Radionuclide mobility in thucholitic hydrocarbons in fractured quartzite

Abstract

Radioactive hydrocarbons (thucholites) fill joints and fissures in Presvecokarelian quartzites around the town of Västervik, 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 radioelements have been added to the examined mineralizations. The δ13C values of the examined hydrocarbons are −32 to −46‰ relative to PDB, with the lower values in the thicker hydrocarbonaceous masses at the surface. Because of complex open system behaviour, the age of the uranium accumulations is open to dispute. A Mesozoic age is favored here, but a Precambrian age cannot be ruled out.