The barite of the Jbel Irhoud deposit in Western Jebilet, Morocco, is a Paleozoic massif known for its mineralization, which occurs mainly in shale, sandstone, and Middle Cambrian limestone. Three main types of barite deposits are known in the area: karst, vein fillings, and limestone replacement. The karst formations make up the majority of the reserves. Barite-quartz-galena and Fe-Cu-Zn- and Ag-sulfides, as well as hematite-carbonates, form the mineral paragenesis. Oxidation and mixing models are proposed for the deposition of barite. To check the effects of oxidation, numerical modeling of aqueous fluid composition for Irhoud barite deposition (at 150 degrees C-250 degrees C, Psat, and 1-6 m NaCl) was performed using a program developed by Professor Moine at the Paul Sabatier University in Toulouse, France. It shows that a large amount of barium can be transported as Ba2+ (barium chloride becomes more significant at relatively high temperatures) and that the decrease in solubility of barium under the given conditions can be caused by an increase in fO2, with or without a decrease in temperature, pressure, and/or salinity. Moreover, it is shown that the mixing of two fluids with different compositions leads to an oxidation (and a partial decrease in temperature) that causes a significant decrease in the solubility of barium (more than 130 ppm) and thus an efficient precipitation of barite in the Jbel Irhoud deposit. This modeling could be used to explain the manifestation of fluids with different compositions associated with the deposition of barite worldwide. The hydrothermal and structurally controlled Irhoud barite is the result of rapid decompression and Ba2+/BaCl+ transport under moderate to high P-T conditions, suggesting an epigenetic, postsedimentary system.

The Nkob talc deposit located in the central Anti-Atlas Pan-African belt is hosted by magnesian and siliceous metacarbonates in the contact aureole of the Ediacaran Amassine granite. It consists of a stratified succession of green serpentine-rich marbles, black dolomitic marbles, talcitites and phlogopitites (variously retrogressed into chloritites) enclosed within andalusite metapelite hornfels and quartzites. Two main metamorphic stages have been defined for the formation of the marbles and talcitites: (1) high-temperature recrystallization occurred under amphibolite facies conditions ( 500 degrees C). Temperature increase during granite emplacement led to the formation of forsterite, tremolite, phlogopite (+/- diopside) bearing assemblages in siliceous marbles and to transformation of dolomitic (calcite-free) marbles from pure dolostones. This high temperature stage is also characterized by the growth of andalusite porphyroblasts in the metapelites surrounding the deposit. (2) A low temperature retrograde hydrothermal stage characterized by the reactive infiltration of aqueous silica-rich fluid (presumably derived from the granitic body), in greenschist facies conditions (<520 degrees C). Hydrous silicate minerals, mostly serpentine and talc (+calcite), were formed during this stage within the rocks (after fluid infiltration along grain boundaries) or within veins (marking channeled fluid circulation). Talc in ores formed by two processes at Nkob: acicular talc near the granite formed after the breakdown of tremolite in a temperature range of 350 to 500 degrees C, while tabular talc in the external aureole formed via reaction between dolomite and siliceous-aqueous fluids in temperatures below 350 degrees C. The different types of marbles derive from pure to slightly siliceous dolomite but green and green-black marbles were strongly affected by interactions with siliceousaqueous fluids forming serpentine-phlogopite veins with calcite reaction zones. The talcitite layers were also affected by similar hydrothermal H2O-Si enrichment but their dolomitic precursor probably contained a more important detrital silicate fraction.

The Lu-Hf isotopic composition of detrital zircons has been used to investigate the crustal evolution of the northern part of the West African Craton (WAC). The zircons were separated from six samples of siliciclastic sedimentary rocks from the main Neoproterozic stratigraphic units of the Anti-Atlas belt, from the Sirwa and Zenaga inliers. The data suggest that the north part of the WAC formed during three cycles of juvenile crust formation with variable amount of reworking of older crust. The younger group of zircons, with a main population clustering around 610Ma, has a predominant juvenile character and evidences of moderate mixing with Paleoproterozoic and Neoarchean crust, which supports that most igneous and metamorphic rocks where zircons originally crystallized were formed in an ensialic magmatic arc environment. The group of zircons in the age range 1.79–2.3Ga corresponds to the major crust forming event in the WAC: the Eburnian orogeny. The isotopic data indicate that the provenance area should represent a crustal domain that was separated from a mantle reservoir at ∼2050–2300Ma, and further evolved with a time-integrated 176Lu/177Hf of ∼0.01, characteristic of continental crust. The evolution of the Eburnian orogeny is, consequently, compatible with new crust formation in an island arc environment, the transition to a continental arc setting and a final continent–continent collision. The Lower Paleoproterozoic and Neoarchean evolution (2.3–2.75Ga) includes a group of detrital zircon ages that has not been identified up to now in the igneous or metamorphic rocks of the north WAC basement. Their Hf isotopic signature points to reworking of mainly juvenile Neoarchean crust with some Meso- to Palaearchean contributions. The significance of these ages is uncertain: they can represent a tectonothermal event not discovered yet in the Reguibat Shield or the zircons can be far traveled from an unknown source.

Detrital zircon dating from Neoproterozoic successions in the Sirwa inlier of the Anti-Atlas belt in Morocco confirms that the maximum depositional age of the main stratigraphic groups is significantly younger than has been previously proposed in lithostratigraphic correlations. This can probably be extended to the whole Anti-Atlas according to other recent data from the Saghro inlier. Although the relative stratigraphic position of the different units remains valid as published previously, a crucial implication of the new ages is that the sequences believed to be contemporaneous with oceanic crust and island arc formation during the rifting and break-up of the northern margin of the West African Craton (WAC), and believed to be involved in the first phases of the Pan-African orogeny, are actually late to post-orogenic. The age of the main deformation associated with the collision of the oceanic- and arc-derived terranes to the WAC, allegedly affecting the sediments of the Saghro Group, has been estimated at around 663–640 Ma. However, the youngest zircon populations of sediments of the Saghro and Bou Salda Groups, obtained in this study, cluster around 620–610 Ma, constraining the maximum age of deposition. This age of sedimentation is indistinguishable from the age of intrusive high-K calc-alkaline plutons of the Assarag Suite, suggesting a very rapid cycle of magmatism, relief formation, erosion and sedimentation in an active geodynamic scenario. Moreover, the proportion of the 610 Ma detrital zircons becomes less with respect to the Paleoproterozoic zircons at higher stratigraphic levels, suggesting that the source of young zircons was progressively eroded and more extensive cratonic areas, that probably underlie the Neoproterozoic rocks, were exposed. We interpret these data in terms of the development of a ca. 610 Ma magmatic arc, built upon WAC basement, and its progressive dismantling. This arc can be correlated with the voluminous late Neoproterozoic (ca. 640–570 Ma) arc magmatism characteristic of the north Gondwana margin and the peri-Gondwanan terranes. The diamictite beds that appear in the Imghi Formation of the Saghro Group have been correlated with the Sturtian glacial period ca. 700 Ma. However, zircons from one sample of these diamictites indicate that this correlation cannot be longer maintained, and instead they should be correlated with the Marinoan glacial period ca. 630–610 Ma, with a widespread distribution of glaciogenic deposits in West Africa. In addition, around 375 U–Pb concordant analyses obtained from Paleoproterozoic zircons from six samples represent a statistically significant population of this area of the WAC basement, which can be a useful database for comparison with the detrital zircon populations of the peri-Gondwanan terranes of Europe and North America, as the WAC margins were one of the major sediment suppliers for these terranes.