Petrology and geochemistry of scandium in New Caledonian Ni … · 2021. 1. 14. · Scandium,...
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Petrology and geochemistry of scandium in NewCaledonian Ni-Co laterites
Y. Teitler, M. Cathelineau, M. Ulrich, J. P. Ambrosi, Manuel Munoz, B. Sevin
To cite this version:Y. Teitler, M. Cathelineau, M. Ulrich, J. P. Ambrosi, Manuel Munoz, et al.. Petrology and geochem-istry of scandium in New Caledonian Ni-Co laterites. Journal of Geochemical Exploration, Elsevier,2019, 196, pp.131-155. �10.1016/j.gexplo.2018.10.009�. �hal-01975996�
PetrologyandgeochemistryofscandiuminNewCaledonianNi-Colaterites
Y.TEITLER1*,M.CATHELINEAU1,M.ULRICH2,J.PAMBROSI3,M.MUNOZ4
1LaboratoireGéoRessources,rueJacquesCallot,BP70239,54506Vandoeuvre-lès-NancyCedex,France
2EOST,1rueBlessig,67084StrasbourgCedex,France
3CEREGE,AMU,CNRS,IRd,INRA,CdF,Technopôledel'Arbois-Méditerranée,BP80,13545AixenProvencecedex4
4GéosciencesMontpellier,placeEugèneBataillon,34095Montpelliercedex05,France
Declarationsofinterest:none
Keywords
Scandium,laterite,Ni-laterite,goethite,NewCaledonia
Abstract
The growing demand for scandium (Sc), essential for several modern industrial applications,
thrives the mining industry to develop alternative Sc sources. In such context, significant Sc
concentrations(~100ppm)wererecentlyreportedinseveralNi-Colateriticoxideoresdeveloped
aftermafic-ultramaficrocks.ThiscontributionexaminesthedistributionofScinNi-Colaterites
fromNewCaledonia, thesixth largestNiproducerworldwide.Representative lateriticprofiles
were selected based on the protolith type and include dunite, harzburgite and lherzolite
protoliths,whereintheSccontent,determinedbytherelativeproportionofolivineandpyroxene,
rangesfrom<5ppminduniteto>10ppminlherzolite.
In Ni-Co laterites, dissolution and leaching of primary Mg-rich silicates leads to the residual
enrichment of iron as ferric oxides/oxyhydroxides in the upper horizons. Downward
remobilisation and trapping of Ni and Co lead to their local enrichment to economic
concentrations,withmaximumgradesreachedintherockysaproliteandinthetransitionlaterite
horizons,respectively.Incontrast,maximumScenrichmentoccursintheyellowlateritehorizon,
where Sc-bearing goethite reaches about ten times the Sc content of the parent rock.
Consequently, harzburgite- and lherzolite-derived yellow laterites yield maximum Sc
concentrationsupto100ppm,togetherwithmoderateNiandCoconcentrations.There,Scisa
potentially valuable by-product that could be successfully co-extracted along with Ni and Co
throughhydrometallurgicalprocessing.
Inadditiontoperidotite-hosted laterites,hornblende-richamphibolitesyieldelevatedScupto
130 ppm. The saprolitisation of amphibolites leads to the formation of a goethite-gibbsite-
kaolinitemixturewithScconcentrations>200ppm.There,goethiteisthemainSccarrierwithup
to800ppmSc.Therefore,despitetheirrelativelylimitedvolumes,amphibolite-derivedsaprolites
mayalsorepresentattractivetargetsforScinNewCaledonia.
1Introduction
Scandium(Sc),the21stelementoftheperiodictable,hasseveralremarkableapplicationsforthe
modern industry. The addition of small (<1 wt%) amounts of scandium to aluminium
outstandingly increases its weldability and its resistance to stress, corrosion and heat, while
maintaininglightweight(RoysetandRyum,2005;Toropovaetal.,1998).Scisthereforemainly
usedashardeningadditivetoaluminiumtoformAl-Scalloys,withcurrentapplicationsranging
fromtheaerospace industry to themanufactureofhigh-qualitysportsequipment. Inaddition,
scandium finds promising application in the development of Solid Oxide Fuel Cells (SOFCs),
whereinthesubstitutionofyttriumbyscandiumimprovesconductivityandlowerstheoperating
temperature,extendingfuelcelllife.Also,Scisnotablyusedinhightemperaturelights,lasersand
ceramicsmanufacturing.
Owingtoitsionicradiusandbehavioralproperties,Schashistoricallybeenregardedasaheavy
rareearthelement.Sc,whichexclusivelyoccurinthe+3oxidationstate,lacksaffinitytocombine
with the common ore-forming anions. Thus, Sc3+ hardly concentrates through geological
processesandrarelyformsSc-richminerals(Dasetal.,1971;Emsley,2014).Rather,itoccursas
a trace element in a number of hydroxide, silicate, fluoride, sulfate and phosphate minerals.
Consequently, although Sc is relatively common on Earth surface with an average crustal
abundance of 22 ppm (Rudnick and Gao, 2014), it is onlymarginally produced (~15t/yr, US
Geological Survey, 2016) and solely as a by-product of uranium, tungsten or bauxite ore
processing,mostexclusivelyinChinaandtoalesserextentinKazakhstan,RussiaandUkraine.So
far, the high quotedprices and the lack of organizedmarket for scandiumhaveprevented its
widespread commercial adoption. However, increasing needs in energy saving technologies
togetherwithrecentdevelopmentofScextractiontechniqueshaveraisedinterestforthismetal.
In the lastdecade, Sc-richoccurrenceswitheconomicallyattractivegradesand tonnageshave
been identified in some oxide-rich laterites developed after mafic and ultramafic rocks
(Aiglspergeretal.,2016;Audet,2008;Baillyetal.,2014;Chasséetal,2017;Hoatsonetal.,2011;
Maulana et al., 2016; Ulrich, 2010). There, scandium enrichment is interpreted to be largely
residual and resulting from the intense leaching ofmobile cations during lateritisation of the
parent rock, while scandium remains trapped and concentrated in neo-formed goethite. The
initial Sc content of the parent rock is therefore of primary importance in controlling the Sc
concentrations in its weathered derivatives. Two main types of Sc-bearing laterites are
recognized, depending on the parent rock type. First, oxide-rich laterites developed after
clinopyroxenitesmayyieldScconcentrationsupto300-600ppm.Todate,twohigh-grade(>300
ppm)Sclateriticcamps,bothlocatedinNewSouthWales,Australia,havebeenidentifiedandare
underdevelopmentforexploitation:theSyerston-Flemington-OwendaleandtheNyngancamps.
These laterites, considered as worldclass Sc deposits, are developed from alaskan-type
clinopyroxeniteswhichyieldunusuallyhighScconcentrationsofabout~100ppm(Chasséetal.,
2017). The lateritisation of these Sc-rich clinopyroxenites resulted in a threefold to sixfold
increase of the Sc concentration, explaining the atypically elevated Sc grades (300-600 ppm)
identified in their lateritic derivatives. Second, oxide-rich Ni-Co laterites developed after
peridotitesmay yield Sc content up to 60-100ppm, as observed in someNi-Co laterites from
Queensland (Australia), New Caledonia, Philippines and the Cuba - Hispaniola islands of the
GreaterAntilles(Aiglspergeretal.,2016,Audet,2008,Maulanaetal.,2016;Hoatsonetal.,2011).
Previousinvestigations(Aiglspergeretal,2016;Maulanaetal.,2016;Audet,2008;Ulrich,2010)
have shown thatmaximum Sc grades are observed in the goethite-dominated, yellow laterite
horizon,representingaboutatenfoldenrichmentcomparedtotheparentperidotite,withhigher
Scgrades(100ppm)beingreachedinharzburgite-derivedlateritescomparedtodunite-derived
laterites(60ppm).Suchconcentrationsaretoolowtobeeconomicallyattractiveasstandalone
Sc resources. Nevertheless, Sc could be a highly valuable by-product ofNi and Co processing,
providedthat(i)Sc-richzonessufficientlyoverlapNi-and/orCo-richzones,and(ii)metallurgical
plantsallowlow-costextractionofScalongwithNi-Cooreprocessing.Recently,severalmining
companies have developed industrialized plants for scandium recovery from Ni-Co laterites,
demonstratingsuccessfulandcost-effectiverecoveryofscandiumfromthehydrometallurgical
extractionoflateriticNi-Coores.Amongtheprimaryproducersoflateriticnickelandcobaltthat
may develop Sc co-beneficiation, opportunities exist for New Caledonia, as (i) its Ni (+Co)
resources,accountingforabout11%oftheworldnickelresources,aredominantlyhostedinoxide
laterites,whichareconsideredasthemostfavorableoretypeforScco-valorisation,and(ii)the
hydrometallurgicalprocessingplantcurrentlyoperatedbyValeintheGoromine(SouthProvince)
couldallowoperatorstoextractScdirectlyonsite.Additionally,someoccurrencesofweathered,
amphibole-richintrusiverockshavebeenrecognisedacrosstheNewCaledonianperidotitenappe
(Cluzeletal.,2006).SuchlithologiesmayhostelevatedamountsofSc,closetothatofthehigh-
grade(300ppmorhigher)clinopyroxenite-derivedlateritesfromtheSyerston-Flemingtonand
Nyngan deposits in Australia. In this contribution we examine the distribution of Sc in
representative lateritic sequences of New Caledonia, accounting for the different types of
protoliths and alteration styles.These include currentlyminedNi-Co lateritesdevelopedafter
peridotites,wherein Scmaybe a valuableby-product ofNi-Coproduction, aswell as laterites
developed after amphibole-rich, mafic intrusive bodies. Using a combination of field
characterization, petrology, whole-rock and in situ major and trace element geochemistry
analysis,weaddressthemajorprocessesandthecriticalcontrolsonthedevelopmentofSc-rich
laterites in New Caledonia and discuss the potential for Sc valorization in New Caledonian
laterites.
2Regionalgeology
TheNewCaledonianarchipelago,composedofseveralislandsdistributedalongtheNorfolkand
Loyaltyridges in theSouthwestPacific,constitutes theemergednortherntipof theZealandia
continent,formerlypartoftheGondwanasupercontinent(Mortimeretal.,2017).Themainisland,
referred to as “Grande Terre” (Figure 01), consists of a 300 km long allochtonous peridotite
ophiolite tectonically overlying continental rock sequences of the Norfolk ridge (Cluzel et al.,
2012a).ThegeologicalevolutionofNewCaledoniaincludesseveraltectonicphases.Importantly,
the North-east-dipping subduction, which appeared to the East of New Caledonia at the
Paleocene-Eoceneboundary,endedduringtheLateEocenewiththeobductionoftheperidotite
ophioliteafterblockingofthesubductionzonebytheNorfolkRidgeatca.35Ma(Cluzeletal.,
2001,2012a;PaquetteandCluzel,2007).Theperidotiteophiolite,whichhoststheNi-Colateritic
resourcesofNewCaledonia,representsabout30%ofthesurfaceofGrandeTerre.Exposedinthe
“MassifduSud”southernunitandinseveralisolatedtectonicklippesmainlyalignedintheN140
directionalongthewestcoastoftheisland(Koniambo,Tiebaghi,Poummassifs),theperidotite
nappe is mostly composed of harzburgite locally interlayered with dunite, except in the
northernmost klippes where lherzolite dominates (Ulrich et al. 2010). In the Massif du Sud,
peridotites are locallyoverlainby layeredpyroxenite andgabbro, representing thebaseof an
oceanic crust (Guillon, 1975; Prinzhofer et al. 1980). The nappe does not exhibit any major
internalthrustsandthereforedidnotexperiencethickeningduringitstectonicemplacement.Low
temperaturehydrationoftheoceanicmantlelithosphereresultedinthepartialserpentinisation
(20–60%) of the peridotite. In the main part of the nappe, serpentines principally occur as
networksofmm-to~10cm-thickfracturesandveinscontaininglizardite±chrysotile.Incontrast,
thebaseoftheperidotitenappeconsistsofa20to200mthick,subhorizontal,porphyroclastic
andmyloniticserpentinesoleformedduringobduction,containingupto100%serpentine(Avias,
1967;Orloff,1968;Guillon,1975;Cluzeletal.,2012a;Quesneletal.,2013).Locally,amphibolite
lensesoccuratthebaseoftheserpentinesole,abovethePoyaBasaltformation.Theselensesare
regardedastectonicslicesoftheSouthLoyaltybasinoceaniccrustthatrecrystallizedatca.56Ma
into the high-temperature amphibolite facies (Cluzel et al., 2012b). Also, ultramafic-mafic
(amphibolite to micro-diorite), and felsic (leucodiorite to granite) coarse grain dykes are
developedthroughouttheperidotitenappe,especiallyinthemassifduSud(Cluzeletal.,2006).
These dykes, dated at ca. 53 Ma, are interpreted as slab melts and supra-subduction zone
magmaticproducts.Subsequently,theemersionofNewCaledoniaduringorsoonafterobduction
lead to the development of a thick regolith cover that likely extended over thewhole island.
Several steppedplanation surfacesweredevelopedduring repeatedupliftphasesand/or seas
level changesuntil theOligocene. In thenorthernpart ofGrandeTerre, the laterite-blanketed
peridotitehas sincebeen intensively erodedanddissected, so that it only remains as isolated
klippesalongthewestcoastoftheisland,whereasitislargelypreservedinthesouthernlowlands
andendorheicbasinsof theSouthMassif,whereweatheringprobablycontinueduntilrecently
(Sevinetal.,2012,2014).Someoccurrencesofpseudo-karsticstructuresintheperidotitenappe
demonstratetheexistenceofanefficientdrainagesystem(Gennaetal.,2005;Trescases,1975).
Importantly,theextensivelateritisationoftheperidotiteophioliteresultedintheformationofthe
worldclasslateriticNi-CoresourcesminedinNewCaledonia.GeneticmodelsforNi-Colateritesin
NewCaledoniainvolve(i)theleachingofmostelementsincludingMgandSiafterhydrolysisof
olivine(0.4%Ni)andpyroxene(0.025%Ni),theredistributionandconcentrationofNiwithinthe
saprolitebysubstitutingNiforMginsecondaryserpentineswithupto5%Niandingarnierite
veins,whichcangradeover20%Ni,and(iii)thedevelopmentofNi-bearingoxide-rich(lateritic)
horizonsat theexpenseof thesaprolite(ButtandCluzel,2013;Cathelineauetal.,2016,2017;
Freyssinetetal.,2005;Golightly,2010;Manceauetal.,2000;Trescases,1975;Wellsetal.,2009).
TheNi-rich, silicate ± garnieritic ore has been activelymined since the late 19th century but
reservesare rapidlybeingdepleted.Theoverlying lateriticoreyields lowerNi concentrations
(1.0–1.5%Ni).However, laterite ore reserves are enormous andwill represent thebulk ofNi
reservesofNewCaledoniainthefuture.Also,Nilateriteoresoftenyieldelevated(>2000ppm)
cobalt concentrationsaddingsignificantvalue to theore.Currently, co-extractionofNiandCo
from oxide laterite ore is successfully applied at the Goro processing plant (Massif du Sud).
Similarly,ScmayaddsignificantvaluetolateriticNi(-Co)oreasitisassumedtobeconcentrated
in the yellow lateritehorizon.Although lateritic profiles inNewCaledoniamostly consist of a
saprolitic zone with mixed hydrous silicates and oxides that directly grades upwards into a
lateriticzoneessentiallycomposedofoxidesandoxyhydroxides,afewlateriticprofilesexhibit
well-developed, Ni-rich smectitic zones below the lateritic zone. The development and
preservationofNismectiticore,mostlyobservedinTiebaghiNi-lateritesandmorelocallyinsome
low-altitudelateritesoftheBoulindamassif(Latham,1986;Trescasses,1975),probablyrelates
to relatively lower drainage conditions during weathering. As suggested by previous studies
documentingthedistributionofScinsomedunite-andharzburgite-derivedNilateritesfromNew
Caledonia,boththetypeofperidotiteprotolithandthestylesoflateriticalterationareimportant
parameterscontrollingthemaximumScgrades(Audet,2008;Baillyetal.,2014).Sofar,nodata
isavailableonScinlherzolite-derivedNi-laterites,althoughUlrich(2010)observedSccontentup
to20ppm in someunweathered lherzolites.Differences in theSc contentofperidotites likely
relatetotheirvariableproportionsofmagnesianolivine(forsterite),orthopyroxene(enstatite)
andclinopyroxene(diopside).WithregardstothemaficintrusiverocksdocumentedbyCluzelet
al.(2006),nodataonScisavailableneitherontheunweatheredlithologiesnorintheirlateritic
derivatives. In the following, we document and discuss the distribution of Sc in some
representativemaficandultramafic-derivedlateriticprofilesofNewCaledonia.
Figure01:GeologymapofNewCaledonia.ModifiedfromMaurizotandVendée-Leclerc(2009).
3Methodology
3.1Samplingstrategy
InordertoinvestigatethescandiumdistributioninrepresentativelateritesfromNewCaledonia,
sampling strategy has been accordingly developed to encompass the diversity of peridotitic
(harzburgite, dunite, lherzolite ± serpentinite) and mafic to ultramafic intrusive (gabbro,
pyroxenite, amphibolite) protholiths. Selected Ni-laterites include (i) six harzburgite-derived
lateritesfromKoniambo,N’Go,NakétyandCapBocage,(ii)twodunite-derivedlateritefromCap
BocageandN’Go,and(iii)twolherzolite-derived,atypicalclay-richlateritesfromTiébaghi(Figure
01). Selected lateritic profiles aremostly preserved from erosion andwere sampled from the
bedrocktotheferricrete.Samplingwasmainlyconductedon15to>60mdeepdiamonddrillholes
thatpenetratesthenear-surfaceweatheringdowntothebedrock.Holeswereloggedintermsof
theirrocktypes,alterationstylesandalterationintensity.Drillholesamplingwastwinnedwith
fieldobservationsincorrespondingopenpitswhenaccessible.Complementarysamplecollection
has been conducted along open pit walls for some key facies and profiles when
accessible/necessary(i.e.nodrillholesavailable).Also, fiveamphiboliteoramphibole-bearing
pyroxenitedykesandtheirlateriticderivativesweresampledinN’GoandNakéty,togetherwith
twoprofilesoflateritisedgabbrodykesandtheirperidotite-derivedlateritehostsinKoniambo
andN’Go.Eventually,discretesamplesofpristineamphiboliticgabbroswerecollectedinN’Go,
Nakéty and Tiébaghi. Location coordinates are given using the RGNC Lambert NC geographic
coordinatesystem.
3.2Analyticalstrategy
Sixty polished thin-sections were prepared from collected samples and examined using
reflected/transmitted light and scanning electron microscopy with a JEOL JSM7600F at the
Georessources Laboratory and at the SCMEM. In situ mineral chemistry analysis for major
elements(Na,Mg,Al,Si,P,Ca,Ti,Mn,Fe,Co,Ni)wasconductedonelectronmicroprobe(EPMA,
WDS analysis) Cameca SX100 at the SCMEMwith typical voltage intensity of 15kV and 10nA,
respectively.InsituanalysisforScandothertraceelementswasconductedusingeitherelectron
microprobe or LA-ICP-MS. Specific conditions for electronmicroprobeWDS analysis of Sc are
25kVand150nA,withbothdetectionlimitandaccuracyofabout50ppm.LA-ICP-MSanalysisof
Sc in silicateswas performedwith theNIST60 standard. For oxides, in-house standardswere
developedandvalidated(Ulrichetal., inprep.).Detection limit isbelow2ppmformost trace
elements.Ablationtimeandspotsizesare45sand60microns,respectively.Dataprocessingwas
conductedusingtheSillsprogram(Guillongetal.,2008).
HandspecimenswerecrushedusinganagatebowlpulveriseratGeoRessourcesLaboratory,and
pulps were analysed for whole-rock major- and trace-element geochemistry at the SARM
analytical service of the CRPG. Major element oxides and Sc were analysed using ICP-OES
iCap6500withLiboratefusion.TraceelementswereanalysedbyICP-MSiCapQwithLiborate
fusionfollowedbynitricaciddigestion.Thelossonignition(LOI)componentwasdeterminedby
dryingsamplepowdersovernightat110C,ignitionat1100Candsubsequentmeasurementofthe
weight loss. Analytical accuracy (2s standard deviation) lies within typical uncertainty of the
analyticaldataforbothmajoroxidesandtraceelements,thatislessthan1%formajoroxidesand
lessthan5%formosttraceelements.
4Geologyofinvestigatedsites
4.1Ni-Co-lateritesdevelopedafterperidotites
Inthefiveinvestigateddistricts(Figure1),Ni-Colateritesexhibitasuiteofalterationfacieswith
specific mineral assemblages and composition. All the Ni-Co laterites are developed after
serpentinisedperidotitesofvariabletype.InKoniamboandNakéty,harzburgiteispredominant.
InN’GoandCapBocage,harzburgitealsodominatesbutitisinterlayeredwithsignificantvolumes
of dunite. In Tiébaghi, lherzolite dominates over harzburgite, although the intensive
serpentinisationoftheperidotitecomplicatesitsunivocalrecognitioninthefield.Theintensityof
serpentinisation inunweatheredperidotites isvariableandrelates,at thedistrict- toregional-
scale,totheverticalproximityoftheperidotitewiththeserpentinesoledevelopedatthebaseof
theophiolitenappe(positionedatca.200minelevation).TiébaghiNi-Colaterites,locatedatlow
altitude (200-400m), are therefore developed after intensely serpentinised peridotites. In
contrast,inNi-Colateritesdevelopedathigheraltitude(e.g.Koniambomassiflateritespositioned
at900m,i.e.wellabovetheserpentinesole),serpentinisationmostlyoccursasmmtodm-large
veinsofserpentinedistributedasfracturenetworkswithintheperidotite(Figure02A).
Saprolitisationoftheperidotiteinitiatesalongfractures,oftenusingpre-existingstructures,and
progressespervasivelyfromthesefracturesintothefreshperidotiteboulders.There,hydrolysis
ofolivineandpyroxeneresultsintoanincreaseofporositywhilelargelymaintaininghardness.
DiffusegreenishcolorationoftherockysaprolitelikelyrelatestoanincreaseoftheNicontent
(Figure2D).Also,veinsofhighgrade,greenishtalc-like(kerolite)arebestexpressedintherocky
saproliteandrootintothefreshperidotite(figure2B).Rockysaproliteisusuallypoorlydeveloped
inthickness(<50cm)andsharplyevolvesupwardsintoearthysaprolite(e.g.intheOPB7profile,
Koniambo,Figure3A)whereinthetotalsilicatecontentdropsbelow20%afterhydrolysisofmost
of the remaining silicates (Figure 2C, 2D, 3A). In some other profiles, rocky saprolitemay be
thicker(~3m)andremainspartlypreservedashardcoreswithintheoverlyingearthysaprolite
(e.g. in thePZ1Bprofile, capBocage, figure3A).Although earthy saproliticmaterial is friable,
primarystructuresremainessentiallypreservedwithminimalcompaction,furtherincreasingthe
porosity so that the bulk density of dry earthy saprolite may drop below 1 (referred to as
“mineraisbouchon”or“corkore”bylocalminersforitsbuoyancyproperties).Noteworthily,some
atypicallateriticprofilesexhibitwelldevelopedNismectite-richzonesbelowtheoxide-richzone
(Figure2E,3A).Suchlateritesaremostlydevelopedinthenorthernmostklippesandparticularly
atTiébaghi(Fantoche,EastandSouthAlphadeposits).Also,similarextensiveoccurrencesofNi-
smectiteswererecognizedduringourinvestigationsintheMa-ouidepositoftheKoniambomine.
Theinterfacebetweentheearthysaproliteandtheoverlyinglaterite,referredtoasthetransition
laterite, ismarkedbytheprogressivedisappearanceofprimarystructuresandthesubsequent
compactionoftheearthysaprolite.Thetransitionlateriteoftenexhibitssignificantconcentrations
of Mn-Co-Ni oxides (lithiophorite, asbolane) either as mm- to cm-thick veins or as diffuse
accumulationintheiron-richmatrix(Figure2F,3A).Thetransitionlateriteevolvesupwardsinto
theusuallywell-developedyellowlateritewhereingoethite(limonite)predominates.Theyellow
laterite then grades into the red laterite wherein hematite forms at the expense of goethite
resulting in a mineral assemblage dominated by goethite and hematite. Eventually, lateritic
profilesarecappedbymassivetovesicularferricreteandpisolithichorizons.Theseuppermost
unitsarelargelydissectedanddismantledbyerosionsothatbouldersoftheferruginoushardcaps
oftenappearencapsulatedintheunderlyinglooselateriticmaterial(Figure2G,2H,3A).
Figure 2: Field photographs of peridotite-derived laterites. (A)Unweatheredharzburgitewith
networks of serpentine (lizardite) veins (Koniambo), (B) Ni-rich talc-like (kérolite) vein
developed at thebedrock– rocky saprolite interface (Koniambo), (C)Typical sharp transition
from silicate-dominated bedrock and rocky saprolite to oxide-dominate earthy saprolite and
laterite (Koniambo). (D) Altered boulder of peridotite grading outwards from unweathered
dunite to rocky and earthy saprolite. The rocky saprolite is poorly developed (<20 cm in
thickness) and readily grades into oxide-dominated earthy saprolite (Cap Bocage). (E) Well-
developedNismectite-richzone(upto>15minverticalthickness)abovelherzoliticserpentinite
atTiébaghi. (F)Mn-Cooxide-rich transition lateritedevelopedat theearthysaprolite–yellow
laterite interface (Cap Bocage). (G) Yellow laterite grading upwards into red laterite. The red
laterite encapsulates boulders of the dismantled ferricrete (Tiébaghi). (H) Neptunian dyke of
pisolithicmaterialintotheyellowlaterite(CapBocage).
Figure3:Stratigraphiclogsandcrosssectionsofrepresentativelaterites(A)Peridotite-derived
laterites.(B)Amphibolite-andgabbro-derivedlaterites.
4.2Lateritesdevelopedaftermafic-ultramaficintrusiverocks
ThemaficdykesidentifiedinN’Go,NakétyandTiébaghidisplayasuiteoftextureandmineralogy.
AmphiboliteandamphiboliticpyroxeniteexposedinN’GoandNakétyoccurassubverticaldykes,
locally forming swarms, with thickness ranging from <10 cm up to >10 m (Figure 3B, 4A).
Amphibolites andamphibole-bearingpyroxenites are commonlymore resistant toweathering
thanthesurroundingperidotites,andthereforeformpositivereliefinoutcrops(Figure4A).These
dykes contain variable proportion of amphibole and pyroxene, from amphibolitic pyroxenites
where pyroxene dominates to amphibolites essentially composed of amphibole, and variable
crystalsize,fromfine-grained(pyroxeneandamphibole<1mm)topegmatitic(amphiboleupto
10cm,pyroxeneupto5mm)textures.Suchvariability isobserveddowntotheoutcropscale,
where two adjacent dykes often display marked differences in thickness, texture and/or
composition. Noteworthily, one large (> 10m in thickness) intrusive body identified at N’Go
shows internal zonation of texture and mineralogy, from zones consisting of monomineralic
amphibolite to zones dominated by pyroxenewith giant (>10 cm) amphibole porphyroblasts
(Figure4E).Similarly,dykesofamphiboliticgabbrosidentifiedinTiébaghi,N’Go,Nakéty(Figure
4F) display variable crystal size, from fine grained to pegmatitic texture, and contain variable
proportionoffeldspar(~50to90vol%)andamphibole(~10to50vol%).
Duetotheirincreasedresistancetoweathering,amphibolitesandamphibole-bearingpyroxenites
exposedinoutcropsexhibitalterationthatgenerallypenetratesonlytoshallowdepth(fewcmto
few m) below surface, contrasting with the well-developed laterite profiles derived from
peridotites(15to60minthickness).Themostevolvedalterationproductofthesedykesconsists
of brownish, goethite-bearing earthy saproliticmaterial, wherein primary silicates have been
largelyleachedawayalthoughprimarystructuresremainpreserved.Strictosensulaterites, i.e.
withobliteratedtextures,arerarelyobserved.Owingtothepositivereliefformedbythesedykes
relativetothesurroundingperidotite-derivedlaterites,fragmentsofamphibolite-orpyroxenite-
derived earthy saprolite may accumulate downslope in outcrops (Figure 4A). Comparatively,
amphibolite-bearing gabbros are more readily altered as they mainly contain weatherable
feldspar,sothattheymayaltertogreaterdepthandformfriablelateriticmaterial(Figure4B).
Importantly, whereas amphibolite and pyroxenite usually alter to form brownish, goethite-
bearing saprolite, gabbro alters to pinkish, hematite- and kaolinite-bearing saprolite/laterite
(Figure4B).Colorzonationpatternsarecommonlyobservedinweatheredgabbros,fromwhitish
orpalepinkishfaciesdominatedbykaolinitetocherrypinkishfacieswithhigherproportionsof
hematite(Figure4D).Suchzonationlikelyresultseitherfromprimaryzonationinthegabbroor
secondary segregation of Fe and Al during weathering. As discussed below, the observed
discrepancy in the nature of secondary iron-bearing phases (goethite vs. hematite) formed
throughweatheringofmafic-ultramaficdykes(amphiboliteandpyroxenitevs.gabbro)isofmajor
importanceincontrollingScdistributionintheirweatheredderivatives.
Figure 4: Field photographs of amphibolite- and gabbro-derived laterites. (A) Weathered
amphibolite dykes cutting across harzburgite-derived laterite. Exposed dykes form positive
reliefsdueto increasedresistancetoweathering(N’Go),(B)Hematite-bearing,gabbro-derived
laterite cutting across lherzolite-derived laterite (Tiébaghi). (C) Goethite-bearing, saprolitised
dykeofamphiboliteoverlyingharzburgite-ordunite-derivedlaterite(N’Go),(D)Internalfacies
zonationwithingabbro-derivedsaproliteexhibitingpalereddishzonesdominatedbykaolinite±
hematite and darker reddish zones with higher proportions of hematite (Tiébaghi), (E)
Unweathered amphibolite containing amphibole (hornblende) megacrystals (N’Go), (F)
Unweatheredamphibole-bearinggabbrointrusions(Tiébaghi).
5Scandiuminparentrocks
5.1Peridotites
5.1.1Mineralassemblages
Unweatheredperidotites exhibit typicalmineral assemblages essentially including (i) primary
(mantellic) magnesian olivine (forsterite), orthopyroxene (enstatite) and clinopyroxene
(diopside), whose relative proportions depend on the type of peridotite, and (ii) secondary
(developed at the expense ofmantellic silicates) serpentine (mainly lizardite),whose relative
proportion depend on the degree of serpentinisation. In dunite, as observed inN’Go and Cap
Bocage,forsteriteisthepredominant(>90vol%)mantellicsilicate(Figure5A-5C)whileenstatite
may only be present as an accessory phase, both mineral forming automorph crystals up to
~8mm.Accessorychromiferousspinelalsooccursasalate-crystallizing,interstitialphase(Figure
5C).Comparatively,harzburgiteexhibitshigherproportionofenstatite(>10vol%andupto~30
vol%)attheexpenseofforsterite.Noteworthily,someharzburgitesidentifiedatKoniamboand
Nakétycontaindiopsideasaccessoryphase(<1vol%),eitherovergrowingenstatitecrystalsoras
a late interstitialphase. In lherzolite,proportionsofenstatiteandforsteriteareabout45vol%
whilethediopsidecontentincreasesupto5-10vol%forminglarge,automorphcrystals,aswell
as exsolutionswithin enstatite. Secondary serpentinisation, characterized by the formation of
lizardite ± chrysotile at the expense of primary mantellic silicates, is ubiquitous within the
peridotite.Inmoderately(~30-50%)serpentinisedperidotites(i.e.distaltotheserpentinesole),
lizardite occur mainly as vein network (or mesh) fragmenting olivine and pyroxene crystals
(Figure5A-5I).Pervasiveserpentinisationofthefragmentedsilicatesmaybeobservedbutrarely
penetratesdeepintotheprimarysilicatecrystals.Instronglyserpentinisedperidotites,suchas
the lherzolite-derived serpentinites typically observed inTiébaghi,most if not all theprimary
silicates are pseudomorphologically replaced by lizardite after extensive, pervasive
serpentinisation(Figure5J-5K).Serpentinisationiscommonlyaccompaniedbytheformationof
magnetite,eitheralong initialcrystallographicplanesofserpentinisedenstatiteoraseuhedral
crystals in serpentinised forsterite (Figure 5K-5L). Also, some calcite veins developed at the
expense of diopside are locally observed inmoderately serpentinised lherzolites (Figure 5H).
Theseveins,whicharenotrecognizedinhighlyserpentinisedperidotites,probablyformedearly
duringtheserpentinisationprocessandweresubsequentlyreplacedbylizarditeaftercomplete
serpentinisationoftheperidotite.
Figure5:OpticandSEMmicrophotographsofmineralassemblagesinperidotiticbedrocks.(A-C)
DunitefromCapBocage,(D-F)HarzburgitefromKoniambo,(G-I)LherzolitefromTiebaghi,(J-K)
serpentinite from Tiébaghi. Mineral abbrev.: Lz = lizardite, Fo = forsterite, Sp = spinel, En =
enstatite,Di =diopside, Ca= calcite, Lz(Fo)= lizardite after forsterite, Lz(En)= lizardite after
enstatite,Ma=magnetite.
5.1.2Majorandtraceelementgeochemistry
Eleven samples of unweathered peridotites including dunite, harzburgite, lherzolite and
lherzolite-derived serpentinite were analysed for whole-rock major and trace element
geochemistry(Table1).Also,sixsamplesofunweatheredlherzolitefromtheBabouillatzoneof
theTiébaghimassifanalysedbyUlrich(2010)wereincludedtothisdataset.Si,Mgandtolesser
extent Fe are the principal constitutive elements in peridotites, with SiO2, MgO and FeO
concentrationsof38-43,35-45and6-7wt%,respectively(Figure6A).Whole-rockconcentrations
formajorelementsareconsistentwiththemineralchemistryoftheprincipalconstitutivephases
(forsterite,lizardite,enstatite)asanalyzedbyEPMA(Figure6A).Moderatelyserpentiniseddunite
andharzburgiteyieldthehighestMgOcontentowingtohigherproportionofMg-richforsterite.
The elevated LOI illustrates the significant contribution of hydrous serpentine (lizardite) in
peridotites,fromabout6wt%LOIinmoderatelyserpentinised(~30%serpentine)peridotitesto
about17%inserpentinites(~100%serpentine).Al2O3andCaOconcentrationsvaryfromabout
~0.25 wt% in dunite to about 2 wt% in lherzolite (Figure 6A) and are therefore inversely
correlatedwithMgO. The Al2O3 content of peridotitesmostly relates to the proportion of Al-
bearing(1.6 to3wt%Al2O3)enstatite (Figure6A).TheCaOcontentalso largelyrelates to the
proportionof enstatite althoughaccessory,Ca-richdiopside (~24wt%CaO)may significantly
contributetothetotalCaOcontentoflherzolites.TheCr2O3contentofperidotitesrangesfrom0.3
to0.5wt%andresultsfromthecontributionofenstatiteandaccessoryCr-spinel.TheNicontent
ranges from0.20 to0.25wt%withhighergrades reached inharzburgiteandduniteowing to
higherproportionofNi-bearingforsterite(0.25-0.40wt%Ni).
SimilartoAl,Scexhibitslargevariationsofconcentrationinperidotites,from3-5ppmindunite
to 6-8 ppm in harzburgite and 10-25 ppm in lherzolite (Figure 6B). Noteworthily, lherzolites
investigatedinthisstudyfromtheAlphaandFantochedeposits(Tiébaghi)haverelativelylowSc
concentrations(~10ppm)whereaslherzolitesfromtheBabouillatzone(Tiébaghi)analysedby
Ulrich(2010)yieldhigherScconcentrations(15-25ppm).LA-ICP-MSanalysis(Figure7)indicates
thattheSccontentinmantellicsilicatesincreasesfromforsterite(3-7ppm)toenstatite(15-30
ppm)anddiopside(40-70ppm),sothattheincreaseoftheSccontentinperidotitesfromdunite
to harzburgite and lherzolite relates to increasing proportions of enstatite (and accessorily
diopside)attheexpenseofforsterite.TheSccontentofenstatiteanddiopsidemayvarydepending
on the consideredperidotitemassif. Enstatite indunite andharzburgite from theN’Gomassif
yieldsSccontentof15-20ppm,whileenstatite inharzburgite fromtheNakétyandKoniambo
massifsyieldsSccontentof20-30ppm,andenstatiteinlherzolitefromtheTiébaghimassifyields
Sccontentof~30ppm.Similarly,diopsideinharzburgitefromKoniamboyieldsSccontentof40-
50 ppmwhile diopside in harzburgite from Nakéty and in lherzolite from Tiébaghi yields Sc
contentof60-70ppm(Figure7).Scconcentrationsinlizardite(5-50ppm)stronglydependon
theSccontentofthemantellicsilicateprecursorsandthereforecovermostoftherangeobserved
inprimarysilicates(Figure7).Itisassumedthatthedegreeofserpentinisationdoesnotaffectthe
whole-rockSccontentofperidotites.
Noteworthily,whole-rockScconcentrationsarewellcorrelatedwithVandtoalesserextentwith
Al concentrations (Figure 6B), so that the whole-rock Sc content of peridotites may be
approximativelyestimatedfromtheAlandVconcentrationsasfollowing:
(1) Sc(ppm)=10.6*Al2O3(wt%)=0.23*V(ppm)
These whole-rock correlations result from the co-increase of the Sc, V and Al content from
forsteritetoenstatiteanddiopside(Figure7).Inenstatite,ScandAlconcentrationsdefineatrend
fromlow-Al-ScenstatiteinN’GoandCapBocageduniteandharzburgite(3000-5000ppmAl,10-
20ppmSc)tohigher-Al-Sc-enstatiteinTiébaghilherzolite(15000-20000ppmAl,~30ppmSc)
withintermediateconcentrationsoccurringinenstatitefromNakétyandKoniamboharzburgites.
Theco-increaseofAlandScinperidotitesisthereforenotonlyduetoincreasingproportionsof
enstatite at the expense of forsterite, but also to increasing Al and Sc concentrations within
enstatitefromduniteandharzburgitefaciesofthesouthernmassifstoharzburgiteandlherzolite
faciesofthenorthernmassifs.InSc-richlherzolite(>15ppm),enstatitealonecanhardlyaccount
fortheelevatedSccontent.Insuchfacies,diopsidecontributessignificantlytothewhole-rockSc
budget. Also,mineral chemistry data indicates that Sc and V concentrations co-increase from
forsteritetoenstatiteanddiopside,sothattheSccontentofperidotiticmineralphasesmaybe
approximativelyestimatedfromitsVcontentasfollowing:
(2) Sc(ppm)=0.26*V(ppm)+6
IncreasingproportionsofSc-andV-bearingenstatiteanddiopsideinperidotiteswellexplainthe
Sc–V correlation identified from whole-rock analysis (Figure 6, 7). Eventually, Sc and Ti
concentrations inprimary silicatesmayexhibit correlation trends from low-Sc-Ti-forsterite to
higher-Sc-Ti-enstatiteanddiopside.However,contrastingwithSc-AlandSc-Vcorrelationtrends,
Sc-Ti correlation trendsdisplayspecific slopesdependingon theconsideredperidotitemassif,
suggestingthatSc-Ticorrelationsmayonlybedefinedatthemassifscale.
Figure 6: Whole-rock geochemistry data for peridotites (dunite, harzburgite, lherzolite and
serpentinite).(A)PlotsofSiO2,LOI,FeO,Al2O3,CaO,Cr2O3andNi(wt%)againstMgO(wt%).Grey
rectangles indicate mineral compositions from electron microprobe data. (B) Plots and
correlationsofSc(ppm)againstAl2O3(wt%)andV(ppm).
Figure7:PlotsandcorrelationsofSc (ppm)againstAl,VandTi (ppm) inperidotiticminerals
(diopside,enstatite,forsteriteandlizardite)fromLA-ICP-MSdata.
5.2Mafic-ultramaficintrusiverocks
5.2.1Mineralassemblages
Unweathered mafic-ultramafic intrusive rocks including amphibolite, amphibolitic pyroxenite
and amphibolitic gabbro exhibit specific mineral assemblages and textures. Amphibolite
occurrences identified inNakéty andN’Go contain over 90 vol%hornblendewith crystal size
rangingfrom<500µmto~10cmtogetherwithminorenstatiteandaccessorytitaniteandFe-Ti-
oxides (Figure 8A-8C). Textural relationships indicate that hornblende develops surrounding
corrodedenstatitecores.Similarmineralassemblagesareobservedinamphiboliticpyroxenite
althoughenstatiteoccursingreaterproportionfrom>10upto80vol%(Figure8C-8E).Whereas
hornblendemaydevelopporphyroblastictextureswithcrystalsupto10cm,enstatitecrystalsize
rarely exceeds 5mm. Amphibolitic gabbro andmicrogabbro fromN’Go, Nakéty and Tiébaghi
contain50to90vol%plagioclaseand10to50vol%hornblende,togetherwithaccessoryFe-Ti
oxides,sulfidesand/orphosphates(Figure8G-8L).Asconfirmedbymineralchemistryanalysis
(seebelow),plagioclaseexhibitvariouscompositionranging fromanorthite toandesine. Inall
mafic-ultramafic dykes, secondary chlorite commonly forms at the expense of enstatite and
plagioclase(Figure8L).Prehnitisationofanorthiteisalsoobservedinsomegabbrooccurrences
fromTiébaghi.
Figure8:OpticandSEMmicrophotographsofmineralassemblagesinmaficintrusivebedrocks
fromN’Go,NakétyandTiébaghi. (A-C)Amphibolite, (D-F)Amphibole-bearingpyroxenite, (G-I)
amphiboliticmicrogabbro, (J-K) amphibolitic gabbro.Mineral abbrev.: Ho = hornblende, En =
enstatite,An=anorthite,Ch=chlorite,Ma=magnetite.
V.2.2Majorandtraceelementgeochemistry
Twelve samples of unweathered amphibolite, amphibole-bearing pyroxenite and amphibole-
bearinggabbrowereanalyzedforwhole-rockmajorandtraceelementgeochemistry(Table1).
Mg and Fe whole-rock concentrations co-increase from 5-10 wt%MgO and 4-8 wt% FeO in
amphiboliticgabbroto15-20wt%MgOand9-11wt%FeOinamphibolite,andupto25wt%MgO
and 15 wt% FeO in amphibolitic pyroxenite (Figure 9A). Conversely, Mg and Al whole-rock
concentrationsareanti-correlatedasAl2O3concentrationsdecreasefrom>15wt%ingabbroto6-
11wt%inamphiboliteanddownto~3wt%inpyroxenite.Thesewhole-rockcorrelationtrends
formajorelementsarewellexplainedbyvaryingproportionsoftheprincipalconstitutivephases,
as (i) Al-rich and Mg-Fe-free plagioclase is predominant in gabbro, (ii) Al-Fe-Mg-bearing
hornblende is predominant in amphibolite and (iii) Al-poor and Mg-Fe-rich enstatite is
predominant inpyroxenite. Similar toAl,CaandNawhole-rockconcentrationsdecrease from
amphibolitetopyroxeniteowingtoincreasingproportionofCa-Na-poorenstatiteattheexpense
ofCa-Na-bearinghornblende. In gabbro, Ca andNawhole-rock contentmayvary significantly
dependingbothontherelativeproportionofplagioclaseandhornblende,andontheCa/Naratio
ofplagioclase.Ticoncentrationismaximuminamphibolitereachingupto1.3wt%TiO2.Mineral
chemistry analysis suggests that hornblende is the predominant carrier of Ti in amphibolite,
althoughsignificantcontributionofaccessorytitaniteand/orFe-Tioxidestothewhole-rockTi
budgetisnotexcluded.
Figure 9: Whole-rock geochemistry data for mafic-ultramafic intrusive rocks (amphibolitic
gabbro,amphiboliticpyroxenite,amphibolite).(A)PlotsofSiO2,CaO,Al2O3,FeO,Na2O,LOIand
TiO2 (wt%) againstMgO (wt%). Grey rectangles indicatemineral compositions from electron
microprobedata.(B)PlotsandcorrelationsofSc(ppm)againstTiO2(wt%)andV(ppm).
Similar to major elements, Sc exhibits large variations of concentration in mafic-ultramafic
intrusive rocks, from 30-50 ppm in pyroxenite to 40-80 ppm in gabbro and 100-130 ppm in
amphibolite(Figure9B).LA-ICP-MSanalysis(Figure10)indicatesthatScisdominantlyhostedby
hornblendewithScconcentrationsrangingfrom50to180ppmandtoalesserextentbyenstatite
(20-25ppm),whileothermineralphases(plagioclase,Fe-Tioxides)yieldlowScconcentrations
(<5ppm).Thewhole-rockSccontentofunweatheredintrusiverocksthereforedependsbothon
therelativeproportionandtheSccontentofhornblende.Noteworthily,amphibolite(>90vol%
hornblende)typicallycontainSc-richhornblende(>100ppm),sothatthewhole-rockSccontent
of amphibolite is accordingly high. Sc-rich hornblende also occurs in some gabbro and
microgabbro occurrences as those identified in Nakéty and Tiébaghi, although the large
proportion of Sc-poor plagioclase in gabbros dilute and buffer the contribution of Sc-rich
hornblendetothewhole-rockSccontent. Incontrast,hornblende-bearingpyroxenite, together
withsomeothergabbrooccurrencesidentifiedinN’Go,containhornblendewithlowerSccontent
(50-80ppm),sothatthewhole-rockSccontentofpyroxenitedoesnotexceed50ppmdespite
significantproportionofhornblendeinthemineralassemblage.
Importantly, whole-rock Sc concentrations are well correlated with Ti and V concentrations
(Figure9B),so that theSccontentofmafic-ultramafic intrusiverocksmaybeapproximatively
estimatedfromtheTiandVconcentrationsasfollowing:
(3) Sc(ppm)=100*TiO2(wt%)=0.17*V(ppm)
Asrevealedbymineralchemistryanalysis(Figure10),thesewhole-rockcorrelationtrendsare
best explained by the co-variation of the Sc, V and Ti content of hornblende, which is the
predominantmineralhostfortheseelements.Atthemineralscale,theSccontentofhornblende
maybeestimatedfromtheTi,VbutalsotheFecontentofhornblende,asfollowing:
(4) Sc(ppm)=0.017*Ti(ppm)=0.16*V(ppm)=163*Fe(wt%)
Whereas Sc, Ti and V and predominantly hosted by hornblende so that their co-variations in
hornblendedirectlytranslateintowhole-rockcorrelationtrends,Feishostedbothbyhornblende
andenstatite, and thewhole-rockFe content cannotbe indicativeof theSc contentexcept for
monomineralicamphibolite.
Figure 10: Plots and correlations of Sc (ppm) against Ti, V (ppm) and Fe (wt%) inmagmatic
minerals(enstatiteandhornblende)fromLA-ICP-MSdata.
6Scandiuminlaterites
6.1Peridotite-hostedNi-Colaterites
6.1.1Parageneticsequence
Lateritisationofperidotitesischaracterizedbythedevelopmentofseveralalterationfaciesfrom
thebedrocktotheferricreteandisassociatedwithcomplexmineralandtexturaltransformations
(Figure11).Theparageneticsequencecommonlyobservedinperidotite-derivedlateritetypically
includesmultiplestagesofdissolutionofprimarysilicatesandformationofsecondarysilicates
andoxides(Figure12).Atthebedrock–rockysaproliteinterface,dissolutionofmantellicsilicates
(olivine, pyroxene) is associated with the development of secondary goethite (referred to as
skeletalgoethite,Go-sk)alongsilicategrainboundariesandcrystallographicplanes(Figure11A-
11C). Lizardite (Lz) is preserved from dissolution in the rocky saprolite although it is largely
transformed into secondary, nickeliferous serpentine/talc-like (Lz-ni) either along fractures
(Figure 11A) or through pervasive replacement (Figure 11B, 11C). Locally, accessory quartz
and/orsmectitesmayformasinfillsofpre-existingsilicates(Figure11C)buttheseareinturn
rapidly dissolved upwards in the sequence. Some atypical clay-rich laterite such as those
developed at Tiébaghi exhibit well-developed smectitic zonesmostly consisting of Ni-bearing
nontronitepseudomorphingpre-existingsilicates (Figure11D).However, inmost investigated
profileselsewhereinNewCaledonia,therockysaprolitedirectlygradesupwardsintotheearthy
saprolitewhereinmost(>80vol%)oftheprimaryandsecondaryserpentinesareepigenisedinto
goethite(Go-lz)(Figure11E).Fine-grainedgoethiticmaterialderivedfromtheoverlyinglaterite
(Go-lt)may occur asmechanic infill of voids andnotablywithin dissolved/skeletized silicates
(Figure 11F). The transition between the earthy saprolite and the laterite is marked by the
accumulation(upto~5vol%)ofMn-Co-Nioxidesmostlyrepresentedbylithiophorite(Lp)which
developsattheexpenseofepigeneticgoethiteGo-lz(Figure11G).Upwards,theyellowlateriteis
characterizedbytheprogressivecompactionanddestructionofpre-existingstructuresthrough
the fragmentation of skeletal (Go-sk) and epigenetic (Go-lz) goethite (Figure 11H). These
fragmentsarelooselycementedbyfinegrained, lateriticgoethite(Go-lt)that likelyunderwent
severalstagesoffragmentation,dissolutionandreprecipitation(Figure11I).Thetransitionfrom
theyellowtotheredlaterite isassociatedwiththeformationof finegrainedlateritichematite
(He-lt) at the expense of the lateritic goethite matrix (Go-lt), together with the progressive
decreaseinsizeandabundanceoftheremainingskeletal/epigeneticgoethiteandMn-Co-Nioxide
clasts(Figure11J).Eventually,theredlateriteiscappedbytheferruginoushardcaporferricrete.
The basal portion of the ferricrete is usually autochtonous and is characterised by the
crystallizationofeuhedralgoethite(Go-fe,upto~5µm)andhematite(He-fe,upto1µm)atthe
expenseofpre-existingoxides(Figure11K).Theupperportionoftheferricreteusuallyconsists
ofaccumulatedgoethiticandhematiticpisoliths(Go-piandHe-pi)cementedbyagoethite-rich
matrix(Figure11L).Overgrowthofpisolithicgoethiteonhematitecortexiscommonlyobserved.
Also, large (3mm-1cm) goethite pisoliths may exhibit marked zonation textures indicating
successivegrowthstages.Throughoutthelateriticprofile,accessorymagnetiteandchromiferous
spinelaremostlyresistanttoweatheringandresiduallyaccumulateupwards.
Figure 11: Backscattered Electron (BSE) microphotographs: (A) Bedrock – rocky saprolite
interface: onset of olivine/pyroxene dissolution and formation of secondary Ni-bearing
serpentine/talc-like (Lz-ni) at the expense of lizardite (Lz). (B-C) Rocky saprolite: effective
dissolutionofolivine/pyroxene,formationofskeletalgoethite(Go-sk)preservingthestructureof
primary silicates, togetherwith secondaryNi-bearing serpentine/talc-like (Lz-ni) and kerolite
(Ke).(D)Atypicalclay-richzonefromTiébaghilaterites:replacementofprimaryandsecondary
serpentines by smectites (Sm), mostly Ni-bearing nontronite. (E-F) Earthy saprolite:
epigenisationofserpentinesintogoethite(Go-lz)andonsetofvoidinfillbyfine-grainedlateritic
goethite(Go-lt).(G)Transitionlaterite:accumulationofMn-Co-Nioxides,mostlylithiophorite.(H-
I) Yellow laterite: fragmentation of pre-existing structures. (J) Red laterite: Advanced texture
obliteration,decreaseinsizeandabundanceoftheremainingskeletal/epigeneticgoethiteand
Mn-oxide clasts, and formationof lateritichematite (He-lt)within the lateritic goethite (Go-lt)
matrix.(K)Basalallochtonousferricrete:formationofcrystallisedferricriticgoethite(Go-fe)and
hematite (He-fe). (L) Pisolithic horizon: formation of pisolithic goethite (Go-pi) locally
overgrowingpisolithichematite(He-pi)and/orencapsulatingCr-spinel(Sp)relics.
Figure 12: Evolution ofmineral relative abundances and assemblages along a typical lateritic
sequencedevelopedafterharzburgite.
6.1.2Distributionofmajorandtraceelements
The present section first describes the whole-rock geochemical changes along three profiles
selectedfortheirrepresentativeness:(i)thedunite-derivedPZ1Bprofile(17samples)fromCap
Bocage (Figure 13), (ii) the harzburgite-derived OPB7 profile (16 samples) from Koniambo
(Figure14)and(iii)thelherzolite-derivedTIEAprofile(14samples)fromTiébaghi(Figure15).
Elementarycorrelationsobtainedfromwhole-rockanalysis(Figure16)arethenpresentedfor
the entire dataset that includes 105 samples collected along ten peridotite-derived lateritic
profiles (Table 1). In situminor (Si, Al, Cr,Ni; EPMAWDS analysis) and trace (Sc; LA-ICP-MS
analysis) element concentrations in goethite and hematite along the OPB7 profile are finally
documented(Figure17)anddiscussedwithregardstowhole-rockgeochemicaldata.
Typicaloxide-richlateriticprofilesPZ1BandOPB7exhibitcharacteristicgeochemicaldistribution
patterns.Inbothprofiles,therockytoearthysaproliteinterfaceconstitutesthemostprominent
geochemicaltransitionwiththesharpdepletionofSiO2andMgOfrom40to<5-10wt%,andthe
concomitantincreaseinFe2O3from~7to~60wt%(Figure13,14).Contrastingwithoxide-rich
laterites,lateriticprofilesfromTiébaghiexhibitclay-richsaproliticzonescharacterizedbymore
progressiveMgdepletionandFeenrichment(Figure15).Fromthebaseoftheearthysaproliteup
to the ferricrete, Fe2O3 concentrations further increase up to ~70wt% together with further
decreaseoftheSiO2andMgOcontents.SimilartoFe,anumberofelementsincludingAl,Cr,Mn,
Ni,Co,Sc,VandREEsexhibita~tenfoldenrichmentinthelateriticprofilecomparedtotheparent
peridotite.Howevermostoftheseelementsdisplaydifferentdistributionpatternscomparedto
Fe in that their maximum concentrations occur at intermediate positions within the lateritic
profile.Niusuallyreachesmaximumconcentrationsintherockysaprolite(~2wt%)andalmost
linearlydecreasesinconcentrationupwardsintheprofile.Similarly,MnandCoconcentrations
aremaximuminthetransitionlateriteanddecreaseupwards,whileAlandScconcentrationsare
maximum in theyellow laterite (Figure13,14).The initial Sc contentof theparentperidotite
stronglycontrolsthemaximumScgradeofitslateritisedderivatives,sothatthedunite-derived
lateritefromthePZ1BprofileyieldsamaximumScgradeof~60ppm,whiletheharzburgite-and
lherzolite-derived laterites from the OPB7 and TIEA profiles, respectively, yieldmaximum Sc
gradesof~100ppm.
Figure13:Geochemicalevolutionalongdunite-derivedlateriticprofilePZ1B(CapBocage):(A)
Majoroxides:SiO2,MgO,Fe2O3,Al2O3,Cr2O3,MnO(wt%),(B)Economicelements:Ni(wt%),Co
(ppm),Sc(ppm),traceandLREE:V,La,Ce(ppm).
Figure14:Geochemicalevolutionalongharzburgite-derivedlateriticprofileOPB7(Koniambo):
(A)Majoroxides:SiO2,MgO,Fe2O3,Al2O3,Cr2O3,MnO(wt%),(B)Economicelements:Ni(wt%),
Co(ppm),Sc(ppm),traceandLREE:V,La,Ce(ppm).
Figure15:Geochemicalevolutionalong lherzolite-derived lateriticprofileTIEA(Tiébaghi): (A)
Majoroxides:SiO2,MgO,Fe2O3,Al2O3,Cr2O3,MnO(wt%),(B)Economicelements:Ni(wt%),Co
(ppm),Sc(ppm),traceandLREE:V,La,Ce(ppm).
PlotofScagainstFe2O3 concentrations for thenine investigated lateriticprofiles (Figure16A)
indicatesthatScandFearepositivelycorrelatedfromtheparentrockuptothelowerportionof
theearthysaprolite(Fe2O3≤60wt%).There,theSccontentmaybeapproximativelyestimated
fromtheFe2O3concentrationsasfollowing:
(5) Sc(ppm)=1.0*Fe2O3(wt%)
Aboveintheprofiles(Fe2O3>60wt%),Scconcentrationsarelargelyvariableanddiscorrelated
withFeconcentrations.Fromtheupperportionoftheearthysaproliteuptotheyellowlaterite,
ScisenrichedcomparedtoFe,sothatScgradesreach90-100ppminharzburgite-andlherzolite-
derivedperidotiteswhiletheFe2O3contentdoesnotexceed70wt%.Intheuppermosthorizons
(redlateriteandferricrete),Scisatfirstorderanti-correlatedtoFe,asScconcentrationsdecrease
downto~10ppmwhileFe2O3concentrationscontinuouslyincreaseupto80wt%.Incontrast,Sc
concentrationsarepositivelycorrelatedwithAlandtoalesserextentwithVconcentrationsfrom
theparentbedrockuptheredlaterite,sothattheSccontentmaybeapproximativelyestimated
fromtheAl2O3andVconcentrationsasfollowing:
(6) Sc(ppm)=11.7*Al2O3(wt%)=0.32*V(ppm)
Figure 16: Plots of Sc (ppm) as a function of Fe2O3, Al2O3 (wt%) and V (ppm) along the 9
investigatedperidotite-derivedlateriticprofiles(whole-rockdata).
At themineral scale, in situ analyses of ironoxides andoxyhydroxides from theOPB7profile
revealthatthedifferentgenerationsofgoethiteandhematite(definedfromopticandscanning
electronmicroscopy,seesectionVI.1.1)yieldspecificminorandtraceelementconcentrations.
Early-formed skeletal goethite yields the highest Si and Ni concentrations (~2 and 1 wt%,
respectively)togetherwithlowAlandCrconcentrations(~1and0.3wt%,respectively)(Figure
17A).Subsequentformationofepigeneticandlateriticgoethiteisassociatedwithadecreaseof
theSiandNiconcentrationsandanincreaseoftheAlandCrconcentrations.Late-stageferricritic
andpisolithicgoethiteandhematitearecharacterizedbyfurtherdepletionofSiandNi,together
withamajor increaseof theCrcontent(upto5wt%)andvariableAlconcentrations.Also,as
revealed by LA-ICP-MS analysis, in situ Sc concentrations of goethite and hematite strongly
depends on their vertical position along the lateritic profile (Figure 17B). Variations of Sc
concentrations in goethite, and to a lesser extent hematite, largely explain the distribution of
whole-rock Sc concentrations, so that the distribution of Sc in the laterite profile is mostly
controlledbyvariationsof theSccontent ingoethite,ratherthanbytherelativeproportionof
goethiteandhematite.
Figure17:(A)PlotsofAl,CrandNiasafunctionofSi(wt%)ingoethiteandhematitefromthe
harzburgite-derived OPB7 profile, Koniambo (WDS data). (B) Evolution of the Sc content of
goethite in theOPB7 profile (LA-ICP-MS data) and comparisonwithwhole-rock Sc and Fe2O3
concentrationpatterns.
6.2Mafic-ultramaficintrusiverocks
6.2.1Parageneticsequence
Weathering of mafic and ultramafic intrusive rocks (amphibolites, pyroxenites and gabbros)
resultsinthedissolutionofprimarymagmaticmineralsandtheformationofspecificsecondary
mineral assemblages. In amphibolites and amphibole-bearing pyroxenites, dissolution of
hornblendecommonlyinitiatesalongcrystallographicplanesandisassociatedwiththeformation
ofearly-stageskeletalgoethite.Skeletalgoethitenetworksareinturncoatedeitherbyeuhedral
gibbsite(Figure18A)orbypoorlycrystallizedkaolinite(Figure18B),eachindividualgoethitic
compartmentexhibitingeithergibbsiteorkaolinitecoatings,butnotboth.Locally,bothgibbsite
and kaolinite may however occur within a single compartment. In such a case, textural
relationshipscommonlyargueforalatterformationofgibbsiterelativetokaolinite(Figure18E),
although rare occurrences of kaolinite developed after gibbsitemay also be observed (Figure
18F). Early-stagealterationofenstatitemayresult intheformationofsmectites(Figure18C),
which are in turn readily transformed into kaolinite (Figure 18G). Further development of
alteration leads to increaseddevelopmentof goethite at the expenseof kaolinite (Figure18E-
18H),eventuallyresultingintheformationofamatured,goethite+gibbsiteassemblage(Figure
18I).Contrastingwithamphibolitesandpyroxenites,weatheringofgabbroscommonlyresultin
theformationofkaoliniteratherthanAl-oxides(Figure18J),and,moreimportantly,theformation
ofhematiteinsteadofgoethite(Figure18K,18L).There,hematitetypicallypostdateskaolinite,as
indicatedbyovergrowth(Figure18K)andreplacement(Figure18L)textures.Suchreplacement
of kaolinite by hematite argue for the downward remobilization of iron, as previously
characterizedinsomeafricanlateriticprofiles(Ambrosietal.,1986).
Figure18:BackscatteredElectron(BSE)microphotographs:(A-I)amphibolite-derivedsaprolite.
(J-L)gabbro-derivedsaproliteand laterite.Ho=hornblende,Go=goethite,Gi=gibbsite,Ka=
kaolinite,En=enstatite,He=hematite.
6.2.2Distributionofmajorandtraceelements
Thesaprolitisationofamphibolite(andamphibole-bearingpyroxenite)dykesisassociatedwith
majordepletionofmobilecationsSi,MgandCa,togetherwiththeresidualenrichmentofFe,Al
andCr(Figure19A).Owingtotheformationandrelativepreservationofkaoliniteinsaprolitized
amphibolites, the observed depletion in Si is progressive and delayed compared to the sharp
depletioninMg(andCa),contrastingwithtypicalperidotite-derivedlateriteswhereinbothSiand
Mgexhibitsharpandcongruentdepletion.FeandCrenrichmentsaremaximalatthetopofthe
alterationprofile,whereasAlconcentrationsaremaximalatanintermediatepositionwithinthe
profile. Such distribution patterns are similar to those observed in typical peridotite-derived
laterites.EconomicelementsNi,CoandScalsoexhibitsimilardistributionpatternscomparedto
thoseobservedinNi-laterites:Niconcentrationsaremaximuminthelowersaprolitealthoughnot
exceeding1wt%.CoandScmaximumconcentrationsoccuratintermediatepositionwithinthe
profile(Figure19B).Importantly,saprolitizedamphibolitesexhibitatwo-tothreefoldincrease
inScconcentrationscomparedtotheparentrock,sothatfertileamphibolites(≥100ppmSc)alter
tosaprolitewithScconcentrationsupto~300ppm.AsobservedinNi-laterites,Scinsaprolitized
amphibolitesexhibitdistributionpatternssimilartothoseofAlandV.
Figure 19: Geochemical evolution along amphibolite-derived laterite profile NGPB (N’Go): (A)
Majoroxides:SiO2,MgO,Fe2O3,Al2O3,Cr2O3,MnO(wt%),(B)Economicelements:Ni(wt%),Co
(ppm),Sc(ppm),traceandLREE:V,La,Ce(ppm).
At the mineral scale, WDS elementary mapping of mineral assemblages in saprolitized
amphibolitesindicatethatScisessentiallyhostedbyearly-stageskeletalgoethiteandlate-stage
goethiteformedafterkaolinite,andtoamuchlesserextentbykaolinite(Figure20).Insaprolites
developedafterfertile(>100ppmSc)amphibolites,goethiteyieldsScconcentrationsuptoabout
0.08 wt% (i.e. 800 ppm) together with Ni and Co concentrations up to 1 and 0.03 wt%,
respectively.There, thespatialdistributionofVappearssimilar to thatofSc. Incontrast,Ti is
mostlyhostedbyearly-formedskeletalgoethiteandislargelyabsentinlate-stagepatchygoethite
formedafterkaolinite.
Figure20:Quantitativeelementarymaps(Fe,Ti,Si,Ni,Co,Al,Sc,V;wt%)ofagoethite–kaolinite
–gibbsitemineralassemblagedevelopedinamphibolite-derivedsaprolite(NGPBprofile,N’Go).
Contrasting with goethite-bearing saprolitized amphibolites, hematite-bearing laterites
developedaftergabbrosexhibitrelativelylow(usually<50ppm)whole-rockScconcentrations
(Figure 21), i.e. lower than their unweathered counterparts (40-80 ppm, see Figure 9 section
V.2.2),suggestingthatScisexpulsedduringtheweatheringofgabbros.SuchremobilizationofSc
fromgabbrosissupportedbytheanomalouslyelevatedScconcentrationsidentifiedinfootwall
peridotite-derivedlateriteswhereintheSccontentexceeds150-200ppm(Figure21).There,the
observedScover-enrichmentextendsdownto~2metersbelowthebaseofthegabbrointrusive
bodies.
Figure21:Geochemicalevolutionalonggabbro-derivedlateriteprofileKNGA(Koniambo):Major
oxides:SiO2,Fe2O3,Al2O3,TiO2(wt%),economicelements:Ni(wt%),Sc(ppm).
7.DiscussionandimplicationsforScexploration
ThisstudyaimstodocumentthedistributionofScinNewCaledonianlateritesandtoassessthe
main factors controlling its distribution in lateritic profiles encompassing for the diversity of
protoliths and alteration styles. Critical conditions for the development of Sc-rich zoneswith
potentialeconomicinterestarediscussedinthefollowingsectionbasedonfieldandanalytical
investigations.
7.1Synthesisoffieldandanalyticalobservations
Combinationof fieldobservation,mineralcharacterization,bulkrockandmineralgeochemical
datarevealsaremarkableevolutionoftheScmineralhostsandabundancesinlateriticprofilesof
NewCaledonia.UnweatheredmaficandultramaficrocksexhibitalargerangeofScconcentrations
from<5ppm indunite to>10ppm in lherzoliteandup to100ppmorhigher inamphibolite,
resulting from variable contributions of Sc-bearing primary phases (mostly enstatite in
peridotites and hornblende in intrusive rocks). The lateritisation of peridotites results in the
progressive increase of Sc concentrations upwards in the profile so that maximum Sc
concentrations are reached in the goethite-dominated yellow laterite, representing a~tenfold
enrichmentcomparedto theparentperidotite.WhereasScconcentrationsco-increasewithFe
concentrations from the bedrock to the earthy saprolite, Sc commonly exhibits a relative
enrichmentcomparedtoFeintheyellowlateriteandarelativedepletionintheredlateriteand
ferricrete.SuchdistributionappearsmoresimilartothatofAlorV.Intheseoxide-richhorizons,
whole-rock Sc concentrations are mostly controlled by the Sc content of goethite which
significantly varies from the earthy saprolite to the ferricrete. The Sc content within these
horizonsisnotonlycontrolledbytherelativeproportionofgoethiteandhematite,asproposed
byChasséetal.(2017),butalsoandtoamuchlargerextentbytheSccontentofgoethiteitself.
Insaprolitizedamphibolites,Scisalsoessentiallyhostedbygoethite.There,owingtothehigher
Al content of the parent rock compared to peridotites, goethite does not represent the
predominant secondaryphasebutoccur inassociationwithSc-poorkaoliniteand/orgibbsite.
Consequently, the Sc concentration factor fromunweathered amphibolites to theirweathered
derivatives is about2 to3 (i.e. lower than the tenfold increaseobserved inperidotite-derived
laterites). Yet,weatheringof someSc-rich (>100ppm) amphibolites leads to the formationof
goethite–gibbsite–bearingsaprolitethatyieldsthehighestSccontentrecognizedsofar inNew
Caledonia(upto>300ppmSc),withgoethitebeingthepredominantSchostwithupto~800ppm
Sc. In contrast, the lateritisation of gabbros leads to the formation of hematite–kaolinite–
dominated assemblages and is associated with significant expulsion of Sc. Remobilised Sc is
trappedinthesurroundinggoethite-richlateritederivedfromperidotite,leadingtoimportantSc
over-enrichmentupto>150-200ppmwithinafewmetersfromthegabbrosalbands.
7.2 Preliminary genetic model and critical controls for the formation of Sc-rich
laterites
Fieldandanalyticaldatadocumentedinthepresentcontributionallowtoproposeapreliminary
geneticmodelforScenrichmentinlateritesdevelopedaftermaficandultramaficrocksofNew
Caledonia.Inperidotite-derivedlateriticprofiles,thedissolutionofprimarysilicatesandleaching
ofSiandMgleadtotheco-enrichmentofScwithotherpoorlymobileelements,mostnotablyFe,
thusformingSc-bearingsecondarygoethites.Suchco-enrichment,recognizedfromthebaseupto
theearthysaprolitic zonesof the investigated lateriticprofiles, is interpreted tobeessentially
residualwithnoormarginalremobilizationofSc.Upwards,theobservedrelativeenrichmentand
depletionofSccomparedtoFesupportacertainmobilityofScwhichis,tosomeextent,expulsed
from theuppermosthorizons (red laterite and ferricrete) and accumulatesdownwards in the
yellowlaterite.Suchbehavior isquitesimilartothatofNi inoxide-richhorizons,althoughthe
lower mobility of Sc together with its strong affinity with goethite result in its maximum
accumulation at a higher position in the profile (i.e. in the yellow laterite rather than in the
saprolite).WithregardstoNi,theupwardsdecreaseofNiconcentrationsinoxide-richhorizons
relatestoitsexpulsionfromgoethiteafter(i)goethiteageingthroughsuccessivedissolutionand
recrystallisation processes forming increasingly crystallised goethite and (ii) goethite
dehydrationtohematite intheuppermost lateritichorizons(Dubletetal.,2015;Schwertmann
andLatham,1986).SuchmodelappearspartlysuitableforexplainingthedistributionofScand
itsmaximumenrichmentintheyellowlaterite,providingthatdownward-remobilizedScfromthe
uppermosthorizons(redlateriteandferricrete)ismorereadilytrappedinthedirecltyunderlying
goethite-richyellowlaterite.FurthersupportforthismodelarisesfromtheobservationthatSc
accumulates in goethite-rich saprolitized amphibolites whereas it is largely remobilized from
hematite-rich lateritized gabbros and accumulates in surrounding goethite-rich lateritized
peridotites.Thestrikingdiscrepancyinthenatureofsecondaryironcarriersbetweenperidotite-
and amphibolite-derived alterites on one hand (mostly goethite) and gabbro-derived alterites
(mostlyhematite)ontheotherisfacies-dependantandcommonlyobserveddowntotheoutcrop
scale(e.gintheSouthAlphapit,Tiébaghi,Figure4B).Suchdiscrepancylikelyresultseitherfrom
differencesintheactivityofwater,pHand/orAl/Feratio(TardyandNahon,1985;Trolardand
Tardy,1989;Ramanaidou,1989).Withregardstotheactivityofwater,highdrainageconditions
inlateriticprofilesfavorthemaintenanceofunsaturatedzonesatgreatdepthswhereinhematite
predominates over goethite. Conversely, poor drainage conditions lead towater saturation at
shallower depths and favor goethite over hematite.With regards to pH, goethite formation is
optimalbothatlow(pH=4)andhigh(pH=12)pH,whereashematiteformationisoptimalatnear
neutral(pH=8)pH.Eventually,increaseintheAl/Feratiotendstofavorgoethiteoverhematite
providingthatthewateractivityremainscomprisedbetween~0.55and0.85.Amongthese,itis
hereproposedthatchangesinwateractivityrepresentthepredominantcausefortheobserved
mineraldiscrepancyat theoutcrop scale. Subvertical, readily alteredgabbros likely constitute
preferential drain pathways for weathering fluids, allowing the maintenance of unsaturated
conditions at much greater depths compared to surrounding lateritised peridotites. At the
opposite, amphibolites and pyroxenites are more resistant to alteration and probably act as
aquitardsforbothlateralanddownwardsfluidcirculation.
It is therefore proposed that three main factors control the distribution and intensity of Sc
enrichmentinperidotite-,amphibolite-andgabbro-derivedlaterites.Thefirstcriticalfactorfor
the formationofSc-richzones in laterites is theoccurrenceofparentrocks thathaveastrong
potential(fertility)forsubsequentupgradeandmineralisation.Thefertilityofagivenparentrock
isprimarilycontrolledbytherelativeproportionofSc-bearingprimaryphases,mostlyenstatite
and hornblende in peridotites and intrusive rocks, respectively. In peridotites, significant
variationsintheSccontentofenstatitearealsoobserveddependingontheconsideredperidotitic
massif,withhigherconcentrationsobservedwithinlherzolitesoftheTiébaghimassif.Inintrusive
rocks,Sc-bearinghornblendeexhibitverylargevariationsoftheSccontentdowntotheoutcrop
scale.ThesecondcriticalelementfortheformationofSc-richlateritesisthemaintenanceoflong-
livedtropicalweatheringallowingacidic,Si-undersaturatedmeteoricfluidstodissolveprimary
mineralsand leachawaymostcations includingSi, thus favouringtheconcentrationof ironas
goethitetogetherwithotherpoorlymobileelements includingSc.Third, furthermaturationof
peridotite-andamphibolite-derivedlateriticprofilesfavorboththedissolution/recrystallization
of goethite and its dehydration into hematite, both processes likely resulting in the local
remobilization and over-concentration of Sc in the directly underlying horizons. In a similar
manner, the preferential hematitisation of gabbros during weathering results in the
remobilizationofScanditsover-concentrationinsurroundinggoethite-richzones.Inanycase,
theformationofgoethiteisofcriticalimportanceasitisthepredominantmineraltrapforSc.The
chemical speciation of Sc in goethite remains to be investigated as it probably has important
implicationsbothforfurtherunderstandingofScmobilityinlateritesaswellasforthefine-tuning
ofmetallurgicalScextractionprocesses.BasedonXANESanalysisof lateriticsamplesfromthe
Syerston-Flemington Sc deposit, Chassé et al. (2017) propose that Sc is mostly adsorbed on
goethite. Alternatively, from the combination of XANES and sequential chemical extractions,
MunozandUlrich(2017)arguethatScismainlyco-precipitatedinFe-Scgoethitesolidsolution.
7.3ImplicationsforScexplorationandmininginNewCaledonia
A possible exploration strategy for identifying peridotite-derived Ni laterites wherein the Sc
contentishighenoughtomakeitapotentiallyattractiveby-productofNi(andCo)production
includes:
- Therecognitionoffertileperidotiticparentrocks.Harzburgiteandlherzolitearethemost
fertiletypesofperidotiteastheycontainsignificantproportionsofSc-bearingenstatite
(andmoremarginallydiopside).ThisstudyhasdemonstratedthatAlandVarereliable
pathfinderelements indicativeof theSccontent inunweatheredperidotitesregardless
theirdegreeofserpentinisation.Atthedepositscale,theanalysisoftheAl(andV)content
of unweathered peridotites may therefore complement field-based recognition of
peridotitefacies,especiallyinstronglyserpentinizedzones.
- TherecognitionofSc-richlateritichorizonsandtheirdegreeofoverlapwithNi(-Co)-rich
zones. As Sc is most enriched in the yellow laterite horizons, zones of maximum Sc
enrichment do not coincide with zones of maximum Ni (and Co) enrichment. Yet,
determination of respective Ni, Co and Sc cut-off grades based on stock prices and
production costs considerations may allow to identify overlapping zones wherein
covalorisationoflateriticNi,CoandScmaybeeconomicallyviablewhileNigradesremain
too low to be attractive for a standalone commodity production. There, as for
unweatheredperidotites,theanalysisoftheAlandVconcentrationsmaybeindicativeof
Sc grades. Also, the occurrence of hematite-bearing, gabbro-derived laterites cutting
across peridotite-derived, yellow lateritic horizons is likely associated with Sc over-
enrichment localized within a few meters from the gabbro salbands. Field-based
recognitionofgabbro-derivedlateritesisrelativelystraightforwardandmaybetakeninto
considerationfortargetrankingpriortodrilltesting.
In addition to the potential covalorisation of Ni, Co and Sc in peridotite-derived laterites,
saprolitisedamphibolitesmayrepresentattractivetargetsforprimaryScproduction.However,
this study has shown that amphibolites exhibit large variations in their Sc content, both in
unweathered facies and in their weathered derivatives, and that they represent discrete
occurrences of relatively restricted volumes. Following the field-based recognition of
amphibolite-bearingintrusions,theidentificationoffertileamphibolitesmaybebasedontextural
observationsdeterminingtherelativeproportionandcrystalsizeofhornblende,withpreference
for pegmatoïd textures. Also, analysis of Ti and V in amphibolites may provide first order
estimatesoftheirSccontent.
8.Conclusion
ThiscontributionexaminesthedistributionandthecriticalcontrolsontheformationofSc-rich
zoneswithinNewCaledonianlaterites.Theyellowlateriticzonesofsomeperidotite-derivedNi-
laterites exhibit elevated Sc concentrations up to 100 ppm, suggesting potential for co-
valorisationalongwithNiandCoproduction.Spatiallyrestricted,amphibolite-derivedsaprolites
arealsohosttoremarkableScconcentrationsupto>300ppm.
TheinitialSccontentofparentrocksrepresentthefirstcriticalcontrolonthemaximumScgrades
reachedintheirlateritisedderivatives.Inunweatheredperidotites,enstatiteisthedominantSc
host, controlling the Sc content of harzburgite and lherzolite. In intrusivemafic to ultramafic
rocks, hornblende is the dominant Sc host, controlling the Sc content of hornblende-bearing
intrusiverocks.Inallinvestigatedlateriticprofiles,residualScenrichmentintheupper,goethite-
dominatedhorizonsconstitutesthesecondcriticalconditionforthedevelopmentofSc-richzones.
Third, the expulsion of Sc from the uppermost horizons and downwards over-concentration
following goethite recrystallisation and/or dehydration leads to further Sc enrichment. In
peridotite-derived Ni laterites, the upgrade of the Sc content corresponds to a ~tenfold
enrichmentcomparedtotheparentperidotite.TheoccurrenceofelevatedScconcentrationsup
to 100 ppm in the yellow lateritic zones of harzburgite- and lherzolite-derived Ni-laterites
suggests potential for co-valorisation alongwithNi and Co production, providing that Sc-rich
zonessufficientlyoverlaplow-gradelateriticNi(-Co)orezones.Inamphibolite-derivedsaprolites,
Sc-bearinggoethiteyieldsScconcentrationsupto800ppmbutoccursinassociationwithgibbsite
and/or kaolinite so thatmaximumbulk rock Sc concentrations only grades up to~300 ppm,
correspondingtoatwo-tothreefoldincreaseoftheSccontentcomparedtotheparentrock.Such
concentrationsmaybeeconomicallyattractiveforstandaloneScproduction,althoughhigh-grade
saprolitisedamphibolitesrepresentmarginalvolumesespeciallywhencomparedtohigh-grade
Sc deposits recognized in Australia (Chassé et al., 2017). Contrasting with peridotites and
amphibolites,thelateritisationofgabbrosleadstothepreferentialformationofhematiterather
than goethite, resulting in significant remobilisation of Sc and subsequent trapping in the
surroundinggoethite-rich,peridotite-derivedNilaterite.There,over-enrichmentupto>150-200
ppmmayaddsignificantvalueintheperspectiveofNi-Co-Scco-valorisation.
Acknowledgments
ThisworkhasbeenfundedandlogisticallysupportedbytheFrenchNationalResearchAgency
through thenationalprogram“Investissementsd’avenir”of theLabexRessources21with the
reference ANR-10-LABX-21-RESSOURCES21 and by the National Centre for Technological
Research“Nickeletsonenvironnement”basedinNewCaledonia.Theauthorswouldliketothank
Dr.BriceSevin,StephaneLesimpleandDr.BernardRobineaufromtheGeologicalSurveyofNew
Caledonia, together with France Bailly and Laurence Barriller from the National Centre for
Technological Research, for their technical and logistical support as well as for sharing their
extensiveknowledgeofthegeologyofNewCaledonia.WearegratefultoSociétéLeNickel(SLN),
KoniamboNickelSAS(KNS),NickelMiningCompany(NMC),SociétédesMinesde laTontouta
(SMT) and Mai Kouaoua Mines (MKM) for support and access to mines and drill cores. In
particular,weacknowledgeDr.MohamedKadar(SLN),Dr.ClémentMarcaillou(SLN),Dr.Maxime
Drouillet (KNS), Willy Foucher (NMC), Mathieu Mazières (SMT), Matthieu Muret (SMT) and
MatthieuDésesquelles(MKM)fortheirinvolvement.WealsothankDr.ChristopheCloquet(SARM
analyticalcentre,CRPG,Vandœuvre-lès-Nancy,France),Dr.Marie-ChristineBoiron,Dr.Chantal
Peiffert, Dr. Andreï Lecomte and Dr. Olivier Rouer (GeoRessources, Vandœuvre-lès-Nancy,
France)fortechnicalsupportinprovidinganalyticaldata.
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