Tim Mersoi Research Articles

Evidence of a Magmatic Intrusion in the Tim Mersoi Basin in Relation With Uranium Mineralizations

The Tim Mersoi Basin is a very rich uranium basin. It is a basin that belongs to the Iullemenden syncline located in the northern part of Niger. Research work in recent years in this basin has revealed significant mineralization. The present study in this basin is based on the QEMSCAN and petrographic analysis of samples particularly highlighted during recent exploration campaigns. This study has shown that the basement of the sector is granitic in nature but the presence of an andesite intrusion has been revealed in this studied area. The presence of this andesite dyke has also shown a close link with uranium mineralization.

Cenozoic oxidation episodes in West Africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies (Imouraren deposit, Tim Mersoï Basin, Northern Niger)

Uranium (U) deposits can undergo oxidizing events, subsequent to which the U may be redistributed or precipitated under distinct mineralogical forms. Occurrences of U6+ minerals can be the result of either (i) in situ oxidation and alteration of previous U4+ minerals or (ii) direct precipitation from U6+-rich solutions. Samples from Imouraren, the largest U6+ore deposit known in the world, located in the Tim Mersoi Basin, Northern Niger, have been studied to determine the nature, distribution, and timing of occurrences of U and related minerals. The host rock is composed of sandstones and analcime-rich siliciclastic rocks of Jurassic age. The U ore is largely dominated by acicular hexavalent U minerals and by a very small amount of uraninite. A comprehensive geochemical model that describes the origin and geochemical setting of this style of economically important mineralization has been developed. The petrographic and mineralogical studies show evidence of in situ uraninite alteration into uranophane and metatyuyamunite, documented by textural relationships and chemical analyses using SEM and electron microprobe. The textural relationship between the U4+ and U6+ minerals is supported by the near-identical REE signatures of the primary uraninite and related secondary U minerals. Ion microprobe U-Pb dating (from 21 to ~ 1 Ma) confirms that primary uraninites were altered to U6+ minerals under supergene conditions during Cenozoic oxidation events in western Africa, in relation with the well-known stages of intense continental weathering that have affected western Africa from the early Cenozoic to recent times.

Hot Fluid Flows Around A Major Fault Identified By Paleothermometric Studies (Tim Mersoï Basin, Niger)

Abstract Faults are mostly considered as low-permeability zones due to their frequent sealing, but they can also contribute significantly to fluid movements in sedimentary basins, and subsequently to temperature anomalies. In favorable cases, paleo-temperatures and pressures can be determined from fluid-inclusion and chlorite thermometry. Using this approach, we estimate the temperature gap between incoming fluids and host rocks along the Arlit Fault, a major fault affecting the whole series of continental siliciclastic formations from Paleozoic to Late Jurassic in the Tim Mersoi Basin (Niger). Strong alteration halos and uranium deposits occur along this regional north–south-trending fault, as well as a secondary network of NE–SW faults. Sandstone porosity is partially reduced by authigenic quartz and Fe-rich trioctahedral chlorites. The chlorites precipitated directly from the solutions under reducing conditions, prior to uranium-oxide deposition. The temperatures and pressures are estimated to be around 115–150 °C and 80–120 bars respectively, and are remarkably similar in all formations from the Carboniferous to the Jurassic. These temperatures are much higher, around 60–80 °C, than those estimated at maximum burial, and document hot fluid flow along the major fault. This thermal anomaly is inferred to be related to the opening of the Atlantic Ocean during the Cretaceous, before the onset of the main exhumation stages during the mid-Tertiary. This study documents the role of faults as significant conduits for fluid flow, and their role in the mass and heat transfer related to the formation of economic deposits.

Biological and physical evidence for extreme seasonality in central Permian Pangea

Climate models indicate increased desertification in the continental interior of Pangea during the Permian, which would have affected the composition of the flora and fauna. We present a multi-proxy paleoenvironmental reconstruction of a terrestrial ecosystem in central Pangea of Lopingian age. The reconstruction is based on biological and physical data from the Moradi Formation, located in the Tim Mersoi sub-Basin, northern Niger. Paleosols and sedimentological evidence indicate that the prevailing climate was semi-arid to very arid with marked intervals of high water availability. Carbon stable isotope data from organic matter and paleosols suggest that both the soil productivity and actual evapotranspiration were very low, corresponding to arid conditions. Histological analysis of pareiasaur bones shows evidence of active metabolism and reveals distinct growth marks. These interruptions of bone formation are indicative of growth rhythms, and are considered as markers for contrasting seasonality or episodic climate events. The macrofossil floras have low diversity and represent gymnosperm-dominated woodlands. Most notable are ovuliferous dwarf shoots of voltzian conifers, and a 25-m long tree trunk with irregularly positioned branch scars. The combined biological and physical evidence suggests that the Moradi Formation was deposited under a generally arid climate with recurring periods of water abundance, allowing for a well-established ground water-dependent ecosystem. With respect to its environment, this system is comparable with modern ecosystems such as the southern African Namib Desert and the Lake Eyre Basin in Australia, which are discussed as modern analogues.

Sedimentology and vertebrate taphonomy of the Moradi Formation of northern Niger: A Permian wet desert in the tropics of Pangaea

Pangaean paleogeographic models place the Tim Mersoi basin of northern Niger in a 5000-km-wide corridor between Gondwana and Laurasia approximately 15 degrees south of the paleoequator. Late Permian paleoclimate models position this basin between tropical summer-wet to the north and desert to the south. Recent investigations of the fossil vertebrates and paleosols in the late Permian (Lopingian) Moradi Formation confirm that the climate was warm and hyperarid with highly seasonal monsoonal rainfall. Possibly as a result of these unusual “wet desert” conditions, the tetrapod fauna shows a high degree of endemism. This study tests existing paleoclimate models by providing additional data on sedimentary environments and vertebrate taphonomic processes. The Moradi red bed sequences accumulated in a gently subsiding sag basin to the west of the tectonically active Massif de l'Aïr. Low angle gravelly alluvial fans prograded westward from the massif and at times impinged on a large stable northward flowing meandering channel system. The interchannel mudrock sequences are over-thickened by the accumulation of loessic silts and preserve isolated skull and post crania of amphibians (Nigerpeton and Saharastega) as well as semi-articulated captorhinids (Moradisaurus). Detailed surface mapping of a fossil-rich exposure revealed an anastomosed network of loess-filled distributary channels incised into the floodplain mudrocks. This provided a locus for the accumulation and rapid burial of at least 15 associated skeletons of the pareiasaurian Bunostegos. Semi-permanent ponds are evidenced by patches of fissile red mudstone containing rare bivalves and spiral coprolites. In the distal floodplains away from the main river channels, the combination of a generally high groundwater table, warm mean annual temperatures, and deflation of fines from the floodplain surface promoted the formation of gypsiferous paleosols and end-point playa lakes. Carbonate-rich mud accumulated around the lake margins and provided ideal conditions for the imprinting and preservation of tetrapod trackways.

DETRITAL MONAZITE IN THE TIM MERSOI BASIN, NIGER: PROVENANCE AND CONTRIBUTION TO THE URANIUM BUDGET IN SILICICLASTIC SEDIMENTS

A detailed study using back-scattered electron imaging and electron-microprobe analysis shows that grains of detrital monazite are present in the Carboniferous, Permian, Triassic and Jurassic sediments of the Tim Mersoi basin, Niger. Despite variations in the size and shape of monazite grains, the U–Th–Pb chemical ages obtained by electron microprobe fall in a narrow range, 425 to 650 Ma. These ages indicate that the monazite grains are issued from the erosion of Pan-African granites and metamorphic rocks from the Air Mountains, and probably also from the basement near Zinder and in Nigeria, a hypothesis in good agreement with paleocurrent reconstruction. The uranium content of monazite from sediments is high (up to 1.7 wt% UO 2 ), but low in adjacent rhyolite. Some monazite grains from the sandstone show evidence of alteration, especially to florencite. We conclude that detrital monazite is a major reservoir of uranium in some sandstones from the Tim Mersoi Basin.

The Permian Moradi Formation of northern Niger: Paleosol morphology, petrography and mineralogy

Three basic paleosol morphologies, named Type A, Type B and Type C, are described from the middle–upper Permian strata of the Moradi Formation, Tim Mersoi Basin, northern Niger. The Moradi Formation is a typical alluvial redbed succession dominated by red mudrocks with fine to coarse-grained pebbly channel sandstones and matrix-breccias. Type A paleosols are hosted by well-sorted fine to medium grained trough cross bedded and massive sandstones and preserve abundant vertical to horizontal micritic and microspar calcite tubules, interpreted as rhizoliths. Lateral variability of rhizoliths in Type A paleosols, and their close association with fluvial channel-fill sediments suggests they are the roots of grove stands of phreatophytic vegetation that grew within unstable anabranching stream systems. Type B paleosols are hosted by mudrocks and preserve well-developed ped structure, abundant micritic calcite nodules and vertically-stacked micritic calcite nodular bodies, as well as rare calcite with satin-spar texture interpreted as a pseudomorphic replacement of pedogenic gypsum. The morphology of Type B paleosols suggests they were formed in well-drained floodplain deposits on stable landforms. Type C paleosols are similar to Type B but preserve pedogenic structures indicative of soil volume expansion and contraction, as well as more abundant Stage II pedogenic carbonate nodules. The morphology of Type C paleosols suggests that they developed periodically rather than seasonally in poorly-drained deposits that nevertheless occupied a relatively stable part of the landscape such as the plains flanking ephemeral lakes or sabkhas.X-ray diffraction analysis of the <2μm fraction from the Moradi Formation strata indicates that paleosol phyllosilicates are composed of illite, smectite, and occasionally kaolinite and talc. Illite is likely a detrital mineral, whereas smectite and kaolinite are likely pedogenic weathering products. The presence of talc in the Moradi Formation paleosols is unusual. It is limited to paleosol horizons that also preserve evidence for pedogenic gypsum accumulation and is therefore most likely related to a pedogenic weathering process. It is possible that this talc is a relatively low-temperature (~50–100°C) diagenetic alteration product of pedogenic Mg–phyllosilicates such as sepiolite.The range of morphologies, petrographic textures and mineralogy of the paleosol profiles indicates semi-arid to hyper-arid climatic setting. This paleoclimatic reconstruction is in agreement with Middle and Late Permian conceptual paleoclimate models and quantitative general circulation models. Nevertheless, and in spite of an arid climate, Moradi paleosols and their host strata also indicate a relatively shallow groundwater table. Importantly, this shallow groundwater resource undoubtedly helped to support the moderately diverse fossil vertebrate assemblage and large-stature macrophytes preserved in the Moradi Formation.

Conulariids of the Upper Talak Formation (Mississippian, Visean) of northern Niger (West Africa)

Conulariids of West Africa have received relatively little detailed attention in the literature on this widespread extinct group of marine cnidarians. Recently, however, Babcock et al. (1995) described two new species, Paraconularia feldmanni and P. sahara, from the upper part of the Mississippian (Visean) Talak Shale Formation in the Tim Mersoi Basin of northern Niger. This was the first report of the genus Paraconularia Sinclair, 1940 from Africa as well as the first assignment of Carboniferous conulariids from that continent to the species level. Since then two of the present authors have collected 24 additional conulariid specimens from the Talak Formation in the same area that yielded the two specimens described by Babcock et al. (1995). Like the previously described material, most of the new specimens, described below, have been worn or weathered to such an extent that it is difficult to identify them to the species level. In some cases, moreover, identification to the genus level is problematical, but this may ultimately be due to the fact that currently recognized conulariid genera (e.g., Moore and Harrington, 1956b) have been defined phenetically, rather than on the basis of prior analysis of the phylogenetic relationships among conulariid species. This means that at least some currently recognized conulariid genera may have been defined by similarities that are primitive or analogous.

Discontinuites et sequences dans la formation de Tarat (Viseen superieur) et l'unite d'Arlit (Namuro-Westphalien) a Arlit (Niger); evolution sedimentaire, climatique et tectonique de la region au Carbonifere

The Tarat Formation is composed at Arlit of three sequences consisting of sandstones and black mudstones rich in organic matter of continental origin. The three of them are interpreted as estuarine sediments deposited in different tectonic frame- works. Tlic Arlit Unit consisls of motley argillaceous sandstones deposited in a very flat lacustrine or alluvial environment and under an arid climate. These formations are separated by a fundamental discontinuity linked to the structuration of tlie Tim Ifersoi Trough. At regional scale, the sediments lying below this discontinuity are grouped in tectono-xdimentary supersequences, which are interpreted as biorhexistasic rhythms ; their deposition was horizontally directed by complex evolving synsedimentary tectonics controlled by the N-S In Azawa Lineament, and was influenced by the permanent proximity of a norllicrn sea. The sediments lying above the fundamental discontinuity belong to tlie i( Continental Intercalaire )) Supergroup ; their deposi!ion was influenced by the Tim Mersoi Trough. The whole Carboniferous sedimentation took place in a climatic context which evolved from cold (Lower Visean) to warm and arid (Upper Carboniferous) through temperate climates.