Search

El distrito Casposo se ubica en el borde oriental de la Cordillera Frontal, departamento de Calingasta, en el oeste de la provincia de San Juan, Argentina. El mismo se conforma de un extenso sistema de vetas epitermales de baja sulfuración, dentro del cual se desarrolla una mina en explotación (actualmente inactiva). Desde el inicio de sus actividades en el año 2011 y hasta el 2015, esta mina produjo un total de 283.200 oz de Au y 9,5 Moz de Ag. La geología del yacimiento está caracterizada por afloramientos de una secuencia marina (Formación La Puerta), depositada entre el Carbonífero y el Pérmico temprano, que fue intensamente deformado durante la fase orogénica San Rafael (Pérmico inferior). Sobre esta secuencia, y mediante una importante discordancia angular (Superficie San Rafael), tuvo lugar un importante magmatismo representado por el Grupo Choiyoi, de naturaleza subalcalina que evolucionó desde términos calcoalcalinos hacia composiciones shoshoníticas. Entre los 269,7 Ma y los 256,7 Ma (Pérmico medio a superior) este magmatismo presentó su mayor intensidad y volumen, resultando en una potente secuencia piroclástica con presencia de discordancias intraformacionales (Sección inferior y superior), que es intruída por cuerpos subvolcánicos (denominados Dacita Casposo y Andesita Cerro Pelado). Este vulcanismo evolucionó de manera sincrónica a la intrusión de importantes cuerpos plutónicos de grandes dimensiones como la Granodiorita Casposo, que potencialmente podrían representar las partes más profundas del mismo sistema magmático, aunque una importante falla N-S separa ambos ambientes. Durante las etapas terminales de este periodo, entre los 263,6 Ma y los 256,7 Ma, se habría instalado el sistema hidrotermal responsable de la formación del yacimiento, en cercana asociación con un conjunto de cuerpos subvolcánicos denominados Dique Félsico e Intrusivo Félsico. Luego del emplazamiento de la mineralización, y mediante un período de actividad magmática nula de ~ 12 Ma, ocurrió una reactivación del magmatismo a partir de los 249,7±2 Ma (Triásico Medio) que resultó en la intrusión de una sucesión de familias de diques de composiciones intermedias a ácidas (denominados Diques Básicos, Diques Riolíticos Porfíricos y Diques de Aplitas). El yacimiento abarca un área de aproximadamente 100 km2 formado por vetas que exhiben una marcada zonación composicional: las puramente silíceas concentradas en la zona de operaciones de mina (Mercado, Aztec, B Vein, Inca), vetas cuarzo-carbonáticos (Cerro Norte, Lucía), y las puramente carbonáticas con pulsos silíceos (Oveja Negra, Aurora, Casposo Norte, Leñador y Julieta). Las texturas predominantes son las de relleno de espacios abiertos, como los bandeados coloformes con desarrollo de ginguros, crustiformes, cocardas, masivas y brechas con cemento de sulfuros. Del detallado análisis estructural realizado sobre las secciones transversales y plantas de la zona de mina, se determinó un vector de máxima apertura NE-SO actuando en simultáneo a la formación de la mineralización (control sin-mineral), que luego habría sido afectada por al menos dos eventos posteriores de fallamiento (control post-mineral). A partir del análisis de detallado de la mineralogía y textura de la veta Inca, se discriminaron 9 pulsos que fueron agrupados en 6 episodios principales, siendo 5 de ellos de naturaleza hidrotermal y 1 de del tipo tectónico-hidrotermal. Los pulsos P1 y P2 son estériles. En el pulso P3 precipitan cuarzo-calcedonia con textura bandeada coloforme, y adularia subrómbica a pseudoacicular, con minerales de mena alojadas en bandas oscuras tipo ginguro. La mena se compone de electrum (10% Au – 90% Ag) y plata nativa, sulfosales como tetraedrita (rica en Ag), pirargirita y polibasita, teluros y seleniuros (naumanita y hessita) y sulfuros de metales base (esfalerita, galena y calcopirita). En el P4 precipitan sulfuros finos a gruesos junto a un cuarzo macizo, los minerales de mena se corresponden con sulfosales como la tetraedrita, y sulfuros de metales base. La evolución de los pulsos continúa hacia una composición donde predominan las arcillas verdes y las texturas brechosas en el P5. Continuo, aparece otro pulso silíceo y estéril (P6), con grandes cristales de adularia subrómbica. La secuencia sigue con un cuarzo microcristalino negro, masivo, de grano fino que posee la particularidad de exhibir molibdenita mostrando una extraña textura dendrítica (P7). En el pulso P8 actúan en simultáneo una componente hidráulica y otra tectónica, que generaron una brecha de matriz negra donde el cemento es masivo y posee anomalías de Mo; en ocasiones presenta pátinas azuladas que tiñen los clastos de los pulsos previos. Por último, se reconoce un evento menor que forma finas vetillas de carbonatos (P9). La mineralización se habría originado a partir del ascenso de soluciones cloruradas reducidas, con pH neutro a levemente alcalino, con salinidades del 7% NaCl eq. y con evidencias de ebullición; en tanto que los datos de isótopos estables establecen un origen producto de mezcla entre aguas meteóricas y magmáticas. La presencia de calcita de hábito hojoso, texturas de reemplazo de cuarzo y adularia pseudoacicular, bandeados coloformes y brechas, sumado a la coexistencia entre inclusiones fluidas ricas en fase vapor y líquido, evidencian a la ebullición como el factor responsable para la depositación metalífera. Por último, por las características observadas se puede clasificar ala Veta Inca como un sistema epitermal clásico de baja sulfuración, aunque ciertos atributos como las salinidades moderadas, la elevada relación Ag/Au, la presencia de sulfosales como tetraedrita, pirargirita y polibasita, y la presencia de esfalerita pobre en Fe, evidencian un estado de sulfuración intermedia., The Casposo district is located on the eastern edge of the Cordillera Frontal, Calingasta department, in the west of the San Juan province, Argentina. It consists of a larguest system of low sulfidation epithermal veins, within which there is an operating mine (currently inactive). Since the beginning of its activities in the year 2011 and until 2015, this mine produced a total of 283.200 oz of Au and 9,5 Moz of Ag. The geology of the deposit is characterized by outcrops of a marine sequence (La Puerta Formation), deposited between the Carboniferous and Early Permian, which was intensely deformed during the San Rafael orogenic phase (Lower Permian). Over this sequence, and through an important angular unconformity (San Rafael Surface), an important magmatism took place, represented by the Choiyoi Group, of subalkaline nature that evolved from calcalkaline to shoshonitic compositions. Between 269,7 Ma and 256,7 Ma (middle to upper Permian) this magmatism reached its highest intensity and volume, resulting in a thick pyroclastic sequence with the presence of intraformational unconformities (lower and upper section), which is intruded by subvolcanic bodies (Dacite Casposo and Andesita Cerro Pelado). This volcanism evolved synchronously with the intrusion of important large plutonic bodies such as the Casposo Granodiorite, which could potentially represent the deepest parts of the same magmatic system, although a major N-S fault separates both environments. During the terminal stages of this period, between 263,6 Ma and 256,7 Ma, the hydrothermal system responsible for the formation of the deposit would have been installed, in close association with a set of subvolcanic bodies called Felsic Dike and Felsic Intrusive. After the emplacement of the mineralization, and through a period of null magmatic activity of ~ 12 Ma, a reactivation of magmatism occurred from 249,7±2 Ma (Middle Triassic) that resulted in the intrusion of a succession of families of dykes of intermediate to acid compositions (Basic Dykes, Porphyritic Rhyolitic Dykes and Aplite Dykes). The deposit covers an area of approximately 100 km2 formed by veins that shows a marked compositional zoning: purely siliceous veins concentrated in the area of mine operations (Mercado, Aztec, B Vein, Inca), quartz-carbonate veins (Cerro Norte, Lucía), and purely carbonate veins with siliceous pulses (Oveja Negra, Aurora, Casposo Norte, Leñador and Julieta). The predominant textures are those of open space filling, such as colloform banding with the development of ginguros, crustiforms, cocards, massive and breccias with sulfide cement. From the detailed structural analysis carried out on the cross-sections and plans of the mine area, a vector of maximum NE-SW opening was determined, acting simultaneously with the formation of mineralization (syn-mineral control), which would have been affected by at least two subsequent faulting events (post-mineral control). From the detailed analysis of the mineralogy and texture of the Inca vein, 9 pulses were discriminated and grouped into 6 main episodes, being 5 of them of hydrothermal nature and 1 of tectonic-hydrothermal type. Pulses P1 and P2 are barren. Pulse P3 precipitates quartz-chalcedony with colloform banded texture, and subrhombic to pseudo-acicular adularia, with ore minerals hosted in dark gingurobands. The ore consists of electrum (10% Au - 90% Ag) and native silver, sulfosalts such as tetrahedrite (Ag-rich), pyrargyrite and polybasite, tellurides and selenides (naumanite and hessite) and base metal sulfides (sphalerite, galena and chalcopyrite). In P4, fine to coarse sulfides precipitate along with massive quartz, the ore minerals correspond to sulfosalts such as tetrahedrite, and base metal sulfides. The evolution of the pulses continues towards a composition dominated by green clays and brecciated textures in P5. Continuously, another siliceous and barren pulse appears (P6), with large crystals of subrhombic adularia. The sequence continues with a black, massive, fine-grained microcrystalline quartz that has the particularity of exhibiting molybdenite showing a strange dendritic texture (P7). In the P8 pulse, a hydraulic and a tectonic component act simultaneously, generating a black matrix breccia where the cement is massive and has Mo anomalies; sometimes it shows bluish patinas that stain the clasts of the previous pulses. Finally, a minor event that forms fine carbonate veins (P9) is recognized. The mineralization would have originated from the ascent of reduced chloride solutions, with neutral to slightly alkaline pH, with salinities of 7% NaCl eq. and with evidence of boiling, while the stable isotope data establish an origin product of mixing between meteoric and magmatic waters. The presence of platycalcite, quartz replacement textures and pseudoacicular adularia, colloform banding and breccias, along with the coexistence between vapor and liquid phase rich fluid inclusions, evidence boiling as the responsible factor for the ore deposition. Finally, due to the characteristics observed, the Inca Vein can be classified as a classic low sulfidation epithermal system, although certain attributes such as moderate salinities, high Ag/Au ratio, the presence of sulfosalts such as tetrahedrite, pyrargyrite and polybasite, and the presence of Fe-poor sphalerite, evidence an intermediate sulfidation state., Facultad de Ciencias Naturales y Museo

Терско-Каспийский прогиб – крупная структурно-тектоническая единица, расположенная на стыке двух разновозрастных и различных по составу элементов земной коры, которые из-за общей геодинами-ческой обстановки приобрели статус единого мегапрогиба с характерными литолого-стратиграфическими, термобарическими, геохимическими и нефтегенерационными характеристиками, что подтверждается гео-лого-геофизическими исследованиями и промышленными скоплениями в пределах пространства изуче-ния., The Terek-Caspian trough is a large structural-tectonic unit located at the junction of two elements of the Earth's crust of different ages and different composition, which, due to the general geodynamic condition, acquired the status of a united mega-trough with characteristic lithological and stratigraphic, thermo-baric, geochemical and oil-generating characteristics, which is confirmed by geological and geophysical studies and industrial accumula-tions within the study area.

International audience; Resolving the timing of initiation and propagation of continental accretion associated with increasing topography and exhumation is a genuinely challenging task using low-temperature thermochronology. We present an integrated thermo-mechanical and low-temperature thermochronology modelling study of tectonically-inverted hyperextended rift systems. Model low-temperature thermochronology data sets for apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He and zircon fission-track systems, which are four widely used thermochronometric systems in orogenic settings, are generated from fourteen locations across a model collisional, doubly-vergent orogen. Our approach allows prediction of specific, distinct low-temperature thermochronology signatures for each domain (proximal, necking, hyperextended, exhumed mantle) of the two rifted margins that, in turn, enable deciphering which parts of the margins are involved in orogenic wedge development. Our results show that a combination of zircon (U-Th)/He and apatite fission-track data allows diagnostic investigation of model orogen tectonics and offers the most valuable source of thermochronological information for the reconstruction of the crustal architecture of the model inverted rifted margins. The two thermochronometric systems have actually very close and wide closure windows, allowing to study orogenic processes over a larger temperature range, and therefore over a longer period of time. Comparison of model data for inverted rifted margins with model data for non-inverted, purely thermally-relaxed rifted margins enables assessing the actual contribution of tectonic inversion with respect to thermal relaxation. We apply this approach to one of the best-documented natural examples of inverted rift systems, the Pyrenees. Similarities between our thermochronometric modelling results and published low-temperature thermochronology data from the Pyrenees provide new insights into the evolution of the range from rifting to collision. In particular, they suggest that the core of the Pyrenean orogen, the Axial Zone, consists of the inverted lower plate necking and hyperextended domains while the Pyrenean retrowedge fold-and-thrust belt, the North Pyrenean Zone, represents the inverted upper plate distal rifted margin (exhumed mantle, hyperextended and necking domains). This is in good agreement with previous, independent reconstructions from literature, showing the power that our integrated study offers in identifying processes involved in orogenesis, especially early inversion, as well as in predicting which domains of rifted margins are accreted during mountain building.; Identifier au moyen de la thermochronologie basse température le moment à partir duquel l’accrétion continentale s’amorce et se propage est une tâche difficile. Dans cette étude, nous couplons modélisation thermomécanique de systèmes hyper-amincis inversés et thermochronologie basse température pour prédire la signature thermochronologique de quatre systèmes basse température communément utilisés en domaine orogénique ((U-Th)/He sur apatite, traces de fission sur apatite, (U-Th)/He sur zircon et traces de fission sur zircon). Notre approche de modélisation permet de prédire les signatures thermochronologiques basse température propres à chaque domaine de marge du système extensif (domaine proximal, zone de neck, domaine hyper-aminci et domaine de manteau exhumé). Ces signatures permettent en retour d’identifier les domaines de marge impliqués dans l’accrétion continentale qui suit l’extension. Nos résultats montrent que la combinaison des signatures thermochronologiques des systèmes (U-Th)/He sur zircon et traces de fission sur apatite renseigne sur les processus mis en jeu lors de la formation d’un orogène collisionnel à double vergence, ainsi que sur l’architecture des marges inversées durant l’orogénèse. Les deux systèmes thermochronométriques possèdent en effet des gammes de température de fermeture voisines et relativement larges, permettant d’étudier les processus orogéniques sur un intervalle de températures plus grand, et donc sur une période de temps plus longue. La comparaison des signatures thermochronologiques prédites dans un modèle thermomécanique d’inversion avec celles prédites dans un modèle de relaxation thermique, sans convergence postérieure à l’extension, permet d’apprécier la nécessaire contribution des processus tectoniques d’inversion par rapport au processus de relaxation thermique postrift. Nous appliquons notre approche à l’un des exemples naturels les mieux documentés de systèmes de rift inversé, les Pyrénées. Les similitudes qui existent entre nos prédictions thermochronométriques et les données de thermochronologie basse température publiées pour les Pyrénées fournissent de nouvelles informations sur la formation et l’évolution de l’orogène pyrénéen. En particulier, ces similitudes suggèrent que la zone interne de déformation pyrénéenne, la Zone Axial, est constituée des domaines de neck et hyper-aminci de la plaque inférieure, inversés durant l’accrétion, tandis que le prisme orogénique qui se développe sur la plaque européenne, la Zone Nord-pyrénéenne, représente la marge distale inversée de la plaque supérieure. Ce résultat est en accord avec les précédentes reconstructions publiées de la dynamique pyrénéenne, démontrant ainsi le potentiel de notre approche dans l’identification des processus impliqués dans l’orogenèse, notamment l’inversion précoce, ainsi que dans la prédiction du type de domaine de marge accrété lors de la formation des chaînes de montagnes.

The Himalaya are among the youngest and highest mountains in the world, but the exact timing of their uplift and origins of their biodiversity are still in debate. The Himalayan region is a relatively small area but with exceptional diversity and endemism. One common hypothesis to explain the rich montane diversity is uplift-driven diversification—that orogeny creates conditions favoring rapid in situ speciation of resident lineages. We test this hypothesis in the Himalayan region using amphibians and reptiles, two environmentally sensitive vertebrate groups. In addition, analysis of diversification of the herpetofauna provides an independent source of information to test competing geological hypotheses of Himalayan orogenesis. We conclude that the origins of the Himalayan herpetofauna date to the early Paleocene, but that diversification of most groups was concentrated in the Miocene. There was an increase in both rates and modes of diversification during the early to middle Miocene, together with regional interchange (dispersal) between the Himalaya and adjacent regions. Our analyses support a recently proposed stepwise geological model of Himalayan uplift beginning in the Paleocene, with a subsequent rapid increase of uplifting during the Miocene, finally giving rise to the intensification of the modern South Asian Monsoon.

Cretaceous porphyry copper deposits of northern Chile (28º-29º30’ S) are genetically related with dacitic to dioritic porphyries and they represent a still poorly-explored target for Cu resources. The porphyries correspond to stocks distributed into two separated discontinuous NS trending belts of different age. The location of these porphyries is generally adjacent to orogen-parallel major fault systems that extend along the studied segment and also have a marked temporal relationship with deformation events registered along these structures. A first episode of Cu-bearing porphyry emplacement took place between 116 and 104 Ma (Mina Unión or Frontera, Cachiyuyo, Punta Colorada, Dos Amigos, Tricolor porphyries). These Early Cretaceous dacite to diorite porphyries are spatially associated with the eastern segments of the Atacama Fault System, which records sinistral transpression that started at 121 Ma producing ground uplift, consequent denudation and exhumation of the Early Cretaceous magmatic arc. This resulted in a change from marine to continental deposition with an angular unconformity in the site of the back-arc basin after of eastward migration of the deformation around 112-110 Ma. At the scale of the continental margin, this deformation is correlated with early stage of the Mochica Orogenic event described in Perú. A second episode of Cu-bearing porphyry emplacement occurred between 92 and 87 Ma (Elisa, Johana, Las Campanas and La Verde deposits), which are spatially and temporally associated with the regional-scale Las Cañas-El Torito reverse fault, active between 89 and 84 Ma, during the Peruvian Orogenic Phase. This fault up thrust to the west part of the Chañarcillo Group rocks (Lower Cretaceous) over the younger upper levels of the Cerrillos Formation (Upper Cretaceous). The integrated geological mapping and geochemical data of the Early to Late Cretaceous volcanic rocks indicates that both Early Cretaceous sinistral transpression and Late Cretaceous east-west compression were not significant in promote changes in magma genesis, except for slight changes in trace element ratios (increase in Th/Ta, Nb/Ta and La/Yb) suggesting that the Late Cretaceous deformation event produced only slightly increase in crustal thickness (>40 km), but far from being comparable to major Cenozoic orogenic phases, at least along the magmatic arc to back-arc domains in the study area. Finally, our study give insights about regional geological parameters that can be used as a first order guide for exploration of Cu resources along Cretaceous magmatic belts of northern Chile, where both Early and Late Cretaceous Cu-bearing porphyry intrusions are restricted to a large structural block bounded to the west and east by Cretaceous fault systems.

International audience; New fieldwork and tectonic analysis has been carried out along the Andean morphotectonic frontline between 46°30’ and 47°30′S. The right lateral transtensional Marques–Zeballos Pass fault system (MZPRS) controlled the 800–1200 m uplift of the Andes at 16.1–18.1 Ma. Our data and analysis indicate that there was no lower Miocene contractile event along the Andean morphotectonic frontline. The Main Andean Thrust (MAT), which is deeply rooted in the upper crust is a retroarc thrust dipping 10–15° westward, and provides evidence for major crustal shortening at ~120 Ma. At that time the arc volcanic strata of the Ibañez Fm (148–178 Ma) was transported eastward above the rift volcanic Quemado Fm (144179 Ma), which lies unconformably on the Deseado basement massif.The geologic records from the fieldwork together with the available data allow us to identify the processes controlling the dynamic evolution of the two main topographic features of the studied Andean segment. (1) Upward convection originating from the subducted South Chile Ridge −1 segment (SCR −1) is dynamically sustaining the high topography of the North Patagonian Icefield (4070 m at the Mt. San Valentin, ~2 km higher than the Southern South Volcanic Zone). This, together with the locus of maximum moisture being at 47°S during glacial events has resulted in producing the two largest glacial lobes of Patagonia. (2) The 180 km long, E–W trending antiformal arch structure of the Mt. Zeballos Ridge (MZR, a hinge zone trending N–S along the MZPRS) straddling the Andes and the Foreland is proposed to be the morphological “twin” of the South American Plate Moho Plateau (SAM MOHO P). The SAM MOHO P controls the location of both the MZPRS and the MZR boundaries location through time. The Patagonia Slab Window and SAM asthenospheric dynamics (upward and corner flows, respectively) and their in–depth interaction appear to control the morphotectonic evolution of the whole studied segment through a powerful feedback loop between tectonics, morphology, and climate, at least for the past 3–4 Myr.