Vincent Roche

Serpentinization is commonly presented as the main source of natural hydrogen (H2) in the continental domains. However, recent works in Australia and Brazil showed that Archean– Paleoproterozoic banded iron formations could be another natural source of H2 gas. Although the reaction that produces hydrogen is similar (Fe2+ oxidation - H2O reduction process), the iron content may be higher in banded iron formations than in mafic igneous lithologies, potentially generating H2 more efficiently. Here, we present structural evidence that reported H2 emissions from Waterberg Basin, Namibia, are associated with underlying Neoproterozoic banded iron formations - the Chuos Formation. The subcircular depressions are numerous in this basin (> 2200) and cover an area of 4,000 square kilometers. The highest density occurs in the central basin a few kilometers away from Waterberg Plateau. Overall, however, the subcircular depressions distribution appears to be random. Magnetite, a known H2‑generating mineral, is ubiquitous and accompanied by other suspected H2‑generating minerals (biotite and siderite) in Chuos Formation. Wherever, Chuos Formation is deformed and metamorphosed as evidenced by the presence of biotite. Magnetite occurs either as pervasive cm to dm continuous metamorphic laminations in foliation and fractures planes and/or diffusely disseminated in metachert and metacarbonate levels. From this, we infer that metamorphism does not negatively affect the Fe2+ content that is required to generate hydrogen. Based on the magnetic response map of the different lithologies around Waterberg Basin, the spatial distribution of Chuos Formation is inferred to underlie much if not most of Waterberg Basin. At some places, Chuos Formation continuously outcrops over several kilometers (>150 km) with an estimated thickness of several hundreds of meters, which is consistent with our field observations. We assume that H2 measurements in Waterberg Basin may be associated to an active H2‑generating system by oxidation of banded iron formations in subsurface. In addition, magmatic dolerite sills are also present in the basin, and may in parallel act as seals to allow accumulation of H2 like in Mali. The banded iron formations that constitute more than 60% of global iron ore reserves, should be targeted for H2 exploration.