Element cycling during the transition from alkaline to acidic environment in an active porphyry copper tailings impoundment, Chuquicamata, Chile

Abstract

In an active tailings impoundment, we studied the evolution of freshly deposited tailings from alkaline, unoxidized to acidic, oxidized tailings tracing changes in pore water quality, mineralogy, and element pathways. The tailings originate from the giant porphyry copper deposit Chuquicamata (Chile), and were deposited in different basins with varying times of surface exposure (0 to five years) to the hyper-arid climate in the Atacama desert.Fresh alkaline tailings (pH9.1) had high concentrations of dissolved Na (1773mg/L), Ca (556mg/L), SO4 (2496mg/L) and Cl (1678mg/L) due to dissolution of primary gypsum, and high dissolved element concentrations in the flotation process water due to recycling of water from the tailings impoundment. High As and Mo concentrations resulted from desorption processes during the flotation, with minor contributions from As-rich river water and recycled tailings water.After draining of free water in freshly deposited tailings, evaporation-driven capillary rise was the dominant water transport in the vadose zone. In younger tailings (up to three years), the pH decreased due to sulfide oxidation to circumneutral values (6.4–8.6). The capillary fringe reached 1m depth, where ongoing evaporation enriched Na (up to 5483mg/L), K (742mg/L), and Cl (6892mg/L). In the vadose zone above 1m, the high daily temperature amplitude resulted in condensation of pore water in the uppermost 80cm of sedimented tailings. Subsequent capillary rise depleted uppermost tailings in soluble phases and increase superficial salt precipitation (halite, gypsum and Na–K–Ca–Mg sulfates).After four years, a 13cm thick oxidation zone with acidic pH (4.7) evolved. After five years, a well-defined oxidation zone (28cm thickness) with low pH (3.8) and high mobility of heavy metals was found (e.g., 247mg/L Fe, 177mg/L Cu, 61.8mg/L Zn). This mobility allowed transport and enrichment in the efflorescent salt crust (e.g., as Cu sulfates devilline, krohnkite and Cu-chlorides eriochalcite and atacamite), with SO4 dominantly from sulfide oxidation (−1.0 to 5.9‰ δ34S, −1.0 to 4.4‰ δ18Osulfates). Stable isotope data suggest that capillary rise from the water table reached the surface, most probably due to the reduction of pore size by precipitating secondary minerals, decreased water loss by clogging of pores and heat isolation by the efflorescent salt crust.The cycles of surface exposure and new tailings deposition in the last decades resulted in buried former oxidation zones, which showed lesser K and Cu concentrations in Fe oxides than recent oxidized tailings, most probably due to the transformation of jarosite and schwertmannite to Fe(3+) oxyhydroxides and dissolution of Cu sulfates.Pore water in the saturated deep tailings was highly dynamic and displayed influences of local groundwater, and mixing with acid rock drainage (ARD) and fresh tailings water, both infiltrating from the surface.At the tailings dam, seepage water varied in time between neutralized ARD and fresh tailings water with groundwater influence, due to the periodic shift of the deposition zone and the resulting shallow groundwater changes. Adjacent efflorescent salts include typical minerals found in the secondary porphyry copper deposits in northern Chile (e.g., atacamite), displaying the similarity between geochemical processes in weathered tailings impoundments and secondary enrichment zones of ore bodies, like Exotica from the Chuquicamata mine.