Zn and Cu isotopic variability in the Alexandrinka volcanic-hosted massive sulphide (VHMS) ore deposit, Urals, Russia

Copper and Zn isotope ratios of well-characterized samples from three ore facies in the Devonian Alexandrinka volcanic-hosted massive sulphide (VHMS) deposit, southern Urals, were measured using multi collector ICP-MS (MC-ICP-MS) and show variations linked to depositional environment and mineralogy. The samples analysed derived from: a) hydrothermal-metasomatic vein stockwork, b) a hydrothermal vent chimney, and c) reworked clastic sulphides. As the deposit has not been significantly deformed or metamorphosed after its formation, it represents a pristine example of ancient seafloor mineralization. Variations in δ65Cu (where δ65Cu = [(65Cu / 63Cu)sample / (65Cu / 63Cu)standard − 1] * 1000) and δ66Zn (where δ66Zn = [(66Zn / 64Zn)sample / (66Zn / 64Zn)standard − 1] * 1000) of 0.63 and 0.66‰, respectively, are significantly greater than analytical uncertainty for both isotope ratios (± 0.07‰, 2σ). Very limited isotopic fractionation is observed in primary Cu minerals from the stockwork and chimney, whereas the Zn isotopic composition of the stockwork varies significantly with the mineralogy. Chalcopyrite-bearing samples from the stockwork have lighter δ66Zn by ∼ 0.4‰ relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. δ66Zn also showed significant variation in the chimney, with an enrichment in heavy isotopes toward the chimney rim of ∼ 0.26‰, which may be caused by changing temperature (hence fractionation factor), or Raleigh distillation. Post-depositional seafloor oxidative dissolution and re-precipitation in the clastic sediments, possibly coupled with leaching, led to systematic negative shifts in Cu and Zn isotope compositions relative to the primary sulphides. Copper shows the most pronounced fractionation, consistent with the reduction of Cu(II) to Cu(I) during supergene mineralization. However, the restricted range in δ65Cu is unlike modern sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3‰ has been observed [Rouxel et al., 2004, Zhu et al., 2000].