Detailed Inventory

Beach and dune deposits on our coast, are some of the largest of this type in the world. The onshore deposits include the present beach and dune sand, and older coastal sand deposits that have been preserved by uplift due to faulting and/or lowering of sea level. Similar deposits in Japan, the Philippines and Indonesia have been mined to a limited extent.

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The coastal sand deposits are subdivided into two geographic areas — a northern Auckland-Waikato region and a southern Taranaki-Wanganui region.Two land based mining operations already exist in the northern region, at Port Waikato and Taharoa. There are eight other medium to large deposits on land in the North Island, each containing a resource of greater than 20 Mt of ironsand concentrate.

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The following descriptions are drawn from the website of NZPAM, and are a review of the ironsand resources of the west coast of the North Island by Brathwaite (1990) and Geological Resource Map reports for Manukau (Kermode et al., 1992) and Te Kuiti (Christie et al., 1992).

An extensive sandspit, some 50 km in length, extends northwards from Muriwai to Kaipara South Head (Schofield, 1973), however only the southernmost 3.2 km carries sufficient titanomagnetite to be of economic interest. Vegetated Holocene dunes cover an area of 1.8 km2 and have an average thickness of 12 m. The resource estimate of Nicholson and Fyfe (1958) of 87.8 Mt of sand at 20% magnetics grade (17.6 Mt of concentrate), was based on 14 drill- holes, 4.5–9 m in depth, none of which reached sea level. A subsequent assessment by Kear (1962), using the Nicholson and Fyfe results together with drilling results from Kaiser Steel Ltd, determined an inferred resource of 90 Mt of sand at 15% magnetics grade (13 Mt of concentrate). This estimate was based on an average thickness of 12 m, which is the average height of the dunes above sea level.

Vegetated and drifting Holocene dune sand fills a small coastal embayment and extends for up to 3 km inland along stream valleys. No drilling or sampling has been done. Very approximate resource estimates of 20 Mt grading 15% magnetics (3 Mt of concentrate) in Kear (1962), are based on conservative estimates of the area and thickness (9 m), and a grade extrapolated from other areas.

A 200–400 m wide strip of vegetated Holocene dune sand extends for about 2.3 km along a shallow coastal embayment. The resource estimate of 3.45 Mt of sand grading 24% magnetics (0.8 Mt of concentrate) (Nicholson and Fyfe, 1958), was based on 5 holes, 3–6.5 m in depth.

A low lying sand flat of mainly drifting Holocene sand extends northwards for 6 km from the north head of Manukau Harbour. The shoreline here is highly susceptible to change, and four phases of erosion and accretion over the past 150 years are recognised. The present 600 ha sand flat was built up only since the 1930s (Williams, 1977). The resource estimate of 17 Mt grading 31% magnetics (5.3 Mt of concentrate) by Nicholson and Fyfe (1958) was based on only 3 holes of 2.7–5 m in depth. Later work was carried out by Winstone Ltd (Carlson, 1972).

The Awhitu Peninsula extends for 40 km from Manukau South Head to the mouth of the Waikato River. It is an ancient sandspit, largely elevated to 150–180 m above sea level, and built up of Awhitu and Karioitahi formations, with Hood, Bothwell and Mitiwai formations mainly restricted to the southern part of the peninsula. Awhitu Formation is generally low in magnetics and only two limited areas of economic grade are known — Hamiltons Gap and Irwins (4.7 km north of Hamiltons Gap).

The Maioro area extends for 16 km along the southern part of the Awhitu Peninsula and is bounded on the south by the Waikato North Head mine site area. A coastal strip of mainly Holocene vegetated dunes (0.5–1 km wide) overlies Bothwell and Karioitahi formations to the east. In 1961, 46 holes were drilled on a reconnaissance 900 m grid pattern. Most holes were drilled to 12 m depth and four went down to about 30 m. Kear (1962) divided the area into two blocks for resource assessment. North of Karioitahi (Coast) Road, the drifting Holocene sand dunes were estimated to contain an inferred resource of 200 Mt at an average grade of 10%. The underlying older sand contained on average 4% magnetics. One hole, sited in the gap in the coastal hills through which the coast road passes, intersected an average of 14% magnetics over 27 m, below 12 m of Holocene sand.
 

The block south of Coast Road was estimated to contain an inferred resource of 150 Mt of Holocene dune sand at an average grade of 21%, with 50 Mt at 12% in the underlying Bothwell Formation. A subsequent round of fill-in, check and deeper drilling, carried out by New Zealand Steel (New Zealand Steel, 1973), showed higher grades extending to slightly greater depths than the earlier drilling, so that a slightly larger resource is indicated.

Ironsand deposits on the northern side of the Waikato River are up to 120 m thick, with local very high grades (up to 75% magnetics). They comprise sands of the Waiuku Blacksand Member (Hood Formation), and the younger Mitiwai Sand, localised within a fault angle depression on the north side of the east-northeast striking Waikato Fault (Waterhouse and MacArthur, 1989). The ironsand deposit is predominantly composed of dune sand, with minor interbedded beach sand, tephra and mud (Christie, 1979).

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Based on a 100 m grid drilling pattern, in situ resources have been estimated at 780 Mt of raw sand at an 18% magnetics grade (140 Mt of concentrate) (Waterhouse and MacArthur, 1989). Airborne magnetic surveys (Lawton, 1979) indicated that additional resources extend below sea level offshore to the west, and southwards under the Waikato River. Mining of the deposit began in 1969, and production to the end of 1996 totalled about 12.75 Mt of concentrate. Current mining produces 1.4 Mt of concentrate per year using two large 1000 tph bucket wheel excavators linked to the concentrating mill by two moveable bench conveyors and a central trunk conveyor (see cover). In the concentrating mill, titanomagnetite grains are separated from non-magnetic minerals and clay in a wet magnetic separator followed by final upgrading by gravity separation using cones and spirals. The concentrate is pumped as a slurry 18 km to the Glenbrook mill.

An active sandspit and overlying sand dunes cover an area of about 7.5 km2, with an average height of about 4.5 m amsl with some parts over 30 m amsl. Kear (1979) noted that a single sample contained 22% magnetics, but he considered this to represent a maximum for the deposit. He tentatively estimated total resources to be more than 30 Mt at less than 22% magnetics.

Kaihu Group sands occur along a low lying stretch of coast in the Waikorea and Waimai streams area, bounded to the north, east and south by Tertiary rock with thin dune cover. Scout drilling by New Zealand Steel indicated that the southern part of the area was the most prospective, particularly south of Waimai Stream, because of preservation of a considerable thickness of the titanomagnetite-rich Waiuku Blacksand. Inferred resources of ironsand in three of four blocks total 20 Mt of concentrate (42 Mt of sand at 47% magnetics); the northern (fourth) area was not included because of low grade (Waterhouse, 1973). The average grade for the Waimai area of 44% magnetics is considerably above average mine grades at Waikato North Head (18%) and Taharoa (35%).

At the north head of Raglan Harbour, coastal sands of the Kaihu Group have accumulated in an embayment on the northern side of a large headland formed by the Mt Karioi basalt-andesite volcano of Early Pleistocene age. The Raglan deposit was investigated with three shallow drill holes in 1949 (Nicholson and Fyfe, 1958), and was later drilled (11 drillholes) to an average depth of 22 m on a reconnaissance 600 m grid pattern in 1960–61 for New Zealand Steel (Kear, 1962; Waterhouse and White, 1994). In 1972, a further 10 holes were drilled to close up the grid to 300 m centres and to deepen some of the earlier holes to about 30 m depth. As shown by the drilling, Bothwell and Mitiwai formations overlie enriched Hood Formation with titanomagnetite-poor Awhitu Formation marking an economic base of the deposit. The average titanomagnetite contents of the sand units were determined as: A whitu (2.5%), Hood (45%), Bothwell (12%) and Mitiwai (16%). The Mitiwai Formation increases in titanomagnetite concentration (up to 24%) inland. A resource of 25.5 Mt of concentrate has been estimated from drilling and is made up of 15.5 Mt indicated (78.7 Mt containing 19.7% magnetics) and 10 Mt inferred (49 Mt at 20.4% magnetics).

Large deposits of Kaihu Group sands occupy part of a coastal depression between the headland formed by the Mt Karioi volcano and greywacke basement rocks at Albatross Point 18 km to the south. The shallow, but extensive Aotea and Kawhia harbours are also contained within the depression. Older ironsand-poor formations (Awhitu Formation) crop out to the north of Aotea Harbour, and large dune fields of Holocene (Mitiwai) sand are found at the north head of Aotea Harbour and between Aotea and Kawhia harbours. Nicholson and Fyfe (1958) estimated a resource of 500 Mt of sand above sea level with an average grade of 5% magnetics (equating to 25 Mt of concentrate) in the dune sand deposit at the north head of Aotea Harbour. However, the grade estimate is based on only three shallow holes and appears low in comparison with an average grade of 9% magnetics at the nearby Kawhia deposit.

The extensive dunefield at Taharoa is the largest of the ironsand deposits. It occupies a coastal depression, some 8 km in length and extending inland for about 2 km. The depression was formed by down-faulting of the basement greywacke to depths of up to 120 m below sea level, as determined by seismic refraction (Lawton, 1979), along east-west striking faults. The greywacke is directly overlain by Pleistocene– Holocene sand formations of the Kaihu Group (Stokes et al., 1989). The currently worked deposits at Taharoa are loose grey aeolian sands of the Mitiwai Formation with titanomagnetite contents averaging about 40%, but up to a maximum of about 70%. These are underlain by less titanomagnetite-rich aeolian and beach sands of older formations of the Kaihu Group.

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Reconnaissance drilling by Nicholson and Fyfe (1958) and Kaiser Steel Ltd (in Kear, 1962), and more detailed drilling by New Zealand Steel in 1969, resulted in an indicated resource figure for the Mitiwai Formation of 593 Mt grading 35% magnetics (208 Mt of concentrate). The subsurface geometry of the ironsand deposit was investigated using geophysical techniques (magnetics, gravity, seismic refraction and electrical methods) by Lawton (1979). From modelling of aeromagnetic anomaly patterns he inferred a total resource of 2560 Mt (565 Mt of concentrate), some of which is below sea level (200 Mt of concentrate). However, subsequent drilling has shown that the bulk of the deeper sands included in his estimates are of sub-economic grade (Stokes et al., 1989).

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Titanomagnetite concentrate has been mined from the site since 1972. An electric cutter suction dredge and floating concentrator extract and process sand, annually producing about 1.4 Mt of titanomagnetite concentrate (Fig. 5). The concentrate is pumped as slurry through a pipe to an offshore loading buoy and into an ore carrying ship. Total exports to the end of 1996 were 31 Mt, mainly to Japan, with small quantities to South Korea and China.

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Harihari area: A high level weathered remnant of Awhitu Formation caps a hill of Mesozoic greywacke about 1 km inland from the coast. The deposit lies at an elevation of about 150 m. Henderson and Grange (1922, p. 183) noted that “the deposit is horizontally bedded, and is evidently part of an ancient beach”. They described it as being about 30 m thick, covering an area of 1.2–1.6 ha, and containing about 1 Mt of “iron-ore”.

The extensive dunefield at Taharoa is the largest of the ironsand deposits. It occupies a coastal depression, some 8 km in length and extending inland for about 2 km. The depression was formed by down-faulting of the basement greywacke to depths of up to 120 m below sea level, as determined by seismic refraction (Lawton, 1979), along east-west striking faults. The greywacke is directly overlain by Pleistocene– Holocene sand formations of the Kaihu Group (Stokes et al., 1989). The currently worked deposits at Taharoa are loose grey aeolian sands of the Mitiwai Formation with titanomagnetite contents averaging about 40%, but up to a maximum of about 70%. These are underlain by less titanomagnetite-rich aeolian and beach sands of older formations of the Kaihu Group.

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Reconnaissance drilling by Nicholson and Fyfe (1958) and Kaiser Steel Ltd (in Kear, 1962), and more detailed drilling by New Zealand Steel in 1969, resulted in an indicated resource figure for the Mitiwai Formation of 593 Mt grading 35% magnetics (208 Mt of concentrate). The subsurface geometry of the ironsand deposit was investigated using geophysical techniques (magnetics, gravity, seismic refraction and electrical methods) by Lawton (1979). From modelling of aeromagnetic anomaly patterns he inferred a total resource of 2560 Mt (565 Mt of concentrate), some of which is below sea level (200 Mt of concentrate). However, subsequent drilling has shown that the bulk of the deeper sands included in his estimates are of sub-economic grade (Stokes et al., 1989).

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Titanomagnetite concentrate has been mined from the site since 1972. An electric cutter suction dredge and floating concentrator extract and process sand, annually producing about 1.4 Mt of titanomagnetite concentrate (Fig. 5). The concentrate is pumped as slurry through a pipe to an offshore loading buoy and into an ore carrying ship. Total exports to the end of 1996 were 31 Mt, mainly to Japan, with small quantities to South Korea and China.

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Harihari area: A high level weathered remnant of Awhitu Formation caps a hill of Mesozoic greywacke about 1 km inland from the coast. The deposit lies at an elevation of about 150 m. Henderson and Grange (1922, p. 183) noted that “the deposit is horizontally bedded, and is evidently part of an ancient beach”. They described it as being about 30 m thick, covering an area of 1.2–1.6 ha, and containing about 1 Mt of “iron-ore”.

A high level weathered remnant of Awhitu Formation caps a hill of Mesozoic greywacke about 1 km inland from the coast. The deposit lies at an elevation of about 150 m. Henderson and Grange (1922, p. 183) noted that “the deposit is horizontally bedded, and is evidently part of an ancient beach”. They described it as being about 30 m thick, covering an area of 1.2–1.6 ha, and containing about 1 Mt of “iron-ore”.

A small area of moving and fixed (Mitiwai Formation) dunes, up to 50 m above sea level in height, occupies a coastal embayment at the mouth of the Marakopa River. The Mitiwai Formation overlies greywacke basement. The larger part of the deposit lies on the northern side of the river, where three holes averaging 4 m deep were drilled by Nicholson and Fyfe (1958). The average grade calculated from these holes is 53% magnetics, but this is biased by the distribution of the drillholes. An overall grade of the order of 45% magnetics is probably more reasonable. The inferred resource estimated by Nicholson and Fyfe was 55 Mt of raw sand, giving 29 Mt of concentrate at 53% grade or 25 Mt of concentrate at 45% grade.

The coastline south of Tirua Point is mainly formed by cliffs of Tertiary sediments, except where the major rivers have cut wide river valleys. At the mouths of these rivers lie beach, sandspit, dune and alluvial sand deposits.

Deposits of Mitiwai Formation at the mouths of the A wakino and Mokau rivers were investigated by the 1949 Nicholson and Fyfe (1958) survey. At Awakino, two holes drilled in the sandspit on the north side of the river mouth, showed grades of 63–65% magnetics over a depth of 2.4–10 m. Nicholson and Fyfe estimated an inferred resource of 9 Mt of sand at an average grade of 63% magnetics (5.8 Mt of concentrate). The three holes drilled in the Mokau deposit showed an average grade of 37% magnetics but only partly tested the full thickness of the sand above sea level. An inferred resource of 10.5 Mt of sand at 37% magnetics (3.9 Mt of concentrate) was estimated.

Older raised beach and dune deposits, equivalent to the Rapanui Formation of the Patea-Wanganui area, cap coastal terraces along 20 km of coastline from Awakino south to Tongaporutu. These older deposits have not been prospected, but, although they contain ironsand at the base, they are overlain by up to 24 m of andesite sand and laharic debris as in the New Plymouth area to the south.

Five small deposits of Holocene Patea Dunesand occur at intervals along 13 km of coastline from Waitara to Fitzroy. The deposits were investigated in the 1940s (Hutton, 1945; Beck, 1947), and in 1961 a reconnaissance drilling programme was carried out to check on the previous work (Kear, 1962). The average thickness of the deposits is 3 m, with an average grade of 24% magnetics, decreasing with depth. The magnetic grains contain a high proportion of composite grains with inclusions of silicate minerals, which reduces the iron content of the concentrate to an average of 46% Fe compared with the overall average of 56% Fe for west coast ironsands.

The high proportion of composite grains (middlings) is a reflection of proximity to the andesite source rocks of Mt Taranaki. A few of the drill holes penetrated older dune sand below the Patea Dunesand, but generally it is underlain by beds of clay, lignite and andesite debris. The potential for discovery of significant additional quantities of economic grade ironsand, either along the coast from, or inland, from the known deposits is regarded as poor.

This is a coastal strip of Holocene Patea Dunesand, 1 to 4 km in width, extending 35 km southeast from Manutahi to Waitotara River. The area, which includes the Waipipi deposit, was drilled and assessed by the Mines Department in 1939 (see Mason, 1945; Kear, 1962). This work was subsequently reviewed and extended by further sampling in 1961 (Kear, 1962). The resource assessment was essentially confined to the Patea Dunesand which was shown by drilling to range in thickness from 2.4–7.3 m. The resource was estimated at 45.5 Mt of raw sand at 31.8% magnetics (14.5 Mt of concentrate) as drill- indicated, plus 28 Mt of sand at 32% (9 Mt of concentrate) as inferred in lateral extensions of the drill-indicated blocks.

The underlying older formation (Rapanui Sand) was reported as being low grade (8% magnetics) although little systematic testing appears to have been done. Kear (1962) noted that there was a volume of over 16 Mt of Rapanui Sand in the Waverley area (Waipipi) that would be worth testing if an ironsand mining operation was developed in the area.

The Waipipi deposit lies within the Patea area described above and consists of Patea Dunesands overlying Rapanui Dunesand, with a lignite-clay unit defining the economic base of the deposit (Judkins, 1989). The Rapanui Dunesand is crossbedded and to some extent cemented by limonite and interstitial clay. The dune sands contain layers of peat, clay, volcanic ash, iron pan and organic material. The ironsand orebody extends over an area of 20 km2 and averages 9.75 m in thickness.

The Waipipi deposit was mined between 1971 and 1987 using a floating dredge similar to that at Taharoa (Fig. 6). During this period, 15.7 Mt of concentrate at a grade of 56% iron and 7.8% TiO2 was recovered from 60 Mt of sand, and exported to Japan where it was used for blending in blast furnace steel production. The project was started by Marcona Corporation which later became part of BHP- Utah International. When the operation closed, some 92 Mt of raw sand at 26% magnetics (24 Mt of concentrate) and 10 Mt of tailings at 24% magnetics (2.4 Mt of concentrate) were left unmined. The ore reserve estimation was based on drilling on sections 250 m apart, with later fill- in drilling at 125 m centres for mine planning and grade control.

This area extends for 27 km between the Wanganui and Waitotara rivers and includes the Westmere, Kai Iwi, Okehu and Nukumaru dune complexes. These dune complexes were investigated by Fleming (1946) and resource estimates of the Holocene dunesands were based on calculation of the volume of the dunes from elevation data and surface sampling. The resulting resource estimate, combined with that for the area between Nukumaru dune complex and the Waitotara River, gives a total tonnage of 236 Mt of sand at 16% magnetics (37 Mt concentrate) Kear (1962).

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Assessments of the ironsand potential of the older sands (Rapanui and Brunswick) were given by Fleming (1953) and Kear (1962), the latter being based on a 1961 survey. For the Rapanui Sands a total inferred resource of 300 Mt at 10% magnetics (30 Mt of concentrate) was estimated by Kear, although the grade figure is based on only limited surface sampling and is conservative. The Rapanui Sands are overlain by 6 m of overburden. For the Brunswick Sands, Kear concluded that the average grade of 7% magnetics was uneconomic although the basal beach sand beds (3–6 m thick) are richer (18.5% magnetics from 4 samples).