Kyushu University Global COE Program
Journal of Novel Carbon Resource Sciences, Vol. 3, pp. 32-39, Feb. 2011

Technical Report

Laterization Process of Ultramafic Rocks in Siruka, Solomon Islands
Christopher V. Sagapoa*1, Akira Imai*2, Koichiro Watanabe*3*1

Graduate School of Engineering, Kyushu University
Faculty of Engineering and Resource Science, Akita University*3
Faculty of Engineering, Kyushu University*2(Received November 11, 2010; accepted January 19, 2011)
The lateritic weathering crusts exposed in Siruka, Choiseul Islands, Solomon Islands, were developed on the
expense of serpentinized peridotite underlain by Choiseul Schists and Voza Lavas with a subhorizontal contact. The
lateritic profile consists of four zones: bedrock, weathered zone, decomposed (or saprolitic) zone and limonitic zones
upwards. The profiles demonstrate variations in depths and continuity but illustrate mineralogy and geochemical
characteristics affinity down profile. They are broadly analogous to saprolite - oxide nickel laterite deposits. Silica
and magnesia in the bed rock and the saprolitic zones have been removed and only the residual elements (iron,chromium, aluminium, manganese, cobalt and nickel) remain in the limonitic zone. These elements are relatively
concentrated by the removal of the soluble elements. However, nickel becomes unstable and is leached towards
the lower part of the profile and concentrated at the saprolitic zone. Nickel is combined with silica and magnesia,
as lizardite or mixed gels (garnierite nickel ore) at the weathering front. Significant supergene nickel enrichment
occurring in the decomposed zone, indicating that water had percolated downward to a very low water table at depth
during the weathering process. Cobalt and manganese are concentrated in the limonitic part of the profiles (~36 m depth) of which are poorly protected against erosion and mechanical transportation. The behavior of nickel iscontradictory to that of cobalt and manganese and its relative mobility is a little higher than them. It is leached out of
the limonitic concretions, where it is first concentrated with Co and Mn, and largely migrates to the bottom part ofthe profiles. The structure and mineralogy of the weathering profile indicate environmental conditions likely to havebeen dominated by alternating wet and dry periods.

1. Introduction

Nickel laterites are an important resource of Ni andferronickel and account for approximately 40% ofworld annual Ni production1). Of the 130 million tonnes(Mt) of Ni in land-based resources containing over 1%
nickel, approximately 60% is in laterite deposits (USGS
Mineral Commodity Summary, 2002). These deposits
are produced by the prolonged and deep weathering ofNi silicate-bearing ultramafic rocks, generally in humid
tropical to subtropical climates. As a consequence,
many recently formed and actively forming deposits are
situated in equatorial latitudes, where major-producing
countries of Ni laterite ores are located (e.g. New
Caledonia, Cuba, Philippines, Indonesia, Colombia, andincreasingly Australia). Lateritic deposits usually consistof three layers, namely the limonite, the saprolite and thegarnierite layer2). Nickel laterites are generally formedby the weathering of ultramafic rocks, such as peridotite
or serpentinite. Studies on nickel deposits coupledwith simultaneous chemical analysis have been widely
documented3-5). During weathering, some elements
become leached (e.g. Mg, Ca and Si) and others either
are secondarily enriched (e.g. Ni, Mn, Co, Zn and Y) or
residually concentrated (e.g. Fe, Cr, Al, Ti and Zr) withinlaterite profiles6). As the rocks are weathered, nickel,
iron, and sometimes cobalt are taken into solution bydescending meteoric water and re-deposited at greater
depth, creating a zone of enrichment. Because laterites
are formed by surface water percolating downward
through ultramafic rock, most laterite deposits are either
in the tropic or in subtropical to temperate coastal regionsthat posses long geologic history of heavy rainfall.
Enrichment and distribution of minor and trace elementsin the laterites are often controlled by weathering
processes and morphology of the peridotite massifs7). The
Ni content of ultramafic rocks, although relatively highcompared with that of other rocks, rarely exceeds 0.3%
by weight on the earth surface8). The formation of lateritic
deposits is controlled by four major factors: parent-rockcharacteristics which include chemical and mineralogicalcompositions; geomorphological conditions (location in
the landscape, drainage conditions, and local erosionrate); paleoclimatic history, and the age of lateritic
weathering9). Laterites above Siruka ultramafic rocks in
Choiseul, the Solomon Islands (Fig. 1) consist of in-situ(autochthonous) with possibly semi-autochthonous upper
most organic horizons. Petrography, mineralogy andgeochemistry of lateritic weathering crusts developed at
the expense of Siruka ultramafic rocks is the focus ofattention in this paper. The study considers the chemical
behavior of several specific elements (e.g. Al, Fe, Ni, Mn,
Co, Mg, Cr and Si) and constituent minerals according totheir depth in the weathering profile which in return should
contributes in understanding lateritization processes in
the Solomon Islands. The annual climatic conditions in
the Solomon Islands are tropical and Siruka in Choiseul
province is characterized by average precipitation of3200 mm/year. The tropical climate is very aggressive
and the peridotites are quickly weathered. The result of
these two combined factors is that the Siruka ultramafic
rocks tend to behave as soluble rocks. Moreover, theserocks are very easily faulted. The stressed serpentine
promotes sliding of blocks against one another, and at
every scale of observation the peridotitic bodies show adense network of joints and minor faults.

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

Table 1

Geochemical composition variation occur with depth at CD0040 drill hole (*representative profile in Siruka)

2. Analytical Methods

Sampling of lateritic profiles was carried out on drill
cores drilled by Summitomo Metal Mining Solomon
Islands, at the Siruka ultramafic rocks. Bulk chemical
analyses of samples were obtained by combined X-ray
fluorescence (XRF) and atomic absorption spectroscopy(AAS) techniques. X-ray diffraction (XRD) analyses ofclay minerals were done on oriented mounds. ScanningElectron Microscope/Energy Dispersive Using X-Ray(SEM-EDX) was used for quantitative mineralogy.

3. Result

A lateritic Ni ore deposit was discovered in Siruka
during mineral exploration and was investigated withthe help of numerous drill holes. Detailed analyses givea general idea of the composition and genesis of thisdeposit. The weathering mantle which covers the Siruka
serpentinite massif consists generally of three layers.
Above the serpentinized harzburgite follows the orezone, consisting of Ni-rich saprolite, which is of variable
thickness. Detail geochemical description of the profile
is as tabulated (Table 1) and on the field observation
column below.

3.1 Geology

The Solomon Islands form an archipelago situatedbetween longitudes 156º to 170ºE, and latitudes 5º
to 12ºS (Fig. 1). This paper concentrates on the larger

islands which form the characteristic NW–SE-trending
double chain of islands comprising Choiseul, the New
Georgia Group, Santa Isabel, Guadalcanal, Malaita andMakira (or San Cristobal), but with special emphasis
on Choiseul and Santa Isabel. It was conventionallyacknowledged that the Solomon block is bounded by two
trench systems; the Vitiaz trench to the Northeast andthe Britain-San Cristobal trench to the Southwest, later
termed as South Solomon Trench System (SSTS)10). The
focused island in this study, Choiseul province, forms
part of the Northwest-Southeast trending Solomon arc
and illustrate pronounced elongation along the trend (Fig.
1B). Convergent plate margin tectonics has much control
over the structural development of the island which
subsequently gives rise to two distinct structural units;(i) the Pre-Miocene igneous and metamorphic basement
complex and (ii) the Miocene-Holocene volcanic andsedimentary cover. The distribution of rock types in
the Choiseul Pre-Miocene basement is as shown on thegeological map of SE Choiseul (Fig. 1). The basement
includes the Voza Lavas, Choiseul Schists (termed as
Siruka Schist in here), Oaka Metamicrogabbro and Siruka
Ultramafic rock11). The Voza Lavas are the predominant
formation of the Choiseul basement together with the
Choiseul Schists. This basement sequence of Choiseul
is thought to represent an ophiolite complex with
MORB characteristics, developed close to a subduction
zone11). The Voza Lavas occur as pillowed, massive andbrecciated basalts11,12) and can be further divided into

Laterization Process of Ultramafic Rocks in Siruka, Solomon Islands

Fig. 1 General Geology of Choiseul (A), Geology of Solomon Islands (B), various drilled locations (C) identifying
CD0040 and Solomon Islands in the Pacific region.

Fig. 2 Schematic section through the lateritic profiles in Siruka, Choiseul Province, in the Solomon Islands.

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

two groups; (i) unmetamorphosed and low grade rocksand (ii) more highly metamorphosed varieties such asamphibolite facies. The Choiseul Schists are considered
to be dynamothermally altered Voza Lavas, and are
distinct from Voza Lavas by their possession of tectonic
fabrics within their textures.
Ridgway and Coulson (1987)11) suggested that
the Siruka Schists were formed by deformation andmetamorphism of parts of the original Voza Lava
sequence. No radiometric age has yet available for the
Choiseul basement sequence but stratigraphic andstructural evidence suggest a probable Cretaceous
age10,11). The Siruka Ultramafic rocks are predominantly
harzburgites and dunite though patches of lherzolites
were also reported present and have undergone varying
degree of serpentinization. The Siruka Ultramafic rocks
form a large sheet lying on Choiseul Schists and Voza
Lavas with a general subhorizontal contact13,14), hence,are perhaps emplaced as coherent thrust sheet on asubhorizontal plane in Late Miocene to Pleistocene14).
Field observation showed that the mentioned relationships
have been modified by post-emplacement faulting andhigh angle fault contacts are common. Estimates of the
thickness of the emplaced sheet based on valley bottom
to hill crest measurement; show an increasing thickness
towards the south11).
A maximum of 560m of the emplaced was reported
present on the south, thinning to 300m on the northwest11). The sedimentary and volcanic cover ranges inage from Miocene to Recent and are arranged in orderof decreasing age11) which includes Mole Formation Kumboro and Maetambe Volcanics (Early – Middle
Miocene); Vaghena Formation (Early Pliocene); Pemba
Formation (Early to Late Pliocene); Nukiki Limestone
Formation - backreef and lagoonal deposits (Pliocene);
Holocene deposits including alluvium, mangrove
and freshwater swamp; backreef and lagoonal facies
sediments and coralline reef limestones (Holocene).
Petterson et al. (1997)15) suggested that stage 1 arc
development is represented by crystal and lithicrich turbidites from the Mole Formation whereas theKomboro and Maetambe Volcanics constitute the stage 2
arc sequence of the Solomon Islands tectonism.

3.2 Field observation and mineralogy

Lateritic profiles in Siruka show large variations inthickness and continuity of individual layers. Differences
are also observed in the mineralogy and chemistry of
the zones over short distances. Intense post-erosion
processes are unlikely after laterization in Siruka,
though possibilities of erosion cannot be absolutely
ruled off considering the incline geomorphology of thestudy area. Due to lack of intense erosion, observationof well developed ferruginous zones, referred to us
limonite 1, limonite 2 and limonite 3 hereafter is enabled.
Though slightly varied (in depths, and serpentinizationlevel) most other drilled hole cores considered wereanalogous to CD0040, hence the usage of CD0040 as
the representative drilled hole for discussion and beencharacterized as; (a) Bedrock (protolith), (b) Weatheringzone, (c) Decomposed zone (Saprolite) and (d) Limonitic
zone. The bedrock is mostly serpentinized harzburgite
which grades into the weathered and decomposed zone.
Consistent to descriptions suggested by previous authors
where the protolith is the original ultramafic rock,most commonly harzburgite, other types of peridotite
or dunite3). The bedrock comprised predominantly ofolivine, pyroxene, serpentine (Fig. 3) and grades into
weathered irregular boundary on the weathered zone.
Weathered serpentinite clasts are apparent and areinterpreted as relicts. Hard and relatively substantial
fragmented rock boulders were observed at the lower
saprolite zone which gradually grades upward into soft
and argillized matrixated saprolite. The weathering zone
is occasionally transected by loosely printed stockwork
talc, garnierite and carbonate veins.
The fragmented boulders in the weathering zone stillretain the mineral textures of the original rock howeverwith slight variable appearances. Locally, within thesaprolite, areas of intense secondary veining andboxworks are commonly observed. These can naturally
go after relic structures, fractures, and grain boundaries
and can contain neo-formed quartz and Ni-rich mineralssuch as the distinguishing green mineral garnierite1).
The saprolite zone is generally yellowish brown withnumerous rock fragments. Filling of voids and cracks
by garnierite is commonly observed in the saprolitic
zone yielding higher nickel concentration (~avg. 1.5 g/t)
at ~7-14 m depth. The transition to the limonitic zone
is marked by gradual disappearance of yellowish darkgreen serpentine into smectite unit at ~8 m depth. The
limonitic zone is categorized into brown homogeneous
argillic soil (L3), banded yellowish orange argillic (L2) togoethite and hematite enriched brown homogeneous silty
soil (L1) with random occurrences of darkish manganeseoxide blebs within the oxide matrix and finally, thesurface organic zone which randomly host visible ironcrusts. Locally the limonitic zone extends downward to
the decomposed serpentinite. The primary mineralogy
and texture of the host rock is totally destroyed in thisencrusted zone. The limonitic zone is soft and friableconsisting of goethite, gibbsite, minor hematite, tracesof magnetite-maghemite and kaolinite. At 5 m depths,
relicts of serpentine and talc were detected by XRD
which are still part of the friable layer. The contact
between the limonitic zones with the decomposed zone
is intertonguing and not as clearly demarcated as inFig. 2. The modal proportion of goethite and hematite
increases toward the surface with ambiguous transition
between the two along the profile, meaning they both
occur coherently with each other on the schematic
profile. Geochemical analysis of the various drilled holes
analyzed to understand the chemical behavior of certain
elements on the profiles during laterization process at
Siruka show geochemical affinities to that of CD0040.
Analogous to other nickel enriched profiles studied inthis local; Fe, Cr and Al show enriching behavior withincreasing weathering intensity. Contrary, Si and Mg got
depleted as weathering increases. Ni, Co and Mn seem topositively correlate with each other at depth but varied on
the upper portion of the profiles.

Laterization Process of Ultramafic Rocks in Siruka, Solomon Islands

Fig. 3 XRD patterns of serpentinized peridotite (specifically CD0040 protolith unit) from Siruka. The spectrum was
obtained using an X-ray diffractometer ‘Rigaku’ at room temperature, using a rotating ‘Cu Kα’ target with avoltage of 40 kV and a current of 20 mA. The scan range (2θ) was 2-65º with a step size of 0.02º. Olivine = Ol,
Chlorite = Chl; Amphibole = Amph, Chrysotile = Ctl, Lizardite = Liz, Orthopyroxene = Opx, Clinopyroxene= Cpx and talc.

4. Discussion

Chemical analyses were performed on the samples
taken for each half a meter thickness from seven drill
holes at Siruka for this study. Distribution model of
elements on the diagnosed weathering profiles are
identical to patterns of nickel laterites elsewherein the world (e.g. New Caledonia and Cuba)1). The
mineralogical and geochemical process which affected
the ultramafic bedrock and gave rise to the weathering
crust in Siruka is distinguished as follows. The incipient
stage encompasses; Amphibolitization, serpentinization,
chloritization, introduction of talc and carbonate during
dissolution of bedrock. The Mg replacement process
could be attributed to an ionic exchange reaction suchas: Mg-serpentine + Ni2+ = Ni-rich serpentine + Mg2+.
Complete destruction of primary silicates took placeand higher in the profiles, collapse and loss of primaryfabric is apparent. Coherently, leaching process of Mg
is attributed to the formation of nontronite depicted by
XRD on the profiles. The incipient dissolution processexplains the apparent depletion of Mg and Si in the upper
portion of the profiles illustrated in Fig. 4. We further
suggest that reactions involving substitution of Mg with
Ni in Siruka may have occurred as a result of Ni bearing
solutions from more alkaline zone (e.g. limonitic zone)
moving to more basic environment (e.g. saprolitic zone,
Fig. 2). The suggestion brewed following previous workon mineralogy and geochemistry of iron-rich laterites,
where it has been articulated that reactions involving

substitution of Ni with Mg require a course where by
Ni bearing solutions do moved from a low pH to a highpH environment16). Following the dissolution process islimonitization or laterization where crystallization of
Fe oxyhydroxides (goethite in particular) dominates theupper saprolite and the overlying limonite horizon of the
Siruka profiles. This oxide rich portion of the profile isreferred to informally as limonite and the volume of theupper collapsed portion may become as little as ~15%of the original rock4). XRF results (Table 1) from the
Siruka profiles illustrate Mn oxides to be concentrated at
the interface between the organic zone and the saprolite.
High EH condition may prevails at such boundary
mentioned above, where the Mn oxides can coprecipitate
with Ni, Co and other elements17). This could explain thecoexisting may prevail at such boundary and the Mnoxides coprecipitate with Ni, Co and other elements17).
This could explain the coexisting relations between Mn
and Co on the profiles. According to Llorca (1993)18)
manganese do leached from concretions of olivine,
pyroxene coupled with their metamorphic products’
concretions and precipitates as Mn oxides however,
was limited to certain horizons or depths. Elevatedconcentration of Mn oxides in Siruka was proposed
to have precipitated and form significant Mn-Co-Ni
enrichment, coherent to those in Ora Banda-Siberia area ofWestern Australia19) and equivalently alluded to by Kuck
(1992)8) in New Caledonia. Considering the significant
correlation between MnO and CoO, as suggested byprevious workers20), Mn oxyhydroxides would be theJournal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

Fig. 4

Vertical distribution of elements major elements in CD0040 drill core obtained from XRF. Concentrations are
in wt %. AAS results are not included in the figure though the experiment was carried out to evaluate major
elements.

main carriers of cobalt and in some cases nickel. In
silicate laterites in New Caledonia21) and even in clayey
laterites in Western Australia22), cobalt mineralization
is principally associated with Mn-oxyhydroxides(asbolane-lithophorite group). Based on these known
deposit characteristics, Mn enrichment in Siruka is

attributed to occurrence of asbolane group manganeseon the limonite zone shown by XRD. Contrary, previous
authors also suggest that nickel is present in an adsorbed
or lattice bound state substituting Fe in less crystalline
goethite23). Nonetheless, following previous intensive
Mn oxides deliberations20) we suggest that nickel, cobalt,

Laterization Process of Ultramafic Rocks in Siruka, Solomon Islands

and manganese at Siruka may have been precipitated
together as oxidized concretions during laterization
stages. Subsequent to the co-precipitation event Ni thenleached out of the limonitic concretions, where it was
first concentrated with Co and Mn during the laterization
and weathering earlier stages, and chiefly migrates to thesaprolitic portion of the profiles at a later weathering stage.
The concept is analogous to the previous concepts where
Ni has been leached from the limonite Fe oxyhydroxide
phases and moves down the profile before redepositing
either as hydrous silicate minerals or substituting for Mg
in altered serpentine5). Major nickel enrichments in thesaprolitic zone (average 1.33 g/t) occurred on fracturesand voids commonly observed on the weathered andsaprolitic columns (Fig. 2). The association of cobalt in
Siruka is such that it occurs with manganese mineralphases particularly on the upper part of the profiles,
where liberated elements (e.g. nickel, silicon andmagnesium) got depleted. Discrete concentration ofcobalt is apparent on the profiles. According to YoungueFouateu et al., (2006)24), cobalt can significantly become
enriched within the less crystalline manganese mineralhosts higher up in the profiles where remobilization andre-precipitations have led to the development of enrichedzones and nodules associated with well-crystalline Mn
mineral of the asbolane-lithiophorite group are observed.
The asbolane of course was detected on XRD peak plots(d=4.75Å) in the upper portion of the profile at Siruka,
specifically on horizons where random dark manganese
nodules are common. Though not substantiated with raw
data on this paper, SEM-EDX analysis confirmed thatnickel is associated with silica and magnesia, as lizardite
and mixed gels (garnierite nickel ore) at the weathering
front along fractures and joints. Depletion of magnesiumin the representative profile (Drill Hole CD0040) (Fig.
4) reflects the leaching process during lateritizationand subsequent exclusion of other elements by draining
waters, which resulted in large concentrations of Fe, Al
and Cr in the upper portion of the laterite profile relative
to the bedrock. This explains the magnesium depletion
evident bottom to top of the profiles with lowest values
in the limonitic zone (Fig. 2). Due to depletion of alkaline
metals and silica, the weathering-resistant chromium
becomes residually enriched in the mid-limonitic zone
(Fig. 2). Gibbsite and manganese oxyhydroxide mineralenrichment in the limonitic layer is attributed to repeat
remobilizations of the residual material in which besides
goethite, magnetite–maghemite is well represented, thismagnetite more or less weathered to maghemite, explain
the high chromium content observed in the residue24).
Aluminium is residually enriched in the granular limonitic
zone as demonstrated in Fig. 4. This explains the presenceof gibbsite in the limonitic zone. Parallel resemblanceof Al2O3 and Fe2O3, as apparently denoted in extracted
XRF results possibly attributes to the relative associationand the occurrence of goethite, hematite and gibbsite in
the upper portion of the profile. Extensive XRD analysis
on the granular limonitic portion of the lateritic profile
confirmed that goethite, hematite and kaolinite are thedominant minerals while gibbsite, asbolane and smectite(nontronite) are sparsely distributed. On the other hand,

our data (Table 1), illustrate Al/Fe ratio increase fromwet to dry areas in the profile, i.e. from the bottom tothe top (limonite zone) of profiles, signifying significant
underground water involvement during the laterization
process. The maximum substitution ratio is attributed tovery close association between goethite and gibbsite.

5. Conclusion

The lateritic weathering crusts which developed at
the expense of ultramafic rocks exposed at Siruka, thesoutheastern part of Choiseul are broadly similar tosaprolite nickel laterite deposit around the world (e.g.
New Caledonia and Cuba1)). The mineralogical andgeochemical processes which affected the ultramafic
bedrock and gave rise to the weathering crust in Siruka
are classified into two stages: Dissolution of bed rock,where nickeliferous serpentine, tremolite, chlorite andtalc and carbonate were inferred to form by dissolutionof bedrock. During the dissolution process, magnesium
and silicon were discharged in solution and Mg was later
replaced with Fe and Ni within the rock forming minerals.
According to Kelepertsis (2002)16), reactions involving
substitution of Ni for Mg require a course where by Ni
bearing solutions migrates from a low pH to a high pH
environment. The Ni enrichment in Siruka is inferredto have attributed to percolation of surface solutions
and could be the result of an ionic exchange reaction
such as: Mg-serpentine + Ni2+ = Ni-rich serpentine +Mg2+. Significant supergene nickel enrichments on thesaprolite zone indicate water movement through thosezones downward to a very low water table. Nickel
was removed by percolating waters and reprecipitatedin the saprolite zones. With pronounced weathering,
all relicts of bedrock and saprolite were obliterated
and the limonite zone was formed. Following thedissolution process is limonitization or laterization
where crystallization of Fe oxides (goethite in particular)
dominates the upper saprolite and the overlying limonite
horizon of the Siruka profiles. In the upper part of the
profile, above the water table, smectite was broken down
as been indicated by related loose of silica in solution.
Leaching of Mg favored formation of nontronite as
depicted by XRD analysis. In the present study nickel
is a marker in the various horizons and appears invarious mineral associations; generally with iron oxides(mainly as goethite but also with magnetite-maghemite
relicts) defining the limonitic zone, unclear association
with smectite (nontronite) and with serpentine in thesaprolitic horizons. It appeared that with respect to nickel
content, silicate horizons are richer than oxidized ones as
illustrated in Fig. 3, consistent with 25). Nickel enrichment
decreases upward the weathering profile as it is removed
during the recrystallization of goethite and the leachingof manganese oxide phases4). Crystallization of ironoxide in the form of goethite has a low capacity for nickel
fixation 24) which explains the depleting behavior of nickelin the limonitic horizon at Siruka. Following the earlier
deliberations it’s apparently evident that Ni has been
substituting Mg in altered serpentine during the earlier
phases of the weathering, leached from the limonite Fe

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

oxyhydroxide phases, moves down the profile under
the influence of gravitated percolating water before
redepositing in the saprolite zone, hosted by hydrous
silicates, at the later intensive weathering stages.
Acknowledge: This work was indebted to the Global
Center of Excellence (G-COE) for its numerous supports
in funding the research. Grateful indeed for the technicalsupport and field expertise provided by the Summitomo
Metal Mining Limited Solomon Islands (SMMLSol)
staffs, Solomon Islands.
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