Deciphering the metal stable isotope record of Sn-W ore deposits: a complementary approach based on experiments and case studies

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Enrichment, transport and crystallisation of metals in Sn-W deposits results from a complex combination of melt- and fluid-driven processes in highly evolved magmatic-hydrothermal systems. However, the exact conditions and controlling parameters in the various stages of ore formation are not yet well constrained. In addition to structural and classical geochemical and mineralogical investigations, the stable isotope fractionation of the metals may provide important information on crucial changes during metal transport that finally result in metal deposition, as their fractionation critically depends on changes in their bonding environment. Thus, provided that the relationship between metal bonding changes and resulting isotopes fractionation is well understood, or even calibrated, isotopic signature may fingerprint the conditions of metal transport and crystallisation. Here, we propose a complementary approach, combining experiments and case studies (1) to better understand and calibrate the fractionation of Li isotopes between melt-fluid and Li-micas in well-designed laboratory experiments and (2) to apply the Li and Sn isotope proxy to two already well-characterized, but chemically and structurally distinct granitic Sn-W-Li deposits (Sadisdorf, Erzgebirge and Argemela, Portugal). For Li isotopes (mostly) Li micas and for Sn isotopes cassiterite will be analysed, both in situ with femtosecond laser-ablation (LA-) MC-ICP-MS. We expect complementary information from these two isotope systems: Li isotopes are expected to be sensitive to the nature of the fluid and to fractionate during melt-fluid exsolution (first and second boiling) or between to fluids (vapour-brine), which will be experimentally calibrated. Sn isotopes are expected to mostly fractionate as a result of a redox change (from Sn2+ to Sn4+), during vapour-brine separation or during the crystallization of cassiterite (as indicated by several studies of the literature). The experimental and analytical set up for the investigations planed in this study are already established in Hannover and application of the Li and Sn isotope systems to their host minerals in Sn-W-Li deposits is expected to provide very valuable information on the conditions of metal transport and crystallization. If time allows we plan to establish in situ W isotope analyses of wolframite with LA-MC-ICP-MS, validated with high-precision solution double spike analyses of the same wolframite crystals (in Cologne). As W isotope fractionation during the crystallization of wolframite, which is frequently associated with cassiterites in hydrothermal veins, is not controlled by a redox process, the isotope compositions of W may allow us to constrain the role of decreasing temperatures during the metallogenic evolution.