13.2-1 Metal fluxes in the oceanic crust and implications on the formation of hydrothermal mineralizations
Future discovery of mineral resources requires a better understanding of the mineralized system at large scale. Metal fluxes in the oceanic crust have direct and indirect impact on the formation and composition of hydrothermal mineralizations in active black smoker systems, but also in their ancient analogues including volcanogenic massive sulfides and possibly in some epithermal-porphyry systems. These fluxes occur at different stages during the evolution of the oceanic crust and in very diverse tectonic environments, such as slow- and fast-spreading ridges, back-arc basins, island-arcs and continental-arcs, strongly affecting the intensity and nature of the fluxes. Seafloor hydrothermal alteration is critical for hydrothermal ore deposit formation, but it is still poorly constrained in many tectonic environments. Of particular importance are magmatic-hydrothermal processes related to crustal formation, especially in arc-related environments. The competitive effect of sulfide saturation and magmatic degassing during magmatic differentiation can strongly affect the metal endowment of a system, but over whole remains poorly understood. Finally, oceanic crust dehydration in subduction zones has strong impact on the overlying mantle composition and its redox condition, but metal fluxes remain elusive and can possibly have far reaching implications on the formation of hydrothermal mineralizations in oceanic and continental arc environments. In this session we welcome field based, experimental or modelling studies which focus on metal fluxes from modern day oceanic crust or ophiolites.
1:30pm - 2:00pm
Compositions of hydrothermal vent fluids as a guide to subseafloor mineralization processes
Universität Bremen, Germany
Studying active hydrothermal systems in the deep-sea provides unique opportunities for furthering our understanding of how polymetallic seafloor massive sulfide accumulations form. The possibility of sampling the ore-forming fluids that are emitted through sulfide-sulfate chimneys is particularly powerful. The use of gas-tight samplers in collecting hydrothermal vent fluids facilitates measurements of the contents of dissolved gases and metals and allows for accurate reconstructions of in situ pH and redox conditions.
Metal transport in seafloor hydrothermal systems is affected by fluid-rock interactions, magma degassing, phase separation, and subseafloor mixing of the upwelling hydrothermal fluids with entrained seawater. The composition of basement hosting deep-sea hydrothermal vent systems, i.e. the type of rock involved in fluid-rock interactions, ranges from ultramafic to felsic. Geotectonic settings of vent systems vary from mid-ocean ridges to backarc spreading centers to island arc and intraplate volcanoes, which show strong contrasts in water depths and influx of magmatic fluids. Our recent compilation of vent fluid data (doi:10.1029/2020GC009385) allows a first complete assessment of how these differences affect the compositions of fluids in the root zones of hydrothermal systems. Beyond an examination of these general differences, valuable insights into processes in the discharge zone of hydrothermal systems can be obtained from detailed fluid sampling in individual vent fields. We present examples from selected arc/backarc hydrothermal vent sites in felsic crust for how vent fluid compositional data and thermodynamic computations can yield detailed insights into km-scale metal transport as well as smaller scale processes of zone refining.
2:00pm - 2:15pm
Three-component fluid mixing: Evidence from trace element and isotope systematics in vent fluids and sulphides from Maka volcano, North Eastern Lau Spreading Centre
1Department of Physics & Earth Sciences, Jacobs University Bremen, Bremen, Germany; 2GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Erlangen, Germany; 3Center for Marine Environmental Sciences (MARUM), University of Bremen, Bremen, Germany; 4Department for Geosciences, University of Bremen, Bremen, Germany; 5Department for Geology and Paleontology, University of Münster, Münster, Germany
The axial volcanic edifice of Maka at the North Eastern Lau Spreading Centre shows intense hydrothermal activity at two vent sites (Maka HF and Maka South) emitting fluids of distinct composition. We present trace element and isotope data for hydrothermal fluids and related sulphide precipitates that actively form on the seafloor at 1525 to 1543 m water depth. Hydrothermal activity at Maka HF is present as vigorously venting black smoker-type fluids reaching temperatures of ~330°C. High metal (e.g. Fe, Mn, Li) and REE contents in the vent fluids, are indicative for a rock-buffered hydrothermal system at low water/rock ratios. At Maka South venting of white smoke at up to 300°C occurs at several sites. Measured fluid pH (4.53-5.42) and Mg, SO4 and Cl concentrations are depleted compared to seawater, whereas Li, Mn and H2S are enriched, indicating a three-component mixing model between seawater, a boiling-induced low Cl vapor and a black smoker-type fluid at Maka South. Trace element systematics in hydrothermal pyrite also report on the contribution of these different fluid-types. Pyrite that precipitates from low Cl vapor-rich fluids at Maka South is characterized by high As/Co (>10) and Sb/Pb (>0.1) values that we relate to a boiling-induced element fractionation between the vapor (As, Sb) and liquid phase (Co, Pb). The Se/Ge ratio in pyrite may be used as a new tracer for fluid-seawater mixing. Sulfur and Pb isotopes in hydrothermal sulphides indicate a common metal(loid) source at the two vent sites by host rock leaching in the reaction zone.
2:15pm - 2:30pm
Spatial variations in submarine caldera-hosted hydrothermal systems: Insights from sulfide chemistry, Niuatahi caldera, Tonga rear-arc
1GeoZentrum Nordbayern, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Schlossgarten 5, 91054 Erlangen, Germany; 2Institut für Geologie und Paläontologie, Westfälische-Wilhelms Universität Münster, Corrensstraße 24, 48149, Münster, Germany; 3Deep-sea and Seabed Mineral resources Research centre, Korean Institute of Ocean Science & Technology, 385 Haeyang-ro, Yeongdo-gu, 49111, Korea
Submarine “black smoker” systems and their associated seafloor massive sulfides (SMS) may represent economic resources for future generations. However, the processes leading to spatial variations in the mineralogical and chemical composition of subduction zone-related hydrothermal systems remain poorly constrained. The large submarine caldera of Niuatahi volcano hosts several active hydrothermal vent sites associated with faults at the caldera wall and with young post-caldera volcanic cones, venting vapor-rich and black smoker-type-fluids with temperatures up to 334 °C. We combine bulk sulfide chemistry with in-situ trace element data and S- and Pb isotopes of pyrite, sphalerite, chalcopyrite to decipher key ore-forming processes causing spatial variations in metal(loid) enrichment.
We refer these spatial variations within the caldera to a continuum between magmatic fluid-dominated venting at the central cones (high Cu, As, Bi, Te, Au, Sb, δ34S = -10.6 - 2.7 ‰) compared to fluid-rock interaction and seawater mixing at the caldera wall (high Au, Ag, Cd, Pb, δ34S = -0.6 - 6.3 ‰). Lead isotopes of sulfide separates suggest a connected hydrothermal circulation cell and/or similar source rock compositions in the central part of the caldera compared to a discrete one at the caldera wall. We conclude that metal(loid)s from distinct sources (magmatic volatiles vs. host rock leaching) combined with hydrothermal fractionation (e.g., boiling) leads to spatial variations in economically relevant elements (e.g., Te, Au, Ag, Bi, Se, Co) in submarine caldera-hosted hydrothermal systems. This has important implications on exploration of fossil SMS or volcanogenic massive sulfide deposits on land.
2:30pm - 2:45pm
Metal sources in the actively forming seafloor massive sulfide deposit of the Kolumbo volcano: Insight from the basement rocks
1Institute for Applied Geosciences, Geochemistry and Economic Geology, KIT, Karlsruhe, Germany; 2National and Kapodistrian University of Athens, Athens, Greece
The shallow submarine Kolumbo volcano , located in the 5 Ma-to-present Aegean volcanic arc in Greece, hosts an active hydrothermal system currently forming polymetallic seafloor massive sulfide (SMS) mineralization on the seafloor, with high As, Ag, Au, Hg, Sb and Tl contents. It is one of the few known SMS deposits associated with continental margin volcanism. The hydrothermal system of the Kolumbo volcano represents an active hybrid analogue style of epithermal and VMS mineralization. The particular geological setting of the Kolumbo volcano in the Anydros basin makes it a great natural laboratory to investigate the metal flux as the underlying units outcrop on the neighboring islands of Santorini, Ios and Anafi . To this day, it is not clear to which extend the metals in the fluid derive from a magmatic source or if they are leached from the basement rocks by magmatic-hydrothermal fluids. Whole rock geochemistry of the basement and sedimentary rocks allows identifying the potential metal reservoirs in the system. The basement rocks can add metals to the system either by leaching through magmatic-hydrothermal fluids or contamination of the melt by assimilation. Sulfur and Pb isotope analysis allow to track contribution of the basement rocks to the metals/ligands budget of the fluids by comparison with the sulfates and sulfides of the Kolombo SMS. Constraining the metal reservoirs involved in marine magmatic-hydrothermal systems is crucial to understand the formation of SMS and variability in the metal endowment between the deposits .
2:45pm - 3:00pm
Linking Laser-Ablation ICP-MS analysis and sulfide textures in identifying gold remobilization and enrichment processes in modern seafloor massive sulfides, Kolumbo arc volcano, Greece
1National and Kapodistrian University of Athens, Faculty of Geology and Geoenvironment, 15784 Athens, Greece; 2University of Erlangen-Nuremberg, GeoZentrum Nordbayern, 91054 Erlangen, Germany; 3University of Gothenburg, Department of Earth Sciences, SE-405 30 Gothenburg, Sweden; 4University of Leicester, School of Geography, Geology and the Environment, University Road, Leicester LE1 7RH, UK; 5Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture, 71003, Heraklion, Crete, Greece
Target settings to secure sustainable access to raw materials include seafloor massive sulphide (SMS) resources. Gold-rich SMS deposits, are often the result of complex interplay of multiple Au enrichment events. Recent studies have shown that high-grade Au ores result from Au remobilization from preexisting mineralization, driven by fluid-induced coupled dissolution-reprecipitation (CDR) reactions; however investigations into this process in modern Au-rich SMS, are lacking. To tackle this issue, Au-rich [AuBULK≤32ppm; Au/(Cu+Zn+Pb)=1.9], polymetallic (Sb, Tl, Hg, Ag, Mo, Te) diffuser chimney samples from the active Kolumbo shallow-water SMS system, Hellenic Volcanic Arc, were geochemically and texturally examined using combined SEM-EDS imaging, and LA-ICP-MS spot analysis and trace element mapping. Recrystallized subhedral auriferous arsenian pyrite2 (≤65 ppm Au, ≤13290 ppm As) records textures, being porosity growth concurrent with the presence of native gold and accessory pore-filling Pb-Sb sulfosalts, indicating that recrystallization proceeded via fluid-mediated CDR reactions. The latter caused replacement of earlier, colloform-banded, Au-rich arsenian pyrite1 (≤130 ppm Au, ≤9057 ppm As) by pyrite2, and liberated invisible Au (nanoparticles and/or lattice-bound) and associated elements (Pb, Sb). Furthermore, textural evidence indicates that porous orpiment with Pb-Sb sulfosalt inclusions, showing extreme Au enrichment (≤861 ppm Au) compared to other SMS deposits worldwide, was formed by replacement of Au- and As-rich Pb-Sb sulfosalts (≤132 ppm Au, ≤6550 ppm As) via CDR reactions. This study provides significant evidence that in arc-related Au-rich polymetallic SMS deposits, native and invisible Au are closely associated to various sulfides/sulfosalts, and CDR reactions may contribute to upgrading Au grades during hydrothermal reworking.