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

Highlights

• 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.

Abstract

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-HCO₃ and Na-SO₄ 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-HCO₃ composition typical of recent infiltration and shallow flow, but they have an anomalous concentration of NO₃. The δ²H and δ¹⁸O indicate a common meteoric source for the warm and cold water plotting along an evaporation line. The waters have higher CO₂ 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.

Introduction

Geothermal energy is an important renewable energy source with a wide range of uses, such as the production of electricity, greenhouses, and building heating and cooling systems (Fridleifsson, 2001; Lund et al., 2005). Geothermal research traditionally focuses on conventional fields associated with recent and active volcanic systems (e.g., Hermanska et al., 2020). In Mexico, these include the fields of Los Azufres, Los Humeros, and Las Tres Vírgenes (e.g., Bruhn et al., 2020). Others like the giant Cerro Prieto field in Mexico and the Salton Sea field in the US, are closely related to magma intrusion in pull-apart basins located along the plate boundary, which provide a high heat flux (Gutierrez-Negrin et al., 2015). Although Mexico still has a number of conventional geothermal sites that can be potentially developed (Gutiérrez-Negrín., 2019), the need to increase the production of dispatchable clean energy requires exploring also novel, unconventional geothermal prospects.

Medium to low-enthalpy hydrothermal manifestations unrelated to Pleistocene-Holocene volcanism have been studied only marginally in Mexico (Wolaver et al., 2013; Morales-Arredondo et al., 2018), whereas their geothermal potential and genesis remain unknown. This is the case for some Oligocene to early Miocene grabens in central-western Mexico, located away from the active volcanic arc. The Juchipila graben hydrothermal system is an example of an unconventional reservoir, located south of the city of Zacatecas (Fig. 1B), which contains at least a dozen thermal springs and groundwater discharge zones with temperatures up to 60 °C distributed along a 150 km-long graben system.

Hydrothermal activity in this area has not been explained to date. Extensional tectonics and felsic magmatism in the area are Oligocene to early Miocene (Ferrari et al., 2013, 2018) and a minor reactivation of the southern part of the graben, associated with mafic volcanism, took place in late Miocene (Martínez-Reséndiz, 2020). Consequently, the thermal anomaly cannot be associated with a cooling magma intrusion in the upper crust or with lithosphere thinning.

During the exploration and assessment of geothermal resources, geochemical and isotope tracers are used to get insights into the reservoir fluid and rock chemistry, the reservoir permeability and temperature, the groundwater flow direction, and the rates of water recharge into the reservoir (Giggenbach, 1992; Arnórsson, 2000; Arnorsson et al., 2007; Spycher et al., 2011; Battistel et al., 2014). The stable isotope composition of water (δ²H and δ¹⁸O) is widely used as a tracer to determine the origin of groundwater, for assessing the degree of water-rock interaction, and as an indicator for mixing of waters from different sources (Arnórsson, 2000; Boschetti, 2013; Battistel et al., 2016). The isotope composition of noble gases, in particular He and Ne, helps to distinguish between the atmospheric, magmatic, and crustal contribution of these gases to the groundwater system (Sano and Wakita, 1985; Kipfer et al., 2002).

In this study, we propose an integrated hydrothermal model derived from the analysis of the geochemical properties from groundwater wells and thermal springs of the Juchipila Basin. Based on the chemical and isotope composition of the water, also including dissolved gases and He isotopes, we determine the origin of cold and thermal fluids, we suggest a possible heating mechanism and we identify possible water flow pathways.