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 (OS)-O-187/(OS)-O-188 isotopic compositions decrease in the ore facial range: sulphide chimneys-stockwork zone-seafloor massive and elastic sulphides-metal I iferous 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 (Os-187/(188)s= 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 ((OS)-O-187/(OS)-O-188 similar to 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 (2 sigma) with initial (OS)-O-187/(OS)-O-188 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 similar to 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. (c) 2007 Elsevier B.V All rights reserved.