Geochronology of magmatism, rock alteration, and ore deposition at the Verkhneurmiisk ore field (Khabarovskii Krai, Russia)

Isotopic geochronology of the Verkhneurmiisk ore field and the Pravourmiisk W-Sn deposit (Badzhal ore district, Khabarovskii Krai) were studied. Forty -three monomineral fractions from 35 various magmatic and ore-metasomatic r ocks of the area were analyzed by K-Ar, Rb-Sr, and Pb isotope methods. Spat ial and timing relations between the hydrothermal rock alteration and the f ormation of the Verkhneurmiisk granitic pluton (95-92 Ma) were established. Metasomatic rocks in some deposits of the Verkhneunniisk ore field were fo rmed in several stages, according to isotopic data. The regional pre-ore pr opylites have the same isotopic age as the granite has (95-92 Ma). Their fo rmation was followed by two stages of the ore deposition and associated wal l rock alteration. The first of these stages (92-88 Ma) occurred at every d eposit in the form of wolframite and cassiterite greisens. At some deposits , feldspatic and tourmaline metasomatic rocks were formed as well. The seco nd stage (83-78 Ma) occurred only at the Pravourmiisk deposit and was accom panied by tour-malinization and sulfidization. The source of components for these altered rocks probably differed from that for greisens. The exoconta ct and endocontact greisens were formed simultaneously with each other and with crystallization of the granitic pluton (96-92 Ma). The source area of ore metals was a mixed mantle-crustal chamber according to the Pb isotopic data for sulfides at the Verkhneurmiisk ore field. A new timing scheme of t he ore-magmatic system at the Verkhneurmiisk ore field is proposed. Accordi ng to this scheme, magmatic and metasomatic processes evolved over more tha n 20 Ma.