Radionuclide mobility in thucholitic hydrocarbons in fractured quartzite
Radioactive hydrocarbons (thucholites) fill joints and fissures in Presvecokarelian quartzites around the town of Vastervik, southeastern Sweden. Uranium series nuclides, including stable end members, and carbon isotopes have been measured in surface samples and in drill-core material originating down to 60 m below the surface. Uranium both is adsorbed onto noncrystalline hydrocarbonaceous material and forms different mineral phases. The amount of hydrocarbons and uranium rapidly decreases with depth: below 60 m the accumulations were too scanty for sampling. Uranium series nuclides show distinct gradients. The surface samples, with uranium contents up to 50 wt.%, have been enriched in uranium, lead, and particularly thorium in geologically recent time, probably during the last glaciation. Along the drill cores thorium has been immobile, whereas 226Ra and 222Rn have migrated. Uranium has been distinctly migratory at and close to the surface but more or less immobile below 20 m depth. Almost no foreign rad...