A study of Zn and Mo attenuation by waste-rock mixing in neutral mine drainage using mixed-material field barrels and humidity cells

We studied the attenuation of Mo and Zn in neutral pH drainage from waste rock at the Antamina mine in Peru. We hypothesized that Mo or Zn contained in leachate produced in one waste-rock type can be attenuated when the Mo or Zn containing leachate is allowed to contact a different waste-rock type. We tested the hypothesis using mixed-material stacked field barrels and humidity cells connected in series, where leachate from a Mo or Zn - releasing waste-rock type flows through a second waste-rock material type. Results from both the humidity cells (laboratory conditions) and field barrels (field conditions) showed the same general attenuation patterns. When drainage from Mo-releasing waste rock flowed through Pb-rich black marble waste rock Mo was removed from solution. Molybdenum attenuation was not observed when the order of the waste-rock materials was reversed such that drainage from Pb-rich waste rock flowed through Mo-releasing intrusive rock. Geochemical modeling, MLA and SEM analyses supported the hypothesis that wulfenite precipitation was responsible for the observed attenuation of Mo. Zinc is shown to be removed from leachate both by contact with Mo-releasing intrusive rock and by contact with calcite-rich grey hornfels material. Results from geochemical modeling in PHREEQC suggested that precipitation of Zn carbonate, or Zn hydroxide minerals could not account for the degree of Zn attenuation observed in this study. Scanning Electron Microscopy suggested that Zn may have been incorporated into the crystal structure of phyllosilicate clay minerals. - Highlights: • Molybdenum in leachate from waste rock is attenuated by the formation of wulfenite. • Molybdenum-release and attenuation spatially segregated in distinct rock types. • Attenuation observed in both humidity cells and in stacked field-barrel experiments. • Zinc in leachate also attenuated, but mechanism is indeterminate. • Suggests judicious blending of waste could reduce leachate concentrations.