Petrography and geochemistry of ferricrete near Shire, northern Ethiopia

A detailed petrographic and geochemical study was conducted on ferricrete (laterite) developed on Mesozoic (?) ferruginous sandstone near Shire, Tigray region, northern Ethiopia. 30 rock samples were analyzed for major and minor elements and studied for petrographic details. Ferruginous sandstone overlying the Neoproterozoic low grade basement rocks dominantly contains quartz followed by orthoclase feldspar and iron oxides/hydroxides as cement. Residual enrichment process has resulted in the development 2-3m thick ferricrete horizon and also sub-horizons: mottled, mixed nodular and psuedo-pisolitic. Mineralogy of the ferricrete includes limonite, goethite and hematite. Arid conditions and dehydration reactions seem to have produced hematite from goethite. The ore minerals show replacement, cavity and fracture filling, remobilization and colloform textures. Si>Al>Fe is the mobility pattern observed in the ferricrete horizon though presence of secondary quartz and kaolinite are also common. Development of ferricrete is related to the climatic condition that existed during Eocene and is comparable with similar deposits in Arabian Nubian Shield (ANS). Key words : Ferricrete, Laterite, Ferruginous sandstone, Shire, Tigray, Northern Ethiopia, ANS