SHRIMP measurements of U and Pb isotopes in the Koongarra secondary ore deposit, Northern Australia

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Author(s)

    • Sato Tsutomu SATO Tsutomu
    • Division of Global Environmental Science and Engineering, Graduate School of Natural Science and Technology, Kanazawa University
    • ZAW Myint
    • Environment Division, Australian Nuclear Science & Technology Organisation
    • PAYNE Timothy E.
    • Environment Division, Australian Nuclear Science & Technology Organisation
    • AIREY Peter L.
    • Environment Division, Australian Nuclear Science & Technology Organisation
    • YANASE Nobuyuki
    • Research Group for Terrestrial Environment, Department of Environmental Sciences, Japan Atomic Energy Research Institute
    • ISOBE Hiroshi
    • Department of Environmental Science, Graduate School of Science and Technology, Kumamoto University
    • OHNUKI Toshihiko
    • Research Group for Terrestrial Environment, Department of Environmental Sciences, Japan Atomic Energy Research Institute

Abstract

SHRIMP analyses have been conducted for rock samples from the Koongarra secondary ore deposit in Northern Australia to obtain activity ratios of <sup>234</sup>U/<sup>238</sup>U and isotopic ratios of <sup>207</sup>Pb/<sup>206</sup>Pb and <sup>204</sup>Pb/<sup>206</sup>Pb and to understand their migration behavior within the secondary ore deposit. Main target minerals for the analyses were iron minerals and kaolinite, which are the main weathering products in this area. The activity ratios of <sup>234</sup>U/<sup>238</sup>U were based on SHRIMP counts at the mass of uranium metal. The <sup>234</sup>U/<sup>238</sup>U activity ratios based on counts of uranium oxides were not satisfactory, because the count rates of <sup>234</sup>U<sup>16</sup>O were affected by interference from the <sup>238</sup>U<sup>12</sup>C fragment. The activity ratios of <sup>234</sup>U/<sup>238</sup>U were approximately unity for crystalline iron minerals, whereas the amorphous iron minerals (precursors of the crystalline iron minerals) had also values above unity. The mean residence time of uranium within the iron nodules was estimated to be up to approximately 2–3 million years. Results of lead isotopes, represented by a diagram of <sup>207</sup>Pb/<sup>206</sup>Pb vs. <sup>204</sup>Pb/<sup>206</sup>Pb, indicated that the present three samples contained radiogenic lead, common lead and mixture of both components. In addition, the recent radiogenic lead component within the primary ore is dominant in the area closer to the primary ore. The radiogenic lead isotope compositions of samples at greater distances from the primary ore have been affected only by the earlier mobilization event of radiogenic lead. All these results indicate that some geologic event causing migration of the radiogenic lead had occurred before formation of the secondary ore deposit due to the weathering. After the weathering commenced at least 2–3 million years ago, uranium and lead have migrated from the primary ore. While the uranium has dispersed throughout the secondary ore deposit, the lead reached only the area closer to the primary ore probably due to its immobility compared to the uranium.

Journal

  • GEOCHEMICAL JOURNAL

    GEOCHEMICAL JOURNAL 34(5), 349-358, 2000-10-01

    GEOCHEMICAL SOCIETY OF JAPAN

References:  25

Codes

  • NII Article ID (NAID)
    10008805514
  • NII NACSIS-CAT ID (NCID)
    AA00654975
  • Text Lang
    ENG
  • Article Type
    ART
  • ISSN
    00167002
  • NDL Article ID
    5544871
  • NDL Source Classification
    ZM41(科学技術--地球科学)
  • NDL Call No.
    Z53-R488
  • Data Source
    CJP  NDL  J-STAGE 
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