The Cu-Mo±Au Mineralizations Associated to the High-K Calc-Alkaline Granitoids from Tifnoute Valley (Siroua massif, Anti-Atlas, Morocco): an Arc-Type Porphyry in the Late Neoproterozoic Series

The Tifnoute valley Cu-Mo±Au mineral occurrences are located NW of Siroua massif in the central Moroccan Anti-Atlas. This mineralization appears to be associated to the Imourkhssan granite and the Asskaoun granodiorite dated respectively 561 ± 3 and 558 ± 2 Ma. These highly potassic granitoids show an arc signature and are in-placed in an extensional tectonic setting typical of the post-collision Late Pan-African period. In places, these rocks are strongly to slightly affected by hydrothermal alterations of phyllic, propylitic and argillic types with development of a pyritic facies around the Imourkhssan granite. The mineralization can be divided into two types: A molybdenite mineralization and some sulphides spatially linked to the Imourkhssan granite. It is disseminated in the granite or appears in lamellar or pluri-millimetric fragments. It also occurs as spots and nets along chloritized fractures planes oriented NNE-SSW and dipping 40 to 60 SE. The paragenesis consists of molybdenite, pyrite, chalcopyrite and sphalerite. Talat N’Lbnour Cu-Au mineralization linked to NS fractures affecting Askaoun granodiorite. The vein is about 0.5 to 2 m thick that extends about 400 m. The mineralized fractures are filled essentially with quartz, siderite and chlorite. Metallographic study reveals a diverse paragenesis that consists of pyrite, chalcopyrite, gold, bornite and chalcocite. The secondary paragenesis consists of chalcopyrite, covellite, bornite, malachite, azurite and hematite. These features of these mineralizations are discussed in the context of an arc-type Cu-Mo±Au porphyry mineralization. Keywords: Anti-Atlas, Siroua, Molybdenum, Copper, Gold, Porphyry

The Ediacaran volcanic rocks and associated mafic dykes of the Ouarzazate Group (Anti-Atlas, Morocco): Clinopyroxene composition, whole-rock geochemistry and Sr-Nd isotopes constraints from the Ouzellarh-Siroua salient (Tifnoute valley)

International audienceBelonging to the huge Ouarzazate volcanic Group that covered the whole Anti-Atlas during the late Ediacaran (580-545 Ma), the Tifnoute valley volcanic formations are mainly pyroclastic and show a large composition, from trachybasalt to rhyolite and are crosscut by dolerite dykes. The Tifnoute valley volcanic rocks are located within a rigid salient of the Anti-Atlas that gives them special extreme characteristics. Due to the heavy greenschist alteration that affects this volcanic group, we focused the more immobile elements, but as REE can also be affected, we used the composition of unaltered clinopyroxene crystals to determine the nature of these volcanic rocks. The clinopyroxene is an augite diopside in the basalt, an augite in the andesite and an augite-salite in the dolerite. Petrography of the Tifnoute mafic volcanic rocks and clinopyroxene compositions indicate the presence of two magmatic series: (i) older high-K calc-alkaline (alkali-calcic) andesite and basalt characterized by the early crystallization of Fe-Ti oxides and of the late fractionation of plagioclase, the modal proportion of the latter increasing from the basalt to the andesite and (ii) younger alkalic dolerite dykes. With clinopyroxene trace element compositions obtained using laser ablation ICP-MS, we calculated the composition of the melts in equilibrium with the pyroxenes.The volcanic rocks of the Tifnoute Valley have positive εNd570 (+1.7 to +5.0), low Sri (<0.7063), and NdTDM model ages ranging from 0.80 to 1.14 Ga, indicating a mostly depleted Neoproterozoic source with limited involvement of the Eburnian lithosphere for the Tifnoute magmas. This depleted source is the young lithospheric mantle for the alkali-calcic series and the asthenosphere for the younger alkalic series. The Tifnoute Valley volcanic rocks emplaced in a Pan-African transtensive post-collisional environment that evolved towards the major rifting event that will give rise to the Rheic ocean, in a similar way to what occurred just after the Variscan orogeny during the Triassic period that evolved to the Tethys ocean opening

The 600 Ma-Old Pan-African Magmatism in the In Ouzzal Terrane (Tuareg Shield, Algeria): Witness of the Metacratonisation of a Rigid Block

The high-level sub-circular North Tihimatine granitic pluton, intrusive in the In Ouzzal terrane, has been dated at 600 ± 5 Ma (LA-ICP-MS U–Pb zircon) and at 602 ± 4 Ma (SHRIMP U–Pb zircon). At this time, while Tihimatine intruded a brittle In Ouzzal without major metamorphism, large high-K calc-alkaline granitoid batholiths emplaced in the adjacent terranes under ductile conditions and regional amphibolite facies metamorphism. Outside In Ouzzal, high-level plutons emplaced under brittle conditions are known only at c. 580 Ma. The In Ouzzal terrane (500 km × 80 to 5 km), made of c. 2 Ga very high-temperature granulitic lithologies with Archean protoliths, is the sole terrane within the Tuareg Shield to have been largely unaffected by the Pan-African orogeny. The field, petrographic, geochemical and isotopic characteristics of the In Ouzzal granitic plutons studied herein, give keys for the understanding of the atypical behavior of the In Ouzzal terrane. The In Ouzzal Pan-African granitoids present chemical compositions varying from medium-K to high-K calc-alkaline to alkaline compositions. This is recorded by the Sr and Nd radiogenic isotopes (−4 < ɛNd < −30; 0.704 < ISr < 0.713), pointing to a mixing between a heterogeneous and old Rb-depleted source, the Eburnean granulitic In Ouzzal crust, and a Pan-African mantle. The latter is represented by the nearby bimodal Tin Zebane dyke swarm (ɛNd = +6.2, ISr = 0.7028; Hadj Kaddour et al. in Lithos 45:223–243, 1998), emplaced along the mega-shear zone bounding the In Ouzzal terrane to the west. Trace element composition and Sr–Nd isotope modeling indicate that 20–40% of different crustal lithologies outcropping in the In Ouzzal terrane mixed with mantle melts. At least two, most probably three, Eburnean granulitic reservoirs with Archean protoliths are needed to explain the chemical variability of the In Ouzzal plutons. The Pan-African post-collisional period is related to a northward tectonic escape of the Tuareg terranes, including the rigid In Ouzzal terrane, bounded by major shear zones. Blocking of the movement of the In Ouzzal terrane, which occurred 20 Ma earlier (at 600 Ma) on the western side than on the eastern side, induced its fracturing along oblique faults inside the terrane. This process allowed asthenosphere to rise and to locally melt the In Ouzzal crust, giving rise to the studied plutons. This corresponds to a metacratonization process. The In Ouzzal terrane demonstrates that a relatively small rigid block can survive within a major orogen affected by a post-collisional tectonic escape at the cost of a metacratonization, particularly at depth along faults

The Stavelot-Venn Massif (Ardenne, Belgium), a rift shoulder basin ripped off the West African craton: Cartography, stratigraphy, sedimentology, new U-Pb on zircon ages, geochemistry and Nd isotopes evidence

The Stavelot-Venn Massif (SVM) is the major Cambrian-Ordovician inlier of the Ardenne Allochthon (Southern Belgium). The SVM belongs to the Avalonian microcontinent and was affected by the Caledonian and Variscan orogenies. The Ardenne Allochthon constitutes the northwestern part of the Rheno-Hercynian domain, just south of the Variscan northern front. By contrast, the comparable Avalonian Cambro-Silurian Brabant Massif (northern Belgium) is located just to the north of this front and is not affected by the Variscan orogeny. In this study, we compiled the SVM available but dispersed data concerning the the Cambrian-Ordovician series (field, geochemistry, isotopes). In addition, we acquired new data especially LA-ICP-MS detrital and magmatic zircon ages and Nd isotopes. The comparison with the Brabant Massif, which also belongs to Avalonia and whose geological history is well known, is particurlarly enlightening. SVM Cambrian and lowermost Ordovician depositional environments are similar to the Brabant Massif but sediment thickness is significantly lower (2000 m vs 9000 m). During the remaining Ordovician and Silurian, after the opening of the Rheic Ocean, Brabant and SVM behaved differently, pointing to the existence of two different basements, whereby the basement underneath the SVM acted more rigidly (metacratonic). The SVM environment matches a rift shoulder while the Brabant was located in the rift itself. Our comprehensive study of detrital zircon ages from Cambrian-Ordovician sediments establishes the relative contributions over time of three composite sources: the West African craton, the Western Amazonian craton and the Pan-African orogen. This allowed us to determine a fine record of the tectonic events, distant or local, at the origin of the supply variations from these major sources. The detrital zircon age pattern of the Pridoli conglomerate (Ardennian unconformity) deposited after a sedimentation hiatus of 45 Myrs, is very distinctive. It shows a continuous record between 467 Ma and 420 Ma (97% zircons) that we relate to the activity of a large igneous province (LIP). The latter would be located along the south-eastern boundary of Avalonia, denying the existence of a Silurian island arc as previously proposed. The geochemical evidence indicates that most magmatic rocks have a crustal origin in relation to melting in an intracontinental setting due to stress applied at the Avalonia plate margin. Finally, we propose a geodynamic model in which the rigid basement of the Rheno-Hercynian domain originated from the tearing of a metacratonic fragment of the West African craton, which left a large scar on its western margin (Mauritania/Senegal). This metacratonic fragment includes only the Silurian Brabant western foreland and extends to England, including the Midlands microcraton.SCOPUS: re.jinfo:eu-repo/semantics/publishe