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

S.G. Tessalina (1, 2) , Bernard Bourdon (1) , Valery V Maslennikov (2) , Jean-Jacques Orgeval (3) , Jean-Louis Birck (1) , Abdelmouhcine Gannoun (4) , Françoise Capmas (1) , Claude J. Allègre (1)

1 IPGP - Institut de Physique du Globe de Paris
2 Institute of Mineralogy of RAS
3 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
4 School of Environment, Earth and Ecosystem Sciences [Milton Keynes]

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.

Domaines: Planète et Univers [physics]; Sciences de la Terre

S.G. Tessalina, Bernard Bourdon, Valery V Maslennikov, Jean-Jacques Orgeval, Jean-Louis Birck, et al.. Osmium isotope distribution within the Palaeozoic Alexandrinka seafloor hydrothermal system in the Southern Urals, Russia. Ore Geology Reviews, 2008, 33 (1), pp.70-80. ⟨10.1016/j.oregeorev.2005.05.003⟩. ⟨hal-00643744⟩