Controls on the enrichment and transport of As and Sb in magmatic fluids: experimental studies and implications for hydrothermal Au-Ag-As-Sb deposits

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The genesis of world-class magmatic-hydrothermal ore deposits is largely controlled by the efficient extraction of metals from a deep magma source and their transport by magmatic volatiles to superjacent environments. However, processes occurring at the so-called ‘magmatic-hydrothermal transition’, as magmas degas and crystallize, remain still poorly understood. Developing a robust quantitative understanding of these processes thus requires better constrains on the partitioning of metals between the magma source and the exsolved volatiles, and on the stability of metal complexes in the conveying magmatic volatile phase. Specifically, it is critical to quantify how changes in pressure, temperature, redox state and chemistry affect the partitioning and speciation of metals in ore-forming fluids. Yet, the experimental data is rather limited at magmatic-hydrothermal conditions ( 300-800 ⁰C, 0.5-3 kbar) due to exceptional experimental challenges associated to sampling unquenchable volatile phases. This proposal will address this gap of knowledge by applying a multidisciplinary experimental approach combining experimental and analytical work with thermodynamic modelling. We will particularly focus on the behaviour of As and Sb, and how they affect the mobility and transfer of precious metals Au-Ag, with whom they are associated in a large number of hydrothermal deposits. Volatiles (fluids) equilibrated with silicate melts will be probed by a combination of in situ synchrotron-based X-ray methods in a hydrothermal autoclave and ex situ fluid entrapment as synthetic fluid inclusions (SFI). Spectroscopic data acquired at P-T conditions relevant for the magmatic-hydrothermal transition, will identify the speciation of metals and allow thermodynamic description of the transfer of elements from the melt to the volatile phase. The obtained data will be employed to construct mass transfer and thermodynamic models to assess the role of granitic intrusions and exsolved volatiles as a source of As-Sb-Au-Ag metals in the formation of a range of hydrothermal ore deposits, from epithermal to Carlin-type Au/Ag-As-Sb deposits, at shallower depths.