Mineralogy and chemical forms of lead and zinc in abandoned mine wastes and soils: An example from Morocco

Abstract

Chemical extractions coupled with quantitative X-ray powder diffraction (XRPD) were used to define the chemical and mineralogical forms of Pb and Zn in abandoned wastes and soils from the Upper Moulouya mining district (Morocco). The aim was to provide baseline data required to assess metal mobility and bioavailability. Wastes and soils were sampled inside the mine sites of Zeïda, Mibladen and Aouli, both in exploitation and processing areas. Additional potentially unaffected soil samples were taken outside the Mibladen site. pH of wastes and soils is alkaline as a consequence of carbonate abundance (on average 36%). Total Pb and Zn concentrations have a wide spread of values (Pb: 0.041–17.25gkg−1; Zn: 0.051–276.5gkg−1), with tailings from all mines and soils from Mibladen processing area exhibiting the highest concentrations. Very low or no detectable contamination characterizes the soils from exploitation areas and those collected outside Mibladen. Zinc contamination is mainly restricted to Mibladen processing area, where Zn ores from other Moroccan mines were possibly processed. The sequential extraction procedure for metal fractionation indicates that in contaminated samples Pb and Zn are mainly present in the acetic acid extractable fraction, likely as carbonates, (Pb up to 80%; Zn up to 52%), while in less or not contaminated soils both metals are mostly associated with the reducible fraction, presumably as iron oxides (Pb up to 68%; Zn up to 80%). Eight minerals containing Pb and Zn were identified: cerussite, anglesite, galena, hydrozincite, smithsonite, sphalerite, willemite and hemimorphite. Cerussite is the most important Pb-host. Hemimorphite and smithsonite account for most Zn. According to the alkaline conditions and to the low solubility of Pb and Zn mineral phases, it can be suggested that within the studied environment mobilization into solution in aqueous systems and bioavailability of Pb and Zn have a low potential. Nevertheless, given aridity and strong winds, inhalation of airborne particulates may be a concern.