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Crustal Structures in the Coromandel Volcanic Zone and Hauraki Goldfield: Insight From Qualitative and Quantitative Potential Field Analysis
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  • Engdawork Bahiru,
  • Julie Rowland,
  • Jennifer Eccles,
  • Richard Kellett,
  • Anthony B Christie
Engdawork Bahiru
University of Auckland

Corresponding Author:[email protected]

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Julie Rowland
Univ Auckland
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Jennifer Eccles
The University of Auckland
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Richard Kellett
New Zealand Energy Corp.
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Anthony B Christie
GNS Science
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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.