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

Major- and trace-element contents of vein calcites in a series of tectonically induced fractures in the Toarcian shales exposed in a tunnel site in S France are positively correlated with those of the host rocks, especially at the shale-marl transition. These relationships are interpreted as resulting from short-distance migration of Fe, Mn, Cu, Mo and other trace elements. The relatively constant chemical composition of the calcite implies negligible variations in the physicochemical conditions of crystallization. Low U and high Fe in the vein calcite suggest circulation of reducing fluids. The (super 87) Sr/ (super 86) Sr ratios of the vein calcite range narrowly from 0.70841 to 0.70858 favouring a buffering by the surrounding shales.