Crustal Structures in the Coromandel Volcanic Zone and Hauraki
Goldfield: Insight From Qualitative and Quantitative Potential Field
Analysis
Abstract
The Hauraki Goldfield of the Coromandel Volcanic Zone (CVZ) is New
Zealand’s premier region, hosting approximately 50 adularia-sericite
epithermal Au-Ag deposits with northeast trending structural corridors
that young towards the southeast. The CVZ is situated in northern New
Zealand with a rich record of Neogene deformation, dominated by lateral
migration of arcs with associated back-arc and intra-arc extensions.
Although considerable research has been undertaken in the region with
the purpose of resolving volcano-magmatic-epithermal evolution, the
nature of basement structures underneath the Miocene-to-Pliocene
volcanic cover remain unclear and is investigated using integrated
potential field data (magnetic and gravity) and surface geology. We
performed both qualitative and quantitative interpretations to locate
and determine the geometry of basement structures that likely favour
high flux flow of gold bearing fluids over the crustal depth to control
the localisation of epithermal ore deposits and associated hydrothermal
footprint. Mapped geology, industry acquired aeromagnetic and gravity
datasets reveal regional NNW-SSE, NNE-SSW, NE-SW trends of mapped faults
and geophysical lineaments. A multi-scale edge detection (worming)
analysis suggests the presence of discontinuities and boundaries of
anomalies indicating lateral density and magnetic susceptibility
contrasts with depth persistence of up to 5 km. Our 2D joint gravity and
magnetic models constrain these geophysical discontinuities as
structural contacts between the uplifted Mesozoic metasedimentary
basement and early-CVZ andesite that exists from ~2-4.5
km depth. The top of the Mesozoic basement is modelled at
~3-4.5 km depth in the central CVZ, and
~1-2 km depth to the east and west. The 2D models also
show the continuation of surface expressed hydrothermal alteration up to
~2 km depth in the central CVZ and decrease to 1 km
depth to the east and west. Overall, deep-seated basement-volcanic fault
offsets resolved correlate with corridors of extensive alteration and
mineralisation underlain by a thick unaltered volcanic sequence and the
basement.