Degree Name

Doctor of Philosophy


Department of Chemistry


Studies were made to identify the principal geochemical mechanisms for the mobilization and retardation of 238U, 234U and 230Th in the weathered zone and groundwaters of the Alligator Rivers Uranium Province, Northern Territory, Australia.

Selective extraction studies showed secondary iron minerals retard migration of U and Th isotopes. 230Th/234u activity ratios >>1 in disseminated crystalline iron oxide grains indicate preferential retention of 230Th over time scales < 238U due to recoil emplacement following alpha-decay.

Ultrafiltration showed that U isotopes migrate in ARUP groundwaters largely as true solute species. Solute U levels and 234U/238U activity ratios in shallow aquifers of orebody dispersion haloes approximate a simple twocomponent dilution-by-infiltration model. Desorption of U to the aquifer probably derives from vertical leaching by infiltration. Uranium adsorption in the aquifer is not significant in the short term.

Thorium isotopes are significantly associated with colloids. There was no evidence for recoil enhancement of 230Th dissolution from U ores. 230Th solute levels were universally lower than expected on thermodynamic grounds. 232Th is not significantly enriched in ARUP deposits. Competition between rates of complexation and rapid adsorption appeared to cause equivalent ambient levels of complexed solute 232Th to exceed those of 230Th.

Colloids were composed principally of iron and silicon species, probably ferrihydrite associated with silicate and discrete colloidal silica. Colloidal clays were not evident.

Radioactive ferruginous pisoliths from above an orebody were evaluated for their potential as permanent "sinks" for U and Th. Fission track autoradiography and chemical tests indicated an absence of postdepositional leaching of U. No 230Th/234U activity ratios in excess of unity were found in oxide phases. Accumulating oxide layers cause continuous net accumulation of U by irreversible adsorption. Oxide crystals are smaller than or commensurate with alpha-decay recoil ranges. Authigenic 234U and 230Th (but not 226Ra) m a y b e quantitatively retained by recoil emplacement into crystals and intense adsorption. Dense, indurated Fe/Mn oxide formations may sometimes be U/Th dated using a correction for detrital (allogenic) 230Th incorporation. Dates derived in this way from sites remote from U mineralization were successfully correlated with 14C and TL dates.