Abstract:
The Hauraki goldfield of the Coromandel Volcanic Zone (CVZ) is New Zealand’s premier epithermal
ore-forming metallogenic province. This province encompasses the north trending Coromandel
Peninsula and its southern extension east of the Hauraki Plains (scale: 200 km x 40 km). Approximately
50 low-sulphidation epithermal Au-Ag quartz-vein deposits are recognised within the province, with
many distributed within a series of NE-trending structural corridors that young to the southeast. There
are numerous deposit-scale, and several regional-scale, studies exploring controls on epithermal
mineralisation in this province. However, little is known about the crustal-scale structural framework or
the control that regional-scale structures may exert on the localisation or ore-forming epithermal
mineralisation.
An integrated qualitative and quantitative structural analysis was carried out involving potential field
data (gravity and magnetics), petrophysical data and surface geological data to investigate the major
basement structures in the CVZ and image their crustal geometry in relation to near-surface geological
patterns and the distribution of epithermal Au-Ag deposits. Geophysical lineaments were extracted at
both district and regional scales from spectrally filtered gradient enhanced Bouguer Anomaly and Total
Magnetic Intensity reduced to pole (TMI-RTP) data by using manual picking and multi-scale edge
detection (worming) techniques. The crustal-scale structures hypothesised from the qualitative analysis
were tested and their geometry explored by developing plausible two-dimensional cross-section models
using constraints derived from the regional gravity data, aeromagnetic data, petrophysical data and
geological information. Slip and dilation tendency analyses were undertaken on a preferred structural
model to provide insight into possible structural controls on paleo-fluid flow and localisation of ore
deposits.
Three distinctive sets of fault patterns were identified: 1) Discrete NNE-to-NE striking rift segments (e.g.
the Waitekauri rift basin) defined by subparallel arrays of normal/oblique faults within a Neogene
volcanic cover sequence. 2) Five major N-to-NNW-striking crustal-scale faults (e.g., Whangamata-
Coromandel Fault) that are interpreted to represent long-lived inherited faults within the Mesozoic
metasedimentary basement, and which behave as transfer structures. 3) NW-striking mostly rightlateral
faults, some of which noticeably offset the basement (e.g Martha-Thames fault), that formed in
response to rotation of eastern North Island following impact of the Hauraki Plateau at the subduction
margin. Collectively, these structures are interpreted to contribute to a province-scale fault system that accommodated tectonic segmentation in association with extension and migration of the Late Miocene
Colville arc
Slip and dilation tendency analysis in 3D Stress® indicates that the NNW and NNE-to-NE-striking faults
have high slip tendency (Ts≥ 0.6) and dilation tendency (Td≥ 0.8) under the inferred paleo-stress fields
(i.e. NW – SE and E – W-oriented σ3) and restored regional tilt. These faults with a high likelihood of
slip and dilation showed a spatial coincidence with surface hydrothermal alteration and associated
epithermal Au-Ag deposit occurrences. This is based on the notion that critically stresses fault segments
have a relatively high likelihood of acting as fluid flow conduits
A useful relationship is established between tectonic segmentation of the crust and the Miocene to
Pliocene epithermal mineralisation in the CVZ. The NNE-to-NE-striking rift basins (e.g. the Waitekauri
rift basin) and the NW and NNW-striking crustal-scale faults are spatially associated with hydrothermal
alteration and associated epithermal mineralisation. This follows the notion that the NW and NNWstriking
crustal faults likely focused deep epithermal fluids into the epithermal zone dominated by
shallow-seated rift segments.