Provenance of titanomagnetite in ironsands on the west coast of the North Island, New Zealand

Accepted Manuscript Provenance of titanomagnetite in ironsands on the west coast of the North Island, New Zealand

R.L. Brathwaite, M.F. Gazley, A.B. Christie PII: DOI: Reference:

S0375-6742(17)30216-9 doi: 10.1016/j.gexplo.2017.03.013 GEXPLO 5910

To appear in:

Journal of Geochemical Exploration

Received date: Revised date: Accepted date:

18 October 2016 23 February 2017 26 March 2017

Please cite this article as: R.L. Brathwaite, M.F. Gazley, A.B. Christie , Provenance of titanomagnetite in ironsands on the west coast of the North Island, New Zealand. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Gexplo(2017), doi: 10.1016/j.gexplo.2017.03.013

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Provenance of titanomagnetite in ironsands on the west coast of the North Island, New Zealand

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Corresponding author: R. L. Brathwaite

Co-authors: M. F. Gazley

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GNS Science, PO Box 30-368, Lower Hutt, New Zealand Email: [email protected]

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CSIRO Mineral Resources, ARRC, PO Box 1130, Bentley 6102, Western Australia, Australia, and School of Geography, Environment and Earth Science, Victoria University of Wellington, PO Box 600, Wellington, New Zealand Email: [email protected]

A. B. Christie

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GNS Science, PO Box 30-368, Lower Hutt, New Zealand Email: [email protected]

ACCEPTED MANUSCRIPT ABSTRACT

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Previous studies on the provenance of titanomagnetite-rich ironsands on the west coast of the North Island, New Zealand, have suggested that the main source is from erosion of andesitic volcanic rocks of the Taranaki volcanoes, with a lesser contribution from ignimbrites of the Taupo Volcanic Zone (TVZ) in, and north of, the Waikato North Head ironsand deposit. Here, we compare the results of electron probe microanalysis of titanomagnetite grains in samples of coastal and river sands (sinks), with a dataset (compiled from the published literature) of analyses of titanomagnetite in volcanic rocks erupted from Mt. Taranaki and TVZ volcanoes (potential sources). A principal components analysis was conducted on a five element (MgO, Al2O3, TiO2, MnO, FeO) dataset of Fe-Ti oxide compositions to identify groupings in multivariate space. The composition of titanomagnetites in the sinks from Whanganui to Waikawau Beach (southern sector) are similar to those of Mt.Taranaki andesites as sampled in the published literature; minor contributions from both Ruapehu andesites and TVZ ignimbrites are also present. The composition of titanomagnetites in the sinks at both Waikato North Head deposit and in the ironsands from Taharoa to Muriwai Beach (northern sector) are consistent with a composition of titanomagnetites that is present in the rivers that drain the Taranaki volcanoes and their ring plain debris avalanche and lahar deposits, but not a composition that is represented in the literature for Mt. Taranaki volcanic rocks. A minor proportion of the titanomagnetite grains in the Waikato North Head ironsand deposit and the coastal ironsands from Port Waikato north to Muriwai Beach appear to be sourced from ignimbrites of the TVZ.

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Keywords: Taranaki andesite, Taupo Volcanic Zone ignimbrite, Ruapehu andesite, electron probe microanalysis (EPMA), principal components analysis (PCA).

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1. INTRODUCTION

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Titanomagnetite ironsand occurs along the west coast of the North Island, New Zealand (Fig. 1), both onshore and offshore (Kear, 1965; 1979; Christie & Brathwaite, 1997; Carter, 1980). Mining of ironsand deposits at Waikato North Head (WNH), Taharoa and Waipipi since 1969 has produced magnetite concentrate for local steel manufacture and for export to Asian steel mills. Recent exploration, both offshore and onshore, has defined potentially economic resources offshore from Patea (Christie, 2016; http://www.ttrl.co.nz/projects/south-Taranakibight/) and onshore at Aotea (Meyers et al., 2010; Wood et al., 2016) (Fig. 1).

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Fig. 1. Distribution of ironsand and volcanic source rocks along part of the west coast of the North Island. The offshore percent ironsand contours are from Carter (1980). The black and

ACCEPTED MANUSCRIPT grey arrows show the magnitude of northwards and southwards longshore current directions (from Gorman & Laing, 2003; Briggs et al., 2009). TVZ = Taupo Volcanic Zone.

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The concentration of titanomagnetite is variable along the coast and therefore knowledge of its provenance is important to define supply points to the coast for modelling coastal concentration mechanisms and where economic concentrations of titanomagnetite may occur. Previous workers (Kear, 1965; 1979; Williams, 1974; Stokes & Nelson, 1991), considered that the ironsands were derived from erosion of the Taranaki andesite volcanoes, although Hamill and Ballance (1985) suggested that all of the ironsand north of the Waikato River mouth could have been sourced from the TVZ via the Waikato River. More recent provenance studies (Laurent, 2000; Swales et al., 2003; Briggs et al., 2009) of the mineralogy, particle size and shape of 200 beach sand samples from Waitotara River in the south along 800 km of coastline north to Cape Reinga, indicated that black sand minerals were mainly derived from Taranaki andesites, with TVZ ignimbrites being a minor source for ironsands north of the Waikato River mouth.

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Coastal sands eroded from the Taranaki volcanoes and their extensive ring plain debris avalanche and lahar deposits have been transported from Cape Egmont northwards and southwards by a long period swell from the southern ocean, coupled with the prevailing southwesterly wind and wave action (Gorman & Laing et al., 2001; Swales et al., 2003; Briggs et al., 2009). Discrete ilmenite grains are a minor component of the coastal sands from the Waikato North Head ironsand deposit north to Muriwai (Nicholson & Fyfe, 1958; Hamill & Ballance, 1985; Barakat & Drain, 2006; Mauk et al., 2006), and do not occur south of the Waikato River mouth. This suggests input from TVZ sources via the Waikato River, because the voluminous TVZ ignimbrites contain minor ilmenite in addition to titanomagnetite. Other potential titanomagnetite and ilmenite sources in the Waikato and west Auckland regions are from the erosion of older volcanic rocks of the Alexandra Volcanics near Raglan (Karioi) and Kawhia (Pirongia) and of the Waitakere Group at Piha and Muriwai, but their contributions to the ironsands appears to be very local (Briggs et al., 2009). Extensive ilmenite-garnet coastal sands between Karamea and Bruce Bay on the West Coast, South Island, are derived from erosion of metamorphic and plutonic rocks (Gill, in McPherson, 1978; Brathwaite & Christie, 2006). However, the absence of ilmenite and garnet in the ironsands from Whanganui north to Cape Egmont rules out significant northward transport of these minerals during Quaternary low sea levels, when the North Island and South Island were joined (e.g. Newnham et al., 1999).

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Here we fingerprint the possible sources of the coastal ironsands along the west coast of the North Island by way of analysing the ironsand mineralogy and the chemical composition of the titanomagnetite. We report the results of electron probe microanalysis (EPMA) of titanomagnetite grains in samples from coastal and river sands and in samples from the sand sequence at the Waikato North Head ironsand deposit. The results of our EPMA are visualised using principal components analysis (PCA) together with the dataset of analyses of titanomagnetite in the volcanic rocks erupted from Mt. Taranaki and TVZ volcanoes. We note that laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses would have given useable precision on a wider range of trace elements (e.g. Dare et al., 2014; Nadoll et al., 2014), but because there are no published LA-ICP-MS data for titanomagnetite from the potential volcanic sources, we have chosen to analyse our samples using EPMA to allow for direct comparison to the published literature. 2. COMPOSITION OF TITANOMAGNETITE IN CALC-ALKALINE VOLCANIC ROCKS

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Titanomagnetite forms a solid solution series between magnetite Fe2+Fe3+2O4 and ulvöspinel (Usp) Fe2+2TiO4 at temperatures above 600°C (Buddington & Lindsley, 1964). Minor amounts of divalent Mg, Mn, Zn, and Ni can substitute for Fe2+, whereas Fe3+ can be replaced by trivalent Al, Mn, V and Cr (e.g. Bowles et al., 2011). The composition of Fe-Ti oxides in igneous rocks is related to the temperature, oxygen fugacity (fO2) and silica activity of the melt, which also governs the composition of coexisting Fe-Mg silicates (e.g. Buddington & Lindsley, 1964; Frost & Lindsley, 1991). Increasing temperature, with fO2 varying parallel to the common fO2 buffers, results in an increase in the TiO2 content of the titanomagnetite (e.g. Devine et al., 2003). Titanomagnetite in tholeiites tends to be enriched in Ti (50–80 mole% Usp), compared with those in calc-alkaline andesites and dacites (18–40 mole% Usp) (Frost & Lindsley, 1991). In the rhyolitic Bishop Tuff in California, the Mg, Al and V contents of magnetite increase with higher crystallisation temperatures, as determined by magnetiteilmenite equilibration temperatures, whereas Mn decreases (Hildreth, 1979; Anderson et al., 2000). Turner et al. (2011b) noted a positive correlation between MgO wt.% in titanomagnetite and in whole rock analyses in eruptive products from the Mt. Taranaki andesite volcano.

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3. FE-TI OXIDES IN TARANAKI AND TVZ VOLCANIC ROCKS

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The Taranaki volcanoes are comprised of three main andesitic volcanic centres (Kaitake, Pouakai and Mt. Taranaki/Mt. Egmont), which young southwards (Fig. 1). Kaitake and Pouakai are eroded stratovolcanoes that have K-Ar ages of 580 ka and 250 ka respectively (Stipp in Neall, 1979). Mt. Taranaki (also known as Mt. Egmont) forms a 2518 m high volcanic cone, which last erupted in AD 1755 (Neall et al., 1986). The oldest eruptive activity from Mt. Taranaki is stratigraphically dated at c. 200 ka (Alloway et al., 2005; Zernack et al., 2012), but much of the cone was constructed between 5 and 6 ka (Turner et al., 2011a). The 12 km3 cone edifice is small compared with the c. 150 km3 ring plain (Zernack et al., 2011). Volcanic activity from Mt. Taranaki is characterised by alternating phases of edifice construction and collapse, producing a thick apron of volcaniclastic rocks (debris avalanche and lahar deposits), with volumetrically smaller tephra deposits and minor lava flows and domes (Alloway et al., 2005; Zernack et al., 2011). The Taranaki volcanic rocks are geochemically classified as high-K andesites, ranging from high-alumina basalt to andesite, with the youngest Mt. Taranaki andesite being the most K-rich (Stewart et al., 1996; Price et al., 1999; Zernack et al., 2012). The andesitic rocks contain phenocrysts of plagioclase, augite, hornblende and minor (1–10 modal%) titanomagnetite, with rare ilmenite as a groundmass phase (Gow, 1967; Neall et al., 1986; Stewart et al., 1996; Price et al., 1999; Turner et al., 2009, 2011a, 2011b; Zernack et al., 2012). The TVZ is an active, 25 to 50 km-wide, volcano-tectonic zone along the Pacific-Australian Plate boundary in the North Island and has been the source of numerous volcanic eruptions since about 2 Ma. The volcanic rocks are calc-alkaline in composition with voluminous tephras, ignimbrites and rhyolites, and minor dacite and andesite lavas (e.g. Wilson et al., 1995). Rhyolitic volcanism, as ignimbrite with lesser rhyolite, is volumetrically dominant (>90%) and is concentrated in the central part of the zone (e.g. Graham et al., 1995; Wilson et al., 1995). At least 25 caldera-forming eruptions, including the 1100 ka Mangakino (Kidnappers), 335 ka Whakamaru I and 25.4 ka Oruanui super-eruptions, interspersed with smaller silicic eruptions, have originated within the TVZ (Wilson et al., 2009). The ignimbrites and rhyolites contain minor titanomagnetite (0.1–1.0 modal%) and lesser ilmenite as grains or phenocrysts (Ewart, 1963, 1968, Briggs et al., 1993; Shane 1998; Smith et al., 2004; Wilson et al., 2006; Saunders et al., 2010). In the southern TVZ, the andesitic

ACCEPTED MANUSCRIPT volcanoes of the Late Quaternary Tongariro volcanic centre have minor titanomagnetite (0.1– 3.8 modal%), averaging about 1.5 modal% in andesites from the Ruapehu volcano, along with rare ilmenite as phenocryst phases (Cole et al., 1986; Price et al., 2012).

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Because of the stability of titanomagnetite during non-tropical weathering, its chemical composition has been widely used to fingerprint the source of volcanic eruptive events in North Island tephra deposits (e.g. Kohn & Neall, 1973; Lowe, 1988; Cronin et al., 1996; Shane, 1998; Turner et al., 2009, 2011a, 2011b). Titanomagnetites in andesites erupted from Mt. Taranaki generally have lower Cr2O3 (<0.1%) and higher MnO (0.2–1.5%) than those from the Tongariro and Ruapehu andesite volcanoes (0.2–0.3% Cr2O3, 0.1–0.7% MnO) (Kohn & Neall, 1973; Lowe, 1988; Cronin et al., 1996; Turner et al., 2008, 2011a, 2011b; Price et al., 2012; Zernack et al., 2012) (Table 1). Cr2O3 has not been reported in EPMA of TVZ ignimbrite/rhyolite titanomagnetites as it is probably below the detection limit of c. 0.05%. TiO2 shows no clear pattern of variation between Mt. Taranaki (2.1–15.6% TiO2) and Ruapehu-Tongariro andesites (3.4–25.2% TiO2) or ignimbrite/rhyolite (6.8–14.5% TiO2) titanomagnetites (Table 1). MgO contents generally exceed 2.0% (mean 2.65%, Table 1) in Mt. Taranaki titanomagnetites (Cronin et al., 1996; Stewart et al., 1996; Turner et al., 2008, 2009, 2011a, 2011b; Zernack et al., 2012), whereas titanomagnetites from Tongariro andesitic tephra (Lowe, 1988) and Ruapehu andesite lavas have lower MgO contents (mean 1.6%, Table 1), which may be attributed to the Ruapehu lavas being derived from dacitic or rhyolitic melts (Price et al., 2012). TVZ ignimbrite/rhyolite titanomagnetites generally have MgO <2.0% (mean 0.77%, Table 1) (e.g. Ewart, 1967; Lowe, 1988; Shane, 1998; Wilson et al., 2006), which distinguishes them from Mt. Taranaki titanomagnetites. We have compiled the titanomagnetite EPMA data from these papers into a dataset (Table S1, summary in Table 1) in order to compare them to our EPMA ‘sink’ data.

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4. FE-TI OXIDES IN THE COASTAL AND RIVER SANDS

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The coastal sands from Whanganui north to Kaipara South Head are composed predominantly of plagioclase, augite, hornblende, titanomagnetite and andesite fragments (Hutton, 1940; Gow, 1968; Stokes & Nelson, 1991; Laurent, 2000; Swales et al., 2003; Briggs et al., 2009), although the sands north from the mouth of the Waikato River also contain significant quartz, hypersthene, K-feldspar, and ilmenite (Hamill & Ballance, 1985). Ilmenite abundances, reported by Nicholson and Fyfe (1958), increase from 0.2–1.5% at Waikato North Head to a maximum of 11–12% at Piha and Muriwai. The extensive dataset of titanomagnetite analyses of Mt. Taranaki and TVZ volcanic rocks that we have compiled can be used to distinguish the source of the titanomagnetite in samples of the coastal and river sands. Previous published analyses of titanomagnetite in the coastal sands are limited (n = 21, mainly averages), comprising four bulk magnetic concentrate, wet chemical analyses in Wright (1964) and six EPMA by Wright and Lovering (1965) from the Patea, Taharoa, Raglan and WNH ironsand deposits, averages of EPMA on a set of eight samples of magnetic fractions from the Waipipi deposit (Graham & Watson, 1980), and three average EPMA from the Taharoa deposit (Macorison, 2003; Macorison, et al., 2003). The analyses for Patea, Taharoa, Raglan and WNH (Wright, 1964; Wright & Lovering, 1965) all plot within the Mt. Taranaki andesite field on an MgO-MnO plot (Brathwaite & Christie, 2015), as do the averages for the Waipipi analyses (Graham & Watson, 1980) and the Taharoa analyses of Macorison (2003). 5. METHODS

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5.1 Sampling and Processing

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Samples used for EPMA were collected from beaches and rivers along the west coast of the North Island between Muriwai Beach near Auckland and the Whangaehu River in the Whanganui region (Fig. 2). Beach-sand samples were collected in the intertidal zone using a plastic pipe sampler (7.5 cm inside diameter) driven 10 cm into the sand to obtain a minimum 3 kg of sample. This sample was split to provide a 25 g subsample for processing by SGS Minerals Services using a Davis Tube apparatus to separate and measure the weight percent of the magnetic fraction. Polished thin sections were prepared from splits of the magnetic fraction, and for selected samples from a >3.3 SG heavy mineral separate. At the WNH deposit, samples were collected from face exposures and from drillholes in the central part of the deposit, and polished thin sections of the samples were prepared from splits of a >3.3 SG heavy mineral separate.

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ACCEPTED MANUSCRIPT Fig. 2. Digital terrain map of part of western North Island, showing location of ironsand samples. Digital data from https://data.linz.govt.nz/ 5.2 Electron probe microanalysis

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Electron probe microanalysis (EPMA) data for titanomagnetite and ilmenite were collected using the JXA-8230 SuperProbe at the School of Geography, Environment and Earth Sciences, Victoria University of Wellington, New Zealand. Elemental concentrations for minerals were analysed with an accelerating voltage of 25 kV, a 20 nA beam current and a focussed beam of 1 µm spot size. A set of natural and synthetic standards were used for calibration. Titanomagnetite grains in polished thin sections from a selection of the beach and river sand samples (n=35) were analysed for SiO2, FeO, TiO2, Al2O3, MgO, MnO, Cr2O3, V2O3 and ZnO. Particular care was taken to analyse homogeneous titanomagnetite grains with no intergrowths (i.e. grains shown in Fig. 3a, but not grains like Fig. 3b, c). Between 12 and 28 grains were analysed in each thin section (total n =602). 5.3 Data analysis

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Principal components analyses (PCA) of the EPMA data were conducted using a centred logratio transform (CLR) in CoDaPack v. 2.01.15 (Thió-Henestrosa & Martín-Fernández, 2005); and heat maps showing data-point density were created using ioGAS™ v. 6.1.

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6. RESULTS 6.1 Petrography

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Examination of polished thin sections of splits of the magnetic or >3.3 SG separates shows mainly grains of homogeneous titanomagnetite (Fig. 3a), and lesser two- and three-phase grains of magnetite with lamellae and spindles of titanhematite or ferri-ilmenite (Fig. 3b), as also noted by Wright (1964), and hematite in relict magnetite grains (Fig. 3c). Fig. 3b also displays fine black rods of aluminous spinel forming a trellis texture (Wright, 1964, Haggerty, 1991) in the titanomagnetite. The lamellae and spindle-shaped blebs of rhombohedral Fe2O3-FeTiO3 titanhematite or ferri-ilmenite (Fig. 3b) have probably been produced by progressive oxidation-exsolution of the titanomagnetite at high temperatures (Buddington & Lindsley, 1964; Wright, 1965; Turner et al., 2008). The end product of the oxidation is grains of hematite with titanomagnetite relicts (Fig. 3c). The homogeneous and two- and three-phase grains contain minor Fe-Mg silicates and apatite inclusions, and tiny (<5 µm) chalcopyrite and pyrite inclusions. Studies of titanomagnetite textures in andesites erupted from Mt. Taranaki show that homogeneous titanomagnetite is produced by fast ascent eruptions that do not allow time for oxidation-exsolution, whereas titanomagnetite with titanhematite or ferri-ilmenite intergrowths is formed by oxidation-exsolution during cooling in slow ascent eruptions (Turner et al., 2008).

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Fig. 3. Photomicrographs. (a) Homogeneous titanomagnetite grains, Urenui River mouth (reflected plain polarised light). (b) Titanomagnetite with oxidation spindles (white) of ferriilmenite/titanhematite and trellis-textured rods of aluminous spinel (black), Ohawe Beach (reflected plain polarised light). (c) Hematite lamellae (white) with relict titanomagnetite, Urenui River mouth (reflected plain polarised light). (d) Pumice fragment with titanomagnetite phenocryst (black), Waikato River at Mercer (transmitted plain polarised light).

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The other minerals found in the magnetic and heavy mineral fractions of the samples are mainly augite and minor hornblende. Grains of andesite with small phenocrysts of homogeneous titanomagnetite and titanomagnetite with titanhematite or ferri-ilmenite intergrowths are present in trace to minor amounts in many of the thin sections. Pumice grains with small phenocrysts of homogeneous titanomagnetite and titanomagnetite with titanhematite or ferri-ilmenite intergrowths are abundant in a sample (Fig. 3d) from the lower Waikato River at Mercer. Ilmenite occurs in minor amounts in samples at and north of the Waikato River mouth, as reported in previous studies (Nicholson & Fyfe, 1958; Hamill & Ballance, 1985; Barakat & Drain, 2006; Mauk et al., 2006). Ilmenite also occurs in the sample from the Waikato River, which is consistent with a source from the TVZ. Ilmenite is a minor accessory mineral in the rhyolitic pyroclastic rocks of the TVZ (Ewart, 1967; Shane, 1998); whereas it is rare or absent as a primary mineral in the Taranaki andesites (Stewart et al., 1996; Zernack et al., 2012). 5.2 Electron probe microanalyses As noted in Section 3 (above), and summarised in Fig. 4 and Table 1, MgO contents generally exceed 2.0% (range 0.89–5.53%) in Mt. Taranaki titanomagnetites, whereas

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titanomagnetites from Tongariro andesitic tephra, Ruapehu andesite lavas and TVZ ignimbrite/rhyolite have lower MgO contents (range 0.1–3.4%). A MgO-MnO plot (cf. Anderson et al., 2000) broadly discriminates between Taranaki, Ruapehu and TVZ ignimbrite/rhyolite titanomagnetite sources, although there is a large overlap between the fields (Fig. 5a). Because the published EPMA data for Mt. Taranaki andesites are sparse and mainly represented by thin, young (<12 ka) tephra units (Turner et al., 2008, 2009, 2011a, 2011b), we have included three source proxies from our EPMA dataset in Fig. 5a. These source proxies are titanomagnetite analyses from black-sand samples from the Patea, Stony and Manganui rivers, which drain directly from the Taranaki volcanoes and their ring plain debris avalanche and lahar deposits (Figs. 1, 2), and thus should only contain titanomagnetite from Taranaki andesitic rocks. The proxy titanomagnetite compositions tend to be more MgO-rich and MnO-poor, than those collected directly from the volcano itself. This is probably a reflection of the limited data available for Mt. Taranaki andesites compared to the proxy titanomagnetites.

ACCEPTED MANUSCRIPT Fig. 4. Box and whisker plots for titanomagnetite source and sink data: (a) TiO2, (b) MgO, (c) Al2O3, (d) FeO, and (e) MnO. The median is defined by a horizontal line, the box spans the interquartile range (IQR), and the mean is represented by the black circle. The upper and lower ‘whiskers’ are 1.5 times the length of the box. Samples outside of the whiskers are outliers (circles; 1.5 to 3 times the IQR) and far outliers (triangles; three times the IQR). Colour representation in all panels as per panel (a).

Table 1. Summary statistics of source and sink EPMA data. (see references in Section 3, above). MgO

Al2O3

0.66 27.72 9.05 8.92 27.06 7.22 10.33

0.12 6.04 2.21 2.32 5.92 1.24 3.09

0.37 11.01 2.97 2.85 10.64 1.88 3.70

Ruapehu (n = 106) Minimum Maximum Mean Median Range 25th percentile 75th percentile

3.36 25.12 12.49 12.19 21.76 10.07 14.34

0.12 3.35 1.60 1.51 3.23 1.15 2.03

2.07 15.57 8.55 8.06 13.50 7.10 9.85

Taupo Volcanic Zone (n = 135) Minimum Maximum Mean Median Range 25th percentile 75th percentile

FeO

MnO

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TiO2 Entire dataset (n = 967) Minimum Maximum Mean Median Range 25th percentile 75th percentile

0.08 1.82 0.60 0.60 1.74 0.47 0.71

0.59 11.01 2.55 2.40 10.42 1.88 3.18

62.67 82.25 75.65 76.09 19.58 73.28 78.73

0.10 0.69 0.37 0.36 0.59 0.28 0.43

0.89 5.53 2.65 2.73 4.64 1.82 3.18

0.90 7.59 3.19 2.98 6.69 2.46 3.95

69.25 93.46 78.08 77.66 24.21 74.98 80.29

0.23 1.48 0.73 0.72 1.25 0.58 0.88

6.81 14.52 10.29 10.29 7.71 10.00 10.54

0.39 2.42 0.77 0.60 2.03 0.57 0.87

1.23 2.95 1.62 1.51 1.72 1.43 1.66

76.72 84.40 80.84 81.16 7.68 80.12 81.64

0.44 0.98 0.68 0.66 0.54 0.63 0.71

Taharoa-Muriwai (n = 173) Minimum Maximum Mean Median Range 25th percentile 75th percentile

1.18 27.72 8.15 7.91 26.54 6.59 9.32

0.38 5.31 2.73 2.81 4.93 2.01 3.54

0.37 10.43 3.59 3.48 10.06 2.68 4.21

64.67 87.65 78.33 78.23 22.98 76.64 80.10

0.17 1.24 0.54 0.54 1.07 0.44 0.63

Waikato North Head (n = 160) Minimum Maximum Mean Median Range 25th percentile 75th percentile

1.53 14.82 7.96 7.95 13.29 6.54 9.15

0.16 6.04 2.40 2.53 5.87 1.58 3.17

0.88 7.64 3.10 3.13 6.77 2.23 3.79

71.57 87.07 78.98 78.67 15.50 77.06 80.75

0.08 1.35 0.61 0.59 1.27 0.48 0.70

Waikato, Mercer (n = 27) Minimum Maximum Mean Median Range 25th percentile

3.88 13.61 10.58 10.28 9.72 10.17

0.29 1.52 0.66 0.54 1.23 0.49

1.14 2.98 1.56 1.47 1.84 1.39

75.04 84.20 80.56 81.44 9.15 78.33

0.36 0.88 0.62 0.62 0.52 0.51

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Mt. Taranaki (n = 124) Minimum Maximum Mean Median Range 25th percentile 75th percentile

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62.67 93.46 78.57 78.68 30.78 76.74 80.80

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0.96

1.76

81.98

0.71

Whanganui-Waikawau (n = 242) Minimum Maximum Mean Median Range 25th percentile 75th percentile

0.66 18.61 8.30 7.98 17.95 6.91 9.52

0.45 5.09 2.75 2.77 4.64 2.25 3.27

0.93 9.67 3.41 3.22 8.74 2.56 4.04

69.53 84.43 78.50 78.55 14.89 77.12 80.23

0.20 1.82 0.62 0.61 1.62 0.48 0.73

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75th percentile

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Fig. 5. (a) MgO vs. MnO for titanomagnetite sources, including source proxies for Taranaki andesites (Patea, Stony, and Manganui rivers), showing outline of fields for TVZ ignimbrites and rhyolites, Ruapehu andesites and Taranaki andesites. (b) MgO vs. MnO for a selection of titanomagnetite coastal sand locations compared with fields for titanomagnetite sources.

ACCEPTED MANUSCRIPT A plot of MgO vs. MnO in titanomagnetite from a selection of samples from the coastal ironsands (Fig. 5b) compared with fields defined by the source volcanic dataset (Fig. 5a), indicates that Taranaki andesites are the predominant source, TVZ ignimbrites are a lesser source, and two analyses plot unambiguously in the Ruapehu andesite field. However, the fields overlap, and this plot only uses a selection of the samples, representing the geographic spread of the samples, because the graph would be too cluttered if all of the samples were used. Furthermore, only two of the analysed elements are used, and importantly, it is subject to the constraints of constant sum compositional data.

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The difficulties associated with using compositional data analysis in geochemistry were recently summarised by Buccianti and Grunsky (2014) and references therein; geochemical data are typically reported as compositions, which are subject to a constant sum (e.g. they must total 100% or 1,000,000 ppm). As such these data are “closed”; that is to say that for a composition of n-components, only n-1 components are required (Buccianti & Grunsky, 2014). The use of the log-ratio transform overcomes these constraints by converting the data into real number space (Aitchison, 1982, 1986). A centred log-ratio transform (CLR) was applied to this dataset to remove closure issues associated with geochemical data.

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6.3 Principal components analysis

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The EPMA dataset collected for the sink samples as part of this study (n =602, Table S1) contained nine elements (MgO, Al2O3, SiO2, TiO2, V2O3, Cr2O3, MnO, FeO, ZnO), but the published data for the possible sources of titanomagnetite only consistently contain five elements (MgO, Al2O3, TiO2, MnO, FeO). A composite dataset of EPMA data for titanomagnetite in Mt. Taranaki and TVZ volcanic rocks (sources) available in the literature (Cronin et al., 1996; Stewart et al., 1996; Shane, 1998; Wilson et al., 2006; Turner et al., 2008, 2009, 2011a, 2011b; Saunders et al., 2010; Price et al., 2012; Zernack et al., 2012) was compiled (n = 414). This gave a total dataset of n = 1020. Some samples did not have all five elements detected (namely, MgO (n = 2), TiO2 (n = 1), and MnO (n = 15), these samples were largely from the Ruapehu titanomagnetite data and were fairly evenly distributed around the data cloud for this source, thus their removal is unlikely to introduce a bias into the dataset. Additionally, samples with a high MgO content (MgO >7%) were removed from the dataset (n = 31), and again the majority of these samples came from the Ruapehu titanomagnetite data (n = 29), and two from Mt. Taranaki titanomagnetite data; they are not present in any of the sink data. Three ilmenites (TiO2 > 44%), and one Fe-oxide (91.8% FeO) were also removed from the sink data. This left a five-component dataset (MgO, Al2O3, TiO2, MnO, FeO) of source and sink titanomagnetite data where each element was detected in each sample of n = 967 (Table S1).

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Fig. 6. Principal components analysis of EPMA data for titanomagnetites from source and sink areas (input dataset is MgO, Al2O3, TiO2, MnO, FeO). Average compositions for Ruapehu, TVZ ignimbrite and Mt. Taranaki titanomagnetites are shown. Waikato River at Mercer is not shown. clr = centred log-ratio transform.

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To better differentiate groupings of titanomagnetite based on EPMA data, a PCA was conducted on a five element dataset using a CLR (Fig. 6). Principal component 1 (PC1) vs. Principal component 2 (PC2) account for 88.8% of the variance in the dataset. The PC1 is dominated by the strong negative alignment of eigenvectors for MgO and Al2O3 (i.e. the sensu strictu MgAl2O4 spinel component) to the axis, while FeO (i.e. magnetite/hematite, Fe3O4/Fe2O3) is associated with positive PC1 values. Principal component 2 is driven by MnO (eigenvector positive PC1 and PC2); and TiO2 (positive PC1 and negative PC2). Average compositions for each source (i.e. Mt. Taranaki, Ruapehu and TVZ ignimbrite) are shown as larger dots on Fig. 6. The large number of data points on Fig. 6 makes it difficult to clearly discern groupings of source and sink samples. To aid with interpretation of this figure, simplified versions were created that only show the average source along with heat maps showing data-point density (Fig. 7). Fig. 7(a-c) shows heat maps for the three sources which are then used to define the fields for these sources that are shown on Fig. 7(d-f) along with the heat map for the three sink areas. The groupings for titanomagnetite compositions for possible sources identified in Fig. 7(a-c) are a significant improvement on those presented in Fig. 5(a) as there is substantially less overlap between the fields.

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Fig. 7. Simplified representations of the PCA presented in Fig. 6, with heat maps showing point density. Source heat maps (a-c) for Ruapehu, TVZ ignimbrite and Mt. Taranaki are used to define the fields shown in panels (e-f) which present heat maps for titanomagnetites collected along the west coast. Average compositions for Ruapehu, TVZ ignimbrite and Mt. Taranaki titanomagnetites are shown as large dots. Outliers are shown as small black dots.

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To enable data exploration of the nine component dataset (MgO, Al2O3, SiO2, TiO2, V2O3, Cr2O3, MnO, FeO, ZnO) that we collected by EPMA, proxies for the different sources were generated by picking samples that plotted closely to means of each source (e.g. Stony, Patea and Manganui rivers represent Mt. Taranaki; Waikato Mercer represents the TVZ; and low MnO samples from Whangaehu River represent Ruapehu). The addition of SiO2, V2O3, Cr2O3, and ZnO to the PCA did not add to our understanding of the dataset, and accordingly that extended PCA version is not presented here. The SiO2 component is generally less than 0.1 wt% and likely represents silicate inclusions and Si in solid solution in titanomagnetite (Newberry et al., 1982). 7. DISCUSSION The bulk of the titanomagnetites in beach sands from the Whanganui to Waikawau Beach sector are chemically similar to the average composition for Mt. Taranaki in the literature, suggesting that this is the main source (Figs. 6, 7d). The analyses of titanomagnetite from the literature are from samples of tephra and lava from the young < 6 ka) Mt. Taranaki (Turner et al., 2008, 2009, 2011a, 2011b) and from two debris avalanche deposits, stratigraphically dated at c. 200–130 ka (Zernack et al., 2012), and thus only represent titanomagnetite compositions from limited time spans in the c. 200 ka eruption history of Mt. Taranaki. There are no published data on the compositions of titanomagnetites from the older Taranaki

ACCEPTED MANUSCRIPT volcanoes. The sinks of the Whanganui to Waikawau Beach sector, which are spatially the closest to Mt. Taranaki in the present study, have titanomagnetites that are dominated by the Mt. Taranaki compositions in the published literature (see references in Section 3, above). Minor TVZ and Ruapehu contributions are also apparent in the sinks of the Whanganui to Waikawau Beach sector. Despite Ruapehu and the TVZ being the most distal sources, both the Whanganui and Whangaehu rivers have their head-waters in volcanic rocks of the TVZ, including Ruapehu (Fig. 1), and could have transported titanomagnetites from the central North Island to the coast near Whanganui.

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Moving further north up the coast from the Whanganui-Waikawau Beach sector, the sinks of the Taharoa to Muriwai sector have a minor population near the composition for the average Mt. Taranaki composition as discussed above (Figs. 6, 7e). However, the bulk of the population sits between the average Mt.Taranaki and Ruapehu compositions, which do not correlate to a source that is present in the published literature.

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Titanomagnetites from the Waikato North Head deposit are considered separately (Figs. 6, 7f). This significant economic accumulation of titanomagnetite has been separated out from the sinks of the Taharoa to Muriwai sector. Given its location on the northern side of the mouth of the Waikato River, which runs from the central North Island, Waikato North Head is the most likely sink thatwill record a TVZ source. As for the two other sink areas discussed previously, there is a minor TVZ ignimbrite contribution, but no titanomagnetites with a composition akin to Ruapehu were identified. This is not surprising because Lake Taupo at the head of the Waikato River would act as a sink for titanomagnetite from Ruapehu. A minor proportion of the titanomagnetite grains in the Waikato North Head ironsand deposit and the coastal ironsands from Port Waikato north to Muriwai have compositions that match those from the TVZ and in the sink at Mercer on the Waikato River, 35 km upstream from the Waikato river mouth at Port Waikato. The absence of ilmenite south of Waikato North Head is consistent with the ilmenites at Waikato North Head, and Maioro Gap and Hamiltons Gap beaches being sourced from the TVZ.

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The contribution from the known compositions of Mt. Taranaki titanomagnetite is consistent with that of the broader Whanganui to Waikawau Beach sector. However, there remains a large population (as at Taharoa to Muriwai) which does not correspond to a known source dataset. Is this unknown population of titanomagnetites (in the Taharoa to Muriwai sector and the Waikato North Head deposit) sourced from eruptions from Mt.Taranaki and the older Taranaki volcanoes that have not been sampled in the literature? The heat map for the PC1PC2 space for titanomagnetite compositions from the Waikato North Head deposit is presented in Fig. 8(a) along with the data points for titanomagnetites from the Stony, Patea and Manganui rivers, which we have already identified as matching titanomagnetites sourced from Mt. Taranaki and its ring plain deposits. Whereas a significant group of these datapoints plot close to the average Mt. Taranaki composition, there are also a significant group of datapoints that plot close to the highest point in the heat map for titanomagnetites at the Waikato North Head deposit. This suggests that there are titanomagnetites that have been transported north from the Taranaki volcanoes that were not previously sampled and analysed in the published literature. We infer that this titanomagnetite composition, present in the rivers draining Mt. Taranaki and its ring plain, represents a time integrated composition of titanomagnetites that have been erupted over the c. 200 ka history of Mt. Taranaki volcano. The rivers will have sampled a spectrum of the debris avalanche and lahar deposits, thus their black sand concentrations may contain titanomagnetite grains sources from deposits ranging in age from <10 ka to c. 200 ka. The distribution of ironsands offshore (Fig. 1) indicates an older Taranaki source, because as described by Carter (1980) they were concentrated under

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littoral conditions during the Late Pleistocene to Holocene post glacial transgression (c. 25 – 6 ka). In this context, Christie et al. (2009) estimated that erosion of the Taranaki volcanoes has produced approximately 485 km3 of volcanic material containing 39 x 109 tonnes of titanomagnetite, much of which would be derived from ‘older Taranaki’ andesites. The observation that the bulk of the titanomagnetites at the Waikato North Head deposit have a different composition from the Mt. Taranaki source dataset is consistent with them having travelled further than those from the Whanganui to Waikawau sector that are dominated by titanomagnetite compositions close to the known Mt. Taranaki source dataset. A greater transport distance (and hence longer transport time) from Taranaki sources is also consistent with a trend of decreasing mean particle diameter of titanomagnetite, plagioclase, augite and hornblende in beach sand samples with distance north from Cape Egmont (Swales et al., 2003; Briggs et al., 2009).

Fig. 8. (a) Heat map for Fig. 7(f) for composition of titanomagnetite from Waikato North Head overlain by data points (small black dots) for Stony, Patea, and Manganui rivers, which flow directly off Mt. Taranaki. The large black, red and green dots are averages for titanomagnetite in Mt. Taranaki, Ruapehu and TVZ rocks, from Fig. 6. (b) Heat map for Fig.

ACCEPTED MANUSCRIPT 7(b) for composition of titanomagnetite from the TVZ overlain by data points (small black dots) for the Waikato River (at Mercer) which drains the TVZ.

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Contributions from the ignimbrites of the TVZ are minor, and minimal from Ruapehu andesites. Fig. 8(b) presents the published source data for the TVZ along with data points for titanomagnetite samples from the Waikato River, at Mercer, which drains the TVZ. This plot shows that the composition of titanomagnetite in the Waikato River is consistent with that in the published literature for the TVZ. Other potential titanomagnetite sources are from the erosion of basalts and basaltic andesites of the Pliocene-Pleistocene Alexandra Volcanics near Raglan (Karioi) and Kawhia (Pirongia) and of the Miocene Waitakere Group at Piha and Muriwai, but they appear to be only a minor, local source of titanomagnetite (Briggs et al., 2009). Middle to Late Miocene andesites of the buried offshore Mohakatino volcanoes and the associated volcaniclastic Mohakatino Formation (King & Thrasher, 1996; Edbrooke 2005), are another potential source. The Mohakatino Formation outcrops along the coast from Mokau north to Waikawau Beach (Fig. 2), but Stokes and Nelson (1991) noted that this formation only supplied a small proportion of material to the Pliocene-Pleistocene coastal sands of South Auckland. No analyses of titanomagnetite from any of the above mentioned potential sources have been published.

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8. CONCLUSION

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The new EPMA dataset that we have presented here for titanomagnetites from sinks along the west coast of the North Island, along with previously published data from potential volcanic source rocks, is consistent with the bulk of the titanomagnetite being sourced from andesites of the Taranaki volcanoes. Contributions from the ignimbrites of the TVZ are minor and minimal from Ruapehu andesites. The composition of titanomagnetites in the ironsands of the Whanganui to Waikawau Beach sector are similar to those from Mt. Taranaki as sampled in the available literature; minor contributions from both Ruapehu and the TVZ ignimbrites are present. Moving further north along the coast, the composition of titanomagnetites at both the Waikato North Head ironsand deposit and in the ironsands from the Taharoa to Muriwai sector are consistent with a population of titanomagnetites that is present in the rivers that drain Mt. Taranaki and its ring plain, but not apparently sampled in the published literature on in situ Mt.Taranaki volcanic rocks. A minor proportion of the titanomagnetite grains in the Waikato North Head ironsand deposit and the coastal ironsands from Port Waikato north to Muriwai appear to be sourced from the ignimbrites of the TVZ. The contributions of other potential titanomagnetite sources such as the basalts and basaltic andesites of the PliocenePleistocene Alexandra Volcanics and the Miocene Waitakere Group, and the andesites of the Miocene Mohakatino Formation, appear to be minor because of dilution by the voluminous supply from erosion of the Taranaki andesites. The occurrence of economic deposits of ironsands is ascribed to localisation in coastal embayments or depressions (e.g. Waikato North Head and Taharoa; Mauk et al., 2006), rather than variations in supply points along the coastline. ACKNOWLEDGEMENTS This research was supported by the Strategic Science Investment Fund of the New Zealand Ministry of Business, Innovation and Employment. Ian Schipper and Melissa Rotella are acknowledged for assistance with EPMA analyses. Jamie Ogiliev and Oliver Pullein of Bluescope NZ Steel are thanked for assistance with sample collection and data from the Waikato North Head ironsand deposit. Ben Durrant, Neville Orr and John Symes provided

ACCEPTED MANUSCRIPT technical assistance. We are grateful for the reviews of Steve Barnes and Ian Graham on an earlier draft of this paper. Helpful reviews by journal reviewers Roger Briggs and Patrick Nadoll improved the paper. REFERENCES

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We examine the provenance of titanomagnetite-rich ironsands, west coast, North Island, New Zealand.

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Electron probe microanalysis (EPMA) of titanomagnetite from coastal/river sands (sinks) were collected.

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A dataset of titanomagnetite EPMA analyses from potential sources was compiled.

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Principal components analysis of MgO, Al2O3, TiO2, MnO, FeO in titanomagnetite finds groups in multivariate space.

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Sink titanomagnetites are mainly sourced from Taranaki volcanoes, with minor Taupo Volcanic Zone contributions.

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