Paleocirculations in shales: a mineralogical and geochemical study of calcite veins from the Tournemire tunnel site (Aveyron, France)
Résumé
Compressional Pyrenean tectonic activities induced fractures in the Toarcian shales of the experimental
site in the Toumemire 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 87Srf86Sr ratios of the vein
calcite range narrowly from 0.70841 to 0.70858, favouring a buffering by the surrounding shales. The two
extreme 87Srf86Sr 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.