Changes in the configuration of late Cenozoic volcanic arcs
related to Pacific-Australian plate convergence in northern New Zealand may have
localised large-scale epithermal gold-silver mineralisation to a particular part
of the volcanic arc system, the Coromandel Volcanic Zone (CVZ). The history of
arc-type volcanism in northern New Zealand began with the development of the
continental margin Northland Volcanic Arc from early-mid Miocene (26-15 Ma).
From 18-15 Ma the Northland Volcanic Arc extended south into the northern and
western part of the CVZ where it is dominantly composed of andesite with lesser
dacite and very minor rhyodacite. In the south, centre and east of the CVZ,
younger andesite-dacite volcanism overlapped in space and time (14-5 Ma) with
voluminous rhyolitic volcanism associated with rift-hosted calderas and minor
basaltic eruptions. This change to bimodal rhyolitic-basaltic volcanism and
extensional tectonics was a result of southwest (SW)-directed propagation of the
Colville-Lau oceanic island arc into the north-northwest (NNW) trending
Northland Volcanic Arc and its underlying continental crust. The extensional
tectonic regime created in the collision zone between the two arcs is also the
site of the 14-6 Ma Coromandel epithermal gold-silver province, which contains
some 50 separate deposits and has a production (1862-1996) of about 300 t Au and
1150 t Ag. Extensional tectonics facilitated the emplacement of silicic magma at
shallow crustal level and led to reactivation of deep basement faults. The
resulting enhanced heat flow and fault-focused fluid flow localised large
hydrothermal systems and associated extensive low-sulphidation epithermal
gold-silver deposits in the southern part of the province, as at Waihi, Golden
Cross and Karangahake.