Supra-subduction arc environments are well-endowed geological settings, hosting major hydrothermal ore deposits. Ore formation requires specific geological chain of processes, generally starting in the metasomatized mantle wedge. During subduction, prograde high pressure-low temperature (HP-LT) metamorphism leads to slab dehydration and enable efficient transfer of mobile elements from the pelagic sediments and the altered oceanic crust into the mantle. These fluid fluxes strongly buffer the composition, redox state and melting conditions of the metasomatized mantle. The behavior of Au and related elements (e.g. Ag, Bi, Mo, Se, and Te), however, is still poorly constrained in subduction zones. Several studies show that supra-subduction metasomatized mantle can be enriched in Au with respect to primary mantle values but none have proven so far if Au is mobilized or not during subduction-related processes.
The aim of this project is to characterize the behavior of Au and a suite of selected metals and semi-metals (Ag, As, Bi, Co, Cu, Mo, Pb, Sb, Se, Te, Tl, and Zn) during subduction processes. We suggest that these elements are partly mobilized during slab devolatilization, leading to a metal-enriched metasomatized mantle which would be a fertile source for magmatic-hydrothermal mineralization in the arc environment. To test this hypothesis, we investigate selected areas of the Cyclades, Greece, which are part of the Hellenides subduction system in the Aegean Sea. The Cyclades host the Cycladic Blueschist Unit (CBU), which contains a passive margin and an ophiolitic sequence, hosting metasedimentary and metavolcanic rocks metamorphosed at lawsonite-blueschist to eclogite facies conditions. These rocks have been rapidly exhumed, preventing extensive retrograde metamorphic overprint and are thus particularly well suited to investigate mass transfer in a subducting slab during prograde metamorphism. Combining state-of-the-art whole rock analysis, in-situ mineral analysis and thermodynamic modelling we will generate a unique systematic dataset for the behavior of Au and related elements in rocks metamorphosed under variable metamorphic degrees from a well characterized HP terrane.