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This study applies high precision 238U/235U measurement techniques two three geologic settings: basalt differentiation, uranium ore genesis, and the remediation of a uranium-contaminated groundwater system. In the latter two cases, 238U/235U is used as a tracer of uranium reduction. 238U preferentially enters the reduced (solid) U phase, thus analysis of U ores can reveal information on the development of an ore body (chapters 2 and 3) while analysis of 238U/235U in contaminated groundwater can be used to monitor the progress of uranium reduction (chapters 4 and 5). 238U/235U measurements are applied in a less orthodox way to the problems of magmatic differentiation, where 235U separates from 238U when a partially-molten basalt is allowed to equilibrate under a temperature gradient. This extends the previous work on the isotopic effects of thermal diffusion into the heavy elements, and presents new research on the mineralogical development of a basalt under a thermal gradient. There are five studies in this work: 238U/235U is first applied to detect the effect of a thermal gradient on a partially molten basalt, with variations of ≈1.0 ‰ found over ≈150°C. 238U/235U is then applied to the case of sedimentary reduced uranium ore deposits. A general survey of finds a shift of ≈1.0 ‰ between magmatic-type and sandstone-type uranium ores. A small-scale study of a uranium roll front deposit finds 238U/235U variation in excess of 1.0‰. In both cases, the shift in 238U/235U is attributed to the nuclear field shift effect during uraniu