The discovery of U mineralization in the late Variscan plagiogranite vein of the Shkhara crystalline massif (Greater Caucasus, Georgia)

The Greater Caucasus belt is the northernmost expression of the Caucasus orogen and is linked to the southern margin of the Precambrian Scythian Platform. In the pre-Jurassic crystalline basement of this belt, a plagiogranite vein, exposed in the headwaters of the Enguri River, with elevated radiation (μSv/h range of ~1–3), has been discovered. The vein is located along the Main Thrust of the Greater Caucasus, in the upper Paleozoic biotite migmatites of the Shkhara crystalline massif. It is ~2–3 m thick and represents hydrothermally altered rock (SiO2 content varies from ~75% to ~85%) predominantly composed of a quartz-plagioclase assemblage. LA-ICP-MS 206Pb/238U dating of zircons from the vein indicates an age of 310.2±7.5 Ma that corresponds to the late Variscan orogenic activity. The vein is slightly fragmented and fractured, and fractures and nests are filled with Th-enriched uraninite veins and impregnations. According to ICP-MS-ES analyses, the Th content varies from ~26 ppm to ~50 ppm, and the U varies between ~55 ppm and ~290 ppm. Based on the conducted research, it was found that there is a full correlation between the studied vein and U-bearing granitic veins of different regions of the world by composition, magma series, geodynamic setting, tectonic location and isotopic age. On this basis, it is supposed that the late Variscan hydrothermally altered plagiogranite veins, which are localized in the shear zones of the Shkhara Massif, and entirely in the Main Range Zone of the Greater Caucasus, are potentially U-bearing.