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Provenance of Ironsands

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Provenance of Ironsands Stratigraphy, mineralogy and provenance of the sand sequence at Waikato North Head, south Auckland Bob Brathwaite 1, Michael Gazley 2 and Tony Christie 1 1GNS Science, Lower Hutt , 2RSCMME, Level 2, 93 The Terrace, Wellington; CSIRO Mineral Resources, Perth Acknowledgement: Ian Schipper, Victoria University Wellington, for assistance with EMPA analyses. Jamie Ogiliev and Edward Smith of NZ Steel for assistance with sample collection and data. Funding from Science and Innovation, MBIE. GNS Science Introduction . The WNH ironsand deposit is contained within a c. 80-m thick sequence of coastal and river sands at the mouth of the Waikato River . The age of the sand sequence is inferred to be late Pliocene to Holocene in age from limited palynology, regional stratigraphic correlations and two carbon-14 (14 C) dates . Here we use mineralogical data, combined with age and environmental information from palynology, to determine the provenance and age of the sand sequence GNS Science Titanomagnetite ironsand occurs along the west coast of the North Island, both onshore and offshore. Mining of ironsand deposits since 1969 at Waikato North Head (WNH), Taharoa and Waipipi has produced titanomagnetite concentrate for export and for local steel production. The titanomagnetite is mainly derived from erosion of Cenozoic andesitic rocks of the Taranaki volcanoes. These andesites contain titanomagnetite as a minor (1-10 modal%) phenocryst phase. The large Quaternary ignimbrite eruptions from the TVZ are a minor source. The ignimbrites contain 0.1- 1.0% modal titanomagnetite. GNS Science Volcanic clasts and minerals eroded from the Taranaki volcanoes have been transported northwards and southwards from Cape Egmont by a long period swell from the southern ocean, coupled with prevailing southwesterly wind and wave action. Titanomagnetite and other heavy minerals are concentrated in beach and dune coastal sands. Ilmenite occurs in sands to the north but not to the south of the Waikato River mouth, Indicating derivation from TVZ sources via the Waikato River. TVZ ignimbrites contain minor ilmenite in addition to titanomagnetite. Other potential magnetite sources are the Alexandra Volcanics near Raglan, and the Waitakere Group, but their contributions are local (Briggs et al. 2009). GNS Science Petrographic Analyses Polished thin sections of magnetic fractions show mainly grains of homogeneous titanomagnetite Minor 2- and 3-phase grains of titanomagnetite with intergrowths of titanhematite and ferriilmenite, and black spinel rods Some grains of hematite with titanomagnetite relicts GNS Science Waikato River at Mercer Pumice particles with phenocrysts of homogeneous titanomagnetite abundant in a sample from the lower Waikato River - sourced from TVZ ignimbrites. Ilmenite occurs in samples from the WNH deposit and the Waikato River, consistent with a source from the TVZ, because ilmenite is present in ignimbrites erupted from the TVZ but is very rare in Taranaki andesites . Ilmenite also occurs in minor amounts north of the Waikato River mouth GNS Science Electron probe microanalyses (EPMA) of titanomagnetite Titanomagnetite grains in polished thin sections from samples of the sand units at WNH were analysed for FeO, TiO2, Al2O3, MgO, MnO, Cr2O3, V2O3 and ZnO using a JEOL JXA-8230 electron microprobe at Victoria University of Wellington. Between 12 and 28 grains were analysed in each thin section. Particular care was taken to analyse homogeneous titanomagnetite grains with no intergrowths. Because of the stability of titanomagnetite during non-tropical weathering, its chemical composition, mainly determined by EPMA, has been widely used to distinguish the source of tephra deposits erupted from Taranaki and TVZ volcanoes. We have used the extensive dataset of published titanomagnetite analyses of the Taranaki and TVZ volcanic rocks to distinguish the different sources of the titanomagnetite in samples of the coastal sands from the sequence at Waikato North Head MgO contents generally exceed 2.0% (1.6–5.2%) in Taranaki titanomagnetites, whereas Mt. Ruapehu andesites and TVZ ignimbrites have lower MgO contents (0.4–2.4%). GNS Science Automated Mineralogy Automated mineralogy data collected using a Tescan Mira field emission gun scanning electron microscope (FEG-SEM) running the Tescan Integrated Mineral Analyser (TIMA) software, located at CSIRO’s Research Centre in Perth. The TIMA software package conducts automated energy dispersive spectroscopy (EDS) mapping over an area of approximately 3 cm2 with a scan duration of 1.5 – 2 hours. A spectra-matching library is then used to automatically classify the EDS spectra as known minerals. The TIMA software package, operating in modal mineralogy mode, then gives mineral abundance in volume% and mass% (wt. %). Analyses of duplicate samples show that there is minimal sampling bias GNS Science Local geology 80-m thick sequence of coastal and river sands of late Pliocene to Holocene age occupies a depression at the north head of the Waikato River mouth Waikato North Head ENE striking Waikato Fault calculated from Port Waikato modelling of gravity data to down-fault Murihiku Terrane greywacke basement by 2.7 km GNS Science Waikato North Head 80-m thick sequence of coastal and river sands of late Pliocene to Holocene age occupies a depression at the north head of the Waikato River mouth GNS Science Stratigraphy . Three formations – Mitiwai, Hood and Awhitu sands - subdivided into units based their heavy mineral and clay contents (Barakat & Drain, 2006) . Predominantly composed of cross- bedded dune sands, with minor intercalated beach sand and interdune clays . Titanomagnetite-rich dune and beach sand units in the Waiuku Black Sand member and the Entrican Sand member, currently mined by New Zealand Steel GNS Science Entrican Sand GNS Science Mitiwai Sands . Mitiwai Sands subdivided into the Alder and Entrican Sand members, both grey dune sands with the Entrican Sand having a higher titanomagnetite content (ave. 14 % magnetics) . Mitiwai Sands underlain by a tephra consisting of a 1 – 4 m thick orange to yellow clay. 14C dates of 1930 ± 48 and 1854 ± 49 y. BP, on wood samples are consistent with a tephra source from the 1850 y. BP, Taupo eruption (Dingley, 2002) GNS Science Entrican Sand overlying Waiuku Black Sand GNS Science Hood Sands . Mainly dune sands with local interdune swamp beds . Waiuku Black Sand member consists of cross-bedded to laminated, titanomagnetite-rich (ave. 47% magnetics) dune sands, with thin mud layers . Paleosols may mark erosional events at times of lowered sea level during glacial periods . Pollen assemblage in peat indicates a warm, interglacial climate and age of <1 Ma GNS Science Awhitu Sands . Top of Awhitu Sands marked in drillholes by fluvial quartz-rich sand with minor pumice - River Sand Member – which overlies weathered tephra (Awhitu Clay) . Awhitu Sands are yellow-brown with minor magnetics (ave. 5%). Shallow marine or lagoon environment . Unconformable on Oruarangi pumiceous beds of late Pliocene age (2.5-1.7 Ma), from palynology and magnetic polarity data GNS Science Mineralogy and provenance EPMA . Comparison of EPMA of titanomagnetites in heavy mineral separates of surface and drillhole samples indicate that most of the titanomagnetite in the Entrican and Hood Sands is derived from andesitic rocks of the Taranaki Volcanoes; active since c. 0.58 Ma. Ignimbrites of the TVZ are a source of minor titanomagnetite and ilmenite, particularly in the Lower Hood Sand GNS Science Mineralogy and provenance TIMA . Suite of silicate and resistate minerals indicate additional sources, eg. minor olivine from late Pleistocene basalts of South Auckland, and almandine and epidote from metamorphic rocks . Titanomagnetite abundance greatest in the Waiuku Black Sand (up to 51%). Resistate minerals (eg. apatite, almandine, ilmenite, rutile, titanite, zircon) also highest (>3%) in this unit . Silicate minerals dominated by pyroxene, feldspar and quartz. Awhitu Sands have abundant quartz and feldspar GNS Science Correlations . Tephra at top of Hood Sands correlates with 1850 y BP Taupo Eruption . Titanomagnetite-rich dune and beach sand units in the Base of Waiuku Black Sand correlated with inception of volcanic activity at the 0.58 Ma Kaitake volcano (Mt Taranaki) . Pollen assemblage in peat at base of Lower Hood Sand indicates age of < 1 Ma GNS Science Summary I . An 80-m thick sequence of coastal and river sands of late Pliocene to Holocene age at the north head of the Waikato River mouth . Sequence contains titanomagnetite-rich dune and beach sand units in the Waiuku Black Sand and the Entrican Sand; currently mined by NZ Steel . South of the Waikato River mouth, the Oruarangi pumiceous beds containing distal ignimbrites derived from eruptions in the TVZ, are of late Pliocene age (2.5 – 1.7 Ma). They are unconformably overlain by the Awhitu Sands . Peat at the base of the Lower Hood Sand, contains a pollen assemblage that indicates a warm, interglacial climate and an age of less than 1 Ma . Mitiwai Sands underlain by a tephra consisting of a 1 – 4 m thick orange to yellow clay. 14C dates of 1930 ± 48 and 1854 ± 49 y. BP, are consistent with a tephra source from the 1850 y. BP, Taupo eruption GNS Science Summary II . EPMA of titanomagnetites indicate that the bulk of the titanomagnetite in the Entrican and Hood Sands derived from andesites of the Taranaki Volcanoes; active since c. 0.58 Ma. Also a contribution of titanomagnetite and minor ilmenite from the ignimbrites of the TVZ, particularly in the Lower Hood Sand . Automated mineralogy data reveal a suite of silicate and resistate minerals that indicate some other sources in addition to the Taranaki andesites and TVZ ignimbrites, eg. presence of minor olivine points to derivation from the late Pleistocene basalts of South Auckland . Relative abundance of the resistate phases vary widely through the section, with the abundance of almandine (along with epidote, presumably from a metamorphic rock source) being the most variable. Titanomagnetite abundance is greatest in the Waiuku Black Sand, comprising up to 51% of the classified grains GNS Science Entrican Sand overlying Waiuku Black Sand GNS Science.
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