Exploration for epithermal Au–Ag deposits in New Zealand: history and strategy

ABSTRACT

More than 80 Cenozoic low sulphidation Au–Ag epithermal deposits and prospects are known in Northland, the Hauraki Goldfield (Coromandel Volcanic Zone) and the Taupō Volcanic Zone of the North Island. Reconnaissance exploration since the 1970s has included magnetic surveys, and stream sediment and rock-chip geochemical surveys. Prospect-scale exploration over the same period has included rock and soil geochemical surveys, magnetic and resistivity/induced polarisation geophysical surveys and drilling, including the study of hydrothermal alteration mineralogy (by thin section and XRD) plus fluid inclusion systematics, to help reconstruct the former hydrothermal system and identify zoning related to mineralisation. Since the 1990s, digital regional exploration and research data sets have enabled GIS-based targeting of prospective areas. At the prospect scale, multi-element ICP and ICP-MS analyses, hyperspectral short wave infrared spectral analyses and portable XRF geochemical analyses of field samples and drill core have been used to complement assay data for identifying spatial trends in geochemistry and mineralogy that help vector to mineral occurrences. Exploration since the 1970s has discovered new Au and Ag resources in extensions of some previously mined deposits (e.g. Golden Cross and Karangahake) and in new deposits in adjacent areas (e.g. Favona, Trio and Correnso in the Waihi area).

KEYWORDS:

• Epithermal
• gold
• silver
• exploration
• geochemistry
• geophysics
• Northland
• Coromandel Volcanic Zone
• Hauraki Goldfield
• Taupō Volcanic Zone

Introduction

New Zealand has a history of significant Au and Ag production from epithermal deposits since the 1860s, and epithermal deposits continue to be one of the main mineral deposit types sought by the exploration industry on the North Island. The deposits are associated with Cenozoic volcanic rocks in Northland (Figure 1), the Coromandel Volcanic Zone (CVZ; Figure 2) and the Taupō Volcanic Zone (TVZ; Figure 3). The distribution of Au–Ag deposits in the CVZ defines the Hauraki Goldfield that encompasses Great Barrier Island and the Coromandel Peninsula, extending south to Te Puke (Figure 4). Exploration for Au–Ag deposits and prospects has been aided by insights gained from nearby active geothermal systems of Northland (Ngawha; Figure 1) and the TVZ (e.g. Waiotapu, Waimangu, Ohaaki and Rotokawa; Figure 3) (Henley and Ellis 1983; Henley et al. 1986), and of volcanic hydrothermal systems related to andesitic volcanoes (e.g. White Island; Hedenquist et al. 1993). As these active hydrothermal systems are readily accessible modern-day analogs to past hydrothermal systems, explorers can directly observe processes that occurred in formation of epithermal deposits, the surface features that would have been present during epithermal deposit formation and appreciate the potential size of the hydrothermal systems.

Figure 1. Geology and location of epithermal Au–Ag prospects in Northland. See Table 4 for a list of the main occurrences.

Figure 2. Geology and location of epithermal Au–Ag deposits and prospects in the Coromandel Volcanic Zone – Hauraki Goldfield. See Figure 4 and Table 3 for a list of the main occurrences.

Figure 3. Geology and location of epithermal Au–Ag prospects and geothermal fields in the Taupō Volcanic Zone. See Table 5 for a list of the main epithermal Au–Ag occurrences.

Figure 4. Relative size of past Au and Ag producers, Au:Ag ratios and dominant vein orientation of epithermal Au–Ag deposits of the Hauraki Goldfield (modified after Fig. 27 of Brathwaite and Pirajno 1993; Fig. 2 of Chrisite et al. 2006). The direction of the bar symbol shows the dominant strike of veins. Deposit reference codes are from Brathwaite and Pirajno (1993). Inset map shows broad geographic groupings of deposits into northern, southern and eastern provinces based on mineralisation style described by Chrisite et al. (2006, 2007).

Most mineral exploration begins with developing a target concept and undertaking an area selection process based on an assessment of mineral potential (Figure 5), as well as factors potentially relevant to development if discovery is successful. Exploration progresses in stages, with each stage ending in a decision point on whether to continue to more intensive investigations or abandon part or all of the area, progressively focussing on the most prospective ground. In areas where there has been significant previous exploration, specific target areas may already be apparent (e.g. known epithermal deposits), and the programme may commence with detailed exploration. This has been the case for most epithermal Au–Ag exploration in New Zealand since 2010. However, development of new concepts or techniques may justify reassessing regional selection and earlier unsuccessful targets.

Figure 5. Path to mineral production. Prospecting is reconnaissance exploration and Exploration is detailed exploration in the text.

In New Zealand, Au and Ag are Crown-owned minerals and therefore gold exploration requires a permit from the New Zealand Government, regardless of land ownership. There are three main types of onshore minerals permits that are granted by New Zealand Petroleum & Minerals (NZP&M – www.nzpam.govt.nz): prospecting, exploration and mining, and these differ in land area, duration and the type of permitted activities (Table 1). The permit heirachy and structure prescribe the framework for exploration programmes.

Table 1. Mineral permits (onshore) under the Crown Minerals Act 1991 and Minerals Programme for Minerals 2013.

Permit type	Area	Duration	Activities
Prospecting (Usually Tier 1)*	Up to 500 km^2	2 years + potential for 2-year extension generally over half of area	Literature searches and data compilation, and low impact activities such as geological mapping, hand sampling (e.g. stream sediment, soil and rock), and ground and airborne geophysical surveys
Exploration (Tiers 1 and 2)*	Minimum 150 ha	5 years + potential for up to 5-year extension generally over half of area	Minimum impact activities such as geological mapping and sampling plus more invasive activities such as drilling and bulk sampling
Mining (Tiers 1 and 2)*	Tier 1 – depends on extent of discovery and work programme Tier 2 – generally no larger than 200 ha	Variable depending on mining plan and size of deposit	Mining and related activities

*Permits are separated into two tiers to distinguish higher-value or higher-risk permits from those for smaller scale operations.

Tier 1: Complex, higher risk and return mineral operations, based on expenditure or production thresholds set out in Schedule 5 of the Crown Minerals Act. Tier 1 permits are subject to closer assessment, monitoring and management than Tier 2 permits.

Tier 2: Lower risk and return industrial, small business, and hobby mineral operations. Tier 2 permits are managed under less rigorous procedures.

Prospecting, exploration and mining permits for epithermal Au–Ag deposits are Tier 1.

This paper reviews exploration for epithermal Au and Ag deposits in New Zealand in terms of the programme structure and exploration techniques employed, and discusses some research that has assisted exploration. The paper concludes with a strategy for exploration for epithermal Au–Ag deposits in New Zealand, as well as a list of recommended future research topics.

Geological setting

Northland, CVZ and TVZ each contain a subduction-related Cenozoic volcanic arc with volcanic rocks overlying a basement of Mesozoic greywacke. Northland also includes a complex Cretaceous and Tertiary succession of autochthonous sedimentary rocks and allochthonous marine sedimentary and basaltic volcanic rocks (Ballance and Spörli 1979; Edbrooke and Brook 2009). In the CVZ, mid-Tertiary marine sedimentary rocks are locally present in the northern Coromandel Peninsula.

Cenozoic volcanism in Northland formed two semi-parallel NW-oriented volcanic belts of Miocene age: a western belt of basaltic volcanism and an eastern belt of andesitic and rhyolitic volcanism; later (Late Miocene-Quatenary) volcanism consisted of intraplate basalt and minor rhyolite eruptions. The epithermal Au–Ag deposits and present-day geothermal systems (e.g. Ngawha) are associated with the eastern belt and have produced small quantities of Ag and Hg (Williams 1974). In addition, three porphyry Cu occurrences and one Pb-Zn skarn deposit (Brathwaite and Pirajno 1993) are exposed on the eastern flank of the eastern belt, where erosion is deepest.

The CVZ volcanic rocks consist mainly of ca. 18 to 10 Ma andesite and dacite in the northern CVZ, and ca. 12 to 2 Ma rhyolite, andesite, dacite and minor basalt in the eastern and southern areas (Skinner 1986; Edbrooke 2001). Andesitic volcanism was mainly related to stratovolcanoes, whereas rhyolitic volcanism was mainly associated with caldera formation and dome extrusion.

The CVZ hosts c. 50 epithermal Au–Ag deposits that have collectively produced c. 12 Moz of Au and 50 Moz of Ag since the 1860s (Figure 4; Brathwaite et al. 1989; Christie et al. 2007). The current annual production of c. 125,000 oz Au and 350,000 oz Ag is almost entirely from mines at Waihi (I76 and I77 area in Figure 4), with additional production from workings at Karangahake and Broken Hills (I75 and I56, respectively, in Figure 4). The porphyry Cu occurrences in the CVZ (I20, I24, I43, I50 and I52 in Figure 4) are best exemplified by that at Ohio Creek near Thames (Merchant 1986; Brathwaite et al. 2001b). Radiometric dating of the epithermal and porphyry Cu deposits indicate that they range in age from ca. 14 to 2 Ma and 16 to 10 Ma, respectively. However, most of the epithermal deposits in the southern CVZ, which account for more than 80% of the total Au and Ag production, were formed between ca. 7 and 6 Ma, during a period of strong regional extension (Mauk et al. 2011).

The TVZ is a NE-trending extensional volcanic-tectonic basin with a basement of Mesozoic greywacke and filled with Pleistocene (ca. 2 Ma) to recent volcanic and volcaniclastic rocks (Wilson et al. 1995; Wilson and Rowland 2016). It is flanked to the west and east by ignimbrite plateaus originating from volcanoes in the TVZ. The TVZ predominantly consists of rhyolite calderas and domes, and ignimbrite, with lesser andesite and dacite. Andesite forms a complex of stratovolcanoes at the southern end of the TVZ (Tongariro, Ngaruhoe and Ruapehu) and a largely submerged stratovolcano offshore at the northern end (White Island). Basalt is a minor constituent. The TVZ has c. 20 known geothermal systems (Rowland and Simmons 2012), some of which have been developed to produce electricity, and c. 20 epithermal Au–Ag prospects, but there has been no Au or Ag production.

Target type

The known epithermal Au–Ag deposits in New Zealand are all volcanic associated, with features characteristic of low (locally to intermediate) sulphidation types, in which the Au and Ag are mainly contained in quartz veins, with little disseminated Au and Ag in the wallrocks except for that in some rhyolite-hosted deposits such as Wharekirauponga (Christie et al. 2003, 2016). There is potential for high sulphidation epithermal Au–Ag deposits associated with porphyry Cu mineralisation at Ohio Creek near Thames, but this remains untested (Corbett Geological Services 1991; Brathwaite et al. 2001b; Christie et al. 2008). In addition to deposit characteristics described in epithermal deposit models (e.g. Brathwaite and Christie 2000; Christie and Brathwaite 2003a; Christie 2019), other criteria for target selection include: (1) whether the exploration will be brownfields, i.e. near past or present mining locations, or greenfields, in areas with no known deposits, or both; (2) the minimum total gold content sought; and (3) the minimum average gold grade. The latter affects the potential mining method, typically open pit bulk mining for low-grade ore, or the option of underground mining for high grade ore, particularly in the case of ore deposits below about 100 m depth. Table 2 shows the evolution of target type in the CVZ from the 1960s to present day. Current community opinion on the North Island is unlikely to accept the development of any new open pit mining of Au and Ag, resulting in a change in the most common target type, from the 1980s exploration for stockwork or sheeted veins that are amenable to open pit bulk mining, to exploration during the past two decades for high-grade Au-bearing quartz veins that can be developed by underground mining.

Table 2. Evolving targets, model concepts and exploration techniques.

	Target	New model concepts	New exploration techniques
			Area selection	Reconnaissance exploration	Detailed exploration
1960s	Porphyry Cu ± Mo ± Au		(Minimal information available from previous exploration)	Stream sediment and rock geochemistry	Stream sediment and rock geochemistry
1970s	Porphyry Cu ± Mo ± Au, Epithermal Au–Ag	Empirical descriptive epithermal model, Partial adoption of the geothermal model in late 1970s	Satellite imagery Manual compilation and visualisation of information (New Government regulations requiring submission of exploration reports increased information available on previous exploration)	Aeromagnetic surveys Satellite imagery	Bulk sampling at Martha Petrographic analysis of hydrothermal alteration
1980s	Epithermal Au–Ag, Mineable by open pit, Exploration under mineralised rhyolite (e.g. Broken Hills and Neavesville), Exploration under post-mineral cover rocks at Golden Cross	Geothermal and empirical models from 1980 – hydrologic model: Hydrothermal alteration, Vein textures (multiple events and bladed minerals), Geochemical vectors (e.g. Au, Ag, As, Cu, Mo, Pb, Sb, Zn), Mineralogical vectors (e.g. pyrite, chlorite, carbonate (acid test), kaolinite, smectite, illite)			Ground magnetic, IP/resistivity and CSAMT surveys, Airborne magnetic and radiometric surveys, Routine photography of drill core in trays, XRD analyses, Fluid inclusion Th & Tm
1990s	Epithermal Au–Ag, Open pit & underground mining targets, Exploration under cover at Waihi	Paleodepth indicators, Post mineralisation tilting	GIS compilation (Exploration reports scanned and made available online by Government)	GIS compilation	Volcanic facies logging of drill core, 3D GIS compilation with mining software
2000s	Epithermal Au–Ag, Beginning of focus on high grade, underground mine	Age control and host rock associations in the Hauraki Goldfield (3 sub-regions)	GIS prospectivity analyses (Digital data available including QMaps of Northland and CVZ) Use of ‘worming’ to model deep structure	GIS prospectivity analyses, Airborne gravity (TVZ), ground-based gravity (CVZ), airborne EM (CVZ), Use of ‘worming’ to model deep structure	GIS compilation, 3D GIS prospectivity analyses, Airborne EM, ground-based gravity surveys Oriented drill core at Waihi enabling better structural modelling Drill core hyperspectral SWIR–PIMA/Terraspec analyses
2010s	Epithermal Au–Ag, high grade, underground mine	Detailed geochemical vectors Detailed mineralogical vectors Mineral systems models	(QMap of TVZ available)	Multi-element assays SWIR – Terraspec, pXRF geochemistry	Drill core multi-element assays, hyperspectral SWIR – Terraspec analyses and pXRF geochemistry (Research alteration mineral quantification)

CSAMT = Controlled Source Audio-frequency Magnetotelluric.

T_h = fluid inclusion homogenisation temperature.

T_m = fluid inclusion final ice melting temperature (and resulting apparent salinity estimate).

Area selection

Regional mineral endowment

A fundamental consideration in selecting a region for exploration is whether the proposed area has potential to contain resources of sufficient size for the commodity or commodities sought. For greenfields exploration, this is the presence of a favourable tectonic and geological history for forming gold deposits. For brownfields exploration, it includes evidence from previous exploration, mining and related research that there is still potential in the area for a significant undiscovered economic resource (termed residual endowment).

The CVZ has a long history of mining of epithermal Au–Ag deposits (Brathwaite et al. 1989; Christie et al. 2007), and current Au and Ag production is from mines at Waihi, Karangahake and Broken Hills. Epithermal deposits in the CVZ have also been mined for Hg at Mangakirikiri and Mackaytown (Ascot mine), and for Zn, Pb and Cu at Tui (Christie et al. 2008).

The main epithermal Au–Ag prospects in the CVZ (Table 3), where several discoveries have recently been made, include extensions of known deposits (e.g. Empire vein, Golden Cross) as well as discoveries near known veins (e.g. Favona, Trio and Correnso veins in Waihi; EG vein at Wharekirauponga), testifying to the exploration potential of the CVZ (Barker and Christie 2016a). In addition, there are large areas with post-mineral volcanic rock cover that have potential for concealed deposits beneath the cover (Christie and Brathwaite 2005). Brathwaite (1981) graphed the past production of known deposits in the CVZ prior to the recent discoveries, and using Zipf’s law, suggested that there was potential to discover deposits of significant size, including four deposits with sizes intermediate between the historic (pre-1953) production of Waihi and Thames.

Table 3. Main epithermal Au–Ag exploration prospects in the Coromandel Volcanic Zone (Hauraki Goldfield) from 1978 (modified from Table 1 of Barker and Christie 2016a).

Location	Prospect	Company	Drilling^a	Date	MR^b	Comments
Kennedy Bay	Whareroa	Amoco Minerals	4 DDH	1981	413
Coromandel Range, east of Coromandel town	Taumatawahine, Tokatea	Adams Developments	3 DDH	1978	359
	Lillis Mine	Union Gold	6 RC	1990	2911
Coromandel	Kapanga	Heritage Gold	5 DDH	1989	2681
Whitianga	Kuaotunu	Barrack Mines	36 RC	1989	2493
	Murphys Hill	ACM NZ	1 DDH	1989	2643
	Opitonui	ACM NZ	1 DDH	1989	2529
Tairua Valley	Broken Hills	ACM NZ	23 RC, 4 DDH	1989–1990	2463, 2502, 2663, 2904
		Broken Hills Gold				Developed as a small scale underground historical demonstration mine in 2000.
	Golden Hills	Heritage Mining/Highlands Gold	5 DDH	1995	3541, 3550,
	Coronation Mine	Mt Martin Gold	3 DDH	1990	3032
Tairua Valley, east of Neavesville	Chelmsford	ACM NZ	5 DDH	1992	3180
Thames	Golden Gem	Heritage Mining	5 DDH	1987	550
	Ohio Creek	Amoco Minerals	4 DDH	1978–1981	353, 383, 403	Porphyry Cu-Au-Mo prospect.
	Lookout Rocks	Austpac Gold	1 DDH	1988	577, 3009
	Punga Reef	Austpac Gold	1 DDH	1988	577, 3009
	Horseshoe-Jupiter	Austpac Gold	8 RC	1988	588
	Waitangi	Gold Resources	11 DDH	1987–1991	533, 3010 3057
Southeast of Thames	Otamakite	ACM NZ	2 DDH	1990	2943
Whangamata	Ohui	Austpac Gold	12 DDH	1986–1988	574, 591
	Onemana	Heritage Gold	28 RC	1993–1995	3356, 3357, 3408
Coromandel Range, southeast of Thames	Neavesville (Trig Bluffs, Ajax Mine)	Amoco Minerals, Homestake NZ, Welcome Gold Mines, ACM NZ	61 DDH	1979–1998	363, 376, 445, 3585, 3609, 3800	Resource estimate by ACM NZ
	Ridge 5, Neavesville	ACM NZ	2 DDH	1992	3138
North of Paeroa	Maratoto	Waihi Mining and Development, Cyprus Gold, Delta Gold	33 RAB, 8 DDH	1989–2000	270, 2617, 3809
Maratoto Valley	Camoola South	Cyprus Gold	2 DDH	1989	2617
Southwest of Whangamata	RNF	Amoco Minerals	4 DDH	1985	489
West of Waitekauri Valley	Komata	Coeur Gold, Newmont/Glass Earth Gold	13 DDH	2001, 2008	3874 4481
Waitekauri Valley	Empire South	Cyprus Gold, Coeur Gold	16 DDH/RC	1991–1995	3055, 3354
	Golden Cross	Amoco Minerals, Cyprus Minerals, Coeur Gold	70 DDH, RC	1981–1999	480, 487, 509, 542, 3714	Open pit and underground mine developed in 1991; ceased production in 1998
	Jubilee Mine	Cyprus Gold, Coeur Gold	7 DDH	1987–1992	3043, 3177, 3515
	Scotia	Cyprus Gold, Coeur Gold	32 DDH	1989–1992	2910, 3874, 3515
	Sovereign	Cyprus Gold, Coeur Gold	27 DDH	1989	2910, 3515
	Waitete	Coeur Gold, Heritage Gold	3 DDH, 3 RAB	1995, 2008	3353, 4374
	Jasper Creek	Cyprus Gold	20 DDH	1990–1991	3071
Karangahake	Mt Karangahake	Freeport Aust. Minerals, Cyprus Gold, ACM NZ	41 DDH 4 RC	1985–1991	517, 2264, 3030, 3031, 3044, 3060,
		Heritage, New Talisman Gold				Underground development of a small scale mine
	Dominion Knoll	Heritage Gold	2 RC	2008	4484
	Rahu	Cyprus Minerals, Heritage Gold	14 DDH, 8 RC	1982–2007	482, 4104, 4282
	Owharoa	Amoco Minerals	7 Rotary	1985	482
Northeast of Waihi	Waiharakeke	Cyprus Minerals	3 DDH	1986	526, 3874
	Wharekirauponga (WKP)	Amoco Minerals, BHP Minerals, ACM Glass Earth Gold, Newmont Waihi Gold, OceanaGold	67 DDH	1982–1985, 2007–2013, 2016-present	415, 496, 3021, 3179, 4481, 4742	Reports on drilling of holes 30–67 not yet available on open file (Streiff et al. 2019). Drilling is ongoing in 2019.
	Monument	BP/Amoco, Amoco/Newmont Newmont Waihi Gold	2 DDH 9 DDH/RC	1983–2012	448, 2257, 4167, 4243, 4542, 4860
Waihi	Correnso	Newmont Waihi Gold, OceanaGold				Discovered in 2009 and is currently being mined
	Favona-Moonlight	Amax Exploration, Waihi Gold, Auag Resources, OceanaGold	86 DDH, 6 RC	1989–2003	2355, 2504, 3894, 3969	Moonlight discovered in 1997 and Favona in 2000, and developed as an underground mine in 2006; ceased production in 2013.
	Gladstone Hill	Union Hill JV, Otter Gold Mines	20 DDH	1997–1999	3439, 3549, 3741, 3742
	Heath Road, Winner Hill	Waihi Gold Mining Co SDC Rabone	6 Rotary	1999–2000	3099, 3748
	Martha	Amax Exploration, Waihi Mine Joint ventures	Extensive	1980–1993		Developed as Martha open pit
	Martha East/Grand Junction	Amoco Minerals, Cyprus Minerals Coeur Gold	49 DDH	1984–1996	472, 511, 513, 516, 530, 559, 589, 2367, 2368, 2519, 2528, 2799, 3581
	Ohinemuri	Waihi Gold Mining Co	13 Rotary DDH	1985	500
	Taieri	Cyprus Minerals	5 Rotary	1987	538
	Union Hill	Auag Resources, Mineral Resources NZ, United Gold Mines,	45 DDH	1985–2001	481, 501, 514, 561, 571, 3854,	Trio vein discovered in 2003 and mined from 2012
	Waihi East	Waihi Mine Joint Venture	10 DDH	2002	3968
	Waihi North	Heritage Gold	4 DDH	2008	4368
	Waihi Northeast	BP Oil	2 DDH	1985	497, 529
	Waihi West	Glass Earth Gold	9 DDH	2006, 2007	4301
	Waione	Cyprus Minerals	2 DDH	1984	515
Te Aroha	Tui	HPD NZ Ltd	6 DDH	2004	4064
	Waiorongomai	Tasman Gold, HPDNZ	6 DDH	1988, 2004	595, 4064
Te Puke	Muirs Reef	BP Oil, Mineral Resources, HPD NZ, Glass Earth NZ	77 RC, DDH, Rotary	1986-2009,	667, 685, 3374, 4088, 4270, 4781, 4532, 5180	Resource estimate by Glass Earth NZ

^aDrilling abbreviations: DDH = diamond drill hole core drilling; RC = reverse circulation and aircore; RAB = rotary air blast using compressed air; Rotary = wash drilling using water. Most diamond drill holes have near-surface intervals of up to 50 m that have been drilled using non-cored methods.

^bNew Zealand Petroleum & Minerals exploration report database (www.nzpam.govt.nz) mineral report (MR) reference numbers.

Although Northland and the TVZ do not have a history of Au and Ag production, each province has significant exploration targets (Tables 4 and 5; Barker and Christie 2016b, 2016c), with exploration at Puhipuhi in Northland and Ohakuri in the TVZ indicating significant Au and Ag resources in these deposits (Grieve 2000; Hamilton and Soengkono 2009; Beach and Hobbins 2016; Hobbins et al. 2018). Regional mineral resource assessments by Christie et al. (2008) for the CVZ and by Christie and Barker (2007) for Northland found that there was potential for significant undiscovered Au and Ag resources in epithermal deposits in these areas.

Table 4. Main epithermal Au–Ag exploration prospects in Northland from 1970 (modified from Table 1 of Barker and Christie 2016c).

Prospect	Company	Drilling^a	Date	MR^b	Comments
Huia	Aurora Minerals	4 DDH (489 m) 21 RC (2102 m)	2006	4190	Trace Au only
Huia Trig	Aurora Minerals	106 RAB (1014 m)	2006	4242, 4219	Trace Au only
Te Pene	GEONZ	2 DDH (130 m)	1990	2954	Peak values (ppm) in DDH1; 0.55 Au, 1.7 Ag, 1600 As, 2660 Sb
Whakarara	Aurora Minerals	320 RAB + auger (1297 m)	2006	4218
Puketotara (Pyrite Creek)	Kiwi Gold	6 DDH (168 m)	1987	113, 114	Followed up NZCC Pt anomaly. Best result for Au 0.03 g/t over 1.5m in DDH2. No Pt or Pd detected
Puketotara	BP Oil	1 DDH (172 m)	1984	106, 65	No Au or Ag detected at 0.02 g/t Au and 0.5 g/t Ag detection limits
	NZ China Clays	8 Rotary/ DDH (424 m)	1971	48	Pyritic clay and unaltered basalt. See MR60 for analyses
	NZ China Clays	20 DDH (762 m)	1971	2251	Pyritic clay locally, see MR60 for analyses
	NZ China Clays	2 DDH (471 m)	1971	60	Includes analyses for all samples to date including Pt to 16.8 g/t in hole AA1
	NZ China Clays	20 Rotary (1500 m)	1971	49	Logs only. No test results
	NZ China Clays	40 DDH (2520 m)	1971	64, 63	Au + Ag + PGM values to 0.85 g/t
	Kaiser Mining and Development	14 DDH (1127 m)	1971	59	Follow up of Pt discovery. No Pt detected with traces of Au and Ag
Puhipuhi	Evolution Mining	8 DDH (3803 m)	2013	­-	Best intersection 0.6 m at 23 g/t Au and 146 g/t Ag from 31.8 m in FDH06. (Hobbins et al. 2018).
	Golden Fern Resources	10 RC (1026 m)	2008	4436	Four targets tested. Weak, pervasive Au/Ag mineralisation in most holes. Peak gold value 0.8 g/t in PRC37
	BHP	1 DDH (619 m)	1989	2478	Au to 0.2 g/t only
	BHP	24 RC (3128 m)	1988	120	Seven targets tested. Best result was 2 m at 12.4 g/t Au and 86 g/t Ag at Plumduff Breccia
	BHP	11 RC (555 m)	1987	188	Four targets tested. Best result 10 m at 5.3 g/t Au and 18.5 g/t Ag from Plumduff Breccia
	Homestake NZ Exploration	3 DDH (463 m)	1985	2413, 185	Peak gold value 0.99 g/t Au over 2 m at Mt Mitchell sinter

^aDrilling abbreviations: Auger = power and some hand auger holes; DDH = diamond drill hole core; RC = reverse circulation and aircore; RAB = rotary air blast using compressed air; Rotary = wash drilling using water. Most diamond drill holes have near-surface intervals of up to 50 m that have been drilled using non-cored methods.

(123 m) Number in brackets indicates total depth drilled in metres.

^bNew Zealand Petroleum & Minerals exploration report database (www.nzpam.govt.nz) mineral report (MR) reference numbers.

Table 5. Main epithermal Au–Ag exploration prospects in the Taupō Volcanic Zone from 1978 (modified from Table 1 of Barker and Christie 2016b).

Prospect	Company	Drilling^a (metres drilled)	MR^b	Gold drill results
Matahana	Cyprus Minerals, Amax Resources 1987–1990	4 RC (791 m) 5 RC (751m)	655, 2905, 2906	RCM 1–4 No mineralisation detected 4 m at 0.24 g/t Au from 122 m in RCM5
Horohoro	Glass Earth Gold 2009	1 Rotary (261 m)	4487	6 m at 0.06 g/t Au from 192 m
Thomsons	Glass Earth Gold 2010	1 DDH (271 m)	4697	No Au mineralisation detected
	BP Oil 1988	1 DDH (353 m)	671, 2406	No Au mineralisation detected
Tahunaatara	Glass Earth Gold 2009	4 DDH (1360 m)	4504	1 m at 2.93 g/t Au from 194 m in TNTDDH05 1 m at 2.25 g/t Au from 224 m in TNTDDH03
Tikorangi	BP Oil 1988	1 DDH (275 m)	678	No Au mineralisation detected
Ohakuri North	Glass Earth Gold 2009	2 DDH (802 m)	4449, 4469	1 m at 1.04 g/t Au from 69 m in hole 1 Resource evaluation of all data to date
	Delta Gold 2000	7 RC/DDH (2286 m)	3802	4 m at 2.79 g/t Au from 62 m in OHDG3 4 m at 2.7 g/t Au from 167 m in OHGD3
	Coeur Gold 1997	1 DDH (520 m)	3519	1 m at 0.68 g/t Au from 145 m in OH34
	Cyprus Gold 1991	19 DDH / wash drilling (4420 m) 13 RC (1532 m)	2482, 3053	5.5 m at 4.05 g/t Au from 168 m in OH8 10 m at 2.0 g/t Au from 65 m in OH19 4 m at 1.66 g/t Au from 106 m in OH32
Ohakuri East	BP Oil 1988	6 DDH (1079 m)	681	1.2 m at 7.9 g/t Au and 180 g/t Ag from 116 m in OH4 1.7 m at 1.6 g/t Au from 124.5 m in OH2 14.1 m at 1.2 g/t Au from 130.7 m in OH6
Ohakuri West	Elders Resources 1990	5 RC (423 m)	2937	10 m at 0.07-0.2 Au g/t from 40 m in Hole 2.
Wharepapa	Cyprus Gold 1989	3 RC (303 m) 1 RC + DDH (137 m) 2 DDH (640 m)	687, 699	4 m at 1.2 g/t Au from 54 m in RC4
Pukemoremore	Glass Earth Gold 2009	2 DDH (571 m)	4524	1 m at 0.4 g/t Au from 138 m in PUKDDH02 10 m at 0.25 g/t Au from 137 m in PUKDDH02 Prospect was explored by Cyprus Gold (MR 668, 1988) but no drilling was undertaken.
Umukuri	BP Oil 1988	3 DDH (552 m)	674, 675	1 m at 0.12 g/t Au from 54 m in UM3
Forest Road	BP Oil 1988	9 Rotary/DDH (1977 m)	676, 677	0.15 m at 0.83 g/t Au from 211.2 m in FR4

^aDDH = diamond drill hole core drilling; RC = reverse circulation.

^bNew Zealand Petroleum & Minerals exploration report database (www.nzpam.govt.nz) mineral report (MR) reference numbers.

Data discovery

Area selection commonly involves a desktop review of the geology and previous exploration, and relevant research results of the area(s) being considered. In New Zealand, prospecting and exploration permit holders must report their exploration results to the government (NZP&M). Since the 1970s, this has generated more than 2000 such reports related to epithermal Au–Ag exploration. The supplied information is held confidential until the permit expires or five years from the time of submission of the information, whichever is sooner. The subsequent open-file reports and data are publicly available for download free of charge from the Exploration Database section of the NZP&M website (www.nzpam.govt.nz). Data generated from regional airborne magnetic and radiometric surveys carried out by Glass Earth in the TVZ in 2004 and in Northland by the government in 2011 (Doyle 2007; Stagpoole et al. 2012, 2015, 2016; Rattenbury et al. 2016) were also compiled and are available. Other major information sources include: geological maps at various scales, produced by GNS Science and its predecessor organisations (e.g. New Zealand Geological Survey), published research papers, reports and university theses, as well as satellite remote sensing data, Lidar data and digital elevation models.

GIS analysis of data and target ranking

Relevant geological, geophysical, geochemical and topographic data are compiled and typically assessed by the explorer with GIS software, and prospectivity assessment may be conducted using weights of evidence and fuzzy logic techniques to aid in area selection (e.g. Rattenbury and Partington 2003; Feltrin et al. 2016; Peters et al. 2016; Hill et al. 2019). A mineral systems approach, assessing metal and fluid source, metal transport and fluid pathways, physical and chemical depositional traps, and preservation factors, has been recommended for this type of assessment because it is conceptual or process based (e.g. Hronsky and Groves 2008; McCuaig et al. 2010; Joly et al. 2012; McCuaig and Sherlock 2017).

At a more detailed stage, NZP&M’s National Core Store may also provide information for desktop prospecting with drill core photographs, geological and geotechnical logs and assay data available online from the NZP&M exploration database.

Government data compilation and prospectivity studies

Data from the open-file exploration reports have been compiled into digital databases of geochemical and airborne geophysical surveys (e.g. Crown Minerals 2009). The exploration data have been incorporated in an epithermal gold package, including a prospectivity weight of evidence study, by Crown Minerals (2003). This analysis was recently updated by Hill et al. (2019). Many of the airborne magnetic surveys with publicly available data (Christie et al. 2012), including government-funded surveys of the Northland region, also have radiometric data that enables geology and hydrothermal alteration (i.e. K-bearing minerals) to be mapped (e.g. Morrell et al. 2011). National reconnaissance ground-based gravity surveys, with an average spacing of 5 km, have been augmented by more localised and detailed surveys by university researchers in the southern CVZ (e.g. Smith et al. 2003, 2006; Morrell et al. 2011) and by a TVZ airborne gravity survey by Glass Earth (Doyle 2007).

Exploration company data compilation and analysis

Some exploration companies have conducted major data compilations and studies. For example, Glass Earth Ltd (later Glass Earth Gold Ltd) carried out an extensive study in the TVZ and southern CVZ in 2004. They compiled previous exploration and research data in a 3D database for interpretation of the geology and structure, and to determine data gaps requiring follow-up exploration (Henderson et al. 2005, 2016). Kenex, generally on behalf of their clients, also carried out numerous data compilation projects and prospectivity studies (Payne et al. 2014; Payne and Peters 2015; Peters et al. 2016).

Target ranking

Targets defined in an area selection process can be ranked based on geological, geochemical, geophysical, prospectivity (e.g. from weights of evidence prospectivity analysis) and other criteria, such as ease of access and an assessment of local societal issues. The geological aspects can be assessed with an empirical system that ranks subjectively-based features associated with known mineral deposits (e.g. geological setting, geochemical anomalies and hydrothermal alteration). This allows features that are considered relevant by the geologist in relation to the mineral deposit model to be targeted. These empirical criteria are combined to achieve a relative ranking of targets (McCuaig and Sherlock 2017). Joly et al. (2012) noted that in a mineral systems approach, ranking may be based more objectively on the probability of occurrence of each critical mineralisation process, depending on the available information.

Reconnaissance regional-scale exploration

Prospecting for epithermal quartz veins in the late nineteenth and early to mid-twentieth centuries was by basic sediment panning and examination of float and outcrops. Since the 1970s, reconnaissance exploration of large areas has typically involved regional airborne geophysical surveys and/or reconnaissance geochemical surveys as a first step prior to ground-based surveys. Aeromagnetic surveys are generally the most cost-effective airborne surveys for epithermal deposit exploration in volcanic terrains and radiometric measurements are commonly collected at the same time (Kellet and Bromley 2018). Airborne electromagnetic and gravity surveys are also useful but generally involve a much higher cost. Satellite imagery, aerial photography and Lidar surveys can assist mapping of structure, and a few epithermal deposits have a topographic expression due to either a quartz vein(s) and/or siliceous hydrothermal alteration that are resistant to erosion (e.g. the Martha vein system at Waihi, Big Buck Reef at Wairongomai, and Muirs Reef). Regional-scale geochemical surveys typically use stream sediment geochemistry with conventional minus-80 mesh or bulk leach extractable gold (BLEG) sediment sampling techniques, the latter characterised by a lower density of sampling. Panned concentrate geochemical surveys are also sometimes used.

Even where exploration uses remote techniques, field prospecting and mapping of outcrops are still important fundamental techniques to characterise the geological and structural setting, as well as style and footprint of hydrothermal alteration and mineralisation. Mapping the geological units can provide an indication of primary permeability and rheological properties of the rocks, which, in turn, may influence the potential to host veins. Important features noted during mapping include veins, hydrothermal breccias, silicified horizons and silica sinters (cf. Simmons 2017, 2019; Hamilton et al. 2018). Furthermore, documentation of vein textures may provide information on paleodepth (colloform-banded versus massive crystalline quartz) and boiling (e.g. bladed quartz or calcite; Simmons and Christenson 1994; Simmons 2019) that is a process conducive for the deposition of gold; breccia textures may indicate multiple hydrothermal events. The use of portable XRF (pXRF) and portable infrared spectrometers in the analysis of samples in the field, as well as for drill hole samples, has greatly assisted mapping of hydrothermal alteration by characterising the geochemical anomalies and mineralogy, respectively (e.g. Halley 2014; Halley et al. 2015). Portable XRD instruments are now also available and can be useful for determining and quantifying alteration mineralogy (e.g. Burkett et al. 2015).

Reconnaissance exploration in Northland

In Northland (Figure 1), reconnaissance exploration programmes during the 1970s to 1990s included rock chip, stream sediment and panned concentrate surveys (Brown 1989; Barker and Christie 2016c). In addition to the previously known Puhipuhi prospect, several new prospects were identified from geochemical anomalies and mapping of hydrothermal alteration and epithermal mineralisation features, particularly hydrothermal eruption breccias. The main discoveries include Hazelbrook (Huia, Toolshed, Backyard), Te Pene, Te Mata, Puketotara and Waikare, as well as the previously known Puhipuhi deposit (Christie and Barker 2007). In 2011, the government funded an airborne magnetic and radiometric survey of the entire Northland region, which was subsequently interpreted by Stagpoole et al. (2012, 2016).

Reconnaissance exploration in the Hauraki Goldfield (CVZ)

A phase of intensive mineral prospecting began in the CVZ in 1976 with reconnaissance surveys by Amoco Minerals that extended into the 1980s (Hay 1989; Rabone 1991). Many other companies carried out reconnaissance prospecting programmes in the 1980s and 1990s (Barker and Christie 2016a). Stream sediment geochemical surveys were the major reconnaissance prospecting technique used on a routine basis, together with selected rock and soil geochemical surveys. Most stream sediment programmes used conventional sampling techniques and analysed the minus-80 mesh sediment fraction; however, in the 1980s, many companies began using panned concentrate and BLEG techniques. Following the Amoco Minerals aeromagnetic surveys in the 1970s, several companies carried out detailed airborne magnetic and radiometric surveys beginning in the 1980s. The results of many of these airborne surveys were compiled by Stagpoole et al. (2001). The total magnetic intensity maps identified low-amplitude magnetic zones that correlate with areas where magnetite has been converted to pyrite during hydrothermal alteration, destroying magnetisation (Allis 1990; Irvine and Smith 1990; Locke et al. 2006; Kellett and Bromley 2018). Newmont Waihi Gold conducted helicopter-borne electromagnetic surveys of many of their prospects to identify resistive alteration and vein zones (Kellet and Bromley 2018).

Reconnaissance exploration in the Taupō Volcanic Zone (TVZ)

The TVZ (Figure 3) was prospected from the 1970s to the early 1980s by a number of companies (Barker and Christie 2016b). This work identified more than 10 epithermal Au and Ag prospects based on the presence of epithermal features such as sinters, hydrothermal eruption breccias, silicified hydrothermal breccias, quartz veins, quartz-and adularia-altered rocks, and/or geochemical and geophysical anomalies. The work included geological mapping, geochemical surveys, ground magnetic surveys and aeromagnetic surveys by BP and ACM. Early reconnaissance geochemical surveys found that the widespread presence of volcanic ash and other cover rocks, particularly Taupō Pumice, limited the usefulness of conventional stream sediment samples, and therefore most subsequent drainage surveys used panning with visual identification of gold grains, panned concentrate sampling with chemical analysis of the concentrate, or BLEG geochemistry. Furthermore, because of limited erosion and exposure of the epithermal systems, there has been considerable emphasis on geophysical prospecting in the TVZ, including use of Department of Scientific and Industrial Research resistivity survey data originally collected for exploration of geothermal resources (Allis 1990).

In 2005, Glass Earth Ltd commissioned regional-scale airborne geophysical surveys of the TVZ. These were a combined aeromagnetic and radiometric survey over a large part of the TVZ and a smaller airborne gravity survey (airborne full tensor gravity gradiometry) within this area. A large database of geological, geochemical and geophysical information, together with the interpretation of the airborne geophysical survey data, identified 22 targets for follow-up exploration (Henderson et al. 2016). Many of these targets have coincident areas of low magnetic and high residual Bouguer gravity anomalies. Wavelength edge detection analyses of the magnetic and gravity data were interpreted to indicate deep crustal structures that were suggested to control the location of some geological features and mineral prospects (Smith et al. 2007). Later, publicly available data were used by Kenex to complete a 2D prospectivity model for epithermal Au–Ag prospects in the TVZ and a 3D model of the Ohakuri epithermal Au–Ag prospect (Payne et al. 2014; Peters et al. 2016).

Research information relevant to area selection and regional-scale exploration

Geothermal analogues of epithermal Au–Ag deposits

Study of geothermal systems in the TVZ provides information to model the footprint, and geological and structural setting of active systems that can help to target epithermal Au–Ag deposits (Table 6). Resistivity surveys that determine the extent of hot fluid and clay alteration in TVZ geothermal systems have shown that their footprint varies in size from 7 to 62 km², with most between 18 and 26 km² (Bibby et al. 1995). Simpson and Mauk (2004) showed that this geothermal size, or footprint, was similar to the footprint of hydrothermal alteration in the Au–Ag deposits of the CVZ.

Table 6. Advances in the understanding of epithermal ore deposits from TVZ geothermal systems.

Feature group	Feature	Sub-features and notes	References
Setting	Tectonic and structural controls on fluid flow		Rowland and Simmons (2012)
	Spacing of geothermal fields	10–15 km (= c. 2 x the inferred depth of circulation)
Architecture	Cross sectional model of a geothermal system		Henley and Ellis (1983); Henley (1985)
	Vertical movement of the paleowater table		Simmons (2002)
	Footprint and deep structure of geothermal systems	Resistivity and magnetotelluric surveys Footprint of 7–62 km^2 Convection cells to >5 km depth, heated by melts from c. 8 km depth and deeper	e.g. Bibby et al. (1984, 1992, 1994, 1995); Risk et al. (1999); Heise et al. (2008)
Fluids and processes	Boiling model and zoning		Henley et al. (1984); Henley and Brown (1985); Hedenquist (1986, 1991); Krupp and Seward (1987); Seward (1989)
	Importance of CO2 in fluids		Hedenquist and Henley (1985b)
	Carbonate deposition		Simmons and Christenson (1994)
	Hydrothermal eruptions		Hedenquist and Henley (1985a); Browne and Lawless (2001); Christenson and Hayba (1995)
Hydrothermal alteration	Occurrence and zonation of key hydrothermal alteration minerals	Zeolites Illite/illite-smectite/smectite K-feldspar (adularia)	Steiner (1953, 1968, 1970, 1977); Browne (1978): Simmons and Browne (2000)
Metals	Mineralisation in geothermal systems	Analyses of discharge waters	Weissberg (1969); Pope et al. (2005); Simmons et al. (2016b)
		Metals in sinters	Weissberg (1969); Hedenquist and Henley (1985a); Krupp and Seward (1987); Pope et al. (2005)
		Sulphides and trace elements in drill core	Browne (1969, 1971); Browne et al. (1975); Ewers and Keays (1977); Weissberg et al. (1979); Simmons and Browne (2000)
	Vertical zonation of metals	Analyses of trace elements in pyrite	Ewers and Keays (1977)
	Metals deposited on backpressure plates by boiling		Brown (1986)
	Downstream metal zonation in precipitates within geothermal wellhead pipes		Reyes et al. (2002)
	Concentration of metals in deep waters		Brown and Simmons (2003); Simmons et al. (2016b)
	Recognition of higher CO2+H2S systems		Ellis and Mahon (1977); Giggenbach (1981); Simmons et al. (2016b)
	Porphyry Cu style and advanced argillic alteration overprinted by modern low sulphidation style alteration at Ngatamariki		Chambefort et al. (2017)
	Age of geothermal systems	10,000s to 100,000s or years	Browne (1979); Hedenquist and Henley (1985a); Grimes et al. (1998); Arehart et al. (2002); Rowland and Simmons (2012); Milicich et al. (2013); Rae et al. (2015)
	Flux of metals	Rotokawa could transport 1 Moz Au in ∼300 and 1,000 years or 32 Moz Au in ∼12,500–15,000 years Flux is high at Rotokawa and Mokai, moderate at Ohaaki and Kawerau, and low at Ngatamariki and Wairakei	Simmons and Brown (2006, 2007); Simmons et al. (2016a, 2016b)
	Effect of boiling on metals	Boiling causes downstream deposition of precious metals and metal sulphides, removing Ag, Au, Cd, Cu, Pb, and Te from solution, in contrast to As, Mn, Mo, Ni, Sb, Tl, and Zn, which are only partially removed by boiling making them available for transport and dispersion to peripheral and shallow parts of the epithermal environment	Simmons et al. (2016b)
	Magmatic input	The high concentrations and fluxes of Ag, Au and Te at Rotokawa and Mokai attributed to direct fluid inputs from magmas	Simmons et al. (2016b)

More than 20 geothermal systems are currently active in the TVZ, spaced at 10–15 km intervals. By analogy, multiple epithermal Au–Ag systems were likely active contemporaneously in both Northland and the CVZ during the Miocene-Pliocene.

The CO₂/H₂S ratios and metal contents of the deep geothermal fluids vary between different geothermal systems, even between adjacent systems such as those at Wairakei and Rotokawa (Henley 1985; Henley and Brown 1985; Henley et al. 1986; Simmons et al. 2016b). This variability may reflect heterogeneity in magmatic source and flux, and/or the composition of the basement and reservoir rocks that the fluid interacted with (cf. Ellis and Mahon 1964, 1967; Hedenquist 1986). Simmons et al. (2016b) argued that input of magmatic fluid is required to develop the highest concentrations of Au and Ag, as well as Te, in TVZ geothermal fluids, which are a function of the concentration of H₂S in the fluid. By analogy, this argument would extend to the source and metal diversity for epithermal ore deposits.

Volcanotectonic setting

The Miocene-Pliocene volcanic rocks of Northland, CVZ and TVZ were formed in a series of volcanic arcs related to subduction along the boundary between the Pacific and Indian-Australian plates. The succession of arcs records a change in orientation of the subduction zone from NNW-SSE at ca. 25 to 15 Ma during the formation of the Northland Arc, to the present ENE-WSW trending TVZ-Kermadec Arc related to subduction along the similarly oriented Hikurangi Trench. There is controversy about the nature and mechanisms of this change in orientation (e.g. Mortimer 2012), but nonetheless the change resulted in a series of events that are recorded by periods of volcanism as well as associated hydrothermal activity (e.g. Mauk et al. 2011). The changing orientation strongly influenced the structural development of the volcanic arcs (e.g. Rowland et al. 2016; Bahiru et al. 2019). The formation of the Hauraki Rift was also a major influence on the structural development of the CVZ and may be partly responsible for the southeastern tilt of the CVZ stratigraphy (Irwin 2004; Spörli et al. 2006; Rowland et al. 2016). Elucidation of this volcanotectonic development has potential to provide an improved understanding of the prospectivity of specific rock groups and ages. Chrisite et al. (2006, 2007) recognised three sub-regions in the CVZ based on mineralisation style and host-rock association (Figure 4), likely related to the volcanotectonic history.

Regional geological mapping

Since the geological setting of the epithermal deposits is crucial to their location, geological maps provide basic information to explorers. Large-scale geological maps of relatively small areas were published by GNS Science and its predecessor organisations (e.g. New Zealand Geological Survey) between 1975 and 1996 at 1:63,360 and 1:50,000 over parts of the provinces prospective for epithermal Au and Ag deposits (e.g. Skinner 1993, 1995; Brathwaite and Christie 1996). In addition, self-published maps cover parts of the CVZ and Northland at 1:100,000 (Rabone 1992, Thames to Te Aroha) and at 1:50,000 (Evans 1993a, 1993b, 1993c; Ahipara and Kaitaia, Herkino and Rawene, Waipoua and Aranga, respectively). Since 2001, GNS Science has produced GIS-based compilations at 1:250,000 (QMap) and achieved national coverage. The relevant QMap sheets for epithermal exploration are Auckland (Edbrooke 2001), Whangarei (Edbrooke and Brook 2009) and Rotorua (Leonard et al. 2010), and seamless QMap digital data are now available.

Regional structural mapping

Remote sensing and aeromagnetic and gravity survey results help to identify major structural features such as fault zones, structural corridors, and calderas in Northland (e.g. Stagpoole et al. 2012), CVZ (e.g. Skinner 1983, 1986, 1993, 1995; Briggs and Fulton 1990; Bromley and Brathwaite 1991; Rabone 1991; Brathwaite and Christie 1996; Malengreau et al. 2000; Smith et al. 2006; Christie et al. 2007; Bahiru et al. 2019) and TVZ (e.g. Leonard et al. 2010; Stagpoole et al. 2015). Several of the CVZ Au–Ag deposits are associated with structurally controlled basins or grabens, including the Waihi vein system (Torckler et al. 2006) and Wharekirauponga (Christie et al. 2016).

Skinner (1986), Rowland and Simmons (2012), Rowland et al. (2017) and Bahiru et al. (2019) suggested that many of the structures in the CVZ and TVZ Cenozoic volcanic rocks are inherited from N- to NW-striking structures in the greywacke basement and some act as potential transfer zones between discrete fault segments in the younger cover sequence. Their analysis suggests that such regional-scale inherited structural fabrics are important conduits for fluid flow and appear to have controlled the distribution of hydrothermal mineralisation and alteration. Moreover, in the TVZ, Rowland and Simmons (2012) suggest NW-striking transfer zones coincide with the alignment of some geothermal systems, rhombic caldera structures and young (<61 ka) silicic vents.

Regional geochemical and hydrothermal alteration mapping

Compilation of the stream sediment geochemical data indicates elevated concentrations of Cu, Pb and Zn in the western part of the CVZ that reflect more deeply formed base–metal epithermal and porphyry mineralisation exposed as a result of the regional eastward tilting of the stratigraphy and a deeper level of erosion on the western side of the CVZ than on the easern side (see Fig. 4 of Christie and Stewart 2016).

Christie and Brathwaite (1986) and Christie et al. (2001) mapped hydrothermal alteration on the Coromandel Peninsula on a regional scale (see Fig. 1B of Simpson and Christie 2019 – this issue). They used information from 1:50,000 geological map sheets (Skinner 1993, 1995; Brathwaite and Christie 1996), open-file mineral exploration reports and university theses on individual prospects, and indications of subdued magnetic variation on aeromagnetic maps (e.g. Rabone 1991). The field mapping was based on visual appearance of the rocks, in many cases supported by petrological and XRD analyses.

Recent processing of high-resolution aeromagnetic and airborne radiometric data acquired by exploration companies complements the above interpretations. Total magnetic intensity maps delineate magnetic quiet zones that correlate with areas where magnetite has been destroyed by strong hydrothermal alteration (Allis 1990; Irvine and Smith 1990; Locke et al. 2006; Kellett and Bromley 2018). Potassium band data from radiometric surveys can potentially be useful for delineating rocks with significant adularia and/or illite (Morrell 2004; Harris et al. 2005; Morrell et al. 2011). Adularia and illite are common K-bearing alteration minerals in the core of deposits in the southern CVZ (e.g. Simpson et al. 2001, 2019; Brathwaite and Faure 2002; Simpson and Mauk 2007, 2011; Christie et al. 2007; Simpson and Christie 2019). Therefore, combined geophysical methods can reveal the extent of hydrothermal alteration, and the K-altered zone that can surround orebodies, provided the host rocks originally contained magnetite and there is limited post-mineralisation cover.

Detailed prospect-scale exploration

In the late nineteenth and early to mid-twentieth centuries, veins were tested by trenching and underground workings excavated with hand tools and explosives. Drilling was rarely used except at the larger prospects and mines. From the 1960s, various geochemical and geophysical techniques were developed for detailed exploration to help identify drilling targets.

Modern follow-up exploration of prospects identified in regional exploration is commonly carried out by grid-based soil geochemical surveys, as well as by geophysical surveys such as magnetic, resistivity/induced polarisation (IP) and controlled source audio-frequency magnetotelluric (CSAMT) surveys (Kellet and Bromley 2018). In rugged terrain where sampling grids are difficult, soil geochemical surveys commonly use ridge and spur sampling.

Once the sources of anomalies have been located, drilling is used to test the targets at depth. Drill testing of prospects typically involves diamond drilling (DDH) that collects continuous core samples. This method is expensive, but it provides the most information as it produces samples of rock that are relatively undisturbed and for additional cost may be surveyed to enable orientation and measurement of dip and azimuth of structures. Lower cost drilling methods, such as reverse circulation (RC) and rotary air blast (RAB), produce crushed rock samples and are typically used as a reconnaissance approach or to determine the continuity of mineral content between the DDH. Drill cores are examined and described in detail, and samples are taken and analysed. All the data are recorded in a digital database and rendered and modelled in 3D.

Geochemistry

There have been significant advances in geochemical analysis of exploration samples over the past 40 years from AAS to INAA to ICP-AES to ICP-MS, with lowering of detection limits and availability of a larger number of elements for similar cost (e.g. Halley 2014). In the 1960s and 1970s, typically only a few elements were analysed (e.g. Ag, As, Cu, Pb, and Zn). Gold and Ag had high lower limits of detection relative to analyses today and therefore, if analysed for Au and Ag, a high percentage of samples returned assays below the lower limits of detection. Commercial laboratories now offer packages of 30 or more elements for their ICP-MS analyses. However, fire assay is still a popular method of Au analysis and some drill programmes will assay only for Au and Ag, and maybe a few selected other elements. Halley (2014) recommended routinely using a 4-acid digestion and ICP-MS/AES analyses of a large suite of elements in exploration samples to provide data for interpreting geology and hydrothermal alteration, as well as metal assays.

Whole-rock geochemistry and mineral chemistry (e.g. chlorite, adularia and pyrite) in samples (typically drill core) may be used to identify the zonation of hydrothermal alteration. This will enable vectoring toward higher grade alteration and, hopefully, Au and Ag mineralisation (Gemmell 2006). Alteration indices such as the Alteration Index (AI; Ishikawa et al. 1976) and chlorite-carbonate-pyrite index (CCPI; Large et al. 2001) using ratios of major element geochemistry (e.g. K₂O, Na₂O, MgO, S and LOI) typically increase toward mineralisation. A study by Booden et al. (2011) at Waitekaui in the CVZ suggested that ratios of K/Sr, Rb/Sr, molar K/(K + Na + 2Ca), and K/Al can be used to vector toward adularia-rich rocks (cf. Barker et al. 2019). Chlorite chemistry changes from Fe-chlorite to Mg-Fe chlorite to Mg chlorite with increasing hydrothermal alteration and proximity to ore in some deposits (note that this property may also be determined by SWIR analyses). Adularia shows an increase in Ba, Pb and, to a lesser extent, Cs, and pyrite shows an increase in Pb, Bi and Mo, towards mineralisation (Gemmell 2006).

Hydrothermal alteration mineralogy

Description of the hydrothermal alteration mineralogy in surface and drill hole samples provides information on hydrothermal fluid composition, fluid temperature and any potential overprinting of hydrothermal events (Simpson and Christie 2019). This is particularly important for identifying spatial relationships and interpreting 3D alteration trends, and possible vectoring to mineralisation. In exploration, routine thin section petrography and XRD analyses are gradually being replaced by hyperspectral SWIR analyses for low cost real-time information. Geochemical mineral proxies identified in multielement analyses and pXRF analyses are also used (see above). Portable XRD analysis may become more useful in the future with further improvements in the technique and reduction in cost. A more traditional technique of staining for the presence of potassium is still useful for visual logging the occurrence of adularia and its abundance in drill core.

Detailed exploration in Northland

Detailed exploration of the more than 20 epithermal Au–Ag prospects identified by reconnaissance exploration in Northland was undertaken by 11 companies (Barker and Christie 2016c) with five prospects tested by drilling (Table 4). This work included mapping of geology and hydrothermal alteration, rock chip and soil geochemical surveys, resistivity/IP and CSAMT surveys, and drilling at the Te Pene, Hazelbrook-Huia, Puketotara, and Puhipuhi prospects. Puhipuhi is the most explored prospect with 12 DDH, 66 RC and 106 RAB drill holes (Barker and Christie 2016c; Hobbins et al. 2018).

Detailed exploration in the Hauraki Goldfield (CVZ)

In the 1960s and 1970s, several Pb–Zn–Cu–Au–Ag epithermal veins were explored, notably at the Tui mine that produced Zn, Pb and Cu from 1967 to 1974 (Brathwaite et al. 1989). With a rise in the gold price in the late 1970s, detailed exploration commenced with follow-up from Amoco’s reconnaissance programmes and, at the same time, a reappraisal of the former Martha underground mine at Waihi by Amax and Waihi Mining and Development for possible open pit mining (Brathwaite and McKay 1989). These two programmes led to a new phase of gold mining that commenced in 1988 with the opening of the Martha open pit mine at Waihi (Brathwaite et al. 2006). Subsequently, the Golden Cross mine was commissioned in 1991 as a combined open pit and underground operation (Simpson et al. 2001) and further discoveries at Waihi led to underground mining at Favona, Trio and Correnso (Torckler et al. 2016; Barker and Christie 2016a).

Detailed exploration has used close spaced rock and soil geochemical sampling, and geophysical surveys including ground magnetic, resistivity/IP and CSAMT surveys, generally on a grid pattern, to outline drill targets (Barker and Christie 2016a; Kellet and Bromley 2018). Diamond drilling has been the most common drilling type, although RC drilling has been used on some prospects (see Barker and Christie 2016a).

Most of the old mining centres and new targets defined by reconnaissance prospecting were investigated, with detailed exploration programmes by more than 20 companies (Table 3). This work resulted in the definition of new Au and Ag resources in previously mined deposits at Monowai, Karangahake (Talisman), and Waihi (Martha), and discovery of previously unknown veins during brownfields exploration at Golden Cross (Empire stockwork in 1981 and deep Empire vein system in 1982; Hay 1989) and Waihi (discoveries of Favona in 2001, Trio in 2003 and Correnso in 2009; Torckler et al. 2006, 2016). Concealed quartz veins with economic grades (EG Vein and T-Stream vein) have recently been discovered at Wharekirauponga (Streiff et al. 2019).

Detailed exploration in the Taupō Volcanic Zone

Follow-up detailed exploration of the more than 20 epithermal Au–Ag prospects identified in the TVZ by reconnaissance exploration was undertaken by seven companies (Table 5). The exploration programmes included geological and hydrothermal alteration mapping, rock chip and soil geochemical surveys, IP/resistivity, CSAMT and E-SCAN resistivity surveys, and drilling of 12 prospects (Barker and Christie 2016b). Three of the drill-tested prospects are at Ohakuri, including Ohakuri North, East and West. At Ohakuri North, more than 40 holes were drilled, returning up to 4.05 g/t Au over 5.5 m (DDH OH8); resources were estimated as 126 Mt at 0.38 g/t Au and 8.5 g/t Ag, for a contained 2.22 Moz Au equivalent (Au + Ag) (Grieve 2000).

Research information to aid detailed prospect-scale exploration

Exploration techniques

In the CVZ, soil geochemical surveys were undertaken at Ohui (Carruthers et al. 2010) and Favona as a part of a research programme (Browne and Mauk 2007). Theron and Mauk (2010) used data from soil geochemical surveys of several prospects in the southern CVZ to determine regional background, threshold and anomaly values. In the CVZ and TVZ, experimental biogeochemical surveys (e.g. Goff et al. 1985; Dunn and Christie 2014, 2017; Dunn et al. 2017, 2018) and soil gas surveys (e.g. Christenson et al. 1988) have been carried out on epithermal prospects and geothermal areas, but to date these techniques have not been adopted in routine exploration.

Geophysical research has ranged from undertaking surveys to reinterpreting industry acquired data. These include gravity surveys at Golden Cross (Locke and de Ronde 1987) and Waihi (Smith et al. 2006), resistivity surveys at Golden Cross and Waihi (Bromley and Funnell 1990; Bromley et al. 1996), multiple geophysical techniques at Puhipuhi (Locke et al. 1999), and new interpretations of existing data at Karangahake (Harris et al. 2005), deposits in the Waihi-Waitekauri area (Morrell et al. 2011) and Golden Cross (Kellett and Bromley 2018).

Geothermal research

During the 1960s and 1970s, TVZ geothermal research provided a wealth of information on the temperature profiles, fluid compositions and hydrologic structure of geothermal systems that could further be correlated with hydrothermal alteration mineralogy. This enabled synthesis into cross sectional models by Henley and Ellis (1983) that show the architecture of geothermal systems and alteration mineral zonation. The zonation of some minerals is temperature dependant with the alteration sequence of smectite, interlayered illite/smectite and illite, indicating temperatures of <130°C, 130 and 230°C and >230°C, respectively (Steiner 1968; Henley and Ellis 1983; Simmons and Browne 2000). Similarly, adularia indicates temperatures >180°C and epidote forms at temperatures >230°C from fluids with generally low CO₂ concentrations (Browne and Ellis 1970). Adularia also correlates with zones of high fluid permeability and is an indirect mineral indicator of boiling, and bladed textured calcite deposited in open spaces is also indicative of formation from a boiling fluid (Browne and Ellis 1970; Simmons and Christenson 1994; Simmons and Browne 2000; Simmons 2017, 2019). Recently active (ca. 19 to 1.5 ka) geothermal systems at Ohakuri and Tahunaatara that host epithermal Au-Ag mineralisation, have a distal alteration assemblage of mordenite-clinoptilolite-K-feldspar-opal-smectite that probably formed at temperatures of 60–150°C (Brathwaite and Henderson 2016; Brathwaite and Rae 2019).

Structure strongly influences fluid permeability and fluid upflow zones (e.g. Grindley 1965) with mineral deposition concentrated by high fluid flow rates and boiling at epithermal depths (>1000 m to <200 m) (Rowland and Simmons 2012). The host rhyolitic rocks are predominantly pyroclastic deposits that have primary permeability in the upper c. 1 km of the system, enabling the fluids to flow outward from feeder structures to produce an upwardly mushrooming fluid profile.

A major advance in understanding the process of epithermal deposit formation was made by Brown (1986) who showed that high concentrations of metals precipitated on back pressure plates in geothermal well heads due to boiling. Brown and Simmons (2003) and Simmons et al. (2016b) directly sampled fluids at depth in the wells, prior to boiling and metal deposition, and determined that high gold grades are deposited from fluids with low metal concentrations at the ppb level. However, in order to deposit a large quantity of gold, a high fluid flux must be coupled with boiling (Rowland and Simmons 2012). Reyes et al. (2002) described metal zonation in amorphous silica scales which precipitated as a result of boiling and rapid cooling in pipes downstream from geothermal wellheads at the Rotokawa geothermal field, as Cu, Pb, Zn, Ag and Au at and near the wellhead, followed downstream by Hg and Sb.

Hydrothermal alteration mineralogy and geochemistry

Early research on hydrothermal alteration in the TVZ geothermal systems encouraged New Zealand explorers to conduct petrological and XRD studies of the mineralogy of altered rocks from the late 1970s, to help identify mineral zonation and indicate areas of fluid upflow and potential gold deposition within large areas of hydrothermal alteration (e.g. Simpson and Mauk 2001, 2007, 2011; Brathwaite and Faure 2002). More recently, hyperspectral SWIR and pXRF instruments have helped in this task. The SWIR instruments provide analyses of some key hydrothermal alteration minerals (e.g. clays and ammonium minerals; Simpson and Christie 2016; Simpson et al. 2019), whereas pXRF provides lithogeochemical data that can be used to determine element zonation as well as be used as a proxy for mineral determination (e.g. adularia) (e.g. Waihi; Hughes and Barker 2017; Barker et al. 2019). The use of automated mineralogy from a scanning electron microscope showed that adularia is more common in hydrothermal alteration at Karangahake than previously known, with many rocks containing >30% adularia and a maximum of 74% adularia (Simpson et al. 2019). Thus, large amounts of adularia may occur near veins and its increasing abundance towards the veins may be used as a potential vector to mineralisation.

Research on the hydrothermal alteration of individual Au-Ag deposits in the CVZ has enabled the development of a model of hydrothermal alteration mineral zonation (Figure 6; Christie et al. 2007; Simpson and Christie 2019). This model identified an assemblage of adularia ± albite + illite ± calcite proximal to the vein followed outward by a zone of interlayered illite-smectite ± albite ± adularia ± calcite and a distal zone of smectite ± calcite ± hematite. Quartz, pyrite ± chlorite are present in all three zones. Ammonium minerals are present at some deposits and occur proximal to veins.

Figure 6. Schematic model of hydrothermal alteration mineral zonation (modified after Christie et al. 2007; Simpson and Christie 2019). Hard bar: miners term for moderately to weakly altered rock enclosed in strongly altered rock.

Structure

The detailed structure of deposits in the CVZ has been described in several theses completed at the University of Auckland and in subsequent publications (e.g. Nowell 1990; Smith 2003; Irwin 2004; Nortje et al. 2006; Begbie et al. 2007; Mortimer 2008; Spörli and Cargill 2011; Masangcay et al. 2019). Studies of deposits hosted in andesite, rhyolite and greywacke indicate that vein patterns develop due to local stress conditions, typically triplanar (Irwin 2004; Spörli et al. 2006; Mortimer 2008). In general, the metalliferous veins formed in extensional to oblique extensional stress regimes, dominated by steeply dipping normal faults (Spörli et al. 2006; Mortimer 2008). Movement on the vein-hosting structures was predominantly dip-slip with only minor strike-slip, whereas strike-slip movement has occurred on many of the post-mineral faults. Fault dilation, with potential for ore shoot development, may occur with changes in fault dip and/or strike and at the intersection of faults (Fraser 1910; Wodzicki and Weissberg 1970; Merchant 1978; Main 1979; Irwin 2004).

Brathwaite et al. (2001a) quantified the differences in vein development in different host rocks. They found that rock type has a significant influence on vein spacing distributions, with veins hosted in rhyolite being thinner and shorter, but with a higher density compared with veins hosted in andesite at the same deposit. Joints are a major control on vein spacing. Overall, andesite lava and andesitic pyroclastic rocks host thicker and more persistent veins than rhyolite lava and tuff. However, rhyolite can host significant veins as exemplified by the discovery of the EG vein, Wharekirauponga (Streiff et al. 2019).

Quartz vein and breccia textures

Quartz vein textures in deposits of the CVZ have been described in many University of Auckland MSc theses mostly following the classification schemes of Dong and Morrison (1995) and Dong et al. (1995). In terms of prospectivity and exploration, probably the most important vein textures are lattice bladed quartz pseudomorphs after platy calcite and colloform textures that indicate boiling, and banded veins that indicate multiple events (e.g. Simmons 2017). The presence of pseudosedimentary textures, such as swirl, ripple marks, channel fill, ponding and graded bedding, are caused by fluid flow and tend to form at shallower depths.

Breccia textures have also been described and classified in theses on CVZ Au-Ag deposits, with a comprehensive description of breccias at Favona presented by Mortimer (2008; Table 4.4). Oatmeal breccia texture, described from Golden Cross and the Waihi vein system, typically has associated high gold grades. The texture consists of fragments of vein material that are slabby to ovoid in section and commonly exhibit preferred orientation, suggesting a settling or imbricate texture (see Fig. 3.4C in Simmons 2019). Hydrothermal breccias with clasts of breccia are another indicator of multiple events.

Tilting and reorientation

Most of the host rocks to the CVZ Au-Ag deposits are tilted to the east and southeast (Skinner 1986). Structural studies by Begbie et al. (2007) concluded that the local 50^o southeast dip of the host rocks at Golden Cross was post-mineralisation. By restoring the vein system to its original orientation, the apparent reverse sense of movement on the steeply dipping Empire vein and numerous low-angle footwall veins are transformed so that the Empire vein is shown to have formed as a southeast dipping normal fault and the low-angle footwall veins were steeply dipping extensional veins in the hangingwall (see Fig. 21 of Begbie et al. 2007). Similarly, the veins in the Waihi vein system are interpreted to have undergone tilting to the southeast post-mineralisation (Brathwaite and Faure 2002). This is confirmed by identification of vein sediments that exhibit tilting from their former horizontal bedding. Masangcay et al. (2019) similarly noted a 15^o east to southeast tilt of the Wharekirauponga veins based on the orientation of vein sediments.

Hydrothermal fluid temperature and salinity – fluid inclusions

Studies of fluid inclusions in quartz deposited in veins have been used to estimate the ore-forming fluid temperatures and compositions. These, in turn, have been used to estimate depth below the paleowater table (see below), the gold deposition depth window, and vectors toward high fluid temperature. The first two applications have been very successful (see below), however, vectoring has not been so successful, partly because the large number of samples required to be measured limits the technique to advanced drilling projects.

Measured homogenisation temperatures are mainly from 225 to 250°C, with individual measurements between 100 and 350°C (Merchant 1978; Christie 1982; Brathwaite and Faure 2002; Christie et al. 2007; Simpson et al. 2015). Salinities are mainly in the 0–2 wt % NaCl eq. range, although maximum salinities exhibited by individual inclusions are as high as 11.9 wt % NaCl eq. at Tui, the deepest deposit (c. 1200–1300 m below the paleo-water table; Christie et al. 2007), and 1.7 wt % NaCl eq. at c. 850 m below the paleo-water table at Waihi.

Paleodepth

Paleodepths of the epithermal Au-Ag mineralisation in the CVZ have been determined for some deposits by paleosurface indicators such as sinters, where preserved (Hamilton et al. 2018), or more commonly have been calculated from fluid inclusion temperatures or boardly estimated from hydrothermal alteration minerals, ore mineralogy, and vein textures (Figure 7; Brathwaite and Christie 2000; Christie and Brathwaite 2003b, 2004; Chrisite et al. 2006, 2017). These indicate the deposits formed over a range of depths from near-surface to >1 km below the surface.

Figure 7. Paleodepth relations of vein textures, and gangue and ore minerals, that along with fluid inclusion data enable the estimated depth range below the paleowater table of some deposits (modified after Christie et al. 2007).

Gold window

Brathwaite and Christie (2000) observed that economic concentrations of electrum in the CVZ deposits are confined to a paleotemperature window of 260–180°C, which corresponds to temperatures generally conducive for gold deposition from geothermal fluids (Brown 1986; Simmons and Browne 2000). This means that for dilute, gas-poor fluids that follow the boiling point with depth curve (Haas 1971), gold deposition is limited to a vertical interval of about 400 m, with its upper limit at about 100 m below the water table. Most deposits in the CVZ were mined over vertical extents of 170–330 m (Chrisite et al. 2006). The greater depth extent of economic gold ore at some deposits such as Waihi (600 m) and Karangahake (700 m) may be due to changes in the concentration of CO₂ in the fluid (cf. Hedenquist and Henley 1985b) or changes in the level of the water table due to uplift, sector collapse or by draining of lakes (Simmons 1991, 2002; Simmons et al. 1993), or to overprinting systems. Brathwaite and Faure (2002) also suggested mixing as a cause for deep ore deposition. Christie and Brathwaite (2005) and Christie et al. (2017) mapped areas of the CVZ where the gold window was likely to be largely preserved versus other areas where it was eroded or deeply buried, providing another criteria for regional and deposit scale prospectivity.

Discussion and conclusions

An exploration strategy for epithermal Au-Ag deposits in New Zealand is presented in Table 7 and includes guides from research studies and prospective features at various stages of exploration, from area selection through to the prefeasibility stage. Legacy geochemical exploration data are an important component in desk-top area and target selection using GIS analyses and GIS prospectivity studies. Research on host rock and mineralisation ages, and regional tectonics, structure and paleodepth provides additional information to map the most prospective areas and aid in area selection and targeting.

Table 7. Mineral exploration strategy for low and low-intermediate sulphidation epithermal Au–Ag deposits in New Zealand.

Exploration stage*	Scale	Permit type	Exploration activities	Research input	Prospective features
Area selection	Regional	Pre-permit desktop activity	Compilation and GIS analysis of available exploration and research data, GIS based prospectivity analysis using a mineral systems model, Optional in-field reconnaissance, Target generation and prioritisation	Geological mapping and conceptual studies, Tectonics, Regional structure, Airborne geophysical surveys, Regional geochemical surveys, Geochronology, Prospectivity, metallogenic and regional resource assessment studies, Mineral deposit studies, Minerals databases (e.g. GERM and REGCHEM), Radiometric isotope studies, e.g Pb/Pb	Both Area selection and Reconnaissance exploration: Favourable tectonic setting, Favourable geological setting and rock types, e.g. andesite and dacite (and rhyolite lava flows e.g. Wharekirauponga), Favourable structural setting, e.g. proximity to large basement structures and structural corridors, structurally controlled basins, or calderas, Geophysical anomalies, e.g. coincident magnetic lows and gravity highs; K radiometric anomalies; resistive features, Geochemical anomalies, e.g. Au, Ag, As, Sb, Hg, (Cu, Pb, Zn, Mo), Preserved vertical profile of the gold depth window, indicated by sinters, hydrothermal eruption breccias and shallow epithermal features, Favourable deposit age, e.g. preferably 8–5 Ma in the CVZ
Reconnaissance exploration	Regional	Prospecting (maximum 500 km^2)	Literature searches and data compilation, Low impact activities such as: Geological, structural and hydrothermal alteration mapping, Hand sampling of stream sediments, soils and rocks for geochemical, petrographic and mineralogic analyses, Ground-based and airborne geophysical surveys	Geological mapping, Mineral deposit studies (structure, geology, hydrothermal alteration mineralogy and geochemistry, ore mineralogy and geochemistry), Exploration technology, Radiometric isotope studies, e.g Pb/Pb
Detailed exploration	District/camp to deposit scale	Exploration (minimum 150 ha = 0.5 km^2)	Minimum impact activities plus higher impact activities such as: Drilling, Trenching, Bulk sampling Geochemical analyses of samples: Multi-element ICP-MS following 4-acid digest, pXRF Mineralogical analyses of samples: SWIR backed up by thin section petrography, XRD and automated SEM analyses	Mineral deposit studies (structure, geology, hydrothermal alteration mineralogy and geochemistry, ore mineralogy and geochemistry), Exploration technology, Fluid inclusion studies, Stable isotope studies, e.g D/H, C and O, Geochronology, e.g. Ar-Ar, (K-Ar), Re-Os, U-Pb and U-Th/He	Andesite and dacite host rocks, Lavas rather than pyroclastic rocks, A large hydrothermal alteration footprint, preferably with evidence of a large zone of quartz-adularia or quartz-illite alteration, A large vein system, identified by structural mapping, Multiple veins in the vein system, Multiple depositional events, evident in vein textures, Evidence of boiling, from vein and mineral textures, Preserved vertical profile of the gold depth window, determined by paleodepth criteria, Fluid upflow zones with high structural permeability, that may be sought by hydrothermal alteration mineral and geochemical vectors, Adularia and NH4-minerals (buddingtonite or NH4-illite)
Prefeasibility studies	Deposit scale	Continuation of Exploration permit	Resource definition and modelling, Metallurgical and geometallurgical studies, Mine planning	Geological and structural modelling, Mineralogical and geochemical analyses, Geometallurgy

*Depending on the level of prior knowledge, an exploration programme may start at the area selection, reconnaissance exploration or detailed exploration stage.

Reconnaissance exploration typically begins with stream sediment surveys along with rock chip sampling of interesting stream float and outcrops, in addition to mapping of geology, hydrothermal alteration, and mineralisation features. Airborne magnetic, radiometric, electromagnetic and lidar surveys can provide useful data for mapping geology, hydrothermal alteration and structure. A major future advance will be the miniaturisation of instrumentation for drone-based geophysical surveys that should reduce the cost of the surveys and enable increased data acquisition and applications. District-scale seismic surveys could help in determining deep structure but have not been used in mineral exploration in New Zealand to date, mostly because of their high cost and logistical difficulties in populated areas. Passive seismic studies may be an alternative.

Detailed exploration typically involves soil and rock chip geochemical surveys and resistivity geophysical surveys to compliment geological and hydrothermal alteration mapping to develop drill targets. Portable XRF and hyperspectral (e.g. Terraspec Halo) analysers are increasingly being used to identify hydrothermal alteration mineralogy and element abundance and zonation, particularly during logging of drill core. Portable XRD instruments are available and can also be used to identify hydrothermal alteration mineralogy and quantify mineral abundances. Hyperspectral core loggers, such as Hylogger and Corescan, have so far not been used in New Zealand because of set-up and running costs, but their application could benefit large drilling projects by identifying hydrothermal mineral vectors to mineralisation. Routine multielement geochemical analyses of surface and drill hole samples enables the use of alteration indices and proxies for alteration minerals (e.g. K/Al for adularia) to help vector toward mineralisation. The combined use of geochemical and alteration mineral determinations by various techniques (assays, pXRF, hyperspectral, XRD) provides the most information about the former hydrothermal system.

Future exploration may benefit from improvements in exploration tools as well as concepts (Table 8). Major challenges include expanding efforts to explore under cover and making informed decisions on where to drill next or whether to abandon the prospect. The recent advances in geochemical and mineralogical vectors work best with information from several to many drill holes. Explorationists would benefit from reliable test(s) on a small suite of samples, such as core from the first drill hole, that can indicate the prospectivity of a prospect. So far this has been elusive, and proposed techniques such as the use of the trace element composition of quartz veins and fluid inclusion salinities have not been fruitful. Exploration continues to rely on traditional methods of evaluating size and grade by successive phases of targeted drilling and analyses of drill core.

Table 8. Some areas of future research that would benefit exploration for epithermal Au–Ag deposit in New Zealand.

Research topic	Research sub-topic
Geological topics
Refinement in age control of host rocks and mineralisation	Re-Os of sulphide minerals, Ar–Ar analyses of alteration minerals and Ar–Ar and U–Pb zircon analyses of host rocks
Refinement of geochemical and mineralogical footprints and vectors	Geochemical signatures and element dispersion models; Adularia occurrence and abundance as vector; NH4-mineral vector; Mineral chemistry vectors e.g. chlorite chemistry
Structural localisation of deposits and structural controls on mineral deposition	Regional and deposit scale studies in Northland and TVZ
Lithological controls on mineralisation	Volcanic facies and proximity to volcanic centres
	Identify intrusive host rocks (e.g. dacite at Golden Cross and rhyolite dike at Komata)
	Compare lava vs pyroclastic vs intrusive rocks as host for veins
Models for exploration under cover	Structural mapping with modelling in 3D; 3D modelling of cover thickness
Origin of mineralising components and regional fertility	Development of local 3D epithermal models from source to surface coupled with seismic and resistivity (e.g. magnetotellurtic) interpretations of deep environment
	Fingerprint magma or greywacke basement sources of components by isotope analyses coupled with analyses of key trace elements in unaltered rocks
	Characterise source magmas (melt inclusion studies) and their spatial distribution
Exploration technique topics
Hydrothermal alteration mineral analysis	Hyperspectral SWIR continuous drill core scanning (e.g. Hylogger and Corescan)
	Adularia mapping and quantification by pXRD and pXRF
	Automated SEM mineral recognition (e.g. TIMA) for alteration types and quantification
Ore mineral analysis	Automated SEM mineral recognition (e.g. QEMSCAN) of ore minerals in individual veins and lateral and vertical spatial variation
Exploration under cover	Geochemical and geophysical techniques to see through cover
Exploration of deep structure	Testing of passive seismic techniques
Geophysical surveys using drones	Conduct trial surveys when suitable equipment becomes available

Acknowledgements

Carolyn Hume and Katy Kelly drafted Figures 1–5. The paper has greatly benefited from reviews by Jeff Hedenquist and Stuart Simmons, and editing by Rich Goldfarb.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The output for the Gold Exploration Models (GEM) research programme (2014–2018) was funded by Science and Innovation contract C05X1405 supported by the Ministry of Business, Innovation and Employment (MBIE).