Potential lithium enrichment above Hercynian migmatite terrains in Europe

Partial melting is a key factor in the transfer of many critical metals from the deep continental crust to mid-crustal levels via granitoids. The transition zone between these crustal levels is characterized by migmatite-rich domains that record melt production, melt loss and melt crystallization. Migmatites are also affected by melt influx from deeper levels, and they release massive amounts of fluid during cooling. Such fluids invade and modify adjacent metasedimentary units - as will be highlighted by examples from Hercynian Europe -, but are also partly transferred to shallower levels, carrying elements like lithium, the key commodity on which we focus in this work. Several processes may act in tandem to enrich Li concentrations by a factor of up to ~100: protolith enichment, partial melting, and fractional crystallization 1. At a late, magmatic-hydrothermal, stage of the latter process, supercritical fluids unmix from the last silicate melt and can transfer up to 50% of the available Li calculated after 2. Where the fluids invade overlying medium-grade domains, the commonly observed retrogression of cordierite and biotite enrich the fluids even further in Li 3. This may eventually lead to epigenetic Li enrichment, and deposits, without the need for directly underlying granite intrusions, although granites are likely to co-exist in migmatite terrains and overlying medium- to high-temperature metamorphic domains at low to medium pressures 4. An even more exciting scenario is when the Li-enriched fluids infiltrate closed basins that are forming evaporites, such as the Permian Zechstein deposits. In that case, salar deposits like those in Bolivia and Chile are possible. In conclusion, we believe that the late-Hercynian period in Europe with its large-scale mid-crustal melting and its extensive overlying salt deposits may harbour significant Li deposits that could help Europe to develop sustainable mining on its own territory. references 1 da Costa E.O., Kriegsman L.M., Kunz B.E., Warren C.J. and Argles T.W. Submitted to Geology. 2 Yuan Y., Chen B., Shang L., Sendula E. and Chou I.-M., 2023. Ore Geology Reviews 162: 105685 3 da Costa, E.O., Warren C.J., Kunz B.E., Kriegsman L.M. and Argles T.W. Metamorphic Studies Group - Research in Progress meeting, University of Oxford, 4-5 April, 2023. Programme abstracts, p. 20. 4 Pesquera A., RodaRobles E., GilCrespo P.P., Valls D. Torres Ruiz J., 2020. Nature Sci. Rep. 10: 10828. This project has received funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement no. 956125.