Highly conductive diatreme aquifer beneath the Cerro Overo maar, Central Andean volcanic zone, Chile

Abstract Cerro Overo maar (COM), located in the Central volcanic zone (CVZ) of the Andes, has an andesitic basaltic composition (~55 wt% SiO2), which is uncommon in the magmas of the CVZ. Its morphology results from a phreatomagmatic eruption in one of the driest deserts in the world. A magnetotelluric study generated a three-dimensional resistivity model that indicates a diatreme-like structure beneath COM. Unlike other diatremes in the world, this structure is highly conductive between 200 and 700 m depth and probably formed by deep explosions that did not expel material. The diatreme would have evolved into a dynamic aquifer, capable of storing and recycling water during and after eruptive events. Leaching studies suggest that water-rock interactions with CVZ ignimbrites release Li and As, contributing to the salinity of the reservoir. Our results highlight COM as a unique case of a deep saline aquifer in a diatreme, offering new insights into these structures and groundwater flow in arid volcanic terrains. This study provides a framework for understanding diatremes as self-contained, mineral-rich hydrogeological systems, particularly in the Altiplano-Puna region, which hosts 40% of the world's lithium reserves.