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Nickel Laterite Ore Deposits: Weathered Serpentinites Charles R. M. Butt1 and Dominique Cluzel2 1811-5209/13/0009-0123$2.50 DOI: 10.2113/gselements.9.2.123 N Some Ni laterites are currently forming and are, in effect, in equilibrium with their present environment. Examples include Ni laterites developed on Miocene ophiolites in Sulawesi, where the present rainforest conditions are considered to have prevailed since the exposure of the ophiolites. The majority of deposits, however, have developed and evolved under climatic and/or tectonic conditions that have changed over time, thereby modifying the ore or deposits, producing a new ore type (Freyssinet et al. 2005; Golightly 2010). Multiphase development is typical of deep lateritic regoliths, especially in cratonic environments that have been exposed to subaerial conditions throughout much of the Phanerozoic (Butt and Zeegers 1992). This paper summarizes the factors controlling the origin and occurrence of these increasingly important deposits and the relationship between their chemical and mineralogical characteristics and their genesis. ickel laterite ores account for over 60% of global nickel supply. They are the product of intensive deep weathering of serpentinites under humid tropical conditions. Nickel is concentrated to over 1.0 wt% and is hosted in a variety of secondary oxides, hydrous Mg silicates and smectites. The formation, mineralogy and grade of the deposits are controlled by the interplay of lithology, tectonics, climate and geomorphology. Most deposits have a multi-phase development, evolving as their climatic and/or topographic environment change. The richest deposits (>3 wt% Ni) formed where oxide-rich regoliths were uplifted and Ni leached downwards to concentrate in neo-formed silicates in the saprolite. KEYWORDS : weathering, regolith, Ni laterite, secondary ore serpentinite, geomorphology, landscape evolution INTRODUCTION Nickel laterites are intensely weathered regoliths with one or more horizons containing exploitable reserves of nickel (Ni), commonly, cobalt (Co) and, rarely, scandium (Sc). They are defined by economic, rather than geological, criteria, namely that the Ni–Co grades and tonnages are sufficient for them to be mined, processed and rehabilitated with financial (and social) benefit. Nickel laterites are formed on serpentinites and, for those on partially or unserpentinized ultramafic rocks, serpentine minerals are commonly some of the earliest weathering products. Their global distribution is shown in FIGURE 1. DEPOSIT CHARACTERISTICS Lateritic Regolith Nickel laterites constitute 60 to 70% of the world’s Ni resources, but although they have been mined for about 140 years, until 2000 they accounted for less than 40% of global Ni production, the remainder being from sulfide ores. Lower grades, complex treatment processes, costly energy requirements and remoteness from centres of industrial demand and appropriate infrastructure contributed to their slow development. Historically, most production from laterites came from the rich deposits of New Caledonia, which have been mined since 1875, and deposits in Greece, the Urals (Russia, Kazakhstan), USA (Oregon, California) and Cuba. More recently, Ni laterites have been discovered and mined in many other regions, and production has increased in response to greater demand, new processing technology and reducing availability of sulfide ores. Total Ni production from laterite ores had risen to 46% of global supply by 2008; it exceeded 50% in 2010 and is expected to reach 60% in 2014 (Nikhil Shah, CRU International Ltd., written communication 2012). Nickel laterites also contribute 20–30% of the total Co supply (Wilburn 2012). Lateritic regoliths generally consist of many or all of the following horizons (from the base): saprock, saprolite, plasmic zone, mottled zone, ferruginous and/or aluminous duricrust or gravels, and soil (Eggleton 2001); saprolite may comprise over 80% of the total thickness of the profile. They have developed under humid tropical to sub-tropical conditions, under present and/or past climatic regimes. A lateritic regolith developed on ultramafic rocks may contain economically significant concentrations of Ni in one or more horizons, and it is these units that define it commercially as a “Ni laterite.” There are three general ore types, based on the dominant minerals hosting Ni: oxides, hydrous Mg silicates and clay silicates (FIG. 2, TABLE 1). This subdivision has important implications for processing and whether a deposit can provide economically viable ore. Most Ni laterite profiles have two ore types, an oxide component and either a hydrous silicate or a clay silicate component (Brand et al. 1998; Berger et al. 2011). Because of the different processing requirements for the different mineral hosts, most mines tend to exploit only one style of mineralization. At the Goro mine, New Caledonia, however, both the oxide and the underlying hydrous Mg silicate resources are exploited (Freyssinet et al. 2005; Golightly 2010). 1 CSIRO Earth Science and Resource Engineering Box 1130, Bentley, Western Australia 6102 E-mail: [email protected] Oxide Deposits 2 Pôle Pluridisciplinaire de la Matière et de l’Environnement Université de la Nouvelle-Calédonie, BP R4 98850 Nouméa, Nouvelle-Calédonie E-mail: [email protected] E LEMENTS , V OL . 9, PP. 123–128 Oxide deposits (limonitic ore) are dominated by Fe oxyhydroxides, principally goethite, in the mid to upper saprolite and extending upwards to the plasmic zone (FIGS. 2A, 2B, 3). Nickel is hosted mainly in goethite, by 123 A PR IL 2013 3), the whole profile is only 20–25 m thick, including about 10 m of ore (Elias 2002), whereas at Cawse, the regolith is commonly about 80 m thick, with up to 30 m of oxide and upgradeable silica–oxide ore. Hydrous Mg Silicate Deposits Hydrous Mg silicate deposits form in the mid to lower saprolite, with Ni concentrated in nickeloan varieties of serpentine, talc, chlorite and sepiolite, some of which are poorly defi ned and known informally as “garnierite” (FIGS . 2 C , 4). These are the highest-grade deposits (locally 2% to more than 5% Ni) and, historically, the majority of Ni laterites were of this type. They represent about 32% of total Ni laterite resources, with a mean grade of 1.44 wt%. Global distribution of Ni laterite. Interactive distribution maps showing deposit names, ore types and reserves can be accessed from Berger et al. (2011). FIGURE 1 substitution for Fe and/or by adsorption. Manganese oxides (e.g. asbolane, lithiophorite) are commonly abundant and are enriched in both Co and Ni. Oxide deposits have mean grades of about 1.0 to 1.6 wt% Ni and represent about 60% of total Ni laterite resources. The overlying upper plasmic horizon and vesicular and/or nodular, ferruginous duricrust are leached, and duricrust rarely contains more than 0.03% Ni. Downwards, the transition from Ni-rich ferruginous saprolite oxide ore to lower saprolite and saprock is marked by a sharp increase in MgO content, from less than 2% to more than ~20% (Mg-discontinuity) and an increasing abundance of silicates, such as secondarily altered serpentine and other hydrous silicates, smectites and remnant primary minerals. Many oxide deposits contain abundant secondary silica, mainly chalcedony and quartz (e.g. Cawse and Ravensthorpe, Australia; Onça and Puma, Brazil), resulting in the dilution of bulk Ni. Such silicification is a typical weathering product of ultramafic rocks, especially dunites and serpentinized dunites, in which the low Al content has restricted the formation of clays. Removal of the silica by crushing and screening is an essential beneficiation step, increasing the effective grade and the available resource. The depth of weathering and thickness of ore horizons in oxide deposits vary; at Moa Bay and East Pinares, Cuba (FIG. A B C Most hydrous Mg silicate deposits are developed on serpentinized, ophiolitic harzburgite peridotite and are best known from tectonically active regions of high relief in the circum-Pacific and Caribbean. Similar mineral assemblages in some deposits in the Urals (Glazkovsky et al. 1977) and Greece were formed by weathering of serpentinites in limestone karst environments, but it is perhaps debateable whether they are true Ni laterites. Spheroidal weathering is a common characteristic of deposits in New Caledonia, and the saprolite (boulder ore) consists of millimetre- to decimetre-scale blocks, with the intensity of weathering increasing from core to margin. Partly weathered primary lizardite, in which Mg has been exchanged by Ni, is a significant host mineral in many deposits (Golightly 1979; Pelletier 1996). In addition, Ni is hosted by a wide variety of neo-formed silicate minerals, some of which can be very Ni rich (3–40% Ni). Many of the latter occur as boxworks and veins, in places with secondary silica, following shears, joints and grain boundaries, and precipitated as coatings on the saprolite blocks. Due to the high relief and high erosion rates, the regolith is rarely thicker than 40 m, including 10–15 m of silicate ore. Where preserved, the upper parts of the profile are similar to those of oxide deposits, with comparable grades and resources, but have to be separated from the silicate ore due to different processing requirements. Clay Silicate Deposits Clay silicate Ni laterite deposits have only recently been recognized and exploited as an ore type, although the presence of thick Ni-bearing clays in regolith developed on serpentinized peridotitic bedrock has long been known. The principal ore minerals are Ni-rich saponite and smectite in the mid to upper saprolite and pedolith (FIGS. 2D, 5). The deposits generally have grades of 1.0–1.5 wt% Ni and represent about 8% of total Ni laterite resources. D Lateritic profiles developed on serpentinized ultramafic rocks showing the principal nickel laterite ores. (A) oxide; (B) partly silicified oxide; (C) hydrous Mg silicate; (D) Clay silicate FIGURE 2 E LEMENTS 124 Clay silicate deposits appear to be confined to sites having relatively low relief, mainly in shield areas in Western Australia (Murrin Murrin; Bulong), the southern Urals (Buruktal, Russia; Kempirsai, Kazakhstan), Burundi and Brazil, but San Felipe (Cuba) is also of this type. Murrin Murrin and Bulong are developed on serpentinized peridotite (mainly after olivine ortho- and mesocumulates) weathered to 40–60 m. The saprock and lower saprolite are composed of primary and weathered serpentine, A PR IL 2013 chlorite and saponite, locally with abundant secondary silica and magnesite. This passes upwards into smectiterich saprolite and plasmic zone that, together, form the ore. At Murrin Murrin, the smectites are intermediate between Al-montmorillonite, Al-beidellite, Fe-montmorillonite and Fe-nontronite, with Ni located in the octahedral layer (Gaudin et al. 2005). The upper boundary of the ore corresponds approximately to the Mg-discontinuity, above which there is a goethitic horizon, the equivalent of oxidetype mineralization, and, locally, hematitic duricrust. TABLE 1 OXIDE ORE Bedrock Geology FIGURE 3 Oxide ore on bedrock, East Pinares, Cuba. PHOTO : M ICK ELIAS, CSA G LOBAL FIGURE 4 Oxide (ox) and hydrous Mg silicate (Mg) ores, Plateau mine, New Caledonia E LEMENTS Goethite Oxide α-(Fe3+)O(OH) 2% Ni, 0.2% Co Asbolane Oxide (Ni2+,Co3+)xMn4+(O,OH)4.nH2O 16% Ni, >4% Co Lithiophorite Oxide (Al,Li)Mn4+O2(OH)2 1% Ni, ~7% Co HYDROUS MG SILICATE ORE FACTORS INFLUENCING THE DISTRIBUTION AND FORMATION OF NICKEL LATERITE Lithology Nickel laterites form almost exclusively on olivine-rich ultramafic rocks and their serpentinized equivalents, which represent a protore that contains 0.2 to 0.4% Ni. Some small deposits in Greece formed by reweathering of sediments containing lateritic debris derived from serpentinized peridotite (Valeton et al. 1987). The deposit type is partly influenced by the lithology of the ultramafic rocks. Peridotites can give rise to oxide and either hydrous Mg silicate or clay silicate assemblages, whereas dunites mainly form oxide deposits, many of which have abundant free silica that may dilute the ore. The degree of serpentinization of peridotites influences the nature and abundance of hydrous Mg silicates that form in profi les developed in free-draining environments (Golightly 1979; Pelletier 1996). On unserpentinized rocks, deposits tend to be oxide rich, with only minor silicate mineralization (e.g. West Soroako, Indonesia). In weakly to moderately serpentinized rocks, the silicate zone is thicker and consists predominantly of neo-formed “garnierites,” as Principal ore minerals in Ni laterite deposits. Most minerals are poorly crystalline and variable in composition. Ni lizardite népouite Serpentine (Mg,Ni)3Si2O5(OH)4 7Å garnierite Serpentine Variable, poorly defined 15% Ni Nimite Chlorite (Ni5Al)(Si3Al)O10(OH)8 17% Ni 14Å garnierite Chlorite Variable, poorly defined Falcondoite Sepiolite (Ni,Mg)4Si6O15(OH)2.6H2O Kerolite-willemseite Talc (Ni,Mg)3Si4O10(OH)2 10Å garnierite Talc Variable, poorly defined 6–33% Ni 3% Ni 24% Ni 16–27% Ni 20% Ni CLAY SILICATE ORE Nontronite Saponite Smectite Na0.3Fe23+(Si,Al)4O10(OH)2.nH2O ~4% Ni Smectite (Ca/2,Na)0.3(Mg,Fe2+)3(Si,Al)4O10 (OH2).4H2O ~3% Ni veins, fracture-fi llings and coatings, and Fe–Mg smectites formed from olivine. However, Ni is also hosted by altered primary lizardite, in which Ni has exchanged for Mg in octahedral sites (Manceau and Calas 1985). In highly serpentinized rocks, Ni-rich altered lizardite is the main ore mineral. Tectonic setting About 85% of Ni laterite resources are located in the accretionary terranes of the CircumPacific belt, the Caribbean and the Balkans, on mainly serpentinized ophiolitic dunite and harzburgite peridotite. The tectonic activity in these environments, especially the effects of uplift, results in a mostly free-draining regolith that promotes the formation of hydrous Mg silicate deposits, but high erosion rates limit their thickness and degree of preservation. The remaining deposits are found on serpentinites in cratonic terranes, formed from basal peridotites and dunites in Archaean to Palaeozoic layered intrusions, such as Niquelandia and Barro Alto (Brazil), Musongati (Burundi) and Wingellina (Australia), and from komatiitic peridotites and dunites, such as Cawse, Murrin Murrin and Ravensthorpe in the Archaean Yilgarn Craton (Western Australia). The antiquity, stability and generally low relief of the cratons have resulted in long periods of weathering and the formation and long-term preservation of deep lateritic regoliths; these factors favour the development of oxide and clay silicate deposits. Deposits in the Urals have characteristics of both tectonic settings, with some formed in regions of intense folding and faulting (“linear” deposits) and others in more stable platform environments (“areal” deposits). FIGURE 5 125 Clay silicate ore, Bulong, Western Australia A PR IL 2013 Structure Fractures, faults and shear zones in bedrock and regolith can strongly influence the thickness, grade and, in places, type of Ni laterite deposit. Mostly, these effects are passive, with pre-existing structures affecting drainage characteristics either by forming barriers to water flow or, more commonly, by increasing permeability and promoting deeper weathering and preferential concentration of Ni along the fracture zone. Similarly, movement contemporaneous with weathering on existing faults in bedrock and new low-angle shears in the regolith formed by slope failure may be the focus of Ni concentration. In New Caledonia, hydrous Mg silicates, quartz and other secondary minerals fi ll veins and tension cracks, and occur as overgrowths and coatings in boxworks and fault breccias (FIG. 6). Multiple striations and differing mineral habits and assemblages indicate that tectonic activity and supergene mineralization were synchronous and occurred under changing weathering conditions (Cluzel and Vigier 2008). High Ni concentrations in saprolite at the La Gloria and Exmibal deposits in Guatemala are interpreted to be associated with shears developed when terrace blocks were downfaulted and became detached from the lateritic plateau on the margin of a graben (Golightly 2010). A B C Climate Most Ni laterite deposits occur in the present humid tropics (FIG. 1). Many of the Indonesian deposits (e.g. Soroako, Weda Bay) and some in West Africa (Sipolou, Conakry) and South America (Onça, Puma, Vermelho, Cerro Matoso) have rainforest climates characterized by >1800 mm of rain per year and dry seasons of less than 2 months. Most deposits, however, including those in New Caledonia, Philippines, northeastern Australia, the Caribbean, Burundi and many in Brazil, are situated in seasonally humid wet savannas (summer rainfall of 900–1800 mm and a 2–5-month winter dry season). Thorne et al. (2012) calculated that Ni laterites develop where rainfall exceeds 1000 mm/y and mean monthly temperatures range between 22–31 ºC (summer) and 15–27 °C (winter). There are also many deposits in warm, semi-arid to arid climates in central and southwestern Australia and in more humid Mediterranean to temperate regions in the USA (Oregon and California), the Balkans, Turkey and the Urals. Each of these regions, however, is considered to have had warmer, humid climates (Scotese 2000; Thorne et al. 2012) when the deposits formed, even though at high latitudes (e.g. southwestern Australia). Modification of the deposits under the later climates has generally been minor, such as precipitation of magnesite and silica under semi-arid to arid conditions in Australia. No clear relationship exists between the present climate and ore type (FIG. 1), grade or size. Although hydrous Mg silicates are most abundant in the present tropics and clay silicates in semi-arid areas, this distribution is due largely to their tectonic, structural and geomorphological settings, which affect drainage status and vulnerability to erosion. onset of weathering (Anand and Paine 2002). However, the majority of dates in this now mostly semi-arid region point to major phases of humid tropical weathering in the Middle to Late Cretaceous, Palaeocene and Late Eocene–Oligocene. Similarly episodic weathering is considered to have occurred in northern Australia and on the shields of central South America and West Africa, but there humid tropical conditions generally continued throughout the Cenozoic (Scotese 2000). Many deposits in accretionary terranes are younger, with the best-constrained dates for those buried by later sediments. In the Urals, weathering of Upper Devonian serpentinites commenced in the Late Triassic, with some Ni laterites buried by the mid-Cretaceous. Nickel laterites developed on Upper Jurassic–Lower Cretaceous ophiolites, and ophiolite-derived sediments in Greece were buried by Middle to Upper Cretaceous sediments (Valeton et al. 1987), whereas the nearby Çaldağ deposit in Turkey probably formed in the Palaeocene to mid-Miocene (Tavlan et al. 2011). Weathering in the newly emergent islands of the Caribbean and western Pacific (New Caledonia, Philippines, Indonesia) similarly date from the Eocene–Oligocene (Sevin et al. 2012). Some deposits formed very rapidly; for example, the ophiolites on Sulawesi are Miocene in age and have probably been exposed for less than 10 My. Geomorphology Age of Weathering Because most Ni laterite deposits, and the landscapes in which they occur, formed and evolved over long periods under different weathering regimes, we can only estimate the period(s) of most intense weathering, rather than ascribe specific times of formation. Direct dating of regolith has only been done in a few regions and rarely of Ni laterites themselves. The oldest deposits occur on cratons, parts of which may have been exposed to subaerial weathering for much of the Phanerozoic. On the Yilgarn Craton in Western Australia, palaeomagnetic dating at Murrin Murrin and sites near Cawse and Bulong give some Palaeozoic ages, indicating a very early E LEMENTS Hydrous Mg silicate mineralization, New Caledonia. (A) “Garnieritic boulder ore”, Nakety. (B) Synkinematic garnierite crack seals, Koniambo. (C) Polyphase crack infill of botryoidal “garnierite”(g), kerolite (k) and quartz (q) (cross-polarizers) FIGURE 6 Nickel laterites occur in regions with a deep, strongly weathered regolith. This implies (1) formation in terrains with sufficient tectonic stability and low relief that the rate of weathering exceeds that of erosion, and (2) preservation due to low relief and protection from erosion by either burial or armouring by ferruginous or siliceous duricrusts. These conditions prevail in cratons but less so in accretionary terranes. In the latter, most deposits occur in dissected plateaux, where uplifted remnants of a previously more extensive regolith cover are being actively eroded and are well preserved only beneath duricrust on crests, upper slopes and terraces. 126 A PR IL 2013 The topography of these different settings affects drainage and, in turn, the nature and grade of Ni laterite, especially on peridotite. On cratons and locally elsewhere, low relief and high water-tables result in impeded drainage, which reduces the rate of removal of weathering solutions and the intensity of leaching. Resultant Ni concentrations are largely residual, with little absolute accumulation. In such environments, serpentinized peridotites weather to form low-grade smectite clay deposits in the saprolite (e.g. Murrin Murrin, Western Australia; San Felipe, Cuba; Kempirsai, Kazakhstan), mostly with poorly developed oxide zones. In areas of high relief in accretionary terrains or on hills in cratons, the regolith is generally free-draining with a deep water-table. This maximizes the rate of leaching and movement of groundwater, giving an absolute enrichment deep in the saprolite in addition to a residual concentration, generally expressed by high-grade hydrous Mg silicates in saprolite and a well-developed oxide zone. In comparison, dunites tend to form oxide deposits irrespective of drainage conditions (Tiebaghi and much of Goro, New Caledonia), although silica accumulation (e.g. Cawse, Western Australia, and possibly Onça and Puma, Brazil) may be greater where drainage is impeded. NICKEL LATERITE FORMATION AND EVOLUTION Direct Formation Many deposits formed during an essentially continuous weathering episode under a dominantly humid savanna climate. These conditions are typical of lateritization on cratons and less active accretionary terranes. High water-tables, fluctuating seasonally in savanna climates, and low erosion rates favour weathering to 50–80 m (FIG. 7A). In free-draining environments, Ni released by the hydrolysis of olivine or serpentinized olivine is largely retained and is hosted by goethite. Magnesium and silica are leached, causing porosity to increase and density to decrease, with a total mass loss of up to 70%. This alone can residually concentrate Ni in the ferruginous oxide zone above the Mg-discontinuity to 0.6–1%, where it is mainly adsorbed to goethite. Higher grades, to over 1.5% Ni, are due to absolute enrichment by Ni leached from surface horizons. Cobalt and some Ni also concentrate in Mn oxides in the lower oxide zone. There is rarely any significant Ni enrichment in the lower saprolite, except in freely drained fault zones. In poorly drained environments, similar oxide zones develop over serpentinized dunite, but as silica may not be fully leached, it can precipitate as quartz in the lower oxide zone, the upper Mg-bearing saprolite and fault zones. Serpentinized peridotites are less strongly leached due to the generally lower porosity and more stable mineralogy, so that some Mg, Si and Ni are retained in smectites in the mid to upper saprolite. Some deposits in juvenile accretionary terranes, such as Soroako, Sulawesi (Golightly 1979), have developed since exposure in the Late Miocene under continuing rainforest climates. The relief is higher than normally associated with the formation of lateritic regoliths, and profiles are shallower and less mature. A similar oxide zone develops but, with a lower water-table and free-draining conditions, Ni adsorbed to goethite is leached and translocated deeper in the profile. There it may be readsorbed to neo-formed goethite, react with Mg and silica released from primary minerals to form secondary hydrous Mg silicates, or exchange for Mg in serpentine, resulting in absolute enrichments of Ni in the lower saprolite. Multi-stage formation Tectonic uplift has played an important role in the formation of some deposits in areas of originally low relief by rejuvenating the topography and lowering previously high water-tables. In the humid tropics, this has typically led to leaching of Ni from the oxide zones and its accumulation in hydrous Mg silicates deeper in the saprolite (FIG. 7B). This process accounts for A B C Formation and evolution of oxide and hydrous Mg silicate deposits. (A) Progressive development of a well-differentiated lateritic regolith under a seasonally humid savanna climate in an area with low relief and tectonic stability. With mean weathering rates of about 8 to 20 m/My (Nahon and FIGURE 7 E LEMENTS Tardy 1992; Freyssinet and Farah 2000), a full profile requires at least 2 to 10 My to form. (B) With uplift and under a similar climate, leaching and reaction/exchange of Ni yield hydrous Mg silicates. (C) The profile is modified during a change to an arid climate, with precipitation of magnesite and silica. 127 A PR IL 2013 many high-grade deposits in uplifted and partly eroded plateaux and terraces. In New Caledonia, the stepped land surfaces indicate that lateritic weathering initially occurred on a landscape of low to moderate relief in the Lower Oligocene and continued as these surfaces were uplifted and partially dissected (Chevillotte et al. 2006; Sevin et al. 2012). A related mechanism may account for the formation of some hydrous Mg silicate deposits in more stable areas of generally low relief. Relief inversion has left isolated hills of free-draining, weathered ultramafic rocks, protected by ferruginous duricrust, in which Ni leached from the oxide zone has concentrated in saprolite. Cerro Matoso, Colombia (Gleeson et al. 2004), is a possible example. Many deposits have been subject to changes in climate since their formation. In Australia, semi-arid climates have prevailed since the mid-Miocene over much of the continent. The principal effects have been a greatly reduced weathering rate and the accumulation of magnesite and, especially over serpentinitized dunite, further silicification. There is little or no change to the profile, or to the distribution or abundance of Ni in either smectite silicate or oxide deposits, except for dilution by silica in the latter (FIG. 7C). At the Onça and Puma deposits, Brazil, oxide ore occurs beneath a protective silica cap rock. Golightly (2010) suggested that silicification occurred during an earlier arid phase and that these deposits represent a separate genetic model. However, silicification can also be a product of poor drainage during an earlier REFERENCES Anand RR, Paine M (2002) Regolith geology of the Yilgarn Craton, Western Australia: implications for exploration. Australian Journal of Earth Sciences 49: 3-162 Berger VI, Singer DA, Bliss JD, Moring BC (2011) Ni-Co Laterite Deposits of the World—Database and Grade and Tonnage Models. USGS Open-File report 2011-1058, 26 pp, http://pubs.usgs.gov/ of/2011/1058/ and http://mrdata.usgs. gov/laterite (accessed January 2013) Brand NW, Butt CRM, Elias M (1998) Nickel laterites: classification and features. AGSO Journal of Australian Geology and Geophysics 17: 81-88 Butt CRM, Zeegers H (eds) (1992) Regolith Exploration Geochemistry in Tropical and Subtropical Terrains. Handbook of Exploration Geochemistry 4, Elsevier, Amsterdam, 607 pp Chevillotte V, Chardon D, Beauvais A, Maurizot P, Colin F (2006) Long-term tropical morphogenesis of New Caledonia (Southwest Pacific): Importance of positive epeirogeny and climate change. Geomorphology 81: 361-375 Cluzel D, Vigier B (2008) Syntectonic mobility of supergene nickel ores of New Caledonia (Southwest Pacific): Evidence from garnierite veins and faulted regolith. Resource Geology 58: 161-170 Eggleton RA (2001) The Regolith Glossary. Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Perth, 144 pp, www. crcleme.org.au/Pubs/monographs. html#books (accessed December 2012) Elias M (2002) Nickel laterite deposits – a geological overview, resources and exploitation. Centre for Ore Deposit Research, University of Tasmania, Hobart, Special Publication 4, pp 205-220 E LEMENTS phase of lateritization, so these deposits may be further examples of continued weathering after relief inversion, without significant climate change. CONCLUSIONS The formation of Ni laterites involves the interaction of numerous geological and environmental factors. It starts with the emplacement and serpentinization of the ultramafic protore, followed by exposure to a humid tropical climate and the development of a deep, intensely weathered regolith. During this phase, Ni is concentrated in goethite and/or smectite and enrichment is largely residual, due to the loss of Mg and Si. Subsequently, most deposits have been subjected to tectonic and/or climatic changes. Where these caused little erosion, led to burial or, with more arid climates, reduced the rate of weathering, the deposits were preserved with only minor modification. Where uplift was significant, loss by erosion increased, but, under humid climates, this is offset, economically at least, by continued weathering and enrichment of Ni in neo-formed hydrous Mg silicates in the surviving regolith. ACKNOWLEDGMENTS Martin Wells, Georges Calas and Richard Herrington are thanked for their constructive comments on the manuscript. The figures were drafted by Travis Naughton. Freyssinet P, Farah AS (2000) Geochemical mass balance and weathering rates of ultramafic schists in Amazonia. Chemical Geology 170: 133-151 Freyssinet P, Butt CRM, Morris RC, Piantone P (2005) Ore-forming processes related to lateritic weathering. 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