Computer Science
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.2833b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 15, pp.2833-2855
Computer Science
3
Scientific paper
Combined organic geochemical and stable isotope (S) analyses of samples from the Cretaceous Bahloul Formation (Tunisia) provide insight to oil accumulation processes, biogeochemical alteration of hydrocarbons, microbial sulfate reduction, and mineral deposition at the flanks of the Triassic Jebel Lorbeus diapir, forming the Bou Grine Zn/Pb deposit. The sulfur isotopic composition of the metal sulfides correlates with the degree of biodegradation of hydrocarbons, with the base-metal content and with the proportion of aromatics in the organic extracts. The 34 S-values are interpreted to reflect bacterial sulfate reduction in a more or less closed system rather than a thermogenic contribution. The extent of H 2 S production by the activity of the sulfate-reducing bacteria probably was limited by the availability of sulfate, which in turn was governed by the permeability of the respective sedimentary sequence and by the distance to the anhydrite cap rock. Evidence is provided that biodegradation of hydrocarbons and microbial sulfate reduction contribute to the formation of the high-grade mineralization inside the Bahloul Formation at the contact with the salt dome cap rock. The metals probably were derived through leaching of deeper sedimentary sequences by hot hypersaline basinal brines, evolved by dissolution of salt at the flanks of the diapirs. These hot metalliferous brines are proposed to migrate up around the diapir, finally mixing with near-surface, sulfate-rich brines in the roof zone. When the fluids came in contact with the organic-rich sediments of the Bahloul Formation, the dissolved sulfate was reduced by the sulfate-reducing bacteria. Hydrocarbons generated or accumulated in the Bahloul Formation were utilized by sulfate reducers. The occurrence of high amounts of native sulfur in high-grade ore samples suggest that the production rate of H 2 S by bacterial sulfate reduction exceeded its consumption by metal-sulfide precipitation. The supply of dissolved metals probably was the limiting reaction. The trend of 34 S of native sulfur (S 0 ) towards lower values to the top of the highly mineralized Bahloul profile provides evidence for a biogenic origin of S 0 by sulfide oxidizers. The variation in isotopic fractionation between native sulfur and sulfide sulfur in the ore minerals reflects the kinetic isotope fractionation factor connected with metabolic processes and the respective reaction rate during biogenic oxidation of sulfur.
Bechtel Achim
Pervaz M.
Püttmann Wilhelm
Shieh Yuch-Ning
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