Abstract
The study aimed at investigating the geochemistry of a sandstone-type uranium deposit in Tarat formation for provenance, paleo-redox, tectonic setting in order to propose a geological model of uranium. X-Ray Fluorescence (XRF) and Induced Coupled Plasma Mass Spectrometry (ICP-MS) analyses were used to determine the sandstone composition. Major, trace and rare earth element compositions of the sandstones have been investigated to determine their provenance, tectonic setting and the influence of weathering conditions. Field studies of Tarat formation revealed that a major accident N30° that can be interpreted as an extension of the Austriche accident ensures the controls of Tarat sedimentation and mineralization deposits and the presence oxidized zone and reduced zone indicate the direction of circulation of the mineralized fluids from West to East. The uranium mineralization is disseminated as pitchblende that gives a yellow color to the sandstone while, molybdenum mineralization gives a blue color to the sandstone. Results of the Geochemical analysis indicate that the sandstone-type uranium deposit of Taratis classified into Protoquarzite (i.e. lithearenite and sublitharenite), arkose, subarkose, wacke and quartz arenite and varied in their maturity. Their Rare Earth Elements (REE) pattern displays high light REE over heavy REE, flat HREE and a significant negative Eu and Ce anomalies and in general correlated well with the NASC and PAAS average composition. The source area may have contained quartzose sedimentary rocks. The geochemical data support deposition in reducing environment of arid to humid climatic conditions rich in organic matter affected by passive margin (PM) tectonic setting and the sediments are derived from felsic and mafic source rocks. Trace and REE data as well as the high values of the weathering indices: Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) suggest intermediate to extreme weathering conditions in the source area or during transportation. The studied sandstones are inferred as highly mature sediments evidenced from their high SiO2/Al2O3 > 1 and the presence of the ultra-stable clay minerals, smectite, kaolinite, chlorite and illite. Also the Th/U < 3 ratio suggests high uranium enrichment of Tarat sandstones over crust average composition. From the factor analysis of the analyzed sandstone, there is no direct relationship between organic carbon and uranium, indicating the absence of organo-uraniferous composite and a major source derived from felsic to the mafic rock of Air Massif. The mineralized fluids of circulating from west to east are usually capped and underlain by impermeable mudstone or clay-rich units and uranium mineralization occurs in the association U-Mo-Pb or U-Zn-Ni-Pb structurally controlled by the major fault in Azaoua lineament fault NS of Arlit in the West, the N70° Tin Adrar “faisceau” in the North, the N30° Mouron accident in the Southeast, N30 Autriche accident in the West and precipitated in areas of suitable reducing environment of arid to humid climatic conditions rich in organic matter affected by passive margin (PM) tectonic setting. The negative correlation of U-Th and U-Na in our samples indicates intense remobilization of uranium in Tarat formation.
Highlights
The Tarat host formation of many deposits from the Tim-Mersoi basin in Arlit is located in the northwestern part of Niger Republic between 18 ̊44'16'' North latitude and 7 ̊23'26'' East longitude, about 200 km north of Agadez, west of the Aïr Massif (Figure 1)
Tim-Mersoi Basin is structured into a synclinal trough, with regional scale, affected by brittle, mixed or by plicatives accident low amplitudes associated with progressive unconformities.Three major structures control the deposit system of Tarat formation such as the Lineament of ArlitIn AzaouaNS accident in the West, the N70 ̊ Tin Adrar “faisceau” in the North and the N30 ̊ Mouron accident in the Southeast (Figure 5 and Figure 6)
The sandstone type uranium deposits of Tarat formation from Tim Mersoi basin, Niger which accumulated in the upper Paleozoic sedimentary part (Carboniferous) of the basin could have been derived from double felsic to mafic rocks of the Air Massif
Summary
The Tarat host formation of many deposits from the Tim-Mersoi basin in Arlit is located in the northwestern part of Niger Republic between 18 ̊44'16'' North latitude and 7 ̊23'26'' East longitude, about 200 km north of Agadez, west of the Aïr Massif (Figure 1). Trace elements such as Y, Sc, Th, Zr, Hf, Cr, Co and REE were expected to be more useful in discriminating source rock compositions and tectonic setting due to their relatively low mobility during sedimentary processes and their short residence times in seawater [17]. These elements are probably transferred quantitatively into clastic sediments during weathering and transportation, reflecting the signature of the parent materials [17]-[26]. The objectives of our research study were to document and update the geological knowledge of these deposits about provenance, paleo-redox and tectonic setting with a view to propose a simplified geological model of uranium for further exploration
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