ニュージーランドの地熱系

• Main Authors: 田口 幸洋, HEDENQUIST Jeffrey W.
• Format: Report
• Language: Japanese
• Published: 鹿児島大学
• Subjects: 450; New Zealand; White Island; Bay of Plenty
• Online Access: http://hdl.handle.net/10232/16104; https://ir.kagoshima-u.ac.jp/?action=repository_uri&item_id=5220; https://ir.kagoshima-u.ac.jp/?action=repository_action_common_download&item_id=5220&item_no=1&attribute_id=16&file_no=1

This paper is aimed at introducing an outline of the geothermal systems of theTaupo Volcanic Zone in New Zealand based mainly on a recent review by Hedenquist(1986).The development of New Zealand's geothermal energy resources began in the late1940s, and the world's first "wet steam" geothermal station was constructed in 1958 atWairakei. Geothermal electricity from Wairakei supplies 150 MW, some 4% of NewZealand's total requirements at present; another 100 MW will be provided by 1989 by the new development at Ohaaki.Twenty major geothermal systems are located in the Taupo Volcanic Zone. The zone extends from the active andesite volcano Mt.Ruapehu to White Island, another active volcano 220km north northeast in the Bay of Plenty; this is a zone of extensive geologic structural depression. A considerable volume of rhyolitic material fills thedepression, erupted during the last ~1.0 Ma. Geothermal drilling and gravity surveyshave revealed a relatively simple set of horst and graben blocks in the basement. Mostof the geothermal systems have boiling springs at the surface and measured heat flows of 50 to 500 MW. They all have areas of low resistivity of a few to several km^<2>, indicating that hot saline water is present at depth. Chemical and isotopic studies of deep hot water indicate that the hot fluids are dominated by meteoric waters which have been heated and have acquired most of their solutes during deep circulation.Gold and silver is presently being deposited in most of the geothermal systems inthe Taupo Volcanic Zone. The study of gold deposition in such active geothermalsystems has allowed us to unequivocally define the chemistry of the metal transporting fluids, and to identify fluid boiling and gas loss as the principal process leading to golddeposition in the analogous fossil epithermal environment.