Uranium dynamics at an iron ore mine site in Northern Sweden: Sources and mobility along the mine value chain

Abstract

Uranium (U) release from mining has been typically associated with former U mine sites, but trace U levels in iron or base metal ores can also lead to U mobilization into ground and surface water posing potential risks due to U's chemical toxicity and radioactivity. This study investigates U sources and mobility at an iron ore mine site in Northern Sweden, where U concentrations (median 1.8 μg/l) exceeding the Swedish annual guideline value of 0.17 μg/l have been detected in a river receiving excess process water from the mine site. Drill core samples were characterized to identify the minerals hosting U in the iron ore and sequential extraction tests were conducted on solid samples from the processing plant to assess U mobility potential. Results indicate that, given its low U content, iron ore is not a significant source of the elevated U levels detected in the process water. Thorite, the main U-bearing mineral remains stable under the neutral to alkaline pH conditions in the processing plant. U speciation calculations on process water monitoring data, performed in PHREEQC with the PRODATA thermodynamic database, revealed dominant calcium uranyl carbonate complexes, specifically Ca₂UO₂(CO₃)₃ and CaUO₂(CO₃)₃²⁻. Mine water from Leveäniemi and Gruvberget open pits, particularly Leveäniemi, was identified as the main source of U to the process water in the recirculation system. The U in mine water originates from groundwater infiltration into the open pits and leaching of U from the open pit wall rocks. Further investigation of these sources and U's geochemical behavior in mine water before it mixes with process water in the processing plant is crucial for understanding the processes driving elevated downstream U concentrations.