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

Svetlana G. Tessalina*, Bernard Bourdon, Valery V. Maslennikov, Jean Jacques Orgeval, Jean Louis Birck, Abdelmouhcine Gannoun, Françoise Capmas, Claude Jean Allègre

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)70-80
Number of pages11
JournalOre Geology Reviews
Issue number1
Publication statusPublished - Jan 2008
Externally publishedYes


  • Geochronology
  • Re-Os isotopic system
  • Urals
  • VHMS deposits


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