| Title | HYDROTHERMAL MINERALS AND HYDROLOGIC EVOLUTION OF THE ROTOKAWA GEOTHERMAL SYSTEM, NEW ZEALAND |
|---|---|
| Authors | M.P. Simpson, A.G. Morales, I. Chambefort, S. Alcaraz, S. Moribe, S.D. Milicich, A. Calibugan |
| Year | 2021 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Hydrothermal minerals, hydrothermal eruptions, SWIR reflectance spectroscopy, XRD, fluid inclusions, Rotokawa |
| Abstract | The Rotokawa geothermal system in the Taupō Volcanic Zone is ~28 km2 in areal extent and has been the site of eight large hydrothermal eruptions. This study documents hydrothermal minerals for 15 wells (3,242 samples). Quartz, chlorite, albite, calcite, pyrite, and illite are the most common hydrothermal minerals. Epidote and smectite are less common. Kaolinite is uncommon. Smectite occurs as a 250 to 900 m thick carapace overlying illite and separated by a narrow intervening interval of mixed-layered illite-smectite. Chlorite is generally ubiquitous at depths ≥700 m, calcite is most common at >750 m, and epidote >1,000 m. Hydrothermal albite is present in all wells, whereas rarer adularia is patchily distributed. Chalcedony unusually occurs at >1200 m depth. Kaolinite patchily occurs at surface and locally to 200 m depth. Kaolinite further occurs up to 1,150 m depth in two wells bordering the resistivity margin. Dickite and / or kaolinite ± alunite is present in three wells (same pad) between 475 and 585 m depth. Hydrothermal minerals correlate with fluid types. Quartz, chlorite, albite, adularia, illite, calcite and epidote coincide with alkali chloride waters. Shallow kaolinite ± sulfur coincide with acid sulfate condensates. Dickite / kaolinite ± alunite at depth are interpreted to have formed from in-situ acid sulfate condensates. Whereas deep marginal kaolinite is inferred to have formed from steam-heated CO2-rich waters. Fluid inclusion and hydrothermal mineral inferred temperatures generally match measured temperatures, but the northern and north-eastern wells are cooler (up to 55°C to 1.5 km depth). A 3-stage hydrologic model is proposed relative to hydrothermal eruptions that occurred between ~20,000 and 3500 years ago. 1) Pre-eruption, with hotter and greater fluid flow to the north. 2) Syn-eruptions with creation of greater permeability in the south. Associated decompressive boiling resulted in chalcedony deposition at depth. Local transient temperature changes resulted in pressure draw down and formation of anhydrite (to 500 m depth). 3) Post-eruptions to present day with enhanced permeability in the south. Greater fluid and heat transfer in the south coupled with reduced fluid and heat transfer elsewhere has resulted in cooler conditions in the north to <1.5 km depth. |