Osmium isotope distribution within the Palaeozoic Alexandrinka seafloor hydrothermal system in the Southern Urals, Russia

The Re-Os distribution and isotopic composition have been studied within different ore facies, host-rocks and sediments from the Alexandrinka volcanogenic hydrothermal massive sulphide deposit, Southern Urals, Russia. The osmium contents increase and the initial ¹⁸⁷Os/ ¹⁸⁸Os isotopic compositions decrease in the ore facial range: sulphide chimneys-stockwork zone-seafloor massive and clastic sulphides-metalliferous sediments. This range reflects variable degrees of reduced hydrothermal fluid-oxidized seawater mixing during the hydrothermal ore-forming process. The Os isotopic composition of the hydrothermal sulphide chimney ( ¹⁸⁷Os/ ¹⁸⁸Os = 1.3) is estimated to be a minimum value of the Devonian hydrothermal fluid, which is an intermediate between initial Os isotopic compositions of island-arc volcanics and interlayered sediments. The initial Os isotopic composition of metalliferous sediments ( ¹⁸⁷Os/ ¹⁸⁸Os ∼ 0.17-0.2) possibly reflects that of the Devonian seawater. The low rhenium concentrations in metalliferous sediments (about 1-2 ppb) could indicate oxic formation conditions. The Re-Os isotope data define a best-fit line corresponding to a Late Devonian age of 355 ± 15 Ma (2σ) with initial ¹⁸⁷Os/ ¹⁸⁸Os of 0.12 ± 0.19. This age could indicate a late Os isotope reequilibration due to ongoing hydrothermal fluid flow from the Givetian (stratigraphic age ∼ 375 Ma) until the closure of the Ural paleoocean in the Late Devonian. The Os contents are higher and Re/Os ratio is lower within Palaeozoic island-arc hosted Urals VHMS deposits compared with TAG deposit in MOR setting.