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

Dolomitization and Hg bearing veins in the lower Triassic limestones of the Espadan Ranges (Iberian Chain, E of Spain) are related to a hydrothermal e vent that occurred during the Santonian (late Cretaceous). Sr-87/Sr-86 rati os of unaltered regional dolostones range from 0.70829 to 0.70945, higher t han those expected for Triassic diagenetic dolostones. Vein filling is cons tituted by a brown cloudy calcite (CC1), quartz, saddle-dolomite, mercurian tetrahedrite ("schwatzite") and a late blocky calcite (CC2). Low delta C-1 3 values of CC1 (-3.7 to -6.1 parts per thousand PDB) calcites resulted fro m the incorporation of light CO2 liberated from the destruction of organic matter within the vein system due to a temperature increase (up to 240 degr eesC-270 degreesC), well above the gas generation window. Quartz has high d elta O-18 values, ranging from 18.4 to 22.5 parts per thousand (SMOW). Sadd le dolomites have low delta O-18 values (20.2 to 17.6 parts per thousand), with also low delta C-13 values, which is consistent with an effervescence of previously formed volatiles (CO2, N-2, H2S). Finally, the isotopically l ight carbon in late calcites (-6.3 to -10.6 parts per thousand) suggests a source related to the oxidation of organic matter whereas the heavy delta O -18 (23.5 to 25.4 parts per thousand) composition reflects O-18 buffering b y the host carbonates. The small variations of Sr-87/Sr-86 ratios in hydrot hermal calcites and dolomites (0.7082 to 0.7089) indicate that strontium is otope composition of the fluids were also buffered by the enclosing limesto nes. (C) 2001 Elsevier Science B.V. All rights reserved.