A geochemical and isotopic approach for geothermal exploration: Deciphering fluid-rock interaction in thermal fluids of San Miguel-Vallecitos fault system, Baja California, Mexico

Thermal fluids are commonly found in Baja California, particularly along active fault systems, despite the lack of recent magmatic activity. This study focuses on the San Miguel-Vallecitos Fault System (SMVFS) in Baja California, Mexico, a region influenced by the now inactive subduction of the Farallon plate beneath the North American plate and the active rifting of Gulf of California. These events have shaped the fault systems and could contribute to the thermal fluids observed today. This study explores the geochemical and isotopic composition of thermal springs, including dissolved and bubbling gases to 1) estimate reservoir temperatures using classic solute and multicomponent geothermometers and 2) characterize the origin of helium and determine if the mantle helium proportion along SMVFS is related to rising mantle fluids, recent magmatic activity or residual ³He from crustal sources (magma aging).

The thermal springs are subdivided into 2 groups based on their geographical distribution and chemical and isotopic compositions. Group 1 waters, localized in the southern sector of the SMVFS with temperatures ranging from 28 °C to 35 °C and total dissolved solids (TDS) from 0.22 to 0.36 g L⁻¹, in contrast Group 2 waters in the northern sector have temperatures of 52 °C and 52.8 °C and TDS values of 0.63 and 0.69 g L⁻¹. The temperature estimated using solute geothermometers allows to identify a low-temperature geothermal system, with the highest temperature mostly estimated for Group 2 waters (up to 112 °C). Both groups show N₂-dominated gas composition, with higher concentrations of He and CH₄ compared to CO₂. Helium isotopes ratio (0.04–0.52Ra) and ⁴He/²⁰Ne ratios (8.4–206.2) allowed to identify that most of the helium is of radiogenic origin. However, the proportion of mantle-derived He increases from south to north along the SMVFS, reaching 6.3 % in Group 2, where the highest outlet temperatures were measured. The ³He/⁴He ratios of fluids along the SMVFS cannot be explained by a magma aging model, especially the higher ratios of Group 2, allowing us to decipher that either a more recent post batholitic magmatic event occurred in the studied area or fluids from the mantle are migrating along the permeable SMVFS, the second hypothesis being more likely considering the regional geological context.