Paper No. 8
Presentation Time: 10:45 AM


BLACKSTOCK, Joshua M., Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand, HORTON, Travis, Geological Sciences, University of Canterbury, Christchurch, 8140, New Zealand and GRAVLEY, Darren M., Department of Geological Sciences, University of Canterbury, Christchurch, 8140, 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.