gas samples collected from a thermal and cold aquifer in the Juchipila Basin, in southern Sierra Madre Occidental, central Mexico. Thermal springs in the Juchipila Basin reach temperatures of 60 °C, these manifestations are not related to recent or active volcanism as are all the known geothermal fields in Mexico. The thermal waters (>32 °C) are Na-HCO 3 and Na-SO 4 type, with an anomalous concentration of F, B, Li, and As. Their chemistry likely results from water-rock interaction processes. The cold waters (<32 °C) have a Ca-HCO 3 composition typical of recent infiltration and shallow flow, but they have an anomalous concentration of NO 3. The δ2H and δ18O indicate a common meteoric source for the warm and cold water plotting along an evaporation line. The waters have higher CO 2 and He concentrations than the air-saturated water. The helium composition is mainly atmospheric and terrigenous with a mantle helium contribution of up to 14%. This suggests that faults affecting the region are deeply rooted, permitting mantle helium uprise. Geothermometry gives mean reservoir temperatures of 58–102 °C. Based on these results, we propose a model of hydrothermal circulation in the Juchipila Basin, in which rainwater infiltrates deeply through the graben edges fault system, dissolves ions and crustal helium, incorporates mantle helium, while heated by the geothermal gradient, and eventually surges and mixes with the cold, shallow aquifer along faults cutting the whole succession within the graben. [ABSTRACT FROM AUTHOR]