The strength and stiffness of lavas, as well as their propensity to fracture and develop permeable flow networks, are all governed by microstructural heterogeneity. In vesicular andesitic–dacitic lavas from the Nevados de Chillán Volcanic Complex (NChVC) in Chile, we show that uniaxial compressive strength (UCS) and Young’s modulus (E) depend not only on connected porosity but also on the distribution, orientation, size and aspect ratio of pores. Pore fabrics were first quantified via 2D analyses (transparent sections and surface imaging) and 3D micro–computed tomography (micro-CT). Then, using cylinders with pores aligned parallel, perpendicular, and inclined to the loading direction, we performed UCS tests instrumented with acoustic emission (AE) monitoring. Loading perpendicular to the pore major axis presents the lowest strength and stiffness and triggers AE output at low stress, whereas loading parallel to the pore major axis initiates damage at higher stress, attains relatively higher strength, and culminates in macroscopic failure with higher rates of AE concentrated near the peak stress. Mixed AE patterns are observed when loading at an inclined angle to the pore major axis. These observations are consistent with the theory of microcrack nucleation and propagation controlled by stress concentration around elliptical pores. While porosity exerts a first-order control on the overall magnitude of strength and stiffness, we conclude that pore alignment, aspect ratio and directional variability fundamentally modulate failure style, damage evolution, and rock mechanical properties. Operationally, this framework anticipates preferential directions of weakness and damage evolution prior to failure, thereby supporting assessments of the mechanical stability of lava domes, levees, and scoria cones composed of high porosity materials.

Active slip partitioning between the subduction megathrust and the upper plate is investigated in the oblique-convergence setting of the Nazca-South American plate boundary between 33° and 47° S. This segment has two major along-strike bends: the Maipo Orocline (~34° S) and the Arauco Peninsula (~38° S), whereas south of 38° S, lies the intra-arc Liquiñe-Ofqui Fault System (LOFS). Here we examine long- and short-term upper-plate deformation by combining a harmonized catalog of about 2,300 fault-slip measurements in the forearc and arc regions, from which we derive P-T axes using kinematic inversions, along with an integrated seismological database for upper-plate events (1976-2025), including global and local networks. These data are categorized by forearc, arc, and back-arc regions. We identify four distinct tectonic segments based on the spatial distribution of P and T axes in the long-and short-term: (1) 33°-34° S, showing both ~E-W and ~N-S subhorizontal shortening in the forearc and arc areas, suggesting active radial shortening; (2) 34°-37° S, dominated by mostly blind, seismogenic, margin-parallel dextral faults along with NW- and NE-trending structures running at a high angle with respect to the plate margin (called transverse faults here); (3) 37°- 41° S, where margin-orthogonal subhorizontal shortening in the submerged forearc coexists with nearly margin-parallel shortening in the emerged forearc, with contemporaneous dextral slip along the LOFS and ~E-W shortening accommodated by transverse NE-trending dextral and NW-trending sinistral seismically active faults; and (4) 41°-47° S, a region governed mainly by the geometry and kinematics of the LOFS strike-slip duplex. Kinematic indicators on mesoscopic faults (mostly slickenfibers) and a lack of pseudotachylytes suggest a considerable aseismic component to upper-plate fault slip, implying that morphotectonic slip rates may overestimate seismic hazard. Future detailed geodetic data may help better constrain the relative contributions of aseismic and seismic slip on the upper-plate faults in this Andean segment.

In geothermal systems the thermo-physical properties of the rocks change as they interact with fluids passing through the volcanic system and during discrete events such as earthquakes and magma intrusion. To characterize a geothermal system and the flow of fluid through a sub-volcanic complex, targeted rock physical tests are needed for the rocks of the area conducted at natural P-T conditions. Here, we present rock property characterization of the main geological units of the active Nevados de Chillan Geothermal System, located in the Southern Volcanic Zone (SVZ), an area with some of the largest geothermal potential in the Andes. Six representative blocks of the geothermal host reservoir and overlying strata were collected from the volcanic basement. The main geomechanical units of this system are (from oldest to youngest): 1) andesites, tuffs and breccia of the Cura-Mallin Formation (Miocene country rocks); 2) granodiorites and diorites of the Santa Gertrudis Bullileo Batholith (15.7 Ma and 5.9 Ma, respectively); and 3) hornfels from the contact between the granitoids and country rocks. Cylindrical core samples (26 mm diameter x 65 mm length) from each block were used to quantify density, porosity, and ultrasonic wave velocities at different confining pressures. All tests were carried out at the Rock Deformation Laboratory, University of Manchester. Polished thin sections were prepared from blocks of the same orientation as the cored directions and analyzed using petrographic.Granodiorite has the lowest porosity at between 2%) to the granodiorite and 4.8 to 5.7 km/s P-wave velocities and 2.7 to 3.3 km/s S-wave velocities. The andesitic lavas have porosities ranging 3-7%, while the tuffs and breccias have porosities of 12-30%. Elastic waves velocities in the andesitic lavas are around 2 km/s faster than the pyroclastic rocks.Tests with cycles of increasing and decreasing hydrostatic pressure (up to approximately 150 MPa) show that granodiorite and diorite exhibit sharp increases in P-wave velocity (Vp). This is attributable to the stiffening of the rock from the progressive closure of pre-existing cracks. Above 40 MPa, the rate of increase in Vp with pressure reduces markedly, implying that the remaining porosity is less compliant. This is consistent with the maximum burial depth of the rocks suggesting that those cracks formed because of bringing the rocks to the surface.Finally, in terms of microstructural observations, the granodiorites, diorites and hornfels have large intragranular and intergranular fractures with very high aspect ratio, which are commonly oriented and therefore impart anisotropy. In contrast, the andesitic, tuffs and breccias porosity is higher than the crystalline rocks and is mainly composed of intergranular pores with low aspect ratios and relatively isotropic.

Numerical models can be utilized to understand and anticipate the future behavior of a geothermal reservoir, and hence aid in the development of efficient reservoir engineering strategies. However, as each system has a unique geological context, individual characterization is required. In this research, the Nevados de Chillan Geothermal System (NChGS) in the Southern Volcanic Zone of the Andes is considered. The NChGS is controlled by the geology of the active Nevados de Chillan Volcanic Complex (NChVC) including their basement units (Miocene lavas and volcaniclastic layers from Cura-Mall & iacute;n Formation and the Miocene, Santa Gertrudis granitoids) as well as the key structural control from crustal scale faults, all of which combine to influence the reservoir characteristics. The presence of faults acts to generate a high secondary permeability which favors the circulation of hydrothermal fluids. Based on previous studies in the NChGS, we designed a thermo-hydraulic model in COMSOL Multiphysics (R) combining equations of heat transfer and Darcy's law in order to determine the distribution of isotherms and surface heat flux. The boundary conditions of the model were informed by a conceptual model of depth 3 km and width of 6.6 km which considers a highly fractured granitic reservoir, a clay cap behavior of Miocene lavas and volcaniclastic units, and transitional zones between a regional zone and the reservoir. A lowangle reverse fault affecting the clay cap unit was also incorporated into the models. Results indicate convective behavior in the reservoir zone and a surface heat flux of 0.102 W/m2 with a local peak up to 0.740 W/m2 in the area affected by the low-angle reverse fault zone. The models suggest hydrothermal fluid residence times of around 9-15 thousand years are required to reach a steady-state thermal configuration, which is consistent with the deglaciation age proposed for the NChVC latitude of the complex (c. 10-15 ka). Permeability in the fractured reservoir is one of the most complex parameters to estimate and the most sensitive and hence requires further constraint. Finally, using the volumetric method and the results obtained in this research, we estimate a geothermal potential of 39 +/- 1 MWe for the NChGS.

Gas geochemistry is key to understanding volcanic processes, offering insights into subsurface magma dynamics and aiding eruption forecasting. We present a 10-year monitoring case study from Andean Southern Volcanic Zone (SVZ), one of the world's most active regions, demonstrating its value in assessing volcanic unrest. The geochemical composition of fumarolic gas emissions from the Nevados de Chill & aacute;n Volcanic Complex (Chile) strongly varied across different phases of volcanic activity, permitting the evaluation of the dynamic interplay between magmatic and hydrothermal processes. During volcanic quiescence periods, (2013 and 2023), fumarolic gases were predominantly controlled by shallow meteoric-hydrothermal circulation, as suggested by Ar (Ar-40/Ar-36 similar to 270-290) and water isotopic signatures, low He-3/He-4 (or Rc) ratios (similar to 3.5 times the atmospheric value Ra), moderate CO2 levels (between similar to 2200 and similar to 9800 mu mol/mol), and the absence of magmatic gaseous species (SO2, HCl, and HF). In contrast, the unrest phase (2016 and 2017) was marked by a rapid and significant gas character shift. Rc/Ra values increased up to >6, CO2 concentrations exceeded 12,000 mu mol/mol, and magmatic gaseous species became detectable, with SO2, HCl, and HF reaching 8.5, 4.3, and 0.21 mu mol/mol, respectively. Additionally, simultaneous Ar (Ar-40/Ar-36 similar to 370-410) and water isotopic signature shifts from meteoric origin toward a mixing with deep components were also detected. Gas geothermometry, computed through the H-2/Ar* - CH4/CO2 and H-2/Ar* - CO/CO2 equilibria systems, revealed a rise from 290 +/- 10 degrees C in quiescence to 340 +/- 10 degrees C during the unrest phase. The long-term geochemical surveillance helped detect a substantial signature of volatile-rich magmatic fluids influx into the hydrothermal system before the eruption phase (2018-2021), offering information on the early detection of unrest.

The spatial distribution of surface geothermal manifestations and hydrothermal alteration reflects the complex interplay of multiple factors controlling fluid circulation. This study aims to understand the different controls on the distribution of surface hydrothermal alteration, using the Nevados de Chill & aacute;n Geothermal System (NChGS) as a case study. Lineament maps were created at 1:2500 and 1:5000 scales, and orientations of faults, fractures, and veins were measured. The pH measured ranges from 2.6 to 5.4. Surface temperature was measured and analyzed via Inverse Distance Weighted (IDW) interpolation in ArcGIS (R). Surface hydrothermal alteration was mapped using drones, and minerals were identified through X-Ray Diffraction of whole rock and clay analysis, including chalcedony, opal, native sulfur, illite, kaolinite, iron oxides, sulfates, and hydrated sulfates. Illite crystallinity measured using the Full Width at Half Maximum index (FWHM) ranged from 0.05 to 1.77, while kaolinite crystallinity measuring using Aparicio-Gal & aacute;n-Ferrell index (AGFI), ranged from 0.73 to 1.15. The results show that the distribution of high temperatures (reaching 95 degrees C) and intense advanced argillic hydrothermal alteration is heterogeneous, controlled by lithological contact and the interaction between similar to E-W-trending lineaments associated with subvertical faults and NNW-striking surficial low-angle faults. Circulation of acid-sulfate waters rich in Fe, Al, and Cu occur primarily along fault/fracture networks, promoting the formation of high-crystallinity illite and kaolinite at the surface. This study proposes a link between surface geothermal manifestations with fault-fracture network and lithological controls within a fractured geothermal system in the Southern Andes.

Detailed structural analysis from representative outcrops is necessary to characterize geothermal reservoir dynamics. Here, we estimate fracture density and intensity, as well as the dimensional properties of individual fault and fracture sets in basement rocks of the Nevados de Chillan Geothermal System. We identified several important structural features that could be responsible for controlling local fluid flow; the high-angle sinistral Las Trancas Fault as well as a series of low-angle reverse faults within the Las Termas-Olla de Mote Fault system. Most fractures identified strike either NE-SW, NNE-SSW, and NNW-SSE. Analysis of fault-slip data, supported by seismicity, indicates the presence of a main transtensional regime with subhorizontal NE-trending sigma 1. Structures sub-parallel to the present-day local maximum horizontal stress show significant dilation tendencies, whilst NW- SE fractures are less prone to dilation. NE and E-W high angle faults could be primary conduits facilitating the upward migration of hot fluids from reservoirs within crystalline and fractured rocks. The fracture length distribution was analysed using power law, negative exponential, and log-normal distribution. The power law with a scaling exponent of about-3 provides the best fit to the data. This study advances our understanding of the structural control of the geothermal reservoir and its associated fracture-controlled fluid circulation and thereby improves the prospectivity in the region by quantifying the optimum fracture sets for fluid flow.

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.

Understanding the controls on crustal fluid-flow in geothermal fractured reservoirs is critical to assessing their occurrence and storage capacity. The Nevados de Chillán Geothermal System (NChGS), located in Southern Chilean Andes, is hosted in volcanic-volcaniclastic rocks and fractured granitoids. In this work, we present evidence of how regional to local fault and fracture networks control the location and size of the NChGS.Fractures in crystalline rocks were analyzed in three sites: 1) Shangri-La diorite, 2) Las Trancas granodiorite and 3) Valle Hermoso hornfels. Linear scanlines have a total cumulative length of 130 meters, in which >1000 fractures were measured. Results show preferential fracture orientations of N60E, N30E, and N45E for the three sites, respectively. Minor families of fractures in NNW and NW directions are also observed. The intensity of fractures (i.e. number of fractures/scanline length) is ~5m-1 at Shangri-La and exhibits minimum and maximum values between 6-13m-1 at Las Trancas, and between 8-13m-1 at Valle Hermoso. Variability in fracture intensity relates to profile orientation, presence of localized shear zones or distance from the geothermal system. These outcrop-scale structures are consistent with regional geometric arrangement and kinematics of major faults.A fourth site was analyzed in the Las Termas-Olla de Mote area. Here, Miocene volcanic and volcaniclastic rocks present an intense argillic alteration in an area ca. 1 km2. Numerous surface geothermal manifestations, such as fumaroles, hot springs, mud pools, mud volcanoes, and heated soils, can be observed. Using a Hanna HI 98509 thermocouple and Fluke TiS45 infrared camera, surface temperatures between 13°C and 95°C were measured. In this site, 85 fractures were measured in a 3-meter-long scanline in a localized cataclastic shear zone. The fracture alignment is essentially isotropic with an intensity of ~28m-1. We noted a hydrothermal alteration pattern associated with centimetric to metric fault planes and fault zones. X-Ray Diffraction on clay minerals related to these fault-controlled alteration zones shows high-crystalline illite (Kübler index as low as 0.096) and kaolinite (Aparicio-Galan-Ferrer index as high as 1.115).Numerical modeling, considering structural, hydrothermal and temperature data, was performed with COMSOL Multiphysics, which allowed us to demonstrate the control of fractures in the development of a crystalline rock hosted geothermal reservoir. The simulated reservoir isothermal pattern can be reproduced consistently with our conceptual geological model after 15 ka.These combined results evidence the first order structural control on the formation of the NChGS. Intersection of regional fault/fracture systems and local dilation areas are the main controls that permit the formation and growth of the active geothermal system. Moreover, the high crystallinity fault-related illite and kaolinite confirms that fluid-flow is mainly controlled by secondary structural permeability. Finally, the surface temperature data, coupled with thermal numerical modelling, allow us to establish a comprehensive theoretical model for the active NChGS relevant for sustainable exploitation.This work is a contribution to the ANID-FONDECYT Project 1220729 and Andean Geothermal Center of Excellence (CEGA). Valentina Mura thanks to ANID -Beca Doctorado Nacional 21210890.

Hot fluid-rock interactions in volcanic-hosted geothermal systems favour the presence of hydrothermal alteration patterns conforming the clay cap, dominated by argillic alteration, and the propylitic zone, currently related to the geothermal reservoir. Clay minerals are ubiquitous phases in these geothermal systems, being the reaction progress from trioctahedral smectite to chlorite, via chlorite/smectite (C/S), and dioctahedral smectite to illite, via illite to smectite (I/S), typical indicators of evolution from clay cap to reservoir conditions with increasing temperature. In fact, different geothermometers have been proposed using chlorite composition highlighting the relevance of clay minerals for a complete understanding of hydrothermal pathways in active geothermal systems.Here, we analyse clay minerals (petrography, XRD, SEM-EDX and HR-TEM-EDX) from a 1000.87 m deep exploration drill core in the active Nevados de Chillán Geothemal System (NChGS) in Southern Volcanic Zone (central Chile). Lithologies are dominated by andesitic lavas and volcaniclastic breccias. Based on hydrothermal mineral assemblages, a transition from argillic to sub-propylitic (c. 350 m deep), beginning the propylitic alteration zone at 680 m deep has been defined. In situ temperature measurements during drilling achieve values up to 200°C at the bottom of the well. Geophysical and geochemical approach suggest a geothermal reservoir at c. 1200 m deep, with temperatures around 250°C, hosted in fractured granitoids.XRD of clay minerals include C/S, corrensite, chlorite, I/S and illite, with a decrease of C/S and I/S with depth. Based on SEM morphologies and sizes, two types of chlorites have been defined: Chl-1, systematically present along all the core and paragenetic with quartz+albite+calcite, characterized by grain size (10-40 mm) and Chl-2, mostly observed as fine-grained flakes (average grain size ~4 mm), only identified in deeper samples, in association with laumontite+epidote±prehnite and rare Ca-garnet. SEM-EDX analyses in Chl-1 suggest an increase in Mg with depth, contrasting with the reverse observed pattern in Chl-2, which is Fe-richer compared with Chl-1. HR-TEM of selected samples at different depths confirms (1) the presence of the Fe-richest chlorites at the shallow levels and a general Mg increase with depth, and (2) the presence of C/S along the core. Cathelineau’s geothermometers using SEM-EDX data provides temperatures of 170-220°C for Chl-1 and 220-240°C for Chl-2, consistent with in situ measured temperatures. However, HR-TEM-EDX chlorite data with (K+Na+Ca)

The Punta del Cobre district near Copiapó is a center of iron oxide-copper–gold (IOCG) mineralization spatially and temporally associated with regional sodic-calcic hydrothermal alteration, the Atacama fault system (AFS), and two phases of Early Cretaceous magmatism. Here, we investigate the spatiotemporal and geochemical relationships between magmatism, ductile deformation, and hydrothermal alteration along the 200 to 300-m-thick steeply NW-dipping Sierra Chicharra shear zone, interpreted to be the major strand of the AFS. Mylonitic fabrics and oblique sinistral-reverse kinematic indicators together record coaxial flattening in a transpressional regime. Deformation on the AFS took place before, during, and after intrusion of the synkinematic Sierra Chicharra quartz diorite of the Coastal Cordillera arc at 122 Ma and terminated before intrusion of the unstrained 114 Ma Sierra Atacama diorite of the Copiapó batholith. Geochemical data show that the Copiapó batholith was more mafic and more K-rich than the calc-alkaline Coastal Cordillera arc. This time period thus overlaps IOCG mineralization in the Punta del Cobre district ( 120 to 110 Ma). Multiple phases of sodic-calcic alteration in and around the AFS shear zone are recognized. Textures of altered rock in the shear zone show both synkinematic assemblages and post-kinematic hydrothermal oligoclase. A 775-m-long andradite vein that cuts the shear zone formed broadly at the end of magmatism in the district ( 95 Ma). Oxygen isotope ratios from the vein indicate that hydrothermal fluids were likely magmatically derived. Together, this work shows the AFS-related shear zone and nearby IOCG mineralization developed in a regional transpressional regime produced by SE-directed oblique convergence across a NE-striking shear zone. IOCG-related magmatic-hydrothermal fluids exploited this transcrustal shear zone to produce multiple episodes of regional sodic-calcic alteration formed from fluids exsolved from magmas or driven by the heat of the Coastal Cordillera arc and Copiapó batholith.

The similar to 173-164 Ma Papudo-Quintero plutonic complex near 32.5 degrees S in central Chile records three deformation events that provide insight into the tectonic development of the early Andean margin. The first event (D-1) includes: (a) high-temperature (>600 degrees C), coaxial-dominated strain along NE- to N-striking subvertical shear zones; (b) widespread emplacement of granitic dikes that dip gently to steeply NE; and (c) development of narrow (<10 cm thick) strike-slip and oblique-reverse shear zones. These D-1 structures record NW-SE to WNW-ESE transpressional shortening with a component of sinistral shear parallel to the N-S trending magmatic arc. Zircon and apatite U-Pb dates and cross-cutting relations constrain most D-1 deformation to similar to 166-164 Ma. The second event (D-2) occurred during postmagmatic cooling in the Late Jurassic and was characterized by development of pervasive E-W-striking veins with alteration halos and minor strike-slip and normal faults that record N-S extension in a transtensional regime. Structures associated with the last deformation event (D-3) include Late Jurassic to Early Cretaceous mafic dikes, veins, and conjugate strike-slip faults that record NW-SE to N-S shortening in a strike-slip regime. D-1 deformation is consistent with studies from other areas that document NW-SE shortening +/- sinistral transpression along the arc throughout the Jurassic, suggesting this deformation was regional in scale and driven by oblique subduction convergence. Deformation associated with oblique convergence was localized within the active magmatic arc, which was an important process in the early Andean orogeny. As the arc migrated eastward, D-2 and D-3 structures formed in a low-stress regime in an arc margin or forearc setting.

Crustal deformation in transpressive tectonic settings is partitioned across fault‐bounded tectonic blocks whose borders may represent ideal loci for enhanced rock exhumation. Field and petrographic analysis, geothermobarometry, zircon U‐Pb geochronology, and zircon and apatite (U‐Th)/He thermochronology were applied to intrusive and metamorphic rocks to investigate exhumation patterns of fault blocks delimited by the Liquiñe‐Ofqui Fault System (LOFS), Southern Andes (39°S). Our integrated analyses document the relative influences of magmatism, fault‐driven differential exhumation, and fault‐controlled geothermal flow along the LOFS. Magmatism was concentrated in the Early to Late Jurassic (∼182–151 Ma), Early Cretaceous (∼116–104 Ma), and Miocene (∼17–6 Ma). Dextral mylonitic deformation was most likely coeval with the Miocene pulse of magmatism. Tectonic exhumation occurred across a positive flower structure during the Late Miocene to Early Pleistocene (∼6–2 Ma), and affected kilometric‐scale tectonic blocks bound by N‐striking, steeply dipping faults of the LOFS. Fault‐controlled geothermal flow occurred from the Early Pleistocene to the present‐day (∼1.5 Ma‐present). Our results suggest that individual faults not only facilitate exhumation of tectonic blocks but also act as pathways for long‐term hydrothermal fluid flow.

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 Chillán 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.

The El Laco iron oxide mineral deposit in the CentralAndes ofChile has attracted significant attention because of its uniquelypreserved massive magnetite orebodies, which bear a remarkable similarityto volcanic products. To date, the outcropping highly vesicular andporous massive magnetite orebodies have received little attentionfrom a microtextural point of view, limiting our understanding aboutthe role of volcanogenic processes on iron mineralization. Here, wereport the chemical composition of vesicular magnetite at El Lacousing EPMA and LA-ICP-MS methods and provide detailed 2D and 3D imagingof the internal structure of these texturally complex magnetite oresby combining SEM observations, synchrotron radiation micro-X-ray fluorescencechemical mapping, and high-resolution X-ray computed microtomography.Our observations reveal the presence of abundant magnetite microsphereswith diameters ranging from & SIM;100 to & SIM;900 & mu;m, aswell as dendritic microstructures forming interconnected networksup to a few millimeters in size. Two-dimensional microtextural andgeochemical imaging of the microspheres show that these features areformed by multiple euhedral magnetite crystals growing in all directionsand occur immersed within a porous matrix conformed by smaller-sized(& SIM;2-20 & mu;m) and irregularly shaped magnetite microparticles.These types of morphologies have been reported in hydrothermal ventsassociated with hydrovolcanic processes and commonly described inhydrothermal synthesis experiments of magnetite microspheres, suggestingprecipitation from iron-rich fluids. A hydrothermal origin for themagnetite microparticles reported here is further supported by theirgeochemical signature, which shows a strong depletion in most minorand trace elements typical from magnetite precipitated from hydrothermalfluids in ore-forming environments. We propose that decompression,cooling, and boiling of fluids triggered massive iron supersaturation,resulting in the nucleation of magnetite microparticles or colloids,followed by self-assembly into larger and more complex microstructures.Our data from El Laco deposit agree with models invoking magmatic-hydrothermalfluids to explain the origin of the deposit and provide new insightson the potential role of iron colloids as agents of mineralizationin volcanic systems.

We combined geoelectric and seismic ambient noise methods to image the shallow depth (<30 m) distribution of thermal waters in two fault-controlled hydrothermal systems located in southern Chile. The bedrock depth was constrained with seismics, while hotsprings and mapped faults were imaged by low-electrical-resistivity domains (<160 omega m) defined with electrical resistivity tomographies (ERT). The distribution and shape of low-resistivitydomains suggest that thermal fluids follow complex pathways, including deep vertical conduits hosted in fractured rock and shallow horizontal bodies hosted in sediments. These results indicate that the studied hydrothermal systems are at least twice longer within the sediments than the superficial area covered by hotsprings.

Arc magmatism in a continental subduction zone facilitates rheological weakening of the rigid upper plate, and can accommodate the partitioned trench-parallel component of oblique subduction into an intra-arc shear zone. We document a shear zone at latitude 25.4 degrees S near Taltal, Chile that was associated with intru-sion of the Matancilla Plutonic Complex at-169 Ma to evaluate intra-arc deformation and possible tectonic plate configurations during this time period. Polyphase folding of Paleozoic metasedimentary rocks is overprinted by mylonitic fabrics that are most extensive in a zone up to 1.4 km wide in the thermal aureole of the granodioritic Matancilla pluton, where contact metamorphic andalusite porphyroblasts are synkinematic with fabric development. Mylonite in metasedimentary rocks is overprinted by a-130 Ma granodiorite (zircon U-Pb) and by-133 Ma postkinematic monazite (U-Pb). Within the Jurassic Matancilla granodiorite, pervasive ductile shear occurs along the intrusive contact while centimeter-scale discrete high-strain zones throughout the pluton are associated with focused hy-drothermal alteration and reaction weakening. Mylonitic foliation in the metasedimentary rocks and within the pluton strikes N-to NE and dips steeply, while stretching lineations are subhorizontal on average. Kinematic indicators record dominantly sinistral shear, though some dextral or symmetric indicators and S > L fabrics suggest a component of coaxial strain and flattening. Sinistral strike-slip kinematics in the Matancilla shear zone may indicate that Middle Jurassic convergence had sinistral obliquity that was locally partitioned into the contemporaneous magmatic arc. Sinistral-oblique convergence would require the Phoenix-Farallon spreading center to be north of-25 degrees S in the Middle Jurassic, providing a constraint to plate reconstructions during the early Andean orogeny.