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Earth and Planetary Science Letters 277, 3-4 (2009) 501-513
Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida (Northern Spain)
Mourad Essalhi ( ) 1, Stanislas Sizaret 1, Luc Barbanson 1, Yan Chen 1, Yannick Branquet 1, Dominique Panis 1, Pierre Camps 2, Pierre Rochette 3, Angels Canals 4

The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn-Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn-Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K2. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterised by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ‹c› axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ‹c› axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE-SW elongation interpreted as the global circulation direction and a NW-SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterised by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.
1 :  GéoHydrosystèmes COntinentaux (GéhCO EA6293)
Université François Rabelais - Tours
2 :  Géosciences Montpellier (GM)
CNRS : UMR5243 – Université Montpellier II - Sciences et techniques
3 :  Centre européen de recherche et d'enseignement de géosciences de l'environnement (CEREGE)
CNRS : UMR6635 – Institut de recherche pour le développement [IRD] – INSU – Université de Provence - Aix-Marseille I – Université Paul Cézanne - Aix-Marseille III
4 :  Departamento Cristalografia, Mineralogia i Deposits minerals
Universitat de Barcelona
Planète et Univers/Sciences de la Terre/Tectonique

Sciences de l'environnement/Milieux et Changements globaux
AMS – carbonates – fluid flow – growth anisotropy – mineral texture
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Essalhi-EPSL-2008.pdf(2.4 MB)