Abstract

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.

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