Modèle de formation du gisement d'argent d'Imiter (Anti-Atlas oriental, Maroc). Nouveaux apports de l'analyse structurale et minéralogique. Model of formation of the Imiter silver deposit (eastern Anti-Atlas, Morocco). New structural and mineralogical constraints .

Based on a combined geometrical and mineralogical analysis, a three-stage model of formation of the mineralized veins of the giant Imiter silver deposit (Anti-Atlas, Morocco) is herein proposed. A first episode is characterized by the development of quartz, pink dolomite and Ag-rich minerals veins formed during a dextral transpressive event. The second episode is associated with a normal left-lateral motion that re-opens previous structures, filled by pink dolomite gangue. Alteration stages contribute to a local Ag enrichment. Basé sur l'étude de nouvelles données géométriques et minéralogiques, un modèle de formation en trois stades des veines du gisement d'argent d'Imiter (Anti-Atlas oriental, Maroc) est proposé. Le premier stade est caractérisé par le développement des veines les plus riches, à remplissage de quartz puis dolomite rose, dans un contexte transpressif dextre associé à une direction de raccourcissement NW–SE à WNW–ESE. Le second stade, à remplissage de dolomite rose, est associé à un cisaillement senestre normal, contrôlé par une direction de raccourcissement nord–sud, réutilisant certaines des structures du stade 1. Des phénomènes d'altération contribuent à un enrichissement local en Ag

Mode of formation of gold-bearing mineralization on top of the boboras (Galicia, Spain) - The combined role of mechanical instabilities, strain localization and vein formation.

International audienceThe Galician Hercynian segment constitutes the core of the Ibero-Armorican orogenic arc, characterized by few tectonic units that record three main phases of deformation (D1 to D3). Four generations of granite, syn- to post-D3 intruded the major tectonic units. From older to younger, we find: i) syn-kinematic biotite-rich granodiorite (G1); ii) syn-kinematic two micas granites (G2) ; iii) biotite-dominant granites (G3) and iii) late-kinematic biotite-rich granodiorite (G4). Numerous sills, dykes and vein systems are widespread within the metasediments of the para-autochton unit. Gold-bearing quartz veins were spatially associated with G3 granites likely the Boborás intrusion, concerned by this work. The Boborás granite (G3) outcrops as a small NS-elongated elliptical intrusion. Granite is homogeneous and exhibits an equigranular texture composed of quartz, oligoclase, microcline, biotite dominant and muscovite

Magnetic fabrics and fluid flow directions in hydrothermal systems. A case study in the Chaillac Ba-F-Fe deposit (France).

This study presents a possible use of anisotropy of magnetic susceptibility (AMS) to describe the mineralizing process in hydrothermal systems. Ba–F–Fe-rich deposits within the Chaillac Basin are on the southern border of the Paris Basin. In these deposits hydrothermal textures and tectonic structures have been described in veins, sinters, and sandstone cemented by hydrothermal goethite. 278 oriented cores from 24 sites have been collected in these formations. In addition, a lateritic duricrust superimposed on the hydrothermal formation has been sampled. Rock magnetic investigations show that the principal magnetic carrier is goethite for the hydrothermal mineralization and for the laterite level. The AMS measurements show distinguishable behaviors in the different mineralogical and geological contexts. The K1 magnetic lineation (maximum axis) is strongly inclined for the vertical veins. For the horizontally mineralized sinters, the magnetic lineation is almost horizontal with an azimuth similar to the sedimentary flow direction. The AMS of goethite-rich sandstone close to the veins shows strongly inclined K1 as they are probably influenced by the vertical veins; however, when the distance from the vein is larger than 1 m, the AMS presents rather horizontal K1 directions, parallel to the sedimentary flow. The laterite has a foliation dominance of AMS with vertically well-grouped K3 axes and scattered K1 and K2 axes. Field structural observations suggest that the ore deposit is mainly controlled by EW extension tectonics associated with NS trending normal faults. Combining the AMS results on the deposit with vein textures and field data a model is proposed in which AMS results are interpreted in terms of hydrothermal fluid flow. This work opens a new investigation field to constrain hydrodynamic models using the AMS method. Textural study combined with efficient AMS fabric measurements should be used for systematic investigation to trace flow direction in fissures and in sand porosity

A new geodynamic interpretation for the South Portuguese Zone (SW Iberia) and the Iberian Pyrite Belt genesis.

International audienceThe South Portuguese Zone (SPZ) constitutes the southernmost segment of the Variscan Iberian Massif. It is bounded to the north by the Beja-Acebuches Ophiolitic Complex and related accretionary wedge. To the south lie the Iberian Pyrite Belt (IPB) and flysch deposits forming the southern extent of the zone. Structural analysis within the Spanish side of the SPZ supports continuous south propagating deformation, evolving from early synmetamorphic thrusting in the internal zone to thin-skinned tectonics in the southern external domain. The accretion of the SPZ to the Ossa Morena Zone is also witnessed by the presence of various mélanges, observed throughout the investigated area. Part of the mélanges observed in the IPB are related to the volcanics and mineralizations setting. A key point to understand the IPB mineralizations genesis is to constrain the volcanogenic model. One underestimated feature is the large amount of submarine calc-alkaline ignimbritic facies, implying the presence of caldera structures within the province. Such correlation between caldera environment and ore deposits strongly suggests that the IPB developed in a continental arc. Our geodynamic model proposes an early north directed subduction associated with the obduction of the oceanic crust toward the south. Southward, this episode is immediately followed by the development of the accretionary prism, while farther south, a second subduction zone responsible for the arc setting of the IPB initiates. Subsequent Visean continental collision is associated with the deposit of the south propagating flysch and the present geometry of the SPZ

Rare earth elements as proxies of supergene alteration processes from the giant Imiter silver deposit (Morocco)

International audienceThe giant Imiter silver mine located at the northern edge of the West African craton in Morocco is assumed to be a late Neoproterozoic epithermal deposit mainly characterized by a hypogene paragenesis of Agrich sulphides and sulfosalts, and Ag-Hg alloys occuring preferentially in quartz-rich veins. The secondary enrichment zone at Imiter reaches a thickness of 50 to 150 m below ground surface. The upper levels, famous because of giant native silver crystals, grade up to 300 kg/t. Metallographic observations, SEM-EDS and XRD analyses reveal the presence of a quite complex secondary paragenesis made of acanthite, cinnabar, imiterite, perroudite, cerussite, mimetite, iron oxyhydroxides, synchisite and coronadite. Supergene alteration processes of the giant Imiter silver mine deposit consist of the remobilisation of the primary hypogene paragenesis by (i) deep and old basinal brines and (ii) downward infiltrations of surficial waters becoming progressively more reduced and F-enriched in response to fluid-rock interactions. Development of such a supergene mineralization strongly suggests prevalence of arid to semiarid conditions

Geochronological constraints on the magmatic and hydrothermal evolution of the tras-os-montes hercynian domain (Galica, Spain) : position of the AU, SN-W mineralizing events.

International audienceThe study area is located in the Galicia-Trás-os-Montes Zone (GTMZ zone, Arenas et al. 1986, Fig. 1), that belongs to the internal zone of the Hercynian belt and is composed of a relative autochthonous and parautochthonous units overthrusted by allochthonous complexes (Ribeiro et al. 1990). This domain of Palaeozoic schists is affected by a low to high temperature – medium pressure metamorphism. These rocks exhibit a well-developed regional schistosity related to nappes emplacement (D1 and D2 events) and are affected by NS-trending crenulation lineation and folds (D3 event) The late D3 event is characterized by a high-temperature metamorphism leading to development of local migmatite. Both parautochthonous and allochthonous units are intruded by syn- and post-kinematic plutons. Four generations of granites (G1 to G4) are identified by their textural and geochemical characteristics and by crosscutting relationships. Gold deposits are spatially associated with the G3 granites whereas Sn-W deposits are represented, in the study area, by disseminated and vein-type mineralization spatially close to the G2 granites (fig. 1)

Geochronological and geochemical characterization of magmatic-hydrothermal events within the Southern Variscan external domain (Ce'vennes area, France)

International audienceGeochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Loze're- Borne plutonic complex and surrounding rocks (Ce'vennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic- hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures

Âge K/Ar et contrôle structural de mise en place des veines épithermales à Au-Ag de la Cordillera Shila, Sud Pérou.

International audienceThe Au-Ag epithermal mineralization of the Shila Cordillera is dated at about 10.7 Ma (K/Ar on adularia). The vein system is characterized by the association of a major east-west vein and N120-135°E secondary fractures. The strike-slip faults controlling the veins indicate an initial NE-SW to ENE-WSW shortening direction, which is compatible with that generally accepted for this period. These structures were reopened during a second phase and channelized mineralizing fluids, the circulation of which may have began at the end of stage 1.Les minéralisations épithermales à Au-Ag de la Cordillera Shila sont datées à environ 10,7 Ma (K/Ar sur adulaire). Le système de veines est caractérisé par l'association entre une veine principale sensiblement est-ouest et des fractures satellites N120 à N135°E. Les décrochements contrôlant les veines indiquent une direction de raccourcissement initiale NE-SW à ENE-WSW, compatible avec celle généralement admise pour cette période. Dans un deuxième stade, ces structures sont ré-ouvertes pour servir de réceptacle aux fluides minéralisateurs, dont la circulation débute probablement dès la fin du stade 1

Les minéralisations Cu-(Ni-Bi-U-Au-Ag) d'Ifri (district du haut Seksaoua, Maroc) : apport de l'étude texturale au débat syngenèse versus épigenèse. <br /> The Cu---(Ni---Bi---U---Au---Ag) mineralization of Ifri (‘Haut Seksaoua' district, Morocco): contribution of a textural study to the discussion syngenetic versus epigenetic

The Cu ore of Ifri is a chalcopyrite stockwork hosted by Cambrian formations and was until now interpreted as a syngenetic massive sulphide deposit. Textural studies highlight two generations of pyrite early (Py I) and late (Py II) with respect to the regional deformation. The chalcopyrite stockwork overprinted Py II, outlining the epigenetic nature of the Cu mineralization. Regarding the origin of Cu-depositing fluids, the presence in the stockwork paragenesis of an U, W, Sn association and preliminary Pb/Pb dating of a brannerite belonging to this association suggest a contribution of the Tichka granite

Mise en place syntectonique des minéralisations cuprifères du gîte d'Ifri (district du Haut Seksaoua, Haut Atlas occidental, Maroc)---Syntectonic emplacement of the Ifri copper mineralization (High Seksaoua, western High Atlas, Morocco)

International audienceL'étude structurale et métallogénique (gîtologique) du gisement de cuivre d'Ifri (Haut Atlas occidental, Maroc) montre qu'une grande partie de la minéralisation est liée à un stockwerk, dont la formation est contrôlée par un cisaillement ductile tardi-hercynien à vergence NNW. Ce résultat significatif remet en question le caractère syngénétique jusqu'alors évoqué pour la minéralisation et permet ainsi de proposer un nouveau guide d'exploration pour ce type de gisement. 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