The Precambrian basement of the Tiris region (F'Derik area, Mauritania) contains two iron-formations. One is in the catazonal Tiris Group and the other in the epi- to mesozonal Ijil Group. The Ijil Group forms allochthonous outliers overlying the Tiris Group. The Ijil Group Iron-Formation is mainly banded iron-formations interbedded with clastic sediments and very few volcanic rocks. Locally, outlines of clastic grains indicate a clastic origin for part of the iron-rich sediments. Structures, believed to be old ooliths or intraclasts, are present. In the iron-bearing sediments, the banding appears to be attributable to postdepositional and metamorphic mechanisms.Currently available geochronological data do not permit an accurate estimate of the age of the Ijil Group. As in numerous other Precambrian areas in the world, in Mauritania there are two superimposed iron-formations, one in the Archaean Tiris Group (2,700 to 3,150 m.y.) and the other in the Proterozoic Ijil Group (1,700 to 2,500 m.y.).Iron-formations with some clastic features, overlying a catazonal basement which also contains iron-rich sediments, suggest that iron originally in the basement rocks has been reworked and concentrated into the younger sedimentary rocks.