Retention of uranium in complexly altered zircon: An example from Bancroft,Ontario |
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| Authors: | Lutz Nasdala John M Hanchar Dieter Rhede Allen K Kennedy Tamás Váczi |
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| Institution: | 1. Geological Survey of Western Australia, Department of Mines and Petroleum, Mineral House, 100 Plain Street, East Perth, WA 6004, Australia;2. ARC Centre of Excellence for Core to Crust Fluid Systems, Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia;3. Western Australian School of Mines, Department of Applied Geology, Bentley Campus, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia;1. CEPS-Department of Earth Sciences, University of New Hampshire, 56 College Road, James Hall, Durham, NH 03824-3589, USA;2. Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, CNRS UMR 5276, 46 Allée d''Italie, 69007 Lyon, France;1. Earth and Environmental Sciences, Department of Earth Sciences, University of Geneva, Geneva, Switzerland;2. Department of Earth, Planetary, and Space Sciences, University of CA, Los Angeles, USA;3. NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK |
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| Abstract: | Mesoproterozoic (~ 1050 Ma; Stenian) zircon crystals from the Saranac Prospect, Bancroft, Ontario, contain up to ~ 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 chemical alteration 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 high porosity and numerous inclusions. Interior regions surrounding the central reworked areas still exhibit primary igneous-type zoning; in those regions the alteration emanates from fractures and then follows the more radiation-damaged growth zones. Altered areas are typically recognized by their high porosity, low BSE intensity, and deficient analytical totals. Those regions often have lost a significant fraction of their radiogenic Pb. They are in general somewhat depleted in Zr, Si, and U, and are notably enriched in Ca and Fe. Element maps reveal elevated concentrations of Al and Y within filled fractures. Our observations indicate that the fluid-driven ion exchange is mainly controlled by the accessibility of micro-areas with elevated levels of radiation damage to transporting fluids via “fast pathways”. Most importantly, there is apparent Zr?Si?U equilibrium between 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 of two major cations Zr (95 ± 4%) and Si (95 ± 2%). In spite of the partially extreme hydrothermal alteration overprinting, the original U zoning in the crystals is well preserved. These observations suggest that preferential chemical leaching of U from zircon is clearly not a general feature of this mineral. This in turn seems to question the general validity of hydrothermal experiments to low-T, fluid-driven alteration of zircon in geological environments. The observed apparent immobility of U may affect the interpretation of U?Pb discordance in zircon, and the performance assessment of this mineral as potential waste form for actinides. |
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