Hydrogen and hydrocarbons associated with the Neoarchean Frog’s Leg Gold Camp, Yilgarn Craton, Western Australia

High concentrations of hydrogen (H₂) and methane have been detected in shallow open-hole exploration wells surrounding the Neoarchean Frog’s Leg gold camp in the Eastern Goldfields, Yilgarn Craton, Western Australia. After corrections for modern air contaminants and excess nitrogen (N₂) the Boomer deposit gases contain: 19.9‒68.7 mol% H₂; 28.7‒76.9 mol% CH₄; 0.47‒1.6 mol% heavier hydrocarbons (C₂‒C₅), which follow an Anderson-Schulz-Flory distribution; 0.11‒3.3 mol% carbon dioxide; and 0.69‒1.87 mol% helium (He). The isotopic composition of the gas components was further investigated: helium has ³He/⁴He ratios of 1.82‒3.33×10-8 (or 0.01-0.02 Ra, where Ra is the atmospheric value) indicating a purely crustal origin; hydrogen has δ²H between -781.3 ‰ and -759.5 ‰; and methane hasδ¹³C between -20.3 and -2.42 ‰ and δ²H between -382.5 ‰ and -342.2 ‰. The C₂-C₅ gaseous hydrocarbons are commonly depleted in ¹³C (up to 22.75 ‰ for ethane) and enriched in ²H (up to 117.3 ‰ for iso-butane) compared to methane while carbon isotope reversal is observed between methane and ethane. These molecular and isotopic characteristics of the gas are consistent with 1) H₂ generation controlled by redox and/or radiolytic reactions within basic and felsic igneous rocks, and 2) methane and C₂₊ gaseous hydrocarbons produced during serpentinization of mafic‒ultramafic rocks. Serpentinization due to ingress of groundwater can produce voluminous free H₂. Subsequent, gas phase Fischer-Tropsch type reactions with limiting CO₂ (from carbonate dissolution) lead to abiogenic methane enriched in ¹³C and to the generation of C₂‒C₅ wet gases. Radiolytic-controlled processes also occur in parallel, where mafics‒ultramafics and granites and their related eroded sediments promote abiogenic radiolysis of water and polymerisation of methane. Using the H₂‒CH₄ hydrogen isotopic fractionations as a geothermometer, isotopic equilibrium temperatures are calculated between 42.2 °C and 57.0 °C, representing minimum 1.4‒2.7 km sub-surface depths for the gas sources, corresponding to the Gleesons Basalt as a major gas contributor. The processes of gold mineralisation billions of years ago laid the foundation for the supply of catalytic metal/metal ions and carbonate (CO₂) source for later independent, abiogenic gas formation. The free gases sampled from exploration wells in the Boomer deposit, Frog’s Leg Gold Camp represent the first positive identification of abiogenic natural gas in Australia, with gas compositions similar to abiogenic gases previously reported from overseas Archean cratons. Interestingly, gases preserved in fluid inclusions associated with some of the region’s other lode gold deposits demonstrate CH₄ production has likely occurred over a period from the Neoarchean to present in the Yilgarn Craton. <b>Citation:</b> Christopher J. Boreham, Jacob H. Sohn, Nicholas Cox, Jodi Williams, Ziqing Hong, Mark A. Kendrick, Hydrogen and hydrocarbons associated with the Neoarchean Frog's Leg Gold Camp, Yilgarn Craton, Western Australia, <i>Chemical Geology</i>, Volume 575, 2021, 120098, ISSN 0009-2541, https://doi.org/10.1016/j.chemgeo.2021.120098.