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

Abstract

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