GEOCHEMICAL STRUCTURE AND POSITION OF THE WAIOTAPU GEOTHERMAL-FIELD, NEW-ZEALAND

The Waiotapu geothermal system occupies the central part of the Taupo Volcanic Zone (TVZ). With a surface area of 17 km(2) and a natural hea t discharge rate of some 550 MW, it is one of the largest in New Zeala nd. Between 1957 and 1962 seven wells were drilled to a maximum depth of about 1000 m. The highest temperature measured in these wells was 2 95 degrees C. In contrast to most other geothermal systems of the TVZ, the rising plume of hot water shows a pronounced lateral component du e to the position of the system on the flanks of a hydrological high. The presence of thermal features generally associated with rising vapo urs, such as fumaroles, mud peals 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. Neutr al CI waters are discharged some 4 ion to the south from a series of b oiling springs and a large, sub-circular pool occupying a hydrothermal explosion crater (Champagne Pool). The chemical and isotopic composit ions of Champagne Pool water reflect extensive non-equilibrium evapora tion of a deep water with delta(2)H = -40 parts per thousand and delta (18)O = -2.5 parts per thousand, in a process similar to that governin g evaporation from steam-heated pools. The CI content of the parent wa ter is 1250 mg/kg, its CO2 content is, at about 0.1 mmol/mol or 240 mg /kg, very low. The S-34 content of H2S corresponds to +5.3+/-1.0 parts per thousand, and the C-13 content of CO2 to -7.3+/-1.2 parts per tho usand. Geochemical evidence suggests that the Waiotapu system is linke d 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.