Unravelling the hydrothermal system of Laguna del Maule restless volcanic field, in the Andean Southern Volcanic Zone (36° 10′S)

Abstract

Laguna del Maule Volcanic Field (LdMVF, 36° 10′ S), located in the Southern Andean Volcanic Zone, concentrates an extraordinary postglacial (<25 ka) rhyolitic volcanism and records one of the highest world rates of surface uplift (>20 cm/yr since 2007). Different papers have been published concerning magmatism and geophysical survey but no research has been performed concerning the associated hydrothermal system present in the area. In this work, we analyze both past and present-day hydrothermal manifestations present in the LdMVF. Seven hydrothermal fossil alteration zones were described, with different mineral assemblages suggesting paleo-fluids temperatures ranging between 150 °C - 250 °C for hydrothermal alteration. Additionally, five active fluid discharge zones were also recognized. Based on their spatial distribution, and new data on surficial water/gas geochemistry including stable isotopic composition (δ18O-δD-H2O and δ13C-CO2), the Laguna del Maule Hydrothermal System is here defined. A conceptual two-layer model is proposed, which consists in a deep reservoir with estimated temperature of 178 °C -195 °C and over this a shallow aquifer system with temperature of 116 °C -143 °C. Water composition from outflow zones revealed fluid-rock interaction, due to carbonates and sulphates dissolution of Mesozoic sedimentary units. The upflow fluid discharges, which are spatially coincident with the ongoing surface uplift, provides geochemical evidence of deep origin, and consequently the arrival of fluids capable of increasing pressure in the upper crust. The spatial distribution of both past and present hydrothermal manifestations suggests a strong structural control promoting fluid circulation due to enhanced permeability, throughout the entire lifetime of the volcanic field.