Timing of magmatic-hydrothermal activity in the Variscan Orogenic Belt: LA-ICP-MS U–Pb geochronology of skarn-related garnet from the Schwarzenberg District, Erzgebirge

[EN] Here, we present in situ U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) ages of andradite-grossular garnet from four magmatic-hydrothermal polymetallic skarn prospects in the Schwarzenberg District, Erzgebirge (Germany), located in the internal zone of the Variscan Orogenic Belt. Within the geochronological framework of igneous rocks and hydrothermal mineralization in the Erzgebirge, the obtained garnet ages define three distinct episodes of Variscan skarn formation: (I) early late-collisional mineralization (338-331 Ma) recording the onset of magmatic-hydrothermal fluid flow shortly after the peak metamorphic event, (II) late-collisional mineralization (similar to 327-310 Ma) related to the emplacement of large peraluminous granites following large-scale extension caused by orogenic collapse and (III) post-collisional mineralization (similar to 310-295 Ma) contemporaneous with widespread volcanism associated with Permian crustal reorganization. Our results demonstrate that the formation of skarns in the Schwarzenberg District occurred episodically in all sub-stages of the Variscan orogenic cycle over a time range of at least 40 Ma. This observation is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal zones of the Variscan Orogenic Belt in central and western Europe. In analogy to the time-space relationship of major porphyry-Cu belts in South America, the congruent magmatic-hydrothermal evolution in the internal zones and the distinctly later (by similar to 30 Ma) occurrence of magmatic-hydrothermal ore deposits in the external zones of the Variscan Orogenic Belt may be interpreted as a function of their tectonic position relative to the Variscan collisional front.Open Access funding enabled and organized by Projekt DEAL. This study was funded by the Federal State of Saxony and the European Social Fund (Grant no. 100339454 received by M. Burisch

Fe Skarn, Iron Oxide Cu-Au, and Manto Cu-(Ag) Deposits in the Andes Cordillera of Southwest Mendoza Province (34°–36°S), Argentina

Several Fe, Fe-Cu, and Cu-Ag prospects hosted by Mesozoic carbonate-rich sedimentary rocks that were intruded by diorite stocks define an approximately 20 × 200 km belt along the Andes Cordillera of southwest Mendoza province, Argentina (34°–36°S). This belt includes the Hierro Indio and Vegas Peladas Fe skarns, Las Choicas, and several other Cu-(Ag) prospects. The main features of the Fe skarns include: association with mantle-derived middle Miocene (~15–11 Ma) diorite stocks and sills; widespread alteration including epidote ± amphibole ± magnetite endoskarns, and zoned garnet (Grs0–66 Adr32.5–100) ± magnetite ± pyroxene (Di24–50 Jo2–9 Hd74–41) exoskarns formed from oxidized, saline, high-temperature brines (530º–660ºC; 60–70 wt.% NaCl equiv.); and magnetite-hematite orebodies associated with quartz ± epidote ± calcite ± actinolite formed at lower temperatures (290º–436ºC) from saline fluids (32–50 wt.% NaCl equiv.) of magmatic origin. West of the Fe skarn belt, the Valle Hermoso district covers more than 300 km2 and hosts several poorly known Cu-(Ag) prospects (estimated 30 Mt with 1.5% Cu and 20 g/t Ag). These prospects share many characteristics with the iron oxide copper-gold-type systems and manto-type Cu deposits of northern and central Chile. One of the prospects (Las Choicas) is located at the apex of a regional 015°-trending anticline, along the contact of stratified calcareous sandstone and limestone (Neocallovian Oxfordian) with a diorite pluton. This pluton has positive Ta and Nb anomalies (on normalized diagrams) and a high Ti content, features uncommon in Miocene diorites associated with Fe skarns and of magmas derived from sub-arc mantle sources. Alteration at Las Choicas includes early widespread chlorite + calcite ± albite ± scapolite ± ilmenite or titanite ± apatite, and local actinolite ± hematite ± biotite alteration of diorite and sedimentary rocks along contacts. Later Cu mineralization (1%–10% Cu) is associated with calcite ± albite ± quartz alteration in fractures, vein networks, and crackle breccias. Mineralization is zoned from bornite ± chalcopyrite ± millerite with Zn, As, Mo, Ag, and U anomalies in the main orebody, through chalcopyrite-rich breccias with Zn anomalies, and chalcopyrite ± tetrahedrite vein networks with Sb, As, Ag, and Zn anomalies in the uppermost zone, to a Cu-depleted pyrite-rich envelope with Co anomalies, and finally to late, barren calcite veins in distal zones.Une série de plusieurs indices de Fe, Fe-Cu, et Cu-Ag dans des roches sédimentaires carbonatées Mésozoïques recoupées par des stocks de diorite forment une ceinture d’environ 20 × 200 km le long de la Cordillère des Andes dans la partie sud-ouest de la province de Mendoza, en Argentine (34°–36°S). Cette ceinture comprend les gîtes de fer de type skarn Hierro Indio et Vegas Peladas, Las Choicas et plusieurs autres indices de Cu-(Ag). Les principales caractéristiques de ces skarns ferrifères sont : l’association avec des sills et des stocks de diorite dérivés du manteau au Miocène moyen (~15–11 Ma); une altération importante comprenant des endoskarns à épidote ± amphibole ± magnétite, et des exoskarns à grenats zonés (Grs0–66 Adr32.5–100) ± magnétite ± pyroxène (Di24–50 Jo2–9 Hd74–41) formés par des saumures oxydées à haute salinité et à haute température (530º–660ºC; 60–70 % poids NaCl équiv.); et des corps minéralisés constitués de magnétite et d’hématite associés à du quartz ± épidote ± calcite ± actinolite formés à basse température (290º–436ºC) par des fluides salins d’origine magmatique (32–50 % poids NaCl équiv.). À l’ouest de cette ceinture de skarns ferrifères, on trouve le district de Valle Hermoso, lequel couvre plus de 300 km2 et contient plusieurs indices mal connus de Cu-(Ag) (estimés à 30 Mt à une teneur moyenne de 1.5% Cu et de 20 g/t Ag). Ces indices partagent plusieurs caractéristiques des systèmes de type oxyde de fer-cuivre-or des gîtes de Cu de type manto du nord et du centre du Chili. Un de ces indices (Las Choicas) est situé à l’extrémité d’un anticlinal régional de direction N015°, le long d’un contact entre des grès calcareux interlités avec des calcaires (NeocallovienOxfordien) et un pluton de diorite. Ce pluton présente des anomalies positives en Ta et en Nb (sur un diagramme normalisé) et un contenu élevé en Ti, ces caractéristiques sont inhabituelles chez les diorites du Miocène associées aux skarns ferrifères et aux magmas dérivés d’une source mantellique sous-jacente à un arc. L’altération à Las Choicas comprend une zone de chlorite + calcite ± albite ± scapolite ± ilménite or titanite ± apatite précoces largement distribuée, et une altération locale de la diorite et des roches sédimentaires en actinolite ± hématite ± biotite le long des contacts. La minéralisation en Cu (1%–10% Cu) est plus tardive et associée avec une altération en calcite ± albite ± quartz dans des fractures, des réseaux de veines, et des brèches de dislocation. La minéralisation présente une zonalité comprenant le corps minéralisé principal constitué de bornite ± chalcopyrite ± millerite accompagné de teneurs anomales en Zn, As, Mo, Ag, et U, une brèche riche en chalcopyrite accompagnée d’anomalies en Zn, un réseau de veines de chalcopyrite ± tétrahédrite avec des anomalies en Sb, As, Ag, et Zn dans la zone supérieure, une enveloppe pyriteuse appauvrie en Cu et anomale en Co et finalement une zone distale marquée par des veines de calcite stériles.Facultad de Ciencias Naturales y Muse

Combined fit to the spectrum and composition data measured by the Pierre Auger Observatory including magnetic horizon effects

The measurements by the Pierre Auger Observatory of the energy spectrum and mass composition of cosmic rays can be interpreted assuming the presence of two extragalactic source populations, one dominating the flux at energies above a few EeV and the other below. To fit the data ignoring magnetic field effects, the high-energy population needs to accelerate a mixture of nuclei with very hard spectra, at odds with the approximate E$^{-2}$ shape expected from diffusive shock acceleration. The presence of turbulent extragalactic magnetic fields in the region between the closest sources and the Earth can significantly modify the observed CR spectrum with respect to that emitted by the sources, reducing the flux of low-rigidity particles that reach the Earth. We here take into account this magnetic horizon effect in the combined fit of the spectrum and shower depth distributions, exploring the possibility that a spectrum for the high-energy population sources with a shape closer to E$^{-2}$ be able to explain the observations

Studies of the mass composition of cosmic rays and proton-proton interaction cross-sections at ultra-high energies with the Pierre Auger Observatory

In this work, we present an estimate of the cosmic-ray mass composition from the distributions of the depth of the shower maximum (Xmax) measured by the fluorescence detector of the Pierre Auger Observatory. We discuss the sensitivity of the mass composition measurements to the uncertainties in the properties of the hadronic interactions, particularly in the predictions of the particle interaction cross-sections. For this purpose, we adjust the fractions of cosmic-ray mass groups to fit the data with Xmax distributions from air shower simulations. We modify the proton-proton cross-sections at ultra-high energies, and the corresponding air shower simulations with rescaled nucleus-air cross-sections are obtained via Glauber theory. We compare the energy-dependent composition of ultra-high-energy cosmic rays obtained for the different extrapolations of the proton-proton cross-sections from low-energy accelerator data

Study of downward Terrestrial Gamma-ray Flashes with the surface detector of the Pierre Auger Observatory

The surface detector (SD) of the Pierre Auger Observatory, consisting of 1660 water-Cherenkov detectors (WCDs), covers 3000 km2 in the Argentinian pampa. Thanks to the high efficiency of WCDs in detecting gamma rays, it represents a unique instrument for studying downward Terrestrial Gamma-ray Flashes (TGFs) over a large area. Peculiar events, likely related to downward TGFs, were detected at the Auger Observatory. Their experimental signature and time evolution are very different from those of a shower produced by an ultrahigh-energy cosmic ray. They happen in coincidence with low thunderclouds and lightning, and their large deposited energy at the ground is compatible with that of a standard downward TGF with the source a few kilometers above the ground. A new trigger algorithm to increase the TGF-like event statistics was installed in the whole array. The study of the performance of the new trigger system during the lightning season is ongoing and will provide a handle to develop improved algorithms to implement in the Auger upgraded electronic boards. The available data sample, even if small, can give important clues about the TGF production models, in particular, the shape of WCD signals. Moreover, the SD allows us to observe more than one point in the TGF beam, providing information on the emission angle

The dynamic range of the upgraded surface-detector stations of AugerPrime

The detection of ultra-high-energy cosmic rays by means of giant detector arrays is often limited by the saturation of the recorded signals near the impact point of the shower core at the ground, where the particle density dramatically increases. The saturation affects in particular the highest energy events, worsening the systematic uncertainties in the reconstruction of the shower characteristics. The upgrade of the Pierre Auger Observatory, called AugerPrime, includes the installation of an 1-inch Small PhotoMultiplier Tube (SPMT) inside each water-Cherenkov station (WCD) of the surface detector array. The SPMT allows an unambiguous measurement of signals down to about 250m from the shower core, thus reducing the number of events featuring a saturated station to a negligible level. In addition, a 3.8m2 plastic scintillator (Scintillator Surface Detector, SSD) is installed on top of each WCD. The SSD is designed to match the WCD (with SPMT) dynamic range, providing a complementary measurement of the shower components up to the highest energies. In this work, the design and performances of the upgraded AugerPrime surface-detector stations in the extended dynamic range are described, highlighting the accuracy of the measurements. A first analysis employing the unsaturated signals in the event reconstruction is also presented