The Rotokawa geothermal system, New Zealand: Hydrothermal minerals and punctuated evolution

The Rotokawa geothermal system is a hot (>300 °C) geothermal resource within the Taupō Volcanic Zone, in a 28 km 2 area of low resistivity, with 36 wells drilled. Breccia deposits from at least eight hydrothermal eruptions cover ~11 km 2 ; thermal features discharge over an area of ~2.5 km 2 . Quartz, chlorite, albite, calcite, pyrite, illite and epidote alteration minerals formed from neutral-pH chloride liquid. Kaolinite ± sulfur at surface and kaolinite ± dickite ± alunite in one location at 475–605 m depth formed from steam-heated acid-sulfate condensates; local chalcedony occurs at >1200 m depth. Steam-heated CO 2 -rich solutions formed kaolinite with calcite or siderite at ~400–1150 m depth. Temperatures from fluid inclusion and mineral stabililties largely agree with measured values, although cooling by ≤50 °C occurred to ≤1350 m in the northern part of the system. Alteration mineralogy and related features record three stages of system evolution: pre-, syn -, and post-hydrothermal eruptions. 1) Pre-eruptions, hot chloride liquid upflow occurred in the north, until dilation along a fault caused a shift to the south. 2) In the south, at least eight eruptions, ~25.4 to 3.4 ka, caused system-wide decompression; this led to incursions of shallow steam-heated acid-sulfate liquid, forming kaolinite ± dickite ± alunite at depth, and locally deeper chalcedony. 3) Post-eruptions to present, fluid upflow is focused in the south, overlain by steaming ground, with kaolinite ± siderite formed by CO 2 -rich steam-heated solution on the margins of the system.