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 craters that formed during the 1886 Tarawera Rift eruption. Boiling chloride spring waters discharge over a 30 m vertical interval from the Steaming Cliffs along the western shore of Lake Rotomahana (∼ 340 m asl) to Inferno Crater (373 m asl) in the upper Waimangu Valley. K-Na and K-Mg ratios, aqueous silica concentrations and Δ 18O(sulfate-water) values of spring waters, indicate deep equilibration temperatures that range from 200 to 280°C, while uniform B/Cl ratios (∼ 0.0075) suggest the chloride waters derive from a single reservoir. Stable isotope data are best explained as the result of boiling and dilution in the upper part (<2 km depth) of the system, with an interpreted parent liquid composition of Cl=585 mg/kg, δ18O = −4.75 ‰, and δD = −37.5 ‰. This liquid is distinct from the parent inferred for the nearby Waiotapu geothermal system and is positively enriched in both δ18O and δD compared to local meteoric water (δ18O = −6.85 ‰, δD = −42 ‰). Boiling upflow is interpreted to dominate beneath all surface features except in the vicinity of Lake Rotomahana, where a cold hydrostatic cap (∼ 100 m thick) depresses the temperature gradient of rising fluids discharging at Steaming Cliffs. The effects of ground water dilution generally increase with hot spring elevation in response to a steepening hydraulic gradient and rugged topography associated with the volcanic craters of Waimangu Valley. A minimum chloride flux estimate of 80–100 g/s corresponds to a thermal output of 150 to 190 MW.