Retention of uranium in complexly altered zircon: An example from Bancroft, Ontario
- Author(s)
- Lutz Nasdala, John M. Hanchar, Dieter Rhede, Allen K. Kennedy, Tamas Vaczi
- Abstract
Mesoproterozoic (~1050 Ma; Stenian) zircon crystals from the Saranac Prospect, Bancroft, Ontario, contain upto ~1 wt.%U and ~0.15 wt.% Th and, correspondingly, they are for the most part extensively radiation-damaged(calculated total α-doses 2.3−35.3×1018/g). The crystals show textures of complex, intense chemicalalteration that is attributed to multiple, low-T replacement events along fluid-controlled reaction fronts.Centers of crystals appear totally replaced; the primary zoning is virtually erased and the material has highporosity and numerous inclusions. Interior regions surrounding the central reworked areas still exhibitprimary igneous-type zoning; in those regions the alteration emanates from fractures and then follows themore radiation-damaged growth zones. Altered areas are typically recognized by their high porosity, low BSEintensity, and deficient analytical totals. Those regions often have lost a significant fraction of their radiogenicPb. They are in general somewhat depleted in Zr, Si, and U, and are notably enriched in Ca and Fe. Element mapsreveal elevated concentrations of Al and Y within filled fractures. Our observations indicate that the fluiddrivenion exchange is mainly controlled by the accessibility of micro-areas with elevated levels of radiationdamage to transporting fluids via "fast pathways". Most importantly, there is apparent Zr−Si−U equilibriumbetween initially existing and newly formed zircon. The retention of U after the chemical replacement (94±14% relative to the original U content in the respective zones) does not significantly fall below the retention oftwo major cations Zr (95±4%) and Si (95±2%). In spite of the partially extreme hydrothermal alterationoverprinting, the original U zoning in the crystals is well preserved. These observations suggest thatpreferential chemical leaching of U from zircon is clearly not a general feature of this mineral. This in turnseems to question the general validity of hydrothermal experiments to low-T, fluid-driven alteration of zirconin geological environments. The observed apparent immobility of U may affect the interpretation of U−Pbdiscordance in zircon, and the performance assessment of this mineral as potential waste form for actinides.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Memorial University of Newfoundland, Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Curtin University
- Journal
- Chemical Geology
- Volume
- 269
- Pages
- 290-300
- No. of pages
- 11
- ISSN
- 0009-2541
- DOI
- https://doi.org/10.1016/j.chemgeo.2009.10.004
- Publication date
- 2010
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/8d9e1696-37a9-4656-8b8a-9aa5a8aee0fd