Fluid sources and the role of abiogenic-CH 4 in Archean gold mineralization: Constraints from noble gases and halogens

Abstract

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 CO 2-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-CH 4 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 H 2O–CO 2 fluid inclusions, pyrite ± magnetite/hematite; and quartz veins containing CH 4-rich fluid inclusions, pyrrhotite ± pyrite. Samples containing H 2O–CO 2 fluid inclusions have higher Br/Cl and I/Cl values and less radiogenic noble gas isotope signatures than suggested for CH 4-rich fluid inclusions. The samples containing CH 4 fluid inclusions have maximum 40Ar/ 36Ar of ∼50,000; 20Ne/ 22Ne of ≤9.8 and maximum 21Ne/ 22Ne of ∼0.56. In contrast, H 2O–CO 2 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 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 ∼2.65 Ga. The combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced H 2O–CO 2 and CH 4 localized high-grade gold mineralization. We suggest that the fluids were intimately associated with crustal magmatism in the St Ives Goldfield and that abiogenic CH 4 could be an important moderator of redox processes in other crustal environments.