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Néogenèses silico-alumineuses en contexte cryptokarstique : l'halloysite de Beez (Namur, Belgique) et de Aïn Khamouda (Kasserine, Tunisie)

Abstract : The cryptokarsts from Beez (Namur, Belgium) were settled in dolomitic Visean limestones, in which vertical Fe-Pb-Zn sulphides veins play an important role as karstic drains. The sedimentary cover is made up of Viseo-Namurian siliceous shales and Oligocene sands. The cryptokarsts from Khamouda (Kasserine, Tunisia) were found in Senonian limestones (Douleb Formation). They expand from a down fault, which has brought limestones into contact with Miocene sands (Béglia Formation), following the sub-horizontal stratification. In both sites, complex paragenesis settled at the limestone/karst-filling interface. White clays, principaly composed of Si-Al and Al phases, and ferruginous crusts are the main paragenesis. In Beez, the white clays are made up of halloysite and gibbsite, while in Khamouda, they are enriched with uncommon zinciferous phases as sauconite (Zn-smectite), a 7Å-phyllosilicate and an amorphous Zn­hydroxide. Sulphates have also been found, as imprints of gypsum crystals in both sites, and as jarosite in Beez. Acid fluids percolated in the overlying sedimentary cover (pH~2 in Beez and pH~4 in Khamouda). The acidity is due to the oxidation of some sulphides (pyrite in Beez; pyrite and sphalerite in Khamouda). Cryptokarsts basically play an important role in chemical elements mobilization and trapping processes. In Beez, Si and Al have mainly been mobilized. Fe, Mn and the Rare Earth Elements (REE) have been mobilized too. In Khamouda, the main mobilized elements are Si, Al and Zn. Fe, Pb and REE have been mobilized too. The acid fluids are neutralized at the limestone karst-wall. It leads first to the neogenesis of sulphates (as gypsum and jarosite) and iron oxi-hydroxides (pH < 4,8). Then, halloysite and Al-oxi-hydroxides are formed (from pH~4,8-5,4 upward). In Khamouda, zinciferous phases developed lately (from pH~9,5 upward). In both studied systems, we clearly show the development of mineral phases from a Si-Al gel to well-crystallized minerals, as halloysite, or badly organized minerals, as some Mn-oxides from Beez. In both deposits, tubular halloysite, which usually develops in cracks, is prominent in comparison to spheroidal halloysite, which usually growths in gel masses. It suggests an extreme fracturing of the gel masses. We established that these gel masses mix cations from the percolating solutions. The main difficulty in cryptokarstic environments is to determine ages of weathering processes, nevertheless the age of overlying deposits give an idea. Thus, the major weathering stage in Beez is post-Oligocene (followed by a Quaternary reactivation), while weathering in Khamouda is post-Miocene. The cryptokarsts from Beez contain several mineral phases, which could be dated with radiometric methods. This "absolute" dating has to be the next step.
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https://tel.archives-ouvertes.fr/tel-00006417
Contributor : Delphine Bruyère <>
Submitted on : Thursday, July 8, 2004 - 10:58:23 AM
Last modification on : Wednesday, November 6, 2019 - 1:26:15 AM
Long-term archiving on: : Friday, April 2, 2010 - 8:30:09 PM

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  • HAL Id : tel-00006417, version 1

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Delphine Bruyère. Néogenèses silico-alumineuses en contexte cryptokarstique : l'halloysite de Beez (Namur, Belgique) et de Aïn Khamouda (Kasserine, Tunisie). Géochimie. Université Paris Sud - Paris XI, 2004. Français. ⟨tel-00006417⟩

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