Abundant occurrence of Aspidiscus cristatus (Lamarck, 1801) in the "Marnes de Smail" Formation from the Batna area (NE-Algeria): index species for the Middle Cenomanian

Les marnes cénomaniennes de la région de Batna (nord-est de l'Algérie) montrent plusieurs niveaux fossilifères, à faune très diversifiée. Parmi les fossiles présents en abondance et en bon état de conservation figure Aspidiscus cristatus (Lamarck, 1801). Cette espèce de polypier a été rattachée à la famille des Latomeandridae Alloiteau, 1952, en raison de la présence de pennulae et de perforations concentrées sur le bord interne des septes. Dans les coupes étudiées, Aspidiscus cristatus se rencontre dans des niveaux allant de la Zone à Cunningtoniceras inerme à la Zone à Acanthoceras amphibolum, avec une relative abondance dans la Zone à Acanthoceras cf. rhotomagense. Cette espèce est, par conséquent, un bon marqueur du Cénomanien moyen de la région de Batna. Nous donnons ici une description paléontologique et discutons des cadres environnemental et paléogéographique de cette espèce. En effet, la morphologie externe de son test, convexe sur la face supérieure, plane à concave sur la face inférieure, évoque une adaptation à des environnements marins de faible énergie, de substrat vaseux et au taux de sédimentation élevé. La répartition paléogéographique de cette espèce, avec des spécimens de taille souvent plus grande sur les marges sud et est du domaine téthysien que sur la marge nord, témoigne d'un gradient thermique et d'une zonation climatique spécifiques, avec des eaux relativement plus chaudes sur la marge sud-téthysienne au Cénomanien, favorisant une bio-érosion beaucoup plus fréquente et plus poussée des spécimens fossiles des régions sud-téthysiennes que celle affectant les spécimens des régions septentrionales de la Téthys.The Cenomanian marls of the Batna region (NE Algeria) display several fossiliferous beds with highly diversified fauna. Among the abundant and well-preserved fossils stands out Aspidiscus cristatus (Lamarck, 1801). This coral taxon is assigned to the Family Latomeandridae Alloiteau, 1952, due to the presence of pennulae and inner septal margins with slight perforations. In the studied sections Aspidiscus cristatus is found in beds ranging from the Cunningtoniceras inerme Zone to the Acanthoceras amphibolum Zone, with a relative abundance within the Acanthoceras cf. rhotomagense Zone. This species is subsequently a good marker for the Middle Cenomanian in the Batna district. We give herein a systematic and paleontological description and discuss the paleo-environmental and paleo-geographic framework of this taxon. Actually the outer morphology of its test, with convex outer surface and planar to concave inner surface, suggests an adaptation to mud-supported, low-energy marine environments with high sedimentary rate. The paleo-geographic range of this taxon, with often larger-size specimens on the southern and eastern margins of the Tethyan Realm, than those on the northern margin, supports very peculiar thermal gradient and climactic zone, with relatively warmer waters on the southern Tethyan margin during the Cenomanian, favoring a more frequent and more effective bio-erosion on fossil specimens from southern Tethyan areas than that affecting specimens from northern Tethyan areas

Origin of the Tala Hamza Igneous Rocks-Hosted Zinc-Lead Deposit, NE Algeria

The Miocene igneous rocks that host the Tala Hamza Zn–Pb deposit are investigated in order to shed light on their petrogenesis and to provide a brief metallogenic description of associated sulphides mineralization. 800 m depth drill hole reveals from top to bottom the following succession: pyroclasitic rocks, andesite, volcanic tuff, microgranite, silicified tuff, kaolinized andesite cross-cut by aplite, metasomatized granodiorite and granodiorite. These rocks are composed of plagioclase, hornblende, biotite, alkali feldspar, sphene, apatite and zircon. Major elements geochemistry shows that these igneous rocks exhibit calc-alkaline, high K to shoshonitic, metaluminous to slightly peraluminous, I-type characters. Multi-elemental spectra, as well as chondrite-normalized rare earth spectra, show LILE and LREE enrichment relative to HFSE and HREE, respectively. The REE shows a slight negative anomaly in Eu. All these features indicate that the rocks originate from an igneous protolith, probably from a metasomatized mantle material that was contaminated by crustal material and emplaced in a post-collision context during the Miocene. Metallogenic study for both massive sulphides and stockworks reveals the presence of replacement textures, open-space-filling texture and pseudomorphism. These textural patterns, in addition to the presence of the anhydrite layer, are common in deposits of VHMS (volcanic-hosted massive sulphides) group formed in the submarine environment. They also indicate hydrothermal processes throughout two main stages: (1) an early stage in which disseminated pyrite and chalcopyrite hosted by metasomatized granodiorite were formed, and (2) a late-stage or mineralizing stage with the deposition of economic ore (mainly sphalerite and rare galena) along faults and fractures

Preliminary data of REE in Algerian phosphorites: a comparative study and paleo-redox insights

This study deals with the preliminary data of rare earth elements (REE) obtained on northeastern Algerian phosphorites from the Tébessa region. These phosphorites are located in two different basins: the northern basin represented by Dj. El Kouif, Dj. Dyr and Tazbant showings and the southern basin with the giant Dj. Onk phosphate deposit. The host sedimentary formation is Late Paleocene to Early Eocene. Twenty-six (26) samples from the four (4) localities were collected and analyzed for their REE contents using the ICP-MS technique. Phosphates from the southern basin (Dj. Onk deposit) show ∑REE contents ranging from 174.41 and 906.39 ppm (average ∑REE = 623.01 ppm), while the northern phosphorites have lower ∑REE contents (from 125.45 to 472.44 ppm; average = 265.57 ppm). PAAS-normalized REE patterns and binary Box plot of (Sm/Pr)N vs (Sm/Yb)N show HREE enrichments for samples from the northern localities while most samples from the southern basin are HREE depleted. Normalized (La/Yb)N vs (La/Sm)N plot shows that (La/Sm)N ratios of all samples are similar to those of modern seawater (from 0.83 to 1.55). However, the (La/Yb)N ratios of the two basins are significantly higher (from 0.67 to 1.18), which indicates an early diagenesis. The Ce/Ce* vs Pr/Pr* diagram shows that the observed Ce anomaly was not affected by diagenesis and more likely represents a proxy for redox conditions. The obtained results substantiate that the northern phosphorites were formed in more oxic environment with more pronounced negative Ce anomalies, whereas the southern phosphorites have lower Ce anomalies. Northern phosphorites are different from those from the south probably because the northern basin was more connected to an open sea as did the Sra Ouartan basin in northern Tunisia. These results have also been confirmed by statistical method studies, such as factorial discriminate analysis

The oceanic anoxic event 2 at Es Souabaa (Tebessa, NE Algeria): bio-events and stable isotope study

At the southern margin of the Tethys, the Es Souabaa area recorded traces of Oceanic Anoxic Event 2 (OAE2) around the Cenomanian-Turonian boundary (C/Tb). The dark, laminated, filament- and pyrite-bearing limestones represent the typical facies of this event. In terms of sedimentary environment, these features reflect a transgressive drowning that had induced hypoxia in these sedimentary environments. Such conditions favored the deposition and preservation of organic matter of marine origin, the distribution of which was controlled by paleogeography and halokinetic tectonics at that period. The OAE2 reached a climax between the last upper Cenomanian occurrence of Rotalipora cushmani and the lower Turonian occurrence of Whiteinella praehelvetica. Positive shift of the δ13C excursion along with relatively high total organic carbon (TOC) contents during OAE2 both indicate palaeo-environmental modifications enhanced by a significant change in primary marine productivity. Meanwhile, negative δ18O peaks in carbonates reflect increasing temperatures. Comparison of the data from this study with those from the neighboring Kalaat Senan section (Tunisia) suggests close similarities of events, although OAE2 is much more enhanced in Algeria

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

The Algerian Glauconite-bearing Phosphorites: REE Variation and Insights on the Depositional Environment

International audienceNew data on rare earth element (REE) analyses are reported here on northeastern Algerian glauconite-bearing phosphorites. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Hand-picked glauconite grains and other phosphate particles (pellets, coprolites, enamel and dentine of marine fish teeth) were embedded in epoxy resin and polished. They were imaged by SEM and major, trace and REE element contents were analyzed “in-situ” by Electron Microprobe and LA-ICP-MS techniques respectively. For comparison, bulk phosphorite samples were also analyzed by solution ICP-MS. The results show that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. These glauconite grains exhibit higher ∑REE contents than the co-existing particles (Kef Essenoun: min = 654 ppm, max = 1760, average = 1146 ppm; El Kouif: min = 543 ppm, max = 623 ppm, average = 584 ppm). The overall whole rock REE content also shows substantial differences between the two deposits. Therefore, whole-rock REE enrichment in the Kef Essenoun main sub-layer can be linked to the abundance of glauconite grains. PAAS normalized-REE patterns of the glauconite grains display signatures similar to those of co-existing particles and whole-rocks in each locality indicating a similar origin for the REE. It is noteworthy that phosphorites grains from the northern deposit show a REE uptake source from oxic-modern seawater, while those from Kef Essenoun deposit exhibit bell-shaped patterns with weak Ce anomaly suggesting a different environment of deposition (i.e. more reduced conditions). These geochemical results along with previous petrographic studies, also confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, i.e., winnowed, moved and re-deposited phosphorites as proposed by some authors

Molluscan and sedimentological sequences of the late Quaternary deposits of Morsott region (NE Algeria) and their paleoenvironmental implication

The late Quaternary sequences of north-eastern Algeria composed of alternating dark and light layers, which are rich in molluscan fauna, could correspond to synchronous environmental phases. The facies variations of the Morsott Region deposits were systematically sampled as part of a sedimentological and malacological study in order to establish a palaeoenvironmental and climatic reconstruction of the late Quaternary. The alternation of sandy clayey silts levels, which contain abundant molluscan hygrophilous organisms, indicates an isostasis cycle followed, at the top, by a succession of rhexistasis and biostasis cycles. The molluscan record shows a dominant succession of xerophilous organisms in relation to the hygrophilous organisms. The faunistic richness in the fine levels is linked to sub-humid periods where the region has suffered numerous floods. The reduction in the number of species in the coarse levels comes is likely due to desiccation when the climate was semi-arid. This study is new in Algeria and will provide an insight into climatic changes. Further research will be required in order to understand the fluvial dynamics during the Quaternary

Glauconite-bearing sedimentary phosphorites from the Tebessa region (eastern Algeria): Evidence of REE enrichment and geochemical constraints on their origin

International audienceRare earth element (REE) analyses are reported on glauconite-bearing phosphorites from northeastern Algeria. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Petrographic examination indicates that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. In both deposits, glauconite grains are richer in REEs than other co-existing particles (pellets, coprolites, enameloid and dentine of marine fish teeth), but the glauconites of the Kef Essenoun deposit exhibit significantly higher REE concentration (min = 654 ppm, max = 1760 ppm, average = 1146 ppm) than those of Djebel El Kouif deposit (min = 543 ppm, max = 623 ppm, average = 584 ppm). The whole-rock REE concentrations also show substantial differences between the two deposits and the REE enrichment in the Kef Essenoun main sub-layer is more likely the result of the high glauconite content. PAAS normalized-REE patterns of the glauconite grains display similar patterns between the northern and southern localities with weak negative Ce anomalies and slight middle REE enrichments. On the other hand, the pellets and whole phosphorite grains from the northern deposit indicate a REE source from oxic-suboxic seawater, whereas those from the Kef Essenoun deposit exhibit slight middle REE enriched patterns with weak negative Ce anomaly suggesting a different environment of deposition (i.e. a tendency to reduced conditions). These geochemical results along with previous petrographic studies confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, where winnowing, transport and re-deposition of previously deposited phosphorites resulted in high accumulation and formation of glauconite grains. The glauconitization process of fecal pellets happened in two stages and was controlled by semi-confined micro-environments. This can explain the abundance of glauconite grains in the phosphorites from Kef Essenoun deposit and their restricted occurrence in the basal levels of the Djebel El Kouif outcrop