Geological and Geochemical Characterisation of Fe-Mn oxide Mineralisation of Wadi Masilah Basin (SE Yemen)

The Wadi Masilah Basin is part of the larger Say’un-Masilah Basin, a Mesozoic sedimentary basin located in the southeastern Yemen. It consists of a sedimentary sequence hosting polymetallic barite, lead, zinc, iron, manganese, and vanadium mineralisations. The area is strongly faulted as a result of extensional and transtensional tectonics connected to the opening of the Gulf of Aden-Red Sea rift system. Quaternary volcanic activity is associated to the post-rift phase. The present paper deals with the Fe-Mn oxide mineralisation hosted by Cretaceous and Eocenic shallow marine carbonates. The mineralogical assemblage consists of hematite, limonite, goethite, hollandite, romanechite, cryptomelane, pyrolusite, and rodochrosite. Mineralisation may be stockwork, vein-like, and disseminated in carbonate rocks. Iron orebodies made up of massive hematite contain ~80-74% Fe2O3. Mineralised veins have tenors of Fe2O3 variable from ~ 52% to ~11%. Pb, Cr, and Zn contents can be high in some samples. Manganese orebodies have mostly high MnO tenors (~65-52%) and are characterised by very high Ba, Pb, and Zn contents. The disseminated carbonate rocks have Fe2O3 contents up to ~13%, and MnO contents up to ~5%. Their Pb and Zn contents can locally be very high. Samples coming from 12 drillcores, mostly limestones and dolomitic limestones are low-grade in both Fe2O3 (~6-0.4%) and MnO (~0.8-0.1%). As for the source of the mineralising fluids, discriminative diagrams based on Fe/Mn and transition metals support a hydrothermal origin with possible contribution from diagenetic processes. The distribution of Zn and HFSE suggests that hydrothermal mineralising fluids are related to the magmatic-hydrothermal systems affiliated to the neighbouring Quaternary volcanic fields. Widespread positive Ce spikes in the trace element patterns suggest the Fe-Mn ore deposition occurred in a relatively shallow, oxidizing environment.