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PLACER Os-Lr-Ru ALLOYS and SULFIDES 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] LM2 THE CANADIAN MINERALOGIST Ru . PapuaNew Guinea Harris& Cabri E S.E. Borneo (1eel) 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<kaido PGM collected proximal to the ultramafic bodies is consideredto be invaluable. Placer PGM originating The nortl-soutl-trending Kamuikotan ophiolitic from relatively recently emplaced ophiolites in complex (Komatsuet al. 1977)in centralHokkaido Hokkaido, Japan,and easternSamar, Philippines, were consistsofultramafic rocks,both high- andlow-pressure studiedfor this pu4)ose. metamorphicrock-types, and Mesozoic volcanic and In this paper, we present a brief description of sedimentary rocks. In tle complex, serpentinite the geology and PGM mineralogy in theseareas, and mdlange (e.g., Mt. Yubari-dakearea; Nakagawa& discuss the role of sulfur in the very early stage of Toda 1987) and dismembered ophiolite (e.9., crystallizationin the ophiolite. Horokanai area; Asahina & Komatsu 1,979) are observedlocally. Although the ultramaIic rocks have Gsot,ocrcAr,SETTING been largely altered to serpentinite, pseudomorphs and relict minerals show they were mostly dunite and The islands of Hokkaido and Samarare located in harzburgitethat had formed part of a shongly depleted tle western part of the Circum-Pacific archipelago ophiolite (Katoh & Nakagawa1986). PLACER OsIr-Ru ALLOYS AND SULFIDES IN OPHIOLITE TM3 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 1444 TTIE CANADIAN MIMRALOGIST 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
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