Sulfur redox reactions and formation of native sulfur veins during low grade metamorphism of gypsum evaporites, Cameros Basin (NE Spain)

Vein deposits containing native sulfur, gypsum, quartz and rare sphalerite are described from Cervera del Rio Alhama, in the very low-grade metasediments of the Mesozoic Cameros Basin of NE Spain. The veins are hosted by lacustrine evaporites which comprise alternations of dolomite and gypsum (anhydrite during metamorphism) layers. Fluid inclusion homogenization temperatures and quartz–sulfate oxygen isotope geothermometry indicate formation of the veins at ≈225°C. Fluid inclusions contain S° along with a gas phase comprising H2S, N2, CO2 and minor CH4. These are all likely reactants and products of S° generation by thermochemical sulfate reduction (TSR) by organic matter, followed by partial re-oxidation of some H2S by SO42− to produce S°. TSR-type reactions during low grade metamorphism are thus concluded to be the origin of the S° veins. The TSR reactions we described differ from those observed in most petroleum-related sour gas settings. Firstly, there is no evidence for secondary carbonate precipitation. Secondly, significant S isotopic fractionations exist between sulfate (around +20‰) and reduced products (S° is around −11‰). This is attributed to relatively increased rates of isotopic equilibration compared to TSR that may be related to low availability of organic matter during the formation.