Sulfoarsenic Mineralization with Gold in the Shear Zone of the Eastern Transbaikalia Branch of the Mongol–Okhotsk Suture (Pogromnoye Deposit, Eastern Transbaikalia, Russia)

The Pogromnoye Deposit related to gold–sulfide-quartz formation of the veinlet-disseminated ore type was formed in the shear zone that occurs between overthrusts in the block confined to the volcanogenic–sedimentary basin along the fragment of the Mongol–Okhotsk suture. The parageneses of zonal quartz–albite–micaceous (muscovite–sericite) metasomatic formation related to the profile of moderate acid leaching are presented in ore-related metasomatites. The ore-related metasomatic replacements are interpreted to be Early Cretaceous and dated as far back as 139.5 ± 1.8 Ma. The absolute age of gold-bearing stockwork formation is associated with the termination of hydrothermal ore activity of the Amudzhikan–Sretensky Complex at 131 ± 1.2 Ma. Ore minerals are dominantly arsenopyrite, pyrite, secondary sphalerite, chalcopyrite, pyrrohotite with sulfosalts, gersdorffite, accessory molybenite and magnetite. The diffusion coefficient of As was calculated in zonal metacrystals of gold-bearing arsenopyrite and pyrite in the ores. The temperature interval of gold ore deposition during the formation of ore-bearing metasomatite equal to 350–150 o C is validated by the physico-chemical data on mineral equilibria and fluid inclusions and the thermodynamic calculations performed using the SELECTOR software. Gold mineralization was generated by sulfoarsenide hydrothermal solution containing gold as thioarsenites AuAs and H 2 AuAs with hydrosulfide AuHS 0 , concentrations of dissolved S ≥ 0.01–0.001 m and As ≥ 0.005–0.0005 m, respectively.