| Abstract |
Our study involved a comparison of hot spring geochemistry data collected between 1905 and 2009 from five geothermal fields in the Taupo Volcanic Zone (TVZ), New Zealand. Atiamuri, Mokai, Ngatamariki, Waikite, and Waiotapu were chosen as the geothermal systems for this study for their east-west and north-south coverage of the central TVZ, their comparative data availability, and for representing development and protected systems. One neutral-chloride hot spring that best represented the deep reservoir fluid was chosen for each geothermal system. Water-type and geoindicator ternary diagrams were utilised for observing changes in each hot spring overtime. Chloride (Cl-), silica (SiO2), sodium (Na), potassium (K), magnesium (Mg), and boron (B) were the main parameters used for interpreting reservoir conditions and associated near-surface influences. Semi-qualitative concentration trend maps were constructed for comparing geochemistry trends between the five fields. The geothermal systems of Mokai and Ngatamariki as well as Atiamuri and Waikite which represented our East-West distribution of systems showed minimal similarities in geochemistry trends. Ngatamariki and Waiotapu were compared as the spatial relationship between these two fields is consistent with the major structural trend of central TVZ. They show definite trend similarities, especially for sodium concentrations between 1978 – 1984. Specific reasons or events causing similarities to occur between Ngatamariki and Waiotapu are not known. It has long been unclear if Waikite and Waiotapu are hydrologically connected. Geochemistry trends identified in our study for these two systems suggest they have dissimilar water chemistry, which may indicate separate geochemical controls. Caldera boundaries and lithological conditions surrounding the caldera-bounded Mokai and Atiamuri systems are identified as important factors for isolating fluids from these systems and minimising external influences on observed geochemical parameters. Our water chemistry study offers insights into comparing geochemistry trend changes at a regional scale by observing different geothermal systems with distinctive conditions. |