Preliminary Geochemical Characterization of a Low-enthalpy Hydrothermal Reservoir Not Associated with Recent Volcanism in Southeastern Sierra Madre Occidental, Mexico

Abstract

This work presents the hydrogeochemical results of water and dissolved gas samples collected from thermal and cold groundwater in two grabens in the southern Sierra Madre Occidental of central Mexico. Thermal springs in the Juchipila and the Santiago Papasquiaro grabens reach temperatures of 60 and 74°C, respectively. These geothermal manifestations are not related to recent or active volcanism, as all the known geothermal fields in Mexico. The thermal waters collected in the Juchipila graben are Na-HCO3 and Na-SO4 type and the waters collected in the Santiago Papasquiaro graben are Na-HCO3 water-type. In both grabens, the thermal samples have anomalous concentration of F, B, Li, and As, associated with rock alteration and dissolution processes. The cold waters have a Ca-HCO3 composition and chemical characteristics of recent recharge and superficial flow. The δ2H and δ18O data indicate a meteoric source for the thermal and cold waters in both grabens, with no volcanic input to the systems. The thermal samples in the Santiago Papasquiaro graben fall on the GMWL, whilst the samples in the Juchipila graben distribute along an evaporation line. In the Juchipila graben, the He composition is mainly of atmospheric origin with a mantle helium contribution up to 14%. On the other hand, the He isotope composition in the Santiago Papasquiaro graben is crustal dominated, with a contribution up to 96%, and characterized by a mantle proportion up to 9%. This implies that in both grabens the fault systems have deep roots or fluids interact with crustal faults, although to a lesser extent in the Santiago Papasquiaro graben. Based on our results we propose a hydrothermal system model for the Juchipila graben, in which rainwater infiltrates deeply through the graben border fault system, dissolve mantle and crustal helium, and eventually surge at surface along faults cutting the whole succession within the graben. The model for the Santiago Papasquiaro graben is similar, although the high contribution of crustal He implies an alternative/additional heat source, probably related to radioactive decay of U-rich geologic units.