Regional crustal-scale structural control on epithermal deposits within the Hauraki Goldfield, Coromandel Volcanic Zone, New Zealand: insight from integrated geological and aeromagnetic structural patterns

Figures & data

Figure 1. Current tectonic configuration of northern New Zealand annotated with loci of volcanism for three different time periods ± 0.5 Ma (circles: black = 0 Ma; grey = 8 Ma; white = 16 Ma) from Seebeck et al. (2014b), geodetic vectors of extension (double arrows: Wallace et al. 2004) and relative plate motion (single arrows: DeMets et al. 1994) in mm/year. Dotted line represents a terrane suture in basement rocks as inferred from the Junction Magnetic Anomaly (JMA). Rotation of eastern North Island shown for period 3 Ma to present (Wallace et al., 2004). HR = Hauraki Rift; HT = Havre trough; NIFS = North Island Fault System; VMFZ = Vening Meinesz Fracture Zone. TVZ = Taupo Volcanic Zone shown in red, Coromandel Volcanic Zone shown in orange, Northland shown in blue.

Figure 2. Structural architecture of the central North Island showing active faults in red (GNS Science Active Fault database (Lamarche et al. 2006), other faults in dashed black lines (Edbrooke 2005; Townsend et al. 2008; Giba et al. 2010), the Taranaki Fault (thick barbed line annotated with position of northern limit of activity through time: Stagpoole and Nicol 2008), the Junction Magnetic Anomaly (thick red dashed line). Periods of activity on extensional faults of western North Island are shown after Giba et al. (2010). Position and timing of epithermal mineralisation and geothermal activity shown by green circles and red square polygons respectively. Red lines within green circles are approximate vein directions. Thick black dashed circles are calderas. Locations referred to in the text are numbered in white circles. WKF = Waikato Fault; OF = Ohura Fault, WP = Waipa Fault.

Figure 3. Geology of the Hauraki Goldfield and its surrounding areas (scale 1:250,000: Edbrooke 2001). Epithermal Au-Ag deposits are indicated by yellow circles, and their size refers to amount of gold production. KOST = Karangahake-Ohui structural trend, TWFZ = Tapu-Whitianga-Fault Zone, THHK = Tairua-Hikuai-Kaueranga Lineament.

Table 1. Description of major rock types in the CVZ (after, Edbrooke 2001).

Main rock types	Description
Coromandel group andesite and dacite rocks	Commonly occur as lava flows and domes with massive and phyric texture composed of phenocrysts of plagioclase, augite and Fe/Ti oxides. As noted by Booden et al. (2012), these rocks are derived from aggregated magmatic systems that evolved either sporadically or rapidly to produce silicic rocks that are widely distributed along the Coromandel and Kaimai ranges, suggesting it is comprised of a chain of large stratovolcanoes. Regional geochronology indicates that the age of these rocks ranges from ca. 18–3.78 Ma (younging to south), and thickness increases towards south reaching up to 1400 m (e.g. Kaimai subgroups)
Tuff breccia and tuffs	Mostly occur as intercalation with lava flows and consist of clasts (maximum of ca. 500 mm in diameter) of andesite and dacite in a matrix composed of plagioclase, pyroxene, quartz and biotite. Most tuff breccias in the southern CVZ appear as syn-eruptive products like debris flow and lahars, instead of primary pyroclastic flow or fall deposits. A thick succession of tuff breccia may reach a thickness of up to 40 m (e.g. Komata valley)
Paritu plutonic rocks	Occur as gabbro, granodiorite and quartz-diorite with andesite-dacite dyke swarms that intrude the Manaia Hill Group basement and early Kuaotunu subgroup andesite at the northern end of CVZ (e.g. Mt. Moehau). Reported K-Ar age range from 16.4–17.6 Ma.
Minden rhyolite	Occur as flow banded and dome complexes (e.g. Maratoto peak) associated with tuff and breccia, marking some of the explosive eruption centres with caldera structures in the central CVZ. The maximum exposed thickness is ca. 280 m at Maratoto peak and crystallisation age range from 2.3–8.9 Ma.
Coroglen ignimbrite	Dominantly composed of rhyolite and andesite lithic, pumice breccia, and associated sediments in the northern and central part of CVZ with thickness up to 160 m. Whereas, poorly-to-strongly welded, pumice-rich, plagioclase-quartz-biotite ignimbrite dominated in the Waihi basin with a thickness of about 80 m. Regional deposition age of these rocks range from ca. 11–1.5 Ma which young towards the south.
Manaia Hill group Metasedimentary basement rocks	Exposed in the northern part of CVZ with characteristic well-indurated, massive, medium-to-course grained lithic volcanoclastic sandstone, interbedded graded sandstone and siltstone, and conglomerates.

Table 2. Nomenclature, definition and use of selected image processing and imaging techniques applied to the aeromagnetic data.

Data processing	Description	Application
Reduced-to-Pole (RTP)	An algorithm that applied on TMI gridded data to remove the effects of magnetic inclination and declination of the main field on the anomaly shapes.	Locating anomalies directly above their possible sources to show the true geometry of the magnetic bodies (Cooper and Cowan 2005).
Analytical Signal (AS)	The square root of the squared sum of the three orthogonal directional gradients. It works independently of the magnetic inclination and declination to removes the effect of remanent magnetisation	Enhance edges/boundaries of magnetised bodies. It also forms peaks over small magnetised bodies (Roest et al. 1992)
Vertical Derivative (VD)	It measures the vertical rate of change in the magnetic field.	It has no directional bias and hence retains peak positions from the TMI. Enhance vertical contacts and geophysical response of weak anomalies (Roest et al. 1992)
Tilt Derivative (TD)	The arc-tangent of the ratio of first vertical derivative and the total horizontal derivative, irrespective of the amplitude or wavelength of the geophysical anomalies.	Improve vertical contacts between the source bodies (geological edges and fault lineaments for both deep and shallow features) (Verduzco et al. 2004)
Total Horizontal gradient (hgm)	The square root of the squared sum of gradients along the two horizontal directions	Emphasise horizontal contacts between source bodies, geological boundaries and structures as points of gradient maxima (Verduzco et al. 2004)
Upward continuations (UC)	Transforms the data by bringing the plane of measurement farther from the source or at different heights above the source	Suppressing the shallower features and highlighting deep geological and structural features (Gibert & Galdeano 1985)
Composite images, and colour-shaded image processing	Involves in layering of two different datasets or differently processed images and overlaying a colour table of the data with shaded greyscale image of the same dataset.	Enabling correlation of spatially coincident features within the datasets. Providing colour and textural ranges that allow the differentiation between geophysical domains (Williams and Betts 2007)

Table 3. Major interpretation criteria to define magnetic lineaments, faults and deformation zones from the regional aeromagnetic data.

Magnetic responses	Anomaly characteristics and interpretation of magnetic structures	Magnetic responses	Anomaly characteristics and interpretation of magnetic structures
	Magnetic contacts interpreted from both deep and near-surface components of TMI-reduced-to-pole data		Dislocation and apparent offset of linear to sub-linear magnetic trends interpreted from tilt and vertical derivatives
	Broad hydrothermal alterations with characteristic magnetic quiet zones interpreted from analytic signal and TMI-reduced to pole image		Linear to sub-linear magnetic edges interpreted from horizontal gradient image.
	Alternating regions of linear to sub-linear magnetic highs and lows interpreted from short wavelength TMI-reduced to pole image		Abrupt terminations, perturbations, and bending of linear magnetic trends interpreted from short wavelength magnetic anomalies

Figure 4. (a) Pseudocolour image of the Total Magnetic Intensity (TMI), (b) TMI, reduced to pole (RTP) of the Hauraki Goldfield, intensity layer illuminated from the northeast. DF = Drury fault, HF = Hauraki Fault, KB = Kerikeri basalt, KV = Kiwitahi volcanics, K = Karangahake. Letters a, b, c, and d refers to high-frequency magnetic anomalies. Polygons with a black outline in (a) indicate calderas (Skinner 1986; Malengreau et al. 2000; Smith et al. 2006). Polygons with a black outline in (b) refer to hydrothermal alteration areas (Christie et al. 2007; Morrell et al. 2011). White cross marks refer to areas with suspected remanant magnetisation, and white lines are rivers and paleochannels. Inset map in (a) refers to different parts of CVZ discussed in the text.

Figure 5. (a) Pseudocolour image of Analytical Signal (AS) showing hydrothermal alterations coinciding with regions of low AS value (black outline polygons: Christie et al. 2007). Polygons with white outlines indicate younger andesites and rhyolites that obscure hydrothermal alterations and exhibit high magnetic frequency. (b) Pseudocolour image of Horizontal gradient magnitude. (c) Pseudocolour image of Vertical Derivative (VD). (d) Pseudocolour image of Tilt Derivative (TD) with values greater than 0.0 to show the approximate width and distribution of features with positive susceptibility (Verduzco et al. 2004). See text for letters R and Y. KV = Kiwitahi Volcanics. Black/white circular feature refers to a high magnetic anomaly related to a deep intrusive body.

Figure 6. (a) Pseudocolour image of TMI-reduced to pole for the Northern CVZ illuminated from the northeast and annotated with interpreted structures. (b) Composite images of the tilt derivative superimposed onto the intensity layer of the vertical derivative of the reduced to pole TMI data for the same area as (a) and illuminated from the northeast and annotated with interpreted structures. (c) The corresponding geology map overlaid with interpreted structures. Abbreviations are: MF = Mangatu Fault; MhF = Matamata harakeke Fault; WF = Waitaia Fault; CF = Coromandel Fault. Labelled numbers (1–21) refer major interpreted faults and lineaments.

Figure 7. (a) Pseudocolour image of TMI-reduced to pole for the Central CVZ illuminated from the northeast overlaid with interpreted structures. (b) Composite images of the tilt derivative superimposed onto the intensity layer of vertical derivative and annotated with structural interpretations. (c) Corresponding geology map overlaid with interpreted structures. Overlaid lines are interpreted structures. Abbreviations: WF = Waihi Fault; OF = Owharoa Fault; WF = Waitekauri Fault; JF = Jubilee Fault; KF = Komata Fault; RF = Rotokohu Fault; MF = Mangakino Fault. Letters from a-to-g are regions of low magnetic anomaly interpreted as fault-angle basins. Labelled numbers (16–34) refer major interpreted faults and magnetic lineaments.

Figure 8. (a) Pseudocolour image of TMI-reduced to pole upward continued to 2 km. (b) Pseudocolour image of TMI-reduced-to-pole upward continued to 4 km. Upward continuation filter calculates the magnetic response of sources that are approximately one-half as deep as the upward continuation distance (Jacobsen 1987). This data shows the progressive change in shape and relative depth continuity of magnetic source bodies. (c) Composite image of analytic signal of aeromagnetic data upward continued to 4 km superimposed onto intensity layer of TMI-reduced-to-pole. Mapped surface alteration zones (Christie et al. 2007). Polygons of black outlines are deep magnetic quiet zones (interpreted in this study). (d) Zoomed pseudocolour image of TMI-reduced-to-pole for the area shown on Fig. b, and annotated with interpreted deep and shallow magnetic lineaments (labelled numbers 12, 21–27, 30, 32) and surface alteration zones. A1-A4 refers to high magnetic rock packages in the subsurface. Th = Thames, W = Waihi, KV = Kiwitahi Volcanics

Figure 9. (a) Main concepts of edge detection in 2D section of data, apparent dip direction of edged and degree of worm sinuosity (Holden et al. 2000). (b) Worms produced using criteria (worm length > 1 km) and (worm sinuosity > 0.95). The degree of sinuosity ranges between 0 and 1; where 1 refers straight and non-sinuous. (c) Trend analysis for shallow and deep worms.

Figure 10. Interpretation of deep structural discontinuities. (a) Crustal-scale lineaments. (b) Deep magnetic worms (depth > 2 km). Wh = Whangamata, W = Waihi, K = Karangahake, GC = Golden Cross, BH = Broken Hills, CF = Coromandel Fault, KOST = Karangahake-Ohui structural trend, TWFZ = Tapu-Whitianga-Fault Zone, THHK = Tairua-Hikuai-Kaueranga Lineament. Labelled numbers (1–5) refer crustal lineaments with depth persistence greater than 3 km. Numbers labelled 6 and 7 refer magnetic lineaments with depth persistence up to 2 km.

Figure 11. (a) Map showing shallow-seated structures interpreted from the short wavelength aeromagnetic anomalies. Overlaid shaded regions and circular features are interpreted discrete fault-angle basins and caldera structures respectively. (b) Map showing a summary of the magnetic fabrics in the Hauraki Goldfield. Surface hydrothermal alteration zones, deep alteration footprints (damage zones at depth) and epithermal deposits are overlaid on deep-seated crustal structures, illustrating the genetic and spatial links between crustal structures and epithermal systems. Surface hydrothermal alteration zones and location of mineral deposits with past gold production in the Hauraki Goldfield (after Christie et al. 2007). Abbreviations: WF = Waihi Fault; OF = Owharoa Fault; WF = Waitekauri Fault; JF = Jubilee Fault; KF = Komata Fault; RF = Rotokohu Fault; MF = Mangakino Fault; Wh = Whangamata, W = Waihi, K = Karangahake, GC = Golden Cross, BH = Broken Hills, CF = Coromandel Fault, KOST = Karangahake-Ohui structural trend, TWFZ = Tapu-Whitianga-Fault Zone, THHK = Tairua-Hikuai-Kaueranga Lineament.

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