The Late Miocene gold-silver bearing epithermal system at Waihi has some close similarities in its paleo-thermal and
-geochemical structure with modern, volcanic-hosted geothermal systems such as Ohaaki-Broadlands. At Waihi an extensive
braided quartz vein system is enveloped by hydrothermally altered andesites with minor tuff breccias and tuffs. A main stage
of vein filling consists of multiple phases of chalcedonic quartz and fine to medium grained quartz, which are accompanied by
pyrite, sphalerite, galena, chalcopyrite and acanthite below the base of oxidation. Platy calcite is present locally. Fluid inclusion
studies indicate that the main stage quartz was deposited at temperatures of 189-273C from dilute (T of -0.1 to -1.0C) fluids
that were locally boiling. Homogenisation temperature Th versus T. data points lie within the field enveloped by dilution and
boiling/gas loss paths determined for the Ohaaki geothermal field, and indicates that the parent hydrothermal fluid for the Waihi
epithermal system had a similar composition to that at Ohaaki, ie 0.2 wt% equiv NaCI and 3.3 wt% equiv CO2. The hydrothermal
alteration mineralogy and the isotopic composition of vein calcite indicate that the hydrothermal fluid contained relatively high
concentrations of CO 2' as at Ohaaki. Oxygen isotope analyses of main stage quartz range from 6.1 to 11.6 %o and of vein calcite
from 2.29 to 9.4% . Calculated d8Ow. values for the hydrothermal fluid that deposited the main stage quartz range from - 2.0 to -5.7%c , with a trend to less negative values with depth. They are similar to those in the deep chloride fluid in active
geothermal systems with a substantial fluid throughput. A high fluid throughput at Waihi system is also evident from the extensive
high intensity hydrothermal alteration and large volume of quartz deposited in the vein system.