Uranium and thorium in carbonatitic minerals from the Guli massif, Polar Siberia

This paper reports a geochemical and mineralogical study on carbonatites from the Guli massif, which hosts rare-metal mineralization. The principal carriers of radioactive elements in the carbonatites are pyrochlore-group minerals, zirconolite, and thorianite, which are described here. They are characterized by elevated concentrations (wt %) of radioactive elements: up to 17.89 UO₂ and 20.01 ThO₂ in pyrochlore, up to 6.49 UO₂ and 94.29 ThO₂ in thorianite, and up to 6.74 ThO₂ in zirconolite. The pyrochlore-group minerals, zirconolite, and thorianite from the early calcite carbonatites occur in intimate association with Ti-Zr oxides calzirtite, perovskite, and baddeleyite. Significant radioactive element fractionation in early-stage derivatives results in the depletion of the residual magmatic products in these elements. The dolomite carbonatites are reported to contain only trace amounts of pyrochlore-group minerals. It was shown that the distribution of U, Th, Nb, and Ta in the calcite and dolomite carbonatites is correlated with the evolutionary trends of pyrochlore composition. Typical schemes of isomorphic substitution are proposed for pyrochlore-group minerals and zirconolite. The pyrochlore-group minerals show an apparent evolutionary trend from U-rich towards more Th- and Ta-rich varieties, and Ba-Sr cation-deficient varieties originate during the latest stage of the evolution. The pyrochlore-group minerals, zirconolite, and thorianite may also accumulate in placers, together with gold. Because of the relative ease of extraction of the accessory minerals, the carbonatites of the Guli massif can be considered as commercial sources of radioactive raw materials.