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 (Tm of -0.1 to -1.0C) fluids
that were locally boiling. Homogenisation temperature Th versus
Tm 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 NaCl 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 CO2 , as at Ohaaki. Oxygen isotope
analyses of main stage quartz range from 6.1 to 11.6 %0 and of vein calcite from
2.29 to 9.4%0.
Calculated 18Owater values for the
hydrothermal fluid that deposited the main stage quartz range from 2.0 to
-5.7%0, 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.