Metal Transport through Century-Old Waste Rock

Abstract

The potential for waste rock from mining activities to leach poor quality drainage has been well documented. However, despite the potential for leaching to occur over many decades or centuries, few studies have investigated the relevant geochemical processes over long timescales. This study looks at the porewater geochemistry from a ∼10 Ktonne waste-rock pile at an abandoned prospecting property, the Ore Chimney mine, that has been in place for approximately 100 years. Waste-rock samples were collected at 0.5 m intervals from three trenches that transected the ∼5 m depth of the pile; samples were centrifuged to extract porewaters and analyzed for standard geochemical parameters, anions, cations, and metals. The geochemical modelling software PHREEQC was used to explore possible secondary controls on porewater chemistry. The results indicate near-neutral pH throughout the depth of the waste rock and abundant alkalinity (average: 58.6 mg/L CaCO₃), indicating that carbonate buffering is still active within the waste rock. Zn concentrations, as high as 87.5 ppm (16.5 ppm on average), accounted for 92% [M] of heavy metals and is attributed to the oxidative dissolution of sphalerite, the primary sulfide mineral in the waste rock. The preferential weathering of sphalerite over pyrite produces less acidity and plays a large role in maintaining carbonates and neutral pH in the system. Despite the long period of weathering within the Ore Chimney waste rock, weathering processes are similar to those observed in other waste-rock studies and differences in geochemical parameters are largely ascribed to differences in lithology, principally sulfide mineralogy and carbonate content, rather than the age of the waste-rock. High alkalinity levels suggest that the dissolution of calcite and dolomite controlpH, and Pb, Zn, Cd, and Cu concentrations in the porewaters. Results from this study indicate that the source lithology of sulfides plays a strong role in weathering. This study shows that a detailed understanding of in situ solid- and aqueous-phase geochemistry is required to delineate waste-rock weathering as a function of long-term exposure.