Hydrothermal Carbonate Geochemistry of the Ngatamariki Subsurface Reservoir, New Zealand

Abstract

Geochemical assessment of rocks and fluids enables improved drilling success and geothermal development. Hydrothermal carbonates are capable of identifying the source(s) of hydrothermal fluids, fluid mixing processes and metasomatic/alteration temperatures. Here, we utilize core samples from the Ngatamariki Geothermal Field located in the Taupo Volcanic Zone, New Zealand, to ascertain the source, migration and temperature history of the hydrothermal fluids via isotopic and thermodynamic geochemistry of the hydrothermal carbonates. Microthermometric analyses and boiling curves demonstrate platy calcite is deposited from a dilute, CO2-rich steam-heated fluid from 250oC to 270oC at ~3km depth. Calcite stable isotope compositions further reveal that platy calcite is deposited from both CO2-rich steam-heated fluids and less evolved magmatic fluids. Calcite δ13C values range between -5.8 and -7.0 % (VPDB) similar to the composition of mantle carbon. Calcite δ18O values range between -17.17 and -29.4 % suggesting calcite within the Ngatamariki reservoir is derived from a variably evolved magmatic fluid between ~250-300oC. Together, the isotope results suggest that Ngatamariki geothermal fluids have a significant magmatic component. Thermodynamic modelling supports the magmatic origin of Ngatamariki fluids and provides a means to evaluate the fluid-rock interactions as these fluids migrate from source to surface.