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abstract 164

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

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:

Poster:

Session: OS1m Water quality 1

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