ZINC ISOTOPE VARIATION IN HYDROTHERMAL ORE DEPOSITS

Little is known about the variability and controls of Zn isotope compositions of terrestrial materials and currently just a handful of analyses of ore samples exist. We have analysed nineteen well-constrained sphalerite samples from the Irish Zn-Pb orefield in order to determine whether significant variations in isotopic composition can be resolved in hydrothermal ore deposits and to test several possible controls of Zn isotope fractionation. The samples, representing a range of precipitation conditions constrained by fluid inclusion and sulfur isotope analyses, were collected from different parts of the ore system including footwall feeder veins, major and minor orebodies and subeconomic prospects.

Zn isotope compositions were determined using a GVi IsoProbe MC-ICP-MS. The results reveal variation in δ^66/64Zn (where δ^66/64Zn=[(⁶⁶Zn/⁶⁴Zn)_sample/(⁶⁶Zn/⁶⁴Zn)_standard - 1] x1000), from -0.34 to +1.15 per mil relative to the Lyon JMC 3-0749L Zn standard. This variation is significant compared to the external reproducibility (±0.12 per mil, 2σ), and the data show very good mass-dependency with δ^67/64Zn, δ^68/64Zn and δ^68/66Zn values. Thus, we can resolve natural variations in the Zn isotope composition of these ore minerals. Our results span the entire range of δ^66/64Zn values measured on terrestrial geological samples to date, and include the lowest δ^66/64Zn value yet determined.

Our results suggest that the observed variability in isotope composition is unlikely to reflect geographic variation in the composition of the primary metal source rocks or temperature-controlled fractionation effects. The variation in zinc isotope composition occurs on an orebody scale and may be due to a kinetic fractionation during rapid sulfide precipitation and/or mixing of zinc from two contrasting metal sources. The results show that significant variations in zinc isotope compositions can occur in relatively low temperature (~100-250°C) hydrothermal systems. Thus, the acquisition of zinc isotope data is likely to prove useful in better understanding ore-forming processes.