Hydrothermal flow direction traced by anisotropy of magnetic susceptibility

Abstract : This study presents an attempt to use the Anisotropy of Magnetic Susceptibility (AMS) to trace hydrothermal flow directions. The Chaillac Basin is located at the contact between the Mesozoic Paris Basin and the Variscan Massif Central, (France). The basin has been formed along N-S normal fault that allowed the sedimentation of Hettangian sand (−203 Ma) on a metamorphic basement. Stratiform F-Ba-Fe rich deposits occur as syn-sedimentary mineralization within silicoclastic Hettangian sediments. The mineralization sequence is formed by the succession of fluorite-baritegoethite. It cements the sand and presents surficial sinter structures. The tectonics shows that sinter crusts have been fed by vertical E-W and N-S veins emplaced during a general E-W extension. This later is coeval with the subsidence of the Paris Basin. The principal magnetic carrier is the goethite that has been precipitated by hydrothermal fluids. Magnetic fabrics measured in goethite-rich veins and in proximate sandstones are dominated by a strongly inclined magnetic lineation (maximum axis). In sinter and in sandstone located far from the vein (>1 m) the magnetic fabric is dominated by horizontal lineation with an azimuth similar to the sedimentary direction. The textures indicate that goethite has a free crystallisation in an open media (vein or sand porosity). Thus no late deformation has affected this deposit and the magnetic fabric is primary. Image analyses, performed on goethite shape and spatial distribution, suggest that the magnetic fabric is the result of oriented goethite growth produced by hydrothermal fluid flow. This work proposes a new approach to constrain hydrodynamic model developed in geology by using the AMS method: textural study combined with efficient magnetic fabric measurements should be used for systematic investigation to trace flow direction in fissures and in sand porosity.
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Conference papers
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Stanislas Sizaret, Yan Chen, Alain Chauvet, Eric Marcoux, Jean-Claude Touray. Hydrothermal flow direction traced by anisotropy of magnetic susceptibility. Geophysical Research Abstracts, 2003, Nice, France. pp.Vol.5, 05250. ⟨hal-00084668⟩

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