Forty-seven samples of deep geothermal well fluids and hot springs from 14 geothermal sites in the Taupo Volcanic Zone (TVZ), North Island, New Zealand, were analyzed for Cl, Br and Li concentrations and delta Cl-37, delta Li-7, delta O-18 and delta D values. The main purpose of this study was to identify the sources of salinity in the TVZ fluids and to assess the processes that control stable chlorine isotopic fractionation in the geothermal waters. The samples were obtained from deep wells in the Mokai, Ohaaki, Wairakei, Rotokawa, Kawerau, Ngatamariki and Tauhara geothermal fields and from the hot springs at Rotorua, Waiotapu, Lake Taupo, Waimangu, Waikite, Orakei Korako and Tokaanu.
Two main meteoric recharge areas were identified in the central TVZ, one located to the NE with delta O-18 and delta D values of -5.5 parts per thousand and -35 parts per thousand, respectively and the other to the SW with a delta O-18 value of -6.7 parts per thousand and delta D value of -43 parts per thousand. Water rock interactions in most of the geothermal fluids resulted in a delta O-18 shift to more positive values; up to 1 parts per thousand for Mokai and over 3 parts per thousand for Kawerau. The delta Li-7 values range from -2.9 parts per thousand to 2.0 parts per thousand and suggest that the deep geothermal fluids have interacted with rocks with low delta Li-7 values. The highest delta Li-7 values (1.4 parts per thousand) in the reservoir fluids were found at Kawerau where the low quartz content of the greywacke may result in the depletion of Li-6.
The delta Cl-37 values of the samples ranged from -1.0 parts per thousand to 0.8 parts per thousand. The delta Cl-37 values of 0.2 parts per thousand, 0.1 parts per thousand and 0.0 parts per thousand were measured in samples from Orakei Korako, Waiotapu and Waimangu respectively, and these are interpreted to have been affected by surface evaporation and distillation processes. Excluding these data, most waters with high Cl/Br ratios had positive delta Cl-37 values. The Cl/Br molar ratios in well samples ranged from 659 to 1664 and the delta Cl-37 values from -1.0 parts per thousand to 0.8 parts per thousand. The hot springs had Cl/Br molar ratios ranging from 858 to 1611 and delta Cl-37 values from -0.8 parts per thousand to 0.7 parts per thousand.
Overall, the Cl/Br molar ratios of the fluids are comparable with published data on basaltic magmas, and these magmas are inferred to be the main source of halogens in the geothermal fluids. However, the geothermal fluids can be separated into two main groups: one associated with rhyolitic magmas characterized by positive delta Cl-37 values and high Cl/Br molar ratios, and other related to andesitic magmas with negative delta Cl-37 values and lower Cl/Br molar ratios. We suggest therefore, that the isotopic fractionation of the stable Cl isotopes is related to the difference in water contents of andesitic and rhyolitic magmas and that the fluid phase is enriched in Cl-37 during magmatic evolution. There is no evidence of isotopic fractionation during the transit of the geothermal fluids from the reservoir to the surface, but evaporation and distillation processes at the surface can affect the delta Cl-37 values. (C) 2013 Elsevier Ltd. All rights reserved.