40Ar/39Ar Dating of Hydrothermal Processes in Large Gold Deposits of the Kochkar Anticlinorium (South Urals, Russia)

The age restrictions for productive mineral associations of the two largest gold deposits of the Southern Urals-Svetlinsk and Kochkar, which are located in the East Uralian megazone-are discussed. The Ar-40/Ar-39 dating method was performed for the first time for potassium-containing hydrothermal minerals (micas and amphiboles) from the ore veins and ore-host alteration, as well as from marbles. The age estimates obtained are in the range of 290-276 Ma; the weighted average value for the Svetlinsk deposit is 276 +/- 2 Ma; for the Kochkar field, 284 +/- 2 Ma. It is supposed that the studied mineral assemblages of the Svetlinsk and Kochkar deposits were formed in the beginning of the postcollision stage corresponding to the tectonic relaxation regime. Age of hydrothermal mineral formation in the gold fields of the Kochkar anticlinorium are consistent with the time of post-tectonic plutonic activity, which was expressed in the Middle-Southern Urals in large-scale granitization (similar to 300 Ma). The ore-bearing alteration of a femic profile (of the same age for two deposits), which are most likely basificates, are near-synchronous with the granitization.