laterization process of peridotites in siruka, …

J. SE Asian Appl. Geol., Jul–Dec 2011, Vol. 3(2), pp. 76-92

LATERIZATION PROCESS OF PERIDOTITES INSIRUKA, CHOISEUL, SOLOMON ISLANDS

Christopher V. Sagapoa*1, Akira Imai2, Takeyuki Ogata3, Kotaro Yonezu1, and KoichiroWatanabe1

1Department of Earth Resource Engineering, Faculty of Engineering, Kyushu University, Japan2Department of Earth Science and Technology, Faculty of Engineering and Resource Science, Akita University, Japan3International Center for Research and Education on Mineral and Energy Resources, Akita University, Japan

Abstract

The lateritic weathering crusts exposed in Siruka,Choiseul Islands, Solomon Islands, were developedon the expense of serpentinized peridotite underlainby Siruka schists and Voza lavas with a subhorizon-tal contact. The lateritic profiles consist of three gen-eralized zones: bedrock, saprolitic zone (weatheredand decomposed zone) and the limonitic zones. Theprofiles demonstrate variations in depths and con-tinuity but illustrate mineralogy and geochemicalaffinity down profile and are analogous to saproliticnickel laterite deposits. Silica and magnesia in thebed rock and the saprolitic zones have been removedand only the residual elements (iron, chromium, alu-minium, manganese, cobalt and nickel) remain inthe limonitic zone. These elements are relatively con-centrated as a result of the removal of the soluble ele-ments. Nickel is associated with silica and magnesia,as lizardite or mixed gels (garnierite nickel ore) at theweathering fronts. On the other hand, nickel, withgenerally low concentrations (<2% NiO), is alsohosted by several secondary mineral phases (goethiteand Mn-oxyhydroxides locally) as residuals in thesurface limonitic zone. Cobalt, on the other hand,is generally of higher grade (up to 0.9% CoO) andis associated with cryptocrystalline and crystallinemanganese-oxyhydroxides. Cobalt and manganeseare concentrated in the limonitic part of the profiles(~3-7m depth) of which are poorly protected againsterosion. The behavior of nickel is different from that

*Corresponding author: C.V. SAGAPOA, Departmentof Earth Resource Engineering, Faculty of Engineering,Kyushu University, Japan. E-mail: [email protected]

of cobalt and manganese as its relative mobility is alittle higher than them. Nickel is suggested to havebeen leached out of the limonitic concretions, whereit is first concentrated with cobalt and manganese,and migrates to the bottom part of the profiles. Sig-nificant supergene nickel enrichment occurring inthe saprolitic zone indicated that water had perco-lated downward to a very low water table at depthduring weathering. The structure and mineralogyof the weathering profile is consistent with the alter-nating wet and dry periods of the studied area.Keywords: Siruka ultramafic rocks, laterization,Choiseul-Solomon Islands

1 Introduction

Nickel laterites are an important resource ofnickel and ferronickel and account for approx-imately 40% of world annual nickel produc-tion (Gleeson et al., 2003). Of the 130 milliontonnes (Mt) of nickel in land-based resourcescontaining over 1% nickel, approximately 60%is in laterite deposits (USGS Mineral Commod-ity Summary, 2002). These deposits are pro-duced by the prolonged and deep weatheringof ultramafic rocks, such as peridotite, generallyin humid tropical to subtropical climates. Asa consequence, many recently formed and ac-tively forming deposits are situated in equato-rial latitudes, where major producing countriesof nickel laterite ores are located (e.g, New Cale-donia, Cuba, Philippines, Indonesia, Colombia,and Australia). Studies on nickel deposits cou-pled with chemical analyses have been widely

76

LATERIZATION PROCESS OF PERIDOTITES IN SIRUKA, CHOISEUL, SOLOMON ISLANDS

documented (Colin et al., 1990, Golightly , 1981and Pelletier, 1996). During weathering, someelements become leached (e.g, magnesium, cal-cium and silicon) and others either are secon-darily enriched (e.g, nickel, manganese, cobalt,zinc and yttrium) or residually concentrates(e.g, iron, chromium, aluminum, titanium andzirconium) within laterite profiles (Brand et al.,1998).

Petrography, mineralogy and geochemistryof lateritic weathering crusts developed at theexpense of Siruka Peridotites is the focus of at-tention in this paper. The study considers thechemical behavior of several specific elements(e.g, iron, cobalt, magnesium, chromium, iron,magnesium, aluminium and silicon) and con-stituent minerals with respect to their depth ofoccurrence in the weathering profiles, at Siruka,Choiseul, Solomon Islands. Drill hole profilewith the ID #CD0040 was used as a represen-tative profile since it share laterization affin-ity with the other 14 profiles considered inthe study. The annual climatic conditions inthe Solomon Islands are tropical and Siruka inChoiseul province is characterized by averageprecipitation of 3200 mm/year.

2 Geology

The Solomon Islands form an archipelago sit-uated between longitudes 156º to 170ºE, andlatitudes 5º to 12ºS (Figure 1). This paper con-centrates on the larger islands which form thecharacteristic NW–SEtrending double chainof islands comprising Choiseul (Figure 2), theNew Georgia Group, Santa Isabel, Guadal-canal, Malaita and Makira (or San Cristobal),but with special emphasis on Choiseul. Itwas conventionally acknowledged that theSolomon block is bounded by two trench sys-tems; the Vitiaz trench to the Northeast and theBritain-San Cristobal trench to the Southwest,later termed as South Solomon Trench Sys-tem (SSTS) (Petterson et al., 1999). Convergentplate margin tectonics has much control overthe structural development of the ChoiseulIsland which subsequently gives rise to twodistinct structural units; (i) the Pre-Mioceneigneous and metamorphic basement complex

and (ii) the Miocene-Holocene sedimentary andvolcanic cover. No radiometric age is yet avail-able for the Choiseul basement sequence butstratigraphic and structural evidence suggest, aprobable Cretaceous age (Petterson et al., 1999;Ridgway and Coulson, 1987).

The distribution of rock types in the ChoiseulPreMiocene basement is shown on the geolog-ical map of southeast Choiseul (Figure 3). Thebasement includes the Voza Lavas, ChoiseulSchists (termed as Siruka Schist locally in here),the Oaka Metamicrogabbro and Siruka Peri-dotites (Ridgway and Coulson, 1987). Thisbasement sequence of Choiseul is thought torepresent an ophiolite complex with mid-oceanridge basalt (MORB) characteristics, developedclose to a subduction zone (Ridgway and Coul-son, 1987). The Voza Lavas occur as pillowed,massive and brecciated basalts (Purvis andKemp, 1975; Ridgway and Coulson, 1987) andcan be further divided into two groups; (i) un-metamorphosed and low grade metamorphicrocks and (ii) more highly metamorphosed va-rieties such as amphibolite facies. The SirukaSchists are considered to be dynamothermallyaltered Voza Lavas, and are distinct from VozaLavas in possession of tectonic fabrics. Ridg-way and Coulson (1987) suggested that theSiruka Schists were formed by deformationand metamorphism of parts of the originalVoza Lava sequence. The Siruka Peridotites arepredominantly harzburgites and dunite thoughlherzolites were also reported by Ridgway andCoulson (1987), and have undergone varyingdegree of serpentinization. The Siruka Peri-dotite form a large sheet (Figures 3 and 4) lyingon top of Siruka Schists and Voza Lavas with ageneral subhorizontal contact (Coleman, 1960;Thompson, 1960), hence, are perhaps emplacedas coherent thrust sheet on a subhorizontalplane in Late Miocene to Pleistocene (Thomp-son, 1960). Field observation showed that thementioned relationships have been modified bypost-emplacement faulting and high angle faultcontacts are common. Estimates of the thick-ness of the emplaced sheet, based on valleybottom to hill crest measurement, show an in-creasing thickness towards the south (Ridgwayand Coulson, 1987). A maximum of 560 m of

© 2011 Department of Geological Engineering, Gadjah Mada University 77

SAGAPOA et al.

Figure 1: Geographical location of Siruka, Choiseul province, Solomon Islands in the southwestpacific region

Figure 2: General geology of Choiseul province, Solomon Islands (Modified after Ridgway andCoulson, 1987)

78 © 2011 Department of Geological Engineering, Gadjah Mada University


the emplaced sheet was reported on the south,thinning to 300 m on the north-west (Ridgwayand Coulson, 1987).

Figure 3: Map illustrating different drill hole lo-calities in this study. Drill hole IDs are repre-sented as 1- 15, where 1 – CD0065; 2 – CD0064; 3– CD0047; 4 – CD0040; 5 – CD0041; 6 – CH0017;7 – CC19D001; 8 – CD0053; 9 – CN18D010; 10 –CN02D004; 11 – CD0001; 12 – CN01D006; 13 –CH004; 14 – CD0066 and 15 – CE15D001. (Mod-ified after SMM Solomon Ltd)

The sedimentary and volcanic cover rangesin age from Miocene to Recent and are arrangedin order of decreasing age (Ridgway and Coul-son, 1987) which includes Mole Formation –Kumboro and Maetambe Volcanics (Early –Middle Miocene); Wagina Formation (EarlyPliocene); Pemba Formation (Early to LatePliocene); Nukiki Limestone Formation back-reef and lagoonal deposits (Pliocene); HoloceneDeposits including alluvium, mangrove andfreshwater swamp; backreef and lagoonal fa-cies sediments and coralline reef limestones(Holocene). However, recent work by Pettersonet al. (1997) suggested that stage 1 arc devel-opment is represented by crystal and lithic-richturbidites from the Mole Formation whereasthe Kumboro and Maetambe Volcanics consti-tute the stage 2 arc sequence of the SolomonIslands tectonism.

Figure 4: Expanded illustration of hand sam-pling sites denoted by ‘dark grey’ square (seeFig 3) on the geological map (modified afterSMM Solomon Ltd)

3 Analytical methods

Sampling of lateritic profiles was carried out oncores drilled by SMM Solomon Ltd, SolomonIslands, in the Siruka ultramafic rocks. Bulkchemical compositions of samples were deter-mined by combined X-ray fluorescence (XRF)technique. Major and trace elements weredetermined using X-ray fluorescence (Rigaku-RIX 3100) at the Department of Earth ResourceEngineering, Kyushu University, Japan. X-ray diffraction (XRD) analyses of clay min-erals were done on oriented mounds, usingan X-ray diffractometer (Rigaku RINT 2100) atroom temperature, using Cu Kα target witha voltage of 40 kV and a current of 20 mA atthe Department of Earth Resource Engineer-ing, Kyushu University. Electromicroprobe an-alyzer (EPMA) was used for mineral chemistryanalysis. Chemical composition of the rockforming minerals in the studied serpentiniteswere acquired using JXA-8800R under operat-ing condition of 15kV and 20nA, with suitablesynthetic and natural mineral (silicates) stan-dards for calibration purposes at the Interna-tional Centre for Research and Education onMineral and Energy Resources, Akita Univer-sity, Japan.


SAGAPOA et al.

4 Petrography of Siruka peridotites

From hand specimen, the Siruka Peridotites ap-pear to be mostly grayish to dark greenish incolor with interlocking textures. The studiedperidotites include massive and sheared vari-eties. Those exposed on the surface appeargreenish to dark colored. The green color ofthe exposed peridotites were presumably dueto their olivine content and their darker featuresare attributed to increasing abundance of ser-pentine and iron oxide mineral content withinthe samples.

Petrographically, the primary constituentminerals of the Siruka ultramafic rocks areolivine, pyroxene and spinel. Mineral assem-blages and abundances of ultramafic rocksfrom Siruka were estimated from observationon thin-sections (Table 1). Obvious unevencomposition of orthopyroxene is attributedto varying degree of serpentinisation and theabundance of orthopyroxene over clinopyrox-ene is probably attributed to the degree ofmelting. The peridotites are typically cross-cutby vein (Figure 5A), recording a complex his-tory of fracturing and fluid circulation. Theabundance and orientation of veins is variablehowever, observed parallel veins is apparent(Figure 5B). In serpentinite schist (e.g, SRK8),the original texture has been completely modi-fied presumably by deformation and recrystal-lization as illustrated by talc, which occurredin very fine grain ground mass as flakes (Fig-ure 5C). Texturally, the peridotite drilled coresare characterized by prevalence of mesh andbastite textures (Figure 5D).

Peridotites associated with laterization inSiruka partially preserve geometric configura-tions of the original primary minerals, whereserpentine developed as an alteration afterolivine has a mesh texture while that formedafter alteration orthopyroxene has a bastite tex-ture. Chrysotile occurs as veinlets traversingmatrixes and lizardite occur as minor grainsreplacing olivine (Figure 5E). Within the meshtextures, broken olivine fragments were re-placed by concentric layers of serpentine ag-gregates commonly associated with magnetitegrains (Figure 5F). The opaque minerals in the

studied peridotites are represented predomi-nantly by Mg-Al-chromite and magnetite. Am-phibole is identified as tremolite and magnesio-hornblende. Spinel is commonly dark andopaque; only the euhedral shaped and finegrained spinels host magnetite in Siruka. Talcand amphibole are commonly associated withaltered orthopyroxene and serpentine (Fig-ure 5G). In general, the sampled Siruka Peri-dotites are characterized essentially of amphi-bole, chrysotile and lizardite, with subordinateamounts of magnetite, talc, magnesite, brucite(Figure 5H), and chlorite. Magnesite is associ-ated with opaques (spinel) as well as chloriteveins.

5 Field observation and mineralogy

Lateritic profiles in Siruka, following observa-tion of fifteen drill holes, show variations inthickness and continuity for individual layersin the profiles. Differences are also observedin the mineralogy and chemistry of the zonesover short distances. Intense post-erosion pro-cesses are unlikely after laterization in Siruka,however, possibilities of erosion cannot be ab-solutely ruled off considering the incline geo-morphology of the study area. Due to lack ofintense erosion, ferruginous zones are well de-veloped, referred to limonite 1 (L1), limonite 2(L2) and limonite 3 (L3), is possible. L3 is abrown homogeneous argillic soil horizon, L2 isbanded yellowish orange argillic horizon andL1 is goethite and hematite enriched brown ho-mogeneous silty soil horizon. Though varied indepths, mineralogy for most other drilled holecores considered were analogous to that of ourrepresentative drilled hole (CD0040). CD0040is used as the representative drill hole for dis-cussion in this paper and is characterized as; (a)bedrock (protolith), (b) saprolitic zone (weath-ering and decomposed zone and (c) limoniticzone (Figure 6).

The bedrock is mostly serpentinized harzbur-gite and grades into the saprolitic zone. Hardand relatively substantial fragmented rockboulders or pebbles were observed at the lowersaprolite zone (referred to as weathering front)which gradually grades upward into soft and



Figure 5: Photomicrograph showing the main textures and minerals associated with the serpen-tinized Siruka peridotite. (A) Sample ID: CD0040, locality: 324695(E), 9185608(N) and (B) SampleID: CD0066; locality: 331098(E), 9190103(N); hand samples illustrate carbonate network veining.(C) Sample ID: SRK8, locality: 326074(E), 9186622(N); altered olivine (Ol) grain with patches ofdark spinel (Spn) grains in association with orthopyroxene (Opx), tremolite (trem) and talc [XPL].(D) Sample ID: SRK5, locality: 326625(E), 9186540(N); orthopyroxene (Opx) with associated ex-solved clinopyroxene (Exslv.Cpx) on grain boundaries coupled with talc matrix [XPL]. (E) Sam-ple ID: CN01D006, locality: 331098(E), 9190103(N); chrysotile (Ctl) and chlorite (Chl) veins withgrains of lizardite (liz) [XPL]. (F) Sample ID: CN01D006; magnetite (mt) grain, chlorite (Chl) grainplus serpentine on the spinel grain boundary [PPL]. (G) Sample ID: SRK2, locality: 326452(E),9186349(N); magnesio-hornblende with disseminated Opx grains [XPL]. (H) Sample ID: SRK7, lo-cality: 326336(E), 9186652(N); typical serpentine lizardite vein bounded by reddish orange bruciteveins [XPL]. Localities are expressed in UTM coordinates (WGS 84)


SAGAPOA et al.

Table 1: Petrological and mineralogical characteristics of Siruka Peridotites, Choiseul, Solomon Is-lands

*Abbreviations: Olivine, Ol; Orthopyroxene, Opx; Clnopyroxene, Cpx; Spinel, Spn; Seperntine, serp; Amphi-bole, Amp; and Chlorite, Chl. Modal composition estimate is after point counting. *’Yes’ indicates presenceof the listed minerals

Figure 6: Schematic laterite profile in the Siruka nickel laterite deposit, Choiseul, Solomon Islands.Note both the weathering zone and the decomposed zone are grouped as ’saprolitic zone’



argillized decomposed zone. The lower sapro-litic zone is occasionally transected by loosetalc, garnierite and carbonate veins. The frag-mented boulders in the lower yellowish greento brown saprolitic zone still retain the mineraltextures of the original rock. Filling of voidsand cracks by garnierite is commonly observedin this horizon yielding higher nickel concen-tration (~avg. 1.5%).

Locally, within the saprolite zone, areas ofintense secondary veining and boxworks arecommonly observed. These can naturally go af-ter relic structures, fractures, and grain bound-aries and can contain neo-formed quartz andNi-rich minerals such as the distinguishinggreen mineral garnierite (Gleeson et al., 2003).The transition to the limonitic zone is markedby gradual disappearance of yellowish dark-green serpentine into smectite unit (observableat ~8 m depth for CD0040). Dark manganeseoxide blebs or nodules are commonly observedin L1 zone within the oxide matrix associatedwith the surface organic zone which randomlyhost visible iron crusts.

Compared to the basement rock mineral-ogy, the primary mineralogy and texture of thehost rock are totally destroyed in the encrustedlimonite zone. XRD results of bulk serpen-tinite reveal distinct peaks for the serpentinepolymorphs, chrysotile and lizardite, trevorite,magnesite and amphibole and talc (Figure 7).Nevertheless, between ~10-12 m depths, relictsof serpentine and talc were still detected byXRD (Figures 8A and 8B). The limonitic zoneis soft and friable consisting goethite, gibbsite,hematite, and traces of magnetite-maghemite(Figures 8C and 8D). Geochemical analysis ofthe various drilled hole profiles at Siruka alsoshows geochemical affinities to that of CD0040(Table 2). The regolith thicknesses of each drillholes still differ regardless of whether the pro-file is on the same parent rock. Basaltic rockscollected from drilled holes (CD0064, CD0065and CE15D001) indicated no nickel enrichment.

Mineralogy evaluation on the representativeprofile (CD0040) show inconsistent mode of oc-currence with respect to the parent rocks inSiruka. Specific individual minerals show vary-ing modal occurrence up the weathering pro-

files. Table 3 shows enriched olivine content onthe basement which is anticipated for harzbur-gites however is gradually depleting as youmove up the profile. The occurrence of spinelminerals (chromite and magnetite) in the par-ent rock and their quick termination indicateswell on their instability nature in highly oxi-dized settings. Goethite, hematite and kaoli-nite are predominant minerals alternating theprimary minerals. Gibbsite on the other handis highly enriched on the most surface samplescoherent to enriching behaviour of Al2O3.

6 Mass balance calculations

Mass balance evaluation was carried out toevaluate element distribution during supergeneweathering processes in Siruka. In very intenseweathering conditions, the mobility of refrac-tory elements such as Al, Zr, Ti and Th wasdemonstrated (Mungall and Martin, 1994; Na-hon and Merino, 1997; Kurtz et al., 2000), thusthey are unreliable to use as immobile element.According to Braun et al. (2005), the mobilityof refractory elements can also depend on thepresence of organic ligands in soil solution.

In the process of evaluating gains and lossesduring supergene weathering at Siruka, wedid not try to estimate volume changes dueto the vast limonite thickness. We choose as-sumed immobile element approach which wasemployed successfully by previous authors onweathered rocks (e.g. Moroni et al., 2001). Thepercentage gain or loss in the concentration ofeach element in weathered samples was com-pared with that of the parent rock according toequation.

% change =

[

Xa/Ia

Xu/Iu− 1

]

× 100 (1)

Where Xa and Xu are the element concentra-tion in the weathered sample and the parentrock respectively. On the other hand, Ia andIu are the concentration of immobile element inthe weathered sample and the parent rock, re-spectively.

It is quite complicated to try and identify anybest suitable element that is truly immobile inthe laterites from Siruka since all components


SAGAPOA et al.

Figure 7: XRD patterns of serpentinized peridotite (specifically CD0040 protolith unit - at 19.50mdepth) from Siruka. The spectrum was obtained using an X-ray diffractometer ’Rigaku’ at roomtemperature, using Cu Kα target with a voltage of 40kV and a current of 20mA. The scan range (2θ)was 2-65◦ with a step size of 0.02◦. Olivine = Ol, Chlorite = Chl; Amphibole = Amp, Chrysotile =Ctl, Lizardite = Liz, Orthopyroxene = Opx, Magnesite = Mgn, Trevorite = Trvt and talc

Table 2: Geochemical composition variation occur with depth, CD0040 drill hole (*representativeprofile in Siruka)


LA

TE

RIZ

AT

ION

PRO

CE

SSO

FPE

RID

OT

ITE

SIN

SIRU

KA

,CH

OISE

UL

,SOL

OM

ON

ISLA

ND

SFigure 8: XRD patterns of CD0040 protolith unit from Siruka. A = 12m, B = 8m, C = 5m and D = 3m depth samples respectively. Olivine = Ol,Chlorite = Chl; Amphibole = Amp, Chrysotile = Ctl, Lizardite = Liz, Orthopyroxene = Opx, Magnesite = Mgn, Trevorite = Trvt, Maghemite= MtHt, Gibbsite = Gibb, Goethite = Gt, Asbolane = Asb, Nontronite = Nte, Kaolinite = Ka and talc

©2011

Departm

entofGeologicalE

ngineering,Gad

jahM

ada

University

85

SAG

APO

Aet

al.

Figure 9: Model graphical representation of elemental distribution in Siruka weathering profiles. Here Ni, Co, Cr and Fe are consistent withcalculated correlation factors on each zone in the profile

86©

2011

Dep

artm

ento

fGeo

logi

calE

ngin

eeri

ng,G

adja

hM

ada

Uni

vers

ity


Table 3: Mineralogical composition of the parent Siruka Peridotite and its weathered products inthe representative profile (CD0040)

*Mode of occurrence abbreviations: ++++, very abundant; +++, abundant; ++, represented; +, poorly repre-sented; +, traces; -, not identified

of the parent rock have been removed to vary-ing degrees. Moreover, most of the trace ele-ment results recovered in this work present aconcentration very near to their detection limit,therefore complicate choices for any best fit ele-ments. According to Ndjigue et al. (2008) if anelement present at its detection limit in the par-ent material is used in equation above (Equa-tion 1), a difference in its concentration in thelast decimal place can lead to wide change inthe calculated % change. Because of this phe-nomenon, a reflective element with relativelyhigher concentration in the parent rock is cho-sen. From which we calculate % changes formost other elements, assuming that the ele-ment chosen is the least mobile than the oth-ers. Applying this decisive factor and accord-ing to Nesbitt and Wilson (1992), chromium isthe opted immobile element for Siruka samplescompared to TiO2 or Zr. The parent peridotitesat Siruka are used for the mass balance calcu-lation. Relative to chromium, practical applica-tion of Equation 1 illustrate apparent removalof most major and trace element in amounts ex-ceeding ~80% of the original concentration in

the parent body prior to weathering (Table 4).Enrichment and distribution of minor and traceelements in the laterites are often controlled byweathering processes and morphology of theperidotite massifs (Marker et al., 1991).

Mass balance evaluation tabulated in Table4 clearly singled out SiO2 and MgO to be themost leached major elements out of all the otherin the profile though MgO is more depleted asweathering intensifies towards the surface ofthe profiles. SiO2 is very unstable in surfaceenvironment and on top of that its depletingbehavior is attributed to heavy leaching dur-ing dissolution and laterization process. Thedepleted behavior of MgO is attributed to itsalmost complete ionic exchange reaction withNi during dissolution and obliteration of thesilicates. On the other hand, MnO illustratevariable behavior however, prominent enrich-ment in the decomposed and Limonite transi-tion zone is consistent with the presence of as-bolane minerals in these zones. Na2O and K2Oare very low in Siruka thus are recorded to bebelow detection limit in the weathering hori-zons. Similar to Na2O and K2O, TiO2 also illus-


SAGAPOA et al.

Table 4: Mass balance evaluation (in %) for CD0040 profile, Siruka, Solomon Islands

*Chromium (Cr) is the assumed immobile element therefore used as the normalizing element

trate low values and its variable behavior in thesurface horizons is attributable to the elevatedoxidation interaction in the oxidized region.

Table 4 shows apparent enrichment of FeO*and Al2O3 as weathering increases in the pro-files at Siruka and this behavior is consistentwith the increased proportion of goethite andgibbsite observed in the surface horizons of thestudied profiles. Chemical analyses and mass-balance calculations suggest that progressiveweathering of the parent rock is characterizedby an enrichment of iron, cobalt, chromium,aluminium and manganese on the limonitezone, an incorporation of nickel at the bound-ary between the bedrock and the saprolite andin the lower part of the limonite, and a deple-tion of magnesium and silicon.

7 Discussion

Chemical analyses were performed on the sam-ples taken for each half a meter (0.5 m) thick-ness from fifteen (15) drill holes at Siruka forthis study, three of which share basaltic base-ment. Distribution model of elements on thediagnosed weathering profiles (Figure 9) sharesimilar patterns of nickel distribution as sapro-

litic laterites elsewhere in the world (e.g, NewCaledonia and Cuba, Gleeson et al., 2003).

Based on petrography, mineralogy andgeochemistry of the parent samples studied,schematic process affecting the peridotiteswhich subsequently gave rise to the devel-opment of the weathering crust in Siruka ischaracterized by, (i) obliteration of bed rockwhich resulted in dissolution and (ii) subse-quent limonitization. During obliteration anddissolution stage, smectites form and incorpo-rated with the chromium presumably releasedduring primary minerals alteration. But, as theweathering proceeds mobile elements like mag-nesium and silicon are preferentially leached(Bonifacio et al., 1997), leaving less mobile el-ements, like iron, chromium and aluminiumin the profiles and form substituted oxides.Coherently, we suggested that the leachingprocess of magnesium is attributed to the for-mation of nontronite depicted by XRD on theprofiles (see Figure 8). Presence of nontroniteis in agreement with the results presented byBosio et al. (1975) and Decarreau et al. (1987),who found nontronite in the Jacuba garnierite.

This post-serpentinization process results inreplacement of magnesium by nickel. The mag-nesium replacement process could be attributed



to an ionic exchange reaction such as: Mg-serpentine + Ni2+ = Ni-rich serpentine + Mg2+.Subsequent to the dissolution process is thelimonitization, which involve crystallization ofFe oxyhydroxides (e.g, goethite and hematite)dominant in the overlying limonite horizon atSiruka. According to Becquer et al. (2006) andGarrnier et al. (2006), aluminium, chromiumand nickel released during the weathering ofprimary minerals can be incorporated into ironoxides. This oxide rich portion of the profile isreferred to informally as limonite and the vol-ume of the upper collapsed portion may be aslittle as ~15% of the original rock (Golightly,1981).

Siruka profiles illustrate manganese oxidesto be mainly concentrated at interface betweenthe limonite zone and the saprolitic zone. Ac-cording to Butt and Sheppy (1975), high Ehand low pH conditions often prevail at inter-faces between the most surface limonite zoneand the saprolite, a region where manganeseoxides can coprecipitate with nickel, cobalt andother elements. This could explain the coex-istence of manganese and cobalt in the pro-files at Siruka. According to Llorca (1993) man-ganese is leached from olivine, pyroxene, cou-pled with their metamorphic products, and pre-cipitated as manganese oxides however, werelimited to certain horizons and depths. Man-ganese oxide in Siruka was suggested to haveprecipitated and form significant manganese-cobalt-nickel enrichment, coherent to those inOra Banda-Siberia area of Western Australia(e.g, Elias et al., 1981) and equivalently alludedto by Kuck (1992) in New Caledonia. Consid-ering the significant correlation between man-ganese oxide and cobalt oxides, Roorda andQueneau (1973) suggested that manganese oxy-hydroxides would be the main carriers of cobaltand in some cases nickel. In silicate laterites inNew Caledonia (Llorca and Monchoux, 1991)and even in clayey laterites in Western Aus-tralia (Wells, 2003), cobalt mineralization isprincipally associated with manganeseoxyhy-droxides (asbolane-lithophorite group). Man-ganese enrichment in Siruka is attributed tooccurrence of asbolane group manganese onthe limonite zone depicted by XRD (Figure

8C). Contrary, previous authors also suggestedthat nickel is present in an adsorbed or latticebound state substituting iron in less crystallinegoethite (Das et al., 1999). Nonetheless, fol-lowing previous intense deliberations of man-ganese oxides (e.g, Roorda and Queneau, 1973),we suggest that nickel, cobalt, and manganeseat Siruka have been precipitated together as ox-idized concretions during laterization stages.

Subsequent to the co-precipitation eventnickel was then leached out of earlier upperlimonitic concretions, where it was first be-lieved to have earlier concentrated with cobaltand manganese during the laterization andweathering earlier stages, and chiefly migratesto the saprolitic portion of the profiles at a laterweathering stage. Major nickel enrichments inthe saprolitic zone (average 1.33 %) occurred onfractures and voids commonly observed on thesaprolitic columns. With respect to nickel con-tents, silicate horizons are richer than oxidizedhorizons in Siruka.

Cobalt is associated with manganese mineralphases particularly on the upper part of the pro-files in Siruka, where liberated elements (e.g.nickel, silicon and magnesium) got depleted.Discrete concentration of cobalt is apparent onthe limonitic portion of the profiles. Accord-ing to Youngue-Fouateu et al. (2006), cobaltcan significantly become enriched within theless crystalline manganese mineral hosts higherup in the profiles where remobilization and re-precipitations have led to the development ofenriched zones and where nodules associatedwith well-crystalline manganese mineral of theasbolanelithiophorite group are observed. Pet-rographic analysis confirmed that nickel is asso-ciated with silica and magnesia, as lizardite andgarnierite nickel ore at the saprolite zone alongfractures and joints. The weathering-resistantchromium becomes residually enriched in themiddle-limonitic zone while depletion of alka-line metals and silica is apparent. This could beattributable to prevalence of spinel on limoniticzone depicted on XRD (Figures 8C and 8D).Prevalence of chrome on zones where dissolu-tion features observed in the profiles, as wellas bulk chemical modification suggest that the


SAGAPOA et al.

chromites are slowly dissolved during pedoge-nesis (Garnier et al., 2008).

On the other hand, enrichment of gibbsiteand manganese oxide mineral in the limoniticlayer could mean repeated remobilizations ofthe residual material in which besides goethite,magnetite– maghemite transformation is likely,where magnetite more or less weathered tomaghemite, this could also explain the highchromium content observed in the residue(Youngue-Fouateu et al., 2006). Aluminium isresidually enriched in the granular limoniticzone as demonstrated in Figure 9. This behav-ior of aluminium is consistent with the low pHvalue of the limonite horizon. This explainedthe presence of gibbsite in the limonitic zone.Parallel resemblance of Al2O3 and Fe2O3 (Ta-ble 2) is attributed to the relative associationand the occurrence of goethite, hematite andgibbsite in the upper portion of the profiles. Ex-tensive XRD analysis on the granular limoniticportion of the profiles confirmed that goethite,hematite and kaolinite are the dominant miner-als while gibbsite, asbolane and nontronite aresparsely distributed. Increasing Al2O3/FeO*ratio from wet to dry portions of the profiles,i.e. from bottom to top of the profiles, signifyactive meteoric water involvement during thelaterization process.

8 Conclusion

The lateritic weathering crust developed atSiruka, the southeastern part of Choiseul isbroadly similar to saprolite nickel laterite de-posit around the world. The mineralogical andgeochemical process which affected the ultra-mafic bedrock and gave rise to the weatheringcrust in Siruka is characterized by oblitera-tion and dissolution of bed rock followed bylimonitization.

Chemical analyses’ mass-balance calcula-tions and quantitative correlation analyses sug-gest that progressive weathering of the parentrock is characterized by an enrichment of iron,cobalt, chromium, aluminium and manganeseon the limonite zone, an incorporation of nickelat the boundary between the bedrock and thesaprolite and in the lower part of the limonite, a

depletion of magnesium and silicon. The Ni en-richment in Siruka is inferred to be attributed topercolation of surface solutions and the resultof an ionic exchange reaction as: Mgserpentine+ Ni2+ = Ni-rich serpentine + Mg2+.

With pronounced weathering, all relicts ofbedrock and saprolite were obliterated andthe limonite zone was formed. Therefore, wesuggest that following the dissolution processis limonitization where iron oxides (goethitein particular) are dominant in the overlyinglimonite horizon at Siruka. In the upper part ofthe profile, leaching of Mg favored formationof nontronite. Nickel is a marker in the varioushorizons and appears in various mineral as-sociations; generally with iron oxides (mainlyas goethite but also with magnetite-maghemiterelicts) defining the limonitic zone, unclear as-sociation with smectite (nontronite) and withserpentine in the saprolitic horizons.

Significant nickel enrichments are very par-tial in limonite zones but are chiefly developedin the saprolite zone indicating water move-ment through these zones downward to a verylow water table, meaning much of the nickelin Siruka deposit percolated downwards andreprecipitated in the saprolite zones. Nickelcontent decreases upward the weathering pro-file since it is removed during the recrystalliza-tion of goethite and the leaching of manganeseoxide phases. Crystallization of iron oxide inthe form of goethite often has low capacity fornickel fixation. This could also allow easy cap-ture of the metal by percolating down waterfrom the limonitic horizon and redeposited inthe saprolitic region at Siruka.

Conversely, nickel in Siruka has been sub-stituting Mg in altered silicate during the ear-lier phases of the weathering, incorporated asconcretions with other elements, leached fromthe limonite concretions, moves down the pro-file under the influence of gravitated percolat-ing water, and finally redeposited in the sapro-lite zone, hosted by hydrous silicates. With thiswe classify the deposit as hydrous magnesium‘silicate type’ deposit.



Acknowledgement

This work was indebted to the Global Cen-ter of Excellence (G-COE), Kyushu Universityfor its numerous supports in funding the re-search. Also grateful indeed for the techni-cal support and field expertise provided by theSMM Solomon Ltd staffs, Solomon Islands.

References

Bonifacio, E., Zanini, E., Boero, V., FranchiniAngela,M. (1997) Pedogenesis in a soil catena on serpen-tine in north-western Italy. Geoderma, 75, 33– 51.

Bosio, N.J., Hurst, J.V., Smith, R.L. (1975) Nickellifer-ous nontronite, a 15 Å garnierite, at Niquelandia,Goias Brazil. Clays Clay Miner., 23, 400–403.

Butt, C.R.M. and Sheppy, N.R. (1975) Geochemicalexploration problems in Western Australia exem-plified by Mt. Keith area. Elsevier, Amsterdam,392-415.

Brand, N.W., Butt C.R.M., Elias, M. (1998) Nickel La-terites: Classification and features. AGSO Journalof Australian Geology & Geophysics, 17 (4), 81-88.

Braun, J.-J., Ndam Ngoupayou, J.-R., Viers, J.,Dupre, B., Bedimo Bedimo, J.-P., Boeglin, J.-L.,Robain, H., Nyeck, B., Freydier, R., Sigha Nkamd-jou, L., Rouiller, J., Muller, J.-P. (2005) Presentweathering rates in a humid tropical watershed:Nsimi, South Cameroon. Geochim. Cosmochim.Acta 69 (2): 357-387.

Colin, F., Nahon, D., Trescases, J.J., Melfi, A.J. (1990)Lateritic weathering of pyroxenites at Nique-landia, Goais, Brazil: The supergene behavior ofnickel: Economic Geology, 85, 1010–1023.

Coleman, P.J. (1960) An introduction to the geologyof the island of Choiseul in the Western Solomons1957. British Solomon Islands Geological Record,16-26.

Das, S.K., Sahoo, R.K., Muralidhar, J., Nayak, B.K.(1999) Mineralogy and geochemistry of pro-files through lateritic nickel deposits at Kansa,Sukinda, Orissa. Joural of Geoogical. SocietyIndia, 53, 649– 668.

Decarreau, A., Colin, F., Herbillon, A., Manceau,A., Nahon, D., Paquet, H., Trauth-Badaud, D.,Trescases, J.J. (1987) Domain segregation in NI-Fe–Mg– Smectites. Clay Clays Miner. 35, 1–10.

Elias, M., Donaldson, M.J., Giorgetta, N.E. (1981)Geology, mineralogy, and chemistry of lateriticnickel-cobalt deposits near Kalgoorlie, WesternAustralia. Economic Geology, 76 (6), 1775-1783.

Garnier, J., Quantin, C., Martins, E.S., Becquer,T. (2006) Solid speciation and availability of

chromium in ultramafic soils from Niquelandia,Brazil. Journal of Geochemical Exploration. 88,206–209.

Garnier, J., Quantin, C., Guimarães, E., Becquer, T.(2008). Can chromite weathering be a source ofCr in soils? Mineralogy Magazine, 72, 49–53.

Golightly, J.P. (1981). Nickeliferous laterite de-posits: Economic Geology 75th Anniversary vol-ume: 710–735.

Gleeson, S.A., Butt, C.R., Wlias, M. (2003) Nickellaterites: a review. SEG Newsletter, Soci-ety of Economic Geology, 54. Available fromwww.segweb.org.

Orquidea, C., Federico, G., Ianeya, H., Jeannette,M., Edgardo, D. (2008) Cobalt and nickel recover-ies from laterite tailings by organic and inorganicbioacids. Hydrometallurgy, 94, 18–22.

Kuck, P.H. (1992) Nickel, Bureau of Mines/Mineralsyear book, 1, 863-894.

Kurtz, A.C., Derry, L.A., Chadwick, O.A., Alfano,M.J. (2000). Refractory element mobility in vol-canic soil. Geology, 28 (8): 683-686.

Llorca, S.M. (1993) Mettalogeny of supergene cobaltmineralization, New Caledonia. Australian Jour-nal of Earth Sciences, 40, 377-385.

Llorca, S., and Monchoux, P. (1991) Supergene cobaltminerals from New Caledonia. Can. Mineral, 29,149–161.

Marker, A., Friedrich, G., Carvalho, A., Melfi, A.(1991). Control of the distribution of Mn, Co, Zn,Zr, Ti and REEs during the evolution of lateriticcovers above ultramafic complexes. Journal ofGeochemical Exploration, 40, 361–383.

Moroni. M., Girardi V.A.V., Ferrario A. (2001). TheSerra Pelada Au–PGE deposit, Serra dos Carajas(Para state, Brazil): geological and geochemicalindications for a composite mineralising process.Mineralium Deposita, 36, 768–785.

Mungall, J.E. and Martin, R.F. (1994) Severe leach-ing of trachyte glass without devitrification. , Ter-ceira, Azores. Geochimica Cosmochimica Acta,58: 75-83.

Nahon, D., Paquet, H., Delvigne, J. (1982) Lateriticweathering of ultramafic rocks and the concentra-tion of nickel in western Ivory Coast. EconomicGeology, 77, 1159–1175.

Nahon, D., and Merino, E. (1997) Pseudomor-phic replacement in tropical weathering: evi-dence, geochemical consequences and kinematic-rheological origin. American Journal of Sciences.297: 393-417.

Ndjigui, P-D., Bilong, P., Bitom, D., and Dia, A.(2008) Mobilization and redistribution of majorand trace elements in two weathering profilesdeveloped on serpentinites in the Lomié ultra-


SAGAPOA et al.

mafic complex, South-East Cameroon. Journal ofAfrican Earth Sciences. 50: 305-328.

Nesbitt H. W., and Wilson R.E. (1992) Recent chem-ical weathering of basalts. American Journal ofSciences, 292: 740-777.

Ogura, Y., Murata, K., Iwai, A. (1986) Relationshipbetween chemical composition and particle-sizedistribution of ores in the profile of nickelifer-ous laterite deposits of the Rio Tuba Mine, Philip-pines. Chemical Geology, 60, 259.

Pelletier, B., (1996). Serpentines in nickel silicate oresfrom New Caledonia, in Grimsey, E.J., and Neuss,I. (eds). Nickel ’96, Australasian Institute of Min-ing and Metallurgy, Melbourne, Publication Se-ries 6/9, 197–205.

Petterson, M.G., Babbs, T., Neal, C. R., Mahoney,J. J., Saunders, A. D., Duncan, R. A., Tolia, D.,Magu, R., Qopoto, C., Mahoa, H. and Natogga, D.(1999) Geological-tectonic framework of SolomonIslands, SW Pacific: crustal accretion and growthwithin an intra-oceanic setting. Tectonophysics,301(1-2), 3560.

Purvis, J.G. and Kemp, G.J. (1975) Final report onreconnaissance permits R33, R35, R36 Choiseul.

Unpublished report Conzinc Riotinto AustraliaExploration pty Ltd (CRA) (open-file, GeologyDivision, Mining Natural Resources, Honiara,Solomon Islands).

Ridgway, J. and Coulson, F.I.E. (1987) The geologyof Choiseul and the Shortland Islands, SolomonIslands. British Geological Survey, HMSO, Lon-don, 134 (+ maps).

Roorda, H.J., Queneau, P.E. (1973) Recovery ofnickel and cobalt from limonites by aqueous chlo-riation in sea water. Trans. IMM C, 82, 79-87.

Thompson, R.B. (1960) The geology of ultrabasicrocks of the British Solomon Islands. PhD thesis,University of Sydney.

Wells, M.A. (2003) Murrin Murrin nickel laterite de-posit. Western Australia. CRC LEME, p. 3.

Youngue-Fouateu, R., Ghogomu, R.T., Penaye J.,Ekodeck G.E., Stendal H., Colin F. (2006) Nickeland cobalt distribution in laterites of the Lomiéregion, south-east Cameroon. Journal of AfricanEarth Sciences, 45, 33-47.


laterization process of peridotites in siruka, …

Documents