ISOTOPIC AND PETROGRAPHIC EVIDENCE OF THERMOCHEMICAL SULFATE REDUCTION, SIERRA MADRE ORIENTAL, MEXICO

 

The Zuloaga Formation, correlative with the Smackover Formation of the northern US Gulf Coast, consists of a series of trangresive carbonates that unconformably overlies Minas Viejas evaporites.  The Zuloaga Formation is exposed in the core of the frontal fold of the Monterrey salient near Saltillo, and presents distinct structural and geochemical features related to a complex history of burial diagenesis and fold related deformation.  Microcrystalline limestone, dolomitic limestone and discontinuous and irregular breccia bodies host vein fillings containing calcite, quartz, gypsum, and rare pyrite.

Fluid inclusion homogenization temperature and vitrinite reflectance indicate formation of vein material at temperatures in excess of 220°C.  Fluid inclusions in calcite and quartz contain H₂S, CO₂, and minor CH₄.  The d¹⁸O_PDB values of dolomitic limestone and vein calcites range from -14‰ to –8.5‰, and those of breccia fragments range from -9.5‰ to –7.5‰.  The d ¹³C_PDB values of the dolomitic limestone, breccias, and early vein calcites range from -4‰ to +3.8‰.  Late stage calcite veins that crosscut older veins and breccia bodies have d¹³C_PDB values ranging from –21‰ to +10‰.  Bed-parallel gypsum veins, spatially related with the presence of late calcites, have d³⁴S_VCDT values between –15‰ to –11‰, significantly different from the Minas Viejas anhydrite (+16.2‰).  There is a strong inverse correlation between d¹³C_calcite and d³⁴S_sulfate.  The light carbon composition indicates that the carbon incorporated in these calcites was partly derived from thermal oxidation of light hydrocarbon gases.  Petrographic observations of paragenetic relations between individual minerals within the veins and d³⁴S_VCDT values of gypsum suggest re-oxidation of TSR produced sulfides.  Reactants and products of TSR have also been identified in carbonates of the Cupido Formation where it is situated directly above an evaporitic diapir in La Paloma Canon.  In this case the fluid inclusion homogenization temperatures are above 150°C, d¹³C_PDB of late vein calcite ranges from -18‰ to -8‰ and the presence of metal sulfides is significant.

The ubiquitous presence of H₂S, the highly variable isotopic composition of late calcite veins, the presence of metal sulfides, indicates that TSR was an important process related to the late stage of folding.