Petrological evolution of the magmatic suite associated with the Coroccohuayco Cu(-Au-Fe) porphyry-skarn deposit, Peru

The petrological evolution of magmatic rocks associated with porphyry-related Cu deposits is thought to exert a first-order control on ore genesis. It is therefore critical to understand and recognize petrological processes favourable to the genesis of porphyry systems. In this study we present new petrographic, geochemical (whole-rock and mineral), and isotopic (Pb, Sr, Nd) data for rocks from the magmatic suite associated with the Eocene Coroccohuayco porphyry–skarn deposit, southern Peru. Previously determined radiometric ages on these rocks provide the temporal framework for interpretation of the data. Arc-style magmatic activity started at Coroccohuayco with the emplacement of a composite precursor gabbrodiorite complex at c. 40·4 Ma. After a nearly 5 Myr lull, magmatic activity resumed at c. 35·6 Ma with the rapid emplacement of three dacitic porphyries associated with mineralization. However, zircon antecrysts in the porphyries show that intra-crustal magmatic activity started c. 2 Myr before porphyry emplacement and probably built a large intra-crustal magmatic body with an associated large thermal anomaly. Our data suggest that all magmas underwent a period of evolution in the deep crust before transfer and further evolution in the upper crust. The gabbrodiorite complex was sourced from a heterogeneous deep crustal reservoir and was emplaced at a pressure of 100–250 MPa where it underwent a limited amount of fractionation and formed a chemically zoned pluton. Its initial water content and oxygen fugacity were estimated to be around 3 wt % H2O and NNO ± 1 (where NNO is the nickel–nickel oxide buffer), respectively. The deep crustal source of the porphyries appears to have been more homogeneous. The porphyries are interpreted to be the product of advanced differentiation of a parental magma similar to the gabbrodiorite. Most of this evolution occurred at deep crustal levels (around 800 MPa) through fractionation of amphibole + pyroxene + plagioclase ± garnet, leading to the development of a high Sr/Y signature characteristic of porphyry-related magmatism worldwide. Subsequent upper crustal evolution (100–250 MPa) was dominated by crustal assimilation, cannibalism of previously emplaced magma batches (proto-plutons) and magma recharge. Water content and oxygen fugacity were estimated to be around 5 wt % H2O and NNO + 1 to NNO + 2, respectively, at the end of the period of upper crustal evolution. This high oxygen fugacity is inferred to have favoured sulphur and metal enrichment in the melt. The high thermal regime generated through 2 Myr of sustained magmatism in the upper crust favoured crustal assimilation, proto-pluton cannibalism, and efficient metal extraction upon fluid exsolution. The Coroccohuayco magmatic suite appears to have acquired its metallogenic potential (high fO2, high Sr/Y) through several million years of deep crustal evolution

Geothermal state of the deep Western Alpine Molasse Basin, France-Switzerland

Over the last few years the Western Alpine Molasse Basin (WAMB) has been attracting large institutional, industrial and scientific interest to evaluate the feasibility of geothermal energy production. However, the thermal state of the basin, which is instrumental to the development of such geothermal projects, has remained to date poorly known. Here, we compile and correct temperature measurements (mostly bottom hole temperature) from 26 existing well data mostly acquired during former hydrocarbon exploration in the basin. These data suggest that the average geothermal gradient of the WAMB is around 25–30 °C/km. We further use these data to build the first well data-driven 3D geostatistical temperature model of the whole basin and generate probabilistic maps of isotherms at 70 and 140 °C. This model highlights a number of positive and negative thermal anomalies that are interpreted in the context of heat advection caused by fluid circulation along faults and/or karst systems. This study confirms that the WAMB has a great potential for low-enthalpy geothermal resources and presents a typology of advection-dominated potential targets

Garnet petrochronology reveals the lifetime and dynamics of phonolitic magma chambers at Somma-Vesuvius

Somma-Vesuvius is one of the most iconic active volcanoes with historic and archeological records of numerous hazardous eruptions. Petrologic studies of eruptive products provide insights into the evolution of the magma reservoir before eruption. Here, we quantify the duration of shallow crustal storage and document the evolution of phonolitic magmas before major eruptions of Somma-Vesuvius. Garnet uranium-thorium petrochronology suggests progressively shorter pre-eruption residence times throughout the lifetime of the volcano. Residence times mirror the repose intervals between eruptions, implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution, thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longer-lived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that long-lived deep-seated reservoirs and transient upper crustal magma chambers are common features of subvolcanic plumbing systems

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

Chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic–hydrothermal systems such as the Coroccohuayco Fe–Cu–Au porphyry–skarn deposit, Peru. Here, we used recent partitioning models to determine the Cl and S concentration of the melts from the Coroccohuayco magmatic suite using apatite and amphibole chemical analyses. The pre-mineralization gabbrodiorite complex hosts S-poor apatite, while the syn- and post-ore dacitic porphyries host S-rich apatite. Our apatite data on the Coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity (from the gabbrodiorite complex to the porphyries) causing the dominant sulfur species to shift from S2− to S6+ at upper crustal pressure where the magmas were emplaced. We suggest that this change in sulfur speciation could have favored S degassing, rather than its sequestration in magmatic sulfides. Using available partitioning models for apatite from the porphyries, pre-degassing S melt concentration was 20–200 ppm. Estimates of absolute magmatic Cl concentrations using amphibole and apatite gave highly contrasting results. Cl melt concentrations obtained from apatite (0.60 wt% for the gabbrodiorite complex; 0.2–0.3 wt% for the porphyries) seems much more reasonable than those obtained from amphibole which are very low (0.37 wt% for the gabbrodiorite complex; 0.10 wt% for the porphyries). In turn, relative variations of the Cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the Coroccohuayco magmatic suite. This confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement, and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100–200 MPa. Finally, mass balance constraints on S, Cu and Cl were used to estimate the minimum volume of magma required to form the Coroccohuayco deposit. These three estimates are remarkably consistent among each other (ca. 100 km3) and suggest that the Cl melt concentration is at least as critical as that of Cu and S to form an economic mineralization

High-resolution geochronology of the Coroccohuayco porphyry-skarn deposit, Peru: A rapid product of the Incaic orogeny

Precise and accurate determination of the timing and duration of ore-forming processes in porphyry systems is a fundamental step in understanding their genesis and placing them in a regional context. Here, we take advantage of the considerable improvements in the field of geochronology over the last decade to provide a robust geochronologic framework for hydrothermal and magmatic events in the Eocene Coroccohuayco porphyry-skarn Cu deposit, and the first robust dating of an ore system in the emerging Andahuaylas-Yauri batholith and metallogenic belt, southern Peru. This batholith and associated porphyry systems were emplaced during the Incaic orogeny, in a context of slab flattening, compression, exhumation, uplift, and the initiation of the bending of the Bolivian orocline. High-precision ages from early skarn (U-Pb, hydrothermal titanite) and later-stage mineralization (Re-Os, molybdenite) in the Coroccohuayco deposit are indistinguishable from each other and from available high-precision U-Pb zircon ages of the porphyries. All together, they indicate that the deposit was formed in less than 100 k.y. between 35.7 and 35.6 Ma. We also highlight a previously unrecognized pre-ore high-temperature hydrothermal event (U-Pb, hydrothermal titanite) that corresponds to the emplacement of a precursor gabbrodiorite complex at ca. 40.2 Ma. A new 40Ar/39Ar age at 26.6 Ma of a post-ore alkali basalt is interpreted as recording the initiation of slab roll-back following the flat slab episode and is therefore not related to the magmatic-hydrothermal system at Coroccohuayco. These data, together with structural measurements at the Coroccohuayco deposit and available regional data, suggest that the Coroccohuayco deposit was formed toward the end of Eocene arc magmatism, in a context of transpressional stress, intense erosion, and exhumation associated with Incaic orogeny. At the scale of the Tintaya ore district (which hosts the Coroccohuayco, Tintaya, and Antapaccay deposits), available data and a new molybdenite Re-Os age obtained for the Tintaya deposit suggest that mineralizing events were spatially focused and episodic over several millions of years, while a single economic deposit may have been formed within less than 100 k.y

Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes

International audienceQuantifying the distribution of granitic melt at all scales in mid-to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = Fraction of newly Precipitated Zircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr] sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr] bulk , the fraction of melt F melt can be determined through F melt = FPZ × [Zr] bulk / [Zr] sat. The such obtained F melt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H 2 O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925°C. Most of the lower temperature migmatites (800°C) retained F melt of 0.35-0.50 (±0.07-0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites