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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 …