Abstract
A geochemical survey of fluids in the Las Tres Vírgenes geothermal field (LTVGF) in Baja California Sur, Mexico, was carried out to describe their origins and evolution within the reservoir. Major and minor elements and noble gas isotopes (He, Ne, Ar, Kr, and Xe) were measured in fluids from three production wells, one injection well, and one fumarole (El Azufre). Stable isotopes of water (δD and δ18O) show mixing between Quaternary recharge, lighter than present-day rainfall, and a deep fluid of andesitic origin. The Na/Br and Cl/Br ratios indicate that deep brines from the LTVGF are seawater having leached evaporite deposits (i.e., halite). These deposits are presently absent in the area, but likely occurred during the Miocene, prior to the breakup of the Gulf of California, suggesting that the saline fluid end-member of the LTVGF is several million year old. Measured 3He/4He ratios of greater than 6.5 Ra (where Ra is the atmospheric ratio of 1.384 x10-6) show that LTVGF fluids are a mixture of meteoric waters and deep magmatic fluids, with the hotter and more pristine mantle fluids found in the southern part of the exploitation zone.
Highlights
The Las Tres Vírgenes Geothermal Field (LTVGF) is the smallest high-enthalpy geothermal field exploited by the Comisión Federal de Electricidad (CFE) in Mexico
It is located in the central portion of the Baja California Peninsula (Baja California Sur; Fig. 1A), 34 km NW of the town of Santa Rosalía, within a magmatic complex that includes La Reforma Caldera, the Aguajito complex, and the Las Tres Vírgenes volcano (Fig. 1B)
The LTVGF is located in a NW-SE-oriented Plio-Quaternary rift, the Santa Rosalía Basin [2], which is related to the deformation zone created by the opening of the Gulf of California [3]
Summary
The Las Tres Vírgenes Geothermal Field (LTVGF) is the smallest high-enthalpy geothermal field exploited by the Comisión Federal de Electricidad (CFE) in Mexico. It is located in the central portion of the Baja California Peninsula (Baja California Sur; Fig. 1A), 34 km NW of the town of Santa Rosalía, within a magmatic complex that includes La Reforma Caldera, the Aguajito complex, and the Las Tres Vírgenes volcano (Fig. 1B). To assess the current state of LTVGF fluids (Fig. 1C), a geochemical survey was carried out in November 2016 During these studies, three production wells (LV-6, LV-11, and LV-13), a reinjection well (LV-8), and the El Azufre fumarole were sampled for major and trace elements, halogens (Cl and Br), water stable isotopes ( D and 18O), and noble gas isotopes (He, Ne, Ar, Kr and Xe). The main results of halogen, He isotope, and stable isotopes of water analyses are presented in this paper
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