Nature and origin of the ferruginous duricrust of the Khat Oummat Elbid area, Inchiri, Mauritania

Formed through oxidation of underlying primary sulfide- and/or oxide-bearing mineralization, gossans remain a major metallotect for mineral exploration. The identification of gossans within arid regions with ferruginous regolith of different origins is not easy and requires a good knowledge of these superficial red indurated formations on a regional scale. Following this approach, we studied some ferruginous cuirasses in the Khat Oummat Elbid (KOB) region of Inchiri in Mauritania, a region actively prospected for gold and copper in particular. The KOB zone belongs to the orogenic segment of the northern Mauritanides. Two hills with a quartzite basement and capped with ferruginous duricrust were selected for this study. The good outcrop quality made it possible to study the basement, to realize logs across the duricruts and to sample for petrographic, mineralogical (optical microscopy, SEM, DRX) and geochemical (multi-elementary total rock) analyses. The duricrusts are ferricretes in the strict sense with no direct link to the development of lateritic profiles. The quarzitic basement contributes only by producing a coarse clastic fraction at the base of the ferricretes. The cement of the ferricretes is goethitic at the base in between the quartzite clasts while the top is marked by very fine grain sizes and dominant authigenic processes (nodules and pisolites with cortex of goethite and hematite). From the base to the top of the ferricretes, rounded ilmenite grains accompanied by small quartz grains suggest aeolian transport "sprinkling " this material from external sources. These ferricretes are therefore characterized by significant mechanical and chemical allochthonies, the source of Fe in solution such as grains of ilmenite being difficult to trace with certainty in this region. The comparison with average contents of major and trace elements of gossans, lateritic cuirasses and bog ore shows that whole rock geochemistry can be used to discriminate KOB ferricretes from gossans at first-order, but that databases should be implemented with analyzes of gossans developing above iron-oxides-copper-gold deposits such as those exploited in Inchiri (e.g. the Guelb-Moghrein deposit, near Akjoujt).