Chemical-Isotopic Evidences for the Origin and Evolution of Geothermal Fluids at the Las Tres Vírgenes Geothermal Field, B.C., NW-Mexico

Abstract

The Las Tres Vírgenes (LTV) geothermal reservoir is a liquid-dominated resource located in the Baja California Peninsula (NW-Mexico) with a currently installed power capacity of 10 MWe. The sustainability of future electricity production at the LTV geothermal field raises concerns about the origin and hydraulic dynamics of exploited geothermal fluids. As part of the present study, water samples were taken from the production wells LTV-4 and LTV-11 with a maximum bottom depth of 2,413 m and 1,974 m, respectively, to analyze a series of chemical (major and trace elements) and isotopic (2H, 3H, 11B, 13C, 14C, 18O, 87Sr/86Sr) parameters. The chemical and isotopic composition of LTV geothermal fluids is compared with surface water types (precipitation, thermal springs, shallow groundwater, seawater) and host rocks from the Baja California region to define their primary provenance, as well as type and degree of hydrothermal alteration. Interpreted results are compared with data from the adjacent geothermal fields in Cerro Prieto (Baja California Norte, Mexico) and Salton Sea (California, USA).
Measured 14C-concentrations from 19.8 to 35.6 pmC - combined with the absence of tritium - support evidence for a Late Pleistocene to Early Holocene recharge event (8,300 – 13,000 yr BP) of surface water into the geothermal reservoir. Elevated precipitation rates by humid climatic conditions in northwestern Mexico at the final stage of the Last Glacial period, combined with enhanced fracture and fault permeabilities through local volcanic eruptions (La Virgen Volcano: 6,515 yr BP) could have enabled and accelerated the meteoric recharge of the LTV geothermal reservoir. Cooler climatic conditions are reflected by depleted deuterium values (D = 19 to 30‰) of the meteoric component in comparison to present-day local precipitation. The intermediate composition of LTV geothermal fluid (2H, 18O, Na, Cl, Ca, Br) between meteoric water and seawater suggests a 60:40 contribution of both water types. In contrast to Salton Sea geothermal water, mineral dissolution to explain elevated fluid salinity (TDS: 10 – 12 g/L) can be excluded as Cl/Br- (245) and Na/Cl-ratios (0.57) are relatively close to seawater and Cerro Prieto geothermal fluid composition.
The assimilation and homogenization of strontium and boron isotopic signatures between LTV geothermal fluids and reservoir host rock, as well as fluid enrichment in K, Li and SiO2 suggest a mature degree for hydrothermal alteration, mainly by de-sorption of exchangeable B in clays, albitization of K-feldspar, chloritization and interaction with siliciclastic rocks.