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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

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