Long-term effects of vegetation cover on the rehabilitation of lead/zinc mine tailings

Pb/Zn sulfide ore is extracted from carbonate host rock by milling to grainsizes <120 μm and separation by flotation. This produces large volumes of near neutral pH tailings that must be carefully managed due to regulatory concern about the residual Pb and Zn concentrations (up to ∼0.3 % by wt. of each) and trace concentrations of other potentially toxic elements. To prevent dust formation, vegetation cover is established on inactive areas of the tailings management facility (TMF). This paper reports the changes in the chemical and microbiological composition of the tailings as a function of both time and depth. Over the course of eight years, there is progressive oxidation of pyrite in the tailings, and accumulation of soil organic matter in the surface layers. The mobility of most potentially toxic elements is reduced due to sorption to ferric oxyhydroxides formed as a result of pyrite oxidation, although Cu is more mobile in surface layers probably due to formation of dissolved organic carbon complexes. The microbial community diversity in the surface layer increased with the age of the tailings and was similar to natural calcareous soil after 8 years. At this age the functional profile (functional diversity) of the community was similar to that of natural calcareous soil, despite differences remaining in taxonomic composition. Development of soil-like properties in the surface layer, such as increased soil organic matter content and a soil-like microbial community, suggest that the vegetation cover will be self-sustaining. Therefore, revegetation is a viable option for TMF closure planning.