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Processus métallogéniques associés aux interactions hydrothermales entre l'eau de mer et les roches ultrabasiques du manteau, exemple de la ride Médio-Atlantique

Abstract : In this thesis , we studied hydrothermal sulfides that form in ultramafic environments along the Mid-Atlantic ridge , to clarify mantle hydrothermal processes and metallogenic processes behind enrichments metal sulphides . At the regional level , vulnerabilities low angle detachment , which bear the mantle rocks outcrop , control fluid circulation , while the engine of hydrothermal convection is activated locally by gabbroic intrusions within the mantle rocks . Hydrothermal sulfides studied in this work from different hydrothermal sites ultrabasic substrate ( Ashadze , Logatchev , Rainbow ) , as well as sites basaltic substrate (Snake Pit, Krasnov ) . The mineralogical characterization of hydrothermal samples by microscopic observation and X-ray diffraction spectroscopy has led to a selection of samples before more detailed analyzes . Chemical analysis by spectroscopy , X-ray fluorescence and ICP for the determination of isotopic composition of copper and trace concentrations of metals, tin and rare earth , were performed on various sulfides of copper ( chalcopyrite , bornite , digenite , covellite , stannite ) the peculiarity of having the copper precipitate in various forms depending on the temperature, pH , redox conditions and fugacity of oxygen and sulfur of the medium. The study of sphalerite, zinc sulfide, has been primarily used as a comparison to the concentrations of trace metals. The study of sulphides along these faults causes a number of geochemical and mineralogical characteristics of hydrothermal mineralization associated with mantle rocks on slow-spreading ridges . These characteristics indicate local metallogenic processes in mineralization formed on the ocean floor. On a more global scale, our results also discuss the characteristics of mantle hydrothermal circuits and the nature of the rocks involved in hydrothermal reactions in depth. The enrichment of copper sulphides primary and secondary hydrothermal sites is characteristic of ultramafic environments. The study of the isotopic composition of the metal 's ability to work on the evolution of hydrothermal structures and trace the chemical reactions that occur there . The lack of variation in the different rocks where copper is extracted does not discuss its source. However the important copper isotope fractionation during oxidation of primary sulfides highlights local processes across the fireplace. The oxidation and reduction of copper, and the dissolution and precipitation of secondary sulfides fractionate the isotopes of copper and will either impoverish heavy isotopes in the case of residual sulfides , or enrich heavy isotopes in case of secondary sulphides from fluid alteration . The acquired data also indicate maturation Mountains by incorporation and reduction of oxidized sulphides and therefore split into copper. Trace metals from sulphides are good tracers of the substrate leached . The abundance of nickel reflects the hydrothermal interaction with the ultrabasic rocks . High concentrations of tin and the REE chondrite normalized and negative europium anomalies can be explained by leaching of acid rock , such as trondhjemites located at the top of gabbroic intrusions . The presence of these rocks in ocean cores complex involves partial melting of gabbros during cooling at depth. This fusion is induced by lowering the melting point of the gabbro associated with the injection of seawater depth along the detachment faults . The concentrations of trace metals also allow you to work on complex involved in the transport of these metals and thus to study the liquid phase at the origin of transport. Phase separation occurring during the ascent of fluids, generates two types of fluids, fluid and high salinity fluid condensed vapor of low salinity . These fluids may take different paths along faults can thus be considered that during boiling , the vapor is separated from the brine and migrates to the top of the wall to power the outputs of fluids. This separation and fluid migration are controlled, and probably facilitated by the presence of the detachment fault on the wall of the rift. The detachment faults generate acid rock trondjhémites rich in tin , which will enrich the ultrabasic mineralization . The tin is then extracted from these rocks by hydrothermal interactions and precipitate on the surface in the form of stannite in the hot zones of hydrothermal vents . Primary concentrations of tin in sphalerite can be remobilized and reconcentrated , in both sphalerite and chalcopyrite when replacing sphalerite by chalcopyrite inside chimneys and probably also in the heart of the cluster sulfide. Geochemistry of minor elements in sulphide mineralization allows to estimate the rock types involved in hydrothermal reactions, and the evolution of certain metals depending on the chemical conditions of the environment . Our studies of hydrothermal mineralization associated with ultramafic hydrothermal systems show the central role of large detachment faults . Indeed, on the one hand they control cable and outputs of hydrothermal fluids and secondly they generate by injecting water depth of acid rock , unusual on the back . In turn, these acidic rocks such as trondhjemites , reflect broader processes controlled by unusual circulation of large volumes of water along the detachment faults , which is an important characteristic of slow spreading ridges . In conclusion , the study of hydrothermal interactions along major detachment faults is a crucial step in understanding and location of hydrothermal systems . Knowledge of polymetallic mineralization is essential to global economic development of land resources .
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Catherine Evrard. Processus métallogéniques associés aux interactions hydrothermales entre l'eau de mer et les roches ultrabasiques du manteau, exemple de la ride Médio-Atlantique. Géochimie. Université de Bretagne occidentale - Brest, 2012. Français. ⟨NNT : ⟩. ⟨tel-00927274⟩

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