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Natural uranium in Forsmark, Sweden: The solid phase
The Swedish Nuclear Fuel and Waste Management Company (SKB), Sweden.
2015 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 59, 178-188 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2015. Vol. 59, 178-188 p.
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URN: urn:nbn:se:su:diva-135736DOI: 10.1016/j.apgeochem.2015.04.020OAI: diva2:1048602
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2016-11-24Bibliographically approved
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