Download Nickel Laterite Ore Deposits: Weathered

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,
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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
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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.
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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.
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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
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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.
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