Updated conceptual model of the hydrothermal system in the Acoculco Caldera, Mexico

Abstract

The Acoculco Caldera hosts a high-temperature geothermal system that has not been exploited due to the lack of permeability revealed by the deep exploration wells. Results of our detailed geological and geochemical exploration, integrated with previous results of electric and electromagnetic surveys, have shown large differences in the structures and hydrothermal activity between the northern and southern parts of the Caldera. Electromagnetic surveys indicate the presence of a large high conductivity anomaly towards the north of the caldera and very high resistivity where the deep wells were drilled, which is probably related to the intense silicification observed in the core samples from the deep wells. These disparities are enhanced by the presence of thermal springs and recent volcanics in the northern part, in addition to intense hydrothermal alteration and extremely high heat flow measured in shallow geothermal gradient wells drilled in the northern part of the caldera. Geochemistry of the thermal springs on the northern border of the Caldera is consistent with a hydrothermal reservoir with temperatures close to 300 °C. The Curie Point Depth (CPD) calculation yields a value 7 km that can be correlated with the geothermal gradient measured in the deep exploration wells only if convection is assumed in a layer at least 1 km thick. All this evidence suggests that a viable reservoir may be present at depth in the northern section of the Acoculco Caldera.