| Title | EVOLUTION OF GEOTHERMAL-GROUNDWATER CIRCULATION SYSTEMS IN THE QUATERNARY TAUPO VOLCANIC ZONE: EXAMPLE OF THE UPPER WAIKATO RIVER CATCHMENT, NEW ZEALAND |
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| Authors | P.A. White, M. Moreau, A. Rae, J.F. Power, B. Davids |
| Year | 2018 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Geothermal systems, Upper Waikato River catchment, Taupo Volcanic zone, geothermal field evolution |
| Abstract | Geothermal-groundwater circulation systems in the Upper Waikato River (UWR) catchment of the Taupo Volcanic Zone (TVZ) have developed in the Quaternary in close association with volcanic activity, sedimentation and topography. The age and evolution of some UWR catchment geothermal features (i.e., active fields and fossil fields) and the Tokaanu geothermal system, were assessed with: geological reconstructions in four epochs (i.e., pre-TVZ Hauraki Rift; ‘old TVZ’; ‘young TVZ 1’; and ‘young TVZ 2’) considering the formation of calderas, tectonic depressions and pre-historic lakes; groundwater elevation; and geothermal spring chemistry. Hierarchical Cluster Analysis was used to classify the ‘evolutionary’ status of active fields (i.e., unevolved, evolved and highly evolved) and a chloride mixing model that calculated the percentage of deep geothermal fluid in spring flow (Pm) with two end-members of deep geothermal fluid and cold groundwater. No fields were classed as unevolved in the catchment; Tokaanu was the closest example of such a field as water quality in unevolved springs was probably very similar to deep geothermal fluid, i.e., median Pm in these springs was approximately 93%. Evolved geothermal fields (Waikite, Waiotapu and Rotokawa) possibly formed in the young TVZ 2 and highly-evolved fields (i.e., Atiamuri, Ngatamariki, Orakei Korako, Te Kopia, and Reporoa) possibly developed in the young TVZ 2 and young TVZ 1 epochs. Fossil fields probably date from the young TVZ 2 epoch. Most fields indicated that evolution of geothermal systems followed progressive mixing, with time, of cold groundwater with geothermal-reservoir fluid. This approach may have applications as a geochronometer for UWR catchment geothermal systems after further research that could aim to provide more understanding of the evolution of fluids, permeability, and groundwater flow in each class of active field, and fossil systems. |