Cu Isotope Fractionation during Dissolution of Cu Sulfide Minerals: Assessing Metal Sources in Acid Mine Drainage with Isotope Analysis and Time-Resolved Synchrotron X-Ray Diffraction

The aim of this study is to understand Cu isotope fractionation during the dissolution of copper sulfide minerals with the eventual goal of using Cu isotopes to determine metal sources in acid mine drainage environments. In this study, we have combined time-resolved X-ray diffraction with Cu isotope measurements to monitor changes to Cu-S mineral structures and isotopic fractionation upon Cu release.

Pure, naturally occurring chalcocite (Cu₂S) powders were placed in flow-through capillary reaction cells and exposed to various concentrations of aqueous ferric sulfate for up to 4 hrs. Real-time diffraction data were collected at intervals of 2 min at beam line X7B, National Synchrotron Light Source. The δ⁶⁵Cu values of the leached Cu as well as the starting and final powders were measured using multi-collector inductively coupled plasma mass spectrometry.

The powder diffraction data showed that the Cu bonding environment changed as Cu was leached from chalcocite, eventually forming covellite. The rate at which the transformation occurred depended on ferric sulfate concentrations. The δ⁶⁵Cu values of the initial leachates sampled in all experiments ranged between 2.46±0.12‰ and 3.86±0.12‰. These values decreased during the reaction. Fractionation factors were calculated by finding the difference between δ⁶⁵Cu values of the aqueous Cu in the leachate and the isotopic value of the Cu in the residual mineral. The fractionation factors (Δ_{aqueous – mineral}) varied from 3.31±0.28‰ to -2.68±0.28‰ depending on reactant strength. The decreasing δ⁶⁵Cu values of both leachate and minerals are not consistent with Rayleigh-type distillation models. This may suggest that more than one mechanism is influencing Cu isotopic fractionation factors throughout the reaction. A combination of effects, such as reaction rate and extent of reaction, influence the isotopic composition of the leachate and mineral residue during the dissolution of chalcocite.