Formation of secondary carbonates and native sulphur in sulphate-rich Messinian strata, Sicily

Microbially formed authigenic carbonates accompanied by native sulphur are present in the 'Calcare Solfifero' below a thick succession of gypsum deposited during the Messinian salinity crisis in Sicily. We sampled these carbonates and associated sulphur in five former sulphur mines to subject them to a detailed petrographic and geochemical study in order to explore their different modes of formation. Native sulphur formed in conjunction with microbial sulphate reduction, which is reflected in its depletion in S-34 (delta S-34 values as low as -2 parts per thousand vs. V-CDT) and an enrichment of S-34 in the residual sulphate (delta S-34 values as high as + 61 parts per thousand). The oxidation of organic matter by sulphate reduction increased alkalinity, inducing precipitation of secondary carbonate minerals. A set of authigenic limestones lacking sulphate minerals, but characterized by pseudomorphs after gypsum and high 6180 values (as high as +9 parts per thousand vs. V-PDB) reflects syngenetic mineral formation within evaporitic settings. Low delta C-13 values (as low as -52 parts per thousand vs. V-PDB) reveal that these carbonate phases were formed by microbial sulphate reduction coupled to the oxidation of biogenic methane. Another set of authigenic carbonates that replaced sulphate minerals is typified by low delta O-18 values (as low as -4 parts per thousand). These carbonates formed epigenetically during later diagenesis following compaction. Dissolution of gypsum or anhydrite by meteoric waters delivered the sulphate for microbial sulphate reduction. Low carbon isotope values of these carbonates (-29 to -5 parts per thousand) indicate that carbonate was derived from the oxidation of crude oil and possibly minor methane, partly involving different degrees of admixture of dissolved carbonate from other sources. Although the studied rocks with their vast amounts of secondary carbonate minerals and sulphur seem to indicate a similar genesis at first glance having formed by biogeochemical transformations of sulphate and hydrocarbons - this study reveals that these processes can occur at different times in variable geological environments. (C) 2010 Elsevier B.V. All rights reserved.