Abstract

Cerro Overo maar (COM), located in the Central volcanic zone (CVZ) of the Andes, has an andesitic basaltic composition (similar to 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 selfcontained, mineral-rich hydrogeological systems, particularly in the Altiplano-Puna region, which hosts 40% of the world's lithium reserves.