Mineralogical Changes and H ₂ Generation Yield During Hydrothermal Alteration of a Magnetite‐Siderite Assemblage

Abstract To date, the generation of natural hydrogen (H 2 ) from the alteration of Fe II ‐bearing minerals has mainly been studied through serpentinization in (ultra)mafic rocks. This study explores Banded Iron Formations (BIF), which are rich in Fe II minerals, as a potential source for H 2 . We conducted a hydrothermal experiment at 200°C with a combined magnetite‐siderite assemblage, two major components of BIF. The experiment, designed with a high water‐to‐rock ratio and a gas phase (W‐R = 300) enabled to assess mineral transformations during the alteration. Thermodynamic simulations were finally conducted, to explore H 2 ‐generating yields in more realistic geological scenarios (no gas phase, lower W‐R). Our experimental findings show that H 2 is produced through complete siderite dissolution and magnetite precipitation. Concomitantly, non‐stoichiometric primary magnetite (Fe II /Fe III < 0.5) did not enhance H 2 yield; instead, it acted as a sink for dissolved Fe 2+ , sequestrating about 10% of the iron from siderite without oxidation to recover a more ideal stoichiometry. This suggests that abundant, non‐stoichiometric magnetite in natural settings may reduce H 2 generation yields. Mass balance calculations indicate that 83% of the expected H 2 generated was unaccounted for, consistent with suspected CO 2 reduction and formation of dissolved organic compounds in the fluids. Thermodynamic simulations at varying W‐R ratios (from 300 to 1) reveal that H 2 yield ranges widely (39 mmol–73 μmol H 2 per kg of siderite), since lower W‐R prevent from siderite dissolution and enhance H 2 consumption via carbon reduction. These findings imply that low W‐R ratios in BIF ‐and other siderite‐bearing lithologies, may limit H 2 resources.