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Origin of Placer Laurite from Borneo: Se and As Contents, and S Isotopic Compositions

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Origin of Placer Laurite from Borneo: Se and As Contents, and S Isotopic Compositions Mineralogical Magazine, April 2004, Vol. 68(2), pp.353–368 Dedicatedto the memory of Dr A. J.Criddle,Natural History Museum, London, who died in May 2002 Originof placerlauritefromBorneo: Se and As contents,andSisotopiccompositions 1, 2 3 4 K. H. HATTORI *, L. J. CABRI , B. JOHANSON AND M. L. ZIENTEK 1 University of Ottawa, Ottawa, Canada 2 99 Fifth Avenue, Suite 122, Ottawa, Canada 3 Geological Survey of Finland, Espoo, Finland 4 USGeological Survey, Spokane, USA ABSTRACT Weexamined grains of the platinum-group mineral, laurite (RuS 2),fromthe type locality, Pontyn River,Tanah Laut, Borneo, and from the Tambanio River, southeast Borneo. The grains show a variety ofmorphologies,including euhedral grains with conchoidal fractures and pits, and spherical grains with nocrystal faces, probably because of abrasion. Inclusions are rare, but one grain contains Ca-Al amphiboleinclusions, and another contains an inclusion of chalcopyrite+borni te+pentlandite+heazle- woodite (Ni3S2)thatis considered to have formed by a two-stageprocess of exsolution and crystallizationfrom a oncehomogeneous Fe-Cu-Ni sulphide melt. Allgrains examined are solid solutions of Ru and Os with Ir (2.71 –11.76wt.%) and Pd (0.31–0.66wt.%). Their compositions are similar to laurite from ophiolitic rocks. The compositions showbroad negative correlations between Os and Ir, between As and Ir, and between As (0.4 –0.74wt.%) and Se (140to 240 ppm). Laurite with higher Os contains more Se andless Ir and As.The negative correlations between Se andAs may be attributedto theiroccupancy of theS site,but thecompositional variations of Os, Ir and As probably reflect the compositional variation of rocks wherethe crystals grew. Ratiosof S/ Se inlaurite show a narrowspread from 1380 to 2300, which are similar to ratios for sulphidesfrom the refractory sub-arc mantle. Sulphur isotopic compositions of laurite are independent ofchemical compositions and morphologies and are similar to the chondritic value of 0 %. The data suggestthat S inlaurite has not undergone redox changes and originated from the refractory mantle. Thedata support the formation of laurite in the residual mantle or in a magmagenerated from such a refractorymantle, followed by erosion after the obduction of the host ultramafic rocks. KEY WORDS: PGM, nugget,alluvial placer, placer deposit, electron-microprob e,stable-isotopes, osmium, SEM data,Kalimantan, trace-element geochemistry. Introduction rocks(Cabri and Harris, 1975; Slansky et al., 1991; Cabri et al.,1996).The rare occurrences of COARSE-GRAINED platinum-groupminerals (PGM) coarse-grainedPGM inigneous rocks led to the havebeen recovered from many placers asso- suggestionthat they formed in late magmatic- ciatedwith ultrama c-ma c igneousrocks. The pegmatiticenvironments (Johan et al., 2000), as viewof their origin prevailing before the mid- wellas low-temperature crystallization of PGM 1980swas that they crystallized in magmas and duringthe weathering of mac rocks(Augustithis, thenwere mechanically eroded from the igneous 1965;Ottemann and Augustithis, 1967; Cousins, 1973;Cousins and Kinloch, 1976; Stump , 1974), andduring sedimentation in placers (Barker and *E-mail: [email protected] Lamal,1989; Bowles, 1986, 1988; Bowles et al., DOI: 10.1180/0026461046820192 2000). # 2004The Mineralogical Society K H HATTORI ETAL Osmiumisotopic data have contributed to the (Fig. 1a,b).Noother PGM wererecovered at this discussion.A relativelyna rrowsp readof site.Metasedimentary and metavolcanic rocks, 187Os/188Osis used to support the mechanical tonaliticgneiss, and schist occur in the watershed derivationof PGM fromAlpine-typean d immediatelyupstream of the laurite occurrence Alaskan-typeintrusi ons(Hattori and Cabri, (Zientekand Page, 1990), but small northwest- 1992; Cabri et al.,1996), and a considerable owingtributaries drain from ophiolite complexes isotopicvariation from PGM associatedwith ofthe Meratus Range to the site. The laurite is largeintrusions ,suchas those in Freetown believedto havebeen derived from the ophiolitic Complex(Hattori et al.,1991),is explained by rocks.The laurite locality is ~4km upstreamfrom theisotopic heterogeneity of host igneous rocks thegold-PGE placers and lode occurren ces (Hattori,2002). However, in contrast, Bowles et describedby Zientek et al.(1992).Burgath and al.(2000)attributed the variation of 187Os/188Os Mohr(1986) and Burgath (1988) described the inPGM associatedwith the Freetown Complex to PGM-bearingophiolites in the area. surfaceprocesses and crystallization of PGM in Hattori et al. (1992)reported the occurrence of placers.Bird et al.(1999)found elevate d lauritegrains and their Os isotopic compositions in concentrationsof 186Osin leachates during acid chromititesof the Meratus Range ophiolite at extractionsof Os from placer PGM associated SungaiKalaan and compared the data with placer withJosephine ophiolite, Oregon, and suggested lauritegrains collected from the streams draining theircrystallizationnearthec ore-mantle fromthese ophiolitic rocks in the same area. boundary.In contrast, Meibom et al. (2002) attributedthe variation of 186Osto heterogeneity ofthe upper mantle. Analyticalprocedure Inorder to constrain the origin of coarse- ThePontyn River laurite grains were selected grainedPGM, wedetermined the concentrations usinga binocularmicroscope. Laurite grains are ofAs and Se andthe S isotopiccompositions, blackwith a metallicblue tinge. After cleaning becausethese values are susceptible to hydro- thesamples in detergent water in an ultrasonic thermaland low-temperature processes. bathfor >20 min, the morphology of mineral grainswas examined using a JEOL 6400digital scanningelectron microscope (SEM) equipped Samplelocations witha LINK EXL energy-dispersiveX-ray Thelaurite grains examined in thisstudy are from analyser.The operating conditions were 20 kV, theTanah Laut subprovince, in the province of acceleratingvoltage and 0.5 nA beam current. SouthKalimant an,Indones ia(Fig. 1 a). The Themajor element compositions were determined grainswere collected from placer deposits at the onthegrains coated with carbon, using a Cameca typelocali ty(BM4050 4)from the Pontyn CamebaxMBX electronmicroprobe equipped (Pontijn)River (sample numbers starting with P) withfour wavelength dispersive spectrometers. nearthe town of Asemasen on thesoutheast coast Theanalytical conditions were 20 kV acceler- ofBorneoand from the Tambanio River (sample atingvoltage and 35 nA beam current using numberstarting with T), upstream of the village Os-Lb, Ir-La, Ru-La, Rh-La, Pt-La, Pd-Lb, ofRiampinang(~3º 40 ’S and 114º 55’ E), also in As-Lb, Ni-Ka.Standardsare pure metals and thesoutheastern Borneo (Fig. 1 a,b). alloysexcept for a pyritestandard for S- Ka. Raw Thelaurite samples from Pontyn were donated X-raydata were converted to wt.% using the tothe Natural History Museum, London, by Dr CamecaPAP matrixcorrection program. Phoebusin 1866 together with native platinum, Minorelement concentrations were determined Fe-Ptalloys, gold, cinnabar and hydrocerussite usinga CamecaSX 50equipped with four Pb3(CO3)2(OH)2.E.Wo¨hlerat Go ¨ttingenidenti- wavelength-energydispers ivespectro meters. edlaurite as a newmineral in the same year Se-La on TAP As-Kb on LiF. As-Ka was used (Wo¨hler,1866). We con rmed the identity of forbase metal sulphides, but not used for laurite lauriteand the last two minerals using X-ray becauseof overlaps with Os- Lb2.Instead,As- Kb diffraction(XRD). wasmeasured with LiF. Analyticalconditions At theTambanio locality, the laurite grains were35 kVand beam curren tof500 nA. werecollected from sediments in the main Measuringtime was 300 s forpeak and 300 s channelof the river, upstream from the junction forbackgrounds. The minimum detection limits withthe westward- owing Buluhembok River arede ned as 3 HB6Ht, where B isbackground 354 ORIGINOF PLACER LAURITE FIG. 1. (a)Map showing the laurite sample locality (open star), towns and villages (open squares) and selected features relevant to the placer PGMoccurrences in southeastern Kalimantan. They include diamond-Au-PGM placers (dotted areas), PGM-Auplacers (solid colour), podiform chromitite deposits (solid circles), ultramac rocks (hatched pattern), and the Manunggul Formation (inclined stripe). The detailed geology of the dotted rectangular area close to the laurite location is shown in Fig. 1 b.The Late Cretaceous sedimentary and mac volcanic rocks of the Manunggul Formation appear to be the source of the diamond-bearing placers. Modied from Zientek et al. (1992). (b)Geological map of the Riam Pinang area, South Kalimantan. The location of the laurite samples is shown relative to the native gold-platinum-iron alloy-bearing placers downstream from Riam Pinang (area with PGM observed in pan concentrate samples). Modied from Zientek et al. (1992). 355 K H HATTORI ETAL FIG.2. Morphology and BSEimages of selected laurite grains. Xmarks the areas chosen for the analyses. Scale bars are all 100 mm. The brighter areas correspond to higher Os and the darker areas to higher Ru contents. ( a) Morphology of grain P-1-1.; ( b)BSEimage of the grain, P-1-1; ( c)spherical shape of P-2-6; ( d)BSEimage of the grain, P-2-6, with several inclusions of amphiboles; ( e)angular shape with abundant fractures of P-2-9; and ( f) BSE image of the grain P-2-9. 356 ORIGINOF PLACER LAURITE counton eitherside of thepeak and t thecounting time.Under the analytical conditions above, the detectionlimits for Se are15 ppm for laurite and 8ppmfor other sulphides, those for As are 370ppm for laurite and 30 ppmfor
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