Mobilization and retention of heavy metals in mill-tailings from Garpenberg sulfide mines, Sweden

The Lilla Bredsjön tailings contain less than 30% residual sulfide minerals, and an average initial content of 4% calcium carbonate. The tailings are divided into an active oxidation horizon near the surface of the tailings impoudment, an underlying intermediate horizon and a water-saturated horizon. Extensive oxidation of sulfide minerals near the tailings surface results in depletion of carbonate minerals and low pore solution pH. Following the increase of carbonate content, the leachates pH rises up to 6.5–7 at the bottom of the active oxidation horizon. The neutral conditions favour a rapid precipitation of Fe oxyhydroxides, and major removal of other heavy metals from the leachates. Fe-precipitates fill tailings integrain pores and cement tailings, modifying the permeability of the tailings matrix. As a result, Fe-precipitated layers play an important role in the formation of a barrier which restricts O2 diffusion through the tailings, and in acting as an accumulation zone to retain heavy metals in the tailings. In addition, the shallow occurrence of the ground water level in the tailings may also contribute to the formation of the O2 barrier. The occurrence of the barrier results in a considerable reduction of sulfide oxidation and of Fe, Cu, Zn, and SO2 concentrations of the leachates below the Fe precipitated layers. Due to intensive weathering, clay minerals are formed in the active oxidation horizon and heavy metals are retained by clays through adsorption. The geochemical, mineralogical and geohydrological data can help to develop remedial strategies for arresting the acid mine drainage and the attendant hazardous environmental impacts.