Kapowai Caldera Complex (KCC), located 10 km west of
Tairua (eastern Coromandel Peninsula), is a late Miocene composite collapse
caldera, formed during eruption of several small and large volume andesitic
rhyolitic ignimbrites. Numerous epithermal Au-Ag mineral deposits and associated
alteration zones at the KCC are related to caldera (rhyolite lava domes,
intracaldera faults) or regional structures (faults). The main epithermal
deposit exposed in the central KCC is associated with an E-W striking structural
boundary fault that cuts the Welcome Jack rhyolite dome. Propylitic (calcite -
chlorite) and sericitic (quartz - sericite) alteration haloes surround this
deposit. Sporadic zones of sericitic alteration are also exposed within and on
the margins of the Bulls Run rhyolite dome complex. Epithermal systems
post-dated emplacement of these rhyolite domes, but were controlled by them.
Propylitic alteration of ignimbrite in the floor of the Kapowai and Kauaeranga
valleys is related to the major NE-trending Kapowai Fault which extends SW to
south of Thames. Alteration along this fault pre-dates alteration
associated with the rhyolite lava domes. Numerous sericitic and propylitic
altered welded ignimbrite, rhyolite and andesite lava lithic fragments have been
sampled from intracaldera ignimbrites, and although no direct source is known,
they infer the presence of buried epithermal systems. Repeated epithermal
activity at the KCC is inferred by the exposed epithermal deposits/alteration
zones and the older buried geothermal systems. Silicic caldera volcanoes are
considered to be critical volcanic environments for epithermal Au-Ag
mineralisation in Coromandel, but are highly variable and need detailed
stratigraphic, structural, petrologic, and geophysical studies to characterise
them.