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Mineralogical Magazine; April 2004; v. 68; no. 2; p. 353-368; DOI: 10.1180/0026461046820192
© 2004 Mineralogical Society of Great Britain and Ireland
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Origin of placer laurite from Borneo: Se and As contents, and S isotopic compositions

K. H. Hattori1,*, L. J. Cabri2, B. Johanson3 and M. L. Zientek4

1 University of Ottawa, Ottawa, Canada
2 99 Fifth Avenue, Suite 122, Ottawa, Canada
3 Geological Survey of Finland, Espoo, Finland
4 US Geological Survey, Spokane, USA

* E-mail: khattori{at}uottawa.ca


   ABSTRACT
TOP
ABSTRACT
Introduction
 Sample locations
 Analytical procedure
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
We examined grains of the platinum-group mineral, laurite (RuS2), from the type locality, Pontyn River, Tanah Laut, Borneo, and from the Tambanio River, southeast Borneo. The grains show a variety of morphologies, including euhedral grains with conchoidal fractures and pits, and spherical grains with no crystal faces, probably because of abrasion. Inclusions are rare, but one grain contains Ca-Al amphibole inclusions, and another contains an inclusion of chalcopyrite+bornite+pentlandite+heazlewoodite (Ni3S2) that is considered to have formed by a two-stage process of exsolution and crystallization from a once homogeneous Fe-Cu-Ni sulphide melt.

All grains examined are solid solutions of Ru and Os with Ir (2.71–11.76 wt.%) and Pd (0.31–0.66 wt.%). Their compositions are similar to laurite from ophiolitic rocks. The compositions show broad negative correlations between Os and Ir, between As and Ir, and between As (0.4–0.74 wt.%) and Se (140 to 240 ppm). Laurite with higher Os contains more Se and less Ir and As. The negative correlations between Se and As may be attributed to their occupancy of the S site, but the compositional variations of Os, Ir and As probably reflect the compositional variation of rocks where the crystals grew.

Ratios of S/Se in laurite show a narrow spread from 1380 to 2300, which are similar to ratios for sulphides from the refractory sub-arc mantle. Sulphur isotopic compositions of laurite are independent of chemical compositions and morphologies and are similar to the chondritic value of 0{per thousand}. The data suggest that S in laurite has not undergone redox changes and originated from the refractory mantle. The data support the formation of laurite in the residual mantle or in a magma generated from such a refractory mantle, followed by erosion after the obduction of the host ultramafic rocks.

KEYWORDS: PGM, nugget, alluvial placer, placer deposit, electron-microprobe, stable-isotopes, osmium, SEM data, Kalimantan, trace-element geochemistry


   Introduction
TOP
ABSTRACT
 Introduction
Sample locations
 Analytical procedure
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
COARSE-GRAINED platinum-group minerals (PGM) have been recovered from many placers associated with ultramafic-mafic igneous rocks. The view of their origin prevailing before the mid-1980s was that they crystallized in magmas and then were mechanically eroded from the igneous rocks (Cabri and Harris, 1975; Slansky et al., 1991; Cabri et al., 1996). The rare occurrences of coarse-grained PGM in igneous rocks led to the suggestion that they formed in late magmatic-pegmatitic environments (Johan et al., 2000), as well as low-temperature crystallization of PGM during the weathering of mafic rocks (Augustithis, 1965; Ottemann and Augustithis, 1967; Cousins, 1973; Cousins and Kinloch, 1976; Stumpfl, 1974), and during sedimentation in placers (Barker and Lamal, 1989; Bowles, 1986, 1988; Bowles et al., 2000).

Osmium isotopic data have contributed to the discussion. A relatively narrow spread of 187Os/188Os is used to support the mechanical derivation of PGM from Alpine-type and Alaskan-type intrusions (Hattori and Cabri, 1992; Cabri et al., 1996), and a considerable isotopic variation from PGM associated with large intrusions, such as those in Freetown Complex (Hattori et al., 1991), is explained by the isotopic heterogeneity of host igneous rocks (Hattori, 2002). However, in contrast, Bowles et al. (2000) attributed the variation of 187Os/188Os in PGM associated with the Freetown Complex to surface processes and crystallization of PGM in placers. Bird et al. (1999) found elevated concentrations of 186Os in leachates during acid extractions of Os from placer PGM associated with Josephine ophiolite, Oregon, and suggested their crystallization near the core-mantle boundary. In contrast, Meibom et al. (2002) attributed the variation of 186Os to heterogeneity of the upper mantle.

In order to constrain the origin of coarse-grained PGM, we determined the concentrations of As and Se and the S isotopic compositions, because these values are susceptible to hydrothermal and low-temperature processes.


   Sample locations
TOP
ABSTRACT
 Introduction
 Sample locations
Analytical procedure
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
The laurite grains examined in this study are from the Tanah Laut subprovince, in the province of South Kalimantan, Indonesia (Fig. 1aGo). The grains were collected from placer deposits at the type locality (BM40504) from the Pontyn (Pontijn) River (sample numbers starting with P) near the town of Asemasen on the southeast coast of Borneo and from the Tambanio River (sample number starting with T), upstream of the village of Riampinang (~3° 40'S and 114° 55' E), also in the southeastern Borneo (Fig. 1a,bGo).



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  		FIG. 1. (a) Map showing the laurite sample locality (open star), towns and villages (open squares) and selected features relevant to the placer PGM occurrences in southeastern Kalimantan. They include diamond-Au-PGM placers (dotted areas), PGM-Au placers (solid colour), podiform chromitite deposits (solid circles), ultramafic 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. 1bGo. The Late Cretaceous sedimentary and mafic volcanic rocks of the Manunggul Formation appear to be the source of the diamond-bearing placers. Modified 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). Modified from Zientek et al. (1992).

 
The laurite samples from Pontyn were donated to the Natural History Museum, London, by Dr Phoebus in 1866 together with native platinum, Fe-Pt alloys, gold, cinnabar and hydrocerussite Pb3(CO3)2(OH)2. E. Wöhler at Göttingen identified laurite as a new mineral in the same year (Wöhler, 1866). We confirmed the identity of laurite and the last two minerals using X-ray diffraction (XRD).

At the Tambanio locality, the laurite grains were collected from sediments in the main channel of the river, upstream from the junction with the westward-flowing Buluhembok River (Fig. 1a,bGo). No other PGM were recovered at this site. Metasedimentary and metavolcanic rocks, tonalitic gneiss, and schist occur in the watershed immediately upstream of the laurite occurrence (Zientek and Page, 1990), but small northwest-flowing tributaries drain from ophiolite complexes of the Meratus Range to the site. The laurite is believed to have been derived from the ophiolitic rocks. The laurite locality is ~4 km upstream from the gold-PGE placers and lode occurrences described by Zientek et al. (1992). Burgath and Mohr (1986) and Burgath (1988) described the PGM-bearing ophiolites in the area.

Hattori et al.(1992) reported the occurrence of laurite grains and their Os isotopic compositions in chromitites of the Meratus Range ophiolite at Sungai Kalaan and compared the data with placer laurite grains collected from the streams draining from these ophiolitic rocks in the same area.


   Analytical procedure
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
The Pontyn River laurite grains were selected using a binocular microscope. Laurite grains are black with a metallic blue tinge. After cleaning the samples in detergent water in an ultrasonic bath for >20 min, the morphology of mineral grains was examined using a JEOL 6400 digital scanning electron microscope (SEM) equipped with a LINK EXL energy-dispersive X-ray analyser. The operating conditions were 20 kV, accelerating voltage and 0.5 nA beam current. The major element compositions were determined on the grains coated with carbon, using a Cameca Camebax MBX electron microprobe equipped with four wavelength dispersive spectrometers. The analytical conditions were 20 kV accelerating voltage and 35 nA beam current using Os-Lß, Ir-L{alpha}, Ru-L{alpha}, Rh-L{alpha}, Pt-L{alpha}, Pd-Lß, As-Lß, Ni-K{alpha}. Standards are pure metals and alloys except for a pyrite standard for S-K{alpha}. Raw X-ray data were converted to wt.% using the Cameca PAP matrix correction program.

Minor element concentrations were determined using a Cameca SX 50 equipped with four wavelength-energy dispersive spectrometers. Se-L{alpha} on TAP As-Kß on LiF. As-K{alpha} was used for base metal sulphides, but not used for laurite because of overlaps with Os-Lß2. Instead, As-Kß was measured with LiF. Analytical conditions were 35 kV and beam current of 500 nA. Measuring time was 300 s for peak and 300 s for backgrounds. The minimum detection limits are defined as 3{surd}Bx{surd}t, where B is background count on either side of the peak and t the counting time. Under the analytical conditions above, the detection limits for Se are 15 ppm for laurite and 8 ppm for other sulphides, those for As are 370 ppm for laurite and 30 ppm for other sulphides. The proximity of Os-Kß to the As-Lß2 line and of the Os-Lß8 line to the Se-L{alpha} line raised the detection limit of As and Se in laurite. Standards used are pure metallic Se and cobaltite for As (45.2 wt.% As). Replicate analyses of 12 spots on different dates during the period between July, 2002, and May, 2003, show a reproducibility of <9% for Se and <5% for As. The detailed analytical procedure is essentially the same as that described by Robinson et al. (1998).

For S isotope analysis, laurite grains from Tambanio River were examined with a SEM-EDS without carbon coating and each grain (0.6 to 1.2 mg) was ground in an agate mortar together with V2O5 (1:3 weight ratio). The mixture containing ~0.05–0.59 mg of laurite was placed in tin foil and injected into a CE Elemental Analyzer. The samples were combusted at ~1700°C to release SO2. The SO2 gas was introduced to a Finnigan-Mat Delta Plus mass spectrometer for the isotope analysis after passing through 7 ml of silica at ~1000°C and Cu at ~600°C to buffer O isotopes and reduce SO3. Duplicate or triplicate analyses of all samples show a reproducibility of 30.2{per thousand}. The standards used were: IAEA-S1 (–0.3{per thousand}) and IAES-S2 (+21.7{per thousand}).

The XRD analyses were carried out on crushed grains and analysed using a Rigaku position-sensitive detector microdiffraction goniometer (PSD-MDG) at 50 kV, 180 mA, scanned over the range 5–70°2{theta}.


   Results
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
 Results
Discussion
 Conclusions
 Acknowledgements
 References
 
Morphology and mineralogy of grains
Most grains are sub-rounded to spherical with abundant pits and conchoidal fractures (Figs 2Go and 3Go, Repository 1 and 2). Several grains form euhedral to subhedral crystals with evidence of abrasion on the crystal faces (Figs 2Go and 3Go, Repository 1 and 2). There are no significant differences in the morphology of samples from Pontyn and Tambanio.



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  		FIG. 2. Morphology and BSE images of selected laurite grains. X marks the areas chosen for the analyses. Scale bars are all 100 µm. The brighter areas correspond to higher Os and the darker areas to higher Ru contents. (a) Morphology of grain P-1-1.; (b) BSE image of the grain, P-1-1; (c) spherical shape of P-2-6; (d) BSE image 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.

 


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  		FIG. 3. Laurite grains used for the S isotope study listed in Table 3Go: (a) Gr-3; (b) Gr-4; (c) Gr-5.

 
The XRD pattern confirmed that the grains are laurite. They contain substantial amounts of the OsS2 component, ranging from 18 to 38%, up to 12% of the IrS2 component, and minor (up to 1.2%) amounts of the PdS2 component (Fig. 5Go, Table 1Go). The concentrations of Os, Ru and Ir are broadly correlated (Fig. 6Go); grains containing more Os show less Ir (correlation coefficient, r = 0.83) and Ru (r = 0.67). Samples from Tambanio River show a narrower spread than Pontyn samples. Tambanio samples plot the end-member of the compositional variation with high Ir and Se and low Os and As (Fig. 6Go)



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  		FIG. 5. Compositions of laurite plotted on the Ru-Ir-Os ternary. The compositions of the laurite studied (open circles for Pontyn samples, solid circles for Tambanio samples) are compared with those from the Bushveld layered igneous complex (South Africa) and Bird River Complex (Canada). The compositional fields of laurite from ophiolitic ultramafic complexes are shown on the right side of the ternary. They include Oman, Othrys (Greece), Skyros (Greece) and Vourinos (Greece), Ray-lz (Polar Ural, Russia), and Rhodope (Bulgaria). Data sources: Bird River (Cabri and Laflamme, 1988; Ohnenstetter et al., 1986), Bushveld (Schwellnus et al., 1976; Kingston and El-Dosuky, 1982), Oman (Ahmed and Arai, 2003); Othrys (Garuti et al., 1999b), Ray-Iz (Garuti et al., 1999a), Vourinos (Garuti and Zaccarini, 1997), Skyros (Tarkian et al., 1992), and Rhodope (Tarkian et al., 1991).

 

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  		TABLE 1. Chemical composition of laurite grains from Pontyn (P) and Tambanio (T).
 


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  		FIG. 6. Compositional variations of laurite grains, Ir vs. As and Ir vs. Os. Lines of calculated correlation and correlation coefficients, r, are shown. Open circles = Pontyn samples, solid circles = Tambanio samples. (a) Iridium vs. As, (b) Iridium vs. Os, (c) Arsenic vs. Se, (d) Osmium vs. As.

 
Laurite grains from Pontyn commonly show intra-compositional zoning due to different concentrations of Os and Ru (Figs 2b,fGo and 4cGo). The zoning patterns are not necessarily symmetrical. Some show inhomogeneity with diffuse boundaries without any growth bands (Repository R 1h (see the data section of the Mineralogical Society website: www.minersoc.org/pages/e_journals/dep_mat.htm)) and some show sharp boundaries between growth bands of different compositions (Fig. 2b,fGo). The growth pattern shows earlier Os-rich bands surrounded by Ru-rich rims (Figs 2bGo and 4cGo), but reverse zoning with exterior Os-rich growth bands is also observed (Fig. 2fGo). One grain shows several Os-rich interiors surrounded by Os-poor rims (Repository R 1n). Laurite grains from Tambanio are mostly homogeneous with no internal textures (Repository R 1q and R 1s).



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  		FIG. 4. Laurite grain, P-1-2, containing a sulphide inclusion: (a) Subhedral shape of the grain; (b) photomicrograph of the grain; (c) BSE electron image of the grain; and (d) BSE image of the sulphide inclusion. The compositions of the areas marked by X are listed in Table 2Go. Abbreviations: Bn = bornite, Cp = chalcopyrite, Hz = heazlewoodite, Pn = pentlandite.

 
There are grains with growth bands truncated by the external shape (Fig. 2b,fGo). The truncation of growth bands in crystals is commonly interpreted by later fragmentation of a once larger grain. This is possible for a grain that shows fracturing (Fig. 2fGo), but it is not applicable for all grains. For example, some grains show truncation of growth bands by a euhedral external shape (Fig. 2bGo). Such a growth pattern may be formed due to the nucleation and crystal growth of laurite in a limited space, or may be the result of fracturing in the sediments.

One grain contains a spherical inclusion of a mixture of chalcopyrite, bornite, pentlandite and heazlewoodite (Fig. 4Go). The texture between sulphide phases and the spherical outer shape (Fig. 4dGo) suggests that the inclusion was once homogeneous Fe-Cu-Ni sulphide and exsolved into monosulphide solid solution (mss) and intermediate sulphide solid solution (iss). Further crystallization formed pentlandite and heazlewoodite from mss and bornite and chalcopyrite from iss. Another grain contains several inclusions of Ca amphibole (Fig. 2dGo).

The crushed material from three grains gave a clean XRD pattern ascribed to laurite, with a cell edge a = 5.6117 Å. Taking an average composition of all laurites analysed (Table 1Go) gives (Ru0.66Os0.28Ir0.06Pd0.01)1.01S2.00, which may be compared to 5.6089 Å, reported for laurite from Senduma Chrome Mines, Sierra Leone, with a more Ru-rich composition of (Ru0.84Os0.04Ir0.04)0.95S1.05 (Bowles et al., 1983).

Arsenic and selenium
All grains contain considerable amounts of Se and As, far greater than the detection limits of 15 and 370 ppm, respectively. The concentrations of Ir show a broad negative correlation with As (r = 0.77; Fig. 6aGo) and positive correlation with Os (r = 0.83; Fig. 6bGo). The As and Se contents show a broad negative correlation (r = 0.57; Fig. 6cGo). Grains with greater Os contain less Ir (Fig. 6bGo) and Se (Fig. 6a,cGo) and more As (r = 0.94; Fig. 6dGo). As mentioned above, the samples from Tambanio plot close to the end-member composition with high Ir and Se, and low Os and As.

The S/Se ratios of laurite show a narrow spread between 1380 and 2300 (Table 1Go). The concentrations of Se in the sulphide inclusion show a considerable variation among different phases, with S/Se ratios varying from 3500 to 6560, but the concentrations are overall lower than the host laurite (Table 2Go). The weighted ratio of As/S of the sulphide inclusion is 38x10–5, which is far lower than those of the host laurite, >1000x10–5. The data suggest that laurite incorporates As and Se in preference to base metal sulphides.


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  		TABLE 2. Arsenic and selenium concentrations of sulphide inclusions in laurite and of laurite (ppm) (sample P-1-2, shown in Fig. 4Go)
 
Sulphur isotopic compositions
The samples from Tambanio are homogeneous and free of inclusions. Thus, they were selected for S isotopic analysis. The values of all grains show a narrow spread in {delta}34S values (+1.16±0.36{per thousand}) and minor enrichment of 34S, but they are very close to the meteorite standard value of 0{per thousand}. The values are independent of the morphology and the composition of the grains (Table 3Go).


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  		TABLE 3. Sulphur isotope compositions of laurite grains.
 

   Discussion
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
 Results
 Discussion
Conclusions
 Acknowledgements
 References
 
Origin of the host rocks
The origin of Alpine-type ultramafic intrusions is controversial, with two principal opinions: they are obducted slices of oceanic upper mantle, or cumulates of mafic magma in supra-subduction zones (e.g. Robertson, 2002). The latter magmas form by high degrees of partial melting in a refractory mantle. Ruthenium and Os are both refractory, Ir-type PGE and are concentrated in the residual mantle (e.g. Guillot et al., 2000); thus laurite is common in refractory mantle rocks and cumulates of melt formed from such a refractory mantle. This is confirmed by several mineralogical studies of PGM from Alpine-type ultramafic intrusions and refractory mantle rocks (e.g. Melcher et al., 1997; Garuti et al., 1999a). Thus, the major-element compositions of the laurites from Pontyn and Tambanio Rivers are consistent with their crystallization in the upper refractory mantle or cumulates of mafic magmas that originated from a refractory mantle.

Sulphur isotopic compositions
Sulphide formation in surface environments requires the reduction of dissolved sulphate. This reduction of sulphate, either by a kinetic or equilibrium process, at low temperature is accompanied by large isotopic fractionation and the product, S2–, has low {delta}34S, as much as 60{per thousand} less than SO42–values (e.g. Hoefs, 1997). In addition, dissimilatory sulphate-reducing bacteria, which probably participate in sulphate reduction in surface environments, produce S2– with variable isotopic compositions because bacterial reduction is affected by many factors, such as nutrient for bacteria, sulphate concentrations and bacterial population. Hence, sulphides formed near the surface show a large variation in {delta}34S, as shown in many sedimentary rocks (Table 4Go). Our data thus rule out laurite formation in near-surface environments, including "element agglutination" (Augustithis, 1965) or "accretion" of fine particles (Cousins, 1973; Cousins and Kinloch, 1976) during weathering and deposition of placers (Bowles, 1986, 1988; Barker and Lamal, 1989).


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  		TABLE 4. Compositional characteristics of sulphide formed under different environments.
 
Sulphur isotopic fractionation between oxidized and reduced S species becomes smaller at higher temperatures, but is still significant. Thus, sulphides formed at high temperatures, even ~500°C, may show a significant isotopic variation when they form from fluids containing significant sulphate (e.g. Cameron and Hattori, 1987). However, isotopic fractionation among reduced S species is small at high temperature. Ripley et al. (2003) show that the variation in {delta}34S is <2{per thousand}, even after extensive degassing at magmatic temperatures. Therefore, the consistent S isotopic compositions, near meteoritic sulphur values of 0{per thousand}, from the laurite samples studied indicate that their formation did not involve any oxidation-reduction of sulphide sulphur. Thus, the laurite formed under reduced conditions at deep levels in the crust or mantle (Table 4Go).

Compositional variations
Laurite belongs to the isometric diploidal system, similar to pyrite. Pyrite and laurite contain divalent metals (Fe2+, Ru2+, Os2+, Ir2+) and two covalently bonded sulphur atoms as a dianion. Pyrite can accommodate significant amounts of As, and most of the As replaces S, forming (As-S)2– (Fleet et al., 1993). Pyrite with high concentrations of As may contain thin, 10 to 15 Å, layers of arsenopyrite-like structure (Simon et al., 1999). Irarsite (IrAsS) and ruarsite (RuAsS) belong to this arsenopyrite group with a crystal configuration similar to that of arsenopyrite.

The correlation between As and Se is, therefore, easily understood, considering that both elements occupy the S site, forming (As-S)2– and (Se-S)2–. There are fewer S atoms in As-rich laurite. In addition, anion pairs of (Se-As)2– may not be easily formed in the structure.

The laurite samples studied also display correlations among metal ions. The atomic ratio of Os to Ir is 1.2, which suggests the two heavy atoms are nearly substituting for each other. However, the atomic ratios of Ir/As and As/Se are 18 and 71, respectively. This means that the loss of 18 atoms of Ir corresponds to an increase of 1 atom of As and 71 atoms of As to 1 atom of Se. The observed compositional variations may be attributed to either crystallographic effects or compositional control of the environment for crystal growth.

We suggest the latter. First, the correlations between Os and Ir and between Ir and As are not easily explained by the crystallographic configuration, because these three metals form divalent cations. In addition, Ru and Os should be interchangeable in the crystal considering that the two have very similar ionic radii and electro-negativities. Thus, high Ir concentrations in low-Os laurite are attributed to an external cause, the compositional variation of rocks where the laurite grains crystallized. Our proposed interpretation is further supported by the compositions of laurite from other locations (Figs 7Go and 8Go). For example, laurite from the Bushveld Complex shows a wide range of Ir almost independent of Os content (Fig. 7Go), but a broad positive correlation between Ir and As (Fig. 8Go). Considerable compositional variation in a single sample is also noted in the western Bushveld Complex (Maier et al., 1999). Laurite grains from other ophiolitic rocks also show a wide scatter without any apparent correlations on the diagrams of Os vs. Ir and Ir vs. As (Fig. 8Go).



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  		FIG. 7. Ir and Os contents of laurite compared to those from the Merensky Reef of the Bushveld Complex (solid triangles), Bird River complex (X), Ray-Iz (open circles), Ojen in Ronda Complex (open diamonds), and Oman (solid squares). The data from Ojen are from Torres-Ruiz et al. (1996). The other data sources are listed in Fig. 5Go.

 


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  		FIG. 8. Ir and As contents of laurite compared to those from other locations. Data sources are listed in Fig. 5Go.

 
Selenium concentration among different sulphides
The S/Se ratios of laurite are much lower than those in the sulphide inclusion. Furthermore, different phases in the inclusion show a wide variation, chalcopyrite has the highest S/Se ratio, 7350, and heazlewoodite the lowest ratio, ~3500. The weighted average value of S/Se is calculated to be 5890 using 82.7 vol.% for bornite, 6.0 vol.% for pentlandite, 11.0 vol.% for heazlewoodite, 0.24 vol.% for chalcopyrite. The volume of each phase was calculated after the measurement of surface area on a digital photograph using the ‘Image J’ computer program.


The apparent distribution coefficient for Se, KSe, is ~3.0. The value is large, probably due to low-temperature re-equilibration, considering the rapid diffusion of Se in sulphides (e.g. Bethke and Barton, 1971). Nevertheless, the data suggest that laurite preferentially incorporates Se compared to base metal sulphides.

Variations of S/Se ratios
Surface waters contain very low Se (i.e. high S/Se ratios) because of the sluggish oxidation of Se. This is reflected by low Se in sulphides in most clastic sedimentary rocks, evaporites and seawater, S/Se > 100,000 (Measures and Burton, 1980; Fig. 9Go). Selenium may be enriched in Fe-oxide chemical sediments due to strong adsorption of SeO32– on an oxide surface and may also be enriched in organic-rich black shales formed in anoxic basins (e.g. Stanton, 1972; Howard, 1977), but these cases are not applicable to the studied samples. Thus, the S/Se data from the laurite grains studied are not consistent with their formation in surface environments (Table 4Go).



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  		FIG. 9. S/Se ratios of the samples studied (laurite and Cu-Ni-Fe sulphides) compared to the values for the primitive mantle, sulphides from sub-arc mantle, mid-oceanic ridge basalts, and sedimentary rocks. Data sources: primitive mantle (McDonough and Sun, 1995), sulphides from sub-arc mantle (Hattori et al., 2002), mid-oceanic ridge basalts (Hamlyn and Keays, 1986; Peach et al., 1990), sedimentary rocks (Stanton, 1972; Leutwein, 1978) and seawater (Measures and Burton, 1980).

 
The values of S/Se in laurite are even lower than that of the primitive mantle (3300; McDonough and Sun, 1995) and those for the mid-oceanic ridge basalts (3000–6000; Hamlyn and Keays, 1986; Peach et al., 1990). Instead, the values are comparable to those of mantle sulphides that originated from sub-arc mantle wedges (Hattori et al., 2002). Selenium and S show a similar geochemical behaviour. The causes of fractionation include: (1) redox change (Yamamoto, 1976); (2) contribution of Se-poor surface waters; and (3) the formation of other phases (e.g. Bethke and Barton, 1971).

(1) Selenium and S are fractionated in solutions near the redox boundary of S. When S is oxidized to form SO42– and HSO4–, Se remains as H2Se, Hse– and Se2–. Thus, the S2–/Se2– ratios in solutions near the redox boundary of sulphur vary widely due to oxidation of S. Sulphides formed from such solutions show a large variation in S/Se ratio. This is not applicable for the studied samples because the sulphur isotopic data indicate no redox change of S.

(2) Incursion of surface waters may cause a large variation of S/Se ratios because surface waters show very high S/Se ratios due to sluggish oxidation of Se. For example, seawater shows ratios > 4x108 (e.g. Measures and Burton, 1980).

(3) The remaining possibility is the removal of different sulphide phases and melt. Laurite preferentially incorporates Se compared to base-metal sulphides, as shown in the fractionation of Se among sulphide grains. Ruthenium and Os are considered to be refractory metals, retained in the mantle during partial melting. In contrast, Cu and Fe are incompatible with mantle minerals and are preferentially removed from the mantle during partial melting. Thus, partial melting and removal of base metal sulphides would lead to the enrichment of Se (lower S/Se) in the mantle. This is further supported by low S/Se ratios of sulphides found in refractory mantle underlying arcs (Hattori et al., 2002). Low S/Se ratios of the studied laurite samples, therefore, probably reflect the origin of S in laurite from a refractory mantle. We, therefore, suggest that the laurite grains crystallized either in the upper refractory mantle or in the cumulates of mafic magmas that were derived from such a refractory mantle.

The proposed interpretation implies high-temperature crystallization of laurite and this is consistent with experimental data on the stability of laurite (Andrews and Brenan, 2002).


   Conclusions
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
 Results
 Discussion
 Conclusions
Acknowledgements
 References
 
The studied laurite grains from placers in southern Kalimantan show a major element composition similar to laurite grains found in ophiolitic ultramafic rocks of other regions. Furthermore, the S/Se ratios of laurite show a narrow range, and with lower values than base metal sulphides. The ratios are similar to those of sulphides from the refractory mantle wedge, suggesting that the laurite grains crystallized in the refractory upper mantle or in cumulate ultramafic rocks of magmas originated from such a refractory mantle. The evidence suggests that the emplacement of laurite-bearing ultramafic rocks in the upper crust was followed by erosion, which liberated the laurite grains to be concentrated in placers.

Values of {delta}34S for all PGM studied are similar, near 0{per thousand}. The consistent S isotopic values near chondritic composition discount their crystallization at moderate to low temperatures at shallow crustal levels. Instead, they formed at depth where S has not undergone redox changes.


   Acknowledgements
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
 Results
 Discussion
 Conclusions
 Acknowledgements
References
 
We are pleased to be able to contribute this paper in the memory of Alan J. Criddle of the Natural History Museum, London, who made numerous contributions to the subject of ore microscopy, and who was a good friend of the second author.

We thank Alan Hart, Natural History Museum, London, for the laurite grains from the type locality (Pontyn River), Monika Wilke-Alemany for sample preparation in the laboratory, Paul Middlestead and Wendy Abdi, the University of Ottawa, for maintenance of the mass spectrometry laboratory, and Peter Jones and Lew Ling, Carleton University, for assisting with the SEM and electron probe analysis, and to John Wilson, CANMET/MMSL for the XRD analysis. We are also grateful to Ernst A.J. Burke of Vrije University for the information on the occurrence of the Pontyn laurite, Chris Stanley, Natural History Museum, London, for sending us a polished section containing PGM from Rio Pilpe, Colombia, that he and Alan Criddle had studied, even though it was not used in this study. LJC acknowledges the technical support provided by CANMET during his tenure as Emeritus Research Scientist. This project was funded by a NSERC grant to KHH. Comments by A. Bookstrom and S. Box, USGS, and by journal reviewers Baruch Spiro and Adrian Boyce helped to improve the manuscript.


  
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
Dedicated to the memory of Dr A. J. Criddle, Natural History Museum, London, who died in May 2002

[Manuscript received 4 August 2003: revised 13 January 2004]


   References
TOP
ABSTRACT
 Introduction
 Sample locations
 Analytical procedure
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 

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