Examination of Core from Drillhole OKM3 on the Western Bank of Orakei Korako Geothermal Field

Authors: M.J. Purnomo, B.Y. Lynne, S.J. Zarrouk and C. Boseley
Keywords: Orakei Korako alteration, shallow subsurface alteration, Umukuri sinter, Orakei Korako sinter
Conference: New Zealand Geothermal Workshop Session:
Year: 2013 Language: English
Geo Location:
Abstract: The present-day surface activity at Orakei Korako dominantly occurs on the eastern side of the Lake Ohakuri with only minor surface activity on the western bank. A 30 m deep drill hole (OKM3) was drilled on the western bank to monitor the shallow groundwater level. Continuous core was recovered providing the first core from the western side of Lake Ohakuri, as previously reported cored drillholes are located on the eastern side of Lake Ohakuri. Examination of OKM3 core involved identifying the lithology and stratigraphy which consists of pyroclastic rocks of Taupo Pumice alluvium and Akatarewa breccias. At 22 m depth a thin (~10 cm) siliceous sinter horizon is present, indicating historic surface discharge of alkali chloride thermal water. X-Ray Diffraction (XRD), petrographic microscopy, Scanning Electron Microscopy (SEM) and Energy- Dispersive X-Ray Spectroscopy (EDS) were used to examine hydrothermal alteration within the core. Hydrothermal alteration minerals identified include zeolites (mordenite and clinoptilolite). These zeolites are common alteration products of pyroclastic rocks and infer low temperature alteration (~220°C) alteration, was identified at 27-28 m depth. The location of the drill site is on an uplifted block of the Emerald fault and this relict alteration indicates temperatures >220 °C where at depth at some stage in the past. These findings reveal low temperature hydrothermal alteration has taken place in the shallow environment (
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