Characteristics and potential environmental influences of mine waste in the area of the closed Mežica Pb–Zn mine (Slovenia)

Abstract

About 19 million tons of Pb–Zn ore were excavated and processed during more than 300 years of mining and metallurgical activities in the Meža Valley, leaving behind large amounts of mine and ore processing wastes, which were deposited as spoil heaps in narrow valleys and on steep slopes of small streams and in abandoned mine shafts at about 30 locations. After the mine and processing facilities were closed in 1995, dumps of low-grade ore and ore processing wastes persisted as a source of potentially toxic elements in the environment. In the presented research, detailed chemical and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analyses of three of the most important mine waste deposits, the Štopar, Kavšak, and Fridrih waste deposits, were carried out with the purpose of determining the influence of mine waste deposits on high contents of potentially toxic elements in environmental media, determining modes of occurrence of metals in waste deposit material, and assessing the stability and mobility of toxic elements on the basis of solubility data of metal-bearing phases and minerals. Chemical analysis showed that the highest contents of Pb, Zn, and Cd occurred in the Štopar waste deposit and the highest contents of Mo, V, and As in the Fridrih waste deposit. SEM/EDS analysis revealed that Pb, Zn, V, and Mo in the waste deposit material occur mostly as primary ore minerals, such as cerussite, sphalerite, smithsonite, descloizite, and wulfenite. A considerable amount of Pb and Zn was found bound to secondary weathering products, such as Fe-oxy-hydroxides, Mn-oxides, Fe-oxy-hydroxy sulfates, and Al-silicates. Solubility of primary ore minerals is generally restricted to the pyrite weathering zones and consequently they are relatively stable in the carbonate environment of mine waste deposits. With the exception of Fe-oxy-hydroxy sulfates, which are considered unstable in high-pH environments, secondary weathering products are stable and thus represent sustainable trapping and stabilization media for Pb and Zn in the investigated mine waste deposits. However, mine waste material is being washed out into the nearby streams, where the changes in environmental conditions may cause leaching and mobilization of potentially toxic elements into water.