International audienceThe Kiggavik project, located 70 km west of Baker Lake (Nunavut), is a major uranium exploration project in the Canadian arctic, with three significant basement-hosted uranium deposits (Kiggavik, End, and Andrew) which spread along a NE SW trend a few kilometers from the south-eastern border of the Thelon Basin. These deposits are closely associated with alteration zones in which clay minerals are abundant. At the scale of the whole structural trend, the alteration paragenesis is composed of illite +/- sudoite +/- hematite +/- aluminum phosphate sulfate minerals (APS). Alteration petrography and mineral paragenesis are similar to those identified in basement-hosted uranium deposits related to Paleoproterozoic unconfornaities in the Athabasca Basin (Canada) or Alligator River (Australia). The alteration haloes are characterized by two similar types of phyllosilicate assemblages (dioctahedral micas or illite and chlorites) corresponding to a regional retrograde metamorphic stage that was overprinted by hydrothermal alteration during the mineralization event. These two assemblages can be distinguished on the basis of crystallographic and chemical properties and mapping of structural parameters, such as the variation of crystallinity along the c-axis or the polytypes of phyllosilicates, which can be used as a vector to mineralization. The crystal chemistry of the hydrothermal phyllosilicates replacing the previous metamorphic minerals indicates a release of iron. This last point is fundamental to the occurrence of hematite in alteration zones and points out the potential effects of the iron redox state in the control of uranium precipitation during the hydrothermal even

This study presents the structural evolution of the N70° trending Paleozoic-Mesozoic DASA graben, which is considered a sub-basin of the Tim Mersoï basin, and is located in northern Niger. The DASA graben is a uranium-rich trough discovered in recent exploration surveys. A tectono-sedimentary analysis of the DASA graben was implemented with a combined use of satellite imagery, field observations, borehole data analysis and available literature. This graben was affected from the Carboniferous to the Early Cretaceous by two major tectonic periods. The first period was an uplifting stage, which prevailed during the Carboniferous-Permian times and the second, ranging from the Triassic to the Early Cretaceous, corresponds to a rifting episode. The particularity of the DASA graben is that the sediments contain very high uranium grades. Lithological and tectonic factors controlled the emplacement of the uranium mineralization in the graben. The successive fracturing phases that affected the DASA graben were associated with a greater circulation of hydrothermal fluids and would have favoured higher grades of uranium mineralization.

In 2010, a drift prospecting study was initiated over the Kiggavik uranium deposit under the Geomapping for Energy and Minerals (GEM) Program. The objective of this study was to document the till geochemical signatures of the Kiggavik uranium deposit and to apply these geochemical characteristics for future exploration for buried, drift-covered uranium deposits. The study area is within the zone affected by the migration of the Keewatin Ice Divide of the Laurentide Ice Sheet. Mineralized bedrock and surface till samples (n=71) were collected directly overlying, up-ice, and at various distances (50 m, 100 m, 200 m, 500 m, 1 km, 2 km, 3 km, 5 km, and 10 km) in a fan-shaped pattern down-ice from the deposit with respect to the dominant north-northwest, northwest, and west ice-flow directions. Samples containing the highest metal contents were located directly to the west of the deposit in locally derived, basement-dominated grey till, which markedly contrasts with the regional red till dominated by material eroded from the Baker Lake and Wharton Groups of the Dubawnt Supergroup. Till geochemistry exhibits a polymetallic dispersal signature down-ice of the Kiggavik Main Zone outcrop. Uranium, Bi, Mo, Au, Ag, Co, Cs, Pb, and W range from elevated to anomalously high concentrations up to 1 km down-ice of the Main Zone and thus can be utilized as pathfinder elements. These pathfinder elements are also present down-ice from other U deposits within the Kiggavik camp, demonstrating their broad applicability to U exploration in basement rocks near the Thelon Basin. Analysis of the <0.063 mm and <0.002 mm fraction of the till matrix shows that elemental abundances are significantly greater in the finer fraction, indicating a strong geochemical partitioning based on grain size. Laboratory gamma-ray spectrometry, Pb isotopic analysis, and X-ray diffraction were conducted on the till samples. Results show that eU and Pb isotope ratios (206Pb/204Pb, 207Pb/204Pb, 207Pb/206Pb) in the till matrix share a strong correlation with U content in till and can be used as a geochemical tool for U exploration. The clay mineralogy of samples with elevated to anomalously high U is enriched in illite and kaolinite relative to quartz. Pebble lithological and Pb isotope ratio analyses have trends that can be applied to deciphering till matrix provenance.

Located in northern Niger, the N70° DASA graben is a trough discovered recently in the Tim Mersoï basin. In this study, a tectonic history of the DASA graben was presented based on the combined use of satellite imagery, field observations and measures, available literature and borehole data. These data were used to analyse the sedimentary facies and the tectonic deformations in the DASA graben, and derive their relative chronology. For this purpose, uplift and rift deformations and their interactions with sedimentation were characterized. Overall, the analyses suggest that the DASA graben was affected from the Carboniferous to the Cretaceous by three major tectonic phases: the first phase was an uplifting stage with extension during the Carboniferous to Permian; the second phase was a rifting stage. The mean extension was ~ N160° and dominantly produced ENE-WSW trending normal faults; and the third phase was a postrift stage. It was characterized by a ~ N130° compression. The structural and sedimentological features defined the DASA graben as a particular type of syn-sedimentary basin evolving from a transpressive tectonic regime during the period ranging from Carboniferous to Permian to an extensive tectonic regime during the period ranging from Triassic to Lower Cretaceous. Thus, the second period marked by an extensional regime would probably be related to the initial stage of the opening of the Atlantic Ocean. After this rifting period, the DASA graben was affected by a compressional phase related to the collision between Africa and Europe.

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