Late (Early–Middle Permian) Stage of Ore Formation in the W–Au Metallogenic Belt of the Southern Tien Shan: Isotopic LA-ICP-MS U–Pb Dating of Zircon from Intrusive Rocks of the Aktau and Koitash Massifs (Uzbekistan)

The paper presents isotopic U–Pb zircon data (LA-ICP-MS method) on the intrusive rocks from the Aktau and Koitash massifs, which the Lyangar and Koitash tungsten-molybdenum skarn deposits are spatially and genetically associated with. These deposits, together with the other W and Au deposits (Muruntau, Zarmitan, etc.), are part of the largest Au (Au–W) metallogenic belt of Tien Shan. The deposits are represented by zones of hydrothermally-altered skarn with scheelite and molybdenite, and later (overprinting) near- and within-intrusive stockworks of veins and veinlets of late metasomatites (propylitic and phyllic alteration) with scheelite-gold mineralization. The concordant isotopic zircon U–Pb data (269.3 ± 2.4 Ma for quartz diorite of the Aktau massif, 268.9 ± 1.9 Ma for granite of the Aktau massif, 278.6 ± 2.7 Ma for granodiorite of the Koitash massif, 276.4 ± 3.6 Ma and 270.0 ± 2.3 Ma for granites of the Koitash massif) were obtained. A greater scatter of the isotopic U–Pb dates was obtained for granodiorite of the Aktau massif, with the two maxima decoupled, 269.1 ± 2.8 Ma and 278.4 ± 3.4 Ma, corresponding to zircon “autocrysts” and “antecrysts,” respectively. All this indicates essentially younger age of the intrusive rocks studied, compared to the rocks of the other tungsten and gold deposits of the Tien Shan that corresponds to the late (Early-Middle Permian) stage of the Late Carboniferous–Permian epoch of formation of these deposits, which occurred under the conditions of a mature post-collisional development of the region. The data obtained highlight the long-lasting emplacement and crystallization of the intrusive massifs studied that occurred under periodic replenishment of magmatic chambers and resulted in a protracted magmatic differentiation, with the fluid and metal accumulation in residual melts.