Fracture-filling minerals as uranium sinks and sources, a natural analoguestudy at Palmottu, Finland

The nucleation of a mineral crystal and its growth in groundwater carrying fractures 300 m above the Palmottu uranium deposit provide an impressive ex ample of geochemical selectivity of uranium. Fracture-filling material was collected from a 3 mm thick fracture at depth 74.8-75 m (drillcore R348). S EM and EDS analyses on a thin section of the original fracture-filling show that the fracture filling is heterogeneous, composing mineral crystal part icles and very porous clay-rich aggregates. The results of INAA on millimet re-sized single mineral crystals and aggregates selected from grinded fract ure-filling show that porous aggregates (composed of clays and micrometer s ized mineral particles) contain up to 1000 ppm U, which is higher than the average of the whole fracture-filling (400 ppm) and host rock related milli metre sized mineral particles (18-100 ppm). U-233/U-238 isotope exchange pr oves that a large fraction of the uranium in the fracture-filling is not ea sily exchanged with uranium in the solution. The amount of U-238 released i n the isotope exchange experiment is too high to be explained by reversible U(VI) sorption. Oxidation state analyses show that 30% of the uranium exis ts as U(IV). Laboratory batch experiment at anoxic conditions proved that p yrite can immobilise U(VI).