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Hydrogeochemistry and balance between minerals and solutions in the mercurial zone of Azzaba, N-E Algeria

Congress: 2008
Author(s): Moussa Benhamza(1) , Nacer Kherici(1) , Francoise Picard- Bonnaud(2)
(1)University Badji Mokhtar, Faculty of Science of the Earth - Department of Geology - B.P 12, Annaba Algerie Tel., Fax: 038.87.65.62 - E mall: moussa_benhamza@yahoo.fr (2) Laboratory of Sciences of the Environment and Amenagement, Faculty of Science, Univers

Keyword(s): water, metallic micropolluants, pollution, simulation, model PhreeqC
Article:
AbstractThe mercury ore deposit of Azzaba is located on the northern flank of the Limestone Chain, south of Azzaba town, North East Algeria. The Northern numidian zone is divided in two parts of different litho chemical composition. The Northern part is made of sandstone and clays of Oligocene age , the Southern part is formed by limestones, calcareous micro breccia and carbonated sandstones of the Kabyle ridge (calcareous mountain range). The different indices of mineralization which outcrop in the kabyle ridge are classified as polymetallics (Pb, Zn, Cu) and mercurial polymetallics (Hg, Pb, Zn, Cu). Clays and sandstones of the Oligocene present a Hg, Pb, Zn geochemical background contents of respectively 0.6 ppm, 30 ppm, and 60 ppm ; on the other hand, the Copper content is relatively high (80 ppm). As for limestones and carbonated sandstones of the ridge, the Hg background contents range from 0.2 to 1.5 ppm ; the Cu, Pb and Zn contents are high as well. In this region, two aquifers were recognized. The Oligocene phreatic aquifer is composed of fissured sandstone alternating with argillaceous layers whereas the deep aquifer is of Paleocene-Eocene age and is made of sandstone, limestone and conglomerate-breccia. The two aquifers are separated by impermeable clay, microbreccia and limestone of Palaeozoic and Lutetian-Priabonian. The latter is the host of the cinnabar mineralization. The results of the analyses of nine drilling wells and two springs of both aquifers show three main facies: the bicarbonated calcic, the sulphated calcic and the chlorided magnesian facies The contamination of the Oligocene aquifer by Hg and Pb is much higher than that of the Paleocne-Eocene. Thermodynamic interpretation using the simulation of the concentration of the major ions, under the effect of isothermal evaporation (25 C) by the thermodynamic software "PhreeqC" in terms of balances salt/solution, shows that water of the area is under saturated compared to halite, leading to its dissolution and the enrichment of water by Na of Cl. Water is in balance and in some locations is in super saturation compared to the gypsum and anhydrite, which causes the precipitation of these evaporitic minerals. The carbonated minerals are supersaturated and tend to precipitate as calcite, aragonite and mainly as dolomite. Water of the area is under saturated with Pb(OH)2 and PbSO4 , which causes the dissolution of these minerals, resulting in a Pb enrichment . Water is supersaturated with PbCO3, which, in the zones of oxidation, causes a precipitation as PbCO3. Water is supersaturated compared to ZnCO3 and Zn(OH)2, the consequence of which is the enrichment of water by Zn following the dissolution of these minerals.
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