Browsing by Author "Marquardt, Carlos"

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    Crustal folds alter local stress fields as demonstrated by magma sheet – fold interactions in the Central Andes
    (2021) Clunes, Matías; Browning, John; Cembrano, José; Marquardt, Carlos; Gudmundsson, Agust
    For magma chambers to form or volcanic eruptions to occur magma must propagate through the crust as dikes, inclined sheets and sills. The vast majority of models that investigate magma paths assume the crust to be either homogeneous or horizontally layered, often composed of rocks of contrasting mechanical properties. In subduction regions that have experienced orogenesis, like the Andes, the crust has been deformed over several million years, resulting in rock layers that are commonly folded and steeply dipping. The assumption of homogeneous properties or horizontal layering then does not capture all of the potential magma path crustal interactions. Here we tackle this problem by determining the effect of a crust made of steeply inclined layers in which sills and inclined sheets are emplaced. We combine field observations from a sill emplaced in the core of an anticlinal fold at El Juncal in the Chilean Central Andes, such as lithologies, sill and fold limbs attitude, sill length and layers and sill thickness, with a suite of finite element method models to explore the mechanical interactions between inclined layers and magma paths. Our results demonstrate that the properties of the host rock layers as well as the contacts between the layers and the crustal geometry all play an important role on magma propagation and emplacement at shallow levels. Sill propagation and emplacement through heterogeneous and anisotropic crustal segments changes the crustal stress field promoting sill arrest, deflection or propagation. Specifically, sills are more likely to be deflected when encountering shallow dipping layers rather than steeply dipping layers of a fold. Mechanically weak contacts encourage sill deflection due to the related rotation of the maximum principal compressive stress and this effect is attenuated when the fold layers are more steeply dipping. This processes may change the amount and style of surface deformation recorded, with significant implications for monitoring of active volcanoes.
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    Diversity of volcanism and evidence of discrete eruption centres in the Miocene Andes of Central Chile
    (2023) Leiva, Camila; Browning, John; Marquardt, Carlos; Clunes, Matias; Villarroel, Matias; Espinosa, Javier; Meyer, Martin; Payacán, Italo; Mpodozis, Constantino
    The geology of the Farellones region in the Andean Cordillera of central Chile, comprise a thick sequence of volcanic and volcaniclastic Miocene rocks. The occurrence of discrete eruption centres within this sequence, whilst suggested, has been difficult to stablish, as in the modern volcanic systems of the SVZ to the southeast of the study area. Here we report, for the first time a previously uncharacterized composite (Cerro Colorado Volcano) associated to a series of intercalated lava flows and pyroclastic deposits of basaltic to rhyolitic composition of Miocene age. Eruptive sequences reach 1.7 km in vertical section, yet the lateral continuity of units is interrupted by a major faults, and gravity collapse deposits. We document a series of large pyroclastic block and ash flows, which are overlain by rhyolitic pyroclastic density currents, ash fall and crystal, lithic and vitric tuffs, all making up more than 300 m of the sequence. A further 300 m thick unit of spherulite-bearing rhyolitic lavas represent the upper most section of the Cerro Colorado volcano. During the early stages of its construction magmas and hydrothermal fluids interacted to form a maar-diatreme system (Quebrada Lunes maar-diatreme). Much of the volcanic units are intruded by E-W and NW-SE striking andesitic and rhyolitic dikes One of the dikes exhibits internal pyroclastic textures evidence of shallow conduit fragmentation. This pyroclastic dike is further evidence of magma-fluid interaction and related explosivity. Similar pyroclastic breccias described elsewhere related to porphyry copper mineralization. Our results indicate that Miocene volcanism in the Andes of Central Chile is linked to discrete eruptive centres which can be identified and characterized through careful field mapping.
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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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    Reconciling the location of lava domes and eruption centers in Paleocene-Eocene calderas in northern Chile
    (2021) Clunes, Matías; Browning, John; Marquardt, Carlos; Cembrano, José; Villarroel, Matías; Rivera, Orlando; Mpodozis, Constantino
    In the Atacama Desert, at the Precordillera of northern Chile, a series of Paleocene-Eocene caldera deposits and ring-faults are exceptionally well-preserved1. Here we aim to build on previous mapping efforts to consider the location, timing and style of pre, syn and post caldera volcanism in the region. We focus on the partially nested caldera complexes of Lomas Bayas and El Durazno2,3 where deposits record several stages of caldera evolution (pre-collapse, collapse/intra-caldera and extra-caldera, resurgence and post-collapse eruptive deposits). The pre-caldera basement is a thick sequence of early Paleocene mafic lavas4, 5. The caldera complex formed between around 63 and 54 Ma4, 5. Both calderas constitute subcircular structures approximately 13 km in diameter and are cut by several NNW to NNE-trending felsic dikes which are spatially related to felsic domes interpreted as resulting from post caldera formation unrest1,4. These calderas have been interpreted as part of the Carrizalillo megacaldera complex2 . We combine field observations, such as the attitude of dikes, as well as information on their dimension and composition, the size, location and composition of domes and lava flows, as well as the evidence of the regional stress field operating during the caldera evolution from measurements of fault kinematics. This data will be used as the input to finite element method models to investigate the effect of nested caldera geometry, ring-faults and crustal heterogeneities on the location of domes and eruptive centers generated during caldera unrest. The results will be potentially useful for constraining models of eruption forecasting during periods of unrest in calderas and ore deposition models which have been shown to be linked to caldera structure and magma emplacement