Black shales, organic matter, ore genesis and hydrocarbon generation in the Paleoproterozoic Franceville Series, Gabon

摘要: Abstract The 2.1 Ga old FB Formation of the Franceville Series, southeastern Gabon, contains one greatest accumulations organic carbon its age. weakly metamorphosed, dominantly pelitic, ranges in thickness from 400 to 1000 m, covers over 35,000 km 2 , and consists about 80% marine shale with 0.5 10% total carbon. Analyses show that, compared black standard SD O-met, average is enriched only Au, Ag, Ba, Cr. General paucities U Mn represent negative anomalies this area world class deposits. However, detailed geochemical profiles lower that Fe/Mn ratios increase upwards base formation. At least locally, build-up sedimentary column may be linked increased oxidation potential caused by photosynthesis. abundance decomposing matter also played a key role further concentrating carbonate protore during early diagenesis. enrichment process continues today on spectacular scale lateritization. Genesis uranium ores closely burial history maturation shales near Series. presence these rocks diverse fossil remains, including Gunflint-type association stromatolites microfossils, various planktonic species Gunflintian age, abundant carbonized microbial-like forms, provides link genesis liquid bitumens Prolific amounts hydrocarbon, remnants which have since solidified, were generated through “oil window”, opened ca. 2.0 Ga ago. coincidence uranium-bearing aqueous solutions main interval hydrocarbon migration led localization petroleum-type structural traps. Some richer deposits served as loci for development Oklo natural nuclear fission reactors. Analysis LA-ICP-MS mineral-free portions solid bitumens, derived shales, confirms metals V, Zn, (possibly organometallic complexes) serves elucidate evolutionary pathways bitumen particular metal associations.