The Mokai geothermal system, New Zealand, is located within a volcanic collapse structure comprised of flat-lying silicic rocks. The deep chloride fluid rises in the south, inside the caldera margin; near the surface the fluid flows laterally more than 12 km to the north, because of the hydraulic gradient caused by 300 m of relief. The maximum measured temperature of the production wells is near 323 ¿C at 25 m depth, whereas temperatures in two wells on the northern margin of the upflow are 25% of yound (thermonuclear, post-1955) water before the mixture dilute outflowing chloride fluid from the deep system.

The deep chloride fluid has a 36Cl/Cl ratio of 2.6¿10-15 and 36Cl concentration of 106 atoms per 10-6 kg. This fluid has a lower ratio (by a factor of 90) and a higher absolute concentration (by a factor of 7) than present-day stream water. However, the marginal wells have been diluted by groundwater with at least 500 36Cl atoms per 10-6 kg, 30 times richer in 36Cl than present-day stream waters. This spike is most likely due to circulation of precipitation formed during thermonuclear testing to depths of 1000 m (though the component with the 36Cl spike only makes a moderate, <25%, contribution to the groundwater diluent; most of the diluent is much older, pre-1955). The amount of 36Cl in the deep fluid at Mokai is 5--10 times less than that predicted from a calculation of the neutron fluxes in the silicic reservoir and underlying greywacke basement rocks. This indicates that the chloride was not derived from host rock leaching in the upper 3 km; rather, the fluids have interacted with more primitive rocks below the drilled depths of the Taupo Volcanic Zone, with the chloride possibly derived from a magmatic brine. ¿ American Geophysical Union 1990