Examination of core from drillhole OKM3 on the western bank of Orakei Korako geothermal field

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 30m 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 (<~100 °C). Zeolite is a late stage alteration product and indicates near-neutral water which may be of meteoric or mixed groundwater and thermal origin. Early stage relict, neutral pH, high temperature, alkali chloride water alteration was identified in the core. Illite-chlorite indicating a series of medium temperature (190-220°C) alteration was identified at 9 to 10 m depth. Illite indicating high temperature (>~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 (<30 m depth) on the western side of Lake Ohakuri. However, historically, high temperature geothermal fluid occurred at depth in this area. Presently, no thermal activity occurs at the surface. The identification of relict high temperature zone and a previously unknown siliceous sinter horizon at 22 m depth indicates historic flow of alkali chloride fluids at surface and high temperature fluid at depth was more widespread than that of today.