The
epithermal gold-silver deposits wall poster is one in a series of posters on
different mineral deposit types being produced by GNS Science to raise awareness
of the characteristics of New Zealand
mineral deposits and their prospectivity. Epithermal deposits consist of quartz
veins, quartz vein stockworks and hydrothermal breccias that are mainly hosted
in subaerial andesitic or rhyolitic rocks of Tertiary age. The deposits were
formed at depths of typically less than 1 km and at temperatures between 180
and320C, although ore grade mineralisation was generally deposited at
less than 270C.

In
New
Zealand, epithermal deposits occur in
association with late Cenozoic volcanic rocks in the Northland, Coromandel and
Taupo volcanic regions. All of the past and present gold production, estimated
at greater than 380 t (along with 1300 t Ag) comes from the Miocene-Pliocene
Coromandel Volcanic Zone, where about 50 epithermal gold-silver deposits occur
over a 200 km long belt. In Northland, silver and mercury have been produced
from hot spring type epithermal deposits, and several gold-silver prospects have
been identified. Ore grade gold-silver mineralisation is
currently being deposited in several active geothermal fields of the Taupo
Volcanic Zone, and extinct geothermal systems occur on the western side of the
zone. The active systems have provided natural laboratories to help understand
the processes of epithermal mineralisation.