Solid-phase control on the mobility of potentially toxic elements in an abandoned lead/zinc mine tailings impoundment, Taxco, Mexico

Detailed mineralogical and geochemical investigations were conducted within abandoned Pb and Zn flotation tailings at “El Fraile” impoundments in Taxco, Guerrero, central-southern Mexico. These tailings are divided into an active oxidation zone near the surface, an underlying transition zone and an unoxidized zone. Although these tailings have undergone 30 a of sulfide oxidation, the active oxidation zone has only penetrated to a depth of 0.2 m in the settling pond, and to 0.6–1.2 m in the dam. The oxidation of sulfide minerals and the insufficiency of pH-buffering minerals have produced low-pH conditions (pH = 1.9–4.4) and high concentrations of dissolved SO42-, As and heavy metals: SO42-(1534–10086 mg L−1), Fe (1.5–2568 mg L−1), Zn (36.7–2435 mg L−1), Cd (0.4–30.6 mg L−1), Pb (<0.01–0.6 mg L−1), Cu (0.5–38.2 mg L−1) and As (0.01–164 mg L−1). These concentrations of dissolved constituents are attenuated by a series of precipitation and sorption reactions. Precipitation of secondary phases, gypsum, goethite, hematite and K-jarosite has led to the formation of cemented layers within the active oxidation zone in the tailings dam. These cemented layers act as a trap for released, potentially toxic elements from the overlying oxidized tailings. Adsorption and coprecipitation on Fe-precipitates play an important role in the mobilization and attenuation of Zn, Cd, Cu and As within the “El Fraile” tailings. Additionally to the well-known ability of Fe-precipitates to strongly trap As and heavy metals, this study shows that precipitation of beudantite (PbFe3AsO4SO4(OH)6) appears to be one of the solid-phase controls on the natural attenuation of As and Pb and other heavy metals in these tailings.