Abstract
This study reports trace element abundances and Pb, Sr, and U isotopic signatures of uraninite from a variety of ore deposits in order to establish baseline forensic information for source attribution of raw, natural U-rich samples. Trace element concentrations, reported here, provide insights into uraninite crystal substitution mechanisms and possible crustal sources of U, including mobility of trace elements between pristine versus altered fractions. Spatially resolved laser ablation (LA) multicollector (MC) inductively coupled plasma mass spectrometry (ICP-MS) analyses were used to determine secondary 207Pb-206Pb isochron ages, and these were validated by corroborative results obtained by solution mode (SM) MC-ICP-MS for the same sample. Secondary Pb-Pb isochron ages obtained, in this study, indicate that uraninite alteration occurs shortly after ore mineralization. Initial 87Sr/86Sr values correlate in general with host craton age, and therefore suggest that uraninite ore formation is closely linked to the nature of the bedrock geology. The δ238U values are explained by invoking multiple physicochemical conditions and parameters such as temperature, nuclear field shift, oxidation, and source rock composition. The δ234U values indicate that the uraninites, investigated here, have undergone recent alteration, but the latter has not perturbed the Pb-Pb secondary isochron ages.
Highlights
Uranium deposits of economic interest are located on most continents and are classified according to their host rock lithology, nearby tectonic structures, and mode of alteration [1]
If the La/Yb ratio serves as a proxy for monitoring the degree of light vs. heavy rare earth elements (LREE/HREE) enrichment, the results shown in Figure 1A suggest the larger LREEs are preferentially incorporated into uraninite alteration products
Ages obtained for uraninite based on secondary Pb-Pb isochrons are in good agreement with those reported in the literature for the U ore deposits investigated here
Summary
Uranium deposits of economic interest are located on most continents and are classified according to their host rock lithology, nearby tectonic structures, and mode of alteration [1]. The physicochemical conditions prevailing during U ore formation are complex and evolve continuously with time as evidenced by the occurrence of several generations of uraninite within one deposit (e.g., [2]), which is the main constituent mineral. Uraninite has been the focus of numerous past investigations to understand its variable chemical nature, and because it is the most important raw material used for the production of fuel destined for nuclear reactors. Natural uranium has three main isotopes, 234 U, 235 U, and 238 U, of which only 235 U is fissile. The latter feature of uranium has prompted the illicit trafficking of this material for the past several decades, in particular subsequent to the demise of the former Soviet Union [4].
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