Browsing by Author "Mura Toledo, Valentina Rossana"

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    Fluid flow in the Nevados de Chillán Geothermal System as an example of fractured reservoir, Southern Andes
    (2024) Arancibia Hernández, Gloria Cecilia; Mura Toledo, Valentina Rossana; López Contreras, Camila Andrea; Oyarzo Cespedes, Isa Paz Belen; Browning, John; Healy, David; Maza, Santiago; Morata, Diego
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    Hydrothermal Alteration in the Nevados de Chillan Geothermal System, Southern Andes: Multidisciplinary Analysis of a Fractured Reservoir
    (2023) Morata, Diego; Gallardo, Romina; Maza, Santiago; Arancibia Hernández, Gloria Cecilia; López Contreras, Camila Andrea; Mura Toledo, Valentina Rossana; Cannatelli, Claudia; Reich, Martin
    The interplay between a heat source, primary plus secondary permeability, and hydrothermal fluids makes geothermal systems a highly dynamic environment where evolving physico-chemical conditions are recorded in alteration mineralogy. A comprehensive characterization of hydrothermal alteration is therefore essential to decipher the major processes associated with geothermal system development. In this study, we defined the hydrothermal mineralogical evolution of the Nevados de Chillan Geothermal System (NChGS), located in the Southern Volcanic Zone (SVZ) of the central Andes, where the regional framework of the system is formed by a direct association with a currently active volcanic complex, a favorable structural control, and vertically inhibited fluid circulation. To characterize the secondary mineralogy present in the NChGS, we integrated optical petrography, Scanning Electron Microscopy (SEM) observations, X-ray Diffraction (XRD) analysis, and microthermometric measurements along a drill core with a depth of 1000 m at the Nieblas-1 well. These mineralogical approaches were combined with a structural field analysis to highlight the relevance of multidisciplinary study in understanding active geothermal systems. The results indicated that the evolution of the system involved four paragenetic stages, with the main processes in each phase being the heating, boiling, and mixing of fluids and re-equilibration to new physico-chemical conditions. Additionally, three hydrothermal zones were recognized: an upper argillic section, an intermediate sub-propylitic zone, and a deep propylitic domain. Sampled thermal springs are characterized by pH values of 2.4-5.9 and high SO4= concentrations (>290 ppm). These acid-sulfate steam-heated waters suggest the contribution of primary magmatic volatiles to the hydrothermal system. Alunite recorded in the alteration halos of veinlets presents at depths of 170-230 m denote the circulation of acidic fluids at these levels which were favored by reverse faults. These findings indicate that, at this depth range, the condensation of magmatic volatiles into shallow aquifers controls the recharge area of the superficial thermal manifestations. Conversely, deep-seated hydrothermal fluids correspond to near-neutral chloride fluids, with salinities ranging from 0.1 to 6.9 wt.% NaCl eq. The distribution of illite/smectite and chlorite/smectite mixed-layered minerals outline the presence of a significant clay cap, which, in this system, separates the steam-heated domain from the deep hydrothermal realm and restricts fluid circulation to existing permeable channels. Our mineralogical and structural study provides critical data for the interpretation of heat-fluid-rock interaction processes in the NChGS. The interplay between hydrothermal fluids and active faults is also discussed in the context of the complex of geological processes in active geothermal systems along the Chilean Southern Volcanic Zone.
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    Mafic Enclaves Reveal Multi-Magma Storage and Feeding of Shangri-La Lavas at the Nevados de Chillán Volcanic Complex
    (2025) Pineda Ramírez, Camila Andrea; Arancibia Hernández, Gloria Cecilia; Mura Toledo, Valentina Rossana; Morata, Diego; Maza, Santiago; Browning, John
    The Nevados de Chillán Volcanic Complex is one of the most active of the Southern Volcanic Zone. It is formed by NW-SE-aligned eruptive centers divided into two subcomplexes, namely Cerro Blanco (basaltic andesitic) and Las Termas (dacitic), and two satellite cones (to the SW and NE of the main alignment). Our study of the Shangri-La volcano, which is located between the two subcomplexes, in alignment with the satellite cones, and which produced dacitic lavas with basaltic andesitic enclaves, sheds light on the compositional and structural diversity of the volcanic complex. Detailed petrography along with mineral chemistry allows us to suggest partial hybridization between the enclaves and the host lavas and that mixing processes are related to the generation of the Shangri-La volcano and to other volcanic products generated in the complex. This is supported by mixing trends between the enclaves and the most differentiated units from Las Termas. We argue the presence of two main magma storage areas genetically related to crustal structures. A dacitic reservoir (~950 °C) is fed along NW-SE structures, whereas a deeper mafic reservoir (>1100 °C) utilizes predominantly NE-SW structures. We suggest that the intersection between these sets of structures facilitates magma ascent and controls the Nevados de Chillán plumbing system dynamics.
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    Microstructural controls on geothermal reservoir host rock responses to elevated pressures and temperatures
    (ESS Open Archive, 2025) Mura Toledo, Valentina Rossana; Browning, John; Arancibia Hernández, Gloria Cecilia; Healy, David; Farrell, Natalie Jane Charlotte; Bigaroni, Nico; Pineda Ramírez, Camila Andrea; Morata, Diego; Mecklenburgh, Julian
    Rock microstructure controls the nature of geothermal fluid flow within reservoirs hosted in active volcanic environments and as such it is necessary to constrain the microstructural evolution of rocks at elevated pressure and temperature conditions. Despite Andean geothermal systems hosting numerous high enthalpy geothermal plays, there remains a paucity of experimentally derived seismic velocity and porosity data at crustal relevant in-situ conditions. Here, we provide novel constraints on the evolution of rock physical properties of five main representative lithologies of the Nevados de Chillán Geothermal System under dry conditions with confining pressures up to 150 MPa and following heat treatment up to 600°C. The variability of P-wave velocity changes with elevated confinement reveals the presence of both different densities of pre-existing cracks and different distributions of pore aspect ratios within the tested lithologies. Two target units, crystalline granodiorite and diorite, were further subjected to heat treatment to recreate potential temperature conditions within and around the geothermal reservoir. The heat treatment generated new populations of low aspect ratio cracks in both rocks, but this pore space was apparently more difficult to close in the diorite than in the granodiorite when resubjected to confinement. This difference is explained in a conceptual model whereby a combination of crack realignment and mineral alteration in the diorite allows pore space to remain open and potentially connected at greater levels of confining pressure, at greater depths in the geothermal reservoir. These results have implications for the propensity for fluid flow in crystalline geothermal reservoir host rocks.