Origin of vein hydrothermal carbonates in triassic limestones of the Espadلn Ranges (Iberian Chain, E Spain)

Dolomitization and Hg bearing veins in the lower Triassic limestones of the Espadán Ranges (Iberian Chain, E of Spain) are related to a hydrothermal event that occurred during the Santonian (late Cretaceous). 87Sr/86Sr ratios of unaltered regional dolostones range from 0.70829 to 0.70945, higher than those expected for Triassic diagenetic dolostones. Vein filling is constituted by a brown cloudy calcite (CC1), quartz, saddle-dolomite, mercurian tetrahedrite (“schwatzite”) and a late blocky calcite (CC2). Low δ13C values of CC1 (−3.7 to −6.1‰ PDB) calcites resulted from the incorporation of light CO2 liberated from the destruction of organic matter within the vein system due to a temperature increase (up to 240°C–270°C), well above the gas generation window. Quartz has high δ18O values, ranging from 18.4 to 22.5‰ (SMOW). Saddle dolomites have low δ18O values (20.2 to 17.6‰), with also low δ13C values, which is consistent with an effervescence of previously formed volatiles (CO2, N2, H2S). Finally, the isotopically light carbon in late calcites (−6.3 to −10.6‰) suggests a source related to the oxidation of organic matter whereas the heavy δ18O (23.5 to 25.4‰) composition reflects 18O buffering by the host carbonates. The small variations of 87Sr/86Sr ratios in hydrothermal calcites and dolomites (0.7082 to 0.7089) indicate that strontium isotope composition of the fluids were also buffered by the enclosing limestones.