Hydrothermal Alteration at the Karangahake Epithermal Au-Ag Deposit, Hauraki Goldfield, New Zealand

Simpson, Mark P.; Gazley, Michael F.; Stuart, Alistair G.J.; Pearce, Mark A.; Birchall, Renee; Chappell, Debra; Christie, Anthony B.; Stevens, Murray Ronald

doi:10.5382/econgeo.2019.4630

Cited by 13 publications

(7 citation statements)

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“…The Taupo Volcanic Zone (TVZ) fits the prerequisites for a regionally extensive source of volcanic and volcaniclastic rocks (Figure 1a) (Cole et al., 2014; Cronin et al., 1999; Downs et al., 2014; Gravley et al., 2016; Hidgson & Manville, 1999; Kohn & Topping, 1978; Lowe et al., 2013; Procter et al., 2014; Wilson & Rowland, 2016). Their chemical weathering products are demonstrably smectite‐dominant, and smectite is ubiquitous in TVZ geothermal and hydrothermal systems (Heap et al., 2020; Hocking et al., 2010; Libbey et al., 2013; Simpson et al., 2019). Heavily weathered tephra is also widespread in coastal exposures around the Bay of Plenty (Iso et al., 1982).…”

Section: Resultsmentioning

confidence: 99%

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

Underwood

Dugan

Cardona

2022

Geophysical Research Letters

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The Hikurangi margin offshore North Island, New Zealand (Figure 1a) conforms to most of the paradigms of subduction zones with high rates of terrigenous sedimentation (e.g., Moore & Karig, 1976;Underwood & Moore, 1995). The margin's bathymetric architecture includes a narrow shelf, forearc basins, imbricate thrusts and folds within the frontal accretionary prism, scattered slope basins, and through-going submarine canyons (Barnes

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Section: Resultsmentioning

confidence: 99%

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

Underwood

Dugan

Cardona

2022

Geophysical Research Letters

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“…Active geothermal systems are widespread; smectite and mixed‐layer clays are ubiquitous products of hydrothermal alteration (Tillick et al ., 2001; Yan et al ., 2001; Libbey et al ., 2013; Simpson & Rae, 2018; Heap et al ., 2020). Smectite, I/S mixed‐layer clays and illite are zoned by temperature in some of the TVZ hydrothermal systems, with smectite as the dominant phase at temperatures <130°C (Simpson et al ., 2019). Heavily altered tephras are also common in Pleistocene outcrops that fringe the Bay of Plenty (McGlone et al ., 1984).…”

Section: Discussionmentioning

confidence: 99%

Composition of fine‐grained sediment in the Hikurangi Trough: Evidence for intermingling among axial gravity flows, transverse gravity flows and margin‐parallel ocean currents

Underwood

2023

Sedimentology

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The International Ocean Discovery Program cored five sites during Expeditions 372 and 375 to investigate the Hikurangi margin, offshore Poverty Bay, North Island, New Zealand. Trench‐wedge deposits were recovered at Site U1520, and the frontal accretionary prism was cored at Site U1518. Based on X‐ray diffraction analyses of the clay‐sized fraction, illite and smectite are the dominant clay minerals. Compositional scatter is unusually large, and a well‐defined mixing trend is evident between illite‐rich and smectite‐rich end members. The proportion of smectite ranges from 9 to 59 wt.% at Site U1518 and from 12 to 62 wt.% at Site U1520. Nearby slope deposits at Sites U1517 and U1519 are more homogeneous, with consistently higher proportions of smectite. The East Cape Current is probably responsible for transporting smectite‐rich suspensions towards the south‐west from sources that include the Taupo Volcanic Zone. Illite‐rich muds require different sources and different routing paths to the trench. Some suspended sediment probably originated in the Waipaoa watershed on the North Island, entering the trench via Māhia Canyon. Other gravity flows probably started in Kaikōura Canyon or Cook Strait and moved towards the north‐east, down the trench's axial channel. The influence of turbidity currents on clay composition changes with water depth, as do the effects of reworking by contour currents and the role of passive hemipelagic settling. The broad continuum of clay compositions in the trench is enhanced by variable amounts of mixing among multiple ocean currents and fluctuating durations of suspension within a bottom nepheloid layer. That complexity deviates from the paradigm of turbidite‐dominated trench wedges. The margin's compositional diversity also complicates the goal of predicting or modelling three‐dimensional variations in frictional, geotechnical and hydrogeological properties. Those lithological properties are important because they affect slope stability along strike and changes in fault‐slip behaviour along the subduction interface.

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“…Table 2.1. Common elements and, the minerals they are commonly found in, for both primary mineralogy in TVZ volcanic rocks and geothermal alteration minerals from experience in geothermal fields and epithermal deposits (Mauk and Simpson, 2007;Mauriohooho et al, 2016;Simpson et al, 2019;Simpson and Mauk, 2000). Elements in brackets are often, but not always, substitutes for the element in that row.…”

Section: Combining Rock Geochemistry and Mineralogy To Interpret Velo...mentioning

confidence: 99%

“…XRD and automated mineralogy techniques have different strengths and weaknesses and hence the two methods can often disagree on mineral quantities (Simpson et al, 2019). Automated mineralogy can detect minerals at much smaller abundances, with detection limits better than 0.1% whereas XRD generally only detects minerals that are >1 % abundance (Simpson et al, 2019). Automated mineralogy on the other hand can under-report some minerals due to finegrained minerals in the groundmass that are smaller than the pixel size (e.g.…”

Section: Quantitative Xrdmentioning

confidence: 99%

“…Automated mineralogy on the other hand can under-report some minerals due to finegrained minerals in the groundmass that are smaller than the pixel size (e.g. quartz, chlorite, albite, etc) (Simpson et al, 2019). In addition to the differences in the methods, there is also potential variation due to differences in the sampling (although the samples come from the same depth in the well, they are not the exact same sample).…”

Section: Quantitative Xrdmentioning

confidence: 99%

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Geologic controls on seismic velocity from well logs and seismic tomography at the Rotokawa and Ngatamariki Geothermal Fields, New Zealand

Sewell¹

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In this thesis, the factors controlling seismic velocity at the log scale (less than a metre) were explored and this knowledge used to interpret seismic velocity images at the field scale (greater than 200 m) obtained via seismic tomography at two geothermal fields in the Taupō Volcanic Zone in New Zealand; the Ngatamariki and Rotokawa fields. Seismic velocity imaging has the potential to provide valuable insights into geothermal reservoirs that can be used for well targeting, resource management and conceptual and numerical models. However, the potential factors influencing seismic velocity in geothermal reservoirs are many. In this thesis, the factors controlling seismic velocity were explored by pairing geophysical logging data, which included P-wave velocity, S-wave velocity (for two wells), neutron porosity, density, gamma, resistivity and formation image logs with newly acquired geochemical (via portable XRF) and mineralogical (via quantitative XRD and automated mineral scanning) data from drill cuttings in three deep wells in the Ngatamariki field. The combined analysis of these datasets demonstrates that the largest factor controlling seismic velocity in the two fields is porosity, which in turn can be influenced by both primary lithology, alteration and ductile deformation above the magmatic intrusions that provide the heat in high-temperature, volcanic, geothermal systems. The results also show that the physical and chemical properties of the tuff-dominated Tahorakuri Formation in the north of the Ngatamariki field were dramatically altered by high-temperature (>375°C) alteration and ductile-deformation processes during an intrusion event that occurred approximately 65 kya. Wide-spread quartz deposition in the Tahorakuri Formation in the north of the field due to the intrusion event appears to have decreased porosity and increased velocity relative to the same formation in the south of the field (quartz abundance of 58% in the north cf. 38% in the south, Vp of 4.34 km/s in the north cf. 3.78 km/s in the south). Ductile deformation that occurred when temperatures in the past were above the brittle-ductile transition (>375°C) has resulted in closing of pore space and a zone of very low porosity (mean of 4%) and consequent high seismic velocity (mean of 4.89 km/s) that extends approximately 400 m above the intrusion in NM9. The knowledge gained from this work provided the framework for interpreting the results of the seismic tomography. To improve the resolution and robustness of the seismic tomography, 30 seismometers were deployed across the Ngatamariki and Rotokawa fields, to complement the existing seismic network of 22 seismometers, for approximately one year. From this deployment, a sub-set of 351 of high-quality earthquakes were used to perform the tomography analysis. A 1D Monte Carlo VELEST analysis was performed using this dataset by randomly generating 1000 different 1D starting models with velocity distributions based on measured velocity data and knowledge of rock types across the two fields. This yielded an average 1D model for the two fields which was then used as the basis for 3D inversion work. VELEST also inverts for station correction terms that can be used to provide an indication of the spatial variation in velocity for Vp and Vs with negative station corrections indicating faster velocity and positive station corrections indicating slower velocity. Three areas of similar station correction terms were identified; an area of negative Vp and Vs station corrections in the northwest of Ngatamariki, an area of positive Vs station corrections to the east of Ngatamariki and positive Vp and Vs station correction terms at Rotokawa. The spatial pattern of the station correction terms agrees well with the spatial patterns of velocity observed in the 3D inversions. 3D inversion of the same earthquake dataset was performed using the program tomoDD. Inversions were performed using three different inversion grids and two different starting velocity models. Model solution robustness and uncertainty were assessed using both ray-path coverage measures and synthetic testing. In all of the inversions performed, a west to east, high (Vp up to 4.8 km/s, Vs up to 2.7 km/s) to low velocity (Vp up to 3.8 km/s, Vs up to 2.2 km/s) transition was observed in the north of the Ngatamariki field for both Vp and Vs. High velocity in the northwest of Ngatamariki was observed over the depth range of 1-3 km bsl (below sea level), corresponding mostly to the Tahorakuri Formation tuffs and volcaniclastics above the intrusion in the north of the field. As was demonstrated with the logging, geochemical and mineralogical datasets, high velocity in this area is likely associated with reduced porosity due to alteration (particularly quartz deposition) and ductile-deformation that occurred during an intrusion event. The area of high velocity in the northwest of Ngatamariki aligns well with high gravity and high resistivity at 1.5 km bsl from a 3D inversion of magnetotelluric data. The transition between high to low velocity, gravity and resistivity all occur approximately across the NM9 well in the north of the field, suggesting that drilling in the northeast of the Ngatamariki field is unlikely to encounter the low permeability that was observed in the NM4 and NM8 wells. The findings of this thesis highlight that alteration and deformation above magmatic intrusions in geothermal fields can cause pronounced changes in the physical, chemical and mineralogical properties of the rock and that these changes may be imaged by seismic tomography methods. This has potential implications for well targeting in geothermal fields and wider implications for seismic imaging of magmatic systems.

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Hydrothermal Alteration at the Karangahake Epithermal Au-Ag Deposit, Hauraki Goldfield, New Zealand

Cited by 13 publications

References 50 publications

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

Composition of fine‐grained sediment in the Hikurangi Trough: Evidence for intermingling among axial gravity flows, transverse gravity flows and margin‐parallel ocean currents

Geologic controls on seismic velocity from well logs and seismic tomography at the Rotokawa and Ngatamariki Geothermal Fields, New Zealand

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