Heavy Metal Behavior During The Oxidation Of The Fankou Pb-Zn Mine Tailings In Guangdong Province Of China: Implications For Environmental Remediation At Mining Impacted Sites

The Fankou Pb-Zn ore field in Guangdong province is the largest Pb-Znproduction base of China, and accumulation of large volume of the sulfide-rich Pb-Znmine tailings has been a major environmental concern. The distribution and chemicalforms of heavy metals in newly released tailings and in the profile of mine tailingsimpoundment No. 1 were studied based on mineralogical and chemical analyses aswell as sequential extraction. The sulfide-ore contains tens of percent carbonates andsignificant amount of quartz as gangue minerals. In the newly released mine tailings,~ 30 % is pyrite, other sulfide minerals include sphalerite, galena and chalcopyriteand arsenopyrite, with Pb, Zn and Cu concentrations being ~ 0.6%, -1.2 %, and-0.03%, respectively. However, variations in the bulk composition, the concentrationand chemical forms of heavy metals were observed in the profile of mine tailingsimpoundment studied. In the ochre colored oxidation zone, which is developeduppermost the impoundment, almost all the sulfide minerals have been oxidized toform ferric oxides and hydroxides as well as other secondary minerals, most of whichare poorly crystallized; gypsum-like phase content is more than 30 % at the expenseof carbonates, and the strongly oxidized tailings has been hardened due to thecementation by secondary phases. The oxidation degree and, thus, the content ofsecondary phases decrease gradually downward. 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...