Browsing by Author "Barra, Fernando"

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    Episodic construction of the early Andean Cordillera unravelled by zircon petrochronology
    (2021) Jara, Jose Joaquin; Barra, Fernando; Reich, Martin; Leisen, Mathieu; Romero, Rurik; Morata, Diego
    The subduction of oceanic plates beneath continental lithosphere is responsible for continental growth and recycling of oceanic crust, promoting the formation of Cordilleran arcs. However, the processes that control the evolution of these Cordilleran orogenic belts, particularly during their early stages of formation, have not been fully investigated. Here we use a multi-proxy geochemical approach, based on zircon petrochronology and whole-rock analyses, to assess the early evolution of the Andes, one of the most remarkable continental arcs in the world. Our results show that magmatism in the early Andean Cordillera occurred over a period of similar to 120 million years with six distinct plutonic episodes between 215 and 94 Ma. Each episode is the result of a complex interplay between mantle, crust, slab and sediment contributions that can be traced using zircon chemistry. Overall, the magmatism evolved in response to changes in the tectonic configuration, from transtensional/extensional conditions (215-145 Ma) to a transtensional regime (138-94 Ma). We conclude that an external (tectonic) forcing model with mantle-derived inputs is responsible for the episodic plutonism in this extensional continental arc. This study highlights the use of zircon petrochronology in assessing the multimillion-year crustal scale evolution of Cordilleran arcs.
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    Fluid-Assisted Aggregation and Assembly of Magnetite Microparticles in the Giant El Laco Iron Oxide Deposit, Central Andes
    (2023) Ovalle, J. Tomais; Reich, Martin; Barra, Fernando; Simon, Adam C.; Godel, Belinda; Le Vaillant, Margaux; Palma, Gisella; Deditius, Artur P.; Heuser, Gert; Arancibia, Gloria; Morata, Diego
    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.
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    Geochronology and petrogenesis of intrusive rocks in the Coastal Cordillera of northern Chile: Insights from zircon U-Pb dating and trace element geochemistry
    (2021) Joaquin Jara, Jose; Barra, Fernando; Reich, Martin; Morata, Diego; Leisen, Mathieu; Romero, Rurik
    Two models have been proposed to explain the early Andean evolution of the southwestern margin of Gondwana; a model that assumes continuous subduction-related magmatism since the Carboniferous and a second involving subduction cessation during the pre-Andean stage (similar to 280-200 Ma) followed by subsequent reactivation at ca. 200 Ma. Here we provide new constraints regarding the onset of the Andean tectonic cycle and the transition between pre-Andean and early Andean stages (210-100 Ma) by performing a comprehensive study of the geochronology and petrogenesis of plutonic complexes from the Coastal Cordillera of northern Chile. We present the first zircon U-Pb geochronology and trace element dataset of intrusive rocks combined with whole-rock geochemistry for the early Andean stage. The oldest unit identified is a syenogranite dated at 246.7 +/- 3.9 Ma with a subduction signature, i.e., slightly peraluminous, enriched in LILE over HFSE, negative Nb-Ta and positive Pb anomalies, and strong REE fractionation, but also shows anorogenic features with an alkali-rich composition and high enrichment in rare earth and HFS elements compared to chondritic values. These characteristics are interpreted as representing a transitional, anorogenic event from the pre-Andean stage. In contrast, the second oldest magmatic eventwas dated at 211.4 +/- 1.2Ma and has a chemical composition consistent with Andean-related magmatismand its zircon composition is similar to those fromLate Triassic-Early Cretaceous units. Consequently, we conclude that the Andean orogeny started at ca. 210 Ma, before earlier estimates. Our study also supportsworks that indicate episodic high-flux magmatism and the eastward migration of themagmatic arc during the Mesozoic. Furthermore, the whole-rock Th/Yb and zircon U/Yb ratios show a trend fromthe Late Triassic to Late Jurassic of increasing depletion of themantle source. However, during the Early Cretaceous more variable and enriched signatures are observed, possibly related to changes in the tectonic regime. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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    Occurrence and Distribution of Silver in the World-Class Rio Blanco Porphyry Cu-Mo Deposit, Central Chile
    (2020) Crespo, Jorge; Reich, Martin; Barra, Fernando; Jose Verdugo, Juan; Martinez, Claudio; Leisen, Mathieu; Romero, Rurik; Morata, Diego; Marquardt, Carlos
    Porphyry Cu-Mo deposits (PCDs) are the world's major source of Cu, Mo, and Re and are also a significant source of Au and Ag. Here we focus on the world-class Rio Blanco PCD in the Andes of central Chile, where Ag is a by-product of Cu mining. Statistical examination of an extensive multielemental inductively coupled plasma-mass spectrometry data set indicates compositional trends at the deposit scale, including Ag-Cu (r = 0.71) and Ag-In (r = 0.53) positive correlations, which relate to Cu-Fe sulfides and Cu sulfosalts in the deposit. Silver is primarily concentrated in Cu ores in the central core of the deposit, and significant variations in the Ag concentration are related to the different hydrothermal alteration types. The concentration of Ag is highest in the potassic core (avg 2.01 ppm) and decreases slightly in the gray-green sericite (phyllic) zone (avg 1.72 ppm); Ag is lowest in the outer propylitic alteration zone (avg 0.59 ppm). Drill core samples from major hydrothermal alteration zones were selected for in situ analysis of Ag and associated elements in sulfide and sulfosalt minerals. To ensure representativeness, sample selection considered the spatial distribution of the alteration types and ore paragenesis. Chalcopyrite is the most abundant Cu sulfide in Rio Blanco, with Ag concentration that ranges from sub-parts per million levels to hundreds of parts per million. The highest concentration of Ag in chalcopyrite is associated with the high-temperature potassic alteration stage. Bornite is less abundant than chalcopyrite but has the highest Ag concentration of all studied sulfides, ranging from hundreds of parts per million up to similar to 1,000 ppm. The Ag concentration in bornite is higher in lower-temperature alteration assemblages (moderate gray-green sericite), opposite to the behavior of Ag in chalcopyrite. Pyrite has the lowest Ag content, although concentrations of other critical elements such as Co, Ni, and Au may be significant. The highest Ag concentrations, i.e., thousands of parts per million up to weight percent levels, were detected in late-stage Cu sulfosalts (enargite, tennantite, and tetrahedrite). The Ag content in these sulfosalts increases with increasing Sb concentrations, from the Sb-poor enargite to the Sb-rich tetrahedrite. The earliest Ag mineralization event is related to the potassic alteration stage represented by early biotite and transitional early biotite-type veinlets and where the predominant sulfides are chalcopyrite and bornite. Silver mineralization during this stage was predominantly controlled by crystallization of Cu-Fe sulfides. The second Ag mineralization event at Rio Blanco is associated with the transitional Cu mineralization stage, which is represented by the gray-green sericite alteration (C-type veinlets). In this alteration type, Ag was partitioned preferentially into chalcopyrite, bornite, and to a lesser extent pyrite. The last Ag mineralization event is related to the late quartz-sericite alteration stage, characterized by D- and E-type veinlets with pyrite-chalcopyrite and enargite-tennantite-tetrahedrite. Our data indicate that Ag was associated with several Cu mineralization episodes at Rio Blanco, with Ag concentration apparently controlled by cooling, changes in pH, fo(2) and fs(2) of the hydrothermal fluids, and the intensity of alteration. Overall, our results provide information on critical metal partitioning between sulfides, plus the distribution of critical element resources at the deposit scale.
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    Origin of carbonatite-related niobium deposits: Insights from pyrochlore geochemistry
    (2024) Velasquez-Ruiz, Felipe; Reich, Martin; Broom-Fendley, Sam; Beard, Charles D.; Barra, Fernando; Romero, Rurik; Cordeiro, Pedro
    The carbonatite-related Nb deposits of the Alto Parana & iacute;ba Igneous Province (APIP) in central Brazil, currently account for similar to 92 % of the global Nb production. In the APIP, pyrochlore is abundant in magnet-ite-apatite-tetraferriphlogopite +/- carbonate rocks or phoscorites, occurring as interbedded layers with carbo-natites in the lower hypogene zone, feeding dike swarms of phoscorite and calcite carbonatite, and late-stage carbothermal veins in the upper hypogene zone. The origin of the phoscorite-carbonatite association can be explained by three hypotheses: (1) crystal segregation from fractional crystallization, (2) liquid immiscibility, and/or (3) phoscoritic magma formation after basement metasomatism (fenitization). However, it is not well understood whether pyrochlore formation is limited to a carbonatitic event, carbohydrothermal, or both, and this gap of knowledge is addressed in this work. To investigate the petrogenesis of pyrochlore-rich phoscorite, cathodoluminescence (CL) images, chemical maps, and LA-ICP-MS data were acquired of pyrochlore crystals from magmatic and carbothermal rocks from the Boa Vista Nb mine, Catalao II Complex. In the Boa Vista mine, oscillatory and patchy zoning were identified as primary pyrochlore textures commonly recorded at the lower hypogene zone, while secondary dissolution, skeletal and zonation-free textures are registered at shallower depths in the upper hypogene zone. Calciopyrochlore is the dominant Nb phase at the Boa Vista mine, with only two kenopyrochlore outliers. The pyrochlore CI chondrite-normalized REE distribution is consistent with geochemical results of the carbonatite and phoscorite rocks, indicating a magmatic origin for pyrochlore and the presence of pyrochlore antecrysts in carbothermal veins. The Sr/Y vs La and Na vs Ce diagrams in pyrochlore indicate a continuous fractionation pattern, with some mixtures of antecrysts and primary phases. An exami-nation of intercumulus calcite using CL provide evidence of carbonatitic magma residues within tetraferri-phlogopite phoscorite dikes and suggests that alkaline-carbonate-rich fluids played a role in transporting heavy minerals (i.e., magnetite, apatite, pyrochlore). Consequently, the textural and chemical evidence in the Boa Vista Nb mine indicates that the origin of pyrochlore-rich phoscorites is the result of physical segregation of heavy minerals from a carbonatite magma by fractional crystallization, leading to the emplacement of pyrochlore-rich carbonatite and phoscorite dikes. The implications at Catalao II may extend to other APIP alkaline-carbonatite complexes, as they share a genetic connection, and should motivate further studies focusing on pyrochlore geochemistry in other carbonatite-related Nb deposits, which will be crucial for advancing our knowledge of global Nb metallogenesis.
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    Rheological, petrophysical and geometrical constraints of a subduction channel from a numerical model perspective: Insights from La Cabana Paleozoic peridotites, Coastal Cordillera of south-central Chile
    (2022) Sanhueza, Jorge; Yanez, Gonzalo; Barra, Fernando; Maringue, Jose; Figueroa, Ronny; Saez, Esteban
    The emplacement of ultramafic blocks in accretionary complexes poses a geodynamic problem due to their negative buoyancy. In this study, we explore plausible emplacement scenarios for peridotite bodies in the Coastal Cordillera of south-central Chile by combining geophysical observations, numerical modeling and available petrological data for ultramafic blocks exhumed along the subduction channel. The La Cabana massif is the largest serpentinized peridotite complex yet recognized in the Coastal Cordillera, however, its size and petrophysical characteristics remain unknown. The geophysical measurements were performed to determine the size of this body, involving magnetic airborne surveys and electrical resistivity tomography. Inversion data show that the largest ultramafic block in La Cabana is 3 km long, 1.5 km wide and at least 1.2 km deep. This result constrains the characteristics of the block transported by the subduction channel. In the second step, we developed a numerical model for the subduction channel assuming a viscous rheology. In this modeling effort we search for tectonic scenarios that provide adequate conditions for the exhumation of the ultramafic body in La Cabana. These scenarios included a combination of key parameters, subduction angle dip and velocity, subduction channel geometry, rheology and density contrast. Scenarios compatible with the exhumation of La Cabana body type includes channel viscosity range of 10(19-20) Pa s, fast exhumation rates at mantle depths (10-20 mm/yr; >30-40 km), steep subduction angles (30 degrees-60 degrees), subduction channel widths of 3-5 km, density contrast between -200 and -400 kg/m(3) and a body diameter of 1.5 km which is consistent with our geophysical inversions. On the contrary, slow exhumation rates (similar to 1 mm/yr), low subduction angles (15 degrees), high-density contrasts (-400 to -600 kg/m(3)) and bodies larger than 1.5 km wide, are not viable exhumation scenarios. The methodology developed provided insights to infer ancient subduction channel geometries and rheologies that include peridotite.