The role of fluids in the formation of rare-element deposits

Granitic pegmatites are important hosts for economically important deposits of rare-elements (Li, Rb, Cs, Be, Nb, Ta, Zr, Hf, Y and the REE). The magmatic to hydrothermal processes responsible for enrichment of rare-elements and formation of economic deposits are controversially debated. Key questions are the significance of fluid saturation and melt-melt immiscibility for metal enrichment processes, and the geochemical windows for rare-element transport in high-temperature fluids. The joint Finnish-Russian research project will address the role of fluids in the formation of rare-element deposits, focusing on two chemically extremely fractionated field examples (the fluorine-enriched Kymi topaz granite stock in SE Finland and the Ploskogorsky rare-element pegmatites in the NW Russian Federation). The project will combine field geology, mineral chemistry, fluid and melt inclusion studies, high-temperature experimental studies and thermodynamic modeling of rare-element transport in hydrothermal fluids. Based on field mapping, the relative time sequence of different stages of fluid and melt evolution will be established, from incipient fluid saturation in the host intrusions to highly evolved distal rare-element pegmatites and hydrothermal veins. Subsequently, detailed fluid and melt inclusion studies will be performed that combine petrography, microthermometry and LA-ICPMS microanalysis of the chemical composition of individual inclusions (major elements, rare alkalis, trace metals, boron, halogens). A new quantification method for chemically complex multi-solute fluid inclusions will be developed that couples LA-ICPMS data with thermodynamic modeling. The fluid and melt inclusion data, in conjunction with selected data on the major and trace element concentrations of key minerals, will be used to reconstruct the fluid and melt evolution pathways of the two granite-pegmatite suites. Based on critical evaluation of experimental solubility and spectroscopic data, a new internally consistent dataset for the solubility and speciation of rare-elements at elevated temperatures and pressures will be developed. The dataset will be used for modeling the control of key fluid parameters such as temperature, pressure, salinity, fluorine concentration and pH on the transport behavior of rare-elements in magmatic-hydrothermal systems.