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 ∼2.7 Ga 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 ∼2.65 Ga, and have not been significantly modified by post-entrapment 40ArR production (40ArR = 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 ∼2.65 Ga. 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.

► We investigate liquid CH4 and H2O–CO2 fluid inclusions related to gold mineralization. ► The fluid inclusions preserve 2.65 Ga noble gas signatures. ► The fluids derived highly radiogenic noble gas isotope signatures from igneous basement. ► Atmospheric noble gases were derived from the seawater-altered greenstone host-rocks. ► Gold mineralization may have been triggered by interaction of distally sourced H2O–CO2 and CH4-rich fluids.