Metallogenic implications of volcanic quiescence during continental flood basalt eruption – The case of sediment hosted Mn-Fe mineralization in the Ethiopian volcanic plateau

Abstract

The north-western volcanic province of Ethiopia is constituted by Continental Flood Basalt and Shield Volcanoes. A paucity in volcanism allowed the deposition of immature continental sediments in fluviolacustrine environment. A recent geologic expedition in the north-western volcanic plateau discovered an Mn-Fe mineralization hosted in nearly 100 m thick mudrock and volcaniclastic sandstone at two localities called Kulfamba and Wogeltena. This research was conducted to investigate the Mn-Fe mineralization hosted in the inter-trappean sediments. Geologic mapping, mineralogic and geochemical methods were employed to investigate the orebodies and host sediments. Mineralogical investigation was carried out using X-ray Diffraction, thin and polished section analysis under petrographic and ore microscopy. Major and trace element analyses was also conducted using ICP-MS. The Mn-Fe orebodies are dominantly layered occupying the transition zone between the mudrock and the sandstone. The orebody is about 1.6 m thick occupying the upper part of the mudrock (the Fe orebody) and the lower part of the sandstone (the Mn orebody). Mn ore is dominated by pyrolusite followed by manganite. Fe is in the form of hematite and goethite. Average ore grade is 34% MnO and 38% Fe2O3. Gangue minerals are quartz, alkali feldspar and clays. The dominantly layered nature of the orebody, absence of typical hydrothermal Mn ore minerals like todorokite and birnessite, high concentration of ∑REE, and other geochemical data support dominantly hydrogenous origin. The presence of distinct orebodies of Mn and Fe suggests the presence of redox-stratified water in the depositional environment. Sporadic occurrences of vein type, concretionary, and botryoidal textures of ores signify later hydrothermal and weathering remobilization and overprinting on the previously formed hydrogenous deposit. The geographic distribution of exposed sediments, their thickness and facies variations throughout the plateau suggests existence of a large shallow lake fed by streams flowing towards northern/north-eastern part of the plateau.