CONTROLS OF CU AND ZN ISOTOPE VARIABILITY WITHIN A VHMS DEPOSIT

Hydrothermal formations are particularly suitable for studying mechanisms of isotope fractionation of transition elements as they can be well characterised for geochemical parameters such as precipitation temperature, mineral phases, biological impact or fluid flows. This paper assesses the isotopic variability of Cu and Zn in the Alexandrinka VHMS deposit, Urals, which was formed during the Middle Devonian, subsequently escaping significant metamorphic or biological post-deposition alterations. We measured Zn and Cu isotopes in silicates and sulphides from the stockwork, chimney and clastic facies. Significant spatial and mineralogical isotope heterogeneity occurred with a spread in isotopic composition of 0.36 and 0.44‰ per atomic mass unit, for Cu and Zn respectively. Isotopic fractionation during the genesis of the primary Cu minerals in stockwork and chimney was negligible. By contrast, Zn isotopic composition showed a dependency on mineralogy with ZnS bearing samples yielding isotopic heavy composition of ca. 0.2‰ in the stockwork due to equilibrium partitioning of Zn between sphalerite and chalcopyrite, and of ca. +0.13‰ in the chimney associated with temperature induced sulphide precipitation. Post depositional weathering in the clastic sediments lead to isotopic variability of Cu during secondary mineralization consistent with the reduction of Cu.