Abstract: Studies of the U decay series (²³⁸U-²³⁴U-²³⁰Th) have been carried out on samples from unaltered bedrock and highly altered fracture/crush zones from drillcore Km3 (Kamlunge test-site). The fracture zones are characterized by abundant iron oxide coatings (hematite and hydroxy iron oxides) resulting from the passage of hydrothermal solutions coeval with the Lina granite intrusion. Enrichments of U and Th, thought to be due to co-precipitation (or preferential sorption) processes together with the iron-oxides, are also present.

The isotopic results show that out of a total of twelve rock samples measured, six indicate isotopic disequilibrium mostly due to unequal depletions of ²³⁴U and ²³⁸U; one near-surface sample indicated some minor assimilation of U. The major fracture zones generally indicate removal of total U. This has resulted from interaction with groundwaters which are still marginally oxidizing, even at depths of 375 m.

Isotopic disequilibrium has occurred within recent geological times, i.e. during the last 0.5 Ma as imposed by the half-lives of ²³⁴U and ²³⁰Th. In terms of radioactive disposal considerations, the results are important in that: (1) the investigated bedrock environment (100–500 m) is generally reducing; however (2) there is some evidence to indicate that rock/water interactions, leading to the removal of total U, have resulted from the presence of less reducing groundwaters within those large-scale fracture/crush zones which intersect the bedrock surface.