Can Weathering of Banded Iron Formations Generate Natural Hydrogen? Evidence from Australia, Brazil and South Africa

Abstract

Oxidation of iron-rich rock is known to generate H2 in oceanic as well as in continental domains. Here we tested the possibility of H2 generation as the result of weathering of banded iron formations (BIF). The BIF constitute more than 60% of global iron ore reserves with low Fe3+ /Fetot and total Fe ranging from 20 to 40 wt% and are therefore good candidates for H2 production potential. In the vicinity of BIF-hosted iron mines in Australia, Brazil and South Africa, satellite imaging has revealed the presence of sub-circular depressions that usually are the proxy of H2-emitting features. A morphological comparison of the sub-circular depressions with the ones observed in previous studies point to probable H2 seeping in these areas. In parallel, a petrological study conducted on altered and fresh BIF samples from the Hamersley Province in Western Australia also suggests H2 generation during BIF weathering. Indeed, mineral transitions from ferrous silicate (riebeckite and/or minnesotaite) to ferric iron oxi-hydroxides (goethite) or from ferrous and ferric oxides (magnetite) to exclusively ferric oxides (maghemite, hematite, goethite) were observed on the samples. The oxidation of ferrous iron by aqueous fluids circulating through and leaching the BIF is promising for H2 generation. The BIF weathering profile suggests that the limiting factor is the presence of water, and that this reaction is happening at, or near, surface temperature. This challenges the idea that high temperatures are required to generate H2 as it is the case during the serpentinization. The link between BIF and H2 will have however to be further investigated to better constrain the reactions and their kinetics.