The Oued Mellègue: Mining activity, stream sediments and dispersion of base metals in natural environments, North-western Tunisia

• Published in: Journal of geochemical exploration 2009-07, Vol.102 (1), p.27-36
• Main Authors: Mlayah, A., Ferreira da Silva, E., Rocha, F., Hamza, Ch. Ben, Charef, A., Noronha, F.
• Format: Article
• Language: English
• Subjects: Mining; Oued Mellègue; Principal Component Analysis; Stream contamination; Tailings; Trace metals
• ISSN: 0375-6742; 1879-1689
• DOI: 10.1016/j.gexplo.2008.11.016

In the Northwest of Tunisia, mining works have occurred in an area of vital water reserves and resources (Oued Mellègue and Oued Mejerda), which represent nearly 81% of total potential of water in the region. Demographic growth, insufficient drinkable water, agriculture and industrial plants are factors responsible for the strong demand of this resource. Water supply is done by an interconnected battery of dams built on the major watercourses, the first of which, the Mellègue dam, was erected in the 1960s. Nowadays, most of the mines are closed for almost two decades although one is still active: (Jerissa for Fe). It is important to emphasise the fact that there are millions of cubic metres of abandoned tailings, mainly from Pb–Zn–Ba mines, which are not surveyed, representing a serious threat to the environment. Exposed superficial mineralised tailings, weathering of sulphide minerals, and the presence of siderite and phosphorites as well, accelerate the oxidation and generate leaching solutions rich in toxic base-metals (Cd, Pb, Hg, Zn, U, among others, are the most important). Trace element analyses performed on tailings and stream-sediments samples and the use of “Principal Component Analysis” statistical method, helped to understand the processes of toxic metals circulation in the natural environment. It showed that the major and trace metals fate and transfer from contamination sources into the river systems are based on three main factors: i) a mineralogical component which is representative of regional rocks (shales, siliciclastics, limestones, and dolostones), ii) a residual assemblage concerning the ore mineral micro phases (phosphorites and common and complex sulphides of Pb and Zn) and iii) hydrated iron oxyhydroxide components representing the complex geochemical processes taking place, which are extremely active in scavenging toxic metals in either weathered tailing profiles or continuously reworked stream depositional systems. In addition, the superficial lime fraction and the Triassic-derived gypsum and halite could play a significant role in precipitating and thus fixing heavy metals in the proximity of various sources of contamination defined by tailing areas which are constantly being leached.