ll4'7

The Canadian Mineralogist Vol. 37, pp.1147-1161 (1999)

PGEDISTRIBUTION IN BASE.METALALLOYS AND SULFIDES OFTHE NEW CALEDONIA OPHIOLITE

THIERRY AUGES

BRGM. B.P. 6009. F-45060 Orl1ans Cedex 2, France

LOUIS J. CABRI$

CANMET. MMSL. 555 Booth Street, Ottawa, Ontario KIA 0Gl, Canada

OLIVIER LEGENDRE

BRGM. B.P 6009. F-45060 Orldans Cedex 2, France

GREG MCMAHON

CANMET,MTL, 568Booth Street, Ottawa, Ontario KIA )Gl, Canada

ALAIN COCHERIE

BRGM, B.P. 6009, F-45060 Orldans Cedex 2, France

Assrnacr

An Upper Eocene ophiolite nappe covering some407o of New Caledonia consists of depleted mantle harzburgites' locally covered by mafic and ultramaficiumulates. These mantle harzburgites in places show an enrichment in sulfide and alloy (heazlewoodite, , , and native copper), accompanied by anomalous concentrations of the PGE, as revealed by the whole-rock analysesofthree sulfide-enriched samplesand by the heavy-mineral concentratesobtained from these samples(up to 1835 ppb Ir, 152i ppb Rh, 9718 ppb Pt, 11,494 ppb Pd and 988 ppb Au). Although studiesto detemine the PGE- carrier minerals Uy SeU were unsuccessful, SIMS depth profiles show that Pt is strongly enriched in awaruite, albeit_varying widely among graitrs, g., from 4 to 1210 ppm for 21 grains in one sample In conffast, 11 grains of pentlandite from the same " ru*pL do noi exceed 10 ppm Pt, whereas i0 out of 12 grains of heazlewoodite show no detectablePt, and the Pt content of the other two grains is estimatedto be very low (<1 ppm). SIMS ion images thus confirm that awaruite is a preferential host for Pt and that the Piconcentration could be related to thJmineral's Fe, Cu, Ni and Co content. These images also show that the awaruite, whether or not Pt-bearing, contains some Au, whereasthe heazlewoodite is Au-free. Textural featuresand trace-elementdistribu- tions of the ore minerals suggestthat the mineralization occurred in a two-step process.In the initial mineralizing event' primary PGE-bearing base-metalsulfides (BMS) crystallized in intergranular positions among the silicates. The second step was related to serpentinlzation and involved transformation of the primary BMS assemblage to the present secondary assemblage of heazlewoodite, awaruite, and pentlandite, but with a different pattem of PGE distribution

Keyworjs: PGE, Au, awaruite, pentlandite, heazlewoodite, secondary-ionmass spectrometry, ophiolite, New Caledonia.

Sorrauanr

Le complexe ophiolitique de Nouvelle-Cal6donie, mis en place d I'Eocdne supdrieur et qui occupe environ 407p de f ile, est constitu6 de p6ridotites mintelliques appauvries, sur lesquelles reposent par endroits des cumulats mafiques et ultramafiques. Localement, les harzburgites mantelliques sont enrichies en sulfures et alliages de m6taux de base (heazlewoodite, pentlandite, awaruite, mi1l6rite et cuivre nati|; cerlaines montrent des teneurs anomales en EGP Des concentr6s de sulfures et alliages r6alis6sd padr de ces 6chantillons ont des teneurs61ev6es en EGP (maximum 1835 ppb Ir, 1527 ppb Rh, 9718 ppb Pt' 11 '494 ppb pd and 988 ppb Au). Une premibre 6tude au MEB n'a pas permis d'identifier de porteurs des EGP. Cependant, une 6tude e la sonde ionique (SIMS) monte que le Pt se concentrefortement dansI'awaruite, avec des teneursvariant sur 21 grains de 4 h 1210 ppm dans un mOme6chantillon. En revanche,des mesuressur 11 grains de pentlandite du meme 6chantillon monfent des teneurs

$ E-mail addresses:[email protected], [email protected] ca

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quin'excbdentpasl0ppmPt Surl2grainsdeheazlewooditeanalys6s,l0ontdesteneursenPtnond6tectables,etdeuxdonnent des valeurs trbs faibles (estim6es d moins de 1 ppm). Des images de r6partition r6alis6es d la sonde ionique confirment que est le principal I _lwgruile porteur de Pt et que la teneur en Pt de I'awaruite pourrait etre li6e d sa composition (teneurs en Fe, Cu, Ni' Co). Les images SIMS montrent 6galement que l'awaruite, platinifdre ou non, contient Au, alorJ que la heazlewoodite n'en contient pas. La texture des min6raux m6talliques et leur composition indiquent que la min6ralisation r6sulte d'un processusd deux 6tapes.Dans un premier temps (stademagmatique), des sulfures de m6taux de base enrichis en EGP vont crist;lliser enre les silicates. Dans un second temps, cette paragenbseprimaire, sous l'effet de la serpentinisation, va donner un assemblage min6ral secondairecompos6 de heazlewoodite, awaruite, et pentlandite, avec une redistribution des EGP.

Mots-cl6s'. EGP, Au, awaruite, pentlandite, heazlewoodite, spectrom6rrie de masse des ions secondaires (SIMS), ophiolite, Nouvelle-Cal6donie.

IwrnooucrroN Ocean, 1500 km east of the Australian Coast), other- wise interpreted as a residual . Mantle peridotitesin ophiolite complexeseither con- Special attention was paid to these base-metal sul- sist of and harzburgite (depleted "residual', peri- fide (and associatedmetallic alloy) occurrencesduring dotite) or lherzolite (undepletedor "fertile" peridotite), an inventory of the platinum-group element (PGE) re- with all intermediate stagesbetween the two. Mantle sources of New Caledonia. Five sulfide occurences can also be depleted rocks impregnated by were sampled and analyzed for the six PGE. Because percolating magma, and therefore show characteristics some of the samples showed an anomalous spectrum similar to "fertile" lherzolites. Base-metal sulfides are relative to sulfide-free mantle peridotites (although with commonly present in trace amounts in these facies. very low values), these were examined in detail. In this However, there are places where sulfides are abundant, article, we present the results of this study, including a to the point of forming occurrences mapped by pros- description of the occurrences,the and com- pectors. Such is the case of the Massif du Sud position of the ore minerals, and the chemistry of the harzburgite (Fig. l) in the mantle series of the New mineralized rocks and ore mineral concentrate.We also Caledonia ophiolite complex (located in the pacific provide preliminary results of a study of trace Pt in the

+ \ \ N

Ti6baghi

Koumac Hienghdne

N6ou Mindiou 1,2

Massif du Sud Yat63

0 20 100km

Ftc 1. Simplified geological map of New caledonia showing the studied sulfide-rich sites in the Massif du Sud. occurrences Yat€ 1,2 and 3 are located around Lake yat6.

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ore minerals using secondary ion mass spectrometry clinopyroxene and chromite constituting the other 30Vo. (SIMS). There is no difference in terms of mineral composition between the sulfide-bearing and sulfide-free residual GsolocIcAr SBrrmc mantle peridotites (Johan & Aug6 1986). Of note, how- ever, is the associationof the ore minerals in two occur- The ultramafic nappe, which covers about 40Vo of rences (Yat6 2 and3), with spectacularexamples of the New Caledonia (Fig. 1), representsthe lower part of an symplectitic texture (Fig. 2) that is conmon in mantle ophiolite complex emplaced during the Late Eocene peridotite (Field & Haggerty 1994), particularly in the (Pais et aI.1979). Threeunits aredistinguished: (1) the peridotitesof New Caledonia. northern massifs (north of Koumac) consisting of mantle Five sulfide occuffences were studied in the Massif peridotite marked by the presence of plagioclase and du Sud, three in the south (Yat6 1, 2, 3) and two in the spinel lherzolite and an abundance of chromitite occur- north (N6ou Mindiou 1, 2). rences (Moutte 1982, Johan & Aug6 1986), (2) the in- termediate massifs (on the west Coast) consisting of Yatd I occurrence harzburgite, and (3) the Massif du Sud forming the southernthird of the Island. The Massif du Sud is com- This is the largest of the five occuffences studied.In posed of mantle rocks (harzburgite with minor dunite the field, sulfides were identified in large, qu.asiin situ and small bodies of chomitite) upon which cumulate boulders of peridotite (up to 1 m in diameter) extending dunite, wehrlite, pyroxenite and gabbro have been rec- over an area ofabout 1000 m2 in a zone ofthin lateritic ognized locally (Pinzhofer et a/. 1980). The other parts overburden. In a given boulder, the sulfides occur in of the ophiolite sequenceare missing. small, highly discontinuous layers, from 20 to 30 cm in length and up to I cm in thickness. The sulfide content Sur-rme OccmnBucBs is low, estimatedatl}Vo maximumin ahand specimen, with some grains up to 5 mm in size. The presence of base-metal sulfides (BMS) in the The ore mineral assemblageconsists of heazle- mantle peridotite of the New Caledonia ophiolite nappe woodite, pentlandite and awaruite. In addition, rods of has been recorded since the early 20th century (Glasser millerite are found in heazlewoodite,and native copper 1904). These sulfides were even once consideredto be occurs on the periphery ofheazlewoodite and awaruite the source of the supergeneNi deposits,which are now grains. Heazlewoodite, the most abundant mineral, oc- known to be derived from the weathering of curs either in isolated grains or in association with containing -0.3 wtToNi in sulfide-poor ultramafic rocks awaruite and pentlandite. Pentlandite rarely occurs as and not from the sulfide-bearingrocks describedbelow. isolated grains, and is most cornmonly presentas a halo Numerous other BMS occurrences in the mantle around heazlewoodite, which suggestsa late crystalli- peridotite have been documented and studied since the zation. Awaruite occurs as isolated grains. in associa- first discovery. Picot (1959) carried out the first miner- tion with heazlewoodite or as rods or irregular grains alogical study on three occunences,describing a sample included within the heazlewoodite. Graphite also is containing heazlewoodite, partly altered to millerite, and found, generally in associationwith chromite grains and another containing millerite and bravoite (possible al- heazlewoodite. teration ofpentlandite). Ramdohr (1967) gave a detailed The associationsand textures of the ore minerals are description of sulfide minerals in peridotite, particularly unusual. The ore minerals are commonly associated in samplesfromNew Caledonia.Guillon (1969) showed with chromite (Fig. 2A) and form in an arc around sili- that the sulfides in the Massif du Sud are concentrated cates, as illustrated in Figure 28 and 2C, where locally preferentially in the pyroxene-enrichedhorizons (defin- chromite forms a link between the sulfides (Fig. 2D). ing the foliation) ofthe harzburgite. A detailed study on The texture exhibited by the BMS resemblesthat of the the distribution of sulfides in the New Caledonia ultra- chromite grains,1.e., occurring interstitially betweenand mafic massifs (Guillon & Saos 1971) was followed by molded to silicates. Aside from these large grains of a description of the main occunencesand their mineral- BMS, small inclusions of BMS are found in the sili- ization characteristics(Saos 1912), with further miner- cates,particularly in orthopyroxene (similar to Fig.2E). alogical descriptions being given by Guillon & Lawrence (1973). At the time, the peridotite was con- Yat62andioccurrences sidered to be a cumulate belonging to a "large strati- form massif'. The fact that this harzburgite is a residual The ore minerals are found in small boulders, which mantle rock belonging to an ophiolite complex was not occur free and virtually in situ, over an area of about recognized until later (Aubouin et al. 1977). 250 m2. The boulders had to be broken in order to see All the sulfide occurrences have been found in the ore minerals, as they are not visible on the weath- mantle harzburgite consisting of olivine (707o) with a ered surface.The Yat6 2 occurrenceis characterizedby relatively low degree of serpentinization of the olivine symplectitic intergrowths of chromite and silicates (between 20 and 407o), with orthopyroxene, minor (mainly orthopyroxene, generally altered) with associ-

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FIc. 2. SEM images in the back-scatteredelectron (BSE) mode. Scale bar is given on each image. A. Typical chomite (dark grey) - base metal sulfide (BMS) association (heazlewoodite: white, pentlandite: pale grey). Sample 165-1D, Yat6 1 occur- rence. B. Typical texture of BMS (heazlewoodite, pentlandite) and awaruite forming an arc of a circle in an olivine and serpentinematrix.Sample165-1A,Yat6loccurrence.C DetailofBshowingawaruite(lightgrey)andheazlewoodite(dark grey) with exsolution of native Cu (white) D. Image showing the relationshipsbetween BMS (heazlewooditeand pentlandite, white), chromite (iight grey) and silicate matrix (olivine and serpentine,dark grey). Sample 165-lD, Yat6 1 occurrence.E. Euhedral composite inclusion (pentlandite: dark grey, heazlewoodite: light grey) in a pyroxene crystal. Sample 1884, N6ou Mindiou 1 occurrence F Type- 1 symplectite; chromite - orthopyroxeneassociation with a core containing crystals of awaruite (white) and chromite (grey) Sample 165-2B, Yat6 2 occurrence.

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Frc.2. (continued). G. Complex associationof a chromite - symplectite containing crystals of chromite (dark grey) and awaruite (white). Sample 165-3A, Yat6 3 occurrence. H. Reaction rim in chromite (dark grey) at its contact with awaruite (white). Sample 165-3A, Yat6 3 occufience.

ated awaruite. Where awaruite is present,it is located at face alteration to covellite and native copper. Abundant the center of the symplectite; the example depicted in smaller grains of BMS are included in the pyroxene Figure 2F shows an adjacentchromite crystal. The ver- (Fie.2E). micular intergrowths of chromite are very fine (type-l The mineralization at N6ou Mindiou 2 includes rare symplectite). The same samples also show finer disseminatedpentlandite, invariably surrounded by an domains of symplectite without awaruite; these are aureole of awaruite. Chromite locally moulds pentland- intergrowths of chromite and of odhopyroxene and oli- ite grains, and the rock is quite heavily serpentinized. vine. No BMS was found in either of the two samples studied. PGE AsLT{oANces The Yat6 3 occurrence also is characterizedby chromite - silicate - awaruite symplectitesof which two Analysis of a sulfide-bearing rock samplefrom each types are present.The first is analogousto that described ofthe five occurrenceswas done by inductively coupled from Yat6 2 (type I, Fig. 2F), and the second (type 2) plasma - mass spectrometry (ICP-MS) after consistsof an assemblageof chromite + awaruite in sili- sulfide fusion in routine conditions by XRAL Labora- cates(Fig. 2G), with the chromite and awaruite forming tories (Ancaster, Ontario, Canada).The results,normal- more compact grains than in the symplectitesof the first ized to a sulfide-free peridotite ofNew Caledonia,show type. An unusual phase,tentatively interpreted as a re- three types of profile (Fig. 3, Table 1): (i) a spectrum action rim, may be seenbetween chromite and awaruite with a positive slope and enrichment in Pt, Pd and Au (Fig. 2H). Heazlewoodite also is present,in some cases (concentrationsup to 110, 4l and14 ppb, respectively) in abundance,with a halo of pentlandite and a rim of for the two N6ou Mindiou occulrences, (ii) a V-shape awaruite and native copper. profile marked by Pt depletion for the Yat6 1 sample, but with values quite close to those of sulfide-free Ndou Mindiou occurrences harzburgite for the other PGE, and (iii) a sawtooth spec- trum with very low values (lower than the reference These two occurrences are about 700 m apart. At harzburgite) for the Yat6 2 and 3 occurrences.It may be N6ou Mindiou 1 (NMl), a 1- to 2-cm-thick layer of dis- noted that the Pt:Pd ratio is low (between0.23 and 0.40) continuous sulfides is visible in several boulders, and for the three Yat6 occurrences,whereas it increasesto the sulfide horizon is conformable to the foliation de- 0.67 for the N6ou Mindiou 1 occurrenceand to 2.68 for fined by the orthopyroxene horizons. In the N6ou the N6ou Mindiou 2 occultence. Mindiou 2 (NM2) occrrrence, boulders enriched in ore minerals were found on a steepslope. The structural ar- PGE GsocHBN{IsrRYoF THEMnIERALS rangementof the mineralization could not be observed. The mineralization at N6ou Mindiou 1 includes large Following the ICP-MS results obtained on the rock grains ofpentlandite (which is abundantand very promi- samples,we carried out a systematicsearch for PGM in nent here) in an orthopyroxene-rich horizon with vein- these samples.As a searchfor PGM by ore microscopy lets of heazlewoodite. Exsolution domains of cubanite and scanning electron microscopy (SEM) was unsuc- and millerite in heazlewooditeoccur locallv. with a sur- cessful, it was decided to concentratethe ore minerals

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100 and to work on the concentrates. The samples were crushed, ground, and sieved for mineral separation; "mini-panner" and heavyJiquid separationswere made to produce concentratesof the ore minerals that were 10 mounted as polished sectionsfor microscopic examina- tion and later remounted in graphite-loaded polished =o sections for micro-analytical procedures. Examination E f of the concentrates by scanning electron microscopy .ct N (SEM) L1 revealed only two PGM, less than five pm in - size.The first is a Pt-Fe alloy (determinedqualitatively) !o adjacentto a crystal of awaruite (sample 165-2,Yat6 l), o G and the secondis a Pd-Cu alloy, attachedto an altered rim of pentlandite (sample 188, N6ou Mindiou l). Our 0.1 preliminary conclusion was that the PGE could be in solidsolution in the BMS. A portion of the ore-mineral concentrateswas then prepared for chemical analyses,using neutron activa- tion (Cocherie et al. 1987) for Ir, ft, Pd and Au, and alpha-particle activation for Rh (Volfinger 1989). The Ru Rh PI Pd Au concentrationsof the other elements shown in Table I were also establishedby neutron activation; the levels .-Yat6 1 r-Yat62 -. yat63 -+-NM1 -,tr NM2 of Cu, Os and Ru were not determined. In two occur- rences(Yat6 2 and N6ou Mindiou 1), the chromite sepa- Frc. 3. PGE content of the different sulfide-bearing rates were also analyzed for Ir, Pt, Pd and Au. peridotites normalized to mantle harzburgite from New In terms of composition, the ore rninerals from the Caledonia. Normalization values (in ppb): Os 4, Ir 2.7, Ru Yat6 occurrencesare clearly distinct from 6,Rh2,Pt9.2, Pd 8.7,Au 1.5(AU,96 et a\.,1995)(NM1, 2: those of the N6ou Mindiou 1 and 2 occurrences).For PGE values be- N6ou Mindiou occurences. The former are character- low the detection limit (DL), half of the DL was used. izedby a low FeArli value, between 0.10 and 0.14, com- pared to 0.58 and 0.73 for the latter, whereas the latter have high Co concentrations,between 2200 and 2400 ppm, in comparison with 820 to 1450 for former. These differences correspond to the difference in the nature of

TABLE 1 GBOCHEMICALDATA FOR SEPARATEDORE MINERALS AND WHOLE ROCK (SI'LFIDE.BEARING), NEW CALEDONIA OPHIOLITE

Os k Ru Rh Pt pd Au Fe Ni CruO: Co Zn As Sb 6*5 ppb ppb ppb ppb ppb DDb DDb % vo % ffi mm Dm DDm %.

Base metal md alloy separate Yat€1 165-15 - 1635 - 795 9657 47'13 98E 660 60.28 0 58 r3M 93 -31 0 84 -1,9 YaEZ 165-25 - 174s - 152'.7 9'118 3761 77'7 630 6047 0.43 144E 26 32 0.62 -2.0 Yare3 165-35 - 568 - 1570 822 29'7 840 6025 EzA 531, 191 219 -0 1 '23'74 N6ouMitrdioul 1885 - 402 - 1r't6 1954 71494 286 l'.l 80 30'71 1 60 tU 463 961, -O2 't'1 NeouMindiou2 1905 - 9U - 6875 1|U 341 2100 28.86 092 2261, -19 012 -23

Chromite separates Yatb 2 165'2Er 21 623 13'l 15 N€ou Mindioul 188Cr 10'1 390 tOO2 35

Whole rock Yate1 165-1 7 85 102313 8 YatEz 165-2 <3 08 <1<1<13 Yaft3 165-3 <3 21 1<'tz5 <1 NeouMindioul 188 6 5-2 r'7 4 48 72 1'7 N6otrMindiou 2 190 <4 46 22 9 110 41 t4

Appfient partition cmfficimt of bm metal sulffde-alloy/whole rock Yal€l 165-15 41'7 398 1050 1085 706 YaEz 165-25 391 7& 1056 855 555 Ya63 165-35 r13 149 501 185 N6ouMindioul 1885 91 s88 212 2512 2n4 NeouMindiou2 1905 210 141 76t9 244

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the sulfides, with heazlewoodite(Ni3S2) predominating tributions being found: (i) Pt/Pd close to 2, with high Pt in the Yat6 material and pentlandite [(Fe,Ni,Co)eSs] concentrations(Yat6 1 and Yat6 2), (i1) a ratio close to predominating in the N6ou Mindiou material. It should 1, with high concentrations (N6ou Mindiou 2) or low be noted that the Cr content of the heavy-mineral con- concentrations(Yat6 3), and (iii) a ratio much less than centratesremains low, between 0.43 and 7.607oCr2O3, 1, with very high Pd concentrations(N6ou Mindiou l). which means that the analyzed samples contain very For all three types of distribution with high concentra- little chromite. tions, the ft + Pd content is surprisingly constant,between High values for precious metals were determined in 13,448 and 14,430 ppb, with good negative correlation. the heavy-mineral concentrates,with maximum values The sample from Yat6 3, with a Pt:Pd ratio of 0.62, has of 1835 ppb h, 1527 ppb I{h, 97 18 ppb Pt, 11,494 ppb a much lower concentration (Pt + Pd = 4092 ppb). The Pd and 988 ppb Au, dependingon the sample (Table 1). sampleswith pentlanditepredominating (N6ou Mindiou The Pt:Pd ratio is discriminant (Fig. aA), with three dis- I and 2) thus appear to have the highest Pd content.

I 2000 ^NMl 3000 A $ C NM1A NM2A 8000 2000 g 4',NM2

v2L

' LYt Yl 4000 L \t 1000 (J AY3 Y3

I 4000 8000 12000 4000 8000 I 2000 Pt (ppb) Pd (ppb)

2000

O NMt NM1A 1500 .15 Ba

I 000 (fi l0 ClNuz tr 6l L Yl CYl 500 \J U] Y2 av2

0 0 4000 8000 I 2000 0 4000 8000 12000 Pt (ppb) Pd a Pt (ppb) o

Frc. 4. Correlation diagrams for ore-mineral separatesfrom the different sulfide-bearing peridotites A: Pd-ft, B: Ir-Pt, C: Co- Pd, D: Cr2O3-Pdand h. Yl, 2,3'.Yat6 1, 2 and 3 occurences;NMl, 2: N6ou Mindiou 1 and 2 occurrences.

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O@r€nce Yl Yl Y1 Yl Y3 't3 Y3 NM1 NMI Y1 Yl Yl Yl Y3 v3 Y3 NMI NMI NMI Analysisno 1l 28 40 HZl* 50 51 64 61 62 29 75 PN1** 56 6'7 69 60 87 88 Hwlewodite Penflandite Weight percent '1229 Ni 13 t2 1286 7337 73 46 72U'728A 7307 7341 29 t2 340r 1333 3461 3082 3203 3358 3341 3434 3446 s 2689 nB 2703 2668 n@ 26EL n $ 2675 2510 3287 33 18 3348 33 13 3350 3346 33,10 333t 3334 3342 Fe 0t1 0.06 000 038 039 o25 0 07 024 0t9 2431 3066 31'.16 3145 3381 32'76 3194 3r9r 3101 3r t5 000 000 000 003 0-16 016 001 003 000 002 003 003 003 0-04 006 000 009 004 002 Cu oo2 000 002 002 0.03 002 006 001 000 0 14 000 009 005 005 002 005 000 023 0-0? Pb 001 0.00 000 ad 006 006 002 002 004 008 012 012 nd 011 Ol2 006 013 007 000 Co 0u 000 001 000 007 005 000 000 000 1306 r32 140 0.75 t34 089 048 059 03t 035 Cr 000 000 000 nd 040 0 35 0 00 0:00 000 000 000 009 Dd 001 000 002 o00 0.00 0.00 Total 100-32 9996 10042 10058 100,49 9994 tOO47 1@12 t0041 9959 9932 10030 10003 996E 9933 9921 9958 994'7 9948 Abnc percent Ni 05 0595 0597 0598 0.588 0591 0591 0597 0599 0227 0265 025'1 026A 0239 0249 0 2 0260 026'7 02,6'7 S o40l 04M 0403 0398 0403 0 402 0 408 0 400 0 399 0 470 0413 0473 0470 0415 0416 0412 0474 04'74 0475 Fe 0 001 0 001 0 000 0 003 0 003 0002 0001 0c02 0w2 0200 025t 0257 0.256 0275 0268 02.62 02.60 0254 0254 0 000 0 000 0 000 0 000 0 001 0 001 0 000 0 000 0 000 0 000 0000 0000 0000 0000 0000 0000 0001 0000 0000 Cu 0000 0000 0000 0000 0000 0 000 0 000 0 000 0 000 0.001 0000 0001 0000 0000 0000 0000 0000 0.002 0001 Pb 0000 0000 0000 - 0000 0000 0000 0000 0000 0000 0000 0000 - 0000 0000 0000 0000 0000 0000 Co 0 001 0 000 0 000 0 000 0 001 0 000 0 000 0 000 0 000 0 102 0010 0011 0006 0010 0007 0004 0005 0003 0003 0 000 0 000 0 000 0 000 0 000 0 000 Rsultsofele€lrcn-prcbemitrcamlyss *Corrpsitionofh€uldoodiEstoMinPigm6 trCitrpositionofpentlmditeslbMonFiSue6

Figure 48 shows a positive correlation between Ir CovposnroN oF THEORE Mrxpner-s and Pt. The Ir concentrations vary between 402 and 1835 ppb, indicating a clear affinity between Pt and Ir The sulfides and awaruite were determined by elec- in this type of environment. The level of Rh was deter- tron-probe micro-analysis (EPMA) either in polished mined in only three samples,with values between 795 sections of rock slices or in concentrates mounted in and,1527ppb that seem ro be independentof the Pt/Pd polished sections.EPMA determinationsfor major and value. minor elementswere done at BRGM (Orl6ans) using a The Au concentrations are about 1 ppm in Yat6 I CamecaSX-50 electron-probemicro-analyzer equipped and between28 6 and717 ppb in the other samples,with- with wavelength-dispersionspectrometers and operated out any significant variations. at the following conditions: accelerationvoltage 20 kV, Two interesting correlations are found. The first is and beam current ca. 20 nA. Co-Pd (Fig. 4C), which indicates that Pd is essenrially associatedwith pentlandite. The second is more enig- Heazlewoodite matic, and concems the Cr content relative to Pt and Pd (Fig. aD). Cr correlates positively with Pd and nega- Heazlewoodite was analyzed from the Yat6 1,Yatl, tively with Pt. These latter correlations remain unclear. 3 and N6ou Mindiou I occurrences(Table 2). Regard- They cannot be attributed to chromite contamination. less of the source, heazlewoodite has a homogeneous Table I shows the averageapparent partition coeffi- composition, close to the ideal stoichiometry Ni3S2. cients obtained (concentration of the element in the ore Trace Fe was found (up to 0.38 wt%o) and, in a few mineral divided by concentration of the element in the grains, trace concentrations of Cr were measured,but rock), which are highly variable and probably reflect these enrichments are not particularly significant. The differencesin the composition of the sulfides. The high- other elementsa.re not presentin any significant amount. est Pd values, for example, were obtained for samples rich in pentlandite. Pentlandite

S Isoropss Pentlandite was analyzed in samples from Yat6 1, Yat6 3 and N6ou Mindiou 1 and found to have a Ni:Fe The S isotope ratios were measuredon concentrates ratio averaging 1, but ranging between 0.86 and 1. I 9 in of base-metalsulfides and alloys at BRGM by the com- one sample(Table2). Similarly, very heterogeneousCo bustion method. The gasesobtained were analyzed for values were found in one sample,where a Co-rich pent- their isotopic composition using a Delta S Finnigan-Mat landite grain (with 13 wrVa Co) was found included in gas-sourcemass spectrometer.The results are reported heazlewoodite. In contrast to this extreme value, the in E units relative to the CDT standard.Reproducibility following ranges in Co content were found: (a) Yat6 1 -l0.3Voo. was The values obtained are consistentand from 0.5 to 2.3 wt%o(average 1.34 wt%o,o 0.56); (b) slightly negative,lying between-0.1 and -2.3 (Table 1), Yat6 3 from 0.4 to 1.3wt%a (average 7.02wt%o, o 0.39), compatible with a magmatic origin or derivation from a and (c) N6ou Mindiou I from 0.4 to 0.6 wt%o(average fluid of magmatic origin. 0.45 wtvo. o 0.08).

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TABLE3 COMPOSTTIONOFAWARIJITE, NEW CALEDONIA OPHIOLITE

omce Yl Yl Yl Y1 Yl Y1 Yl Y1 Y1 \2 Y2 Y2 \2 Y3 Y3 Y] YJ IJ '7 Analvsisno | 16 18 19 20 9? AWI* AW2** 8tc1 t4rl a5c? A6A 42 47 65 66 93 Awaruite Weight ptrcent ',71 '7947 'n Ni 7149 63 8215 E513 81 19 7658 7898 ao 8085 94 5t 8061 91 62 9601 99.46 1448 7405 98 80 s 000 000 001 002 001 002 006 001 OM 000 002 004 002 002 000 011 010 000 Fe 1983 r91l t't 7t 5 03 il 45 15 67 1999 18 54 1969 t7 51 418 r7 52 1 79 I 65 081 2372 23'.75 187 004 001 003 001 004 004 005 0ol1 005 000 006 001 000 000 0,00 008 001 000 I 55 111 1 80 1330 302 2@ 2.21 | 17 220 oo7 0 16 006 003 t47 042 017 045 022 Pb 000 000 006 000 000 0,00 009 nd nd 000 005 013 009 000 013 001 007 007 Co 03E 038 04r 034 028 03r 029 034 021 o'75 029 072 042 005 000 006 009 009 Cr 000 000 000 000 000 000 000 Dd nd 001 069 000 0-33 {Jll 000 000 0.00 002 Total 9929 99.M 99 49 10085 99 94 99 32 99 13 99 08 99 98 99 24 10055 99 09 10029 99 91 10082 9862 9A 52 10107 Atoflc lrcren! Ni 0'774 0774 0793 0821 0849 0812 0765 0790 0',712 0807 093't 080? 0910 0967 0987 0'146 0742 0911 S 0000 0000 0000 0000 0000 0000 0001 0000 0001 0000 0000 0001 0000 0000 0000 0002 0002 0000 Fe 0208 0201 0.186 0053 0120 0165 0.210 0195 0205 0.184 0050 0184 0081 0017 0008 0250 0250 0019 0000 0000 0-000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0001 0000 0000 Cu 0014 0016 0017 0123 0028 0019 0021 0011 0021 0001 0001 oool 0000 0014 0004 0002 0004 0002 Pb 0000 0000 0000 oo00 0000 0000 0000 o00o 0ooo 0000 oo00 0000 0000 0000 0000 0000 0004 0004 o0o4 0003 ooo3 0oo3 0003 0003 0.002 000? 0003 oo07 0-004 0000 0000 0001 0001 0001 cr 0000 0000 oooo 0000 0000 ooo0 00oo 0000 0o0s 00oo 0.004 0.001 0.000 0.000 0.000 0.000 ' +a R6ults of €ldon-pmbe niqo{oabss Cotr4ositim of Srain l, Figw 6 Compositim of grEitr 2, FiEu€ 6; r: rirq c 6r€

Awaruite reaction-rim up to 10 pm in width has developedin the chromite, characterizedunder the ore microscope by a Awaruite was analyzed from the three Yat6 occur- reflectanceintermediate between chromite and awaruite. rences(Table 3). It is found as isolated crystals or asso- This phaseresulted from transformationof the chromite, ciated in a symplectitic intergrowth with chromite, and occurred prior to its fracturing (Fig' 2H). EPMA where it occurs either as an isolated crystal in the core analysis of this component shows that it is an oxide of of the symplectite (type 1, Fig. 2F), or as small crystals Fe and Ni. The analysesgive an average of 23.5 wt%o in the symplectite (type 2, Fig. 2G). Awaruite varies NrO,22.3VoFeO,5l.\VaFe2O3 and traces of Si andMg, greatly in composition from one crystal to another and with only 0.89 wtTo CrzO: (Table 4), as opposedto be- within a given crystal. The composition is essentially tween 54 and 58 wtVoCr2O3 for the host chromite. The dominated by Ni-for-Fe substitution (Fig. 5); it ranges compositions obtained suggesta spinel of the (Ni,Fe") from close to metallic Ni (Ni 98.77oat.) to a more - Fe3'2Oatrevorite type, but the structural formulas rich composition(Ni14.2Vo,Fe 2O.5Voat., a composi- tion close to Ni3Fe, or stoichiometric awaruite). Trace Co and Cu also are found. At Yat6 1, the awaruite sys- 03 tematically yields higher Cu concentrations,up to 13.30 A waru tte wtvo Cl (av. 1.83 wt%o,o 0.39). The Yat6 2 awaruite has the lowest Cu concenffations(av 0.08 wt%, o 0.04), @ A Yatd1 (0.58 whereas it has higher Co concentrations wt%o,o c) A Yatcz 0.18, comparedto 0.38 wt%o,o 0.09 for Yat6 1). The t / 0.2 c1 Yar63 Yat6 3 awaruite gives variable Cu values (between 1.47 A O

and 0.17 wt%o),btttwith very low Co concentrations c) ",4 l\" (0.09 wt%a,o 0.07). There is no correlation between variations in the major and minor elements. Moreover, it was found that the Ir content of these samples of 01 awaruite is below the calculated detection threshold for (D A EPMA (around 1000 ppm), in contrast to iridian -l R2

awaruite and a Ru-Os-Ir-Ni-Fe alloy that were AR1 described from an ophiolite from Pakistan (Ahmed & ob- Bevan 1981). Furthermore, no relationship was 0,0 served between the habit of the grains (1.e.,isolated or in symplectites) and their composition. However, sig- 01 08 0.9 1.0 nificant zoning occurs; Ni is enriched from the grain Ni (at.conc.) center to the perimeter, but was examined only in cer- (Fig. tain grains 5). Fic. 5. Fe-Ni (atomic concentration, i e., atoms per formula An interesting phenomenonwas observedin certain unit) correlation diagram for awaruite from the different type-2 symplectites(and is therefore not systematic).At sulfide-bearingperidotites. C, Core, R, Rim for zoned crys- the contact between awaruite and chromite, an apparent tals (analytical results given in Table 3).

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TAXII-E4 COMPOSnON OF SECONDARYFet.{i OXIDE AND CHROMITE. the Ni-Fe alloy, it loses all its chromium (through re- NEW CALEDONIAOPHIOLITE placement by Fer+) and all its Mg (which is replacedby O@ufien@ Y3 Y3 Ni), whereasits FeO content remains constant.The phe- nomenon occurs abruptly becauseno chemical zoning Weight pdcent is seenin the chromite composition, unlike the awaruite. Nio 2323 2490 2060 2278 2326 04'7 057 sio2 o_59 0 35 051 000 003 0 l2 0.01 The latter is enriched in Ni around its edges,which ne- FeO tot 66 48 68 58 7385 6140 66,57 Eatesa priori drffitsion of Ni from the awaruite to the FeO 21 66 2253 u26 2262 2230 2271. 2503 chromite. The suggestedmechanism is F%O, 50,80 5l 11 5512 52',17 52m 000 000 as follows: Ni CoO 021 002 030 003 0 00 0 45 0't7 and Fe diffuse from the awaruite to the chromite and Alro3 009 000 001 00t 005 1272 t320 oxidize, and they replace Cr and Mg in the chromite. MnO 0r0 013 000 0M 000 022 0t7 CaO OU 001 000 000 0M 002 000 This process must occur relatively early in the history MgO 063 042 056 0 14 018 681 5t'7 of the rock, as it is independentof the fracturing of the vzor 0 09 0,00 o22 000 o19 t28 121 chromite. Tio, 000 000 0 00 0_03 000 000 004 Cr2O3 088 082 094 0,90 095 5812 5411 'lbtal 98 40 100 36 10262 99.41 9926 10293 L@28 EPMA aNo SIMS Alrar-ysss Atonc trHcent FoR Ni 6173 6469 5 156 6 175 6 311 0098 0r22 THETnacp Er-nueNrs si 0 195 0.113 0 t1't 0000 o0t2 0030 0003 5 986 6 088 6314 637'l 6.292 4947 5@7 Becauseof the almost total absenceof PGE carriers Fe4 12382 12436 12909 t2620 12491 0000 0000 associatedwith the 0057 0004 oo14 ore minerals, it was decided to de- '00 009 0 001 0 093 0 t64 AI 0 036 0 001 0 00.{ 030 0 019 3 872 4 168 termine whether trace amounts of PGE occur in solid Mn o 027 0 036 0000 00t2 0 000 0 049 0 039 solution in the ore minerals. We limited this work to Pt, Ca oo24 0004 0000 0000 0.015 0005 0000 Mg o3l2 0 20r 0 258 0 070 0089 2622 2064 using both EPMA and SIMS (secondaryion massspec- 0020 00fi) 0046 0000 0(43 0219 0214 trometry) and, for this preliminary investigation, stud- Ti 0000 0000 0.000 0007 0 000 0 000 0 00r o23t 0 210 o23t 0240 0253 tr E1t 11461 ied only one sample, from Yat6 I (sample 165-l). The Total 4s9 25562 25.182 25551 2555',7 21't69 21R52. EPMA analysesfor major and minor elements,as well Rqdts of el*troFpmbe nieuabres. The prcpotio of Fd+ is cal@lated as for trace Pt, were carried out at BRGM (Orl6ans) oo th€ b6is of chilg$balre comidqations in th€ rpiael stluctw.. using a Cameca SX-50 electron-probe micro-analyzer equipped with wavelength-dispersion spectrometers. For awaruite, the analytical conditions were: accelera- calculatedon the basis of 32 atoms of oxygen and equili- tion voltage 20 kV, beam current 200 nA, with count- bration of chargesare far from the theoretical formula. ing times of 600 s for Pt (peak + background) and 10 s On the basis of average v-alues,the formula may be for all the other elements. Under such conditions, the written as (Ni6 3Fe62Mg6.z)2*trzt(FensCre 2Si6 r)3*>rz.r calculated limit of detection for Pt is 100 ppm (2o con- O32,thus showing a marked deficiency in trivalent eie- fidence level). The following standardswere used: py- ments relative to spinel. Calculation of the structural rite for S, arsenopyrite for As, and pure metals for all formula based on the NiFe2Oa type does not give any the other elements,using the following X-ray lines Ptlct, better results. This phenomenon (observed systemati- FeKa, NiKcr, CuKa, CoKa, Aslct and SKct. cally where awaruite and chromite are in contact) is Most of the SIMS analyseswere done at CANMET paradoxical because,where chromite is in contact with (Ottawa), with a limited number performed at the Universit6 de Montpellier II. At Montpellier, some analysesfor trace Pt were done using a tryUriOCameca TABLE 5 SIMS ANAIYTICAI CONDTUONS (CANMET) secondary-ionmass spectrometerdescribed as partly an IMS-4f model with features of an IMS-5f model. Op- Strondary ion polility Negative erating conditions were similar to those at CANMET Matrix mmses merowd th", ttF"r*"g, ot11ir+32s,62rrli (Table 5), except that very low currents of 60 to 70 nA Primilybemcrent 350-500nA were used.Local calibrations of the magnet were made, Primary bem aderating yoltage 10 kV followed by depth profiles on implanted standards,as Imprct energy 145kv Field described by Cabri & McMahon (1995). To provide a aperture dimeter 750 Um Contlff t diaphragm dimeter 400 pm reference mass for subsequentcalculations of the Rela- Rtrter 250 pm tive Sensitivity Factor (RSF), as well as to monitor the Image field 150pm system's stability and the integrity of the sample,one or Dimeter of malysis ilea 62.5 pm two other masseswere recorded(e.g.,56Fe, 57Fe2 + 32S, Implmt standild Pentlmdite 62Ni2+ 32S;.Calibrations and some computations knplantationfl uence,sp@ies I oxlol3ions,l98pt/cm2 were Counting time per mms per cycle ls done using the CamecaSIMS version 5.2 program, and Mass resolution -2,O@rtl{n calculationsofRSF, peak and curve concentrationswere Energy offset trone done off-line using Excel. The depth profiles of the Minimum detection _ limits ft: pentlmdite 230 305 ppbw implant standards were measured at the Centre Depth of analyzedprofiles - 0.J 2.0 um d'Electronique de Montpellier.

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At CANMET, the analysesfor trace Pt and Au were TABLE 6, RESULTS OF SIMS ANALYSES OF AWARTIIIE FROM YATE 1I done using a CamecaIMS-4f double-focusingmagnetic sector secondary-ionmass specffometer.Details of the Date Gr No Pt Date Gr No Pt Date Gr No Pt operating conditions are listed in Table 5. The usual practice was to perfom two consecutive depth-profiles 2GFeb AW-l 0 0O2 2GMr AW5*' 0,014 AW9 0 005 on an implant standardbefore the mineral analyses,fol- 26-Feb AW-z 0 018 2GMq AWI 0002 26Mar 26-Feb AW-3 0 011 264/1il AWz 0024 26-M{ AWl2 0 0004 lowed by one or two additional depth-profiles of the 26-Feb AW-4 0l2l 26'lvlrl Aw3 0015 26.Ms AWl3r 0002 same implant standard at the end of the series of min- 26-Feb Aw-5 0 014 2GMr A\{4 0 006 26-Mtr AWl5 0 006 26-Feb Aw-6 0 010 26.I\,Iar A\tr6 0 055 2cltfar AW16 0 009 eral analyses.Crater depthsin the implant standardwere 26-Feb AW-7 0 004 26-M{ AW? 0009 2GMu AWIT 0009 measuredusing a Tencor Alpha-Step 200 profilometer 2GFeb IIK-1 0 004 26-Me AWB 0 00E several times in both directions, and RSF values were calculated using the SIMS Instrument Control System *Sflple 165-1,sstion #53636(sI'2); * rcnin ualyzed by elwtron niooprcte in makingit software. version 4.0. from Charles Evans and Associ- tgsl. Not., polishedswtim ru repolishedfor the 26 Mqtch ualyses, rqosble to'l@gei4 gairo ppiosry oahzed Theonmtration of Pt is quotcd ates. For the analyses of pentlandite, the integrated nwe/t counts for 1e8lt and the reference mass(es)were mea- sured from each spectrum and used, together with the average RSF value, to calculate the concentration of are given in Table 6 and show a rafige from 0.002 to platinum in ppm by weight. Trace concentrations rn 0.121 wt%oPt. awamite and heazlewoodite could not be calculated in SIMS ion images were performed on a section con- the same manner becauseno standardswere available. taining two grains of awaruite, one of which (grain 1) is For awaruite, a grain (AW5) found to contain relatively euhedral, and the other (grain 2) anhedral with a small high Pt was later analyzedby electron microprobe (in inclusion of pentlandite (Fig. 6). The analyzed section the above-describedanalytical conditions). Sixteen also contains part of a large crystal of heazlewoodite. point-analysesgave an averageof 0.014 wt% Pt (with a The major-element compositions of these grains are standarddeviation of 0.009), and the following average given in Tables 2 and 3. composition(inwtVa): Fe 19.861(o 0.367), As 0.018 A slight difference in composition is noted between (0.029),Co0.4ri (0.015),Ni 78.832(0.32s), Cu 1.684 awaruite 1 and2. Awaruite I is slightly enriched in Ni (0.089). The RSF was calculated using the concentra- and Co and impoverished in Fe and Cu compared to tion of 0.014 wt% Pt obtained by EPMA. Thus, the in- tegraled counts for iesPtand the 62Nireference mass for each spectrum were used with this RSF to calculate the Pt concentration of each grain. It was not possible to quantify the Pt concentrationof heazlewooditebecause no grain was found with sufficient h to analyze with the electron microprobe. Direct-ion images of selected elemental and molecular ion specieswere acquired by operating the instrument in the ion microscope mode while retaining high mass-resolution. For the present study, acquisition times of 60 s were used.

Rssulrs or rue SIMS AN.ql-vsss

The first SIMS analyses,which were done at Montpellier, showed that awaruite and pentlandite con- tain Pt, but we did not detect Pt in heazlewoodite. Fur- ther depth profiles and ion images done at CANMET confirmed these preliminary observations.These depth profiles were done on eight grains of awaruite, all of which were found to contain Pt, and were followed by ion images. However, difficulties in focusing the im- agesrequired that the polished section be repolished. A 14 awaruite grains were then analyzed on the further Frc. 6. BSE image showing enlargement of awaruite and new polished surface, and ion images prepared for se- heazlewoodite association in sample 165-1 (Yat6 1). The lected grains containing both awaruite and heazle- area within the ellipse representsthe area shown on SIMS woodite. A grain with relatively high Pt counts was ion images in Figure 7, approximately 62.5 pm in diam- chosen for an EPMA analysis in order to have a refer- eter. Areas 1 and 2 represent awaruite, and area 3 repre- encestandard for Pt in awaruite (grain AW5, seeabove). sentsheazlewoodite; pn: pentlandite inclusions in awaruite The results of the SIMS analysesfor all awaruite grains atea2-

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awaruite 2, but these differences (tess than l%o for the TABLE 7 RESI]LTS OF SIMS ANALYSES OF PENILANDME major elements) are very minor compared to the large rr.ovvars r* Ni-for-Fe substitution recorded in Figure 5. The SIMS ion images confirm that Pt favors awaruite over Date Sample Gr No Pt RefsenG ro Steded Location heazlewoodite,but clearly show that awaruite 1 is much '95 more enriched in Pt than awaruite 2 (Figs. 6, 7); note 7 Ds SI.I 178 99 156 PN-92-12 Montpellier that the Pt content has not been quantified. It 7Ds'95 sr-8 1E5 E 5 156 PN-92-12 Montpelliq seemsthat 23Feb'96 sI-8 PN-1 53 144 PN-92-IE Ottaw Pt is preferentially concentratedin awaruite containing 23Feb'96 SI-E PN-2 2.4 144 PN-92-18 Ottaw less Fe and Cu and more Ni and Co, but this has not 23 Feb'96 sI-8 PN-3 5 9 144 PN-92-18 Otraw 23 Feb'96 SI.E PN-34++ I 3 144 PN-92-t8 Ottsw been systematically studied. In addition, it can be seen 26M6'96 sI-8 PN-3 <0 3 156 PN-92-18 Ottaw that both awaruite grains also host gold (Fig. 7F), rn 26Mw'96 sI-8 PN-5 51 156 PN-92-1t Ottaw contrast to heazlewoodite, 26MN'96 sI-8 PN-6 1 3 156 PN-92-18 Ottaw which hosts none. 26MN'96 SI-E PN-7 <0 3 156 PN-92-18 Ottaw Eleven grains of pentlandite were analyzedby SIMS 26Mt'96 SI-E PN-E <0 3 156 PN-92-18 OttEm (Table 7). The analysesshow that pentlandite contains

from about 1 to l0 ppm Pt, considerably less than *Suple 165-1, srction #53636 (SI-2); **we grain as #185, but middle of gru awaruite. This range is comparableto a range of I to 18 Note polished siim rcpolished for the 26 Mrch malyres, mafting it iapossible to rrcognire grains pwiously malyzed The concentntion of Pt is quoted in ppm ppm Pt recently reported for most pentlandite grains analyzedfrom the Main Sulfrde Zone,GreatDyke,Zim- babwe (Oberthir et al. 1997).ln contrast to pentlandite and awaruite, l0 of 12 grains of heazlewooditeare with- samples,we have demonstrateda clear preferenceof Pt out detectable Pt in SIMS depth profiles. For the two and Au for awaruite over heazlewoodite.A later. more heazlewoodite grains with a few Pt counts, it was not comprehensivepublication (Lindsay et a|.7994) shows possible to quantify their Pt content, but the concentra- that Pt and other PGE are enriched in Fe-Ni alloy in tion is estimated to be less than or close to I ppm. furnace (Fe-Ni-S) matte and in Cu-Ni alloy in con- The preferential association of Pt in awaruite over verter (Cu-Ni-S) matte, which is more in agreement heazlewooditeis in contrast to the preliminary findings with our results. In referenceto their earlier exDloratorv on sampleson unknown origin reported by Lindsay & qualitative work, they wrote: "SIMS imaging of con- Sellschop(1988). Theseauthors showed a SIMS line- vefter matte revealedthat trace (partsper million) quan- scan through heazlewoodite, bornite-chalcocite, Ni- tities of PGE are homogeneously distributed in Cu-Fe alloy and back into heazlewoodite and inter- sulphides.Heazlewoodite hosts Pt, Pd, Ir and Os, bomite 1e4Pt preted the concentration as being highest in hostsAu and minor Rh, and chalcocitehosts Rh...". A heazlewoodite and lowest in bornite-chalcocite. They quantitative study of trace PGE in different mattes also interpreted Au as being most enriched in bornite- would help provide more precise data and should also chalcocite. Though we have no Cu sulfides in our benefit the pyrometallurgical processing of PGE ores.

62Ni. 62Ni 32S 34S. Frc.7. -^ SIMS-ion images, a11625 pm in diameter. A. B. + + C. Overlay 62Ni 32s 31s of + + on image of lesPt.The Pt can be seen to concentratemainly in thL awarulte crystal (area 1) and not in heazlewoodite or the pentlandite inclusions in awaruiteof area2. D 63Cu.E. 19spt.F. le7Au.

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DrscussroNervo CoNcI-usroNs has not been studied at the trace-element scale in the minerals, although we have noted a strong Pd enrich- In this study, we have shown that PGE enrichment ment in a sample with abundant pentlandite. occurs in sulfides and metallic alloys in mantle The conditions for PGE redistribution in the second- harzburgites, and that Pt concentratespreferentially rn ary ore assemblageare totally unknown, but this redis- awaruite as opposedto sulfides. The origin of these sul- tribution remains at the scale of the ore-mineral fide and metallic-alloy minerals is, however, still con- assemblage.If fluids were involved in the transforma- troversial. Their texturessuggest that they appearedvery tion of the primary magmatic assemblage,they must early, at magmatic conditions,but their composition was also have beeninvolved in the redistribution of the PGE. probably modified later, particularly during serpen- This study has shown for the first time that Pt can be tinization. The PGE enrichment of the sulfides probably strongly reconcentratedin the alloy, and that the Pt con- occurred contemporaneouslywith crystallization of the tent in awaruite is very varied (from 20 to 1210 ppm). primary sulfide assemblage,with most of the PGE ini- This heterogeneity occurs at the scale of the crystal, as tially in solid solution in the sulfides. Had primary plati- shown in Figures 6 and'l. Grain 1 (euhedral) is much num-group minerals beenpresent, they would probably more effiched in Pt than grain 2 (anhedral)' We do not not have been entirely affected by the transformation believe, at this stage of the study, that the small differ- processesduring the serpentinization that affected the encein composition between grains 1 and 2 can account BMS, and remnants of PGM would have been found. for the distribution of Ft. We believe rather that Pt was Some authors (e.g., Krishnarao 1964, Sinton 1976) concentrated first in the early euhedral crystals then, consider that the assemblagedescribed here, consisting when the residual Pt content of the fluid was very low, of heazlewoodite,pentlandite, and awaruite, is specific awaruite 2 was formed. The presenceof Au in awaruite to serpentinization.Peretti et al. (1992) have shown an was also not suspected(and has not been quantified)' assemblageconsisting of pentlandite,awaruite, magnet- Both generations of awaruite seem to be equally en- ite, native copper and silicatesthat formed during meta- riched in Au, whereas the sulfides (heazlewoodite and morphism in a peridotite under strongly reducing pentlandite) seem to be totally devoid ofAu. conditions. Experimental studies(Filippidis 1985) have High levels of Pt had already been recorded in shown that serpentinization of olivine at very low sul- samplesfrom the New Caledonia ophiolite, with a typi- fur fugacity produces an assemblageconsisting of cal PGE distribution characterized by a strong Pt heazlewoodite,Ni-poor magnetiteand Ni-poor awaruite, anomaly (Aug6 & Maurizot 1995)' These enrichments whereas the opaque assemblagein the -free sys- are associatedwith chomite-bearing rocks occurring as tem consistsof Ni-rich magnetite and Ni-rich awaruite. dykes at the base of the cumulate series;these are typi- Many other authors have described the effect of cally devoid of sulfides. It was shown that they belong serpentinization on sulfide assemblagesand the possi- to a late-magmatic stage,but the relationship between bility of generating Ni-Fe sulfide and alloys from sili- this PGE mineraltzingevent and the PGE enrichment in cates(Eckstrand 1975,Lorand 1989).On the other hand, the mantle cannot be evaluated. Lorand et al. (1993) suggestedthat the PGE enrichment The PGE distribution in the sulfides and metallic for the Lanzo lherzolite massif was due to an ascending alloys is variable and seemsto be partly controlled by magma (S-saturatedmelt) that percolated through the the mineral assemblage,Pd-rich samples being rela- peridotite (i.e., magmatic conditions). tively enriched in pentlandite. This characteristic is The hypothesis favored here is that an immiscible probably a remnant of the primary mineralization, with magmatic sulfrde liquid (i) formed from a percolating the strong affinity of Pd for pentlanditebeing well dem- ascendingmagma in the mantle (or from an early fluid onstrated.Thus the Pt/Pd value in the ore minerals var- under subsolidus conditions as opposed to a serpen- ies between 0.17 and 2.58, the Pt-rich samples being tinization-related fluid), (ii) becameenriched in PGE as also enriched in Ir, whereashigh Rh values are obtained a result of a high R factor (i.e., a large amount of a S- both in Pd-rich and Pd-poor samples. saturated magma percolating through the residual The PGE values in the ore minerals obtained in this mantle provoking collection of the PGE by sulfides), study are well above some published values for other and (iii) then crystallized, giving a "magmatic" sulflde ophiolites containing sulfide mineralization in the assemblagewith Ni-sulfides, Fe-sulfides and minor Cu- mantle series. For example, Economou & Naldrett sulfides. In a second stage,because of reducing condi- (1984) gave an averageof 179 ppb Pt, 9 ppb Pd and 16 tions presumably imposed during serpentinization (cf. presence of graphite), this mineral assemblagewould be modified to give the obser.redmineral association. The transformation of the magmatic sulfides and the formation of the Ni-Fe alloys resulted in a redistribu- tion of the PGE in the ore minerals. Thus Pt is strongly being the rocks themselves. partitioned in the Ni-Fe alloy (in agreementwith its si- Enrichments in PGE are also rather common in the derophile character).The distribution ofthe other PGE series of mafic-ultramafic cumulates in ophiolites.

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Lachize et al. (1991) described a maximum of 37 ppb oc6anique repr6sentatif des stadespr6coces de l'6volution Pt, 130 ppb Pd and 4 ppb Rh in the sulfide-rich samples alpine. C.R. Acad. Sci. Paris 285, 56r. D,953-956. from gabbro cumulates of the Oman ophiolite. The sul- fides have a magmatic origin, having precipitated as Auc6, T. & Mlunrzor, P. (1995): Stratiform and alluvial plati- num mineralization droplets of immiscible liquid from a mafic magma. in the New Caledonia ophiolite com- plex Can. Mineral 33. 1023-1045. Ohnenstetteret al. (1991) described Pd enrichment in cumulate dunite enriched in pentlandite (Albanian BREroN, J., EeBnr_6,J.M., Gtr_LEs,C., ophiolite), (1993) pr-pd and Pedersenet al. described Jfzfquw,P.,MfzrEnn, J. & Rorenr, M. (1995): Magmatic enrichment in a sulfide-bearing horizon from olivine and supergeneplatinum-group minerals in the New Cal- cumulates in the Leka ophiolite complex, Norway. Ob- edonia ophiolite . Chron Rech. Min. 63(520),3-26. viously, PGE enrichment in "residual" mantle rocks cannot be explained in terms of simple magmatic segre- Caenr, L.J. & McMauoN, G. (1995): SIMS analysis of sulfide gation of a cumulate phase. minerals for Pt and Au: methodology and relative sensitiv- ity factors (RSF). Amongst the unsolved problems is the origin of the Caz. Mineral. 33,349-359. awaruite grains associatedwith the symplectite, and the CocrBnrr, A. Vor-nNcrn, M. & MEvBn, G. (1987): Determr- conditions of formation of the reaction rim between nation of the noble metals in chromites and other geologi- awaruite and chromite in certain types of symplectite. cal materials by radiochemical neutron activation analysls. There is no indication that the awaruite associatedwith J. Radioanal. Nuclear Chem. 113,133-143. the BMS assemblagewas derived by the same process as the awaruite associatedwith the chromite-silicate EcxsrnaNo, O.R. (1975): The Dumont : a model symplectite. for control ofnickeliferous opaquemineral assemblagesby In conclusion, we have shown that PGE (other than alteration reactions in ultramafic rocks. Econ. Geol. 70.183-201. Os, Ir and Ru, which are classically associated with chromite) can be found in an ophiolitic environment, EcoNorraou,M.I. & NlI-nnerr, A.J (1984): Sulfides assocr- and that ophiolite sourceswere not necessarily devoid ated with podiform bodies of chromite at Tsangli, Eretria, of ft, Pd and Rh. Our findings also confirm that SIMS Greece.Mineral. Deposita 19,289-297 . is a suitable tool in the study of trace concentrationsof PGE and Au ore in minerals and for obtaining images FEI-n, S.W. & Heccsnrv, S.E. (1994): Symplectites in upper on the distribution of these elements amons different mantle peridotites: development and implications for the minerals. However, much more work is needid to fully growth of subsolidusgamet, pyroxene and spinel. Contrib. document the trace-elementdistributions in this and re- Mineral Petrol. lI8, 138-156. lated associations. Fnpprnts, A. (1985): Formation of awaruite in the system Ni- Fe-Mg-Si-O-H-S and olivine hydration with NaOH solu- AcxNowr-eocsN4pNrs tion, an experimental study. Econ. Geol 80,1974-1980.

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