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TitreCharacterization of uranium and REE mobility downstream of a uranium tailings impoundment near Bancroft, Ontario
AuteurLaidlow, A M; Parsons, M B; Jamieson, H E; Gault, A G
SourceAssociation géologique du Canada-Association minéralogique du Canada, Réunion annuelle, Programme et résumés vol. 36, 2013 p. 126
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120415
ÉditeurGeological Association of Canada - Mineralogical Association of Canada (GAC MAC)
RéunionGeological Association of Canada - Mineralogical Association of Canada Joint Annual Meeting; Winnipeg; CA; mai 22-24, 2013
Documentpublication en série
Lat/Long OENS-78.5000 -78.0000 45.0000 44.7500
SujetsArchéen; ceintures de roche verte; résidus; géochimie des résidus; analyses des résidus; uranium; terres rares; géochimie des terres rares; contamination des métaux lourds; substances polluantes; Ceinture d'Abitibi Greenstone ; géologie de l'environnement
ProgrammeOutils d'adaptation et d'impacts sur l'environnement pour les mines de métaux, Géosciences de l'environnement
LiensOnline - En ligne
Résumé(disponible en anglais seulement)
Attenuation of uranium (U) and rare earth elements (REEs) has been observed in stream and wetland sediments, but the processes involved in sequestering these elements in natural systems are not well understood. The decommissioned Bicroft Uranium Mine near
Bancroft, ON uses a modified stream and wetland system to reduce the concentrations of U and other metals in tailings pond effluent to levels below the Provincial Water Quality Objectives. The Bicroft Mine was operated from 1957 to 1963, and processed low-grade, isseminated U hosted by pegmatite dykes in amphibolite gneiss. In this study, we used tangential flow filtration (TFF), ICP-ES/MS, canning electron microscopy, and synchrotron techniques (bulk and µ-XANES, µ-XRF, and µ-XRD) to characterize sediment, tailings and colloid samples. In conducting TFF on waters downstream of the tailings impoundments, orange-brown staining of the filter membrane was
observed, suggesting that dissolved Fe(II) was oxidized and precipitated during TFF processing. TFF reservoir mass balance alculations identified losses of Fe, Mn, and Al on the filter membrane, as well as similar losses of REEs (Ce and La). In contrast, U present at concentrations of 18-27 µg/L (median = 21 µg/L; <0.45 µm fraction), showed little to no filter loss during TFF, which suggests that U is not
primarily associated with colloidal Fe- and Mn-oxy-hydroxides in solution. These results indicate that reduced forms of Fe and Mn
remain in solution at least 200 m downstream of the Bicroft tailings impoundments, and that the mobility of U and REEs may be governed
by different processes. Analyses of colloids by SEM, µ-XRF, and µ-XRD confirm that REEs are strongly associated with Fe- and Mn-oxyhydroxides, including goethite, lepidocrocite, 2-line ferrihydrite, and birnessite.
The lack of diffraction under the micro-focused synchrotron beam and poor element correlation between U and Fe- and Mn-xyhydroxides
suggests that U is most likely present in coarser detrital crystals, such as the primary ore minerals uraninite [UO2] and uranothorite [(Th,U)SiO4]. Micro-XANES analyses show that U in the colloids from Bicroft is primarily (80%) U(IV), providing further evidence that U is mainly hosted in detrital minerals and has not been extensively oxidized and re-precipitated in the colloids. This observation is important, as U(IV) is relatively insoluble in neutral pH environments and less mobile than oxidized hexavalent U(VI) species. The results of this study will help to develop better monitoring strategies for U tailings sites and should reduce the impacts of future U mining operations.