Natural uranium in Forsmark, Sweden: The solid phase

•Dissolved U was found at depths where reducing conditions would be expected.•U-bearing solid phases have been characterized in order to understand the source and timing of U oxidation.•Altered primary uraninite and uranothorite were found in local pegmatites.•Secondary CaU(VI)-silicates are associated with calcite and >1 Ga fracture minerals.•Coffinitized uraninite is associated with (Ca)FeAl-silicates and sulfides/sulfates of Palaeozoic age.

U-bearing solid phases from Forsmark, Sweden, a proposed host for radioactive waste repositories, have been identified and characterized. Elevated dissolved U was found in some groundwater samples during the site investigations, prompting a need to study the local U geochemistry. Previous hydrochemical and whole-rock geochemical studies indicated that U was derived from local pegmatites, and mobilized and re-deposited during several geological events. In this study, down-hole gamma logs guided sampling of local pegmatites, cataclasites, and fracture fillings. Back-scattered electron-imaging, petrographic microscopy, and electron microprobe analyses were used to find and analyze U phases in thin sections. The results show that the principal U sources at Forsmark include pegmatitic uraninite (PbO up to ∼22 wt%) and metamict uranothorite. These primary minerals show variable degrees of alteration such as enrichment in Ca and Al and/or replacement by secondary Ca–U(VI)-silicates, haiweeite and uranophane. The haiweeite contains up to ∼5 wt% Al2O3, a chemical signature reflecting early (Proterozoic) events of hydrothermal fluid migration. Coffinitized, secondary uraninite is found in association with FeAl-silicates or Palaeozoic sulfide/sulfate minerals, indicating remobilization-precipitation and/or a secondary, sedimentary source of U. It is inferred that U was oxidized during geologically early periods. Later, U(IV) phases formed in fractures open to fluid circulation during the Palaeozoic. This study establishes the phases available as local U sources and/or sinks, and which will be considered in future isotopic and hydrochemical studies aimed to constrain the mechanisms and timing of water–U phase interaction.