The varied and potentially rewarding geology of northern New Zealand
The Northland region is geologically diverse and has a history of mining dating from the early years of European settlement.
A range of metallic minerals have been produced from a variety of deposit types. Mercury, silver and manganese were produced in the late 19th century, and in the early 20th century, antimony and copper were produced on a small scale. Deposits of non-metals diatomite, kaolinite clay, kauri gum, peat, serpentine and silica sand have also been worked. Five small coalfields have produced about 7.3 million tonnes of coal between 1856 and 1982. Currently mining is dominated by limestone produced for New Zealand’s largest cement plant at Portland, south of Whangarei, and for agriculture. High quality halloysite clay for ceramic manufacture is produced near Matauri Bay in the north of the region by industrial minerals company Imerys.
Over the last 50 years, mainly reconnaissance mineral exploration has been carried out in two main phases. During the 1960s and 1970s, exploration for metals was carried out by a number of companies, notably CRA Exploration Pty Ltd for copper, Western Mining Corporation Ltd for mercury, Consolidated Silver Company of NZ for antimony, Winstone Minerals Ltd for manganese, and Consolidated Zinc and Magellan Minerals for aluminium (bauxite).
The discovery in the early 1980s of the world-class McLaughlin deposit in California in a near-surface epithermal environment led to a reappraisal of gold-silver prospects with associated surface hot spring sinter deposits and mercury mineralisation. Recognition of similar epithermal style Hg-Sb-As-Ag mineralisation and hydrothermal alteration in eastern Northland led to a surge of precious metal exploration by several companies including BHP, Homestake and BP. When this was unsuccessful, exploration became sporadic.
To encourage further exploration, in 2011 the New Zealand government funded an airborne magnetic and radiometric geophysical survey over an area of 12 000 km2 that covered the Northland region (Barker and Robson, 2012; NZP&M, 2011; NZP&M, 2012; Stagpoole et al, 2012) (Figure 1). This has led to renewed interest in exploration with Tai Tokerau Minerals exploring the Te Mata and Hazelbrook prospects near Kaeo in the north and more recently, Evolution Mining has acquired permits to explore the Puhipuhi gold-silver prospect in the east of the region (Figure 2). The available data on the mineral potential and past exploration is summarised in a 2007 mineral resource assessment of Northland (Christie and Barker, 2007) that lists the original references for the information presented here. The New Zealand Petroleum & Minerals (NZP&M) exploration report database (www.nzpam.govt.nz) contains about 250 reports on Northland. Geochemical data for the region has been compiled by Carver (2011).
The oldest rocks that have been recognised to underlie Northland are northerly extensions of the Paleozoic Dun Mountain ophiolite belt that is exposed in the north of the South Island. Its extension into the North Island beneath Northland shows as a linear geophysical anomaly – the Junction Magnetic Anomaly (Figure 1), but these rocks are not exposed at the surface in Northland.
The oldest rocks exposed in the region are a sequence of Permian–Jurassic greywacke and argillite sediments that were accreted onto the margin of the Gondwana continent during the Mesozoic (Figures 2 and 3), and form the basement of the region. The sequence also includes basaltic volcanic rocks, chert and quartzite, and associated manganese deposits.
These basement rocks are overlain by mid-Eocene to Oligocene terrestrial and shallow marine sediments that host coal and limestone deposits. During the Early Miocene these rocks were overridden by thrust sheets of Early Cretaceous to Late Oligocene sedimentary and volcanic rocks of the Northland Allochthon. These include the Tangihua Complex of Cretaceous to Eocene age that consists of submarine basic volcanic and intrusive rocks and sediments. They form slabs (massifs) up to a kilometre thick and 40 km in diameter, and host copper mineralisation that has been worked in the past. Twelve of these are of significant size (more than about 1 km across).
Post allochthonous volcanic rocks form two north-northwest trending belts about 40 km apart. The western belt consists of basalt and andesite with minor dacite and rhyolite intruded by dykes and plugs. The eastern belt is interpreted to be a north-northwesterly extension of the Coromandel arc that hosts the highly productive Hauraki Goldfield containing epithermal gold-silver and mercury deposits that are also present in the correlative rocks in Northland. The volcanic rocks and basement greywacke are intruded by andesite and dacite dykes and small stocks of diorite and granodiorite of Early to mid-Miocene age with occurrences of porphyry copper style mineralisation.
A second phase of volcanism continued from the Late Miocene to Late Quaternary producing extensive basalt flows and scoria cones. The basalt volcanism is an alternative heat source for the genesis of the mercury epithermal mineralisation at Ngawha and Puhipuhi. Alkaline rhyolite domes in the north have weathered to white halloysite clay. Most of the rocks have been affected by deep subtropical weathering which has impeded exploration.
Mineral exploration and potential
Gold and silver
Gold and silver exploration for epithermal hot spring-type deposits has been the target of most recent exploration. All of the known gold-silver prospects in Northland (about ten in total – see Figure 2) are of this type. The two most extensively explored prospects are at Puhipuhi and Hazelbrook, near Kaeo. However, there is also potential in Northland for low sulfidation gold-silver, sedimentary gold and skarn gold deposits (Christie and Barker, 2007).
At Puhipuhi, in the east of the region (Figures 2 and 4), a group of cinnabar deposits (Mt Mitchell, Puhipuhi and Rising Sun; Figure 5) define a northerly trend over a distance of 6.4 km. Between 1917 and 1945 a total of 31.1 tonnes of mercury was recovered from here (Williams, 1974). Greywacke basement is unconformably overlain by Pliocene lake sediments (Purua Beds in Figure 4) and basalt flows.
Silver-bearing quartz veins in the basement greywacke, 2 km to the northwest and 100 m below the level of the cinnabar deposits, probably represent deeper level exposure of the Puhipuhi hydrothermal system. Recorded production amounts to only 42 kg of silver, and payable ore was confined to the upper or oxidised portion of the veins.
Recent exploration of the Puhipuhi field for epithermal gold has outlined a zone 4.5 km long by 1.0 km wide of geochemically anomalous Au, Ag, Sb, As, Hg and Ba contents in soil samples extending from the Plumduff Breccia (Figure 6) north to the Williams sinter (Grieve et al, 2006). BHP Gold reported that reconnaissance reverse circulation drill testing of fossil hydrothermal fluid feeder zones intersected 5.3 g/t Au and 18.5 g/t Ag over 10 m in a chalcedonic quartz vein cutting an eruption breccia. A nearby hole returned 12.4 g/t Au and 85 g/t Ag over 2 m. A deep (619 m) diamond drill hole beneath this zone intersected only weak mineralisation with gold values reaching a maximum of 0.2 g/t and silver 4.8 g/t.
To the north near Kaeo, cinnabar float and small quartz veins were recorded from streams draining the Huia area a century ago. Reconnaissance soil sampling and stream sediment surveys in the early 1970s revealed areas of alteration at Taraire and Huia, and Western Mining Corporation drill tested three small cinnabar orebodies at Huia, which contained values of up to 6.7 per cent Hg.
The Toolshed, Backyard and Eastern Anomaly areas of the Hazelbrook prospects define the 8 km long Huia Trend that is hosted in basement greywacke, overlain by Eocene greensand, and Miocene andesite. From 2004, exploration by Aurora Minerals identified the Backyard and Toolshed prospects. At Backyard, highly variable gold values of up to 6.9 g/t were recorded in outcropping quartz veins, the main vein cropping out at five locations over a 400 m strike length. Sixteen RC drill holes totalling 1600 m and four diamond drill holes totalling 1600 m intersected quartz veins and brecciation, but assays returned only trace gold.
Six other gold-silver prospects have been investigated in the east of the region where anomalous soil and rock sample results for gold, silver and base metals have been recognised, but only two, Te Pene and Puketotara, have been tested by limited drilling.
Copper occurs in volcanogenic massive sulfide deposits that are associated with the Tangihua Complex (Brathwaite et al, 2012). The mineralisation occurs as stratiform lenses of pyrite, chalcopyrite and sphalerite at the contact of, or within, mudstone and claystone interbedded with, or in close proximity to, the volcanic rocks. About 2500 tonnes of high-grade ore have been mined at three locations in the north of the region: Parakao, Pakotai and Pupuke. A fourth copper prospect, Purua, was discovered in 1971 by a soil geochemical survey that followed up a weak stream sediment anomaly. Drilling of five diamond holes totalling 125 m gave values up to 2.1 per cent Cu. Channel samples of trenches returned assays up to 3.5 per cent Cu.
Porphyry-style mineralisation has been recognised at three locations: Kerr Point, Knuckle Point and Coppermine Island, but only the latter deposit has been investigated in detail. Copper mineralisation (mainly pyrrhotite, pyrite and chalcopyrite) is associated with dioritic intrusive rocks (Miocene) in greywacke basement. A four-hole diamond drilling program by CRA Exploration in the 1960s indicated that grades were 0.03-0.11 per cent Cu, and that grade generally decreases with depth.
Bauxite deposits in the north of the region are New Zealand’s only significant occurrences. Resources total about 50 M tonnes in a number of deposits. The maximum thickness of the gibbsite-rich profiles is about 12 m. The bauxite is developed over basalt and is iron-rich.
Antimony has been produced from Lanigan’s Mine where stibnite is present within a 3 m wide silicified crush zone in basement greywacke. The deposit was first mined in 1907 and 115 tonnes of ore, at an average grade of 50-60 per cent Sb, were extracted and exported.
Stibnite also occurs at Puhipuhi, as radiating aggregates in sinter and silicified greywacke associated with mercury deposits. The mineralisation represents a fossil geothermal system. At Ngawha minor stibnite is associated with mercury in Pleistocene lake sediments and is related to the active geothermal system there.
Mercury deposits related to Plio-Pleistocene and active geothermal systems have been worked at Puhipuhi plateau and at Ngawha, which has been developed on a small scale to generate electricity by Top Energy since 1998.
At Puhipuhi, a group of cinnabar deposits occur in silicified greywacke basement, silcified breccia and overlying siliceous sinter. A resource estimate of 85 000 kg of mercury has been inferred.
Other mercury deposits have been prospected at Huia Hill near Kaeo and at Puketi (Doar prospect).
Lead-zinc skarn mineralisation has been recognised at Motukokako (Piercy) Island where disseminated sphalerite and galena occur in calc-silicate skarn minerals and in adjacent recrystallised Oligocene limestone. At Kauri Mountain near Whangarei a 1 m wide vein contains mixed sulfides, mainly sphalerite and pyrite, with chalcopyrite and galena. This vein is hosted in greywacke and may be associated with nearby andesite porphyry intrusives.
Manganese occurs in small manganese oxide deposits that are associated with mafic lava, chert and volcanic argillite of the Permian-Jurassic basement greywacke extending along strike southeast from Whangaroa Bay to Whangarei Harbour. Total manganese production between 1878 and 1911 from the most significant deposits was 19 364 tonnes.
In 1970-71, precious metal values including platinum were reported from drill holes at Puketotara west of Kerikeri in altered basalt where the Maungaparerua rhyolite dome was being drill tested for china clay. Forensic investigations following a complaint to the police determined that the samples had been deliberately contaminated by platinum wire. The broad zone of alteration was investigated in some detail including drilling over the following ten years but produced only low order gold-silver values and base-metal anomalies.
Industrial minerals that have been worked include halloysite ceramic clay (currently producing), kaolin clay for brick-making and near-shore silica sand deposits for glass-making. In addition, potential has been recognised for bentonite, feldspar sand, phosphate and zeolite.
Northland’s diverse geology and mining history has led to the region being explored for a wide range of metallic and non-metallic minerals. Most recent exploration has been for gold and silver with tantalising results being obtained from the Puhipuhi and Hazelbrook prospects. As a result of the combined radiometric and aeromagnetic survey that was carried out by the Government in 2011, new high-quality data is now available over the region (Figure 7) that identifies both deep and shallow geological features. Although the worldwide downturn in exploration activity since then has limited the application of this data to new exploration programs in Northland, interest in the mineral potential of the region is likely to revive when the investment outlook improves.
Carolyn Hume drafted the diagrams. David Skinner and Patricia Durance reviewed the manuscript. Funding for this review was provided by Science & Innovation of the Ministry of Business, Innovation and Employment.
Barker R G and Robson R N, 2012. New airborne geophysical data for the Northland Region, NZ. AusIMM Bulletin August 2012: 34-36.
Brathwaite R L, Christie A B and Lukovic B, 2012. Desktop study documenting the occurrence and geological characteristics of known VMS deposits associated with Northland’s Tangihua Complex. GNS Science Report, 2012/08, 43p. NZP&M open file report MR 4820. (www.nzpam.govt.nz)
Brathwaite R L and Pirajno F, 1993. Metallogenic map of New Zealand. Institute of Geological & Nuclear Sciences Monograph 3, 215 p.
Carver R, 2011. Notes on the Northland geochemical data. NZP&M open file report MR 4819. (www.nzpam.govt.nz)
Christie A B and Barker R G, 2007. Mineral resource assessment of the Northland Region, New Zealand. GNS Science Report 2007/06, 183p.
Grieve P L, Corbett G J and Leach T M, 2006. Conceptual models for gold exploration at Puhipuhi, Northland. In: Geology and Exploration of New Zealand Mineral Deposits (eds: A B Christie and R L Brathwaite), The Australasian Institute of Mining and Metallurgy Monograph, 25: 65-70.
NZ Petroleum & Minerals, 2011. Logistics report for a detailed airborne magnetic, radiometric and digital terrain survey for the Northland Project. NZP&M open file report MR 4800. (www.nzpam.govt.nz)
NZ Petroleum and Minerals, 2012. Northland 2012 – Northland region aeromagnetic survey. Data DVD data pack available from http://www.nzpam.govt.nz/cms/tools-and-services/geoscience-exploration-data?searchterm=Northland.
Stagpoole V M, Edbrooke S W, Christie A B, Davy B W, Caratori Tontini F, Soengkono S, Cox S C and Isaac M J, 2012. Northland airborne magnetic and radiometric survey: a geological interpretation. GNS Science Report 2011/54, p113. NZP&M open file report MR 4806. (www.nzpam.govt.nz). Report MR 4821 is an appendix that lists available reports and digital data.
Williams G J, 1974. The Economic Geology of New Zealand. Australasian Institute of Mining and Metallurgy monograph 4.