A U–Th calcite isochron age from an active geothermal field in New Zealand

We report here the first U–Th disequilibrium age for a hydrothermal mineral from an active geothermal system in New Zealand. Vein calcite recovered from a depth of 389 m in Well Thm-1 at the Tauhara geothermal field has an age of 99±44 ka BP. This age was determined using a leachate–leachate isochron technique on four silicate containing sub-samples of calcite from a single vein. Although the error on this isochron age is considerable, it is significantly younger than the earlier estimated age of ∼200 ka BP for the onset of activity at the Tauhara system and probably records the date of brecciation and veining, which may be associated with volcanic activity at the adjacent dacitic Tauhara Volcanic Complex. These results demonstrate that hydrothermal vein calcite can now be dated directly, and opens the way for more detailed studies of the evolution of the New Zealand geothermal systems.