AN ESTIMATION OF THE AGE AND GEODYNAMIC POSITION OF THE MALO-OINOGOR W-Mo DEPOSIT (SOUTHWESTERN TRANSBAIKALIA) BASED ON DATING OF ZIRCONS FROM ORE-HOSTING GRANITE-PORPHYRIES

The large Malo-Oinogor W-Mo deposit is located in the Zakamensk ore cluster of the Dzida ore district of the Western Transbaikalia. The W-Mo mineralization area is located within the Malo-Oinogor granite porphyry stock and surrounding metamorphic rocks of the Khasurta formation. The features of geological relationships with the surrounding rocks, as well as mineralogical-petrographic and geochemical features of the granite porphyries, indicate that the stock relates to the pre-ore stage and that the development of ore-bearing greisens occurred much later than the stock formation. The U-Pb (LA-ICP-MS) dating of zircons from granite porphyries of the Malo-Oinogor stock yielded an isotopic age of 310±2.5 Ma which does not correlate with the Mesozoic age of the granite porphyries from the Gudzhir intrusive complex, genetically associated with the W-Mo mineralization of the Dzhida ore district and, in particular, the Zakamensk ore cluster. The widespread occurrence of zircons, prone to lead loss, in the granite porphyry stock made it possibe to assess the probable threshold time of development of high-temperature hydrothermal metasomatism (132±6 Ma ago) in which U-Pb isotopic re-equilibration could take place. Within the margin of error this threshold time is comparable to the formation time of the Gudzhir ore-magmatic system (133–123 Ma). The formation of the Malo-Oinogor granite porphyry stock 310 Ma ago is due to the beginning of the formation of the Mongolian-Transbaikalian volcanoplutonic belt (PZ₃–MZ₁) during Mongol-Okhotsk orogeny which, in its turn, was related to the subduction of the Mongolian-Okhotsk paleooceanic plate beneath the southern margin of the Siberian continent. It is believed that W-Mo mineralization is related to hydrothermal activity, the source of which was the Early Cretaceous ore-bearing Gudzhir intrusive. The Gudzhir ore-magmatic system of the Zakamensk ore cluster was formed under tectonic extension during the Early Cretaceous Mongol-Okhotsk orogenic collapse. The arguments are presented for the fact that the Zakamensk ore cluster, to which the Malo-Oinogor W-Mo deposit is confied, is a promising area for the discovery of new deep rare metal reserves.

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