Organic geochemical indicators of biological sulphate reduction in early diagenetic Znsingle bondPb mineralization: the Bois-Madame deposit (Gard, France)

Organic geochemical indicators of biological sulphate reduction in early diagenetic Znsingle bondPb mineralization: the Bois-Madame deposit (Gard, France) Original Research Article Pages 419-435 Hu Ming-An, J.R. Disnar, J.-F. Sureau Close Close preview | Purchase PDF (3603 K) | Related articles | Related reference work articles AbstractAbstract | ReferencesReferences Abstract At Bois Madame (Gard, France), Znsingle bondPbsingle bondS mineralization (Zn/Pb >4) occurs in dolomitized Kimmeridgian carbonates very near layers which are rich in organic matter of planktonic origin. Previous geological and geochemical studies have suggested early syn- or diagenetic ore deposition controlled by sulphate reducing bacteria, the metals probably having originated from regionally mineralized Triassic or Liassic formations. Rock-Eval pyrolysis data, complemented by biomarkers, show that the organic matter reached only the organic diagenesis-catagenesis transition owing to burial (<1500 m sedimentary cover). The β,β-sterane concentrations are abnormally high for this maturity, and are postulated to have arisen from low temperature catalytic transformation of α,α-steranes by reduced or elemental sulphur. Bitumens are absent from the Bois-Madame deposit, but are present in the nearby Les Maliens Znsingle bondPb deposit, as in many Pbsingle bondZnsingle bondS deposits. The hydrocarbons, almost certainly autochthonous, show no indication of relatively late alteration by biodegradation or water washing. The presence of hopanes (C27–35), with high abundance in the C27–32 range, indicate that intense bacterial activity occurred during early diagenesis, under su☐ic-anoxic conditions. This interpretation is reinforced by the presence ofn-alkanes in the rangen-C22 ton-C28 dominated by compounds with an even number of carbon atoms which may have derived from the reduction of bacterial fatty-acids. Except for these even numbern-alkanes, the organic geochemical evidence for sulphate reducing bacteria during early diagenesis must remain equivocal.