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Bouabdellah, Mohamed
Collection EDYTEM. Cahiers de géographie, 2009, Volume: 9, Issue: 1Journal Article
Les minéralisations Pb-Zn de type Mississippi Valley du district de Touissit-Bou Beker (Maroc Nord oriental) se présentent sous forme de remplissage d'un réseau interconnecté de paléo-karsts hydrothermaux et de brèches d'effondrement. Les études géologiques montrent que les dolomies encaissantes ont subi les effets de trois phases de dissolution, dont une phase majeure reliée à la circulation des fluides minéralisateurs. Les structures de dissolution qui en résultent se distinguent les unes des autres aussi bien par leur distribution spatiale que par la nature de leurs produits de remplissage. Les paléo-dolines relevant de la première phase de dissolution présentent des shales de teinte vert noirâtre interprétés comme des paléosols. En revanche, les structures issues de la phase de dissolution majeure montrent des produits de remplissage constitués de dolomite en selle, de sulfures et de pyrobitumes, reliés à la circulation des hydrothermaux (saumures bassinales) libérés au cours de l 'enfouissement profond. Enfin, la troisième phase de dissolution a conduit à l 'enrichissement du minerai primaire et au développement des karsts verticaux modernes (mérokarsts) et aux zones de collapse. Mississippi Valley-type lead-zinc orebodies at Touissit-Bou Beker district of northeastern Morocco are hosted by paleokarst features in a Middle Jurassic (Aalenobajocian) carbonate platform. The ore occurs mainly as open-space fillings of karsts and collapse breccias. Geological investigations coupled to core sample examinations show that the carbonate platform were subjected to three main stages of dissolution and brecciation ; one of which (stage II) is related to the Pb-Zn mineralization. The resulting dissolution structures are distinguished from each other on the basis of their spatial distribution and the nature of their infilling materials. The spatial distribution of sinkholes, whose formation are related to stage I, are interpreted to have been formed by the downward circulation of meteoric waters during the exposure of the carbonate platform at the end of the Aaleno-Bajocian time. The created voids are commonly filled with green to black shale interpreted as paleo soils. In contrast, the stage II structures, commonly fdled with saddle dolomite, sulphides, and pyrobitumen, form a paleokarst solution network. The observations suggest that these structures are linked to the ore-forming hydrothermal fluids that circulated during deep burial. Later recent kartification events have produced the vertical modern continental karsts (merokarsts) and collapse-zones that enriched the primary ore.
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