Quantitative constraints on the formation of hydrothermal rare earth element deposits

Hydrothermal transport of the rare earth elements (REE) in natural geological fluids is essential for the formation of economic REE mineral deposits and fluid-rock interaction in magmatic and metamorphic environments. The processes that control the geochemical behavior of the REE in hydrothermal systems are not well understood, with key questions being the chemical composition of the REE transporting fluids, the relative importance of different complexing ligands for REE solubility, and the importance of key system parameters for hydrothermal REE mobility and deposition. The proposed project will quantitatively address the formation of hydrothermal REE mineral deposits by linking field-based studies of rich and economically viable REE ore mineralization in SE Sweden (the Olserum area deposits) with fluid inclusion and stable isotope studies and geochemical-thermodynamic modeling. Based on detailed field geology and petrography, the hydrothermal evolution of the REE mineralization will be reconstructed. Subsequently, the chemical composition of the REE transporting fluids will be determined with fluid inclusion analysis that combines microthermometry, Raman spectroscopy and LA-ICPMS microanalysis. A novel internally-consistent thermodynamic dataset for REE solubility in hydrothermal fluids will be developed that will make it possible to model fluid-facilitated REE transport in a wide range of geological settings. The dataset will be used for modeling the effect of key system parameters (temperature, salinity, and pH) and to develop a general quantitative understanding of the hydrothermal processes that control the formation of economic REE mineral deposits. The project involves international collaboration with leading research universities in Sweden (Uppsala) and Germany (Tübingen) and active interaction with the exploration industry (Tasman Metals).