Mineralisation processes in
epithermal deposits in the North Island of New Zealand are best illustrated by
considering the two most productive deposits: Waihi and the Thames Bonanza Zone
(TBZ), which have different mineralogies and ore shoots geometries. These two
deposits can be taken as typical of certain styles of epithermal deposits
worldwide, and recognition of their differences can be used to develop
mineralisation models of predictive value. Waihi, the major economic producer in
the Coromandel Volcanic Zone (CVZ), has laterally near-continuous ore shoots
over a vertical interval of 600 m which consist of quartz, chalcedony, pyrite,
base metal sulphides and electrum. They are found in the foot wall of earlier
quartz veins which contain quartz pseudomorphs after platy calcite and adularia.
The Thames Bonanza Zone which was the second biggest producer in the CVZ has
discrete near-vertical ore shoots over 300 m vertically, which consist of
quartz, pyrite, base metal sulphides, electrum and in their upper parts barite,
that are enclosed in veins of quartz.

The
gold mineralisation in both deposits is considered to be a co-precipitate with
pyrite, with that at Waihi produced by prolonged boiling in an upflow of a
hydrothermal system. That at the TBZ was produced by mixing a deep water primed
for gold deposition by prior limited boiling, with an oxidised condensate of the
steam released by the boiling, in an outflow proximal to the system's
upflow.