- Extensive noble gas survey was carried out in the Cerro Prieto Geothermal Field.
- 3He/4He indicates magmatic and old sedimentary fluids, with little recharge.
- Cl, Br, and U-Th/He fluid ages suggest that old fluids are connate seawater.
- Stable isotopes and halogens indicate a third old groundwater from shallower aquifers.
- Active modern recharge is limited, indicating Cerro Prieto is a fossil geothermal system.