CONSTRAINING THE AGE AND DURATION OF GEOTHERMAL ACTIVITY AT THE EXTINCT OHAKURI HYDROTHERMAL SYSTEM, TAUPO VOLCANIC ZONE, NEW ZEALAND

Abstract

The Ohakuri fossil hydrothermal system is located near the western margin of the Taupo Volcanic Zone (TVZ), and at the southwestern boundary of the Ngakuru Graben, Taupo Fault Belt, New Zealand. Valley erosion along the Waikato River has exposed extensive zones of hydrothermal alteration within Ohakuri Ignimbrite including mineralised quartz-pyrite-adularia bearing vein breccias that formed in response to episodic hydraulic fracturing. The exposed alteration zones have mineral assemblages that formed from chloride waters (quartz, adularia, illite, mordenite, clinoptilolite, smectite), which are overprinted by acid condensate minerals (kaolinite, alunite). Scattered silica sinter float blocks in the area prove that the geothermal system had surface hot springs. Until recently, the timing of these different alteration zones have relied upon maximum and minimum ages of altered lithologies and overlying units. A date of ca. 244 ka for the Ohakuri Ignimbrite places a maximum age on the alkaline chloride alteration. Acid steam altered rocks underlie the Rotoehu Ash, implying a minimum age of ca. 45 ka. Our paper reports the K-Ar radiometric ages of hydrothermal adularia from altered Ohakuri Ignimbrite, as well as radiocarbon ages of organic material entombed in silica sinter. The results show that the adularia formed ca. 190 ± 3 ka and the sinters between 20.1 ± 0.3 ka and 22.5 ± 0.2 ka. These dates imply that the Ohakuri geothermal system was active for at least 170 ka, between 190 and 20 ka, at a period after the Maroa eruption of the Pukeahua Formation pyroclastics (ca. 196 ka). Activity persisted through deposition of the Rotoehu fall deposit (Rotoiti Formation: ca. 45 ka) and the Oruanui Formation (25.4 ± 0.2 ka). Our results provide some valuable constraints on the duration of geothermal activity in the TVZ, which previously could only be estimated using stratigraphic relationships.