Fluid sources and the role of abiogenic-CH4 in Archean gold mineralization: Constraints from noble gases and halogens

The St Ives Goldfield of Western Australia hosts high grade gold mineralization in a variety of lithologies within ∼2.7Ga greenstone belts. The low salinity CO₂-bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones, or from external magmatic sources, and independently sourced abiogenic-CH₄ has been suggested as an important control on gold mineralization. In order to better constrain possible fluid sources, we applied a combination of He-Ne-Ar-Kr-Xe isotope and Cl-Br-I analyses to quartz and carbonate veins containing H₂O-CO₂ fluid inclusions, pyrite±magnetite/hematite; and quartz veins containing CH₄-rich fluid inclusions, pyrrhotite±pyrite.Samples containing H₂O-CO₂ fluid inclusions have higher Br/Cl and I/Cl values and less radiogenic noble gas isotope signatures than suggested for CH₄-rich fluid inclusions. The samples containing CH₄ fluid inclusions have maximum ⁴⁰Ar/³⁶Ar of ∼50,000; ²⁰Ne/²²Ne of ≤9.8 and maximum ²¹Ne/²²Ne of ∼0.56. In contrast, H₂O-CO₂ fluid inclusions have maximum ⁴⁰Ar/³⁶Ar of only ∼20,000 and maximum ²¹Ne/²²Ne of ∼0.14. The use of ⁴⁰Ar-³⁹Ar methodology to measure K simultaneously with ⁴⁰Ar/³⁶Ar, enables us to show the highest ⁴⁰Ar/³⁶Ar ratios are representative of the fluids trapped at ∼2.65Ga, and have not been significantly modified by post-entrapment ⁴⁰Ar_R production (⁴⁰Ar_R=radiogenic ⁴⁰Ar). Furthermore, the ⁴⁰Ar/³⁶Ar ratio is correlated with the ²¹Ne/²²Ne and ¹³⁶Xe/¹³⁰Xe ratios, demonstrating that other heavy noble gas isotope ratios are also close to their original composition.The highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host-rocks, because seawater altered-volcanic rocks have high abundances of non-radiogenic atmospheric noble gas isotopes. In contrast, the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at ∼2.65Ga. The combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced H₂O-CO₂ and CH₄ localized high-grade gold mineralization. We suggest that the fluids were intimately associated with crustal magmatism in the St Ives Goldfield and that abiogenic CH₄ could be an important moderator of redox processes in other crustal environments.