Benjamin Malvoisin, Fabrice Brunet

A review of the localities in continental rocks where H2-rich gases have been reported, showed that they are mainly located near gold deposits. Two types of geomorphological features known as markers of gas venting in sedimentary basins were also systematically observed near orogenic gold deposits on satellite images. They consist in both barren ground depressions and high densities of small (< 20 m in diameter) circular- and comet-shaped white spots in 32 and 7 localities, respectively. Point pattern analysis revealed that the white spots are self-organized, and similar to previously described vegetation patterns including termite mounds and fairy circles. We proposed a geochemical model to account for this relationship between orogenic gold deposits, H2 emanations and geomorphological features. Fe-carbonates are ubiquitous mineral products associated with gold mineralization. They can further dissolve in the presence of aqueous fluid due to their high reactivity below 200°C to produce magnetite and up to ~ 1 mole of H2 per kg of rock along with ~ 3 mol/kg CO2. This process induces a solid volume decrease of 50 %. Therefore, we propose that Fe-carbonate dissolution is (1) the primary source of H2 in orogenic gold deposit areas, and (2) involved in the formation of the geomorphological structures reported here, providing a new framework to understand their seemingly complex formation. Ground depressions and white spots are possible tools for gold explorations. Actually, we identified four new areas where we suspect possible orogenic gold deposits. The association between H2-rich gas and ground depressions was also made 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 3.10^5 mol/yr.