The Ordovician iron ore of the Anti-Atlas, Morocco: Environment and dynamics of depositional process

Abstract

In Morocco, iron deposits of sedimentary origin are mainly developed in the Ordovician where they are interbedded at different levels of the stratigraphic column. A detailed analysis of the sedimentary facies shows that the iron ores are not randomly distributed but are concentrated at omission surfaces of the sequences thus raising the question of their sedimentological significance within the sequences that shelter them. This study determines three morphological types of ore, which provide a possible avenue for economic exploitation. These morphological types are: firstly, from metric to decametric bodies of regional extension (examples are “Caid Rami” and “Tachilla”), consisting of a set of detrital facies organized in two stratigraphic stages that cyclically recur to form genetic sequences. Within each sequence, the maximum mineralization is at the top of the prograding basal stage and is expressed by diagenetic iron cement and at the base of the regressive stage in the form of strata of iron oolites, or in massive beds. Secondly, lenses with a thickness up to 30 m over a kilometer width (example of “Imi n’Tourza” field), corresponding to incisions cut along the ravinement surfaces. The maximum mineralization of this second type is within the median sandstone stage in connection with a large transgressive event. Thirdly, there is a type of reworked material consisting of discontinuous layers of ferruginous conglomerates and bioclastics lining the ravinement surface that separates the higher order sequences. It is this third type that has generated the most economic interest. The iron-bearing mineral phases identified are: magnetite, hematite, iron chlorite, maghemite, iron hydroxides (goethite, lepidocrocite) and siderite. The ultimate source of iron would probably be the large reserves of banded iron formation of the West Africa Craton (Mauritania in particular). Intense tropical weathering would have leached the silica and left a highly enriched iron residuum that was carried as particulate iron to shallow marine shelf environments where Fe3+ was converted to Fe2+ during early diagenesis. This reduced iron was recycled into the water column by tidal currents or storm waves where it precipitated as ferric oxide or ferrous silicate ooliths. This depositional mechanism accounts for the positioning of ironstone beds at times of stasis and reworking.