CRUSTAL FLUID SOURCES IN MAGMATIC-HYDROTHERMAL SYSTEMS OF THE TAUPO VOLCANIC ZONE, NEW ZEALAND

Knowledge of the source, transport, and fate of hydrothermal fluids in the upper crust informs our understanding and interpretation of ore-forming processes, volcanogenic hazards, and geothermal resources. Co-variation between fluid inclusion CO₂/CH₄ and N₂/Ar ratios is an established tracer of magmatic, meteoric, and crustal fluid end-members. Yet, this tracer has had limited application to macroscopic fluid reservoirs accessible via geothermal wells and hydrothermal features (e.g. pools). In this study, we compared the covariance CO₂/CH₄ and N₂/Ar ratios of gases collected throughout the Taupo Volcanic Zone, New Zealand (TVZ) with corresponding δ¹³C and ³He/⁴He values. Our findings show that there is good agreement between these proxies for different end-member contributions at coarse scales. However, some samples classified as meteoric water according to the CO₂/CH₄ and N₂/Ar ratios also show more positive δ¹³C values (~ -7.0 per mil) and relatively higher ³He/⁴He ratios indicative of magmatic input. This unexpected result may be related to magmatic fluids, CO₂ in particular, mixing with predominantly derived meteoric waters. The potential to identify magmatic CO₂ in groundwater samples overlying geothermal systems using simple and cost-effective gas ratios is a promising step forward in the search for ‘surface blind’ but developable geothermal systems. The general agreement between the co-variation of CO₂/CH₄ and N₂/Ar ratios with other isotope and geochemical proxies for magmatic, meteoric, and crustal end-members is encouraging to employ expanded use of these ratios for both the exploration and monitoring of gases at Earth’s surface, particularly for the exploration and management of geothermal fields.