The Northern Kazakhstan Uranium Province, Kokchetav Massif: U–Pb (ID-TIMS) and Rb–Sr Geochronology of Rocks of Ore-Hosting Volcanotectonic Depressions

Geochronological studies have been conducted for ore-hosting rocks (quartz rhyolites, felsitic rhyolites, porphyry granites) of some U deposits of the Yakshi-Yangistau complex within the Balkashino ore center of the Northern Kazakhstan uranium province. According to geological maps of various scales, the rocks were traditionally ascribed to the Early–Middle Devonian (quartz and felsitic rhyolites) and Middle–Late Devonian (porphyry granites). The results of U–Pb, Rb–Sr, and K–Ar dating have shown that, according to the current version of the Geochronological Chart, the age of quartz rhyolites is Early–Late Silurian and that of felsitic rhyolites and porphyry granites is Late Silurian. The features of the chemical composition and different initial Sr isotopic composition of these rocks suggest that quartz rhyolites, felsitic rhyolites, and porphyry granites of the Yakshi-Yangistau complex are hardly comagmatic. They are characterized by different ages and formed from independent magmatic sources. Our data indicate the minimum possible age of the formation of primary U ores within the Balkashino ore center.