4:15pm - 4:30pm
Significance of epidosite alteration for seafloor sulphide deposits and for fluid fluxes through the oceanic crust
Institute of Geological Sciences, University of Bern, Switzerland
Epidosites are a prominent type of subseafloor hydrothermal alteration of basalts in ophiolites and Archean greenstone belts, showing an end-member mineral assemblage of epidote + quartz + titanite + Fe-oxide. Epidosites are known to form within crustal-scale upflow zones and their fluids have been proposed to be deep equivalents of ore-forming, black-smoker seafloor vent fluids. Proposals for the mass of fluid per mass of rock (W/R ratio) needed to form epidosites are contradictory, varying from 20 (Sr isotopes) to > 1000 (Mg mobility). To test these proposals we have conducted a petrographic, geochemical and reactive-transport numerical simulation study of the chemical reaction that generates km3-size epidosite zones within the lavas and sheeted dike complex of the Semail ophiolite, Oman. At 250–400 °C the modelled epidosite-forming fluid has near-neutral pH, it is highly oxidized and has low S and extremely low Fe contents. These features argue against the proposal that epidosite fluids are equivalents of black-smoker fluids. The Semail epidosites formed by replacement of lavas already altered to albite–chlorite–actinolite (spilite) assemblages, with the rare end-member epidosites requiring enormous W/R ratios of 700 to ~40000, depending on initial Mg content and temperature. Thus, the variably altered Semail epidosite zones record flow of ~1015 kg of fluid through each km3 of precursor spilite rock. This fluid imposed on the epidosite an Sr-isotope signature inherited from the previous rock-buffered chemical evolution of the fluid through the oceanic crust, thereby explaining the apparently contradictory low W/R ratios based on Sr isotopes.
4:30pm - 4:45pm
Permeability available for VMS source fluids in altered and fractured lavas in the oceanic crust, Semail ophiolite, Oman
Institute of Geological Sciences, University of Bern, Switzerland
The distribution of permeability in the upper oceanic crust controls hydrothermal circulation and the water–rock interactions that feed seafloor mineralization. A prevailing view is that lavas behave as fractured aquifers whose permeability is dominated by major extensional faults flanked by damage zones. Comparatively little is known about the permeability of the km-wide blocks of crust that lie between major faults, yet such blocks constitute huge sources of leachable metals. Our field mapping of hydrothermal veins and pervasive alteration in spreading-axis lavas in the Semail ophiolite enables quantification of the permeability of distal blocks. Fracture length intensities are only ~0.005 m per m2 of outcrop, an order of magnitude lower than in major fault zones. Laboratory measurements show the rock-matrix permeability of lava outcrops is ~2.5 x 10−16 m2. Numerical hydraulic simulations using dfnWorks software yield bulk permeability of ~5 x 10−16 m2 when flow through the fracture network and the rock-matrix are coupled. This demonstrates that the rock-matrix is as permeable as the sparse and unconnected fracture network, consistent with the thoroughly pervasive, rather than fracture-controlled, nature of greenschist-facies hydrothermal alteration observed in the distal Semail lavas. Our observations and calculated bulk permeabilities provide an updated view of fluid flow through the upper crust, in which matrix-flow controls circulation through large blocks of lavas, enhanced by fault-damage zones at km-scale intervals. This new perspective explains how the rock matrix in oceanic lavas is accessible for leaching of metals for seafloor sulphide deposits.
4:45pm - 5:00pm
Geochemistry, mineralogy, Cu, Zn and Fe isotopic composition of Gossans found in Cyprus-type VMS systems from the Troodos ophiolite.
1Laboratoire G-Time, Universite Libre de Bruxelles, Brussels, Belgium; 2Royal Belgian Institute of Natural Sciences, B-1000, Brussels, Belgium; 3Juniata College, 1700 Moore Street, Huntingdon, Pennsylvania 16652, USA; 4Geological Survey Department, 1 Lefkonos Street, 2064 Strovolos, Lefkosia, Cyprus
The Troodos ophiolite hosts significant Volcanogenic Massive Sulfide (VMS) systems, well-known as Cyprus-type sulfide deposits. They are mafic type VMS deposits, mainly enriched in copper and zinc and they have been deposited from seawater derived-hydrothermal fluids. Along the Troodos ophiolite, the VMS system is covered by thick, Fe- rich altered caps, known as gossans, which are likely due to weathering of the VMS under oxidizing conditions. However, the conditions for their formation remain largely debated, suggesting either a submarine weathering origin or mineralization weathering on land. Gossans represent a valuable part of the Troodos ophiolite, presenting not only significant amount of extractible copper and zinc, but also, gold and silver. The studied gossans present as mineral assemblage: goethite, jarosite, hematite, alunite, silica, clays, anatase and siderite. Magnetite, ilmenite and gypsum occur as accessory phases. In this study, we show combined data of Cu, Zn and Fe isotopes from three different mines of the Troodos ophiolite (West Apliki, Skouriotissa and Agrokipia), which indicate δ65Cu values varying from -3.55 ±0.01‰ to -0.05 ±0.02‰ and δ66Zn values ranging from -1.24 ±0.02‰ to +0.34 ±0.05‰. In addition, δ56Fe values vary from -0.65 ±0.07‰ to +0.80 ±0.02‰. We aim to investigate the debated origin of the Troodos ophiolite gossans influenced by physicochemical conditions, fluid composition, hypogene ore (e.g., pyrites) and examine the supergene weathering process in VMS systems, based on the redox-sensitive behavior of Cu and Fe, as well as the pH-sensitive behavior of Zn in supergene-weathering environments.
5:00pm - 5:15pm
Molybdenum isotope evidence for forearc mantle recycling at the Tongan subduction zone
1University of Bern, Switzerland; 2University of Tübingen, Germany
Molybdenum isotope ratios (δ98/95Mo) of marine sediments constitute an important tracer for paleoredox reconstructions of the ancient ocean. Due to its redox-sensitivity, significant mass-dependent Mo isotope fractionation is induced in present-day low temperature environments leading to distinct Mo concentrations and isotope compositions in different marine lithologies. Subduction and recycling of such fractionated material can thus be potentially traced in arc magmas. Indeed, Mo isotope variations are observed in mafic arc lavas that are attributed to reflect recycled crustal components, but open questions remain to what extent different subducted lithologies contribute to the Mo isotope signature of arc magmas.
We present a comprehensive Mo isotope dataset covering input to output at the Tongan subduction zone, together with exhumed eclogite-facies oceanic crust and sediments from the Western Alps and Alpine Corsica. Pelagic Mn-rich metapelites and MORB-type eclogites reveal that Mo is largely lost during early subduction metamorphism. Moreover, rutile hosts most of the remaining isotopically light Mo in the slab at higher metamorphic degrees where it remains fixed during slab-dehydration processes at subarc depths. Thus, direct recycling of this fractionated material cannot account for the observed positive covariations of Mo/Ce and δ98/95Mo with fluid indices (e.g., Ba/Th) in Tongan arc lavas. We propose that Mo systematics in Tongan arc lavas are the result of shallow fluid-induced Mo mobilization and forearc mantle serpentinization during early stages of subduction. Subsequent mechanical transport and devolatiziation of this metasomatized forearc mantle material towards subarc regions is a plausible alternative process to recycle Mo and other metals.
5:15pm - 5:30pm
Ultramafic-hosted volcanogenic massive sulfide deposits: an overlooked sub-class of VMS deposits forming in complex tectonic environments?
1Institute of applied geochemistry, KIT, Germany; 2Laboratoire de Géologie, CNRS-UMR 8538, Ecole Nationale Supérieure de Paris, France; 3Mineralogical State Collection Munich, Germany; 4Department of Geological Sciences, Stockholm University, Sweden; 5Institut Terre et Environnement de Strasbourg, CNRS-UMR 7063, Université de Strasbourg, France
Volcanogenic massive sulphide (VMS) deposits have been recognized both in fossil and present-day settings (e.g. mid-ocean ridges (MORs), back-arcs, island-arcs, fore-arcs) and are associated with different lithologies leading to variable metal enrichments. More recently, a sub-type of VMS associated with ultramafic rocks has been discovered at MORs. These ultramafic-hosted VMS (UM-VMS) form in genetic relationships with detachment faults exhuming mantle rocks and are commonly enriched in base (Cu, Zn, Ni), critical (Co) and precious (Au, Ag) metals. However, they are thought to be scarce in the geological record since they are unlikely to obduct from MOR settings.
We propose, based on an extensive review of worldwide UM-VMS deposits described in ophiolites, that this scarcity is only apparent. Previously, UM-VMS have been commonly misclassified for three main reasons: i) the tectonic settings in which they form has been misinterpreted (e.g. tectonic mélanges), ii) their origin may be disputed (hydrothermal vs. magmatic) and iii) orogenic-related metamorphism and deformation locally obliterated seafloor-related mineralogical and structural features. Also, the strong focus on UM-VMS formed in MORs prevented to recognize them in other settings such as ocean-continent transition or supra-subduction zones which are more easily preserved in the geological record. Here, we discuss discriminant features applied to fossil UM-VMS worldwide which allow us to classify them as such. We show that UM-VMS are not as scarce as previously thought and, hence, represent possible undiscovered metal resources. Further genetic and exploration models are needed for new discoveries.
5:30pm - 5:45pm
The Marmorera-Cotschen hydrothermal system (Platta nappe, Switzerland): A Jurassic analogue to present-day oceanic ultramafic-hosted mineralized systems
1Laboratoire de Géologie, CNRS-UMR 8538, Ecole Nationale Supérieure de Paris, France; 2Géosciences Rennes, CNRS-UMR 6118, University of Rennes 1, France; 3Institut des Sciences de la Terre d’Orléans, UMR 7327, University of Orléans, France; 4IFREMER Centre de Brest, DRO/GM, France; 5Institut Terre et Environnement de Strasbourg, CNRS-UMR 7063, Université de Strasbourg, France
Mid-Oceanic ridges are places of intense fluid-rock interactions. At (ultra)slow-spreading ridges where mantle rocks are exhumed along detachment faults, this notably leads to the formation of mineralized systems. They commonly form massive sulphides at the seafloor which are enriched in base (Cu, Zn, Ni), critical (Co) and precious (Au, Ag) metals. However, the limited conditions of observation at the seafloor lead to partial rather than integrative understanding of these hydrothermal systems, especially concerning deep hydrothermal processes. Alternatively, the study of fossil analogues preserved on-land offers the opportunity to study these systems in 3D and to access the deep hydrothermal plumbing system of such mineralizations.
We adopted this strategy here and focused on a mineralized system preserved in the Platta nappe (SE Switzerland), a remnant of the Jurassic opening of the Alpine Tethys Ocean. As a rule, the hydrothermal system escaped strong Alpine overprint. In the Platta nappe, detachment faulting led to mantle exhumation against basalts. Associated HT fluid circulations led to the formation of mineralizations in the serpentinized footwall at the lithological interfaces with mafic intrusive rocks, suggesting the latter acted as preferential pathways for fluid flows. The Cu-Fe-Co-Zn-Ni mineralization forms massive, semi-massive sulphides and stockwork structures. It mainly consists of chalcopyrite, pyrrhotite, pentlandite, isocubanite and magnetite associated with Fe-Ca-silicates (ilvaite, hydro-andradite and Fe-diopside). Based on structural and petrographic features, the hydrothermal system of the Platta nappe is inferred to represent the root zone of present-day hydrothermal systems.