Epigenesis versus syngenesis: a contribution to the debate based on the stratiform tungsten skarn,nnineralizations of Djebel Aouam, central Morocco1

Abstract. Stratiform scheelite-biotite banded mineralizations in the polymetallic W-Pb-Zn-Ag district of Djebel Aouam, central Morocco, are interpreted as skarn bodies resulting from a two-stage process: (1) Metamorphic reactions in interbedded pelitic and impure limestone layers produce a characteristic zonation by cation exchange; (2) Later infiltration of hydrothermal fluids causes the replacement of the previously constituted calc-silicate layers and the formation of the stratiform tungsten mineralization. This stratiform skarn development occurs away from the granitic contact

Mise en évidence d'une tectonique compressive Éocène-Oligocène dans l'Ouest de la Cordillère orientale de Colombie, d'après la structure en duplex des gisements d'émeraude de Muzo et de Coscuez (Evidence for an Eocene-Oligocene compressive tectonics in the western part of Eastern Cordillera of Colombia, from the duplex structure of the Muzo and Coscuez emerald deposits)

The structures of the emerald deposits of the western part of Eastern Cordillera of Colombia (thrusts, ramps, duplexes, tear faults) have been formed by three deformational events forming a single thrust-tectonics phase, synchronous with the emerald-forming hydrothermalism. As this hydrothermalism is Late Eocene-Early Oligocene in age, it is also the age of the thrust tectonics

Isotopic and geochemical constraints on lead and fluid sources of the Pb-Zn-Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context

International audienceThe Wsingle bondAu, Pbsingle bondZnsingle bondAg, and Sbsingle bondBa mineralizations of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The Pbsingle bondZnsingle bondAg Tighza deposit formed at 254 ± 16 Ma, and is clearly disconnected from the late-Variscan Wsingle bondAu deposit (295-280 Ma). The Pbsingle bondZnsingle bondAg mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized 3He/4He ratio (0.018–0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (δ18O = +19 to +25‰; δ13C = −3.6 to −11.2‰) and the ɛNd values (−4.84 to −9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rocks, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the Pbsingle bondZnsingle bondAg mineralization has 40Ar/36Ar values in the range 284–315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (207Pb/204Pb = 15.70–15.80 and 206Pb/204Pb = 18.30–18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures (207Pb/204Pb = 15.40 and 206Pb/204Pb = 18.05) provide evidence of a deeper source attributed to the lower crust.Crustal thinning related to extensional tectonics in late-Permian and Early-Triassic lead to high-K calc-alkaline to alkaline magmatic activity, which is evidenced by a dense SW-NE-trending dike network that pre-dated the Atlantic Ocean opening (early Liassic times). This magmatic event induced a regional heat flux increase that triggered the circulation of a complex hydrothermal fluid, which has a strong crustal component, but also a meteoric and a lower crustal components. The polymetallic district of Tighza-Jbel Aouam thus results from superposition of an intrusion related porphyry-gold mineralization (Wsingle bondAu, 286 Ma) followed by a Pbsingle bondZnsingle bondAg epithermal mineralization (254 Ma), during two distinct magmatic-hydrothermal events.The proposed metallogenic model for the Pbsingle bondZnsingle bondAg Tighza-Jbel Aouam deposit provides new constraints for the Pbsingle bondZnsingle bondAg exploration in the Moroccan Meseta. Exploration targets must take into account the following geological features: (i) Permo-triassic high-K calk-alkaline to alkaline dikes, (ii) extensional tectonics and reactivation of ancient crust-scale faults and shear zones, and (iii) Paleozoic series containing organic matter (e.g., black shales) subjected to low grade metamorphism (e.g., greenschist facies)

Évolution volcanologique du mont Manengouba (Ligne du Cameroun) ; nouvelles données pétrographiques, géochimiques et géochronologiques<br /> Volcanological evolution of the mount Manengouba (Cameroon line), new petrographical, geochemical, and geochronological data

Le mont Manengouba est un complexe volcanique polygénique de la Ligne du Cameroun édifié au Quaternaire, entre 1,55 et 0 Ma, en trois phases principales. La première phase, de 1,55 à 0,7 Ma, correspond à la construction du volcan Elengoum. La deuxième, entre 0,7 et 0,56 Ma, est marquée par l'effondrement de la partie sommitale de l'Elengoum. La troisième, de 0,56 à 0 Ma, voit l'édification du volcan Eboga, puis la formation de la caldeira, et comporte une activité fissurale adventive à partir de 0,48 Ma. Les produits émis définissent une série alcaline sodique, allant des basaltes à néphéline à des trachytes. Toutes ces laves ont évolué par cristallisation fractionnée dans une chambre magmatique périodiquement réalimentée. The mount Manengouba is a polygenic volcanic complex of the Cameroon line, which was built in the Quaternary, between 1.55 and 0 Ma, during three stages. The first stage, from 1.55 to 0.7 Ma, corresponds to the building of the Elengoum volcano. The second, between 0.7 and 0.56 Ma, points to the collapse of the Elengoum summit. The third, from 0.56 to 0 Ma, fits with the erection of the Eboga volcano and its caldera sinking, and, from 0.48 Ma, with the adventive fissural activity. The products define an alkaline sodic series, from nepheline-bearing basalts to trachytes. All the lavas evolved by fractional crystallization in a periodically replenished magmatic chamber

Caractéristiques géochimiques et thermobarométriques des fluides associés à la scheelite et au quartz des minéralisations de tungstène du Jbel Aouam (Maroc Central)

Fluid inclusions from quartz and scheelite associated with Jbel Aouam vein-type tungsten occurrences in Central Morocco are studied by microthermometric, Raman spectroscopic and scanning electron microprobe techniques. Global composition of two different types of inclusion (V and L) and isothermal molar volume/CO2 molar fraction diagrams allow the determination of trapping conditions at T = 400-450 °C and P = 1 kbar for fluids associated with tungsten deposition. Subcritical evolution with dilution by aqueous fluids of CO2-rich early fluids is the main mechanism proposed at the time of trapping.Les inclusions fluides du quartz et de la scheelite associées aux minéralisations filoniennes de tungstène du Jbel Aouam au Maroc Central sont étudiées au moyen des techniques microthermométriques, spectroscopic Raman et microscopie électronique à balayage. La détermination de la composition globale de deux types d'inclusion (V et L) et l'utilisation de diagrammes isothermes volume molaire/fraction molaire de CO2 dans le système ternaire H2O-CO2-NaCl permettent de fixer les conditions de piégeage des fluides associés au dépôt des minéralisations à T = 400 à 450 °C et P = 1 kbar. Le mécanisme essentiel mis en évidence au moment du piégeage correspond à une évolution subcritique avec dilution par des fluides aqueux de fluides précoces enrichis en CO2.Cheilletz Alain. Caractéristiques géochimiques et thermobarométriques des fluides associés à la scheelite et au quartz des minéralisations de tungstène du Jbel Aouam (Maroc Central). In: Bulletin de Minéralogie, volume 107, 2, 1984. Inclusions fluides

Les minéralisations stratiformes à scheelite-biotite du Djebel Aouam (Maroc Central). Exemple de skarn d'infiltration développé par remplacement de séries sédimentaires gréso-pélitiques

Stratiform scheelite-biotite mineralizations of Djebel Aouam (Central Morocco) are classified into skarn deposit type. Infiltration and replacement of the sedimentary series by hydrothermal solutions induce banded lenses with dominant biotite-actinolite-plagioclase calc-silicate association. The greso-pelitic replaced sequences are remarkable because of their porous structure facilitating the infiltration and diffusion of fluids along the stratification planes.Les minéralisations stratiformes à scheelite du Djebel Aouam (Maroc Central) sont répertoriées dans les gisements de type skarn. L'infiltration et le remplacement des séries sédimentaires par les solutions hydrothermales conduit au développement de lentilles rubanées dans lesquelles l'association calco-silicatée biotite-actinolite-plagioclase est dominante. Les niveaux remplacés, de nature gréso-pélitique, sont remarquables par leur structure poreuse facilitant l'infiltration et la diffusion des fluides le long des plans de stratification.Cheilletz Alain. Les minéralisations stratiformes à scheelite-biotite du Djebel Aouam (Maroc Central). Exemple de skarn d'infiltration développé par remplacement de séries sédimentaires gréso-pélitiques. In: Bulletin de Minéralogie, volume 108, 3-4, 1985. Minéralogie dans les Sciences de la Terre et l'Industrie - Hommage à François Permingeat

Reply to the comment by M. Bouabdellah et al. on the paper " A late Triassic age for the El Hammam high-REE fluorite deposit (Morocco) : mineralization related to the central Atlantic magmatic Province ? "

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Compte-rendu de visite sur les occurrences cuprifères du Haut-Atlas de Béni Mellal, Maroc

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