Browsing by Author "Leisen, Mathieu"

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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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    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.