Age of granodiorite porphyry and beresite from the Darasun gold field, eastern Transbaikal region, Russia

The Darasun ore field situated in the southern West Stanovoi Terrane near the Mongolia-Okhotsk Suture comprises the Darasun (> 100 t Au), Talatui (similar to 38.2 t Au), and Teremki (3 t Au) lode gold deposits. In the opinion of many researchers, the Darasun deposit is spatially and paragenetically linked to granodiorite porphyry of the Amudzhikan Complex and related metasomatic rocks (beresites). Whole-rock samples of granodiorite porphyry, monomineralic fractions of plagioclase, K-feldspar, and biotite, as well as sericite from beresite (26 samples in total), were analyzed by the Rb-Sr method. Eight biotite and sericite samples were analyzed by the K-Ar method. The Rb-Sr mineral isochrons obtained for individual granodiorite porphyry samples yielded initial Sr-87/Sr-86 ratios varying from 0.70560 to 0.70591. The consistent results of both methods allowed us to accept the ages of granodiorite porphyry and beresite as 160.5 +/- 0.4 and 159.6 +/- 1.5 Ma, respectively. The age of granodiorite porphyry of the Amudzhikan Complex of 160.5 +/- 0.4 Ma corresponds to the boundary between the Early and Middle Jurassic and marks the completion of collision between the East Siberian and Mongolia-China continents and related orogeny. Since that time, the eastern Transbaikal region has been involved in the postorogenic (within-plate) stage of evolution, characterized by the formation of large gold, uranium, and other ore deposits.