Introduction

Siliceous sinters are surface expressions of epithermal systems in which fluids derived from deep reservoirs with temperatures >175 °C are discharged at the Earth’s surface (Fournier and Rowe 1966). Upon cooling, these fluids deposit non-crystalline opal-A that precipitates onto biotic and abiotic surfaces to form erosion-resistant sinter mounds and sheet deposits, metres to tens of metres thick (Fournier and Rowe 1966; Fournier 1985; Lynne et al. 2008; Guido and Campbell 2011; Lynne 2012). Sinter deposits are geothermal features that form where hot fluids are expelled at the intersection of the water table with the Earth’s surface (Sillitoe and Hedenquist 2003; Simmons et al. 2005), typically above or adjacent to epithermal deposits occurring at shallow depths in the Earth’s crust (Sillitoe 2015). The geographic area occupied by a given sinter deposit is typically small in comparison to the often extensive lateral and vertical footprint of epithermal mineralisation (e.g. veins, regional late silicification), and therefore sinters provide precise information about the depth level of exposure of a palaeo-epithermal system. Because vent areas in particular represent conduits of fluid flow from the reservoir to the surface, they help pinpoint where epithermal mineralisation is most likely to be found. Indeed, siliceous sinter deposits have been shown to have potential as reliable vectors for epithermal mineralisation at shallow depths in mining districts elsewhere (Buchanan 1981; White et al. 1989; Sherlock et al. 1995; Guido and Campbell 2011, 2014; Lynne 2012). For example, at the McLaughlin Mine in California, USA, exposed Quaternary sinter was the only surface expression of a bonanza deposit (Sherlock et al. 1995). The textures, mineralogy, petrography and chemical composition of sinter deposits may be utilised in palaeoenvironmental and palaeo-hydrological reconstructions of temperature, pH, fluid composition, flow directions and relative discharge volumes of extinct hot-spring systems, from high-temperature (∼100–75 °C) vent geyserites to ambient (∼25 °C), plant-rich, thermally influenced silicified marsh deposits (Fig. 1) (Fournier and Rowe 1966; Walter 1976; Fournier 1985; Cady and Farmer 1996; Campbell et al. 2001, 2015a, b; Guido and Campbell 2011). This potential hot fluid-conduit link between epithermal deposits and siliceous sinters at the land surface may allow sinters to be used in assessing the mineralisation environment of an epithermal system.

Fig. 1
figure 1

Schematic cross-section of a siliceous sinter deposit showing the transition of textures associated with a hot-spring discharge temperature gradient (100 °C–ambient), based on facies identified in the Deseado Massif, Argentina, as compared with their analogues in Quaternary to Recent geothermal systems in the Taupo Volcanic Zone (New Zealand) and Yellowstone National Park (Wyoming, USA); modified from Campbell et al. (2015a)

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The Kohuamuri sinter is a well-preserved, fossilised, siliceous hot-spring deposit located in the eastern province of the Hauraki Goldfield, hosted in Miocene–Pliocene rocks of the Coromandel Volcanic Zone (CVZ), New Zealand (Fig. 2). The Kohuamuri siliceous sinter is the largest known fossil hot-spring deposit in the region, comprising the Kohuamuri deposit, a large in situ outcrop (47,000 m2) and its associated sinter boulder field (4500 m2), and the Kaitoke deposit situated 900 m to the southwest, constituting a landslide-related sinter boulder field (49,000 m2) located on a normal fault. The Kohuamuri sinter has been of interest in the past for its association with gold mineralisation, with reports of minor amounts of gold being extracted from the sinter in early mining attempts (McKay 1897). In a regional geological study of the Mercury Bay area, Skinner (1995) also recorded the Kohuamuri sinter and briefly described its physical appearance. Within the CVZ, half a dozen sinters are documented (Fig. 2), which were used during early exploration of regional epithermal systems as special indicators, and geochemical analyses were conducted on some sinter deposits (Fraser and Adams 1907; Bell and Fraser 1912). A more recent study (Stevens and Boswell 2006) analysed the Onemana hydrothermal eruption breccia, which contains sinter clasts, and revealed anomalous Au, Ag, As and Sb concentrations. Arsenic and antimony contents reach up to 650 and 114 ppm, respectively, with the highest gold concentrations centred around the inferred vent locality.

Fig. 2
figure 2

Geological setting of the Kohuamuri sinter, Coromandel Volcanic Zone (CVZ). a Map of North Island, New Zealand, showing location of the CVZ, and the Northland and Taupo volcanic zones. b Geological map of the Coromandel Peninsula showing the three major geological groups of the CVZ, mapped major faults, inferred large calderas based on structural and geophysical evidence, and known epithermal, porphyry copper and sinter deposits (BJ Blackjack, OH Ohui, ON Onemana, AS Ascot) and active hot springs; location of the Kohuamuri sinter is highlighted by a blue rectangle; modified from Skinner (1986), Adams et al. (1994), Rattenbury and Partington (2003) and Christie et al. (2007)

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This study examines the preserved textures, mineralogy, petrography and elemental concentrations of the Kohuamuri sinter. We compare collected data to modern analogues and other studies to reconstruct the palaeoenvironment and to evaluate its worth as a vector for geothermal fluid flow and mineralisation.

Geologic context

The Kohuamuri sinter is the largest (0.2 km2) known sinter in the Hauraki Goldfield, a 200 × 40 km region of 50 gold-bearing, adularia-illite type epithermal deposits and several porphyry copper prospects associated with the Coromandel Volcanic Zone, North Island, New Zealand (Fig. 2) (Brathwaite et al. 2001; Christie et al. 2007). The CVZ is a product of Cenozoic volcanism driven by subduction of the Pacific Plate beneath North Island (Australian Plate) since 25 Ma, with subaerial andesite-dacite sequences and late rhyolite overlying a basement of Mesozoic meta-sedimentary sequences (Skinner 1986). This period of volcanism (20–4 Ma) relates to the rotation of the locus of arc magmatism from Northland to Coromandel by ca. 16 Ma, and its southeastward migration from Coromandel to the Taupo Volcanic Zone (TVZ) by 2 Ma (King 2000; Christie et al. 2007; Mauk et al. 2011; Rowland and Simmons 2012; Seebeck et al. 2014).

The CVZ was constructed during the Miocene and Pliocene by the products of three volcanic phases (Christie et al. 2007; Booden et al. 2012). The first is recorded by the widespread Coromandel group (18–3.8 Ma) and constitutes andesite and dacite that formed massive lava flows, breccias, lahars, tuffs, dykes and tonalite plutons. The second phase is encompassed by the Whitianga group (9.1–6 Ma), consisting of rhyolite produced by lava flows, ignimbrite sheets, dome volcanoes and large silicic calderas. The third phase comprises the localised Mercury Bay basalts (6.0–4.2 Ma), which represent restricted, mostly Strombolian volcanoes and dykes (Skinner 1986; Adams et al. 1994). As noted by Christie et al. (2007), the epithermal deposits of the Hauraki Goldfield are predominantly hosted in the phase I Coromandel group andesite, but gold-producing deposits also occur in phase II Whitianga group rhyolite and meta-sedimentary greywacke. The goldfield produced 320,000 kg Au and 1.5 million kg Ag between 1862 and 2006 from a total land area of approximately 2900 km2 (Christie et al. 2007). These deposits are controlled by steeply dipping extensional fault arrays (Brathwaite et al. 2001) and accumulated during the Miocene (16–5 Ma). Furthermore, the regional volcanism and epithermal deposits exhibit a punctuated southerly migration over time (Mauk et al. 2011; Rowland and Simmons 2012; Wilson and Rowland 2015).

The Kohuamuri sinter is located within the Whitianga Volcanic Centre, an area of the Whitianga group volcanic rocks, which includes Miocene and Pliocene rhyolite and ignimbrite that make up large calderas and stratovolcanoes (Adams et al. 1994). The major feature of the Whitianga Volcanic Centre is the Whitianga caldera (15 km wide × 1–1.5 km deep), as inferred by geophysical data (Briggs et al. 2005; Fig. 2). The in situ Kohuamuri sinter (Fig. 3) is spatially associated with the Pumpkin Rock ignimbrite (5.9 ± 0.7 Ma, Adams et al. 1994), a plagioclase-phyric, pumice-rich, lithic-poor ignimbrite. It overlies the Motutapere rhyolite (7.4 ± 0.85 Ma), a banded and spherulitic rhyolite, and the Flaxmill (7.9 ± 0.1 Ma) rhyolitic dome, a glassy porphyritic rhyolite (Skinner 1995). The volcanic units surrounding the sinter deposits are silicified. Sinter deposition and associated silicification post-date the surrounding volcanic units; therefore, a late Miocene or early Pliocene age is probable (Skinner 1995).

Fig. 3
figure 3

Geological map of the local Kohuamuri sinter area, comprising the Kohuamuri and Kaitoke deposits, and showing sites (numbered) where sinter samples were collected (Table 1)

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The Kohuamuri deposit (47,000 m2) is in sharp unconformable contact with the silicified Pumpkin Rock ignimbrite and Motutapere dome (Fig. 3). Kohuamuri Stream intersects the deposit along its northern side to form white sinter bluffs up to 10 m high. The bluffs are highly weathered but contain some original porosity as well as visible centimetre-scale bedding that dips gently <3° west. On the northern side of the stream, an elongated black sinter deposit (∼80 × 20 m) is exposed. A boulder field of eroded sinter (boulders up to 2 m3) lines the southern branch of Kohuamuri Stream, from the in situ deposit to where the stream enters Whitianga Harbour. The Kaitoke deposit is a boulder field situated on a normal fault 900 m southwest of the Kohuamuri deposit and extends over the adjacent Pumpkin Rock ignimbrite (Fig. 3).

Methods

A total of 49 texturally representative sinter samples were collected and analysed from the Kohuamuri sinter (i.e. Kohuamuri and Kaitoke deposits) from 11 locations (7 in situ, 4 ex situ; Fig. 3) in order to map its areal distribution and reconstruct the palaeo-geothermal setting. In general, in modern hot springs, temperature gradients are produced, away from the vent, by the cooling and evaporation of thermal fluids that deposit varied litho- and bio-facies types down the discharge apron (Fig. 1). Sinter textures were examined at macro- and micro-scales using hand sample slab study, transmitted light microscopy of thin sections and scanning electron microscope (SEM) analysis of freshly broken rock chips. For SEM analysis, samples were coated with a thin layer of platinum using a Hitachi E1045 ion sputter coating unit. The coated surfaces were then scanned using a Hitachi SU-70 field emission scanning electron microscope with embedded software and acceleration voltages between 5 and 10 kV. To determine the likely conditions of formation, textural comparisons were made with Quaternary to Recent examples from the TVZ and Yellowstone National Park (YNP), USA, to aid interpretation of the Coromandel samples with respect to their geothermally related palaeoenvironmental gradients.

The bulk mineralogy of eight representative samples was analysed via x-ray diffraction (XRD), following Herdianita et al. (2000), using a Philips PW1050/25 diffractometer with CuKα radiation (λ = 1.5405) at 40 kV and 20 mA operating conditions, and fitted with a PW 1752 graphite crystal monochronometer. To determine the silica phase maturation state of the deposit, a Witec alphas300 R+ confocal Raman microscope fitted with a Princeton Instruments SP2300 spectrograph with a Nd:YAG laser was used at a 532 μm wavelength at power levels up to 37 mW to scan along 50 μm of selected cut nodules and vugs. Additional minerals were identified using transmitted light microscopy and were compared with other sinters associated with epithermal deposits.

To determine trace element concentrations, 18 samples were analysed by Bureau Veritas, New South Wales, Australia, using the Aqua Regia Analysis method. Specifically, 40 g of dried and pulverised sample was dissolved with Aqua Regia in a boiling water bath. An aliquot of this acid was then analysed using atomic absorption spectroscopy (AAS), or inductively coupled plasma mass spectrometry (ICP-MS), to determine their gold contents, with ICP-MS having higher sensitivity than AAS. ICP-MS and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to determine the concentrations of the other elements (Ba, Ca, Fe, Mg, Mn, S, Ag, As, Bi, Cu, Hg, Li, Mo, Pb, Sb, Se, Tl, Te, W, Zn).

Results

Textures of the Kohuamuri sinter

Fourteen different palaeoenvironmentally significant textures similar to actively forming sinters from near-neutral pH alkali chloride waters were identified from nine locations associated with the Kohuamuri deposit and two from the Kaitoke deposit. Textures of the Kohuamuri deposit represent an entire geothermal gradient, from high temperature vent and proximal slope areas, through moderate temperature sinter apron pools, to cooler distal apron and thermally influenced marsh settings (Table 1), whereas the Kaitoke deposit only contains textures associated with proximal hot-spring areas.

Table 1 Facies associations of the Kohuamuri sinter (asterisks (*) indicate that these samples were collected in place)
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In detail, several high-temperature (vent to proximal slope facies) sinter fabrics were found in both the Kohuamuri and Kaitoke deposits (Fig. 4). Geyserite with similar textures forms today in spring vent areas (>75 °C) intermittently wetted or splashed by the eruption of geysers to form localised, layered, botryoidal sinter mounds and rims around hot-spring vent pools (Walter 1976; Walter et al. 1996; Braunstein and Lowe 2001; Lowe and Braunstein 2003; Campbell et al. 2015b). At Kohuamuri, vent and proximal slope settings are represented by spicular, nodular and pisoidal (‘beaded’) geyserite, finely laminated sinter and a silicified, inferred hydrothermal eruption breccia containing clasts of nodular geyserite (Fig. 4). The Kohuamuri hydrothermal breccia contains mixed clasts (sinter, volcaniclastic rocks, hydrothermally altered clasts) with poor sorting (clast size ranges from <5 mm to 10 cm in diameter), in a massive silica matrix (Fig. 4a), similar to the Quaternary Mangatete hydrothermal eruption breccia, TVZ (Fig. 4b; Drake et al. 2014). For example, a clast of nodular geyserite was found in the Kohuamuri hydrothermal eruption breccia (Fig. 4c), akin to nodular geyserite texture forming today at YNP around vent rims (Fig. 4d). Moreover, the Kaitoke deposit contains dense, fine laminae of pisoidal geyserite (Fig. 4e), nearly identical to pisoidal geyserite textures from recently extinct vents (<100 years old) at Geyser Valley, TVZ (Fig. 4f). In addition, spicular geyserite was identified in the Kohuamuri deposit (Fig. 4g), which is similar to spicular geyserite microtextures from present-day sinter (YNP; Fig. 4h). Furthermore, dense, very finely laminated textures develop on proximal vent slopes (∼75–90 °C) due to episodically high flow rates that do not allow rapid build-up of sinter (Cady and Farmer 1996; Walter et al. 1996). In the Kohuamuri and Kaitoke deposits, the lamination is composed of white vitreous quartz up to 2 mm thick and laterally continuous for up to 50 cm (Fig. 4i). In places, the layers display small wavelets with amplitudes up to 5 mm and crest distances up to 1 cm apart, inferred as ripples on proximal outflow channel surfaces, similar to textures sampled from geyserite blocks in a steaming landslide deposit at Te Kopia, TVZ (Fig. 4j). At the micro-scale, the size of the microcrystalline quartz crystals within each horizon made it possible to differentiate between individual laminae. This fabric differs from low-temperature distal apron laminated palisade fabric (<40 °C) by its thin and continuous, very regular laminae, and known spatial association with spring-vent areas. The rather thickly laminated palisade fabric bears closely packed, vertical micro-pillars of coarsely filamentous silicified cyanobacteria (Cady and Farmer 1996; Walter et al. 1998; Guido and Campbell 2011; Campbell et al. 2015a). The Kaitoke sinter also has a texture of wavy lamination (2 mm thick), with voids (100 × 50 mm) that are in-filled by siliceous white, brown and dark red sediment in cavities up to 10 mm in diameter. The geopetal fills have an irregular outer morphology that is not parallel to bedding, suggesting the textures represent late stage alteration and dissolution. The red geopetal silicified fill occurs in contact with pisoidal geyserite, so we infer that it formed proximal to a vent.

Fig. 4
figure 4

Vent and proximal facies associations of the Kohuamuri sinter compared to Quaternary analogues. a Cut slab of poorly sorted, polymitic clasts of sinter (black colour) and volcanic material (yellow) in a siliceous matrix; Kohuamuri deposit. b Cut slab of hydrothermal breccia from the Quaternary Mangatete sinter, TVZ, showing poorly sorted polymitic clasts in a silica matrix (Drake et al. 2014). c Cut slab of poorly sorted breccia containing clast of nodular geyserite (white square outline); Kohuamuri deposit. d Nodular, pseudocolumnar geyserite from Yellowstone National Park (YNP) (USA), forming at high temperatures (>75 °C) proximal to a vent. e Geyserite pisoids (‘beads’) from the Kaitoke deposit. f Holocene geyserite collected proximal to a recently extinct vent at Geyser Valley, Wairakei, TVZ, New Zealand, indicating a similar texture to that found in (e). g Microtexture of geyserite spicules forming perpendicular to bedding, plane polarised light; Kohuamuri deposit. h Thin section (plane polarised light) of modern spicular geyserite from YNP, exhibiting a similar size and morphology of textures compared to the Kohuamuri deposit in (g). i Cut slab from the Kohuamuri deposit of finely laminated macrofacies showing low amplitude wavy lamination. j Photograph of cut slab of finely laminated macrofacies associated with geyserite (not shown), from high temperature sinter of the Te Kopia steaming landslide deposit, TVZ, New Zealand

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The Kohuamuri deposit also contains bubble mat, network and conical tufted fabrics (Fig. 5), likely formed in moderate temperature (∼40–60 °C), mid-apron pools and discharge channels (Fig. 1). Network fabrics consist of silica threads less than 1 mm thick that are discontinuous in a webbed pattern within stacked horizons (2 cm thick) of bedded sinter. Network fabrics are inferred to form at the drying margins of moderately hot pools, where extracellular polymeric substances (EPS), exuded by cyanobacteria, become dried out and silicified (e.g. Fig. 5b; Guido and Campbell 2011). Bubble mat texture consists of elliptical voids occurring in small, bedding-parallel lenses within wavy laminated to conical tufted sinter (Fig. 5c). In modern-day sinters, bubbles (Fig. 5d) form and become trapped as microbial mats de-gas during photosynthesis (Hinman and Lindstrom 1996), which then become silicified due to the oversaturation of silica in the cooling thermal fluid (Campbell et al. 2001; Guido and Campbell 2011). At Kohuamuri, large fossilised tufts perpendicular to bedding occur as irregularly laminated, cone-shaped features (up to 5 mm wide, 5 cm long) (Fig. 4e). In the present day, conical tufted textures form in mid-apron, terraced pools (e.g. Fig. 5f) via columnar building of pool stromatolites by finely filamentous cyanobacteria (e.g. Leptolyngbya) (Walter 1976; Cady and Farmer 1996; Walter et al. 1996; Guido and Campbell 2011; Lynne 2012). At the microscopic scale, these features at Kohuamuri (Fig. 5g) are very similar to conical tufted examples from modern spring pools at Whakarewarewa geothermal area, TVZ (Fig. 5h).

Fig. 5
figure 5

Mid-apron sinter facies at Kohuamuri compared to modern examples. a Cut slab of network facies, comprising white webbed silica; Kohuamuri deposit. b Modern-day network facies forming on the dried margin of a hot pool at Tokaanu, TVZ. c Cut sinter slab showing a wavy laminated to low amplitude conical tufted fabric containing lenticular voids (arrows) produced by bubbles trapped within the Kohuamuri sinter deposit. d Modern-day example of ‘bubble mat’ forming within microbial mats at Waipahihi, TVZ. e Kohuamuri conical tufted facies showing individual tufts up to 5 cm in length. f Modern conical tufted sample from Whakarewarewa showing individual tufts. g Tall, steeply conical, tufted sinter microtexture in plane polarised light showing fossilised, stringy, fine microbial filaments (dark) surrounded by microcrystalline euhedral quartz (white); Kohuamuri deposit. h Modern conical filaments (dark) with high primary porosity (i.e. no mineral cement in the clear [white] areas); Whakarewarewa

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Warm to tepid (<40 °C) distal apron and thermally influenced marsh textures recognised at Kohuamuri included palisade lamination, plant-rich and clotted sinter (Fig. 6). Palisade textures are filamentous structures perpendicular to lamination, in which coarsely filamentous cyanobacterial mats (∼0.5–2 mm thick) today produced by the cyanobacterium Calothrix are entombed by silica (e.g. Fig. 6a, b) (Cady and Farmer 1996; Lynne 2012; Campbell et al. 2015a). Within the Kohuamuri sinter, palisade fabric was identified as discontinuous, thick, planar to wavy lamination, up to 1 cm thick, containing truncated micro-pillar structures in microcrystalline quartz (Fig. 6a), very similar to Holocene palisade fabric from Orakei Korako, TVZ (Fig. 6b). Moreover, silicified plant material included branches, twigs and reeds without preferential orientation within a very fine-grained quartz matrix (Fig. 6c). Plants become silicified in modern sinters at the cool margins of discharge aprons and in marsh areas (Fig.6d) (Channing et al. 2004). At Kohuamuri, clotted fabric is present in the black sinter on the northern side of Kohuamuri Stream, occurring within very fine-grained quartz with patches of irregular lamination, clots and rare plant material (Fig. 6e, f). In cool, stagnant, marshy margins of modern geothermal areas at YNP, buff coloured, fluffy, clotted microbial encrustations upon plants and sinter fragments are quite common (Fig. 6g), and clotted fabrics are associated with distal apron settings in the Holocene Umukuri sinter, TVZ (Campbell et al. 2001).

Fig. 6
figure 6

Distal apron sinter facies and silicified, thermally influenced marsh fabrics at Kohuamuri compared to Holocene examples. a Preserved palisade filaments (arrows) within the Kohuamuri deposit. b Modern palisade fabric comprising pillar structures perpendicular to wavy lamination; Orakei Korako, TVZ. c Large fossilised reeds in plant-rich sinter from the Kohuamuri deposit. d Plant material silicified today on the distal apron at Tokaanu, TVZ. e Cut slab of clotted fabric (dark, irregular layers) forming around plant remains; black sinter north of Kohuamuri Stream. f Thin section image of dark clotted microtextures encrusting darker filaments at Kohuamuri in plane polarised light. g Modern-day example of clotted fabric of fluffy microbial encrustations (buff greenish colour) forming within a thermally influenced marsh, Yellowstone National Park. h Preservation of filaments within wavy palisade fabric in plane polarised light with examples of well-preserved filaments (white arrow) and poorly preserved filaments (black arrow); Kohuamuri deposit

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In the inferred distal sinter apron facies at Kohuamuri, preservation in fine-grained quartz is rather patchy (e.g. Fig. 6a). For example, in Kohuamuri palisade fabrics, centimetre-scale bands of well-preserved, densely packed, vertical filaments commonly alternate with horizons of poorly preserved filaments (Fig. 6h), possibly reflecting variations in original porosity within laminae, differentially preserved by secondary silica infill. Alternatively, this depositional stacking pattern (filament-rich versus filament-poor, solid silica bands) could have been controlled by a ‘pulse-pause’ style of sheet flow across terrace areas, which may have shifted from continuous, to gradually or abruptly declining, or to locally ceasing altogether owing to channel switching, climatic conditions or episodic dormancy of a spring vent (Campbell et al. 2015a).

The locations of ten different textural types—nodular/spicular/pisoidal geyserite, finely laminated, network, bubble mat, conical tufted, palisade, plant-rich and clotted—from in situ samples reveal the palaeo-flow direction and palaeo-temperature gradient at Kohuamuri (Fig. 10). Facies mapping shows that vent fluids were expelled proximal to sites 9, 10 and 2. Furthermore, it can be inferred that the geothermal fluid expelled near site 10 flowed westwards to form a cool but thermally influenced marshland at site 5. The only in situ, moderate temperature, mid-apron texture was network fabric at site 11, 5 m to the southeast of the deduced vent area. This may have been an ephemeral pool that formed and then dried. It is also inferred that the extensive bedded sinter bluffs were produced in a mid- to distal apron setting, although textural details are obscured owing to strong diagenetic recrystallisation.

Mineralogy

Bulk XRD analysis utilising powdered representative samples showed the Kohuamuri sinter to be composed predominantly of quartz with no silica polymorphs. Optical microscopy verified that rims of some vugs contain chalcedony. Examination of these chalcedonic horizons using confocal Raman microscopy indicated the presence of minor moganite (<5 %), a distinctive monoclinic polymorph of silica. The chalcedony silica polymorph is believed to be a transition phase that forms between opal-C and quartz, and which typically obliterates primary textures (Rodgers et al. 2004).

At a late stage in the Kohuamuri paragenetic sequence, hydrothermal minerals deposited within vugs of several sinter textures, as revealed under plane- and cross-polarised light, and using SEM (Fig. 7). These include adularia, mordenite, iron oxides, jarosite, sulphur and pyrite which can form at relatively low temperatures in epithermal environments. For example, mordenite and jarosite develop at <120 °C (Browne 1984).

Fig. 7
figure 7

Mineral microtextures within vugs of the Kohuamuri deposit. a Mordenite (Mor) entombed in euhedral quartz (Qz). b Jarosite (Jrs, yellow) in high relief, post-dating quartz microcrystal formation. c Thin section of pyrite (Py, black) in geopetals, followed by quartz (Qz) microcrystal fill of vug. d SEM image of halloysite (h) growing on quartz microcrystals (Qz). e Grain of primary quartz (Qz) formed in a volcanic setting. f Dark, tuffaceous material (t) formed in a volcanic environment. g Clast of grey twinned plagioclase (Pl) altering to highly birefringent illite clay (Ilt). h Grey clast showing alteration of adularia to illite (Ilt) with adularia (Adl)

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The poorly sorted clasts within the hydrothermal breccia include volcanic materials, such as primary quartz crystals, plagioclase, tuffaceous sediments, as well as hydrothermal minerals (Fig. 7e–h). Plagioclase was partially to completely altered to illite (Fig. 7g); adularia also was partially altered to illite (Fig. 7h). The significance of these mineral occurrences is evaluated further in the discussion section below.

Trace element concentrations

Analysis detected most of the 21 targeted elements, with the exception of Ca, Mg and Se. The accepted pathfinder elements of Au, Ag, As and Sb in the Kohuamuri sinter showed typical signatures within the same range as other fossilised hot spring deposits (Table 2). The concentration of trace elements varied greatly (Supplementary Table 1), and while this may have had a primary cause, the differences also could have been influenced by spatially variable intensity of diagenesis, as observed by patchy preservation quality of filaments within the outcrops. Trace element concentrations are plotted in Fig. 8 along an inferred palaeo-flow path deduced from textural analysis in order to evaluate whether enrichment changed with distance away from the implied vent area, along the interpreted temperature gradient. Relatively high concentrations were measured for all elements for textures associated with vent and proximal slope facies, and the lowest values were detected in sinter facies of the distal sinter apron (Fig. 8). Furthermore, sinter facies analysed for trace elements from in situ outcrops and float blocks show a trend of decreasing enrichment from high- to low-temperature facies with respect to Au, Ag, As, Ba, Hg, Mo, S and Tl. These trends are skewed by the mixed mineralogy of the poorly sorted breccia. Excluding the breccia samples reveals that Au, Ba, Hg, Li, S and Tl were highest in concentration in spicular geyserite, which forms in the splash zone of geysers and vents. The proximal facies of finely laminated textures contained the most Ag, Fe, Mn, Wo and Zn. By contrast, the lowest concentrations of Ag, Au, As, Ba, Li and Mo occur within the plant-rich facies formed in distal sinter apron areas. Elemental associations indicate that certain metals display a positive correlation within the Kohuamuri deposit, such as Au and Ag; As and Au; and Ag, Ba, Cu, Hg, Pb, S, Tl and As (Fig. 9). Strong correlations also were noted with Ba, Hg, Pb and Sb. Antimony has a moderately positive correlation with Au and Ag.

Table 2 Comparative trace element concentrations (ppm) in the Kohuamuri sinter and other fossilised hot-spring deposits elsewhere; modified from Guido et al. (2010)
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Fig. 8
figure 8

Trace element concentrations of Au, Ag, As, Ba, Hg, Md, S, Sb, Th and Zn compared to distance from the inferred vent position at the Kohuamuri sinter based on textural analysis (see Fig. 10)

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Fig. 9
figure 9

Graphs showing positive correlation between certain elements within the Kohuamuri and Kaitoke sinter deposits, with covariation values. a Au vs. Ag; b Au vs. As; c Ag vs. As; d Hg vs. As; e Pb vs. As; f Ag vs. Sb; g Au vs. As

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Discussion

The identified ancient Kohuamuri deposit textures are similar to those now forming in active hot springs, as inferred from macro- and micro-scale comparisons (e.g. Figs. 4, 5 and 6). This enabled interpretation of a palaeo-thermal gradient (100 °C–ambient) of the discharging fluids. It also allowed the reconstruction of five facies assemblages containing 12 textural types within a proximal, middle and distal sinter apron setting (Table 1). Spatial facies relationships were mapped to delineate the inferred paleo-flow direction and paleo-temperatures of the in situ deposit (Fig. 10). Thus, palaeo-geothermal fluids were expelled proximal to site 10, possibly from a single vent.

Fig. 10
figure 10

Map showing the spatial relationship of in situ lithofacies for the Kohuamuri sinter. An arrow indicates the predominant direction of geothermal fluid flow during sinter formation. Inferred lithofacies associations also are outlined

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The vertical extent of bedded sinter, more than 10 m thick in some places, suggests that the Kohuamuri sinter was a relatively long-lived system (Guido and Campbell 2014). The 90 m vertical extent of the deposit also suggests that it had a high hydraulic head, as the deposit is both vertically and laterally extensive and shows no evidence of post-depositional faulting or uplift. The textures also suggest high volumes of water discharged from the system. For example, the large conical tufts (Fig. 5e) required maintenance of relatively deep pools of moderate temperature (40–60 °C) in which to form, similar to the large conical tufted features in the Jurassic Claudia palaeo-geothermal system of the Deseado Massif epithermal province, Patagonia, Argentina (Guido and Campbell 2014).

Based on textural, biotic and compositional evidence, the Kohuamuri siliceous sinter is inferred to have developed from near-neutral pH alkali chloride waters (Henley and Ellis 1983; Cady and Farmer 1996; Campbell et al. 2001, 2015a, b; Guido and Campbell 2011; Lynne 2012; Drake et al. 2014). Such sinters are far more common in the geological record than deposits of much thinner (millimetres to centimetres thick) silica residue or sinter formed in acid-sulphate-chloride waters, which are superficially similar in appearance but have quite different origins as products of acidic waters (Rodgers et al. 2004; Schinteie et al. 2007). The Kohuamuri sinter is composed predominantly of silica that preserves textures similar to those associated with organisms living in near-neutral pH alkali chloride waters today (Cady and Farmer 1996; Guido and Campbell 2011; Handley and Campbell 2011; Lynne 2012). The enrichment of Au, Ag, As and Sb in the Kohuamuri sinter is associated with near-neutral alkali chloride waters in active geothermal systems (Henley and Ellis 1983; McKenzie et al. 2001; Pope et al. 2005) and also occurs in the fossilised sinter of the Rhynie cherts, Scotland (Rice et al. 1995). Hydrothermal minerals can develop at relatively low temperatures in epithermal environments, e.g. mordenite and jarosite form at <120 °C, and the zeolite mordenite in the Kohuamuri sinter further implies that the deposit originated from thermal waters of near-neutral pH (Browne 1984). The presence of iron oxides and hydroxides, associated with pyrite and jarosite, are likely products of late oxidation produced either by weathering or overprinting by steam heated acid condensate as thermal activity waned (Browne 1991; Rogers et al. 2002).

It is not clear whether the Kohuamuri and Kaitoke deposits formed contemporaneously as part of the same geothermal system, as no detailed age information is available. Sinters can build up over large areas, such as at the Wai-O-Tapu field, TVZ or the Clepsydra/Fountain/Red geyser group and its extensive sinter apron at Fountain Paint Pots, Lower Geyser Basin, YNP. In addition, the focus of activity can laterally migrate over time due to sealing of subsurface ‘plumbing’ and changes in hydrologic flow paths produced by micro-earthquakes and fluctuations in the water table (Rowland and Simmons 2012) such that age relationships may neither be straightforward nor stratigraphic (e.g. Drake et al. 2014).

Entrained clasts from the inferred hydrothermal eruption breccia in the Kohuamuri deposit provide useful mineralogical information about the palaeo-hydrothermal system at depth (Browne and Lawless 2001). For example, plagioclase alteration to illite is evident (Fig. 7g) and occurs within active geothermal systems at temperatures >210 °C (Browne 1984; Junfeng and Browne 2000). Illite occurs within epithermal deposits such as at Kohuamuri, partially replacing plagioclase and adularia (Fig. 7g, h), and may be found close to ore-producing quartz veins (Simmons et al. 2005; Tosdal et al. 2009), such as in the Golden Cross Au-Ag epithermal deposit of the Hauraki Goldfield (Simpson and Mauk 2011). Adularia indicates the presence of highly permeable rocks hosting thermal waters of near-neutral pH that had boiled (Browne and Ellis 1970; Simmons and Browne 2000).

A focus on textural analysis, such as in this study, enables differentiation of sinters from other siliceous palaeo-surface features (e.g. silicified lake sediments, silicified volcanic rocks, pseudosinters or silicified travertines, water table fluctuations infusing silica into sediments), which may lead to misidentifications (Guido and Campbell 2011), such as has occurred within the CVZ (e.g. the Ohui ‘sinter’; Brathwaite et al. 2001). Where hydrothermal breccias contain sinter clasts, such as at Kohuamuri and Onemana (Stevens and Boswell 2006), and if they also show anomalous metal enrichment, it is necessary to determine if the metals are part of the brecciation event or were incorporated within the sinter clasts prior to brecciation. Spatially detailed studies with good outcrop and/or core coverage are required to determine the specific relationships between metal enrichment in surface sinters and breccias with respect to any ore deposition at depth. Furthermore, the metal enrichment in some sinters may be controlled by factors other than proximity to the vent, which also must be taken into consideration in any exploration study involving sinters. For instance, in modern sinters, metals are concentrated within certain stromatolitic facies, owing to the physiochemical influence of microorganisms (Seward and Sheppard 1986; McKenzie et al. 2001; Mountain et al. 2003). This is due to EPS associated with microbes providing highly reactive sorption surfaces for nucleation of inorganic phases, such as heavy metals (McKenzie et al. 2001). The magnitude of enrichment also may vary in individual sinter deposits owing to local temperature of silica precipitation, the original composition of the geothermal fluid (Ewers 1991) and the intensity of late silica overprinting or later diagenesis (Guido and Campbell 2009, 2011).

The Kohuamuri sinter contains above ambient concentrations of Au, Ag, As, Pb, Hg, Mo and Sb, as occur also in some modern and fossilised geothermal deposits (Henley and Ellis 1983; McKenzie et al. 2001; Groves 2007), and is similar to that reported from sinter clasts associated with the inferred hydrothermal breccia of the Onemana deposit in the Hauraki Goldfield (Stevens and Boswell 2006). At Kohuamuri, the average detected values of trace elements typical of epithermal mineralisation are within the range of those reported from commercial mining operations of the Moonlight sinter (Drummond Basin, Australia; Groves 2007), Red Butte sinter (Oregon; Zimmerman and Larson 1994) and McLaughlin sinter (California; Sherlock et al. 1995). The concentrations of these elements in the Kohuamuri sinter also are within the same range as those measured in the distal alteration halo of the Empire Vein, Golden Cross, Waihi, CVZ (Simpson and Mauk 2011). At Kohuamuri, a wide range in values was measured (>4 orders of magnitude), similar to modern active and extinct sinters at Wai-O-Tapu, TVZ (Wilson et al. 2012), and the Rhynie cherts, Scotland (Trewin 1993). A depletion in dissolved metals in thermal waters with distance from a vent was reported for Sb at Wai-O-Tapu, Champagne Pool, TVZ, where its concentration in the discharge channel dropped from approximately 150 μg/l at 100 m from the pool to below detection at 300 m distance (Pope et al. 2004). This suggests that the metals incorporated within some sinters, including Kohuamuri, may have been concentrated during sinter formation.

The positive correlation of certain trace element associations (Fig. 9) at Kohuamuri is similar to that observed today at Wai-O-Tapu (Pope et al. 2005) and Broadlands-Ohaaki (Simmons and Browne 2000), where this relationship is believed to be due to co-precipitation with colloidal sulphides containing these elements (Simmons et al. 2005). Other studies suggest that trace elements do not always precipitate in sulphides but may encrust opal-A in sinter deposits (Jones and Renaut 2003). The correlation of these metals within the Kohuamuri sinter suggests that sulphides may have been present within the epithermal-geothermal system. This is important because the precipitation of sulphides is required to form epithermal ore deposits at depth (Simmons et al. 2005), and such a positive correlation has been observed in other epithermal deposits in the Coromandel region, such as at Golden Cross, Hauraki Goldfield (Simpson and Mauk 2011).

This study is the first to analyse the trace element geochemistry of an ancient sinter across a suite of textures indicative of vent to marsh environments. The identified palaeo-geothermal gradient in the sinter fabrics shows a trend of increased metal concentrations (Au, Ag, As, Ba, Hg, Mo, S, Tl) with proximity to the vent, providing evidence that the fluid forming the sinter contained anomalous levels of precious metals rather than the enrichment being a product of late-stage hydrothermal or burial fluids. In the latter case, we would expect the trace element distribution to be more irregularly distributed throughout the sinter rather than relatively higher in the near-vent facies. In comparison, trace elemental studies of other sinters elsewhere (Table 2) have not listed environmental or diagenetic textures of sinter in relation to trace element distributions, which has introduced uncertainty in the use of siliceous hot spring deposits as indicators of mineralisation at depth. Thus, it is essential to establish a paragenetic framework for each sinter deposit in order to decipher the fluid history of a given palaeo-geothermal system. The Kohuamuri results illustrate the importance of identifying primary and diagenetic textures to determine at which point in the lifetime of an epithermal system metals may have precipitated or whether they migrated into it from elsewhere, such as in a late-stage hydrothermal overprinting event (McLaughlin deposit; Sherlock et al. 1995). A textural-geochemical approach of reconstructing the relative spatial and temporal distribution of trace metal concentrations with respect to the surface hot-spring discharge gradient opens up new possibilities for sinters to be effectively utilised as vectors for epithermal mineralisation.

Conclusions

The Kohuamuri sinter, enriched in precious metals (Au, Ag) and pathfinder elements (As, Sb) mineralized as sulphides, represents an ancient near-neutral pH alkali chloride hot-spring deposit where fluids at ≤100 °C were expelled through a vent located at the land surface, derived from reservoir temperatures >210 °C in which boiling appears to have occurred. This study shows how analysis of textures, mineralogy and element concentrations of siliceous sinter deposits may allow recognition of thermal fluid flow sites from reservoir to the land surface by recognising discharge areas in temperature-dependent palaeoenvironmental gradients. These data, which may be extracted from modern and fossil sinter, have implications for epithermal exploration as they provide a low-impact and relatively simple method for targeting possible sites of mineralisation at depth. This method could be used throughout the Hauraki Goldfield, and it has proven useful in exploration of other goldfields elsewhere.