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Dolomitization and Hg bearing veins in the lower Triassic limestones of the Espadan Ranges (Iberian Chain, E of Spain) are related to a hydrothermal event that occurred during the Santonian (late Cretaceous). Sr-87/Sr-86 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 delta C-13 values of CC1 (-3.7 to -6.1 parts per thousand 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 degreesC-270 degreesC), well above the gas generation window. Quartz has high delta O-18 values, ranging from 18.4 to 22.5 parts per thousand (SMOW). Saddle 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 light 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 by the host carbonates. The small variations of Sr-87/Sr-86 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. (C) 2001 Elsevier Science B.V. All rights reserved. |
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