Origin of fluids in the Araró-Simirao geothermal system, Central Mexico

The Araró-Simirao geothermal system is located in the eastern part of Cuitzeo Lake in Michoacán, Mexico. It is a liquid-dominated convective system featuring a high salinity, rapid discharge, and heat loss derived into a self-sealing process. The reservoir temperature is higher than 200 °C, associated with a fracture zone linked to the Araró-Simirao fault. Samples of thermal fluids (water and gases) were collected from springs, wells, a mud pool, and runoff water in the study area. The waters had temperatures ranging from 31 °C and 76 °C and near-neutral pH values. Three hydrogeochemical facies types were identified, Na⁺- Cl⁻, Ca²⁺-Na⁺ - HCO₃⁻, and Na⁺ -HCO₃⁻, associated with deep thermal water, groundwater, and recently infiltrated water, respectively.

According to the stable isotopes systematics, a binary mixture of thermal water and groundwater was calculated at a proportion from 61 % to 80 % of the thermal component. In the mud pool sample, the chemical composition of bubbling gas is CO₂-dominated, with a mostly magmatic origin according to the Rc/Ra (5.19), CO₂/³He (4.12 × 10⁹) ratios, and the isotopic composition of δ¹³C_(CO2) (−8 ‰). The gases dissolved in waters are characterized by higher N₂ and CO₂ concentrations compared to the other gases, representing possible mixing processes between an end-member enriched in CO₂ and the chemical composition of air-saturated water. Regarding helium isotopes, the gases are the result of mixing origins between crustal, atmospheric, and mantellic helium, and 3 groups were identified: 1) high contribution of mantle helium ranging from 21 to 66 % up to (Mud pool, LS07 and LS07A) 2) high contribution of crustal helium (LSA-02, LSA-08A, Araró) ranging from 55 to 80 % and 3) high contribution of atmospheric helium of 84 % (LSA-08). The δ¹³C_(CO2) results show a primary interaction with magmatic environments and a slightly sedimentary origin. The origin of CO₂, MORB-type, as well as other gases, may be similar to that in Los Azufres. However, these gases reach the surface through different vertical paths, resulting in varying fluid compositions. In the study area, the Araró-Simirao and Huigo faults would act as zones for these gas ascents from the degassing source (magma) at depth.

According to the estimates of the magma aging model, the age of the mud pool sample is estimated to be 2.7 × 10⁶ ± 1.2 × 10⁶ years, which is younger than the andesitic basement rocks. Additionally, there is evidence of a more recent ³He input in the sample. However, the Los Azufres geothermal system has a ³He input from rocks that are even younger (1.5 × 10⁵ years). This suggests that Los Azufres may have a more recent ³He input than Araro-Simirao.