The Late-Archean Magmatic Gold Event: Toward a Non-Uniformitarian Approach.

International audienceGold is exceptionally abundant at the end of Archean. Three main steps are recognized: (1) a late TTG event at the core of calderas, producing small porphyries and gold-rich VMS deposits; (2) a sanukitoïd event, producing alkaline and felsic subvolcanic plutons with large gold hydrothermal systems, and (3) a hydrothermal/structural event. A large part of the gold mineralization may be directly related to the unique geodynamic situation, including the first subduction with two generations of gold-enriched intrusions

La reconstruction et la résilience urbaine : l'évolution du paysage urbain

Malgré l'impression de pérennité qui se dégage de leur structure, les villes sont vulnérables: en l'espace de quelques heures, elles peuvent être détruites durant une catastrophe naturelle ou un conflit. L'histoire urbaine regorge d'exemples de villes ainsi détruites qui ont par la suite été reconstruites. Loin d'être de simples phoenix renaissant de leurs cendres, ces milieux urbains illustrent toute la complexité du processus de résilience urbaine, à savoir la capacité des villes de retrouver un état de stabilité après avoir subi un traumatisme plus ou moins important. Partant de l'idée que les regards contemporains portés sur la reconstruction sont directement hérités des registres d'interprétation mis en place depuis la destruction des villes, la thèse explore le processus de transmission de la ville reconstruite comme paysage urbain. En émettant l'hypothèse que la reconstruction des villes ne dépend pas uniquement d'une démarche pragmatique destinée à rétablir un cadre bâti tangible mais qu'elle est également issue d'un ensemble de discours desquels ressortent des représentations de la ville qui est reconstruite, la recherche propose de mieux comprendre les dimensions physique et symbolique qui font partie intégrante du processus de résilience urbaine. À travers l'analyse des regards portés sur les villes de Dunkerque (France) et de Coventry (Angleterre), toutes deux détruites à plus de 80% durant la Deuxième Guerre mondiale et dont les reconstructions ont été largement présentées dans les médias, la recherche dégage plusieurs registres discursifs: l'étude des plans de reconstruction, des articles des quotidiens locaux et des revues spécialisées de l'époque, des guides touristiques, des journaux municipaux, voire même des cartes postales, des expositions et des affiches permet de mettre en lumière une pluralité des trames narratives et une polysémie de la reconstruction. Ces discours, mis en place par les divers acteurs de l'urbain, ne sont pas linéaires: certains procèdent par rétroaction positive, d'autres apparaissent puis disparaissent, etc. Leur analyse révèle des configurations spatio-temporelles différentes. On comprend ainsi comment l'image de la ville reconstruite est mise en place et évolue. La reconstruction sous-tend des mécanismes complexes de créations et de re-créations, d'interprétations et de réinterprétations du fait urbain. La superposition et la succession des discours illustrent la quête d'une vision commune de la ville reconstruite (modification des temps et des rythmes de la ville en fonction de l'urbanisme choisi, des vitesses de reconstruction, du vieillissement de la reconstruction, etc.). La portée du modèle proposé peut aussi bien être appliquée aux cas anciens pour permettre aux acteurs de l'urbain une intervention adaptée sur la ville d'aujourd'hui que sur les cas récents de milieux urbains détruits pour une meilleure compréhension des enjeux de la reconstruction. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Reconstruction, Résilience urbaine, Paysage urbain, Patrimoine, Urbanisme

Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex

The Richat structure (Sahara, Mauritania) appears as a large dome at least 40 km in diameter within a Late Proterozoic to Ordovician sequence. Erosion has created circular cuestas represented by three nested rings dipping outward from the structure. The center of the structure consists of a limestone-dolomite shelf that encloses a kilometer-scale siliceous breccia and is intruded by basaltic ring dikes, kimberlitic intrusions, and alkaline volcanic rocks. Several hypotheses have been presented to explain the spectacular Richat structure and breccia, but their origin remains enigmatic. The breccia body is lenticular in shape and irregularly thins at its extremities to only a few meters. The breccia was created during karst dissolution and collapse. Internal sediments fill the centimeter- to meter-scale cavities. Alkaline enrichment and the presence of Cretaceous automorphous neoformed K-feldspar demonstrate the hydrothermal origin of these internal sediments and their contemporaneity with magmatism. A model is proposed in which doming and the production of hydrothermal fluids were instrumental in creating a favorable setting for dissolution. The circular Richat structure and its breccia core thus represent the superficial expression of a Cretaceous alkaline complex with an exceptionally well preserved hydrothermal karst infilling at its summit

Relationship between karstification and burial dolomitization in Permian platform carbonates (Lower Khuff - Oman)

Large breccia fabrics associated with karst constitute an important structure in massive limestone successions. The dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids, but subsequently breccia fabrics can also govern the migration of later fluids. Therefore, breccias are highly relevant features to capture for reservoir characterisation. Outcrop analogues for Lower Khuff units in the Middle East present in the Central Oman Mountains reveal brecciated fabrics up to 10’s of meters in diameter. These brecciated units are closely associated with dolomite bodies of late diagenetic origin. Based on an integrated set of data, the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids. The exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids. Based on the composition of the clasts and matrix in the breccias, two dolomitization events are interpreted to have affected the succession, one prior to (early diagenetic [ED] dolomite) and one after brecciation (late diagenetic [DT2] dolomite). Dolomite of shallow burial origin (ED dolomite) was only observed as clasts within breccia and is much more frequent than late diagenetic (medium to deep burial) dolomite clasts. Thus, the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization. Late diagenetic replacive dolomite (DT2 dolomite) forms 90% of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization, but is rarely present as clasts. Stable isotope measurements [δ18O: − 2.5‰ to − 6‰ VPDB and δ13C: 2.9‰ to 4.8‰ VPDB] suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids. The dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities. Thus, early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the Oman Mountains, and potentially in other similar settings elsewhere

Le magmatisme de la région de Kwyjibo, Province\ud du Grenville (Canada) : intérêt pour les\ud minéralisations de type fer-oxydes associées

The granitic plutons located north of the Kwyjibo property in Quebec’s Grenville Province are of\ud Mesoproterozoic age and belong to the granitic Canatiche Complex . The rocks in these plutons are calc-alkalic, K-rich,\ud and meta- to peraluminous. They belong to the magnetite series and their trace element characteristics link them to\ud intraplate granites. They were emplaced in an anorogenic, subvolcanic environment, but they subsequently underwent\ud significant ductile deformation. The magnetite, copper, and fluorite showings on the Kwyjibo property are polyphased\ud and premetamorphic; their formation began with the emplacement of hydraulic, magnetite-bearing breccias, followed by\ud impregnations and veins of chalcopyrite, pyrite, and fluorite, and ended with a late phase of mineralization, during\ud which uraninite, rare earths, and hematite were emplaced along brittle structures. The plutons belong to two families:\ud biotite-amphibole granites and leucogranites. The biotite-amphibole granites are rich in iron and represent a potential\ud heat and metal source for the first, iron oxide phase of mineralization. The leucogranites show a primary enrichment in\ud REE (rare-earth elements), F, and U, carried mainly in Y-, U-, and REE-bearing niobotitanates. They are metamict and\ud underwent a postmagmatic alteration that remobilized the uranium and the rare earths. The leucogranites could also be\ud a source of rare earths and uranium for the latest mineralizing events

Ghost towns as markers of the resilience conditions along extractive frontiers

International audienceSince the Neolithic revolution, mining occurs in the semiperiphery of civilizations. It provides wellness outside the core urbanized zone and exchanges knowledge, money and infrastructures against minerals. The dynamic establishment of such a pattern goes throughout the development of an extractive Frontier moving outward from the economic core zone. However, extractive Frontiers may fail and leave ghost towns and remnants of mining workings. Thousands of ghost mining towns (GMT) have occurred on all continents since the Chalcolithic times.The study of mining ghost towns allows to distinguish several extractive Frontiers based on their environment. Most of them are in unpopulated highly inhospitable or poorly accessible environments such as hot and cold deserts, and high mountain ranges. Three types could be recognized along the mining cycle: (1) GMT where, although it was claimed that resources were exhausted, mining development failed early due to lack of capital and/or knowledge. These GMT are often related to gold rushes in the 19th century (Macetown, New Zealand; Bannack, Montanna; Kolmanskop, Namibia) and especially abundant in Australia and West America. (2) GMT occurring during or after the production, due to disruptive innovations that make the product outdated such as asbestos, micas, saltpeter and sulfur (Atacama, Sicilia); this is the probable future for petroleum-related cities. (3) GMT occurring after mining that had exploited most of the ore for decades: this has occurred several times since the Bronze age, especially in Central Europe and south-west Asia, but also since the 20th century (Hashima Island, coal, Japan; Joutel, copper, Canada).What are the elements that make or fail the resilience of a Frontier mining town? What lessons can we learn from history? How can they help understand the resilience of mining territories and communities? The presentation will offer an insight from history through the lens of resilience thinking

Le district filonien à Pb-Zn-Ag et carbonates du Jebel Aouam (Maroc central)

The Jebel Aouam district is the site of a succession of veins containing Sb, W and Pb-Zn-Ag. The latter mineralizations are concentrated in two major veins extending over several kilometres at depths up to 850 m. Four mineral formations may be distinguished within the Signal vein : (1) siderite-sphalerite-argentiferous galena, quartz and Ni-Co minerals. Anhydrite and adularia have also been identified. This formation was emplaced during an ENE-WSW compression ; (2) calcite I-ankerite-galena, corresponding to a sporadic tension opening ; (3) barite, galena ; (4) calcite II, pyrite. The fluid inclusions of the quartz and sphalerite of formation (1) display low temperature homogeneizations and high salinities. The Pb-Zn mineralized carbonates present very high contents in rare earth elements, contrasting with the barren carbonates. This may reflect a leaching of the elements at depth.Le district du Jebel Aouam montre une succession de filons à Sb, W puis Pb-Zn-Ag. Le dernier groupe est représenté par 2 filons majeurs de longueur plurikilométrique, avec des dénivelés atteignant 850 m. Quatre formations minérales sont distinguées dans le filon du Signal : (1) sidérite-blende-galène argentifère, quartz et minéraux de Ni-Co, anhydrite et adulaire sont mis en évidence. Cette formation se met en place au cours d'une compression ENE-WSW ; (2) calcite I-ankérite -galène, correspondant à une ouverture saccadée en tension ; (3) barytine, galène, au sein des formations précédentes ; (4) calcite II, pyrite terminale. Les inclusions fluides du quartz et de la blende de la formation (1) montrent des homogénéisations à basse température et des salinités élevées. Les carbonates minéralisés en Pb-Zn présentent des teneurs très élevées en terres rares qui les opposent aux carbonates stériles, ce qui pourrait témoigner d'un lessivage de ces éléments en profondeur.Jébrak Michel. Le district filonien à Pb-Zn-Ag et carbonates du Jebel Aouam (Maroc central). In: Bulletin de Minéralogie, volume 108, 3-4, 1985. Minéralogie dans les Sciences de la Terre et l'Industrie - Hommage à François Permingeat

Economic Geology: Then and Now

The science of mineral deposits (economic geology) has greatly evolved since its inception in the late 19th century, and has subsequently been strongly influenced by mining discoveries. It is a science that has moved from a descriptive phase to a deeper understanding of ore-body genesis. As a result, deposit types are increasing in number, classification systems are improving, and we are beginning to recognize how spatial and temporal distributions relate to plate tectonic mechanisms. Our understanding of ore-forming mechanisms has broadened, thanks, in part, to widely available isotopic dating methods and to advances in analytical techniques that determine the ore-element sources, transport conditions and depositional processes. It also appears that the climate and fundamental geodynamic processes (i.e. mantle plumes) play important roles in ore-deposit formation. SOMMAIRE La science des gîtes minéraux (métallogénie) s'est beaucoup développée depuis sa création à la fin du XIXème siècle, et a été fortement influencée par les découvertes minières. On est passé de la description à la compréhension des gisements, avec une augmentation du nombre de type, de meilleures classifications et les débuts d'une compréhension des distributions spatiale et temporelle en liaison avec les mécanismes de la tectonique des plaques. Les mécanismes de formation ont été mieux compris, en partie grâce aux nombreuses datations isotopiques disponibles, et aux progrès dans l'analyse des processus de source, de transport, et des conditions de dépôt. Le climat et les processus de crises géodynamiques (plumes mantelliques) semblent jouer un rôle significatif dans la formation des gisements

The “eye of Africa” (Richat dome, Mauritania): An isolated Cretaceous alkaline–hydrothermal complex

The Richat dome is a spectacular circular structure located in the Mauritanian part of the Sahara Desert. The current erosion level of this igneous complex presents a wide variety of contrasting extrusive and intrusive rocks from shallow to deep source regions providing insight into the magmatic process at the origin of the complex. The Richat is the superposition of a bimodal tholeiitic suite crosscut by carbonatitic and kimberlitic magmatic rocks. The bimodal series is characterized by two concentric gabbroic ring dikes and two extrusive rhyolitic centers representing the remnant of two maar systems. Silica undersaturated magmas occur as carbonatite dikes, a kimberlite plug, and kimberlite sills extruded along the old regional anisotropies filling NNE–SSW dextral strike-slip faults and en-echelon tension gashes. An intense low-temperature hydrothermal event affected the Richat area. It is responsible, notably, for the karst-collapse central mega-breccia, the alteration of the rhyolites, the potassic alteration of the gabbros and the stable isotope enrichment in the carbonatites. A piston-like collapse is proposed to explain the contrast existing between the central and outer part of the Richat. Structural inheritance played an important role in the history of the Richat complex. Pre-existing anisotropies acted as a pathway for the ascent of asthenospheric and sub-continental melts and allowed the coexistence of alkaline and tholeiitic magmas within the same igneous complex