Meanwhile, the concentration of Pb,Zn and other heavy metals is rather high in the oxidation zone and decreasesdownward to levels that are unbelievably low for Pb-Zn mine tailings. Theseobservations suggest that heavy metals should be re-immobilized by secondaryminerals, rather than primary minerals, in the oxidation zone, and could be leachedout from suboxic environment due to low rate oxidative dissolution which preventsthe precipitation of secondary minerals in significant amount. Thus, cover scenarioswhich establish a suboxic environment beneath the cover layer might not always beeffective in preventing heavy metal release from mine tailings.Sequential extraction studies indicate that, in addition to heavy metal leaching,oxidation of the mine tailings increased the mobility of residual Zn, and Cu, while Pb remains immobile due to the formation of anglesite (PbSO4). Using the seven-step extraction technique developed by Dold (J. Geochem. Explor., 2003, 80: 55-68), galena dissolves in significant amount at step 2 most probably due to the strong complexing of Pb2+ by acetate, while other metal sulfides dissolve at step 6, which suggest that this extraction technique could not be used to evaluate the mobility of lead in natural environment

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