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Tracing the palaeoredox conditions at Forsmark, Sweden, using uranium mineral geochronology
Swedish Museum of Natural History, Department of Geology.ORCID iD: 0000-0003-2377-8272
Swedish Museum of Natural History, Department of Geology. (Nordsim)
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2019 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 506, p. 68-78Article in journal (Refereed) Published
Abstract [en]

U-Pb isotope systems have been used to constrain the timing of formation, alteration, and oxidation of U minerals from the meta-granitic bedrock at Forsmark, eastern Sweden. Secondary ion mass spectrometry (SIMS) has been used to collect U-Pb data from uraninite. Discordant data suggest a ~1.8 Ga emplacement of uraninite-bearing pegmatites and an event of uraninite alteration at ~1.6 Ga. The latter age is contemporaneous with the Gothian orogeny in Scandinavia, which was associated with hydrothermal fluid circulation in the Fennoscandian Shield. Ca-uranyl silicates haiweeite and uranophane predominately formed 1.3–1.2 Ga, contemporaneous with the emplacement of the Satakunta complex of the Central Scandinavian Dolerite Group. A Palaeozoic group of Ca-U(VI)-silicates is also present, which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time. Low Pb concentrations in the U(VI) silicates of several samples are compatible with a recent (<100 Ma) alteration or precipitation of these minerals in connection to reaction with carbonate-rich fluids. The results support a geologically early oxidation of U(IV) to U(VI) and provide insight into the palaeoredox conditions that may impart an on-going influence on the mobility of natural U in the Forsmark fracture network.

Place, publisher, year, edition, pages
2019. Vol. 506, p. 68-78
Keywords [en]
Geochronology, Uranyl silicates, Uraninite, Deep groundwater, Water-rock interaction, Hydrothermal fluids, Oxidation
National Category
Earth and Related Environmental Sciences
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-3652OAI: oai:DiVA.org:nrm-3652DiVA, id: diva2:1379534
Available from: 2019-12-17 Created: 2019-12-17 Last updated: 2019-12-17Bibliographically approved

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