| Abstract |
The Rotokawa geothermal system is up to 20,000 years old as evidenced by hydrothermal eruptions occurring after this time and up to 3,700 years ago. Recent drilling has provided an opportunity to examine hydrothermal mineral alteration that may have recorded magma degassing subsequent to the systems inception. The current reservoir fluids are near-neutral, non-saline and between 320 and 330°C. They contain unusually high H2S concentrations compared to other Taupo Volcanic Zone geothermal systems. Paragenetic sequences in veins from the recently recovered cores below 1850 m indicate that propylitic type quartz ± chlorite ± epidote deposition has been interrupted by intermittent SO2- bearing saline fluids, CO2 enriched fluids and open-system boiling at temperatures >285°C. Propylitic veins have been overgrown and cross-cut by anhydrite veins and infillings (not related to supergene alteration) which have in turn been replaced or infilled by intergrown bladed calcite, chalcedony and hematite. It is suggested that magma degassing events could have initiated these fluid changes and overpressured the reservoir, resulting in deep-focal depth hydrothermal eruptions and decompressive boiling. Fluid changes driven by degassing are suggested by the presence of hydrothermal alteration and vein calcite while current reservoir fluids are undersaturated with respect to calcite. Up to 2.6 million tonnes of native sulphur has been deposited in the near surface as recently as 6060 ± 60 years ago. This is calculated to be greater than the current flux of H2S could deposit in the elapsed time, also suggesting significant changes to reservoir fluid chemistry. Over pressures in the reservoir released by hydrothermal eruptions and associated decompressive boiling would have resulted in vapour losses, oversaturating the reservoir fluid with silica resulting in chalcedony precipitating at temperatures >225°C; and undersaturated the fluid with respect to hydrogen and hydrogen sulphide, precipitating hematite with bladed calcite from the dissolution of pyrite or from chloride and bisulphide complexes in solution. |