Hydrothermal Alteration as an Indicator of Temperature and Flow Regime in the Cerro Prieto Geothermal Field of Baja California

Abstract

More than 65 boreholes, 1.5-3.5 km deep, have encountered temperatures of 350°C and brines with <1% total dissolved solids in the Cerro Prieto geothermal field of the Salton Trough intercontinental rift system. Temperature dependant dehydration an decarbonation reactions have produced progressive hydrothermal alteration in the deltaic sediments of the reservoir to form greenschist facies rocks. We have applied various methods of geothermometry, including authgenic mineral assemblages and light stable isotopes, to study temperature distribution and fluid rock interaction in the subsurface. These data have revealed a pattern of mineral zones and isotopic ratios in the rocks which defines the shape of the reservoir and records the pattern of hydrothermal circulation prior to production. We infer that a thermal plume, elongated in a NW-SE direction, dips at 45° to the NE; it is recharge from the NE and discharges to the SW.