Geochemical structure and position of the Waiotapu geothermal field, New Zealand

Abstract

The Waiotaprothermal system occupies the central part of the Taupo Volcanic Zone (TVZ). With a surface area of 17 km 2 and a natural heat discharge rate of some 550 MW, it is one of the largest in New Zealand. Between 1957 and 1962 seven wells were drilled to a maximum depth of about 1000 m. The highest temperature measured in these wells was 295°C. In contrast to most other geothermal systems of the TVZ, the rising plume of hot water shows a pronounced lateral component due to the position of the system on the flanks of a hydrological high. The presence of thermal features generally associated with rising vapours, such as fumaroles, mud pools and acid sulphate springs, suggests that the major upflow of hot water occurs over the northern sector of the field, close to two rhyo-dacite domes. The magma bodies associated with these domes may represent the heat sources for the system. Neutral Cl waters are discharged some 4 km to the south from a series of boiling springs and a large, sub-circular pool occupying a hydrothermal explosion crater (Champagne Pool). The chemical and isotopic cympositions of Champagne Pool water reflect extensive non-equilibrium evaporation of a deep water with ° 2 H = −40‰ and δ 18 O = −2.5‰ , in a process similar to that governing evaporation from steam-heated pools. The Cl content of the parent water is 1250 mg/kg, its C0 2 content is, at about 0.1 mmol/mol or 240 mg/kg, very low. The 34S content of H 2S corresponds to +5.3 ± 1.0‰, and the 13C content of C0 2 to −7.3 ± 1.2‰. Geochemical evidence suggests that the Waiotapu system is linked hydrologically to its neighbouring systems Reporoa and Waikite. Each of these, however, is likely to receive additional input of heat and chemicals from separate sources.