SIMS Analysis of Lead Isotopes in the Primary Ore Body of the Koongarra Deposit, Australia: Behavior of Lead in the Alte
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ABSTRACT Lead, the final decay product of uranium, is found in natural uranium ore deposits. The isotope composition of lead in uranium-bearing minerals reflects their age and the migration behavior of lead. Secondary Ion Mass Spectroscopy (SIMS) can be used to analyze the isotopic composition of minerals. SIMS analysis of lead contained in the Koongarra uranium deposit, Australia, revealed that uraninite and uranyl minerals with different chemical composition in the primary ore region have homogeneous lead isotope, from the highest grade area to a uranyl silicate zone at a distance of 6.1m. Uranyl minerals, which have good crystallinity and retain stoichiometric composition, have "old" lead isotope composition identical to that of uraninite. Uranyl minerals keep exchanging lead with ground water in the primary ore region through metamictization and recrystallization by alpha-decay damage. Sulfide minerals just outside the primary ore body contain only radiogenic lead with an isotopic composition different from that of uranium minerals. Lead that migrated from the primary ore body formed sulfide minerals in a specific geological event.
Since then, lead
produced from uranium decay has been retained in the primary ore body, in spite of recrystallization of uranyl minerals and exchange and homogenization of lead isotopes. Uranium minerals may retain minor elements despite intense alpha-decay dose.
INTRODUCTION Uranium ore deposits are important study subjects because uranium is not only a direct analog of nuclear waste constituents but also it is the parent of two decay series with different half-lives. Moreover, behavior of minor elements in minerals under intense alpha dose is great concern because such alpha-decay in minerals causes metamictization and a tendency to release trace elements from the minerals. Uranium minerals containing radiogenic lead are suitable subjects for evaluating the behavior of trace elements, including lead, for the long-term evaluation of the geological disposal of nuclear waste. For quantitative evaluation of migration of elements in natural uranium deposits, we have to link geological events causing such the migration with chronological information. To 687
Mat. Res. Soc. Symp. Proc. Vol. 506 ©1998 Materials Research Society
DDH58 drill hole
126.6 and 129 Uraninite zone Uranyl silicate
Figure 1 Schematic cross section of the Koongarra deposit modified from Snelling (1980) [2]. The primary ore body occurs in a quartz chlorite schist between a graphite layer and a reverse fault. Secondary ore is developed as a dispersion fan in a shallower, weathered zone. Uraninite is preserved just below the graphite layer. Most of the primary ore contains veins of uranyl silicate minerals, mainly sklodowskite.
derive a chronology of uranium and lead migration in the deposits, isotopic analysis of uranium and lead should be carried out coupled to detailed petrologic observation. Secondary ion mass spectrometry (SIMS) is the most suitable method for analyzing isotope compositions with the require
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