Les formations évaporitiques triasiques des hautes plaines Sétifiennes (NE Algérien)

Abstract

tract The main purpose of this work was the characterization of minerals associated with evaporite Triassic formations of the Setifian High Plains. In the north eastern of Algeria and between the two barriers of mountains, Hodna in South and Babors in North, situated a vast plains, commonly known as the Setifian High Plains. This zone occupies the central part of the wilaya of Setif, and it's a part of the externe areas of the Alps, eastern Algeria, between the Tellian slicks in the North and the forward land autochton in the South. It is considered a closed continental basin, filled by Neogene and Quaternary deposits. The genesis of evaporite deposits caused by the lagoon system that prevailed in the Triassic era with an arid climate and a high degree of evaporation which led to the precipitation of ions, Ca ++ So3--, Na +, Cl-, giving salt deposits and gypsum. The spatial distribution, and the surface movement of the Triassic evaporite formations in the Setifian High Plains would probably caused by the phenomenon of overlapping slicks of alloctons to the south. We find five (5) evaporite formations dating from the Triassic; Guellal, Koudiat el Bassour, Methgoub Ras, Draa El Djebbas and Jebel el Garsa. The principal minerals related to Triassic outcrops are represented essentially by evaporite deposits (gypsum and rock salt) .The particularity is marked by the presence of metallic minerals (iron and copper) in the blocks associated with evaporites Koudiat El Bassour. The iron mineralization associated with copper, is hosted mainly in carbonate rocks, considered by the ancient writers as an infraliasiques formation. The morphology of the mineralized bodies is characterized essentially by a vein shape and stockworks type. The mineralogical characterization by XRD and metallographic microscopic observation shows that the mineralization is usually oxidized brown-black (hematite, goethite, limonite) developed on iron carbonate (siderite, ankerite) associated with a few sulphides (chalcopyrite covellite, pyrite) and copper carbonate (malachite). Gypsum deposits are widespread in all the Triassic outcrops of the region; it comes in four (4) forms (fibrous, crystalline, microcrystalline and grainy). The Gîtologics characters, the mineralogical characterization by XRD and chemical analysis by fluorescence X show that the gypsum is characterized by a variation of quality and quantity in an outcrops to another. As such, the economic importance gypsum deposit is observed in the Triassic of Guellal. Contrary to gypsum, rock salt (halite) is prevalent only in the depth of the Triassic outcrop in Guellal, and is forming a deposit of a great economic importance. Geological, gîtologic and chemical characteristics of the rock salt deposits are the source of economic interest. Geophysical, mining and hydrogeological works of research in the diapir, which resulted an exploitation of salts for human consumption and industrial purposes. The exploitation is more methodical, it is provided by the ENASel.