Résumé: Pb–Zn–Ba–Sr–F mineralization hosted by the Albian–Aptian carbonates of Hameimat North and Hameimat South massifs is an example of many peridiapiric deposits of the ‘Diapir Zone’ of the Eastern Saharan Atlas, Algeria. In this area, the mineralization develops either at the contact between the Triassic diapiric evaporite bodies and the Creatceous sedimentary cover or along the tectonic faults and karstic cavities within these Albian–Aptian often reef limestones. At Hameimat massifs, the mineralization consists of abundant barite and galena, and minor sphalerite, celestine and fluorite. Gangue minerals are composed of calcite, dolomite and rare quartz. Stable S, C and O isotope analyses were caried out on sulphur-bearing phases and gangue carbonates to shed more light on the genesis and the nature of the mineralizing fluids. Sulphur isotopic data from the Triassic gypsum (δ³⁴SV−CDT), which range from + 13.4 to + 15.1‰, are similar to those obtained from regional Triassic evaporites, and are consistent with the values of Triassic seawater sulphates worldwide. Sulphur of the barite and celestine minerals display higher δ³⁴SV−CDT values, varying between + 18 and + 24‰, whereas sulphide phases (galena and sphalerite) exhibit lower δ³⁴S values (between − 2.2 and + 10.4‰) than those of the Triassic sulphates. This most likely points to the derivation of sulphide-derived sulphur from Triassic sulphates through thermochemical sulphate reduction, a process supported by existing fluid inclusion data. The C and O isotopic compositions of the host carbonates (δ¹³CV−PDB = + 0.2 to + 4.7‰; δ¹⁸OV−SMOW = + 20.6 to + 23.1‰) reflect an inorganic carbon source and oxygen derived from fluid-rock interactions. Combined isotopic and metallographic evidence indicates that the ore-forming fluids originated from deep-seated brines. The ore isotopic signatures resemble those obtained on the Mississippi Valley-type deposits.

Résumé: In northeast of Algeria, the stratigraphical features and rudist descriptions from the Turonian succession of the Gaaga Formation are studied in detail along a 475 m stratigraphical section at Djebel Essen. The Gaaga Formation that is divided into lower and upper Turonian by a significant hardground consists of shallow-water carbonates. The lower Turonian sequence, of 255 m thick, is composed of bioclastic limestone and marlstone with rare miliolids and inoceramid genus Mytiloides, suggesting an early Turonian age and a restricted lagoonal environment with moderate hydrodynamic conditions. The upper Turonian sequence of 220 m thick is characterised by rudist-bearing limestone, dolomitic and bioclastic limestone, devoid of other invertebrate fossils, indicating an inner carbonate platform with moderate storm conditions. Five rudist taxa are first described in Djebel Essen, belonging to the Hippuritidae and Radiolitidae, and suggest a late Turonian age based on the stratigraphical range and biogeographic distribution of these species, previously known in the Mediterranean Tethys. Three main rudist associations are vertically distinguished according to the rudist content and morphotypes: (i) the Vaccinites rousseli association with this species and rare Hippurites resectus, creating a lenticular thicket; (ii) the Durania arnaudi association with this species and rare Sauvagesia sp., and nerinacean gastropods, showing isolated individuals and clusters; and (iii) the Biradiolites lombricalis association with a monospecific lateral clinger of this species. The top of the formation consists of marly facies with Durania sp. These data bring significant reference information for the re-examination of previously identified Turonian limestones in northeastern Algeria.

Résumé: The El Aouana igneous rocks are part of the Miocene magmatic suite that extends from Northern Tunisia to Morocco through the Algerian coast in the Maghrebin chain. These rocks are composed of volcanic (andesites and dacites) and subvolcanic (micodiorites and microgranodiorites) lithologies that intruded both the Cretaceous and Oligo-Miocene nappes, and the Miocene post-nappe sediments. The andesites are composed of plagioclase, amphibole and pyroxene phenocrysts that are set in a microlithic groundmass. The dacites are plagioclase-rich and rare, highly altered ferromagnesian minerals. The microdiorites and microgranodiorites are hornblende-bea­ring rocks with plagioclase, pyroxene and rare biotite phenocrysts that are set in a microcrystalline groundmass. Geochemical observations show that the rocks are calc-alkaline with I-type affinity. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) compared to high field strength elements (HFSE) and heavy rare earth elements (HREE). The negative Nb, P and Ti anomalies observed on the multi-element patterns are imprints of magmas originated from subduction zones. Field, petrological and geochemical investi­gations show that the El Aouana Miocene igneous rocks are emplaced in a post-collisionnal setting. These rocks show similarities with the metaluminous, post-collisionnel granitoids of north-eastern Algeria which are thought to have been derived from a metasomatized mantle source as a consequence of ‘slab break-off’ underneath the North African margin.

Résumé: Abstract The Hameimat North and South massifs, located in the peri-diapiric saline province of the Eastern Saharan Atlas, host polymetallic Zn–Pb–F–Ba–Sr mineralizations. These mineral concentrations develop either directly above the Triassic evaporitic bodies or within the overlying Albian–Aptian sedimentary cover. The paragenetic assemblage is more diverse in the Hameimat North massif, reflecting the greater extent and proximity of Triassic outcrops. Sulfur isotopic values obtained from Triassic gypsum (δ³⁴SCDT = +13.4 to +15.1‰, n=3) are consistent with the Triassic seawater sulphates and those of regional Triassic evaporites. Sulfate minerals (barite and celestine) display relatively high δ³⁴SCDT values (+18 to +24‰, n=6), consistent with a Triassic evaporitic sulphate source. In contrast, sulphide minerals exhibit lower δ³⁴SCDT values (−2.2 to +10.4‰, n=22), indicating that sulphide sulphur was derived from Triassic sulphates through thermochemical sulphate reduction (TSR), a process supported by fluid inclusion data, show hot fluids, with homogenization temperatures around 115°C. The carbon and oxygen isotopic compositions of the host carbonates (δ¹³CPDB= +0.2 to +4.7‰; δ¹⁸OSMOW= +20.6 to +23.1‰) reflect an inorganic carbon source and oxygen derived from the leaching of Cretaceous carbonates, implying significant fluid–rock interactions. Combined isotopic and metallographic evidence indicates that the ore-forming fluids were mainly of basinal origin, having circulated through Triassic evaporites and Cretaceous carbonates. Their isotopic signatures closely resemble those of Mississippi Valley-type (MVT) deposits, highlighting the major role of Triassic diapirism and fluid–carbonate interactions in the genesis of these polymetallic mineralizations.