Paleocirculations in shales: a mineralogical and geochemical study of calcite veins from the Tournemire tunnel site (Aveyron, France)

Compressional Pyrenean tectonic activities induced fractures in the Toarcia n 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 characteris ation 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 h ost rocks, especially at the share-mart transition. These relationships are interpreted as resulting from short-distance migrations for the considered elements (Fe, Mn, Cu, Mo, ...). The relatively constant chemical compositi on of the calcite implies negligible variations in the physico-chemical cry stallisation conditions. Low uranium and high iron contents of the vein cal cite suggest circulation of reducing fluids. The Sr-87/Sr-86 ratios of the vein calcite range narrowly from 0.70841 to 0.70858, favouring a buffering by the surrounding shales. The two extreme Sr-87/Sr-86 Values were determin ed in samples located in or near the main faulted and brecciated zone, sugg esting either external inputs or different water-rock ratios. With the exce ption of this main faulted and brecciated zone, microfissuring most probabl y favoured local and limited fluid migrations.