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During the Albian, the hyperextension of the Pyrenean passive margin led to a hyperthinning of the continental crust and the subsequent subcontinental mantle exhumation. The giant Trimouns talc-chlorite deposit represents the most prominent occurrence of Albian metasomatism in the Pyrenees, with the occurrence of the largest talc deposit worldwide....
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In two companion papers we report the detailed geological and mineralogical study of two emblematic serpentinized ultramafic bodies of the western North Pyrenean Zone (NPZ), the Urdach massif (paper 1) and the Saraillé massif (this paper). The peridotites have been uplifted to lower crustal levels during the Cretaceous rifting period in the future...
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... Then, we characterize the prominent basin conductor ( ρ < 1 ·m) with a statistical analysis of the posterior model distribution and compare the probabilities with the anomaly in 3-D deterministic models. Finally, we use the Hashin-Shtrikman upper conductivity bound model (Hashin & Shtrikman 1962 ) constrained by fluid salinity and mineralogy of rock samples from detachments faults in palaeomargins (Pinto et al. 2015 ;Salardon et al. 2017 ;Corre et al. 2018 ;Quesnel et al. 2019 ;Nteme Mukonzo et al. 2021 ) to unveil two conceptual models of brine circulation in hyperextended rift systems. ...
... In the Pyrenees, investigations of fluid inclusions advocate that brine fluids were squeezed out from Triassic halite layers and leached out to the syn-post rift Albian sediments during the hyperextension phase (Salardon et al. 2017 ;Corre et al. 2018 ;Quesnel et al. 2019 ;Nteme Mukonzo et al. 2021 ). Pre-kinematic Triassic salt facilitated the extension by acting as decollement level and permitting fluid-salt interaction. ...
... Pre-kinematic Triassic salt facilitated the extension by acting as decollement level and permitting fluid-salt interaction. The studies reveal fluid salinity ranging from 10 to 35 wt per cent (Quesnel et al. 2019 ;Nteme Mukonzo et al. 2021 ), compatible with salinity values explaining Aptian-Albian marine shales high conductivity in the SW Barents Sea using the HS + model (Fig. 10 ). In addition, seismic interpretation shows salt diapirs piercing through Early Cretaceous sediments in the Bjørnøya Basin (Fig. 9 ). ...
In this article, we use a new workflow to substantiate the characterization of a prominent, deep sediment conductor in the hyper-extended Bjørnøya Basin (SW Barents Sea) previously identified in smooth resistivity models from 3D deterministic inversion of magnetotelluric data. In low dimensionality environments like layered sedimentary basin, 1D Bayesian inversion can be advantageous for a thorough exploration of the solution space but the violation of the 1D assumption has to be efficiently handled. The primary geological objectives of this work is therefore preceded by a secondary task: the application of a new machine learning approach for handling the 1D violation assumption for 21 MT field stations in the Barents Sea. We find that a decision tree can adequately learn the relationship between MT dimensionality parameters and the 1D-3D residual response for a training set of synthetic models, mimicking typical resistivity structures of the SW Barents Sea. The machine learning model is then used to predict the dimensionality compensation error for MT signal periods ranging of 1 to 3000 s for 21 receivers located over the Bjørnøya Basin and Veslemøy High. After running 1D Bayesian inversion, we generated a posterior resistivity distribution for an ensemble of 6000 1D models fitting the compensated MT data for each 21 field stations. The proportion of 1D models showing ρ < 1 Ω.m is consistently beyond 80% and systemically reaches a maximum of 100% in the Early Aptian - Albian interval in the Bjørnøya Basin. In hyper-extended basins of the SW Barents Sea, the dimensionality compensation workflow has permitted to refine the characterization of the deep basin conductor by leveraging the increased vertical resolution and optimal used of MT data. In comparison, the smooth 3D deterministic models only poorly constrained depth and lateral extent of the basin anomaly. The highest probability of finding ρ < 1 Ω.m is robustly assigned to the syn-tectonic Early Aptian - Albian marine shales, now buried at 6 to 8 km depth. Based on a theoretical two phase fluid-rock model, we show that the pore fluid of these marine shales must have a higher salinity than seawater to explain the anomaly ρ < 1 Ω.m. Therefore, the primary pore fluid underwent mixing with a secondary brine during rifting. Using analogue rift systems in paleomargins, we argue that two possible secondary brine reservoir may contribute to deep saline fluid circulation in the hyper-extended basin: (1) Permian salt-derived fluid and, (2) mantle-reacted fluid from serpentinization.
... When penetrating downward into the basement, these sedimentary brines may have their composition modified through water-rock interactions (Banks et al., 1991;Boiron et al., 2010;Bons et al., 2014;Lüders et al., 2010;Richard et al., 2016;Scharrer et al., 2021;Walter et al., 2016Walter et al., , 2017. Among the most common processes, albitisation of plagioclase releases calcium into the fluid, and the formation of Mg-rich phyllosilicates such as talc (Quesnel et al., 2019) or Mg-clays may deplete magnesium concentration in fluids. The lithium behaviour in these processes is only partially understood due to the lack of data on the lithium distribution in fresh minerals and their alteration products. ...
... Information for the hyperextension stage is fromManatschal et al. (2000),Lescoutre et al. (2020), andSaspiturry et al. (2020). Information for the exhumation stage is fromPinto et al. (2015),Quesnel et al. (2019),Coltat et al. (2019Coltat et al. ( , 2021, and NtemeMukonzo et al. (2021) Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
Although many studies link mineral deposit formation to rifting and hydrothermal processes, we present a study that focuses on the relationship between crustal necking and mineral deposit formation. Necking corresponds to the timing, location, and process of rift localization and abrupt crustal and lithospheric thinning. Although necking is well identified and described from present-day rifted margins and has been modeled numerically, little is known about the necking process and its possible link to ore deposit formation. We present observations from the Mont-Blanc fossil detachment system, one of the few exposed examples of a necking detachment fault. We show that fluids flowed along the fault zone and leached metals (mainly Pb and Zn). This process was associated with the hydrothermal breakdown of feldspar and biotite at temperatures of 200 °C and salinities ranging from 5 to 20 eq. wt% with a H2O-NaCl (-KCl) composition. The resulting metal-rich fluids reacted with mainly carbonate-rich units to form Pb-Zn ore deposits in basement and sedimentary cover rocks. A direct link can, therefore, be demonstrated between fluid and reaction-assisted breakdown of silicates, metal transfer and trapping along detachment faults, and the overlying sedimentary rocks during necking. Similar ore deposits can be found throughout the inner External Crystalline Massif of the Western Alps, interpreted as the former necking domain of the Alpine Tethys. This leads to the suggestion that necking and Pb-Zn deposit formation may be closely linked, a hypothesis, if correct, that has the potential to predict additional Pb-Zn-Ba-F resources in rifts, rifted margins, and reactivated fossil rifted margins forming collisional mountain belts.
... However, differences in fluid composition and temperature arise when considering the structural position of each studied locality within the extensional system. The model in Fig. 12 summarizes the fluid flow knowledge of the Mesozoic Pyrenean and Basque-Cantabrian extensional system (Mendia and Ibarguchi, 1991;López-Horgue et al., 2010;Iriarte et al., 2012;Dewit et al., 2012;Clerc et al., 2015;De Felipe et al., 2017;Salardon et al., 2017;Perona et al., 2018;Ducoux et al., 2019;Quesnel et al., 2019;Renard et al., 2019;Incerpi et al., 2020;Cruset et al., 2021, Motte et al., 2021Bahnan et al., 2021). In this model, we use for the North Pyrenean Zone the restored cross-sections of Teixell et al. (2018) and Ducoux et al. (2021). ...
We propose a review to discuss the large number of studies dealing with the fluid history in extensional and compressional sedimentary basins that evolved along the Iberian-Eurasian plate boundary during the full Mesozoic-Cenozoic Wilson Cycle in the Pyrenean fold belt and the Basque-Cantabrian Basin. We integrate classic and modern geochemical and geochronological datasets used in fluid studies with the current tectonic knowledge of the studied area.
Late Hercynian fluid systems were dominated by Carboniferous-early Permian magmatic intrusions related to large-scale lithospheric delamination at the end of the collision, which caused the accumulation of skarns at depths of 8000–10,000 m during contact metamorphism. During the Mesozoic extension, early and widespread shallow burial dolomitization of Jurassic and Early-Cretaceous carbonates occurred at burial depths of 500–1000 m due to seawater influx. From Albian to Cenomanian, along the North Pyrenean extensional fault zone, contact metamorphism processes occurred in association with mantle-derived and deep-crustal fluids at temperatures higher than 300 °C, which interacted with Triassic evaporites and formation and marine waters and depths of 2000–3000 m. Away from this fault, fluid systems were dominated by hydrothermal dolomitization and the accumulation of Znsingle bondPb mineralization along diapir walls and faults, whereas in the less extended and proximal domains of the extensional system, fluids were formation waters at temperatures up to 150 °C. The Alpine compressional fluid history registers the increasing influence of meteoric fluids as the foreland basin became overfilled and fluid flow occurred at depths of 2.5–4 km in tectonic units detached in Triassic evaporites and of >4 km in units rooted at depth with the Paleozoic basement. Along and across strike differences in the fluid evolution of the Pyrenees are attributed to changes in the structure of the cover and basement tectonic units, the westward decrease of shortening and in the oblique directions of Upper Triassic successions, which acted as very efficient seals for deep-sourced fluids.
Subvertical walls of diapirs are baffles for fluid flow, whereas fracturing and deposition of porous halokinetic successions are effective conduits. Evaporite detachments compartmentalize paleohydrological systems during tectonic deformation, although they may be breached by fluids reaching lithostatic pressures. In large evaporite-bearing provinces, fluid systems may share common patterns during successive extensional and compressional tectonic events, as documented in the Western Mediterranean Mesozoic extensional rift system. In this area, metal-bearing and deep-sourced fluids interacted with Triassic sulphates and organic matter, triggering the accumulation sulphides in rock porosity. However, more research is needed in other large-scale evaporitic provinces of different ages to identify common fluid flow patterns.
... Hauptdolomit lagoon also argue in favour for an active spreading ridge at that time, and the serpentinization of the mantle peridotites (Quesnel et al., 2019). ...
The Neo-Tethys margin evolution is preserved in the Northern Calcareous Alps (Eastern Alps), from Late Permian crustal stretching to Late Triassic oceanization. The Northern Calcareous Alps represent the salt-floored fold-and-thrust developed from the salt-influenced Triassic carbonate sedimentary cover of the ancestral European margin of the Neo-Tethys Ocean. A crustal scale model for the margin has been obtained by restoration of regional cross-sections of the Northern Calcareous Alps carbonate platforms. Lithospheric break-up was investigated from remnants of exhumed mantle found within an evaporitic melange, suggesting hyperextended crust underneath the distal Triassic platforms of the Northern Calcareous Alps preceding breakup. By modelling the thermal evolution of the margin in combination with excellent stratigraphic control, a detailed timeline has been established for the evolution of the Neo-Tethys margin, especially around the period of rapid mantle exhumation. Our study indicates that salt-floored carbonate shelfs can be used as a proxy to characterize the margins evolution, from crustal stretching to continental breakup. Diagnostic stratigraphic records are preserved in the carbonate platforms: pre- mantle exhumation carbonates are represented by aggrading isolated carbonate platforms first, followed by expanding and margin wide prograding carbonate shelfs once thermal subsidence dominates. In addition, a distinct clastic sequence is deposited as an immediate response to mantle exhumation, in between the pre- and post-mantle exhumation carbonate factory. Our study proposes a new refined model for the formation of the Neo-Tethys margin and provides new insights for the dynamic coupling of salt-controlled carbonate shelfs and the underlying lithosphere during continental breakup
... Les études de la transformation des dolomies en minerai talqueux et des fluides responsables de la minéralisation commencèrent avec Mengaud (1908), et surtout Capdecomme (1950) et ont été poursuivis notamment par Zwart (1953), Thiebaut et al. (1965), Aranitis (1967), Fortuné (1971), Moine (1982), Moine et al (1979Moine et al ( ,1982Moine et al ( et 1989, Parseval (1992), Boiron et al. (2005), Boulvais et al. (2006), et Quesnel et al. (2019. Ces travaux ont permis de caractériser les transformations métasomatiques à l'origine du gisement, la métasomatose correspondant à une transformation métamorphique sous l'action d'un fluide induisant des changements majeurs de composition chimique et minéralogique entre les roches de départ et celles transformées (dolomie vs talc). ...
... Par la suite, d'autres travaux ont affiné la compréhension des modalités de formation du gisement en suggérant i) que le magnésium nécessaire à la formation du gisement provenait initialement des saumures et ne résultait pas de leurs interactions avec les roches percolés (Boiron et al., 2007), ii) que la dolomitisation des marbres calcitiques puis la talcification des dolomies résultaient d'un même système de circulation de fluide et non d'évènements hydrothermaux distincts (Boulvais et al., 2006;Quesnel et al., 2019) et enfin iii) que les saumures étaient de l'eau de mer évaporée (Boiron et al., 2007;Leisen et al., 2012;Quesnel et al., 2019) directement expulsées des niveaux évaporitiques triasiques (Quesnel et al., 2019). Récemment, il a été suggéré qu'une source alternative de saumure aurait pu être impliquée dans la formation du gisement. ...
... Par la suite, d'autres travaux ont affiné la compréhension des modalités de formation du gisement en suggérant i) que le magnésium nécessaire à la formation du gisement provenait initialement des saumures et ne résultait pas de leurs interactions avec les roches percolés (Boiron et al., 2007), ii) que la dolomitisation des marbres calcitiques puis la talcification des dolomies résultaient d'un même système de circulation de fluide et non d'évènements hydrothermaux distincts (Boulvais et al., 2006;Quesnel et al., 2019) et enfin iii) que les saumures étaient de l'eau de mer évaporée (Boiron et al., 2007;Leisen et al., 2012;Quesnel et al., 2019) directement expulsées des niveaux évaporitiques triasiques (Quesnel et al., 2019). Récemment, il a été suggéré qu'une source alternative de saumure aurait pu être impliquée dans la formation du gisement. ...
Évolution des Pyrénées au cours du cycle varisque et du cycle alpin présente l’évolution des connaissances géologiques pyrénéennes découlant de grands programmes de recherche scientifique du début du XXIe siècle.Cet ouvrage, consacré au cycle varisque et au rifting crétacé, retrace l’évolution du domaine pyrénéen entre 340 Ma et 90 Ma. Il analyse, dans un premier temps, l’état des connaissances du socle des Pyrénées dont la structuration est héritée de l’évolution varisque de ce domaine. Puis il retrace l’évolution cinématique, depuis le Paléozoïque, du domaine méditerranéen occidental. Il traite enfin de l’évolution des connaissances sur le rifting crétacé et les processus sédimentaires et métasomatiques associés à l’individualisation de la limite de plaque Ibérie-Eurasie.
... En profondeur, les fluides peuvent être piégés dans les espaces poreux pour former des eaux connées (Wiltschko et al., 2009 ;Evans et al., 2012). Les saumures primaires de bassin peuvent être expulsées et précipiter dans les espaces poreux (Quesnel et al., 2019). L'exhumation des bassins sédimentaires marque le début de la télogenèse et de l'influence de fluides météoriques (Cathelineau et al., 1999) à l'origine de la dissolution des roches carbonatées superficielles et du transfert d'éléments chimiques (Swart and Oehlert, 2017). ...
... The extension of evaporitic Triassic was widespread across the European basins (Ortí et al., 2017) and commonly distributed in the North-Aquitaine Basin (BRGM et al., 1974;Biteau et al., 2006). The primary brines in Triassic deposits contain a large amount of Mg (Quesnel et al., 2019) and favored the precipitation of Mg-rich minerals, such as dolomite. During the northern Aquitaine transtensional regime (Asti et al., 2022), major convective fluid movement occurred and primary brines were expelled from the Triassic series (Quesnel et al., 2019). ...
... The primary brines in Triassic deposits contain a large amount of Mg (Quesnel et al., 2019) and favored the precipitation of Mg-rich minerals, such as dolomite. During the northern Aquitaine transtensional regime (Asti et al., 2022), major convective fluid movement occurred and primary brines were expelled from the Triassic series (Quesnel et al., 2019). In the study area, the dolomite cements are mainly located at the vicinity of faults in the north-eastern Aquitaine Basin (Boichard and Drullion, 1982;Bouilhac, 1987). ...
Dans un contexte de changement climatique et de pression croissante sur la ressource en eau des aquifères carbonatés, une meilleure compréhension du fonctionnement et de la recharge des réservoirs est nécessaire afin d'optimiser leur exploitation. Une caractérisation sédimentaire et diagénétique des calcaires s'avère indispensable. Ce travail pluridisciplinaire se concentre sur les roches carbonatées du Jurassique moyen et de l'Oxfordien de la bordure nord-est du Bassin Aquitain. Le premier objectif est de quantifier l'influence couplée des fluctuations climatiques à long terme et de la création d'accommodation sur les producteurs carbonatés et l'accumulation de carbonates. Le second objectif est de reconstruire l'histoire diagénétique et paléohydrologique du bassin et de proposer une méthode pour dater les phases de dissolution et les périodes de création de porosité dans les calcaires. Les études menées dans le Bassin Aquitain et l'ouest de la France montrent que l'augmentation du taux de création d'accommodation provoque (1) l'augmentation du taux d'accumulation de carbonates et (2) des changements de producteurs carbonatés, quel que soit le climat. En période de climat aride, la création d'espace disponible est comblée par une production micritique-microbienne. En climat humide, les taux d'accumulation de carbonate diminuent drastiquement et provoque le déclin des producteurs oolithiques, photozoan et micrite-microbien. La combinaison d'observation pétrographique et de la méthode géochronologique U-Pb permet de dater les phases de dissolution et de création de pores, mais également d'établir une paragenèse à grande échelle. Les processus de néomorphisme et dolomitisation sont datés au cours de la période Jurassique supérieur à Crétacé inférieur. Les processus de dissolution postérieure affectent préférentiellement les corps dolomitiques bajociens. Les âges U-Pb mettent en évidences une succession de phases de dissolution-recristallisation au Paléocène-Éocène. Les phases de cimentation calcitique sont synchrones des processus de dissolution lors de périodes de karstification et peuvent se produire en profondeur formant et comblant des pores vacuolaires de taille millimétriques à métriques.
... The Pyrenean metallogenic domain (talc, barite, magnesite, Fe, Pb, Zn) is set in a mid-Cretaceous hyperextensional passive margin leading to exhumation of subcontinental mantle that triggered ubiquitous fluid migration. Although supporting data are yet rare, the genetic context of the Trimouns giant talc deposit with formation of this newly recognized plate-tectonic ocean-continent transitional (OCT) setting, also termed continent-ocean transition (COT), is very likely (Quesnel et al 2019). COTs with exhumed mantle domains are common and form about half of the world's present rifted continental margins (Sapin et al. 2021). ...
Metallogeny is the science of ore and mineral deposit formation in geological space and time. Metallogeny is interdisciplinary by nature, comprising elements of natural science disciplines such as planetology to solid state physics and chemistry, and volcanology. It is the experimental forefront of research and bold thinking, based on an ever-growing foundation of solid knowledge. Therefore, metallogeny is not a closed system of knowledge but a fast-growing assemblage of structured and unstructured information in perpetual flux. This paper intends to review its current state and trends. The latter may introduce speculation and fuzziness. Metallogeny has existed for over 100 years as a branch of Earth Science. From the discovery of plate tectonics (ca. 1950) to the end of the last century, metallogeny passed through a worldwide phase of formally published ‘metallogenetic’ maps. In the last decades, a rapidly growing number of scientists, digitization and splendid new tools fundamentally boosted research. More innovations may be expected by the growing use of an evolving systematic ‘Geodata Science’ for metallogenic research by an increasingly global human talent pool. Future requirements for metallic and mineral raw materials, especially the critical natural elements and compounds that are needed for the nascent carbon-free economy, already drive activities on stock markets and in the resource industry. State geological surveys, academia and private companies embrace the challenges. The new age requires intensified metallogenic backing. In this paper, principles of metallogeny are recalled concerning concepts and terms. A metallogenic classification of ore and mineral deposits is proposed, and the intimate relations of metallogenesis with geodynamics are sketched (ancient lid tectonics and modern plate tectonics). Metallogenic models assemble a great diversity of data that allow an ever better understanding of ore formation, foremost by illuminating the geological source-to-trap migration of ore metals, the petrogenetic and geodynamic–tectonic setting, the spatial architecture of ore deposits and the nature and precise timing of involved processes. Applied metallogeny allows companies to choose strategy and tactics for exploration investment and for planning the work. Based on comprehensive metallogenic knowledge, mineral system analysis (MSA) selects those elements of complex metallogenic models, which are detectable and can guide exploration in order to support applications such as mineral prospectivity mapping, mineral potential evaluation and targeting of detailed investigations. MSA founded on metallogenic models can be applied across whole continents, or at the scale of regional greenfield search, or in brownfields at district to camp scale. By delivering the fundamental keys for MSA, supported by unceasing innovative research, the stream of new metallogenic insights is essential for improving endowment estimates and for successful exploration.
... All mineralogical and geochronological data acquired in recent years in the NPZ and in the peri-Pyrenean basins confirm the strong volume reduction of the Triassic by mechanical crushing and dissolution of evaporites, whose elements participate in the massive metasomatosis of the continental cover and crust up to the major faults of the outer margin where temperatures sometimes reach 550°C. These interactions are accompanied by transfers of chemical elements (major and trace) in huge quantities and are at the origin of world-class mineral deposits such as the talc-chlorite associations of the NPZ bedrock in the Saint Barthélémy massif (Trimouns deposit) (Schärer et al. 1999;Boulvais et al. 2007;Fallourd et al. 2014;Boutin et al. 2015;Quesnel et al. 2019;Odlum and Stockli 2020). ...
The margins of the North Atlantic Ocean, including the Newfoundland and Iberian Margins, present two distinct episodes of rifting: at Permian–Trias times and in the middle Cretaceous. In the South Atlantic Ocean, rifting occurred on the location of the Pan‐African suture, more than 450 Ma after its formation, and the two events are clearly dissociated. At first order, the geodynamic segmentation of the South and Equatorial Atlantic Oceans leads to the formation of different types of passive margins, showing a relationship between the regional geodynamic context and the structural architecture of passive margins. The Central Segment of the South Atlantic Ocean is characterized by sedimentary basins with pre‐ and syn‐break‐up magmatism, and the presence of an approximately 1–2 km‐thick salt layer in the so‐called continent‐ocean transition, overlying a mainly non‐marine sequence.
... similaires à Nkob mettent en évidence des températures d'homogénéisation de l'ordre de 250 à 400°C pour les fluides minéralisateurs à l'origine de la formation du talc (Tornos et Spiro, 2000 ;De Parseval et al., 2004 ;Joshi & Sharma, 2015 ;Boutin, 2016 ;Quesnel et al., 2019). L'étude des inclusions fluides pour les dépôts de talc de Puebla de Lillo, en Ibérie, de H2O (probablement lors de la décomposition de la trémolite) ait été chassé du système sous forme de fluide (Shin et Lee, 2002). ...
Le gisement de talc de Nkob, situé dans la boutonnière de Siroua de l'Anti-Atlas central, est encaissé par les formations métasédimentaires du Groupe de Taghdout dans l'auréole de contact du granite édiacarien d'Amassine. Il consiste en une alternance stratifiée de marbres verts riches en serpentine, de marbres dolomitiques noirs, de talcitites et de phlogopitites (éventuellement rétrogradés en chlorites), le tout enclos dans des métapélites à andalousite et des quartzites. Deux stades métamorphiques principaux ont été définis pour la formation des marbres et des talcitites : (1) un stade à haute température s'est produit dans des conditions de faciès amphibolite (> 500°C). L'augmentation de la température pendant la mise en place du granite a conduit à la formation d'assemblages contenant de la forstérite, de la trémolite et de la phlogopite (±diopside) dans les marbres, ainsi que la formation de marbres dolomitiques purs, dépendant de la nature du protolithe pré-métamorphique, et la distribution de la température dans l'auréole de l'intrusion. Ce stade à haute température est également caractérisé par la croissance de porphyroblastes d'andalousite dans les métapélites entourant le gisement. (2) Un stade hydrothermal rétrograde à basse température caractérisé par l'infiltration d'un fluide aqueux riche en silice (vraisemblablement dérivé du pluton). Il s'est produit dans des conditions de faciès schiste vert (<520°C). Ce stade a généré la formation de silicates hydratés, principalement de la serpentine et du talc (+ calcite). La formation de ces minéraux silicatés hydratés, a eu lieu à l'intérieur des roches (le long des limites des grains) ou à l'intérieur des veines. Le talc s'est formé par deux processus au sein du gisement de Nkob : le talc aciculaire dans les talcitites près du granite s'est formé après la décomposition de la trémolite dans une gamme de température de 350 à 500°C, tandis que le talc tabulaire dans l'auréole externe s'est formé par réaction entre la dolomite et les fluides aqueux siliceux à des températures inférieures à 350°C. Les marbres verts (et noirs-verts) dérivent de dolomies siliceuses affectées par des interactions avec des fluides aqueux riches en silice, tandis que les marbres noirs dérivent de dolomies pures (à légèrement siliceuses). Les couches de talcitite ont également été affectées par des circulations hydrothermales similaires en H2O et Si mais leur précurseur dolomitique contenait probablement une fraction plus importante de silicates détritiques. De nouvelles données géochronologiques indiquent une mise en place du granite d'Amassine vers 601 Ma, et des âges allant de 1.70 à 3.0 Ga pour la population des zircons détritiques des métapélites du Groupe de Taghdout. Bien que nous ne disposions pas d'une datation directe de la minéralisation, les datations 40Ar/39Ar sur les muscovites séparées de deux échantillons de cornéennes à andalousite situées dans l'auréole du granite, donnent des âges de 592 ± 2 et 591 ± 3 Ma. Les datations 40Ar/39Ar d'hornblende séparée d'un échantillon d'amphibolite, documentent des âges apparents (380-457 Ma) correspondant probablement à une réouverture du système isotopique au cours du Paléozoïque. L'intégration de l'ensemble de ces résultats dans le contexte anti-atlasique régional, met en évidence l'importance de l'événement magmatique felsique post-collisionnel ayant lieu durant l'Ediacarien pour la formation du gisement de talc de Nkob. Cependant, bien que la majorité des gisements associés à cet événement soient formés vers l'Ediacarien terminal, le gisement de talc de Nkob, quant à lui, apparait plus précoce et reste associé au début de la mise en place de ce magmatisme (entre 600 et 603 Ma). Enfin, les résultats obtenus durant ce travail fournissent des guides susceptibles d'orienter efficacement les futures campagnes d'exploitation.
