I44L The CanadianMineralogist Vol. 35, pp. 1,Ml-1,452(1997)

PLACEROs-lr-Ru ALLOYS AND SULFIDES:INDIGATORS OF SULFUR FUGACITY IN AN OPHIOLITE?

MITSURUNAKAGAWA1 GeolagicalSuney of Japan,Holckaido Branch Kita-8,Nishi-2, Kita-ka, Sapporo, 060 Japan

IIELENTIA E. A. FRr\I{CO 1

Mines and Geosciences Bureaw Philippines, NorthAvenue, Diliman, Quezon City, Philippines

AssrRAcr

Tlvo types of placer assemblagesof platinum-group minerals are recognizedfrom ophiolite settings.One is exclusively composedof grains of Os-k-Ru alloy, and occurs in Hokkaido, Japan.The alloys consist of Ru-poor and Ir-rich osmium, iridium (up to 11 at.7aRu), rutheniridosmine,and ruthenium(up to 92 at.7oRu). The other type comprisesosmium and iridium with lower Ru content, and primary laurite with significant amountsof Os and Ir. Samar,in the Philippines, is the typical locality. The differencein the assemblagesis attributedto the prevqiling condition of sulfur fugacity at the initial stageof their crystallization in the upper manfle.

Keywords: platinum-group minerals, Os-k-Ru alloys, laurite, placer, sulfur fugacity, ophiolite, mantle, Hokkaido, Japan, Samar,Philippines.

SovnranE

On trouve deux types d'assemblagesde min6raux du groupe du platine en alluvions dansles milieux ophiolitiques. Dans le prenier type, il s'agit uniquementde grains d'alliages h Os-Ir-Ru; c'est ce que nous trouvons d Hokkaido, au Japon.Dans ce gloupe setrouvenl osmiumpauwe en Ru et riche en k, iridium (usqu'd 117oRu, proportion atomique),rutheniridosmine, et rutlr6nium(atteignant927oRu). Liauhe type comprendosmium et iridium i faible teneuren Ru, et laurite primaire contenant des quantit6simFortantes d'osmium et d'iridium. Samar,aux Pbilippines, en est une localit6 typique. La diff6rence dansces assemblagesserait due i la fugacit6 du soufre au stadeinitial de cristallisation, dam le manteausup6rieur.

(Traduit par la R6daction)

Mots-cl6s: min6raux du gtoupe du platine, alliages i Os-h-Ru, laurite, alluvions, fugacit6 du soufre, ophio[te, manteau, Hokkaido, Japon,Sanar, Philippines.

INrnopucrrou in the systemOs-Ir-Ru (Hanis & Cabri 1973),the alloys from PapuaNevr' Guinea were shown to have a Refiactory Os-Ir-Ru alloys are regarded as the wide compositionalrange. In contrast,eleven samples typical platinum-groupminerals (PGM) commonly of alloy out of twelve from southeasternKalimantan associatedwith mantle-derived ophiolitic ultramafic @orneo)are poor in Ru Qessthan5 wt%o,Stumpfl & rocks (Contstantinideset aL L98O,Ang6 1985,Prichard Tarkian 1973) and,therefore, plot along the h-Os join. et al. L986,Tarkran1987, McElduff& Stumpfl 1990). Although compositionaldata on PGM alloys havebeen Recently,Harris & Cabri (1991) compiledcomposi- accumulating(4. Calcrj,et al. 1996),the regional vmiation tional datafor more than four hundredsamples of PGM in their Ru contentshas not beenwell explained. alloy from worldwide localities, and presentedthem The rarity of PGM hostedby ophiolitic ultramafic on an Os-Ir-Ru diagram, together witl tleir revised rocks doesnot allow a detailed and comprehensive nomenclature(Frg. 1). The data plot in two groups: assessmentof tleir modeof formationand the condition one along the Os-k join, and the otler along tle of the melt from which they crystallized. In practice, Os5ek5s-Ruline. In anearlier compilation of compositions most of our knowledge on ophiolite-relatedPGM

r E-mail addressss; [email protected], [email protected]

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FIc. l. Os-k-Ru diagram showing the composition of alloy in this systemfrom world- wide occurrences.Data from PapuaNew Guinea(Harris & Cabri 1973),southeastern Bomeo (Stumpfl & Tarkian 1973), andthe compilationof Ha:ris & Cabri (1991).The nomenclature(IR: iridium, OS: osmium"RU: ruthenium,RIO: rutheniridosmine)and miscibility gap (shaded)are those ofHarris & Cabri (1991).

has been obtainedfrom studiesof associatedplacer (Fie.2).Both constifute an arc-type association,and concentrates.However, this approachhas disadvan- have in corrmon a tectonically disruptedophiolite tages,because the uncertainty of their origin usually sequencein the arc system. puts a constraint on ttre scopeofthe investigation. A very thorough and systematicinvestigation of placer Hol

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Flc. 2. Map of PGE and Au placers and serpentinite in Hokkaido, Japan, showing localities of placer PGM for this study. Modified from Watanabe et al. (1986) and Yanaga(1984). T: TeshiogawaRiver, U: Ury'ugawaRiveq I: IshikaxigawaRiver, Y: YubarigawaRiver, S: SorachigawaRiver.

The complexis well known for numerousplacer Eastern Samar depositsofPGM in and aroundthe lenticular bodiesof serpentinite(Fig.2; Suzuki 1952, Urashima et al. Prior to the discovery of platinum-group elements 1974);however, serpentinite-hosted PGM had not been (PGE) minepfization in an ophiolitic complex by detectedprior to the study of Nakagawaet al. (L99L). Constantinideset aI. (L980)othe occurrenceof PGB Placer PGM samples were obtained from alluvial was already known at the Acoje chromite mine in tle steam sedimentsnear (ess than 10 km) the ulftamafic Zambalesmountain range on tle western coast of bodiesin two representativelocalities: the westernfoot Luzon Island, Philippines (Hulin 1950).The Acoje of Mt. Yubari-dakeand the Horokanaiplacer chromite mine produced commercial quantities of PGE as a mine (plg. 2). The samplesfrom the Mt. Yubari-dake by-productover the period L97VL975,and is the only were collected by local amateur collectors witl known PGE producerin the Philippines (BMG 1986). traditional skills, and those from the mine comprise a Previous studies concerning PGE in the Philippine part of the Bamba collection of the Geological Survey archipelagohave focussedexclusively on the Acoje ofJapan (GSI), Hokkaido Branch. mine (Bacutaet al. 1988,1990,Orberger et al. 1988, Part of both ultramafic bodies exhibit a well- 1990). In addition to the Zambalesophiolite, eight preserved layering of dunite and harzburgite. Both other ophiolite belts are recognizedin the archipelago high- and low-pressure-type metamorphic blocks (BMG 1986),and host someof the major depositsof are included in the serpentinite matrix of the Mt. chromite in the counfy. A majority of thesechromite Yubari-dakebody (Nakagawa& Toda 1987).On the deposits are of the high-chromium type (Batce et al. other hand.numerous dikes of diorite are found within 1981),and arethus also expectedto becomegood PGE the serpentinite body near the Horokanai mine producers(Yumul 1993). (Igarashiet aL.7985).Lithological characteristicsof Recently, an occlurence of placer platinum-group tlese ultramafic bodies are the absence of minerals (PGM) was found in laterite on Samar and clinopyroxene-bearinglithofacies and podiform-type Dinagatislands@ranco et al. 1993),where an ophiolite chromite deposits,and the occlurenceof accessory sequenceis situated.The ophiotte complex is part of chromite witl high Crl(Al + Cr). the EasternBicol - EasternMindanao ophiotte belt on

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mEastem Bicol- Eastem Mindanao Ophlollte belt E120"

FIc. 3. Simplified geological map and samplinglocalities of easternSamar, Philippines. Modified from Franco et al. (L993). D: Dinagat Island, EB: EasternBicol, SS: Samar-Surigao,PP: PujadePeninsula, B: Bigan River.

tle Pacific coastof the archipelago(Fig. 3). Mining consists of volcanic and pyroclastic rocks with and prospect'ngfor chromitehave been active in the sandstone,mudstone, and limestoneinterbeds dated ophiolitebelt (JNDP: Louca& MakelaL995,IICA & asLate Oligocene(BMG 1981).The catchmentarea is MMAI 1990).Altlough theseislands are well known clrainedby minor tributaries of the Llorente and Bigan for placer gold, none of the small-scaleminers paid rivers, and grains of PGM were pannedtogether with attention to the PGM associatedwith tle gold and placer gold and chromite from an eluvial laterite chromite. cappingthe ultrama.ficsuite. The Eastern Bicol - Eastern Mindanao ophiolite belt runs from the easternpart of Bicol Peninsulato Mnw.narocv op Os-k-RuAnoys ANDSurfloss EasternMindanao through SamarIsland and Dinagat Islandgroup (Frg. 3).This belt constitutesa complete The PGM grains mounted in an epoxy resin were but tectonically disruptedophiolite sequencebounded polishedfor optical observationand coatedwift a rhin on tlre east by the Philippine Trench; the belt is layer of carbonfor electron-microprobeanalysis using composedof tlree segments,namely, Eastern Bicol, the ShimadzuEPMA-8705 instrumentat the Geological Samar-Surigaoand PujadePeninsula. Samar Island Survey ofJapan, Hokkaido Branch.An accelerating occupies ttre northern portion of tle Samar-Surigao voltage of20 kV was usedwith a 10-nA beam current ophiolite segment, and hosts several productive on an MgO standard.Pure metals were employed as depositsof chromite(BMG 1981,1986). standardsfor all elements,except S, for which pyrite The Late Cretaceousophiolite block in Eastern was used. The concentration of ten elements was Samaroutcrops on the soutleasternpart of the island establishedusing the following X-ray lines: PtLc, hZa" (Garcia & Mercado 1981).It is composedpredomi- OsMu, PdZc, RuZcl, CuKa, NiKcr, CoKcl, FeKo, nantly of harzburgite tectonite and cumulate dunite and SKcl. The X-ray intensities were treated using with minor dunite-hostedmassive to disseminated ZAF corrections.Representative analytical results are chromitite, layered gabbro and altered basalt. These presentedin Table 1. ulframafic faciesunderwent pervasive serpentinization. The ophiolite block is characterizedby a seriesof Hokkaido northwesftrendingthrust slicesthat generallydip to the east,constituting a repetitive sequenceof the ophioliie More than 60 PGM grainswere examinedfrom each suite. The oldest overlying non-ophiolitic formation locality Qable2). Theyrange in sizefiom 0.2 to 1.3mm.

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TABLE 1. RNPXTSNMAflVE CIIEMICAL CONPOSMON ANALYSES OF PGM FROM HOKKAIDO ANDSAMAR

Horokanai (Hokkaido) Sanar (Phil No. H52 H87a H88 Yl8 Y82 Y88 iAvll AB45 RB51 AB42 Vo Os-h-Ru allovs Laurite Os M.82 25.t4 7.71 90.74 43.16 26.66t 19.31 55.45 2553 2154 Ir 45.43 65.22 6.m 6.n $.82 7r.611 73.U7 33.13 6.95 3.88 Ru 8.24 4.46 78.75 L.94 t2.69 1.431 0.74 1sL 36.L3 42.64 Rh 0.26 0.33 2.05 0.09 0.43 0.601 0.63 0.65 Pt 0.70 3.52 4.58 - | 6.29 3.05 Pd - o3t | 0.16 Fe 0.t2 0.42 0.r7 0.r0 o.?3 0371 0.36 o.r2 Ni o.L2 o.t2 0.15 0.191 Cu 0.10 - 0.12 0.10; s -l 30.94 31.85 TOTAL 9957 100.16 99.91 100.60100.96i 99.93 99.371100.18 100.56 atonic proportion Os 0.4t9 0.250 0.045 0.888 0.383 O.257t0.192 0523i 0.0S2 0.067 Ir 0.427 0.616 0.036 0.067 0.384 0.685; 0.718 03101 0.022 O.O12 Ru 0.145 0.080 0.864 0.036 o.2r2 0.026i 0.014 0.13010.217 0.247 Rh 0.005 0.006 0.022 0.fiI2 0.007 0.011i 0.0M 10.004 n 0.@6 0.033 0.v26 - 0.061 0.0281 Pd - - 0.m3 0.00s, 0.001 ] Fe 0.004 0.014 0.003 0.003 0.007 0.01210.012 0.oo4l Ni o.w2 0.004 0.004 0.0061 : Cu 0.m2 - 0.003 0.oo3l I s - | 0.675 0.670 -: below detectionlimil

The grains from both localities consist mainly of TABLE2. NUMBEROFDISCRETEPGM GRAINS INVESTIGATED Os-Ir-Ru alloy. All the grains from Mt. Yubari-dake consist of homogeneousOs-Ir-Ru alloy showing a euhedralto subhedralshape @g. 4A); no S- or As-containingphase was observed.This appearsto be a common characteristic of PGM grains from the southernpart of the complex,such as in Mukawa (Ohta & Nakagawa1990). On the other hand, aboutone-half grains of the PGM from Horokanai show similar *: the alloy is partly replaed by PGE $lfide, mtimoide, mide mineralogical affinities (Frg. 4B) to tlose from Mt. 0t: mi@textural replam@t ploduct of alterd Os-Ir-Ru alloy Yubari-dake, but the cry$tals are partly replaced by Q*': incluion in Os-h-Ru alloy microtexturalintergrowths of PGE sulfide, antimonide or arsenide@gs. 4C, D). Although the chemical compositions of the grains of Os-k-Ru alloy from dikes found in the serpentinitebody, the intrusion of both localities occupy a similar field in the Os-Ir-Ru thesedikes may haveinnoduced fluids that wererich in diagram (Fig. 5), the bulk of the material from S, As, Sb and other highly volatile elements,which Horokanaiplots closerto the rutheniridosmine- iridium may also have led to the formation of postmagmatic miscibility gap. laurite, irarsite (hS), and an unknown phasewhose Most of the replacementof individual alloy grains composition is close to hSbS (Nakagawa& Ohta occursat marginsand along cracks.This suggeststhat 1995). laurite @uS), as well asthe arsenidesand antimonides Although the alloy compositionsplot over a wide found along cracks and rims of grains of Ir-Os-Ru region of the system Os-Ir-Ru revised by Hanis & alloy from Horokanai area,are products of alteration Cabri (1991),most of themplot alongthe Os56Ir56-Ru similar to the ones described by Stumpfl & Tarkian join (Fig. 5). The regional differencein the mineral (1976).As thereare an exfraordinarynumber of diorite assemblageis not reflected in the compositional data

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Frc. 4. Photomicrographsof placer PGM. A andB: Optical imageof os-h-Ru alloy fron Mt. Yubari-dakeand Horokanai,respectively, in Hokkaido.Note the hexagonaloutline and angularshape. Scale bar, 0.1 mm, reflectedlight. C and D: Optical and BSE imagesof Os-Ir-Ru alloy (A) rimmed by an aggrega.teof irarsite andlaurite (IL) from Horokanai, Hokkaido. The area of D is marked in C (white arrowhead). Scale bar; 0.1 mm and 5 p*, respectively.E and F: optical imagesof os-Ir-Ru alloy (A) and laurite from eastern Samar. Note the subrounded shape but still recognizable hexagonalor octahedraloutlines of the crystals.Negative hexagonal inclusions in the allovs of E consist of Ir-rich allov.

for the Os-Ir-Ru alloy. Pt-Fe alloys occur only as Eastem Samar inclusionsin, or as aggregateswith, otherPGM phases. Tiny (5-20 pm) inclusions of heazlewoodite(Ni3St, The PGM are composedmainly of a Pt-Fe alloy, olivine (Foes.5)and calcic amphibole are occasionally but significant amountsof Os-Ir-Ru alloy and sulfides observedin the grainsof Os-Ir-Ru alloy, suchas in the also are present (Table 2). The grains of Os-Ir-Ru placerPGM from westernTasmania @eck et aL 1992). alloys and sulfldes from Eastern Samar belong to

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o Mt. Yubri-dake

. Horokanai

Ftc. 5. Plots on Os-Ir-Ru diagram for the alloy compositions from Hokkaido. The nomenclafirre(IR: iridium, OS: osmium,RU: ruthenium,RIO: rutheniridosmine)and miscibility gap (shaded)are those ofHarris & Cabri (1991).

the smallersize-fraction of tle placermaterials, and commonin laurite, and occur eitherintermittently or as measureup to 1 mm, with the majority being around clusters within a grain. The inclusions are composed 0.5 mm in diameter.They exhibit perfectly euhedral mostly of bornite, chalcopyrite and PGE-bearing to subhedral morphology (Fig. 4E). The grains of pentlandite. Os-Ir-Ru alloy are compositionally homogeneous osmium and iridium, and usually contain less than 10 DIScUSSIoN atomic%oof Ru (Fig. 6). Osmium-and iridium-bearing laurite, which also is common, occurs mainly as In general,Os, Ir, and Ru (I-PGE) are concenfrated discrete euhedral grains @g. 4F), rarely as minute in the most refractory cumulates,whereas Rh, Pt, and (<50 pm) inclusions within Pt-Fe nuggets (Table 2) Pd (P-PGE) behaveas incompatibleelements and are and chromite. The bulk of the laurite grains are fairly progressivelyconcentrated in the melt (e.g.,Batnes et homogeneous,and contain 19 to 38 wtToof Os, and 3 al. 1985,Keays 1995).That is why ophiolites,which to t5 wtVoofh (Fig. 6). are assumedto represent mantle-derived materials, haveessentially unfractionaled PGE patterns character- Despite a discrepancyin the size of tle grains, the izedby a relative enrichmentof I-PGE and depletion morphology and chemistryof laurite in the placersare of P-PGE @arneset al. L985). Several studies (e.9., similar to thoseof chromite-hostedlaurite in associated Stockman& Hlava 1984, Talkington & Watkinson ophiolitic bedrock (Franco et al. 1993). Thesegrains 1986) have also shown a positive correlation between are thus consideredto be magmatic in origin. Some Cr and I-PGE in ophiolitic chromitites.This is usually grains exhibit complex microtextures and composi- manifestedby the occurrenceof minute inclusions of tional inhomogeneities,which may be a result of rapid subhedral to euhedral laurite or Ir-Os-Ru alloy in growth in the melt. Mineral inclusions are rather chromitites.These inclusions are believed to havebeen

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. Alloy

til. ro t '

Flc. 6. Plots on Os-Ir-Ru diagramfor the alloy compositionand laurite from Samar.The nomenclatureand miscibility gap (shaded)are those ofHarris & Cabri (1991).

trapped as solid crystals during the formation of magmatic origin. Such magmatic inclusions in placer chromite (Ialkington et al.1984). PGM are known from WesternTasmania @eck et al. The origin of placer PGM, however, remains L992) aadFifield (Slanskyet al. 1991),Australia. In controversial. Whether ttre grains are primary the samemanneq the spongytextures and inclusionsof magmaticin origin (Ca}rj,et al. 798L,P*k et al. L992) hydroxides, both suggesting surface environmental or secondaryproducts formed during river transport productsofplacer PGM (Cabri et al. L98L,Burgatl (Bowles1986, 1988, Burgath 1988) is still opento 1988),are not observedin the Os-Ir-Ru minslals. The argument. But in light of several studies on placer alloys and sulfides,particularly in EasternSamar, show concentrates,many investigatorshave concurredtlat severalcomplex microtextures that can only be related Os-h-Ru minerals, and especially tleir alloys, are to crystallization at magmatic temperatures(Franco pdmary and magmaticin origin (Cabri & Hanis 1975, et aI. 7993). Therefore, thesemineralogical features, Feather1976, Bird & Bassett1980, Hagen et al. L99O). coupled with the geological aspectsof the two areas, Furthermoreo osmium isotopic studies on placer strongly suggestthat the Os-Ir-Ru minerals in this Os-k-Ru mineralsfrom severallocalities supportthis study are of magmatic origin. With regards to the hypothesis(Hattori & Hart 1991,Hattori et al. 1992). preponderanceof placer Pt-Fe alloy from Eastern As noted above, placer PGM from Hokkaido and Samar, detailed description of tlese features and EasternSamar were obtainedproximal to the ophiolitic discussionoftheir genesiswill be presentedelsewhere. bodies.Their distinct euhedralmorphology, such as the As mentionedabove, two fypes of regional assem- prismatic and angularshapes of tle Os-h-Ru minerals blagesofplacer PGM from ophiolites are recognized: (Fig. 4), suggestslittle or no effect of abrasionduring Os-k-Ru alloys without pdmary laurite (namedtype transport.Clearly, this is indicative of their magmatic H, after Hol&aido), and Os-Ir alloys with associated origin. The presenceof olivine and amphibole inclu- primary laurite (6?e S, after Samar).Placer PGM from sions in the minerals also provides evidencefor their Burma (Hagenet al. 1990)consist mostly of Os-k-Ru

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alloys, which rarely include small $ains of laurite. 1992).Therefore, it is expectedthat, under highflS), Also, a majority of fhe PGM from PapuaNew Guinea Ru in the magmais consumedduring crystallizationof consist of Os-Ir-Ru alloys, with minor amountsof laurite, resulting in the formation of a Ru-poor alloy in small inclusions (Cabri et al. 1996) and products the later stage,whereas conditions of lowflS) lead to of secondary alteration (Harris 1974) of laurite. appearanceof aRu-rich alloy in the magmaprior to the Laurite-free placer PGM, including Os-Ir-Ru alloys, crystallization of laurite. are recognized in the ophiolite complex of the Aras Although the original Os-Ir-Ru content of tle mountains,T[rkey (Cabi et aI. 1996).These assem- magmais a principal factor that controls composition blagesare rather similar to type-H alloys. In contrast, of the alloys and laurite, the order of crystallization,as the osmium and iridium from the Pirogues alluvium, determinedby.(S), may be more important to the Ru New Caledoniaophiolite complex (Aug6 & Maurizot content in the alloys. Coexist'ng alloys and laurite 1995), are almost free of Ru. In addition, a latge could give information on theflS) in the magma,but it proportion oflaurite grains are found associatedwith is very difficult to find a rock-hostedalloy-sulfide pair. chromitite in the area.Thus, this assemblageof PGM Only statistical examination of the paragenetic and correspondsto the type-S alloys. compositional data on placer PGM can reveal tle As mentioned in the inroduction, almost all regional difference offlS) in the very early-stage Os-Ir-Ru alloys of placer PGM from southeastern magmaresponsible for the ophiolite, nearits sourcein Kalimantan are Ru-poor (Stumpfl & Tarkian L973, the mantle. Cabi et al. 1996).A numberof laurite grainshave been detected in chromitite from the Meratus-Bobaris CoNcLUsIoNs ophiofrc, southeasternKalimantan (Burgath 1988).If the placer concentrateoriginated from the ophiolite, as TWotypes of assemblageof Os-h-Ru minerals of Burgath (1988) proposed,this assemblagemay also ophiolitic origin are recognized.One type consists correspondto type S. However, Zientek et al. (1992) of Os-h-Ru alloy,which showsa wide rangein chemical suggestedttrat those in the southeasternKalimantan composition, and is representedby the assemblagein placers were derived from Alaskan-t14recomplexes. Hokkaido, Japan.Primary laurite is essentiallyabsent Tasmanianplacer PGM, including Ru-poor Ir-Os-Ru in this type of assemblage.The other, Ru-poor Os-Ir- alloys (Ford 1981)and laurite (Peck& Keays 1990), Ru alloy vrith laurite, is typically observedin Eastern originated from Alaskan-typeintrusions. Samar,Philippines. The differencesin the assemblages presumablyreflect the order of crystallization of the These observations,however, are hamperedby a alloy and laurite. As fle order largely dependsonflS2) lack of data on placer Os-Ir-Ru minerals from other in the magma, this difference can be an indicator of ophiolitic suites. Even the extensive comFilation of in the ophiolite. over one thousandcompositions of alloy (Cabi et al. "(S, 1996) does not include adequatedata from otler ACKI.IOWI;EDGEI\,IErrs ophiolites,and is not sufftcientfor a thoroughstatistical treatment.However, this may well be a start to a new The investigation of Eastern Samar is part of direction ofresearch to enhance our understandingof the joint research and development project of tle placer PGM, not only from ophiolites, but also Geological Survey of Japanand the Philippine Mines Alaskan-typecomplexes, and their probablegenesis. and GeosciencesBureau (MGB) under the framework The associationof an Os-Ir-Ru alloy with laurite of the Institute for Transfer of Industrial Technology within a chromite grain is reportedfrom ophiolites in, (ITIT) Program.The authorsare grateful to the staffof for example,Greece (Aug6 1985) and New Caledonia the GSJ, Hokkaido Branch for their support in this (Legendre& Aug6 1986). Their microscopic textures research,particularly to Dr. E. Ohta for his invaluable suggest epitactic growth of Ru-rich laurite on a commentsand suggestionsduring the courseof study, euhedral grain of alloy. Thus, crystallization of the and to Mr. Takumi Sato for making the polished alloy must have occuned prior to the formation of sections.Sincere thanks are glven to Dr. Y. Togashiof laurite. Phasediagrams indicate that melting tempera- GSJand Mr. E. G. Domingo of MGB for their manage- turesof Os-Ir-Ru alloys arebetwenn2443 and 3050'C ment of the program.Apart of this paperwaspresented @ird & Bassett1980). Thermodynamic data (Westland at the GEOSEA95, held on February1.5-17,7995, in 1981) show that laurite is the first PGE sulfide that Maniln. Publication of this paper has been authorized crystaltzes in a cooling magma. On the other hand, by the committeeof GEOSEA95. We thank Emeritus Ru-poor alloys that epitacticly grew on euhedral and ProfessorK. Yagi of the Hokkaido University and Dr. zonedlaurite from ophiolitic chromitites are reported K. Haftori of the University of Ottawa for their fruitfrrl from Quebec(Corrivaux & Iaflamme 1990)andAlbania suggestions.The manuscriptwas improved by the (Amoss6et aL I992).T\e sulftr tugacity,flS), in these helpful commentsof two referees,Dr. L. Cabri and an chromitites is estimatedto have been relatively high anonymousrevieweq and AssociateEditor Dr. R.R. (betweenRu-RuS2 and Pt-ftS buffers; Amossl et al. Keavs and Interim Editor Dr. W.H. Macl-ean.

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WESTLAND,A.D. (1981): Inorganic chemistry of the ReceivedSeptember 19, 1996, revised manuscipt accepted. platinum-group elements..lz Platinum-Group Elements: March2. 1997.

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