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Paleocirculations in shales

¹ CREGU and EP 578, BP23, 54501 Vandoeuvre-lès-Nancy Cedex, France
² Centre de Géochimie de la Surface (CNRS-ULP), 1 rue Blessig, 67084 Strasbourg Cedex, France
³ Institut de Protection et de Sûreté Nucléaire/DPRE-SERGD, BP6, 92265 Fontenay-aux-Roses Cedex, France
^* present address: UMR G2R-7566 Géologie et Gestion des Ressources Minérales et Energétiques, BP239, Université H. Poincaré Nancy 1, 54506 Vandoeuvre-lès-Nancy Cedex, France
^** present address: Département des Sciences de la Terre, UMR ORSAYTERRE, Université Paris-XI, 91405 Orsay Cedex, France. - e-mail: pagel{at}geol.u-psud.fr

Compressional Pyrenean tectonic activities induced fractures in the Toarcian shales of the experimental site in the Tournemire tunnel (Aveyron, France). A combined petrological, mineralogical and geochemical study of oriented fractures, mainly filled with calcite, allowed observation and characterisation of four mineral parageneses (calcite, calcite and framboidal pyrite, calcite and cubic pyrite, and calcite and barite). Major’ and trace-element contents of the vein calcite are positively correlated with those of the host rocks, especially at the shale-marl transition. These relationships are interpreted as resulting from short-distance migrations for the considered elements (Fe, Mn, Cu, Mo, ...). The relatively constant chemical composition of the calcite implies negligible variations in the physico-chemical crystallisation conditions. Low uranium and high iron contents of the vein calcite suggest circulation of reducing fluids. The ⁸⁷Sr/⁸⁶Sr ratios of the vein calcite range narrowly from 0.70841 to 0.70858, favouring a buffering by the surrounding shales. The two extreme ⁸⁷Sr/ ⁸⁶Sr values were determined in samples located in or near the main faulted and brecciated zone, suggesting either external inputs or different water-rock ratios. With the exception of this main faulted and brecciated zone, microfissuring most probably favoured local and limited fluid migrations.

Key-words: calcite veins, shales, Sr isotopes, fluid circulation, Tournemire site, radioactive waste.

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