Geochemistry and origin of high-pH thermal springs in the Pacific coast of Guerrero, Mexico

Abstract

Thermal waters (40-43°C) discharging from crystalline rocks of the Xolapa complex, near Acapulco, Mexico, within an area of approximately 100 km × 50 km are characterized by a low salinity (TDS<0.5 g/kg) and very high pH (9.5-10). They represent a regional aquifer or a group of aquifers with similar conditions of water circulation and water-rock interaction. Spring gases are nitrogen- and He-rich, with a high proportion of radiogenic He (R/Ra=0.12 to 0.3). Methane from a spring with a high concentration of CH4 (10-2%)  is relatively enriched in 13C (δ13C=-26‰, PDB). All gases are very low in CO2 (<0.5%) and relatively low in Rn (8 to 25 Bq/l). Numerical simulation of water-rock interaction in a multistep flow-through reactor, using a "1st wave" approximation, suggests that this type of high-pH diluted water can be produced by step-by-step dissolution of granite and redeposition of secondary equilibrium minerals along a flow-path. This models the infiltrating meteoric water to the depth of 100°C and its ascending to the surface with conductive cooling to 40°C. Closed system conditions in respect to CO2 and a low concentration of the carbonate carbon in the aquifer rock are needed to attain the high pH in thermal water from initially neutral or slightly acidic rain water. The presence of the carbonaceous material in the aquifer rocks can explain the observed concentrations of methane in the CH4-rich spring, as well as its carbon isotopic composition.