Subduction fluids and the source of metals in porphyry deposits

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Porphyry deposits are the main economic source of copper and molybdenum and a major source of some other metals and elements, including rare materials like rhenium and selenium. They always form in a subduction zone environment, but it is not known what the ultimate source of metals in these deposits is: The asthenospheric mantle, the subducted basaltic crust, subducted sediments, or perhaps the crust of the upper plate. We will address this question by experimentally determining the partitioning of Cu, Mo, Ag, Au, and W between eclogite, meta-sediments, and aqueous fluids under subduction zone conditions. These high-pressure experiments will allow quantitative predictions to what extent the subducted basaltic crust or subducted sediments contribute to the metal budget of porphyry deposits. If sediments are the main metal source and if the metal contents in subducted sediments are highly variable, this would allow predicting which arc segment is most likely to host major deposits. As we note in the proposal, there is strong geochemical evidence that this is so for molybdenum, but the behavior of the other elements also requires further investigation.