U-bearing solid phases from Forsmark, Sweden, a proposed host for radioactive waste repositories, havebeen identified and characterized. Elevated dissolved U was found in some groundwater samples duringthe site investigations, prompting a need to study the local U geochemistry. Previous hydrochemical andwhole-rock geochemical studies indicated that U was derived from local pegmatites, and mobilized andre-deposited during several geological events. In this study, down-hole gamma logs guided sampling oflocal 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. Theresults show that the principal U sources at Forsmark include pegmatitic uraninite (PbO up to22 wt%) and metamict uranothorite. These primary minerals show variable degrees of alteration suchas 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 withFeAl-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 establishesthe phases available as local U sources and/or sinks, and which will be considered in future isotopicand hydrochemical studies aimed to constrain the mechanisms and timing of water–U phase interaction.
2015. Vol. 59, 178-188 p.