1. PhD, Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand (corresponding author).
2. PhD, U.S. Geological Survey, 333 W. Nye Ln., Carson City, NV 89706.
3. Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand.