TY - JOUR
T1 - Zn and Cu isotopic variability in the Alexandrinka volcanic-hosted massive sulphide (VHMS) ore deposit, Urals, Russia
AU - Mason, Thomas F D
AU - Weiss, Dominik J.
AU - Chapman, John B.
AU - Wilkinson, Jamie J.
AU - Tessalina, Svetlana G.
AU - Spiro, Baruch
AU - Horstwood, Matthew S A
AU - Spratt, John
AU - Coles, Barry J.
PY - 2005/10/5
Y1 - 2005/10/5
N2 - 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 θ65Cu (where θ65Cu = [(65Cu/63Cu)sample/(65Cu/ 63Cu)standard - 1]* 1000) and θ66Zn (where θ66Zn = [(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 θ66Zn by 0.4‰ relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. θ66Zn 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 θ65Cu is unlike modern sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3° has been observed [Rouxel, O., Fouquet, Y., Ludden, J.N., 2004. Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev sea-floor hydrothermal fields on the Mid-Atlantic Ridge. Econ. Geol. 99, 585-600; Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S., Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma source mass spectrometry: implications for use as geochemical tracers. Chem. Geol. 163, 139-149].
AB - 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 θ65Cu (where θ65Cu = [(65Cu/63Cu)sample/(65Cu/ 63Cu)standard - 1]* 1000) and θ66Zn (where θ66Zn = [(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 θ66Zn by 0.4‰ relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. θ66Zn 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 θ65Cu is unlike modern sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3° has been observed [Rouxel, O., Fouquet, Y., Ludden, J.N., 2004. Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev sea-floor hydrothermal fields on the Mid-Atlantic Ridge. Econ. Geol. 99, 585-600; Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S., Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma source mass spectrometry: implications for use as geochemical tracers. Chem. Geol. 163, 139-149].
KW - Copper isotopes
KW - Isotope fractionation
KW - MC-ICP-MS
KW - Seafloor hydrothermal vent system
KW - VHMS deposits
KW - Zinc isotopes
UR - http://www.scopus.com/inward/record.url?scp=25144460056&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2005.04.011
DO - 10.1016/j.chemgeo.2005.04.011
M3 - Article
AN - SCOPUS:25144460056
SN - 0009-2541
VL - 221
SP - 170
EP - 187
JO - Chemical Geology
JF - Chemical Geology
IS - 3-4
ER -