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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

*yoram.teitler@univ-lorraine.fr

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.

References

Aiglsperger,T.,Proenza,J.A.,Lewis,J.F.,Labrador,M.,Svojtka,Rojas-Puron,A.,Longo,F.,Durisova,

J.2016.Criticalmetals(REE,Sc,PGE)inNilateritesfromCubaandtheDominicanRepublic.Ore

GeologyReviews76(1)127-147.

Ambrosi,J.P.,Nahon,D.,Herbillon,A.J.1986.Theepigeneticreplacementofkaolinitebyhematite

inlaterite-Petrographicevidenceandthemechanismsinvolved.Geoderma37,283-294.

Audet, M. 2008. Le massif du Koniambo, Nouvelle-Calédonie. Formation et obduction d’un

complexeophiolitiquedutypeSSZ.Enrichissementennickel,cobaltetscandiumdanslesprofils

résiduels.Ph.Dthesis,universitédelaNouvelleCalédonie/UniversitéduQuebecàMontréal,326

pp.

Avias, J. 1967. Overthrust structure of the main ultrabasic New Caledonian massives.

Tectonophysics4,531–541.

Bailly,L.,Ambrosi,J.P.,Barbarand,J.,Beauvais,A.,Cluzel,D.,Lerouge,C.,Prognon,C.,Quesnel,F.,

Ramanaïdou,E.,Ricordel-Prognon,C.,Ruffet,G.,Sevin,B.,Wells,L.,Yans,J.2014.ProjetNICKAL:

“TypologiedesmineraislatéritiquesdeNouvelle-CalédonieetfacteursdeconcentrationdeCoet

Ni”,rapportfinal,BRGM/RP-63482-FR,402pp.

ButtC.R.M.,CluzelD.2013.Nickellateriteoredeposits:weatheredserpentinites.Elements9.123-

128.

Cathelineau, M., Quesnel, B., Gautier, P., Boulvais, P., Couteau, C., Drouillet, M. 2016. Nickel

dispersionandenrichmentatthebottomoftheregolith:formationofpimelitetarget-likeoresin

rockblockjoints(KoniamboNideposit,NewCaledonia).MineraliumDeposita51(2),271-282.

Cathelineau,M.,Myagkiy,A.,Quesnel,B.,Boiron,M.-C.,Gautier,P.,Boulvais,P.,Ulrich,M.,Truche,

L.,Golfier,F.,Drouillet,M.2017.MultistagecracksealveinandhydrothermalNienrichmentin

serpentinizedultramaficrocks(Koniambomassif,NewCaledonia).MineraliumDeposita52(7),

945-960.

Chassé,M.,Griffin,W.L.,O’Reilly,S.Y.,Calas,G.2017.Scandiumspeciationinaworld-classlateritic

deposit.GeochemicalPerspectivesLetters3,105–114.

Cluzel,D.,Aitchison,J.C.,Picard,C.2001.Tectonicaccretionandunderplatingofmaficterranesin

the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic

implications.Tectonophysics340(1-2),23-59.

Cluzel,D.,Meffre,S.,Maurizot,P.,Crawford,A.J.2006.EarliestEocene(53Ma)convergenceinthe

SouthwestPacific:evidence frompre-obductiondikes intheophioliteofNewCaledonia.Terra

Nova18,395–402.

Cluzel,D.,Maurizot,P.,Collot,J.,Sevin,B.2012a.AnoutlineoftheGeologyofNewCaledonia;from

Permian–MesozoicSoutheastGondwanalandactivemargintoCenozoicobductionandsupergene

evolution.Episodes35(1),72-86.

Cluzel,D.,Jourdan,F.,Meffre,S.,Maurizot,P.,Lesimple,S.2012b.ThemetamorphicsoleofNew

Caledoniaophiolite:40Ar/39Ar,U-Pb,andgeochemicalevidence forsubduction inceptionata

spreadingridge.Tectonics31,1-18.

Das,H.A.,Zonderhuis, J.,vanderMarel,H.W.1971.Scandiuminrocks,mineralsandsediments

anditsrelationstoironandaluminium.ContributionstoMineralogyandPetrology32(3),231-

244.

Dublet, G., Julliot, F., Morin, G., Fritsch, E., Fandeur, D., Brown Jr., G.E. 2015. Geochimica et

CosmochimicaActa160,1–15.

Emsley,J.2014.UnsportingScandium.NatureChemistry6,1025.

Freyssinet,P.H.,Butt,C.R.M.,Morris,R.C.,Piantone,P.2005.Ore-FormingProcessesRelated to

LateriticWeathering.In:HedenquistJW,ThomsonJFH,GoldfarbRJ,RichardsJP(eds),Economic

Geology 100th Anniversary Volume. Economic Geology Publishing Company, New Haven,

Connecticut,pp.681-722.

Genna,A.,Maurizot,P.,Lafoy,Y.,Augé,T.2005.Roleofkarstinthenickeliferousmineralisations

ofNewCaledonia.CompteRendusGeoscience337(3),367-374.

Golightly, J.P.2010.Progress inunderstandingtheevolutionofnickel laterites. In:GoldfarbRJ,

MarshEE,MoneckeT(eds)TheChallengeofFindingNewMineralResources:GlobalMetallogeny,

InnovativeExploration,andNewDiscoveriesVolumeII.SocietyofEconomicGeologistsSpecial

Publication15,pp.451-475.

GuillongM.M.,MaierD.L.,AllanM.M.,HeinrichC.A.2008.SILLS:aMATLABbasedprogramforthe

reductionoflaserablationICPMSdataofhomogeneousmaterialsandinclusions.SylvesterP(ed)

LaserAblationICPMSintheEarthSciences.CurrentPracticesandOutstandingIssues,328-333.

Guillon,J.H.1975.LesmassifspéridotitiquesdeNouvelle-Calédonie,Mémoires,vol.76,120pp.,

ORSTOM,Paris.

Hoatson,D.M., Jaireth,S.,Miezitis,Y.2011.Themajorrare-earth-elementdepositsofAustralia:

geologicalsetting,exploration,andresources.GeoscienceAustralia,204pp.

Latham,M.1985.AltérationetpédogenèsesurrochesultrabasiqueenNouvelleCaledonie.Genèse

etévolutiondesaccumulationsdeferetdesiliceenrelationaveclaformationdumodéle,Thèse

d'état,Dijon,331pp.ORSTOM.

Manceau,A.,Schlegel,M.L.,Musso,M.,Sole,V.A.,Gauthier,C.,Petit,P.E.,Trolard,F.2000.Crystal

chemistryoftraceelementsinnaturalandsyntheticgoethite.GeochimicaetCosmochimicaActa

64(21),3643–3661.

Maulana, A., Sanematmatmatsu, K., Sakakibara, M. 2016. An Overview on the Possibility of

ScandiumandREEOccurrenceinSulawesi,Indonesia.IndonesianJournalonGeoscience3,139–

147.

Maurizot,P.,Vendé-Leclerc,M.2009.NewCaledoniageologicalmap,scale1/500000,Direction

del’Industrie,desMinesetdel’Énergie–ServicedelaGéologiedeNouvelle-Calédonie,Bureaude

RecherchesGéologiquesetMinières.

Mortimer,N.,Campbell,H.J.,Tulloch,A.J.,King,P.R.,Stagpoole,V.M.,Wood,R.A.,Rattenbury,M.S.,

Sutherland,R., Adams, C.J., Collot, J., Seton,M. 2017. Zealandia: Earth’sHiddenContinent. GSA

Today27(3),27-35.

Munoz,M.,Ulrich,M.2017.DistributionandspeciationofScinlateriticprofileofNewCaledonia

usingsynchrotron-XRFandScK-edgeXANESspectroscopy.GeochemicalcycleofNi,CoandSc:

fromminingexplorationtoecotoxicityInternationalWorkshopOctober17-19,2017.

Orloff,O.1968.Etudegéologiqueetgéomorphologiquedesmassifsd’ultrabasitescomprisentre

HouailouetCanala(Nouvelle-Calédonie),PhDthesis,Univ.Montpellier,France,189pp.

Paquette,J.L.,Cluzel,D.2007.U–Pbzircondatingofpost-obductionvolcanic-arcgranitoidsanda

granulite-faciesxenolithfromNewCaledonia.InferenceonSouthwestPacificgeodynamicmodels.

InternationalJournalofEarthSciences96(4),613-622.

Quesnel,B.,Gautier,P.,Boulvais,P.,Cathelineau,M.,Maurizot,P.,Cluzel,D.,Ulrich,M.,Guillot,S.,

Lesimple, S., Couteau. C. 2013. Syn-tectonic, meteoric water-derived carbonation of the New

Caledoniaperidotitenappe,Geology41,1063–1066.

Prinzhofer, A., Nicolas, A., Cassard, D.,Moutte, J., Leblanc,M., Paris, J.P., Rabinovitch,M. 1980.

Structuresinthenewcaledoniaperidotites-gabbros:Implicationsforoceanicmantleandcrust.

Tectonophysics69(1-2),85-112.

Ramanaidou, E.R. 1989. Genèse d’un gisement latéritique, Evolution supergène des itabirites

ProtérozoiquesdelaminedeferdeCapanema(MinasGerais,Brésil),PhDthesis,Universityof

Poitiers,183pp.

Royset,J.,Ryum,N.2005.Scandiuminaluminiumalloys,InternationalMaterialsReviews50(1),

19-44.

Rudnick,R.L.,Gao,S.2014.CompositionoftheContinentalCrust.In:Holland,H.,Turekian,K.K.

(Eds.)TreatiseonGeochemistry,2ndEdition.ElsevierLtd,Amsterdam,Netherlands,1–51.

Schwertmann,U.,Latham,M.1986.Propertiesof ironoxides in someNewCaledonianoxisols.

Geoderma39,105-123.

Sevin, B., Ricordel-Prognon, C., Quesnel, F., Cluzel, D., Lesimple, S., Maurizot, P. 2012. First

palaeomagnetic dating of ferricrete inNewCaledonia: new insight on themorphogenesis and

palaeoweatheringof‘GrandeTerre’.TerraNova24,77-85.

Sevin, B., Cluzel,D.,Maurizot, P., Ricordel-Prognon, C., Chaproniere, G., Folcher,N.,Quesnel, F.

2014.AdrasticlowerMioceneregolithevolutiontriggeredbypostobductionslabbreak-offand

upliftinNewCaledonia.Tectonics33(9),1787-1801.

Tardy,Y.,Nahon,D.1985.Geochemistryoflaterites,stabilityofAl-goethite,Al-hematiteandFe3+-

kaolinite in bauxites and fericretes: an approach to the mechanism of concretion formation.

AmericanJournalofScience285,865-903.

Toropova,L.S.,Eskin,D.G.,Kharakterova,M.L.,Dobatkina,T.V.1998.AdvancedAluminiumAlloys

Containing Scandium. Structure and Properties, Gordon and Breach Science Publishers,

Amsterdam,pp.175.

Trescases, J.-J. 1975. L'evolution Géochimique Supergène des Roches Ultrabasiques en Zone

Tropicale: Formation des Gisements Nickélifères de Nouvelle-Calédonie Edited, Paris,

O.R.S.T.O.M.,France.

Trolard, F., Tardy, Y. 1989. A model of Fe3+-kaolinite-Al3+-goethite-Al3+-hematite equilibria in

laterites.ClayMinerals24,1-21.

Ulrich,M.,Picard,C.,Guillot,S.,Chauvel,C.,Cluzel,D.,Meffre,S.2010.Multiplemeltingstagesand

refertilization as indicators for ridge to subduction formation: The New Caledonia ophiolite.

Lithos115,223-236.

Ulrich,M.2010.PéridotitesetserpentinitesducomplexeophiolitiquedelaNouvelle-Calédonie.

PhDthesis,UniversitéJosephFournier–UniversitédelaNouvelle-Calédonie,272pp.

Ulrich M., Cathelineau M., Boiron M.-C., Muñoz M., Teitler Y. In prep. LA-ICP-MS analysis of

Scandium:calibration,andapplicationtoinsituanalysisofScinsilicatesandoxides.Submitted

toJournalofGeochemicalExploration.

U.S. Geological Survey (2016) Scandium. In: Mineral Commodity Summaries. U.S. Geological

Survey,Reston,USA,146–147.

Wells,M.A.,Ramanaidou,E.R.,Verrall,M.,Tessarolo,C.2009.Mineralogyandcrystalchemistryof

“garnierites”intheGorolateriticnickeldeposit,NewCaledonia.EuropeanJournalofMineralogy

21(2),467-483.