Localized Sulfide Oxidation Limited by Oxygen Supply in a Full‐Scale Waste‐Rock Pile

Core Ideas An operational, full‐scale waste‐rock pile was geochemically characterized. Multiyear experimental data were collected on the evolution of pore‐gas composition in the pile. Sulfide oxidation rates were determined on practice‐relevant spatial scales. Major gas‐transport mechanisms and sulfide weathering rate controls were assessed based on experimental evidence. Open‐pit mines produce large amounts of waste rock that is placed on‐site, often in tall, mixed‐composition piles. These waste‐rock piles pose environmental risks because their weathering (i.e., sulfide oxidation) may generate acidic and metal‐laden drainage. However, the reaction and transport limitations controlling sulfide oxidation remain poorly described at field scales. Here, we present comprehensive multiyear data from two instrumented boreholes in an operational waste‐rock pile at the Antamina mine in Peru. Localized but significant (∼20 m) sections of reactive waste rock with up to 20% (w/w) sulfide existed at depths of up to 100 m in the pile, and their oxidation with rates of up to 1 × 10 −7 kg S m −3 s −1 completely consumed pore‐gas O 2 and generated temperatures >40°C. Using mass‐transport calculations, we show that advective rather than diffusive O 2 ingress controlled the sulfide oxidation rates in reactive regions of the waste‐rock pile. Sulfide weathering rates in the less reactive zones of the pile were not limited by O 2 ingress, independent of the gas transport mechanism, with the exception of fine‐grained waste rock at or near complete water saturation. Our results demonstrate the pronounced effects of physical and mineralogical heterogeneity on O 2 supply to waste‐rock piles and controls of gas transport on waste‐rock weathering rates.