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

Abstract

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 O2 and generated temperatures >40°C. Using mass‐transport calculations, we show that advective rather than diffusive O2 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 O2 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 O2 supply to waste‐rock piles and controls of gas transport on waste‐rock weathering rates.