Abstract Seafloor massive sulfides form in various marine hydrothermal settings, particularly within volcanic arcs, where magmatic fluids may contribute to the metal budget of the hydrothermal system. In this study, we focus on the Kolumbo volcano, a submarine volcanic edifice in the central Hellenic Volcanic Arc hosting an active hydrothermal system. Diffuse sulfate-sulfide chimneys form a Zn-Pb massive sulfide mineralization with elevated As, Ag, Au, Hg, Sb, and Tl contents. These elements have similar behavior during magmatic degassing and are common in arc-related hydrothermal systems. Trace-element data of igneous magnetite, combined with whole rock geochemistry and numerical modelling, highlights the behavior of chalcophile and siderophile elements during magmatic differentiation. We report that, despite early magmatic sulfide saturation, chalcophile element contents in the magma do not decrease until water saturation and degassing has occurred. The conservation of chalcophile elements in the magma during magmatic differentiation suggests that most of the magmatic sulfides do not fractionate. By contrast, upon degassing, As, Ag, Au, Cu, Hg, Sb, Sn, Pb, and Zn become depleted in the magma, likely partitioning into the volatile phase, either from the melt or during sulfide oxidation by volatiles. After degassing, the residual chalcophile elements in the melt are incorporated into magnetite. Trace-element data of magnetite enables identifying sulfide saturation during magmatic differentiation and discrimination between pre- and post-degassing magnetite. Our study highlights how magmatic degassing contributes to the metal budget in magmatic-hydrothermal systems that form seafloor massive sulfides and shows that igneous magnetite geochemistry is a powerful tool for tracking metal-mobilizing processes during magmatic differentiation.
Understanding the disruption of a tectonic nappe that experiences a subduction-related pressure temperature (P-T) loop is challenging. Thrust imbrications may disrupt single nappes during its subduction and/or exhumation which can be revealed by detailed petrological and geochronological work. Garnet commonly forms during subduction. It most likely hosts early prograde, peak high-pressure (HP) and subsequent metamorphic mineral inclusions making such assemblages a useful tool for detailed petrological and geochronological investigations. Multiple approaches were used to determine the detailed P-T loop of the Cycladic Blueschist Unit passive margin sequence (Greece) such as Zr-in-rutile thermometry coupled with quartz-in-garnet elastic barometry, average P-T and phase equilibrium thermodynamic modeling. U-Pb garnet and zircon geochronology age data were in addition determined to complement already existing age data.The results of this approach reveal that the passive margin sequence in Thera (Santorini), Ios and Naxos was subducted as a coherent continental fragment at a subduction rate of ~2.1 km/My and a heating rate of ~12 °C/My. Prograde and peak HP metamorphism occurs at c. 50 and c. 40 Ma respectively.  Along Thera, Ios and Naxos, prograde and peak P-T condition increase from sub-blueschist to upper blueschist facies metamorphism. Subsequently, the sequence was disrupted by one or several thrust faults during its exhumation. The passive margin sequence of Naxos was thrust onto the Ios sequence during the Oligocene at c. 30 Ma. This imbrication is revealed by different exhumation rates of ~6 km/My for the passive margin sequence of Naxos and of ~3 km/My for the one of Ios. The passive margin sequence of Thera, Ios and the upper part of Naxos was exhumed to upper crustal levels, whereas the lower part of the Naxos passive margin sequence was exhumed to the lower crust leading to thermal relaxation of 9–96°C following tectonic accretion. This indicates that thermal relaxation following tectonic accretion in the Cyclades controlled the thermal evolution of the evolving Cycladic orogen during a tectonically quiet period before lithospheric extension.
Abstract Efficient transfer of S and chalcophile metals through the Earth’s crust in arc systems is paramount for the formation of large magmatic-hydrothermal ore deposits. The formation of sulfide-volatile compound drops has been recognized as a potential key mechanism for such transfer but their fate during dynamic arc magmatism remains cryptic. Combining elemental mapping and in-situ mineral analyzes we reconstruct the evolution of compound drops in the active Christiana-Santorini-Kolumbo volcanic field. The observed compound drops are micrometric sulfide blebs associated with vesicles trapped within silicate phenocrysts. The compound drops accumulate and coalesce at mafic-felsic melt interfaces where larger sulfide ovoids form. These ovoids are subsequently oxidized to magnetite during sulfide-volatile interaction. Comparison of metal concentrations between the sulfide phases and magnetite allows for determination of element mobility during oxidation. The formation and evolution of compound drops may be an efficient mechanism for transferring S and chalcophile metals into shallow magmatic-hydrothermal arc systems.
Abstract Precambrian greenstone belts are prospective terrains for orogenic Au deposits worldwide, but the sources of Au, base metals, metalloids, and ligands enriched within the deposits are still debated. Metamorphic devolatilization is a key mechanism for generating Au-rich hydrothermal fluids, but the respective role of the metavolcanic and metasedimentary rocks present within these belts in releasing ore-forming elements is still not fully understood. The Central Lapland Greenstone Belt (CLGB), Finland, one of the largest Paleoproterozoic greenstone belts, hosts numerous orogenic Au deposits and is composed of variably metamorphosed volcanic and sedimentary rocks. Characterization of element behavior during prograde metamorphism highlights that (1) metavolcanic rocks release significant Au, As, Sn, Te, and possibly S; (2) metasedimentary rocks release significant S, C, Cu, As, Se, Mo, Sn, Sb, Te, and U, but limited Au; and (3) metakomatiite releases C and possibly Au. Throughout the CLGB metamorphic evolution, two main stages are identified for metal mobilization: (1) prograde metamorphism at ~ 1.92–1.86 Ga, promoting the formation of typical orogenic Au deposits and (2) late orogenic evolution between ~ 1.83 and 1.76 Ga, promoting the formation of both typical and atypical orogenic Au deposits. The complex lithologic diversity, tectonic evolution, and metamorphic history of the CLGB highlight that metal mobilization can occur at different stages of an orogenic cycle and from different sources, stressing the necessity to consider the complete dynamic and long-lasting evolution of orogenic belts when investigating the source of Au, ligands, metals, and metalloids in orogenic Au deposits.
Abstract Reconstructing the original geometry of a high‐pressure tectonic unit is challenging but important to understand the mechanisms of mountain building. While a single nappe is subducted and exhumed, nappe‐internal thrusts may disrupt it into several subunits. The Middle‐CBU nappe of the Cycladic Blueschist Unit (Hellenide subduction orogen, Greece) shows evidence of such disruption along a Trans‐Cycladic‐Thrust (TCT), however, the timing of this thrusting is unknown. Here, we report multi‐petrological and geochronological data from the Middle‐CBU nappe from the Thera and Ios islands (Greece). Using Zr‐in‐rutile thermometry coupled with quartz‐in‐garnet elastic barometry, average P–T and phase equilibrium thermodynamic modelling, we show that garnet growth in Ios occurred during prograde metamorphism at 6.7 ± 1.4 kbar to 13.0 ± 1.6 kbar and 326 ± 20°C to 506 ± 13°C (2σ uncertainty) followed by early exhumation to 10.1 ± 0.6 kbar and 484 ± 14°C and a greenschist facies overprint at 5.7 ± 1.2 kbar and 416 ± 14°C. For Thera, we constrain peak HP conditions of 7.6 ± 1.8 kbar and 331 ± 18°C, followed by exhumation and equilibration at ~2 kbar and ~275°C using average P–T and phase equilibrium thermodynamic modelling. For Ios, Uranium‐Pb garnet geochronology provides ages of 55.7 ± 5.0 Ma (2σ uncertainties) for prograde and 40.1 ± 1.4 Ma for peak HP metamorphism. Combining our new P–T–t data from Thera and Ios islands with existing data from Naxos island, we conclude that the studied nappe segments represent remnants of a former coherent nappe. The P–T–t data define an Eocene subduction rate of 2.1 ± 1.0 km/Ma, which is distinctly slower than the current subduction rate of 40–45 km/Ma. After subduction, the exhumation of the Middle‐CBU nappe occurred during the Oligocene at different rates for different localities. The Middle‐CBU nappe of Naxos was exhumed at a rate of ~6 km/Ma, contrasting with the exhumation rate of ~3 km/Ma calculated for Ios. This result suggests that the Middle‐CBU nappe of Naxos rocks was thrust on the Ios one during the Oligocene. Using P–T–t data and assuming realistic subduction angles during the Eocene and the Oligocene, we present a 2D structural reconstruction of the Middle‐CBU nappe of these islands. This reconstruction helps to understand the mechanisms of subduction of a continental margin and its disruption during exhumation.
The Laivakangas Au-Cu metallogenic area is characterised by orogenic Au deposits with both Au-only and with atypical (Au ± Cu, Co, Ni) metal associations. Here we study and compare four examples to better constrain the parameters controlling enrichment in base metals in addition to Au. We selected two typical Au-only deposits, the Laivakangas and the Huhta deposits and two orogenic Au deposits with atypical metal association, the Jouhineva Au-Cu-Co-Ag and the Kurula Au-Co deposits. All four deposits record multiple successive mineralisation events with local variations in their respective metal association. Two auriferous mineralisation events are identified, (1) a ubiquitous As-Au-(Co, Ni) event close to peak metamorphism (620–430 °C) where Au occurs either as invisible Au in arsenides or as inclusion in arsenopyrite; (2) a later Cu(-Au)-rich sulfide event on the retrograde path where Au locally occurs as free, native grains along with chalcopyrite. From S isotope studies of the sulfide and sulfarsenide minerals and relations between the deposits and surrounding rocks, we propose that the variation in metal association of the ore fluid is linked to the diversity of lithologies involved in metamorphic fluid production. Multi-event hydrothermal mineralisation and relatively reduced redox conditions appear critical to increase the Au endowment in a deposit and to introduce atypical metals. Results of this study provide a new comprehension of the variability of metal association in orogenic Au deposits of the Laivakangas Au-Cu metallogenic area and elsewhere.
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