Structural and tectonic controls on the epithermal Au-Ag deposits in the Coromandel Volcanic Zone (CVZ), New Zealand: Insight from joint geological and geophysical interpretation

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.