THE CHEMICAL AND ISOTOPIC COMPOSITIONS OF THERMAL WATERS AT WAIMANGU,NEW-ZEALAND

The surface expression of the Waimangu Geothermal System comprises hot springs and steaming ground which occur in the vicinity of volcanic c raters that formed during the 1886 Tarawera Rift eruption. Boiling chl oride spring waters discharge over a 30 m vertical interval from the S teaming Cliffs along the western shore of Lake Rotomahana (similar to 340 m asl) to inferno Crater (373 m asl) in the upper Waimangu Valley. K-Na and K-Mg ratios, aqueous silica concentrations and Delta O-18((s ulfate-water)) values of spring waters, indicate deep equilibration te mperatures that range from 200 to 280 degrees C, while uniform B/Cl ra tios (similar to 0.0075) suggest the chloride waters derive from a sin gle reservoir. Stable isotope data are best explained as the result of boiling and dilution in the upper part (<2 lan depth) of the system, with an interpreted parent liquid composition of Cl=585 mg/kg, delta(1 8)O = -4.75 parts per thousand, and delta D = -37.5 parts per thousand . This liquid is distinct from the parent inferred for the nearby Waio tapu geothermal system and is positively enriched in both delta(18)O a nd delta D compared to focal meteoric water (delta(18)O = -6.85 parts per thousand, delta D = -42 parts per thousand). Boiling upflow is int erpreted to dominate beneath all surface features except in the vicini ty of Lake Rotomahana, where a cold hydrostatic cap (similar to 100 m thick) depresses the temperature gradient of rising fluids discharging at Steaming Cliffs. The effects of ground water dilution generally in crease with hot spring elevation in response to a steepening hydraulic gradient and rugged topography associated with the volcanic craters o f Waimangu Valley. A minimum chloride Bur estimate of 80-100 g/s corre sponds to athermal output of 150 to 190 MW.