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 mi neral from an active geothermal system in New Zealand. Vein calcite re covered 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 l eachate-leachate isochron technique on four silicate containing sub-sa mples of calcite from a single vein. Although the error on this isochr on age is considerable, it is significantly younger than the earlier e stimated age of similar to 200 ka BP for the onset of activity at the Tauhara system and probably records the date of brecciation and veinin g, which may be associated with volcanic activity at the adjacent daci tic Tauhara Volcanic Complex. These results demonstrate that hydrother mal vein calcite can now be dated directly, and opens the way for more detailed studies of the evolution of the New Zealand geothermal syste ms. (C) 1998 Elsevier Science B.V. AU rights reserved.