Scandium transport in hydrothermal systems: New insight from experimental, theoretical and field-based studies

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Quantitative data for the transport of ore metals in hydrothermal fluids are essential for understanding fluid flow and formation of economic mineral deposits in the Earthʼs crust. Experimental and theoretical first-principles studies of the solubility and complexation of metals at elevated P-T conditions are critical for developing quantitative process models for hydrothermal ore-forming systems. Experimental research has provided quantitative data for the hydrothermal behavior of common base and precious metals, but many economically important rare-metals have not been adequately studied. Scandium is a rare-metal that is becoming increasingly important for emerging green climate-neutral technologies, but the processes that drive hydrothermal mobilization, transport and enrichment of Sc in ore-forming systems are poorly understood. Scandium shares chemical similarities with Al but also with the REE and Y, which makes it a promising proxy for elemental fractionation in magmatic and hydrothermal systems. Key unresolved questions are the relative importance of chloride and fluoride as ligands for Sc complexation, the differences in solubility and speciation between Sc and the REE and Y, and the relative role of magmatic and hydrothermal processes that lead up to Sc enrichment in mineralized systems. The proposed project will address the hydrothermal transport of Sc through an integrated approach that links high-temperature solubility experiments and first-principles simulation of Sc complexation with fluid inclusion studies of the Sc concentration in magmatic-hydrothermal systems and geochemical modeling of hydrothermal Sc transport. The project is organized as four work packages, which will jointly result in fundamentally new understanding of the hydrothermal geochemistry of Sc. Work package A will study the solubility of Sc in aqueous chloride and fluoride solutions through batch solubility experiments at temperatures of 100-300 ºC. Work package B will investigate the Sc complexation in hydrothermal chloride and fluoride solutions using first-principles simulation. The experimental and first-principles results will be used to develop a consistent thermodynamic model for Sc solubility and speciation in hydrothermal fluids. Work package C will provide fluid inclusion constraints on the chemical composition of Sc transporting magmatic-hydrothermal fluids, including Sc concentrations in natural fluids. Work package D will address the impact of key system parameters (temperature, pressure, salinity, fluorine concentration, pH) on transport of Sc in magmatic-hydrothermal systems through geochemical-thermodynamic modeling, and will also look at the differences in solubility and speciation behavior between Sc, the REE and Y.