Hydromorphic dispersion of uranium in a surficial environment in northern Finland

Abstract

The Ylijoki area in the commune of Sodankylä, northern Finland, stood out as a peak in an airborne radiometric survey. The area lies on a Precambrian granite containing an average of 8 ppm U. The chemical composition of this granite is comparable to the U-rich granites in Sweden. The weathering crust, till, sorted sand and peat contain high U concentration, up to 99 ppm in the weathering crust, 420 ppm in the till, 497 ppm in the sand and 1.3% in peat ash. Although Th also has anomalous values the correlation between U and Th is poor. Cold extractability tests (0.1 M citric acid) show that most of the U in the till and sand is weakly bound and that in the weathering crust more firmly bound. Uranium was possibly fixed in the weathering crust via scavenging by the clay minerals and iron oxides, in the till via adsorption to the surfaces of the mineral grains and in the peat via complexation with organic matter. The predominant dispersions mechanism is hydromorphic. The high U content of the groundwater (maximum 520 ppb) shows that dispersion is still active. The most coherent U anomaly occurs in the groundwater discharge area, where the peat is also enriched with U. Some of it nevertheless, owes its final dispersion pattern to a multicyclic history with several clastic (glacigenic) dispersion phases, each of them followed by postglacial weathering and hydromorphic dispersion.