Chemical transformations of type-III organic matter associated with the Akouta uranium deposit (Niger): Geological implications

Geologic and Metallogenic Framework of North Africa

“…Relatively high Nb and Zr concentrations have been reported in the ore (Forbes 1984;Turpin et al 1991). Fluid inclusion studies indicate that the ore-forming fluids were evolved, basin-derived, hot (80-180°C), saline brines (Forbes et al 1987;Forbes 1989), consistent with vitrinite reflectance measurements, Rock-Eval pyrolysis data, and C, H, and O stable isotope values (Forbes et al 1988). U-Pb and K-Ar data constrain the age of uranium mineralization to between *260 and 130 Ma, with two distinct peaks occurring at *190 and *150 Ma (Turpin et al 1991;Pagel et al 2005).…”

“…This major fault, which is interpreted as a polyphase Pan-African structure that was reactivated during the Visean, late Carboniferous, and Early to Late Cretaceous (Valsardieu 1971;Forbes et al 1988), appears to have been a critical factor in ore genesis, providing a fluid channel for U-bearing fluids. In addition to faults or faulted rocks, main ore controls include stratigraphic contacts and lithology, paleogeography, availability of organic matter, movement of underground waters, and redox state (i.e., Eh vs. pH).…”

“…In the Arlit area, the host rocks are a succession of red clastic strata composed of interlayered conglomerate, sandstone, and siltstone with intercalated shale and volcano-sedimentary units (Cazoulat 1985), unconformably overlain by Permian-Triassic arkosic sandstone and conglomerate. Stratigraphic and sedimentological constraints indicate that these rocks were deposited in organic-rich fluviatile and fluoviodeltaic environments, in which westward paleocurrents carried detritus, minerals, and plant debris (Cazoulat 1985;Yahaya et al 1986;Forbes et al 1988). Tectonic uplift, anorogenic magmatism, and subsequent weathering and erosion of the volcanic cover, together with sedimentation in nearby continental basins, all contributed to the development of the uranium mineralization (Bowden et al 1981).…”

“…Roll-front-type structures have been reported in the Akouta deposit (e.g., Cazoulat 1985;Pagel et al 2005, and references therein). Principal uranium minerals are pitchblende and coffinite (Bowie 1979) accompanied by varying proportions of sulphides (pyrite, marcasite, and sphalerite), and montroseite (VOOH) that is a highly oxidized vanadium hydroxide (Forbes et al , 1987(Forbes et al , 1988. Gangue minerals include vanadiferous chlorite, corrensite, and jordisite (Turpin et al 1991).…”

Mineral Radioactivity in Sands as a Mechanism for Fixation of Organic Carbon on the Early Earth

“…Penkov, 1980;Bonnamy et al, 1982). In some cases this may be due to reduction of mobile oxygenated uranium complexes, but the radiolysis of water can also yield oxidizing radicals that can be incorporated into organic materials (Landais et al, 1987;Forbes et al, 1988).…”