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

Abstract

High concentrations of hydrogen (H2) and methane (CH4) have been detected in shallow open-hole exploration wells surrounding the Neoarchean Frog's Leg gold camp in the Eastern Goldfields Superterrane, 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 (CO2); 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 (from 0.013 to 0.024 RA, where RA is the atmospheric value) indicating a purely crustal origin; H2 has δ2H between −781.3‰ and − 759.5‰; and CH4 has δ13C 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 a carbon isotope reversal is observed between methane and ethane. The molecular and isotopic characteristics of the gas are consistent with 1) H2 generation controlled by radiolytic reactions within basic and felsic igneous rocks and by redox reactions including serpentinization in mafic-ultramafic lithologies, and 2) methane and C2+ gaseous hydrocarbons produced by interaction of H2 with a single carbon source (mainly CO2) most likely in Fischer-Tropsch type reactions. Using the H2–CH4 hydrogen isotopic fractionations as a geothermometer, isotopic equilibrium temperatures are calculated between 42 °C and 57 °C, which based on the modern geotherm represents a depth of ~1.4–2.7 km, corresponding to the Gleesons Basalt beneath the Frog's Leg gold camp. 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, CH4-rich gases preserved in fluid inclusions associated with some of the region's lode gold deposits have distinctly different isotopic characteristics, suggesting that methane production has occurred by different pathways over a period from the Neoarchean to present day in the Yilgarn Craton.