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

Abstract

SHRIMP analyses have been conducted for rock samples from the Koongarra secondary ore deposit in Northern Australia to obtain activity ratios of234U/238U and isotopic ratios of207Pb/206Pb and204Pb/206Pb, 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 of234U/238U were based on SHRIMP counts at the mass of uranium metal. The234U/238U activity ratios based on counts of uranium oxides were not satisfactory, because the count rates of234U16O were affected by interference from the238U12C fragment. The activity ratios of234U/238U 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 of207Pb/206Pb vs.204Pb/206Pb, 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.