The role of continental crust in the formation of uraninite-based ore deposits

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 secondary207Pb-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. Initial87Sr/86 Sr 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.