Contribution to the sedimentology of the Messinian carbonates of the Chelif Basin (Boukadir, Algeria)

The Messinian (Upper Miocene) is characterized at the level of its marginal basins by the development of numerous carbonate platforms. This study concerns the Messinian platform of the Boukadir region in the south of the Chelif Basin in Algeria. It is composed of a lower prograding rimmed platform and an upper aggradational homoclinal ramp resting upon the Tortonian-Lower Messinian Blue Marl Formation, and its thickness reaches ~280 m in the Chelif Basin. The upper red-algae unit is uniform and subhorizontal with a minimum thickness of 90 m. Petrographic analysis of the upper ramp reveals three different microfacies, characterized by Lithothamnium, foraminifera, high porosity and a microsparitic matrix. MF1 is a packstone, MF2 a packstone/bindstone deposited above the fair weather wave base and MF3 is a wackestone to packstone deposited below this level. The upper unit is made up entirely of autochthonous biogenic elements without significant external fluvial contribution. It was formed in a shallow marine environment, with very high productivity and a significant export of the sediments produced. This aggradation was followed by a rapid exhumation (regression) transforming all the aragonite into calcite. The platforms correspond to the T2 complex reef formation (6.7-5.95 Ma) documented on the other Messinian carbonate platforms in the South of the Alboran Sea (Cornée et al, 2004) that formed just before the Messinian Salinity Crisis

Fluid inclusion and stable isotope studies of the Mesloula Pb-Zn-Ba ore deposit, NE Algeria: Characteristics and origin of the mineralizing fluids

In the Saharan Atlas (NE Algeria), the Triassic evaporitic formation was brought to the surface through the thick Cretaceous and Tertiary sedimentary cover as diapirs due to the effect of Atlasic tectonic events. The diapir piercing began in the Jurassic and has continued through present day. Many outcrops of several square kilometres are distributed in a large area (approximately 80 km wide) that extends northeasterly over 300 km towards Tunisia. The diapiric evaporitic formation is often accompanied by the emplacement of Pb-Zn-Ba-F mineralization. The Mesloula massif is an example of these deposits. Fluid inclusion and sulphur, carbon and oxygen isotope studies were carried out on Pb-Zn-Ba mineralization and associated gangue carbonates. Gypsum of the Triassic formation was also analysed for its sulphur isotope composition to show the role of evaporates in the generation of this typical peridiapiric deposit. Gypsum from the Triassic formation showed a narrow range of δ34SVCDT values, ranging from +14.6 to +15.5‰ (n = 8). This range is comparable to that of Triassic seawater sulphates. Sulphide minerals yielded δ34SVCDT values between 0 and + 11.7‰ (n = 15), indicating that sulphide sulphur was likely derived from Triassic sulphates through thermochemical sulphate reduction (TSR) because fluid inclusion microthermometric measurements yielded a mean temperature of 150 °C. Residual sulphate in such a system would have been enriched in 34S; this is reflected in the barite δ34SVCDT values, which range from +21.1 to +33.5‰ (n = 5). The δ13CVPDB values of calcite minerals, ranging from +2.1 to +6.3‰ (n = 4), indicate an inorganic carbon origin, likely from the host carbonate rocks. δ18OVSMOW values were between +21.9 and + 24.9‰, indicating that the most likely source of mineralizing fluids was formation water

Soil metal pollution from former Zn–Pb mining assessed by geochemical and magnetic investigations: case study of the Bou Caid area (Tissemsilt, Algeria)

International audienceAbstractFormer zinc and lead mines that have beenoperating for half a century are located in the massif of BouCaid (Tissemsilt, Algeria). Hazardous heavy metals emit-ted from the mines are abundant in the surrounding soil andcause strong metal pollution in the region. This paperinvestigates the extent of lead and zinc mine activityderived pollution by characterizing both magnetic andgeochemical properties of samples collected in the vicinityof the mines. The results of the magnetic study show thecoexistence of magnetic minerals such as magnetite,hematite and goethite. Analyses on surface soils andweathered rocks suggest that hematite and goethite haveore-related lithogenic origins. Magnetic susceptibilityshows a positive correlation with lead content when presentin low-to-medium concentrations (\*500 mg/kg). Athigher lead concentrations, there is no correlation withmagnetic susceptibility. The relationship between magneticsusceptibility and zinc content is not straightforward. Theseobservations are explained by the higher affinity of Pb toiron oxides at lower pollution levels and their preferentialbonding to carbonates when Pb and Zn contents areextremely high, as demonstrated by Iavazzo et al. (J Geo-chem Explor 113:56–67,2012) in a study of former Zn–Pbmine in Morocco. Based on the general features of thespatial maps of field-measured magnetic susceptibility,mass-specific magnetic susceptibility, Pb and Zn contents,it is concluded that field magnetic measurements provide agood qualitative proxy of pollution spread out of themining galleries, while laboratory measurements afford amore detailed investigation of the links between iron oxi-des and the main heavy metals in the ore