Key role of Fe-carbonates in natural H2 production? Evidence from the spatial link between barren ground depressions, gold deposit and H2 emissions

Even if measurements of high H₂ concentrations in continental rocks have significantly increased in the last decade, the origin of H₂ remains enigmatic in this context. Here we show that the localities in continental rocks where H₂-rich gases have been reported are mainly located near orogenic gold deposits. Two types of geomorphological features were identified near orogenic gold deposits on satellite images. They consist in both barren ground depressions and high densities of self-organized, small (< 20 m in diameter) circular- and comet-shaped white spots in 32 and 7 localities, respectively. Fe-carbonates commonly occur near gold deposits since gold is transported in CO₂-rich fluids. Thermodynamic modelling reveal here that they can further dissolve in the presence of aqueous fluid to produce magnetite and up to ~ 1 mole of H₂ per kg of rock. This reaction leads to a volume decrease of ~ 50 %. Based on these findings, we propose that Fe-carbonate dissolution could be the primary source of H₂ in orogenic gold deposit areas, and involved in the formation of the geomorphological structures reported here. The association between H₂-rich gas and ground depressions was also observed near other formations containing Fe-carbonates such as iron formations and carbonatites. This suggests that H₂ production through Fe-carbonate dissolution is not restricted to gold deposits. The global H₂ production in crustal rocks associated with Fe-carbonate alteration is estimated to 3 x 10⁵ mol/yr.