Aspects of the geochemistry of zinc: a journey to sphalerite

Boyce, A.J. , Barrie, C.D., Samson, I.M. and Williams-Jones, A.E. (2015) Aspects of the geochemistry of zinc: a journey to sphalerite. In: Archibald, S.M. and Piercey, S.J. (eds.) Current Perspectives on Zinc Deposits. Irish Association for Economic Geology: Dublin, pp. 17-35. ISBN 9780950989457

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Our journey to sphalerite begins with a review of the occurrence and behavior of Zn in a wide spectrum of natural fluids. Measured Zn concentrations in crustal fluids vary over at least six orders of magnitude, from around 0.01 to 20,000 ppm (2 wt %). Whist water-rock interaction and boiling influence Zn concentration, the origin of the fluids and in particular their temperature, pH and ligand concentrations are major factors in their capacity to transport Zn. However, in contrast to previous reviews, our compilation shows no significant overall correlation between Zn and Cl concentrations. Fluid chemistry (mainly mCl-, mΣS, pH and fO2) and temperature, and the resultant speciation determine the solubility of Zn and control precipitation of sphalerite. Bisulphide complexes only dominate under relatively high ΣS, low Cl-, and high pH, and are favoured by low temperatures, in most natural systems though chloride complexes dominate. The limited oxidation state of Zn leads to a narrow range of Zn isotope variation in ore systems, with a mean of all published ore-related δ66Zn data around 0.2 ± 0.3 ‰. Nonetheless, kinetic fractionation of Zn isotopes leads to enrichment in 66Zn in residual fluids, following precipitation of sphalerite from hydrothermal fluids. This has resulted, in a wide variety of systems, in a potentially useful exploration vector to feeder zones. We describe the basic crystallography of sphalerite, and show that commonly occurring layered sphalerite, in particular the colloform texture, has the ability to record the extraordinary dynamism of the ore depositional environment through careful petrographic studies followed by in situ laser S isotope and EMPA trace element analyses. Our journey ends in the Galmoy deposit in the Irish orefield, where we show that three distinct ore sulphide-precipitating events and the full isotopic spectrum of the dual ore sulphide source of the Irish-type deposits are recorded across a 2 cm colloform specimen.

Item Type:Book Sections
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian and Barrie, Dr Craig
Authors: Boyce, A.J., Barrie, C.D., Samson, I.M., and Williams-Jones, A.E.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Publisher:Irish Association for Economic Geology

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