Late Cretaceous structural control and Alpine overprint of the high-sulfidation Cu-Au epithermal Chelopech deposit, Srednogorie belt, Bulgaria

The Chelopech epithermal high-sulfidation deposit is located in the Panagyurishte ore district in Bulgaria, which is defined by a NNW alignment of Upper Cretaceous porphyry-Cu and Cu-Au epithermal deposits, and forms part of the Eastern European Banat-Srednogorie belt. Detailed structural mapping and drillcore descriptions have been used to define the structural evolution of the Chelopech deposit from the Late Cretaceous to the present. The Chelopech deposit is characterized by three fault populations including ∼N55, ∼N110, and ∼N155-trending faults, which are also recognized in the entire Panagyurishte district. Mapping and 3-D modeling show that hydrothermal alteration and orebody geometry at Chelopech are controlled by the ∼N55-trending and ∼N110-trending faults. Moreover, the ∼N155-trending faults are parallel to the regional ore deposit alignment of the Panagyurishte ore district. It is concluded that the three fault populations are early features and Late Cretaceous in age, and that they were active during high-sulfidation ore formation at Chelopech. However, the relative fault chronology cannot be deduced anymore due to Late Cretaceous and Tertiary tectonic overprint. Structurally controlled ore formation was followed by Senonian sandstone, limestone, and flysch deposition. The entire Late Cretaceous magmatic and sedimentary rock succession underwent folding, which produced WNW-oriented folds throughout the Panagyurishte district. A subsequent tectonic stage resulted in overthrusting of older rock units along ∼NE-trending reverse faults on the Upper Cretaceous magmatic and sedimentary host rocks of the high-sulfidation epithermal deposit at Chelopech. The three fault populations contemporaneous with ore formation, i.e., the ∼N55-, ∼N110- and ∼N155-trending faults, were reactivated as thrusts or reverse faults, dextral strike-slip faults, and transfer faults, respectively, during this event. Previous studies indicate that the present-day setting is characterized by dextral transtensional strike-slip tectonics. The ∼NE-trending overthrust affecting the Chelopech deposit and the reactivation of the ore-controlling faults are compatible with dextral strike-slip tectonics, but indicate local transpression, thus revealing that the Chelopech deposit might be sited at a transpressive offset within a generally transtensional strike-slip system. The early WNW-trending folds require a roughly NNE-SSW shortening, which is incompatible with the present-day dextral strike-slip tectonic setting and the ∼NE-trending thrust formed during the tectonic overprint of the Chelopech deposit. This reveals a rotation of the principal stress axes after Late Cretaceous high-sulfidation ore formation and post-ore deposition of sedimentary rocks. The nature of the sedimentary rocks interlayered and immediately covering the Upper Cretaceous magmatic rocks hosting the Chelopech deposit indicates sedimentation and associated volcanism in an extensional setting immediately before ore formation. It is concluded that the Chelopech deposit was formed when the tectonic setting changed from extensional during Late Cretaceous basin sedimentation and magmatism, to compressional producing WNW-trending folds under a roughly NNE-SSW compression, possibly in a sinistral strike-slip system. Thus, like other world-class, high-sulfidation epithermal deposits, the Chelopech deposit was formed at the end of an extensional period or during a transient period of stress relaxation, which are particularly favorable tectonic settings for the formation of high-sulfidation epithermal deposits. The exceptional preservation of the Upper Cretaceous Chelopech epithermal deposit is explained by the combined deposition of a thick Senonian sedimentary sequence on top of the Upper Cretaceous magmatic host rocks of the deposit, and the later overthrust of older rock units on top of the deposit. Our study at Chelopech supports previous studies stating that post-ore basin sedimentation and tectonic processes provide the favorable environment to preserve old epithermal deposits from erosion. The tectonic evolution of the Chelopech deposit is similar to that of the entire Panagyurishte ore district. This coherence of the magmatic, hydrothermal, and tectonic events from north to south suggests that the ore deposits of the entire Panagyurishte ore district were formed in a similar tectonic environmen

Petrology, geochemistry and U-Pb geochronology of magmatic rocks from the high-sulfidation epithermal Au-Cu Chelopech deposit, Srednogorie zone, Bulgaria

The Chelopech deposit is one of the largest European gold deposits and is located 60km east of Sofia, within the northern part of the Panagyurishte mineral district. It lies within the Banat-Srednegorie metallogenic belt, which extends from Romania through Serbia to Bulgaria. The magmatic rocks define a typical calc-alkaline suite. The magmatic rocks surrounding the Chelopech deposit have been affected by propylitic, quartz-sericite, and advanced argillic alteration, but the igneous textures have been preserved. Alteration processes have resulted in leaching of Na2O, CaO, P2O5, and Sr and enrichment in K2O and Rb. Trace element variation diagrams are typical of subduction-related volcanism, with negative anomalies in high field strength elements (HFSE) and light element, lithophile elements. HFSE and rare earth elements were relatively immobile during the hydrothermal alteration related to ore formation. Based on immobile element classification diagrams, the magmatic rocks are andesitic to dacitic in compositions. Single zircon grains, from three different magmatic rocks spanning the time of the Chelopech magmatism, were dated by high-precision U-Pb geochronology. Zircons of an altered andesitic body, which has been thrust over the deposit, yield a concordant 206Pb/238U age of 92.21 ± 0.21Ma. This age is interpreted as the crystallization age and the maximum age for magmatism at Chelopech. Zircon analyses of a dacitic dome-like body, which crops out to the north of the Chelopech deposit, give a mean 206Pb/238U age of 91.95 ± 0.28Ma. Zircons of the andesitic hypabyssal body hosting the high-sulfidation mineralization and overprinted by hydrothermal alteration give a concordant 206Pb/238U age of 91.45 ± 0.15Ma. This age is interpreted as the intrusion age of the andesite and as the maximum age of the Chelopech epithermal high-sulfidation deposit. 176Hf/177Hf isotope ratios of zircons from the Chelopech magmatic rocks, together with published data on the Chelopech area and the about 92-Ma-old Elatsite porphyry-Cu deposit, suggest two different magma sources in the Chelopech-Elatsite magmatic area. Magmatic rocks associated with the Elatsite porphyry-Cu deposit and the dacitic dome-like body north of Chelopech are characterized by zircons with ɛHfT90 values of ∼5, which suggest an important input of mantle-derived magma. Some zircons display lower ɛHfT90 values, as low as −6, and correlate with increasing 206Pb/238U ages up to about 350Ma, suggesting assimilation of basement rocks during magmatism. In contrast, zircon grains in andesitic rocks from Chelopech are characterized by homogeneous 176Hf/177Hf isotope ratios with ɛHfT90 values of ∼1 and suggest a homogeneous mixed crust-mantle magma source. We conclude that the Elatsite porphyry-Cu and the Chelopech high-sulfidation epithermal deposits were formed within a very short time span and could be partly contemporaneous. However, they are related to two distinct upper crustal magmatic reservoirs, and they cannot be considered as a genetically paired porphyry-Cu and high-sulfidation epithermal related to a single magmatic-hydrothermal system centered on the same intrusio

Amphibole Geochemistry of the Yanacocha Volcanics, Peru: Evidence for Diverse Sources of Magmatic Volatiles Related to Gold Ores

The Yanacocha mining district is located in the Andes of northern Peru in an area of relatively thick continental crust (~35 km) and long-lived Cenozoic subduction-related volcanism. Volcanic activity in the district began at ~20 Ma, and gold deposits (total resource of ~1500 tonnes of gold) are spatially and temporally associated with eruption of the ~80 km³ Miocene Yanacocha Volcanics from 14.5 to 8.4 Ma. The Yanacocha Volcanics consist of five successive eruptive groups: the Atazaico Andesite lavas, the Colorado Pyroclastics (andesite-dacite), the Azufre Andesite (and dacite) lavas, the San Jose Ignimbrite and related domes (dacite), and small volumes of Coriwachay Dacite dikes, domes and rhyolite ignimbrite. Most dacite magmas likely did not erupt, but rather are inferred to have episodically crystallized to granite at depth to produce ore fluids. Two distinct populations of amphiboles, distinguished by their aluminum content, are found in the dacites. On the basis of phase equilibrium, the low-aluminum (low-Al) amphiboles were formed at 750–840°C and 110–240 MPa, whereas the high aluminum (high-Al) amphiboles are estimated to have formed at 900-950°C and P[subscript H2O] > 250 MPa. The trace element contents of amphibole and whole-rocks are consistent with crystallisation of the high-Al amphibole at near-liquidus temperatures from a basaltic-andesite to andesite magma, whereas the low-Al amphibole crystallized at lower temperatures in equilibrium with a rhyolitic melt derived from a crystal-rich dacite magma. Hydrogen isotopic compositions of both high- and low-Al amphibole exhibit a large range from -40‰ to -120‰ for the 12.5 to 11.0 Ma andesite-dacite and have a restricted range from -100‰ to -112‰ for the younger Coriwachay Dacite (10.8 to 8.4 Ma). The high δD values of some high-Al amphiboles (-41‰) likely represent subduction-derived water dissolved in a water- and fluorine-rich, chlorine-poor, and sulfate-saturated basaltic andesite magma. This magma was injected into an upper crustal, chlorine-rich, silicic magma chamber characterized by low δD of low-Al amphibole (-60‰ or lower). Short residence times (1 year) in the shallow chambers prior to eruption to equilibrate isotopically with the predominant low- δD dacite. The young Coriwachay Dacite magmas likely assimilated meteorichydrothermally altered low-δD rocks to generate the low δD of the low-Al amphibole (-100‰ to -120‰). These dacites are related to the main gold ore stages, and contain low-Al amphibole that is zinc- and chlorine-rich, but copper- and fluorine-poor, compared to the associated high-Al amphibole. These results imply that deep mafic magmas may have supplied much of sulfur, fluorine, copper, and by inference gold, whereas upper crustal recycling may have supplied a significant proportion of the water and chlorine to the late dacite magmas and ore fluids.This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/. This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/. Supplementary Data is available on the Journal of Petrology article site.Keywords: amphibole, volatiles, gold ores, mixing, hydrogen isotopes, assimilatio

The Cu-Au Chelopech deposit, Panagyurishte district, Bulgaria : volcanic setting, hydrothermal evolution and tectonic overprint of a late cretaceous high-sulfidation epithermal deposit

Le gisement épithermal de type "high-sulfidation" à or et cuivre de Chelopech en Bulgarie s'est développé au Crétacé supérieur, dans les roches andésitiques, phréatomagmatiques, pyroclastiques et sédimentaires, qui témoignent d'un environnement côtier du volcanisme. L'étude géochronologique a donné un âge maximum de 91.45 Ma pour la formation du gisement. Cette étude permet d'établir la forte influence de ces différentes lithologies lors de la mise en place de la minéralisation et du développement des brèches hydrothermales encaissantes. De plus, l'étude structurale du gisement de Chelopech a mis en évidence le contrôle par des failles des fluides minéralisateurs et la déformation du gisement pendant l'orogenèse Alpine tertiaire. Les fluides à l'origine de la minéralisation sont essentiellement des vapeurs volcaniques avec une composante d'eau météorique. Le gisement de Chelopech est comparable aux grands gisements épithermaux de type "high-sulfidation" mondiaux

Fluids in Geothermal Systems

Hot fluids are nearly ubiquitous in volcanic environments in the Earth’s crust. Magma at depth heats groundwater which then ascends towards the Earth’s surface through faults, fractures, and otherwise permeable rocks. Fluids in geothermal systems offer direct insight into the many complex chemical and physical processes that occur in these extreme environments. They are also analogues of many ore-forming systems. Scientists have advanced our understanding of fluids in geothermal systems by studying wells sunk ~2–3 km deep into many geothermal fields. Today, we are targeting deeper and hotter reservoirs, at or near the contact of magmatic bodies, which provide unique opportunities to study, and potentially utilize, super-critical fluid resources in the near future.</jats:p

Late Cretaceous structural control and Alpine overprint of the high-sulfidation Cu–Au epithermal Chelopech deposit, Srednogorie belt, Bulgaria

Abstract The Chelopech epithermal high-sulfidation deposit is located in the Panagyurishte ore district in Bulgaria, which is defined by a NNWalignment of Upper Cretaceous porphyry–Cu and Cu–Au epithermal deposits, and forms part of the Eastern European Banat–Srednogorie belt. Detailed structural mapping and drillcore descriptions have been used to define the structural evolution of the Chelopech deposit from the Late Cretaceous to the present. The Chelopech deposit is characterized by three fault populations including ∼N55, ∼N110, and ∼N155-trending faults, which are also recognized in the entire Panagyurishte district. Mapping and 3-D modeling show that hydrothermal alteration and orebody geometry at Chelopech are controlled by the ∼N55-trending and ∼N110-trending faults. Moreover, the ∼N155-trending faults are parallel to the regional ore deposit alignment of the Panagyurishte ore district. It is concluded that the three fault populations are early features and Late Cretaceous in age, and that they were active during high-sulfidation ore formation at Chelopech. However, the relative fault chronology cannot be deduced anymore due to Late Cretaceous and Tertiary tectonic overprint. Structurally controlled ore formation was followed by Senonian sandstone, limestone, and flysch deposition. The entire Late Cretaceous magmatic and sedimentary rock succession underwent folding, which produced WNW-oriented folds throughout the Panagyurishte district. A subsequent tectonic stage resulted in overthrusting of older rock units along ∼NE-trending reverse faults on the Upper Cretaceous magmatic and sedimentary host rocks of the high-sulfidation epithermal deposit at Chelopech. The three fault populations contemporaneous with ore formation, i.e., the ∼N55-, ∼N110- and ∼N155- trending faults, were reactivated as thrusts or reverse faults, dextral strike–slip faults, and transfer faults, respectively, during this event. Previous studies indicate that the presentday setting is characterized by dextral transtensional strike– slip tectonics. The ∼NE-trending overthrust affecting the Chelopech deposit and the reactivation of the ore-controlling faults are compatible with dextral strike–slip tectonics, but indicate local transpression, thus revealing that the Chelopech deposit might be sited at a transpressive offset within a generally transtensional strike–slip system. The early WNW-trending folds require a roughly NNE–SSW shortening, which is incompatible with the present-day dextral strike–slip tectonic setting and the ∼NE-trending thrust formed during the tectonic overprint of the Chelopech deposit. This reveals a rotation of the principal stress axes after Late Cretaceous high-sulfidation ore formation and post-ore deposition of sedimentary rocks. The nature of the sedimentary rocks interlayered and immediately covering the Upper Cretaceous magmatic rocks hosting the Chelopech deposit indicates sedimentation and associated volcanism in an extensional setting immediately before ore formation. It is concluded that the Chelopech deposit was formed when the tectonic setting changed from extensional during Late Cretaceous basin sedimentation and magmatism, to compressional producing WNW-trending folds under a roughly NNE–SSW compression, possibly in a sinistral strike–slip system. Thus, like other world-class, high-sulfidation epithermal deposits, the Chelopech deposit was formed at the end of an extensional period or during a transient period of stress relaxation, which are particularly favorable tectonic settings for the formation of high-sulfidation epithermal deposits. The exceptional preservation of the Upper Cretaceous Chelopech epithermal deposit is explained by the combined deposition of a thick Senonian sedimentary sequence on top of the Upper Cretaceous magmatic host rocks of the deposit, and the later overthrust of older rock units on top of the deposit. Our study at Chelopech supports previous studies stating that post-ore basin sedimentation and tectonic processes provide the favorable environment to preserve old epithermal deposits from erosion. The tectonic evolution of the Chelopech deposit is similar to that of the entire Panagyurishte ore district. This coherence of the magmatic, hydrothermal, and tectonic events from north to south suggests that the ore deposits of the entire Panagyurishte ore district were formed in a similar tectonic environment

Subaqueous environment and volcanic evolution of the Late Cretaceous Chelopech Au–Cu epithermal deposit, Bulgaria

A detailed field and petrographic study constrains the volcanic evolution and environment setting of the volcanosedimentary-hosted Chelopech Cu–Au epithermal deposit, Bulgaria. Magmatic activity and associated highsulfidation epithermal mineralization occurred at about 91 Ma in the Panagyurishte ore district of the Eastern European Banat–Timok–Srednogorie metallogenic belt. Volcanic and hydrothermal activity took place in a complex subaqueous setting, resulting in the intercalation of quartz sandstone with andesitic volcanic and volcaniclastic breccia. There are also hypabyssal andesite intrusion, phreatomagmatic breccia and interbeds of pyroclastic, oolithic and bioclastic rocks. The presence of altered cerebroid ooid-bearing sedimentary units characteristic of salty environment is in accordance with a lagoon environment predating the mineralization at Chelopech. Four principal stages of evolution for the Chelopech district are proposed based on field and petrographic observations. Initial volcanism occurred in a lake or in a coastal, shallow lagoon environment above crystalline basement. The Chelopech “phreatomagmatic” breccia and subsurface andesites were emplaced at this time. Subsequent hydrothermal activity produced the different hydrothermal breccia types, advanced argillic and quartz–phyllic alteration, and Au–Cu vein and replacement mineralization. The end of volcanismand hydrothermal activity was associated with opening of a pull-apart basin that covered the Chelopech environment with a sedimentary flysch. Tertiary compression faulting juxtaposed various rocks and tilted the ore deposit during the Alpine orogeny