Silicate laterite nickel deposits are distributed throughout the island of New Caledonia, including the following:
• The Plateau mine at Thio is located on the northeastern coast of New Caledonia,
~75 km, NNW of Nouméa.
• The Nakety mine is ~15 km to the NW of Thio.
• The Canala mine is 17 km to the WNW of Nakety.
• The Mea/Kouaoua mine is 16 km to the WNW of Canala.
• The Kouaoua mine is ~50 km NW of Thio, and ~6 km north of Mea, and 6 km from the NE coast.
• The Népoui and Kopéto mines are near the SW coast of New Caledonia, ~190 km NW of Nouméa.
• The Koniambo deposit is located in the northwestern part of the island of New Caledonia, ~225 km NW of Nouméa.
• The Ouaco mine is ~45 km NW of Koniambo.
• The Etoile du Nord mine is ~22 km NNW of Ouaco.
• The Tiébaghi mine is ~23 km NNW of Etoile du Nord and 5 km from the SW coast
in far northern New Caledonia.
In 2015, the Plateau/Thio, Mea/Kouaoua, Népoui-Kopéto, Etoile du Nord and Tiébaghi mining centres were operated by Société le Nickel (SLN, a subsidiary of ERAMET). Ouaco, Poya, Nakéty and Kouaoua, operated by Nickel Mining Company (NMC, a joint venture between Société Minière du Sud Pacifique, SMSP and POSCO), whilst Koniambo is operated by Koniambo Nickel, a joint venture between the state owned Société Minière du Sud Pacifique (SMSP) and Glencore.
Tectonic and geological setting
The main island of New Caledonia is elongated in a northwest direction (bearing 305°) and is some 400 km long and 50 km wide. It lies ~1500 km east of Australia, ~2000 km north of New Zealand and just south of Vanuatu. It is an emergent section of the Norfolk Ridge, the eastern of two parallel ridges, the second being the broader Lord Howe Rise to the west, separated by the New Caledonia Basin.
These ridges are part of a large continental plate called Zealandia which drifted away from the Australian continent during the Late Cretaceous Gondwana break-up (Dubois et al., 1976; Crawford et al., 2003; Lafoy et al., 2005).
The Norfolk Ridge is a 32 km thick sialic crust (Dubois, 1969) covered by Oligocene to Recent sedimentary rocks (Dupont et al., 1975; Dubois et al., 1976; Daniel et al., 1976).
New Caledonia was attached to the eastern coast of Australia during the Palaeozoic and most of the Mesozoic, located somewhere near Brisbane (Gympie Terrane; Cluzel and Meffre, 2002). The primary basement was covered by several accreted terranes composed of volcanic, sedimentary and metamorphic units which produced a complex mosaic (Aitchison et al., 1992). This tectonic collage was developed from the Late Jurassic to Early Cretaceous and is related to the New Zealand Rangitata orogeny (Bradshaw, 1979; Paris, 1981). These Pre-Rangitata terranes form the basement to much of of New Caledonia.
At the close of the Cretaceous, the Tasman Sea began to open, accompanied by internal extension which produced the various ridges and intervening narrow basins that are characteristic of Zealandia (Davies and Smith, 1971). The New Caledonia basin lies on the west side of the Norfolk Ridge. It is a 3000 m deep basin, filled with 200 to 4000 m of sediments, dated as Palaeocene to Recent (Goslin et al., 1972.), best developed in the proximity to faults which bound the basin (Dubois et al., 1974; Launay et al., 1979).
The Loyalty ridge lies to the east of the Norfolk Ridge, and is interpreted to be an Eocene island arc with a back-arc basin (Cluzel et al., 1999, 2001). The intervening Loyalty basin is a remnant of the fore-arc basin formed during subduction. It is 2000 m deep, partially filled by major Oligocene to Recent terrigenous deposits, covering important extension faults on the edge of the basin (Weissel and Watts, 1975). The Mohorovicic discontinuity of this area occurs at 17 km under the basin and 24 km under the volcanic arc (Collot and Miségue, 1977; Weissel et al., 1977).
At the end of the Eocene, part of the Loyalty basin was obducted onto the Norfolk Ridge. This event is characterised by a variety of metamorphic rocks of blueschist to eclogite facies, and by widespread ophiolites which extend discontinuously from the Papuan Peninsula of Papua New Guinea to the South Island of New Zealand.
This succession of large scale events has produced 4 main groups of rocks in New Caledonia, as follows:
• The Late Carboniferous to Jurassic basement, resulting from the accretion of oceanic terrane on the eastern margin of Gondwana from the Permian to Late Jurassic. This forms a terrane that occupies the core of the island, bounded to the NE, SW, NW and SE and includes up to 3000 m of ophiolitic gabbro, dolerite, boninite, tholeiite, plagiogranite, chert, sandstone, siltstone; >7000 m of volcaniclastic sandstone, black tuffaceous siltstone and conglomerate; as well as an unknown thickness of calc-alkaline volcaniclastic sedimentary rocks, and several thousand metres of late schist and metagreywacke derived from volcanic-arc rocks.
• High pressure metamorphic terranes, in the northern section of the island, bounding the basement core. It is composed of Upper Cretaceous to Middle Eocene subducted metabasic volcanic and sedimentary rocks that have been metamorphosed to eclogite facies, grading into similar rocks, metamorphosed to blueschist facies. These are overlain by 200 to 1000 m of Upper Cretaceous coal, conglomerate, arkose, sandstone and siltstone, which is concordantly followed by >200 m of Upper Cretaceous to basal Eocene chert, limestone, marl and flysch.
• Obducted terranes, that occupy almost the entire southeastern 30% of the island. They also bound the central core as a broad strip along the southwestern side of the island and form a narrower strip along the central section of the northwestern coastline. Together these form a SE plunging antiformal structure draped over the central core. These include the:
Poya Nappe - <2000 m of Senonian (Upper Cretaceous) to Paleogene basalts, hyaloclasites, diverse sedimentary rocks, representing obducted marginal basin oceanic crust, largely confined to the SW coastal margin.
Ultramafic Terrane - <4000m of Upper Cretaceous peridotite, gabbro, chromitite and felsic rocks, representing obducted oceanic crust and mantle.
• Post-obduction Terranes, developed on the far northern tip of the island, and along the southwestern coastline. These terranes occurred after the subduction and obduction events and is largely characterised by late intrusions and sedimentary rocks and important tropical alteration processes of the older rocks. They include,
Post-obduction granitoids - Late Oligocene granodiorite, adamellite.
Post-obduction Fluvial and Lacustral Formations - Late Oligocene to Lower Miocene conglomerate, breccia to sandstone and calcarenite, the result of fluvial erosion of ultramafic terrane.
Since at least the middle Miocene, the whole of Zealandia has been carried along by the Australian plate, its eastern margin being subducted beneath the Pacific plate along the Vanuatu trench, 200 to 400 km to the east of New Caledonia (Auzende et al., 1995).
Structure
With the exception of the main transcurrent zone, no major tectonic features are evident in New Caledonia. Most of the terranes have faulted boundaries. Thrust faults underlie the obducted Poya and Ultramafic nappes terranes. These faults are easily visible by their low dips and the associated intense serpentinisation and mylonitisation at the contact between these terranes and the basement. Widespread serpentinisation is also evident throughout these terranes on the margins of dip-slip faults. The southwestern margin of the South Massif of obducted rocks is marked by a dextral transcurrent fault (Brothers, 1974), offsetting the western margin of the obducted mafic sheet, and spreading ophiolites massifs along the SW coast. Since the early Miocene, New Caledonia has undergone extensional faulting with a NW-NNW trending direction, controlling fluvial Neogene formations (Lagabrielle et al. , 2005; Chardon & Chevillotte, 2006).
The geological, structural and tectonic setting above is drawn from "Pirard, C., 2007 - Chapter 7, New Caledonia: Geology & Mining, Published by JogMeg Mineral Resource Information."
Mineralisation
The lateritic nickel ores of New Caledonia are developed over variably serpentinised peridotites (predominantly harzburgites and some dunites) of the Obducted terrane. Much of the ore in New Caledonia is now exposed in steep terrane, with deep erosion of the laterite/saprolite profile on the steep mountainsides. A typical profile from the SLN deposits for instance, can have up to 40 m or more of saprolite and laterite developed above bedrock. A generalised profile, from the top, is:
• a few meres of iron crust (0.3% Ni, 0.01% Co, 52% Fe, 1% SiO2, 0.1% MgO), underlain by
• a 0 to 10 m thick layers of red limonite with pisolites in the upper section (0.9% Ni, 0.08% Co, 50% Fe, 1% SiO2, 0.6% MgO),
• yellow limonite (1.4% Ni, 0.10% Co, 49% Fe, 3% SiO2, 2% MgO),
• earthy ore (2 to 3% Ni, 0.15% Co, 22% Fe, 30% SiO2, 16% MgO),
• soft saprolite (2.3% Ni, 0.08% Co, 10% Fe, 35% SiO2, 21% MgO),
• ore with boulders (2.5% Ni, 0.05% Co, 12% Fe, 39% SiO2, 28% MgO),
• rocky ore (3% Ni, 0.02% Co, 8% Fe, 43% SiO2, 33% MgO), and
• fresh rock (0.3% Ni, 0.01% Co, 6% Fe, 44% SiO2, 45% MgO).
The nickeliferous saprolites of New Caledonia are principally silicate 'garnierite' ores. These silicate ores are preferentially derived from serpentinised ophiolites. Serpentinite has a higher Ni content than the olivine of the protolith. Primary serpentinisation in turn is most strongly developed in shear zones, particularly those related to the late Eocene (37 Ma) over thrusting event where structures, including serpentinite mylonites, may be several hundred metres wide. These structures control both the development of serpentinite and the depth and degree of weathering. The degree of serpentinisation in the un-weathered rock also determines the type of saprolite produced, from earthy saprolite over poorly serpentinised peridotite, to smectite rich greenish earthy saprolites on moderate serpentinisation, to 'hard' pink to green nickel rich 'garnierite' silicate saprolites from strong serpentinite development. Colour however is not indicative of grade which may only be determined by assay. The nickeliferous 'garnierite' ores are composed of three Ni-Mg sheet silicates, i). serpentines containing the bulk of the nickel, both residual primary (lizardite) and secondary (nepouite); ii). talc (willemseite); and iii). smectite-kerolite (pimelite). Nickel is also contained in limonite derived from the direct weathering of olivine. The nickel is enriched both through leaching and from concentration of nickel into serpentine and limonite from downward migrating Ni rich aqueous solutions leached from the overlying laterites. The original 'garnierite' ores mined from 1875 and which contained the mineral garnierite, were largely depleted by the 1930's.
Société le Nickel (a subsidiary of ERAMET) operates a 60 000 tpa smelter at Doniambo, near Noumea in New Caledonia which it feeds from its four mines at Thio, Kouaoua, Népoui-Kopéto and Etoile du Nord. This facility produces both ferro-nickel and nickel matte. The smelter treats around 3 Mt of ore per annum. SLN, which was formed in 1880 to mine lateritic nickel in New Caledonia is the dominant producer on the island. It is 90% owned by the French company Eramet, which is in turn 55% owned by the French State. The lateritic ores mined by SLN are principally silicates within nickel enriched saprolites.
The Koniambo deposit is operated by Koniambo Nickel, a joint venture between the state owned Société Minière du Sud Pacifique (SMSP), and Glencore. The deposit was tested by Falconbridge Nickel and SMSP, who completed 35 000 m of drilling testing of the lateritic nickel deposit in the late 1990s. Construction began in 2007, with the first production in 2014.
The small mines of New Caledonia, e.g., the SMGM Tontouta Mine, produce a large percentage of the nickel from the island, commonly as high grade direct shipping ore.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)
