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

SHRIMP analyses have been conducted for rock samples from the Koongarra sec ondary ore deposit in Northern Australia to obtain activity ratios of U-234 /U-238 and isotopic ratios of Pb-207/Pb-206 and Pb-204/Pb-206, and to under stand their migration behavior within the secondary ore deposit. Main Large r minerals for the analyses were iron minerals and kaolinite, which are the main weathering products in this area. The activity ratios of U-234/U-238 were based on SHRIMP counts at the mass of uranium metal. The U-234/U-238 a ctivity ratios based on counts of uranium oxides were not satisfactory, bec ause the count rates of (UO)-U-234-O-16 were affected by interference from the (UC)-U-238-C-12 fragment. The activity ratios of U-234/U-238 were appro ximately unity for crystalline iron minerals, whereas the amorphous iron mi nerals (precursors of the crystalline iron minerals) had also values above unity. The mean residence time of uranium within the iron nodules was estim ated to be up to approximately 2-3 million years. Results of lead isotopes, represented by a diagram of Pb-207/Pb-206 VS. Pb- 204/Pb-206, indicated that the present three samples contained radiogenic l ead, common lead and mixture of both components. In addition, the recent ra diogenic lead component within the primary ore is dominant in the area clos er to the primary ore. The radiogenic lead isotope compositions of samples at greater distances from the primary ore have been affected only by the ea rlier mobilization event of radiogenic lead. All these results indicate tha t 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 through out the secondary ore deposit, the lead reached only the area closer to the primary ore probably due to its immobility compared to the uranium.