Chemical Variations in Uraninites from Various Types of Uranium Deposits

English Abstract:

Uraninite, with an ideal formula UO₂, is the most common uranium mineral. It has been shown that natural uraninite contains such elements as Pb, Th, Zr, rare earth elements (REE), Y, Ca, Fe, and Si. Moreover, uranium in uraninite can be in U⁴⁺ and U⁶⁺ oxidation states, which makes its formula not so simple: (U⁴⁺_{1–x–y–z}U⁶⁺_xREE³⁺_yM²⁺_z)O_2+x–y–z (Janeczek and Ewing 1992, Finch and Murakami 1999). Conventional electron-microprobe analysis (EMPA) and laser-ablation inductively-conducted plasma mass spectrometry (LA-ICP-MS) studies show that natural uraninite is chemically heterogeneous at the micrometer scale in regard to major elements, but (1) it has not been established in what form ‘impurity’ elements occur in uraninite and where they reside, i.e. which elements substitute for U in the crystal structure of uraninite and which ones form micro-inclusions of other minerals such as zircon, thorite, thorianite, or galena, nor (2) has the variability of trace elements been well studied. Additionally, the U⁶⁺/U⁴⁺ ratio in uranium minerals has been overlooked despite the fact that this ratio is a superb tracer of redox conditions. Combining results of electron microprobe analysis (EMPA) with element mapping, performed with the use of field emission gun scanning electron microscopy (FEG SEM) and synchrotron X-ray fluorescence microscopy (SXRF), allows to document rare micron-scale features and the distribution of trace elements in heterogeneous natural uraninites, and provides important information in which mineral phases elements reside and with which other trace elements they are associated. This in turn, aids in understanding the element mobility and constraining fluid chemistry and genesis of uraninites from various types of uranium deposits.