Luc Barbanson

Late Paleozoic pre- and syn-kinematic plutons of the Kangguer–Huangshan Shear zone: Inference on the tectonic evolution of the eastern Chinese north Tianshan

Permian large-scale transcurrent tectonics and massive magmatism are prominent features of the Tianshan belt and neighboring regions of the Central Asian Orogenic Belt. Structural, geochronological and geochemical analyses of Carboniferous and Permian intrusive rocks associated with the Kangguer–Huangshan Shear Zone (eastern Chinese North Tianshan) provide constraints on their tectonic setting and the tectonic evolution of the Tianshan belt as well. Carboniferous granitic rocks were emplaced at 338 ± 4 Ma and 347 ± 2 Ma, respectively, and show geochemical features typical of the calc-alkaline series. These arc-type granites do not display ductile deformation, probably because they were completely cooled at the time of shearing tectonics, and are only offset by brittle strike-slip faults. In contrast, Permian granitoids display pervasive ductile tectonic features diagnostic of synkinematic emplacement. Four gabbro and diorite samples from the East Huangshan intrusive complex yielded zircon U-Pb ages of 267 to 275 Ma, and a granitic dike is dated at 290 ± 1 Ma. The granitic dike is cut by en-echelon right-lateral strike-slip faults, and the mafic intrusive complex displays a sigmoidal shape with mylonitic foliation localized at its margins. Other specific pluton shapes (such as tongue and tadpole-like) and syn-magmatic deformation can be observed in intrusions of the same age, showing similar fabrics and kinematics consistent with that of the Kangguer–Huangshan Shear Zone. Numerous mafic to felsic dikes occur within and off the shear zone with a dominant SE-NW orientation and minor varieties in N-S or NNE-SSW directions. One gabbro dike that intrudes the early Carboniferous granite of the East Kanggurtag area yielded a magmatic age of 274 ± 4 Ma, and contains older zircons (∼340 Ma, ∼390 Ma, ∼450 Ma, and 1.3-2.2 Ga) probably inherited from intruded rocks. The Permian intrusive rocks have variable chemical compositions suggesting derivation of these rocks from depleted and undepleted (or enriched) mantle sources with involvement of subduction-related components. We conclude from our integrated analysis of the geological, structural, geochemical and geochronological data that the Permian magmatic rocks were formed in a post-collisional/post-orogenic setting from multiple sources, and were emplaced under the control of large-scale dextral transcurrent tectonics.

Mineralized hydrothermal solution cavities in the Co-As Aït Ahmane mine (Bou Azzer, Morocco)

In the F53 Aït Ahmane Cobalt-deposit (Bou Azzer, Anti-Atlas, Morocco) a new type of “contact mineralization” is described. It is carbonate hosted but displays similar depositional sequence and fluid inclusion patterns to the well-known vein mineralization from the same mine. Field studies suggest that such a “contact mineralization” formed early by infilling solution cavities in formerly calcified serpentinite.

Mise en place syntectonique des minéralisations cuprifères du gı̂te d'Ifri (district du Haut Seksaoua, Haut Atlas occidental, Maroc)

Abstract Structural and metallogenic studies of the Ifri copper deposit (western High Atlas, Morocco) demonstrate that a great part of the mineralization was linked to a stockwork developed during a Late-Variscan NNW-verging shearing event. With this significant result, the until now accepted syngenetic character for this mineralization has to be reconsidered and allows to suggest a new guide of prospection for this kind of deposit.

Structural, mineralogical, and paleoflow velocity constraints on Hercynian tin mineralization: the Achmmach prospect of the Moroccan Central Massif

The Achmmach tin mineralization (NE of the Moroccan Central Massif) is associated with tourmaline-rich alteration halos, veins, and faults hosted in sandstones and metapelites of the Upper Visean-Namurian. These deposits are reported to be late Hercynian in age and related to the emplacement of late-orogenic granite not outcropping in the studied area. Structural and paragenetic studies of the Achmmach tin deposit were conducted in order to establish a general model of the mineralization. From field constraints, the late Hercynian phase is marked by a transition from transpression to extension with deformation conditions evolving from ductile to brittle environments. The transpression (horizontal shortening direction roughly trending E-W) is coeval with the emplacement of the first tourmaline halos along several conjugated trends (N070, N020, and N120). Thereafter, a tourmaline-rich breccia formed in response to the fracturing of early tourmaline-altered rocks. Subsequently, during the extensional phase, these structures were reactivated as normal faults and breccias, allowing the formation of the main tin mineralization (cassiterite) associated with a wide variety of sulfides (arsenopyrite, chalcopyrite, sphalerite, galena, pyrrhotite, bismuthinite, pyrite, and stannite). This evolution ends with fluorite and carbonate deposition. The hydrothermal fluid flow velocity, calculated by applying statistical measures on the tourmaline growth bands, varies with the lithology. Values are lower in metapelites and higher in breccia. In the general evolution model proposed here, tourmaline alteration makes the rock more competent, allowing for brittle fracturing and generation of open space where the main Sn mineralization was precipitated.

Les minéralisations aurifères des Cévennes (Massif central français): étude comparative des filons d'Alteyrac/Pont-de-Rastel et du paléoplacer du Bulidou

Abstract The comparative analysis of gold mineralization of the Cevennes area (Bulidou boulder and Alteyrac/Pont-de-Rastel veins) shows that these deposits resulted from the same polyphase process characterized by successive opening, crystallization and brecciation. The mineralized pebbles exhibit opening facies (geodic structures) with specific aquocarbonic three-phase fluid inclusions that do not occur within the veins. It is thus proposed that the original site for mineralized pebbles is not directly the Alteyrac/Pont-de-Rastel quartz veins.

Mise en place du complexe filonien aurifère de la région d'Ouro Preto (Quadrilatère ferrifère, Brésil): contraintes minéralogiques et pétrographiques

Petrological and mineralogical analysis of gold-bearing quartz veins of the Ouro Preto area (Minas Gerais, Brazil) demonstrates: i) that gold, always associated with bismuth, is located within open fractures of quartz vein arsenopyrite; ii) that mineral association related to the mineralizing event developed later with respect to the regional metamorphism. These results are consistent with the structural studies that suggest a gold-bearing veins emplacement posterior to the regional metamorphism.