Geochemical and mineralogical assessment of reactivity in a full-scale heterogeneous waste-rock pile

Mine waste rock is typically stored on-site in large piles that are heterogeneous in terms of material composition and grain size. This leads to variable reactivity within the pile and makes prediction of drainage quality difficult. Here, we present data from two > 100 m continuous boreholes through an operational waste-rock pile at the Antamina mine (Peru). Reactive zones with elevated sulfide and metal content exhibited increased temperatures, reduced oxygen content, as well as pervasive secondary mineral precipitation. Sharp contact between altered and unaltered layers at the meter to centimeter scale indicated strong variability in mineral reactivity. Unaltered waste rock persisted within high-sulfide zones due to low oxygen levels, and surface passivation and galvanic interactions between sulfide couples also limited alteration. Waste-rock layers with gradated alteration that resemble hardpans, as observed in tailings, were in direct contact with non-altered, less-permeable layers. Finally, leaching tests show that sulfate, Cu and Zn were efficiently leached from reactive and mineralogically altered zones, whereas As and Fe were retained through sorption, surface passivation, or secondary mineral formation. This study shows that, over and above bulk geochemical and lithological data, waste-rock reactivity may also be defined by mineralogical data and thereby contribute to improved drainage quality predictions.