Barren ground depressions, natural H2 and orogenic gold deposits: Spatial link and geochemical model.

A review of the localities in continental rocks where H2-rich gases have been reported, showed that they are mainly located near orogenic 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 associated with termite mounds and fairy circles. We proposed a geochemical model to account for this relationship between orogenic gold deposits, H2 emanations and geomorphological features. Fe­carbonates 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 mol H2 per kg of rock along with ∼3 mol/kg CO2. This process induces a solid volume decrease of 50 %. Therefore, we propose that Fe­carbonate 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 exploration. 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 Fe­carbonates such as iron formations and carbonatites. This suggests that H2 production through Fe­carbonate dissolution is not restricted to gold deposits. The global H2 production in crustal rocks associated with Fe­carbonate alteration is estimated to 3 × 105 mol/yr.

Real-time monitoring of aqueous Hg2+ reduction dynamics by magnetite/iron metal composite powders synthesized hydrothermally.

An iron-based powder material composed of zerovalent iron (8 wt.%) and magnetite (92 wt.%), has been synthesized hydrothermally at 200 °C from zero-valent iron. Its effect on the reduction of aqueous Hg2+ into gaseous Hg0 has been investigated at ambient conditions for pH comprised between 4 and 8.5. The production of Hg0 was monitored with an online mercury vapor analyzer at the picogram level for concentrations of iron-based composite of a few tenths of mg L-1. Starting from a solution having an Hg2+ concentration of 25 ng L-1 at pH = 4, a succession of two Hg0 production events was recorded. The first event is related to the Hg2+ reduction by ZVI which fully dissolved within the first hours. Upon ZVI consumption, pH drifted towards the pH window where magnetite can efficiently reduce Hg2+ at the hour timescale, resulting in a second Hg0 production peak. The combined use of ZVI and magnetite to remove aqueous Hg2+ by formation of Hg0 (volatile) under mild acidic pH allows (1) to maximize the Hg2+ reduction rate and (2) to take benefit of the longer lifetime of magnetite compared to ZVI.