Geochemistry and Th–U–Total Pb Chemical Ages of Late Variscan Uranium Mineralisation at Shkhara, Greater Caucasus

We present the chemical composition and the U-Pb chemical age of the recently discovered uranium mineralisation occurrence in the crystalline Shkhara Massif in the Greater Caucasus. The mineralisation consists of hydrothermal uraninites from veins that intersect into the Late Variscan biotite–plagioclase-rich granite and migmatites. The chemical composition and the Th–U total Pb chemical age of the uraninites were determined. Results show thorium-rich, (∑LREE/∑HREE)N unfractionated uraninites that had been formed under higher temperatures above 450 ± 50 °C. Fifty-eight measurements on 14 grains revealed homogeneous and unaltered uraninites. Th–U–total Pb ages of the uraninite were calculated from 55 chemical analyses, among which 37 plot between 275 and 305 Ma. The weighted median age of the 55 data points corresponds to 291 ± 14 Ma: the boundary between the Carboniferous and Permian periods. These dates suggest that uraninite mineralisation is related to the late orogenic extensional process of the Great Caucasus structure. During this process, hydrothermal fluids permeated the biotite-+ and plagioclase-rich magmatic rocks of the Main Range zone and formed U- and Th-rich veins and uraninite mineralisation. This study shows that the Shkhara uranium occurrence correlates with most of the late Variscan uranium deposits in Central and Western Europe in terms of geodynamic setting, composition, age and type of mineralisation.