Products of oxidative dissolution on waste rock dumps at the Pb-Zn Rudnik mine in Serbia and their possible effects on the environment

Waste material from two active and two abandoned waste rock dumps from the Pb-Zn Rudnik Mine (central Serbia) was studied by optical, scaning microscopy, and X-ray powder diffraction methods. The weathering minerals in the active dumps are represented by anglesite, Fe-sulphates and Fe-oxyhydroxides. In the abandoned dumps they appear as a Pb-Cu group (beaverite, brochantite +/- Fe-sulphoarsenates) and a Zn-Fe group (boyleite, Zn-melanterite, and hydronium jarosite) of minerals. On the basis of textural and compositional evidence a schematic model of oxidative dissolution of primary sulphides and precipitation of byproducts is proposed. The model includes an initial stage dominated by oxidation of pyrrhotite, which is subsequently followed by decomposition of galena, sphalerite, arsenopyrite and chalcopyrite. The environmental impact of the weathering phases is assessed by analyzing the concentration of toxic elements in water that is directly linked to the waste rock dumps. The results show that the arsenic content in the river water increases up to ten times after the inflow of the waste water. One of the reasons is that soluble As-bearing phases (i.e., Fe-sulphoarsenates) were found in the surrounding waste material. By contrast, because of the presence of almost insoluble weathering phases, such as beaverite and anglesite, the concentrations of lead and copper in the studied aquaeous samples are relatively low. We argue that careful studying of oxidative dissolution of sulphides and formation of particular weathering phases can be of large significance in assessing the environmental impact of waste rock dumps associated to mining activities.