Zn, Fe and S isotope fractionation in a large hydrothermal system

The genesis of hydrothermal ore deposits is of crucial economic importance. This study investigates the extent, causes and consequences of zinc and iron isotopefractionation in alargehydrothermalsystem at the world-class Navan Zn–Pb orebody, Ireland. Large variations in Zn, Fe and Sisotope compositions have been measured in microdrilled sphalerite (ZnS) at the millimetre scale. δ66Zn and δ56Fe display a well-defined positive correlation and both also correlate with δ34S. These relationships represent the combined effects of kinetic Zn and Feisotopefractionation during sphalerite precipitation, and Sisotope variation through mixing of hot, metal-rich hydrothermal fluids and cool, bacteriogenic sulfide-bearing brines. Combined with Sisotope data, δ56Fe and δ66Zn data on mine concentrates confirm that hydrothermal sulfide is a minor component of the overall deposit signature. Our data suggest that incoming pulses of metal-rich hydrothermal fluid triggered sulfide mineralisation, and that rapid precipitation of sphalerite from hydrothermal fluids will lead to strong kinetic fractionation of Zn and Feisotopes at very short time and length scales, thereby limiting the use of Fe and Znisotopes as exploration tools within deposits, but revealing the possibility of detecting new deposits from isotopically heavy Zn–Fe geochemical halos