Strontium isotope composition of rotorua geothermal waters

Abstract

Strontium isotope compositions of Rotorua geothermal waters, taken in conjunction with chemical and stable isotope data, are broadly consistent with existing hydrogeologic models, and provide new insights into complex water-rock interactions and fluid-fluid mixing processes. Primary alkali chloride waters upwelling in the eastern part of the field have 87 Sr/ 86Sr ratios higher than their presumed aquifer wall rocks, implying a component of more radiogenic strontium from metasedimentary basement. Secondary waters, in contrast, have relatively low 87Sr/ 86Sr ratios consistent with interaction of primary fluids (variably diluted by deep groundwaters) with rhyolite country rock, followed by shallow dilution by young groundwater. Although complicated by the non-conservative behaviour of strontium, the strontium isotope data require significant water-rock interaction in the central and western parts of the Rotorua geothermal field, a feature not made obvious by other chemical or isotope studies.