"The high field strength elements (HFSE) are considered to be immobile in many geochemical environments, within which hydrothermal fluids at elevated temperature play a key role in their transport. Of the metals considered in this thesis, niobium is often used as a reference element, against which the mobility of other elements is compared, tantalum, the geochemical "twin" of niobium, is assumed to be equally difficult to transport, and uranium, known to be highly mobile in oxidizing environments, is deemed to be immobile in reducing systems. Through the use of field-based and experimental studies, this thesis evaluates the actual mobility of these metals and, through qualitative and quantitative means, determines their solubility and speciation. The behaviour of niobium and tantalum in a hydrothermally altered setting was first described in the Nechalacho rare metal deposit, situated in the Northwest Territories of Canada. Whole rock geochemical data, petrography, and electron microprobe analyses were used to separate the magmatic and hydrothermal components of the behavior of these metals in a deposit that underwent pervasive hydrothermal alteration. Niobium and tantalum were found to be hosted in zircon, columbite-(Fe), fergusonite-(Y), uranopyrochlore and samarskite-(Y) and a genetic model incorporating the magmatic crystallization of these minerals followed by a hydrothermal overprint was proposed to explain their occurrence. Local remobilization of the two metals was observed, but was limited to a sub-meter scale. Further work was carried out experimentally, in order to determine the solubility and speciation of niobium in solutions containing the ligand most likely to form complexes with this metal, fluoride, at 150, 200, and 250 °C. These experiments, carried out in titanium autoclaves, identified the presence of the species Nb(OH)4+ at low HF concentration and NbF2(OH)3° at high HF concentration. The presence of the latter hydroxyl-fluoride species increases the solubility of niobium in acidic fluoride-rich solutions, by orders of magnitude. Destabilization of this complex is most easily accomplished through a reduction in the HF concentration of the fluid, for example, by fluid-rock interaction involving carbonate rich formations. Similar autoclave experiments were carried out with tantalum at 100-250 °C. At low HF concentration Ta(OH)50 predominates, whereas at high HF concentration TaF5, at ≤150 °C, and, more commonly, TaF3(OH)3-, result in rapid increases in tantalum solubility. However, in the same fluid, tantalum is almost invariably less soluble than niobium. Geochemical modeling of hydrothermal alteration involving acidic brines demonstrated that Nb/Ta ratios may decrease in hydrothermally altered crystals and that removing HF from the fluid, or increasing its pH, result in niobium and tantalum mineral deposition. These experimental data, together with the above observations from the Nechalacho deposit, demonstrate that hydrothermal mobilization of niobium and tantalum is indeed possible, particularly in fluoride-rich acidic systems. Whereas niobium and tantalum are found predominantly in the 5+ valence state, redox conditions play a major role in the geochemical behavior of uranium. The solubility and speciation of uranium was evaluated at oxidizing and reducing conditions, at 200-350 °C, using autoclave experiments containing redox buffers employed to control oxygen fugacity. The experimental data revealed that the species UO2Cl2° dominates at oxidizing conditions and UCl4° at reducing conditions. Both species increase rapidly in abundance with increasing chloride activity, but UCl4° does so more rapidly than UO2Cl2°. Geochemical modelling of an iron-oxide copper gold (IOCG) system, demonstrated that the formation of uranium ore deposits could arise from an influx of reducing fluids, and the long-held belief that uranium is immobile under reducing conditions is unwarranted. " --