Cerro Prieto Geothermal Field (Baja California, Mexico) – A fossil system? Insights from a noble gas study

Results of a survey of noble gases, water stable isotopes (δD and δ18O), and chlorine (Cl) and bromine (Br) contents measured in geothermal waters from the high-enthalpy, liquid-dominated Cerro Prieto Geothermal Field (CPGF) are reported. Helium isotopic ratios (3He/4He) help to identify two distinct fluids: a magmatic fluid, with 3He/4He (R) ratios of up to 7.3 times that of the atmospheric ratio (Ra = 1.384 × 10−6), and a 4He-rich fluid, with 3He/4He ≤ 3.5 Ra. The helium isotopic systematics point to limited modern freshwater recharge in the CPGF, and thus to the presence of a nearly fossil geothermal system. U-Th/4He-estimated minimum residence times for the 4He-rich fluid component vary from 1.5 to 1.8 Ma, corresponding to the age of the Colorado River paleo-delta sediments, suggesting the presence of connate waters. Cl and Br concentrations indicate that this connate water component is seawater with minor halite dissolution. Together with Cl and Br, the isotopic signature of the water (δD, δ18O) indicates the presence of a third old fluid of meteoric origin, from Late Quaternary to Holocene, corresponding to recharge during colder and wetter periods than today. This is identified as being old groundwater from the Colorado Delta unconsolidated sediments, which cap the Cerro Prieto reservoir. Our results suggest little direct recharge to the CPGF. The spatial distribution of helium isotopes in the reservoir shows that connate waters dominate the central portion of the field and mix with the magmatic fluid containing mantle He, with both fluids rising at the center of the basin where the heat source is located. Deep magmatic fluids containing mantle helium enter laterally into the reservoir, through the main lateral strike-slip faults (i.e., the Cerro Prieto and Imperial faults).