Diversity of platinum-group minerals in podiform chromitites of the late Proterozoic ophiolite,...

32 (2007) 1–19www.elsevier.com/locate/oregeorev

Ore Geology Reviews

Diversity of platinum-group minerals in podiform chromititesof the late Proterozoic ophiolite, Eastern Desert, Egypt:

Genetic implications

Ahmed Hassan Ahmed

Department of Geology, Faculty of Science, Helwan University, 11795 Ain Helwan, Cairo, Egypt

Received 19 April 2004; accepted 10 May 2006Available online 12 February 2007

Abstract

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt.They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan–African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix oras variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromititeshave a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have avery restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter.

Platinum-group element (PGE) mineralization has been recently reported in podiform chromitites from the late ProterozoicPan–African ophiolite of the Eastern Desert of Egypt. The populations of platinum-group minerals (PGM) in the studied CED andSED chromitites are quite distinguishable; they are mainly sulfides (Os-rich laurite) in the former, and Os–Ir alloy in the latter.Sulfarsenides and arsenides are also found in subordinate amounts from both chromitites. The most abundant base metal sulfides(BMS) in the Eastern Desert chromitites of Egypt are millerite, heazlewoodite, pentlandite, chalcopyrite and pyrite. The sulfurfugacity and temperature conditions are the main controller of PGE mineralogy in the host chromitite at the initial stage within theupper mantle. Os-rich laurite is stable at high sulfur fugacity and low temperature conditions, whereas Os–Ir alloy is stable at lowersulfur fugacity and higher temperature conditions. The diversity of PGE mineralogy combined with the differences in petrologicalcharacteristics of chromian spinels from CED chromitites to SED ones suggests different degrees of partial melting of the mantlerocks of this ophiolite which, in turn may be attributed to different tectonic settings. During the post-magmatic processes, i.e.,serpentinization, the primary PGM, e.g., Os-rich laurite and Os–Ir alloy, can be modified at low temperatures to secondary PGM.© 2007 Elsevier B.V. All rights reserved.

Keywords: Chromitite; PGE; PGM; Laurite; Os–Ir alloy; Precambrian ophiolite; Egypt

1. Introduction

Due to the compatibility of Ir-subgroup (IPGE=Os, Irand Ru) of platinum-group elements (PGE) duringmantle melting, they tend to be concentrated in theearly magmatic precipitates (i.e., chromian spinel). The

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geochemical behavior of IPGE in the mafic andultramafic systems is very different from those of thePd-subgroup (PPGE=Rh, Pt and Pd) which areincompatible during mantle melting and tend to beretained in the residual melt (e.g., Crocket, 1981; Barneset al., 1985). Ophiolitic chromitites which are linked todeep-seated mantle processes are occasionally dominat-ed by Ru–Os–Ir platinum-group minerals (PGM) phases

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Fig. 1. Geologic sketch map showing the distribution of Precambrian rocks in Egypt and northern Sudan (Modified after El-Gaby et al., 1990). Thestudied localities are shown by rectangles.

2 A.H. Ahmed / Ore Geology Reviews 32 (2007) 1–19

(e.g., Augé, 1988; Tarkian et al., 1991, 1992; Garuti andZaccarini, 1997; Garuti et al., 1999a; Ahmed and Arai,2003). The IPGE mineral phases, e.g., Ru–Os–Irsulfides and alloys, therefore, are the most commonPGM in the mafic and ultramafic igneous rocks (e.g.,Legendre and Augé, 1986). Their mineralogical appear-ance, as sulfides or as alloys, in the mafic–ultramaficsystem is mainly controlled by the degree of partialmelting, temperature and sulfur fugacity (Tredoux et al.,1995; Brenan and Andrews, 2001; Andrews and Brenan,2002). The paragenesis of PGM, combined withcharacteristics of chromian spinel, is therefore veryuseful to constrain the early magmatic conditions andtectonic setting of chromitite formation (e.g., StockmanandHlava, 1984; Brenan andAndrews, 2001). It can also

provide constraints on the behavior of PGE in theophiolitic upper mantle.

In general, there are very few studies available onPGE concentration and distribution of the late Pre-cambrian ophiolitic chromitites in the Eastern Desert ofEgypt (Ahmed, 2001; Ahmed and Arai, 2002). Veryfew descriptions also have been given of PGEmineralogy in podiform chromitites of the Pan–Africanophiolite of Egypt (El Haddad, 1996; Styles et al.,1996; Zaccarini and Garuti, 2001). In this study, thedescription and geochemical analysis of the identifiedPGM in podiform chromitites from the Eastern Desertof Egypt is comprehensively presented. This mayprovide insights into the genesis of PGM and hostchromitite.

3A.H. Ahmed / Ore Geology Reviews 32 (2007) 1–19

2. Geological background and sample locations

The basement complex of the Eastern Desert ofEgypt constitutes the western side of the Arabian–Nubian Shield, which was cratonized in the latePrecambrian (e.g., El-Gaby et al., 1990). In general,three categories of basement rocks are found in theEastern Desert of Egypt (Fig. 1): (1) Pre-Pan–Africanrocks, composed of high-grade metamorphic rocks, i.e.,gneiss; (2) Pan–African rocks, comprising ophiolitesand island arc associations and; (3) Phanerozoic alkalinerocks. The late Precambrian Pan–African rocks, occupyca. 10% of the Egyptian land surface, covering a hugearea in the Eastern Desert, and small areas in southernSinai and southern parts of the Western Desert (Fig. 1).Several ophiolite suites have been reported from the

Fig. 2. Distribution of late Precambrian ophiolitic fragments and the studiedEastern Desert (SED) of Egypt (after Shackleton et al., 1980).

Eastern Desert of Egypt generally located to the south oflatitude 26° N (Fig. 2), e.g., along Qift–Quseir road(Nasseef et al., 1980; Stern, 1981), along the Idfu–Marsa Alam road (Shackleton et al., 1980; Ries et al.,1983), from Wadi Ghadir area (Takla et al., 1982), fromEl-Rubshi area (Khudeir, 1983), and from the southernEastern Desert at Wadi Bitan, Wadi Rahabah and WadiNaam (Ashmawy, 1987). Ophiolite complexes in theEastern Desert of Egypt are usually found as highlydismembered mafic to ultramafic bodies; the mostcomplete ophiolite sequence is described from the WadiGhadir area (El Sharkawy and El Bayoumi, 1979).

Chromitite pods of sub-economic value are commonin the central and southern parts of the Eastern Desert ofEgypt (e.g., Khudeir et al., 1992; Ahmed et al., 2001).They occur mainly as lenticular bodies of variable

chromitite localities in the central Eastern Desert (CED) and southern

Fig. 4. Chondrite-normalized whole-rock PGE patterns of centralEastern Desert (CED) and southern Eastern Desert (SED) chromititesof the late Precambrian ophiolite of Egypt. Note the wide range of PGEin the former compared to the latter.


dimensions; however the southern Eastern Desert (SED)chromitites, in most cases, are larger than the centralEastern Desert (CED) examples. Podiform chromitites,in most cases, are hosted by fully serpentinizedperidotite, mainly harzburgite host and subordinatedunite which forms an envelope around chromititepods. The mantle harzburgite and dunite envelopes arecompletely serpentinized in the CED; no primarysilicate minerals have survived alteration, whereasthey are relatively fresh at some localities in the SED(e.g., Abu Dahr and Abu Syeil areas; Fig. 2), containingfresh primary olivine and orthopyroxene.

Six chromitite localities in the CED of Egypt alongthe Qift–Quseir and Idfu–Marsa Alam roads (Fig. 2)have been examined; all of them have found to includePGM grains. They named Wadi El-Lawi (LW), Wadi El-Zarka (ZK), Um-Huitate (UH), Wadi Bezah (BZ),Barramiya (BR) and, Gebel El-Rubshi (RB). Chromi-tites from five additional localities in the SED of Egypt;Abu Dahr (AD), Abu Syeil (AS), Belmhandite (BL),Arais (AR) and, Um-Thagr (UT), have also beeninvestigated for their PGE mineralogy.

3. Spinel chemistry and PGE concentrations

The SED chromitite exhibits a very restrictedcompositional range of spinel with Cr# (=Cr/(Cr+Al)atomic ratio) of spinel of ca. 0.85, whereas the CEDchromitites show wide compositional variations from

Fig. 3. Variations of Cr# (=Cr/(Cr +Al) atomic ratio) versusFe3+# (=Fe3+/(Fe3++Al+Cr) atomic ratio) of chromian spinel inpodiform chromitites of the central Eastern Desert (CED) and southernEastern Desert (SED) of Egypt. Note the alteration trend of chromianspinel toward the high Fe3+# from the primary low Fe3+# chromianspinel.

high-Cr to high-Al varieties (Fig. 3) (Ahmed et al.,2001). The Fe3+# (=Fe3+/(Fe3++Al+Cr) atomic ratio)of spinel is relatively low in the SED chromitite, N0.08,while it increases up to 0.20 in the CED examplesshowing the alteration trend (Fig. 3). The spinels withlow Fe3+# are often intact and represent the primarycompositions (e.g., Quick, 1990; Ahmed et al., 2001).This compositional trend is common in chromian spinelaltered to various degrees at low-temperature conditions(e.g., Roeder, 1994). The chromian spinel of both CEDand SED chromitites is generally low in TiO2 content,resembling ophiolitic chromitite.

Consistent with chromian spinel chemistry, the SEDchromitites show uniform chondrite-normalized PGEpatterns with general negative slope from Ru to Pt asthe ophiolitic chromitites (Fig. 4) (e.g., Page et al., 1982;Page and Talkington, 1984). The total PGE contents of theSED chromitites range from 140 up to 320 ppb (Ahmedand Arai, 2002). The CED chromitites, on the other hand,exhibit variable PGE contents and distribution patternsfrom locality to locality. Some chromitite pods in the CEDshow unusually high PGE concentrations reaching up to3.2 ppm and their dunite envelopes also have unexpect-edly high PGE concentrations up to 2.3 ppm. The PGE-rich chromitites of the CED usually show gentle negativeslope of PGEpattern (Fig. 4), being enriched in both IPGE(Os, Ir, Ru) and PPGE (Rh, Pt, Pd), while the associateddunites exhibit U-shaped patterns showing an enrichmentin both Os, Pt and Pd (unpublished data). The PGE-poorchromitites from other CED localities show low to


intermediate PGE concentrations ranging from 58 to365 ppb and generally display negative slopes (Fig. 4) asin almost all ophiolitic chromitites.

4. Analytical techniques

One hundred and thirty six polished thin sections (70from the CED and 66 from the SED) of chromitites andassociated ultramafic rocks were carefully examined athigh magnifications using the optical microscope foridentification of various PGM grains with different sizes.Quantitative analysis of PGM grains was carried outusing a JEOL JXA-8800 electron probe microanalyzer atthe Center for Cooperative Research of KanazawaUniversity, Japan. Analytical conditions were 25 kVaccelerating voltage and 20 nA probe current. Standardsused were pure metals for the elements Os, Ir, Ru, Rh, Pt,Pd, Cu, and Cr, gallium arsenide for As, and pentlanditefor S, Fe and Ni. The count time was 10 s for eachelement, except S and Fe (20 s). The KαX-ray lines wereused for Ni, S, Fe, and Cr, Lα lines for Os, Ir, Ru, Rh, Ptand Cu, and Lβ lines for Pd and As.

5. PGM mineralogy

Although almost all of the chromitites studied werefound to include PGM, the Wadi El-Lawi (LW)chromitite (see Fig. 2 for locations), CED of Egypt, isthe most enriched in PGE and PGM of all the podsstudied. Altogether, 208 grains of various PGM species

Fig. 5. Frequency distribution histograms of different PGM in the central EastPrecambrian ophiolite of Egypt. Note the common abundance of laurite in t

were petrographically and geochemically examined inboth the CED and SED chromitites; 141 grains from theformer and 67 grains from the latter. The domains ofPGM grains in the CED chromitites were observed inWadi El-Lawi area (95 grains from 20 polished thinsections of three chromitite pods); up to 9 PGM grainsper thin section. Other chromitite localities in both CEDand SED contained comparable numbers of PGMgrains, ranging from nil up to 3 PGM grains per thinsection. Os-rich laurite, the most common PGM in theCED chromitites, comprises 54% of the total PGMgrains found in all studied areas (Fig. 5), whereas theleast abundant PGM are (Pt–Pd)–Fe alloys (Fig. 5). Ingeneral, three PGM groups have been found in thepodiform chromitites from the Proterozoic ophiolite,Eastern Desert of Egypt: sulfides, alloys and sulfarse-nides–arsenides in decreasing order of abundance. Inmost cases the PGM grains are predominantly mono-phase and subordinately show polyphase PGM associa-tions. The most common associations are laurite+osarsite and Os–Ir alloy+ laurite.

5.1. PGM in the CED chromitites

5.1.1. PGE sulfidesPGE-sulfides form the main PGM found in the CED

chromitites. 108 PGM grains, out of 141 grains, arelaurite; mainly Os-rich laurite and, to a lesser extent, Os-poor or even sometimes Os-free laurite, comprising 77%of the total PGM found in the studied CED chromitite

ern Desert (CED) and southern Eastern Desert (SED) chromitites of latehe former and Os–Ir alloy in the latter.


localities (Fig. 5). The grain size of laurite occasionallyexceeds 5 μm, up to 40 μm across. In most cases Os-richlaurite is found exclusively as solitary euhedral crystalsembedded within chemically fresh, but commonlycracked, chromian spinel (Fig. 6A–C). There is nosystematic distribution of Os-rich laurite grains withinchromian spinel, in the center (Fig. 6A) or in theperiphery of chromian spinel grains (Fig. 6C). Some Os-

Fig. 6. Back-scattered electron images of different PGM in the CED chromititegrains within chemically fresh, but sometimes cracked, chromian spinel. Samlaurite intergrown with irarsite (I) and native Ir within cracked chromian spichromian spinel (Wadi Bezah). (F) Os-rich and Os-poor laurites surround(G) Unnamed (Ir–Cu–Fe–Pt–Rh)S associated with CuS in fresh-cracked chPt–Fe alloy, Pd–Fe alloy, sperrylite (Pt–As), native Pd and hollingworthite(I) Irarsite (I)–hollingworthite (H) solid solution series in the interstitial silic

rich laurite grains are associated along the chromianspinel fractures with other PGM such as irarsite and Ir–Rh alloy (Fig. 6D). Upon weathering, Os-rich laurite inthe altered part of chromian spinel has been changedcompletely to porous and ragged Os-poor laurite(Fig. 6E) or to heterogeneous composite grains of Os-rich and Os-poor laurite surrounded by osarsite(Fig. 6F). Two grains of unnamed (Ir, Pt, Rh)(Cu, Fe)

s of Egypt. (A), (B) and (C) are solitary perfect euhedral Os-rich lauriteples A and C are from Barramiya, B is from Wadi El-Lawi. (D) Os-richnel (Wadi El-Lawi). (E) Porous Os-poor laurite within slightly altereded by osarsite (O) within altered chromian spinel (Wadi El-Lawi).romian spinel (Wadi El-Lawi). (H) Composite PGM grains containing(H) within altered chromian spinel and along cracks (Wadi El-Lawi).ates (Wadi El-Lawi).


sulfide of ca. 5 μm across were found associated with asmall CuS grain (Fig. 6G). Similar to those reportedfrom the Oman ophiolite (Ahmed and Arai, 2003), theunnamed (Ir, Pt, Rh)(Cu, Fe) sulfide also shows grey colorand lower reflectivity than the associated laurite grains.

5.1.2. PGE alloysDespite careful observation, Os–Ir alloy was not found

in the CED chromitites. One grain of Ru–Os–Ir alloy withlow analytical total was found in a cracked chromian spinelgrain, resembling the mode of occurrence of PGE oxidesreported from the Oman ophiolite (Ahmed et al., 2002).Although someof theCEDchromitites are notably enriched

Fig. 7. Back-scattered electron images of different PGM (A–G) and BMS (Hperfect euhedral Os–Ir alloy within fresh chromian spinel. Sample A is fromwith small Os-rich laurite and native Cu (Abu Dahr area). (D) Ru–Os–Ir allirarsite (I) solid solution series associated with millerite (M) within alteredunnamed (Rh–Ir–Cu–Fe)S within fresh chromian spinel (Abu Dahr area).interstitial silicates of chromitite (Abu Dahr area). (H) Euhedral millerite ((I) Composite inclusion from millerite (M) and pyrite (Py) within slightly al

in Pt and Pd, very few grains of Pt and Pd phases have beenfound in the interstitial silicate matrix and chromian spinelfractures. Pt–Fe, Pd–Fe alloys and native Pd are themain Ptand Pd phases found as small aggregates, usually b10 μmacross, of composite grains in the altered chromian spinelparts and along spinel cracks (Fig. 6H). A small grain,N5 μm across, of native Ir is found associated with Os-richlaurite and irarsite in a fractured chromian spinel (Fig. 6D).

5.1.3. PGE sulfarsenides–arsenidesIrarsite (I) and hollingworthite (H), with wide range of

solid solution between the two end-members, are the mainPGE sulfarsenides in the CED chromitites of Egypt. Large

and I) in the SED chromitites of Egypt. (A) and (B) Solitary grains ofBelmhandite and B from Abu Syeil areas. (C) Os–Ir alloy associated

oy within fresh chromian spinel (Belmhandite area). (E) Osarsite (O)–part of spinel (Um-Thagr area). (F) Os-poor laurite intergrown with(G) Composite grain of Os-poor, Os-free and Os-rich laurites withinM) crystal within slightly altered chromian spinel (Um-Thagr area).tered chromian spinel (Belmhandite area).


patches and clots of I–H solid solution N50 μm across arefound mainly in the interstitial silicate matrix of chromititespinel (Fig. 6I). Small grains and blebs of I and H are alsofound associated with Os-rich laurite and along withcomposite Pt–Fe, Pd–Fe alloys, and native Pd in thecracked and altered chromian spinel (Fig. 6D, H). Inaddition to the alteration product of Os-rich laurite(Fig. 6F), solitary small grains, ca. 5 μm across, of osarsiteare also found in the interstitial matrix of chromitite spinel.The PtAs (sperrylite) and unnamed (Pd–Pt)As are found asa part of composite PGM grains in the altered parts ofchromian spinel (Fig. 6H).

5.2. PGM in SED chromitites

The PGE mineralogy in the SED chromitites is muchsimpler than those of the CED examples, comprising

Table 1Representative microprobe analyses of PGE sulfides in the CED chromitites

S. No. Laurite

Os-rich Os-poor

LW9 LW10 BR50 LW10 BZ37

wt.%S 33.15 32.47 33.78 32.52 37.09Os 27.87 25.24 14.74 9.36 3.79Ir 5.61 6.80 7.31 8.99 3.03Ru 29.92 29.34 36.13 38.22 50.31Rh 0.72 0.49 0.81 1.21 1.10Pt n.d n.d n.d n.d n.dPd 0.26 0.31 n.d 0.21 n.dAs n.d 0.92 n.d 5.82 3.75Ni 0.01 0.05 0.04 0.67 0.44Cu n.d n.d n.d n.d n.dFe 0.57 0.89 1.01 0.59 0.64Cr 1.54 1.78 3.83 1.08 1.17Sb 0.37 n.d 0.01Bi 0.30 0.82 0.28Total 99.65 98.96 97.65 99.49 101.61

at.%S 67.78 66.96 67.83 63.17 65.31Os 9.61 8.78 5.00 3.07 1.13Ir 1.91 2.34 2.45 2.91 0.89Ru 19.40 19.19 23.01 23.55 28.10Rh 0.46 0.32 0.51 0.73 0.60Pt – – – – –Pd 0.16 0.19 – 0.12 –As – 0.81 – 4.84 2.83Ni 0.01 0.05 0.05 0.71 0.42Cu – – – – –Fe 0.67 1.06 1.16 0.65 0.64Sb 0.20 – –Bi 0.09 0.25 0.08

CED: central Eastern Desert. S. No.: sample number. n.d: not detected. SampHuitate area, samples BZ37 and BZ42 are from Wadi Bezah, and sample BR

mainly Os–Ir alloy, a few small grains of Os-poor, Os-free,and Os-rich laurite, and osarsite–irarsite sulfarsenides.

5.2.1. PGE alloysFifty PGM grains, out of 67, are Os–Ir alloy

accounting for 75% of the total PGM grains found inthe SED chromitites (Fig. 5). In most cases, Os–Ir alloywas found as solitary small perfect euhedral crystals,N10 μm across, enclosed by fresh chromian spinel(Fig. 7A, B). In a few other cases, Os–Ir alloy isassociated with small grains of Os-rich laurite (Fig. 7C).Os–Ir alloy in the SED chromitites is commonlyassociated with voids (Fig. 7A, B), similar to thosereported from the Oman ophiolite (Ahmed and Arai,2003). A single small rounded grain of Ru–Os–Ir alloy,ca. 10 μm across, was also observed within chromianspinel (Fig. 7D). This Ru–Os–Ir alloy is quite different

, late Precambrian ophiolite, Egypt

Unnamed

Os-free (Ir, Pt)(Cu,Fe)S

BZ42 LW10 UH17 LW9 LW9

35.46 38.08 37.15 21.73 22.666.86 0.59 n.d n.d n.d1.34 1.79 2.66 29.60 39.4550.82 50.94 54.79 4.39 0.150.84 1.42 1.15 3.90 5.47n.d 0.25 n.d 10.84 14.77n.d 0.12 0.11 0.07 0.21n.d 3.58 0.71 n.d n.dn.d 0.33 0.05 0.12 0.100.03 n.d n.d 19.19 10.371.50 0.46 1.13 6.65 1.784.59 1.01 3.94 3.65 3.60

n.d0.23

101.44 98.80 101.69 100.14 98.56

65.55 66.61 65.94 48.68 56.952.14 0.17 – – –0.41 0.52 0.79 11.06 16.5429.79 28.26 30.84 3.12 0.120.48 0.78 0.64 2.72 4.28– 0.07 – 3.99 6.10– 0.06 0.06 0.05 0.16– 2.68 0.54 – –– 0.31 0.05 0.14 0.130.03 – – 21.69 13.151.59 0.47 1.15 8.55 2.57

–0.06

les LW9 and LW10 are from Wadi El-Lawi, sample UH17 is from Um-50 is from El-Barramiya area (see Fig. 2 for the locations).

Fig. 8. Compositional variation of laurite and Os–Ir alloy in terms of Ru–Os–Ir composition (atomic ratio) in the central Eastern Desert (CED) andsouthern Eastern Desert (SED) chromitites of the late Precambrian ophiolite of Egypt. Os–Ir alloys from Vourinos ophiolite (Augé, 1988), Ray–Izultramafic complex (Garuti et al., 1999b), and Oman ophiolite (Ahmed and Arai, 2003) are presented here for comparison.


from that in the CED chromitite, which was hosted by thealtered chromian spinel and show low analytical total.

5.2.2. PGE SulfarsenidesSulfarsenide grains are occasionally zoned, with

osarsite in the core and irarsite in the rim. This ispossibly an alteration product of a precursor primaryPGM, i.e., Os–Ir alloy (Fig. 7E). The osarsite–irarsiteassociation is usually located in the altered parts of

Fig. 9. Variation of Ru versus Os (atomic ratios) of laurite in the centralEastern Desert (CED) and southern Eastern Desert (SED) chromititesof the late Precambrian ophiolite of Egypt. Note the perfect negativecorrelation.

chromian spinel (Fig. 7E) or in the interstitial silicatematrix between chromian spinel grains.

5.2.3. PGE sulfidesA few small grains of Os-free, Os-poor and, to a lesser

extent, Os-rich laurite, b2 μm across, are usuallyassociated with Os–Ir alloy (Fig. 7C) or with otherPGM species like unnamed Cu-rich (Rh–Ir)S (Fig. 7F)within fresh chromian spinel. Despite careful investiga-tion, large euhedral solitary grains of Os-rich laurite arenever found in the SED chromitites. However, largeaggregates of Os-poor, Os-free and Os-rich laurite arefound to be located in the interstitial matrix of chromititespinel (Fig. 7G). Only a single grain of unnamed Cu-rich(Rh–Ir)S, b5 μm across, is associated with Os-free toOs-poor laurite within fresh chromian spinel (Fig. 7F).The mineral has a greenish grey color which is lessreflective than the associated laurite grain.

5.3. Base metal sulfides (BMS)

Two modes of occurrence are found for the basemetal sulfides (BMS) in the Eastern Desert chromititesof Egypt; small inclusions within chromian spinel andlarge aggregates within the interstitial silicates and alongcracks of chromian spinel. The BMS in the CEDchromitites consist of the following minerals, indecreasing order of abundance: millerite, heazlewoo-dite, pentlandite and chalcopyrite. Millerite is also verycommon in the SED chromitites as small euhedral grainsca. 10 μm across within slightly altered chromian spinel


(Fig. 7H), or associated with osarsite–irarsite minerals(Fig. 7E) in the altered chromian spinel. It is alsosometimes associated with pyrite in the same inclusionwithin chromian spinel (Fig. 7I). Pyrite is usually foundas relatively large grains, ca. 10 μm, either as single orcomposite grains with millerite enclosed by chromianspinel (Fig. 7I).

6. Mineral chemistry

6.1. PGE sulfides

Except the two grains of unnamed (Ir, Pt, Rh)(Cu, Fe)sulfide in the CED chromitite, the rest of PGE sulfidesare laurite with different compositions; i.e., Os-rich, Os-poor and Os-free laurite. The Ru# (=Ru/(Ru+Os+Ir)atomic ratio) of laurite varies from 0.59 to 0.98 with anaverage of 0.72. The Os content of laurite in the CEDchromitites varies from nil up to 27.9 wt.%, with anaverage of 16.2 wt.% (Table 1, Fig. 8). The Os-poor andOs-free compositions are usually found in the altered

Table 2Representative microprobe analyses of PGE alloys and PGE sulfides in the

S. No. Alloys

Os–Ir alloy "osmium" Ru–Os–

GL11 AD122 AS50 BL52 GL1

wt.%S n.d n.d 0.01 n.d n.dOs 70.77 60.36 62.57 58.21 17.42Ir 21.89 33.03 30.72 23.46 14.07Ru 4.18 4.52 3.89 9.26 62.57Rh 0.21 0.06 0.33 0.59 3.59Pt n.d n.d n.d n.d n.dPd 0.31 0.04 0.04 n.d n.dAs 0.13 n.d n.d n.d 0.02Ni n.d 0.04 n.d n.d 0.03Cu n.d n.d n.d n.d n.dFe 0.59 0.38 0.45 1.33 0.52Cr 2.09 1.34 1.76 6.80 1.77Total 100.17 99.77 99.77 99.65 99.99

at.%S – – 0.03 – –Os 68.34 58.56 61.05 55.75 11.06Ir 20.90 31.68 29.63 22.21 8.83Ru 7.59 8.23 7.15 16.67 74.67Rh 0.37 0.10 0.59 1.04 4.21Pt – – – – –Pd 0.53 0.07 0.07 – –As 0.33 – – – 0.04Ni – 0.13 – – 0.05Cu – – – – –Fe 1.95 1.24 1.48 4.33 1.13

SED: southern Eastern Desert. n.d: not detected. Samples GL1 and GL11 arefrom Belmhandite area, and sample AS50 is from Abu Syeil area (see Fig. 2

laurite grains located in the interstitial matrix and alongthe fractures of chromian spinel (see Fig. 6E, F). The Oscontent in laurite displays a perfect negative correlationwith Ru content (Fig. 9), indicating substantial substi-tution of Ru by Os. The Ir content of laurite varies from1.19 up to 14.44 wt.%, with an average of 7.36 wt.%,but does not exhibit any systematic relationship with Osor Ru contents (Table 1, Fig. 8). Laurite compositionvaries from a pure RuS2 in the altered, Os-free laurite,up to (Ru0.59Os0.29Ir0.06)Σ=0.94S2.06 in the most Os-richvariety. Relatively high amounts of As and Rh aredetected in the Os-poor and Os-free varieties of thealtered laurite grains (Table 1). Two grains of unnamed(Ir, Pt, Rh)(Cu, Fe) sulfide are found associated withCuS within relatively fresh chromian spinel of CEDchromitite. The optical properties of this mineral aresimilar to those reported from the Oman ophiolite(Ahmed and Arai, 2003), but it has excess amounts of Pt(from 10.84 to 14.77 wt.%) and Cu (from 10.37 to19.19 wt.%), and its PGE:BM (base metals) ratio isapproximately one (Table 1). This phase is characterized

SED chromitites of the late Precambrian ophiolite, Egypt.

Sulfides

Ir Laurite Rh–IrS

GL1 AD129 AD122 AD122 AD122

n.d 36.16 38.77 30.49 26.0317.82 1.87 0.25 34.79 3.5814.53 3.65 1.52 6.10 23.5061.98 52.75 54.68 23.62 2.743.69 1.42 2.45 0.76 24.62n.d n.d n.d n.d 0.62n.d 0.28 0.29 n.d 0.52n.d 1.24 1.95 n.d 0.070.03 0.13 0.08 n.d 1.42n.d n.d n.d n.d 8.540.46 0.18 0.52 0.54 3.061.68 0.73 1.31 2.23 3.12100.19 98.41 101.81 98.53 97.82

– 65.69 66.33 67.14 56.3111.34 0.57 0.07 12.93 1.319.14 1.11 0.43 2.24 8.4874.14 30.39 29.67 16.5 1.884.34 0.81 1.30 0.52 16.60– – – – 0.22– 0.15 0.15 – 0.34– 0.96 1.43 – 0.060.06 0.13 0.08 – 1.68– – – – 9.320.99 0.19 0.52 0.68 3.80

from El-Gallala area, AD122 and AD129 from Abu Dahr area, BL52for the locations).

Table 3Representative microprobe analyses of PGE alloys and native metalsin the CED chromitites of the late Precambrian ophiolite, Egypt

S. No. Pt–Fe alloy Pd–Fe alloy Ir Pd Ru–Os–Ir

LW10 LW10 LW10 LW10 LW1 LW1 LW10 BR53

wt.%S 0.18 0.17 0.01 0.03 n.d n.d 2.59 n.dOs n.d n.d 0.08 n.d n.d n.d n.d 20.33Ir n.d 0.04 n.d n.d 86.17 87.10 n.d 12.39Ru n.d 0.11 n.d 0.04 0.44 0.42 n.d 50.06Rh 0.39 0.42 0.05 n.d 5.38 5.05 8.20 3.16Pt 74.45 42.05 0.34 0.22 1.30 1.25 4.42 n.dPd 3.01 1.67 78.77 80.99 0.09 0.07 75.48 n.dAs 0.08 0.20 0.33 0.22 n.d n.d 6.18 0.02Ni 0.64 1.93 0.62 0.33 0.43 0.41 0.09 0.03Cu n.d n.d n.d n.d n.d 0.01 n.d n.dFe 13.89 44.74 17.03 12.05 2.61 2.33 1.21 0.52Cr 2.23 2.32 1.56 3.77 3.04 2.78 1.16 1.77Sb n.d n.d n.d 0.03 0.03Bi 2.71 1.44 n.d 0.31 0.13Total 97.58 95.09 98.79 97.99 99.46 99.42 99.49 88.28

at.%S 0.81 0.49 0.02 0.10 – – 8.08 –Os – – 0.04 – – – – 15.13Ir – 0.02 – – 79.14 80.61 – 9.11Ru – 0.10 – 0.04 0.78 0.74 – 69.99Rh 0.56 0.37 0.04 – 9.22 8.72 7.98 4.34Pt 55.06 19.86 0.16 0.11 1.18 1.14 2.27 –Pd 4.08 1.44 69.65 76.92 0.15 0.11 71.00 –As 0.16 0.24 0.41 0.29 – – 8.26 0.04Ni 1.58 3.02 0.99 0.57 1.28 1.24 0.17 0.07Cu – – – – – 0.01 – –Fe 35.88 73.81 28.69 21.80 8.25 7.43 2.16 1.31Sb – – – 0.02 0.02Bi 1.87 0.64 – 0.15 0.06

Samples LW1 and LW10 are from Wadi El-Lawi, and sample BR53is from El-Barramiya area (see Fig. 2 for the locations). n.d: notdetected.

Fig. 10. Compositional variation of sulfarsenides in terms ofRhAsS–IrAsS–OsAsS–RuAsS tetrahedron (atomic ratios) in thecentral Eastern Desert (CED) and southern Eastern Desert (SED)chromitites of the late Precambrian ophiolite of Egypt. Note thegenerally high Ru content in sulfarsenides from the former comparedto the latter.


by monosulfide stoichiometry (XS) giving an averagecomposition of (Ir0.29Cu0.38Pt0.11Fe0.12Rh0.08)Σ0.98S1.02,which is very close to the composition of xingzhongitereported by Cabri (1981), and Ferrario and Garuti(1990).

PGE-sulfides in the SED chromitite are representedmainly by Os-poor to Os-free laurite and rarely by smallgrains of Os-rich laurite. Laurite composition varies frompure RuS2 to (Ru0.50Os0.39Ir0.07)Σ=0.96S2.04 for Os-richvarieties which is close to the laurite–erlichmanite bound-ary (Table 2, Fig. 8). Laurite from the SED chromititesrepresents the most Ru-rich variety in the analyzed PGMgrains (Figs. 8 and 9). The unnamed Cu-rich (Rh–Ir)sulfide has countable amounts of Os, Ru, Fe and Ni(3.58, 2.74, 3.06, 1.42 wt.%, respectively), in addition

to the high amount of Cu, 8.54 wt.% (Table 2). ThePGE:BM ratio is approximately 2:1 giving the aver-age stoichiometry of (Rh1.01Ir0.51Ru0.11Os0.08)Σ = 1.71

(Cu0.56Fe0.23Ni0.10)Σ= 0.89S3.40. Its mode of occurrence,associated with primary Ru-rich laurite within freshchromian spinel, suggests that it is an unnamed mineralphase rather than an alteration product.

6.2. PGE alloys

The Pt–Fe and Pd–Fe alloys are found mainly ascomposite grains in the altered parts and along cracksof chromian spinel grains of the CED chromitites.The Pt–Fe alloys display variable compositions fromtetraferroplatinum (Cabri, 2002) with a stoichiometry(Pt1.12Pd0.08)Σ = 1.20 (Fe0.73Bi0.04Ni0.03)Σ = 0.80, to Pt-rich –Fe alloy (Table 3) of composition close to (PtFe3)of stoichiometry (Pt0.80Pd0.06)Σ = 0.86(Fe2.99Ni0.12Bi0.03)Σ=3.14. The Pt–Fe alloy contains minor amountsof Pd, Ni and Bi; 2.36, 1.29 and 2.08 wt.% on aver-age, respectively (Table 3). According to Cabri (2002),there is no Pd–Fe alloy mineral phase. The Pd–Fealloy in the CED chromitites which is a part of com-posite PGM grains has a general composition of“isoferroplatinum” type (X3Fe), giving the averagecomposition Pd2.90Fe1.10 (Table 3). One grain of Ru–

Table 4Representative microprobe analyses of PGE sulfarsenides–arsenides in the CED chromitites, late Precambrian ophiolite, Egypt

S. No. Sulfarsenides Arsenides

I H I–H series O Pt–PdAs Pd–PtAs PtAs

LW9 LW9 LW10 LW9 LW9 LW10 LW10 LW10 LW10 LW10

wt.%S 11.85 12.98 12.52 13.45 14.65 12.85 13.14 0.64 0.33 1.27Os 0.06 0.05 n.d 0.06 n.d 42.49 46.00 n.d n.d n.dIr 53.59 53.09 0.02 43.49 34.01 1.49 1.50 n.d n.d n.dRu 3.03 3.33 0.02 2.16 3.58 15.81 13.39 n.d n.d n.dRh 3.10 3.46 39.26 12.40 21.27 0.29 0.20 1.93 0.84 3.63Pt 0.60 0.45 5.53 0.18 0.25 n.d n.d 47.40 30.05 52.57Pd n.d 0.19 13.69 n.d n.d n.d 0.12 11.54 38.38 0.74As 18.65 19.44 25.15 21.79 21.88 24.58 22.49 30.09 24.70 33.13Ni n.d 0.09 0.16 0.05 n.d 0.32 0.25 0.04 0.07 0.02Cu 0.13 n.d n.d n.d n.d n.d n.d n.d n.d n.dFe 0.42 0.55 0.88 0.54 0.39 0.67 0.68 1.13 1.65 0.96Cr 0.69 1.29 1.26 1.09 0.54 1.45 1.38 1.72 1.62 1.65Sb 0.04 0.06 1.22 n.d 0.10 0.05 0.50 2.58 n.d 2.45Bi 4.99 4.45 0.10 3.08 2.86 n.d n.d 2.55 1.46 2.61Total 97.15 99.43 99.81 98.29 99.53 100.00 99.65 99.62 99.10 99.03

at.%S 37.17 38.75 30.18 37.96 38.39 35.12 36.68 2.37 1.15 4.68Os 0.03 0.03 – 0.03 – 19.60 21.67 – – –Ir 28.04 26.44 0.01 20.48 14.87 0.68 0.70 – – –Ru 3.01 3.15 0.01 1.93 2.97 13.71 11.86 – – –Rh 3.03 3.22 29.49 10.91 17.37 0.25 0.17 2.21 0.90 4.18Pt 0.31 0.22 2.19 0.08 0.11 – – 28.72 17.10 31.95Pd 0.00 0.17 9.95 0.00 0.00 – 0.10 12.82 40.05 0.83As 25.04 24.84 25.95 26.32 24.54 28.76 26.87 47.47 36.61 52.44Ni – 0.15 0.21 0.08 – 0.47 0.39 0.07 0.12 0.03Cu 0.20 – – – – – – – – –Fe 0.75 0.95 1.21 0.88 0.59 1.06 1.08 2.39 3.28 2.03Sb 0.03 0.05 0.77 – 0.01 0.36 0.48 2.50 0.00 2.38Bi 2.40 2.04 0.03 1.33 1.15 – – 1.44 0.77 1.48

I: irarsite. H: hollingworthite. I–H series: irarsite–hollingworthite solid solution. O: osarsite. n.d: not detected. Samples LW9 and LW10 are fromWadi El-Lawi area (see Fig. 2 for the location).


Os–Ir alloy, redefined as “Ruthenium” by Cabri (2002),ca. 7 μm across, displays less reflectivity and loweranalytical total than the associated laurite (Table 3). Itgives a composition of (Ru0.70Os0.16Ir0.09Rh0.04Fe0.01).The petrographical and geochemical properties of thismineral suggest that it is PGE oxide/hydroxide likethose reported from the Vourinos (Garuti and Zaccarini,1997) and Oman ophiolite (Ahmed et al., 2002).Palladium and iridium are the native metals foundassociated with composite PGM grains in the CEDchromitite. Native palladium contains appreciableamounts of Rh, Pt, As and S; 8.20, 4.42, 6.18 and2.59 wt.%, respectively (Table 3). It has an averagestoichiometry of (Pd0.73Rh0.08Pt0.02As0.09S0.08), com-parable to native palladium reported by Cabri (2002).Native iridium also contains appreciable amounts of

Rh, Pt and Fe (respectively 5.22, 1.28 and 2.47 wt.% onaverage; Table 3), with an average chemical formula of(Ir0.81Rh0.09Pt0.01Fe0.08).

PGE alloys in the SED chromitites, on the otherhand, are mainly represented by Os–Ir alloy, which isredefined by IMA as “Osmium” (Cabri, 2002). The Oscontent of “Osmium” varies from 51.10 to 70.77 wt.%,with an average of 60.13 wt.% (Table 2). The Ir contentalso varies from 20.08 up to 39.09 wt.%, with anaverage of 30.97 wt.%. It is noteworthy that the Rucontent of “Osmium” in the SED chromitites isremarkably high compared with those reported fromother ophiolites (Fig. 8), e.g., Vourinos ophiolite (Augé,1988), Ray–Iz complex (Garuti et al., 1999b), Omanophiolite (Ahmed and Arai, 2003). The Ru contentvaries from 1.28 up to 9.26 wt.%, with an average of

Table 5Representative microprobe analyses of PGE sulfarsenides in the SEDchromitites of the late Precambrian ophiolite, Egypt

S. No (Os–Ir)AsS series (Ir–Os)AsS series

. UT65 UT65 UT65 UT65 UT65 UT65 UT65 UT65

wt.%S 13.24 12.99 12.02 11.98 12.51 13.23 13.40 14.07Os 28.28 36.13 50.89 60.40 29.07 12.32 6.57 4.27Ir 23.55 14.17 8.67 1.59 30.2 46.12 52.10 52.60Ru 8.28 8.46 0.82 n.d 0.57 0.48 1.74 0.88Rh 0.21 0.17 0.11 0.03 0.28 0.20 0.45 0.46Pt n.d n.d n.d n.d n.d n.d 0.03 0.24Pd n.d n.d n.d 0.12 n.d 0.60 n.d 0.01As 24.39 24.57 23.16 22.92 21.88 22.70 20.84 19.47Ni 0.13 0.18 0.14 0.04 0.32 0.32 0.48 2.56Cu n.d n.d n.d n.d n.d n.d n.d 0.14Fe 0.95 0.79 0.90 0.92 0.86 1.11 0.99 1.70Cr 1.46 1.13 1.51 1.41 1.15 2.57 2.11 1.64Total 100.49 98.59 98.22 99.41 96.84 99.65 98.71 98.04

at.%S 37.11 36.85 36.59 36.47 38.20 39.00 39.83 40.42Os 13.37 17.30 26.14 31.04 14.98 6.13 3.30 2.07Ir 11.01 6.71 4.40 0.81 15.39 22.68 25.83 25.21Ru 7.36 7.61 0.79 – 0.55 0.45 1.64 0.80Rh 0.18 0.15 0.11 0.03 0.27 0.18 0.42 0.41Pt – – – – – – 0.01 0.11Pd – – – 0.11 – 0.54 – 0.01As 29.26 29.83 30.17 29.87 28.59 28.63 26.51 23.94Ni 0.20 0.28 0.23 0.07 0.53 0.51 0.77 4.02Cu – – – – – – – 0.20Fe 1.52 1.28 1.58 1.61 1.51 1.88 1.69 2.81

Sample UT65 is from Um Thagar area (see Fig. 2 for the location). n.d:not detected.


4.24 wt.% (Table 2, Fig. 8). The chemical compositionof “Osmium” in the SED chromitites ranges from(Os0.54Ir0.39Ru0.07) to (Os0.71Ir0.21Ru0.08); the averagestoichiometry is (Os0.60Ir0.33Ru0.07). The “Ruthenium”in the SED chromitite contains a relatively high amountof Rh (3.64 wt.% on average; Table 2), with an averagestoichiometry of (Ru0.74Os0.11Ir0.09Rh0.04). The Rucontent of “Ruthenium” in the SED chromitites isremarkably higher than those of the CED ones.

6.3. PGE sulfarsenides–arsenides

Irarsite (IrAsS), and to a lesser extent, hollingworthite(RhAsS) and osarsite (OsAsS), are the main sulfarsenidesfound in the CED chromitites. Irarsite and hollingworthitehave a relatively wide compositional range along theIrAsS–RhAsS join of the RhAsS–OsAsS–IrAsS triangle(Fig. 10). The irarsite richest in Ir contains ca. 1.36 wt.%ofRh. Irarsite has appreciable amounts of Bi andRu, up to5.34 and 13.22wt.%, respectively (Table 4, Fig. 10). It hasthe general formula (Ir0.93 Ru0.06Rh0.04Fe0.06)Σ=1.09

(As0.76Bi0.08)Σ=0.84S1.06. Substitution of Ir by Rh inirarsite reaches 21.27 wt.% Rh, and the mineral containsca. 2.86 wt.% Bi (Table 4), giving the formula(Rh0.52Ir0.45Ru0.09)Σ=1.06 (As0.75Bi0.03)Σ=0.78S1.16. Hol-lingworthite has relatively high Pd and Pt contentsand low Ir content (13.69, 5.53 and 0.02 wt.%, re-spectively; Table 4). It contains minor amounts ofSb, Fe and Bi (ca. 1.22, 0.88 and 0.09 wt.%, respec-tively; Table 4), giving the average formula (Rh0.90Pd0.29Pt0.07 Fe0.04)Σ=1.30As0.79S0.91. Osarsite in the CEDchromitite is found as alteration products of Os-richlaurite. It has appreciable amount of Ru (14.16 wt.% onaverage; Table 4, Fig. 9), giving the general formula of(Os0.63Ru0.39Fe0.03Ir0.02Ni0.01)Σ = 1.08As0.84S1.08.Arsenides in the CED chromitites are represented byunnamed Pt–PdAs and Pd–PtAs minerals and sperrylite(PtAs2), as parts of composite PGM grains. There is asolid solution series in the (Pt–Pd)–As composition fromPt-rich to Pd-rich varieties (Table 4). The Pt content variesfrom 47.4 wt.% in the most Pt-rich variety, to 28.92 wt.%in the most Pd-rich one. Both the Pt-rich and Pd-rich arsenides contain minor amounts of Sb, Bi andRh (Table 4). The Pt-rich arsenide has the generalchemical formula (Pt0.59Pd0.26Rh0.05Fe0.05)Σ = 0.95

(As0.97Sb0.05Bi0.03)Σ = 1.05. The most Pd-rich arse-nide is almost close to the stoichiometry of palla-doarsenide (Pd2As) reported from various localities(Cabri,2002). It has the average chemical formula(Pd1.21Pt0.53Fe0.20Rh0.04)Σ = 1.98(As0.99Bi0.03)Σ = 1.02.Sperrylite (PtAs2) has detectable amounts of Sb, Biand Rh (on average 2.34, 2.46 and 2.80 wt.%, re-

spectively) (Table 4). The average chemical formula is(Pt0.97Rh0.10Fe0.07)Σ=1.14(As1.64S0.11Sb0.07Bi0.04)Σ=1.86,which is very close to the sperrylite stoichiometry reportedfrom various localities (Cabri, 2002).

No PGE arsenide phases were found in the SEDchromitites, and the PGE sulfarsenides are entirelylocated along the OsAsS–IrAsS join of the RhAsS–OsAsS–IrAsS–RuAsS tetrahedron (Fig. 10). There is acomplete solid solution series between osarsite andirarsite endmembers (Fig. 10). The most Os-rich osarsitehas ca. 1.59 wt.% Ir, with the general chemical formula(Os0.96Ir0.02)Σ=0.98As0.91S1.11, whereas the irarsite rich-est in Ir contains ca. 4.27 wt.% Os and minor amounts ofNi and Fe (Table 5), giving the average chemical formula(Ir0.77Ni0.12Fe0.09Os0.06)Σ=1.06 As0.72S1.22. Except for afew analyses, the osarsite–irarsite series of the SEDchromitites have a remarkably low Ru content, ca.1.37 wt.% on average, compared with sulfarsenides inthe CED chromitites which have much higher Rucontents (ca. 3.35 wt.% on average (Fig. 10). This may

Table 6Representative microprobe analyses of base metal sulfides (BMS) in the CED and SED chromitites of the late Precambrian ophiolite, Egypt

S. No. Millerite Heazlewoodite Pentlandite Pyrite Chalcopyrite

LW10 GL11 LW13 LW13 LW13 GL13 LW10

wt.%S 39.04 35.54 26.16 26.19 31.68 47.23 34.01Os 1.52 n.d n.d n.d n.d n.d n.dIr 0.16 n.d n.d 0.06 n.d n.d n.dRu n.d n.d n.d 0.12 n.d n.d 0.03Rh n.d n.d n.d n.d n.d 0.07 0.09Pt n.d 0.04 n.d n.d 0.01 n.d n.dPd 0.12 0.01 0.15 0.01 n.d n.d 0.11As 0.06 0.02 n.d n.d 0.03 0.12 n.dNi 49.64 58.35 68.67 69.86 41.29 4.31 0.01Cu 6.08 0.10 0.34 n.d 0.20 0.19 28.86Fe 1.44 1.72 0.62 0.67 18.21 37.58 29.57Cr 3.28 4.22 1.97 2.30 4.90 6.18 2.75Total 101.34 100.00 97.91 99.21 96.32 95.68 95.43

at.%S 55.45 51.9 40.72 40.42 48.88 66.22 51.83Os 0.36 – – – – – –Ir 0.04 – – 0.02 – – –Ru – – – 0.06 – – 0.01Rh – – – – – 0.03 0.04Pt – 0.01 – – – – –Pd 0.05 – 0.07 – – – 0.05As 0.04 0.01 – – 0.02 0.06 –Ni 38.52 46.56 58.39 58.91 34.81 3.30 –Cu 4.36 0.07 0.27 – 0.15 0.13 22.19Fe 1.18 1.45 0.55 0.60 16.13 30.25 25.87

Sample LW10 and LW13 are from Wadi El-Lawi, CED chromitite, and samples GL11and GL13 are from El-Galala area, SED chromitite (see Fig. 2for the locations). n.d: not detected.


be due to the association of sulfarsenides with Os–Iralloy in the former and with laurite in the latter.

6.4. Base metal sulfides

Millerite (NiS) and heazlewoodite (Ni3S2) are themainbase metal sulfides (BMS) identified in the Proterozoicophiolitic chromitites of Egypt. Subordinate amountsof pentlandite (Fe,Ni)9S8, pyrite and chalcopyrite arealso found associated with composite PGM grains. TheBMS, in general, have very low PGE contents, usuallyb2 wt.% total PGE (Table 6). Millerite has variable Fecontents ranging from 0.82 up to 8.01 wt.%, with anaverage of 2.27 wt.% and an average chemical compo-sition of (Ni0.91Fe0.04)Σ = 0.95S1.05. The Fe content ofheazlewoodite is usually b1 wt.% (Table 6), giving thegeneral chemical formula (Ni2.93Fe0.03)Σ=2.96S2.04. Pent-landite has up to 18.21wt.%Fe (Table 6), with the averagechemical formula (Ni5.93Fe2.75)Σ=8.68S8.32. Pyrite has upto 4.50 wt.% Ni (Table 6), giving the average chemicalcomposition (Fe0.91Ni0.10)Σ=1.01S1.99.

7. Discussion

7.1. Laurite vs. IPGE alloys: stability conditions

In general, the highly siderophile elements (PGE, Auand Re) show good correlations with Cr and Ni which, inturn,mainly controlled by spinel and/or olivine inmany ofthe magmatic suites (e.g., Irving, 1978; Brügman et al.,1987; Puchtel and Humayun, 2000, 2001; Righter et al.,2004). In sulfur-undersaturated magmatic rocks, espe-cially deep-seated ultramafic rocks, chromian spinel isconsidered as one of the main controllers of behavior ofthe highly siderophile elements; there is a strong positivecorrelation between Cr content of spinel and PGEcontents of the rock (e.g., Crocket, 1979; Economou,1986; Leblanc, 1991; Ahmed and Arai, 2003). Thissuggests co-precipitation of chromian spinel and PGM indeep-seated rocks of the upper mantle (Arai et al., 1999).Most of the primary PGE-sulfides, i.e., laurite, and Os–Iralloy are found as perfect euhedral crystals includedwithin fresh chromian spinel, suggesting their formation


at an early magmatic stage (e.g., Stockman and Hlava,1984; Talkington and Watkinson, 1986; Augé, 1988;Melcher et al., 1997; Garuti et al., 1999a; Bai et al., 2000).These highly refractory minerals may be formed initiallyas minute clusters in the host melt and act as nucleus forthe early-crystallized chromian spinel crystals (Tredouxet al., 1995). Formation of the IPGE sulfides or alloysmainly depends on the availability of sulfur in the hostbasaltic magma before entrapment by the early-crystal-lized chromian spinel.

In some cases, laurite represents the sole primaryPGM inclusions in chromian spinel, both in theophiolitic and layered intrusion chromitites such asthe Ojen ophiolite, Spain (Torres-Ruiz et al., 1996),Othrys ophiolite, Greece (Garuti et al., 1999a), Still-water intrusion, Montana, U.S.A. (Talkington andLipin, 1986), and Bushveld complex, South Africa(Maier et al., 1999). Many other associations of lauritewith IPGE alloy are reported in the same ophiolitic orAlaskan-type ultramafic complexes. Examples includethe Josephine ophiolite, Oregon, U.S.A. (Stockmanand Hlava, 1984), Vourinos ophiolite, Greece (Augé,1985, 1988), Tiébaghi ophiolite, New Caledonia(Augé, 1988), Thetford ophiolite, Quebec, Canada(Corrivaux and Laflamme, 1990), Samar ophiolite,Philippines (Nakagawa and Franco, 1997), Omanophiolite (Ahmed and Arai, 2003), Guli ultramaficmassif, Russia (Malitch et al., 2002), and Kraubathultramafic massif, Austria (Malitch et al., 2003).However Ru–Os–Ir alloy is the only primary IPGEminerals in very few cases, such as Kamuikotanophiolite, Japan (Nakagawa and Franco, 1997). In thepresent study, the CED chromitites are the first exam-ple where the Os-rich laurite is the primary PGM inchromian spinel. In contrast, the SED examples showthe opposite situation, where Os–Ir alloys representthe sole primary PGM in chromian spinel with only avery few small Ru-rich laurite grains.

The composition of primary IPGE alloys mainlydepends on the proportions of Os:Ir:Ru in the parentmagma, whereas the composition of sulfides, i.e.,laurite, depends mainly on the availability of sulfurand temperature conditions prevailing during thechromite formation (e.g., Legendre and Augé, 1986).In some cases, PGE sulfides, i.e., laurite, can overgrowon IPGE alloy confirming the order of crystallization;alloy±sulfide, and then sulfide alone (Legendre andAugé, 1986). The coexistence of sulfides and inter-grown alloys does not represent the product ofsulfurization or remobilization of alloys, but instead isdue to a continuous crystallization of sulfides afteralloys as a result of increasing sulfur content in the

magma. This case can be clearly shown in the SEDchromitites where small laurite grains are intergrownwith perfect euhedral Os–Ir alloy (See Fig. 7C). At hightemperature and low sulfur fugacity f (S2) conditions, amelt with a specific composition, OsNRuN Ir, willcrystallize first a PGE alloy with rutheniridosminecomposition (Ir–Os–Ru), then followed by Ru-richlaurite with increasing f (S2) (e.g., Westland, 1981;Stockman and Hlava, 1984). Recent experiments(Brenan and Andrews, 2001; Andrews and Brenan,2002) also demonstrated that temperature and f (S2) arethe main factors controlling the formation of laurite and/or IPGE alloy at the early magmatic stage. At a hightemperature, e.g., 1250 °C, laurite, if any, will be veryclose to the RuS2 end-member, showing a slightincrease in Os and Ir, up to 3 wt.%, with decreasingtemperature to 1200 °C (Andrews and Brenan, 2002). Itis therefore suggested that, at such high temperature andlow f (S2) conditions, the two phases, laurite and IPGEalloy, are restricted to Ru-rich compositions. By in-creasing the f (S2) and decreasing temperature, thestability field of laurite will expand to accommodatemore Os and Ir to produce the Os-rich laurite prior to itsentrapment within chromian spinel. Consequently, Ru inthe magma will be consumed in the formation of lauriteand the resultant IPGE alloy, if any, will be Ru-poor(Nakagawa and Franco, 1997). This case is well shownin the CED chromitites of this study where the Os-richlaurite represents the sole primary PGM inclusions inchromian spinel. Further increasing of f (S2) could leadto the formation of more Os-rich sulfides (i.e., erlich-manite), and then Ir-rich sulfides (i.e., xingzhongite)(e.g., Legendre and Augé, 1986). The presence solely ofIPGE alloys and sulfides as the main primary PGMinclusions in chromian spinel, therefore, means that thef (S2) was very low along with high temperatureconditions in the former case, and was initially highenough along with low temperature conditions in thelatter case. The coexistence of PGE sulfides and IPGEalloys, therefore, took place at a relatively wide range oftemperature and f (S2), at which the alloys crystallizedfirst and then the sulfides were formed with decreasingtemperature and increasing f (S2). The approximatelyabsence of this coexistence in the CED and SEDchromitites of Egypt reflects the confined rangeof temperature and f (S2) conditions during the formationof host chromitite.

7.2. Low-temperature alteration of PGM

The PGM included within fresh chromian spinel aretypically interpreted as primary magmatic phases (e.g.,


Stockman and Hlava, 1984; Augé, 1988; Melcher et al.,1997; Garuti et al., 1999a; Bai et al., 2000; Ahmed andArai, 2003). These primary PGM are mostly groupedinto sulfides, alloys and, to a lesser extent, sulfarsenides.Their euhedral habit and sporadic distribution evenwithin the same chromitite pod strongly support theinterpretation that they represent high-temperatureliquidus phases of a primitive magma. When theyundergo post-magmatic processes, e.g., metasomatismor serpentinization, at low temperature conditions theybecome unstable and altered to secondary phases(Stockman and Hlava, 1984; Bowles, 1986, 1987;Bowles et al., 1994). For example, radial cracks aredeveloped throughout the chromite grains due to theirbrittle failure and some of them connect the enclosedprimary Os-rich laurite grains to the altered silicatematrix. As a result, these primary laurite grains will beexposed to oxidative weathering, which desulfuratethem to the Ru–Os–Ir oxides with a significant volumereduction (e.g., Garuti and Zaccarini, 1997; Ahmedet al., 2002). Likewise, formation of native PGE as aresult of low-temperature alteration has also beenobserved in various PGE-bearing chromitites (e.g.,Prichard and Tarkian, 1988; McElduff and Stumpfl,1990; Nilsson, 1990; Garuti and Zaccarini, 1997; Baiet al., 2000). In addition to the primary ones, somesulfarsenides can also be produced by the partial orcomplete alteration of precursor primary sulfides and/oralloys (e.g., Feather, 1976; Stumpfl and Tarkian, 1976).

In the CED chromitites of the late Precambrianophiolite of Egypt, the secondary or “altered” PGM aremainly an Os-poor or-free laurite, osarsite, irarsite,hollingworthite and (Pt–Pd)–Fe alloys, whereas onlyosarsite and irarsite are identified in the SED cases.Most of them are located in the altered zones and alongcracks of chromian spinels, confirming their secondaryorigin. In many cases these secondary minerals show areplacive relationship with the primary magmatic PGMalong the altered parts of chromian spinel.

The association of Os-rich and Os-poor laurite inophiolite complexes has been described from theVourinos and Shetland ophiolites (Augé, 1985; Tarkianand Prichard, 1987). In the present study, the Os-richlaurite in the altered chromian spinel parts is occasion-ally associated with Os-poor and Os-free laurite, whichare, in turn surrounded by and/or intergrown withosarsite. In these cases, Os and, to a lesser extent, Ircontents remarkably decrease with increasing Rucontent giving very high Ru/Os and Ru/Ir ratios alongwith a remarkable increase of As content in the resultantsecondary PGM assemblages. This textural and com-positional association suggests that the process respon-

sible for Os removal from laurite was linked with theaddition of As (e.g., Tarkian and Prichard, 1987). Theselective removal of Os from the primary Os-rich lauritealong with their very low Ni and Fe contents stronglyindicates that this process happened at the very earlystage of serpentinization. The Ni–Fe alloy is commonlyassociated with PGM in highly altered chromitites (e.g.,Garuti and Zaccarini, 1997; Arai et al., 1999). Stockmanand Hlava (1984) also concluded that the porous Ru-richalloys from the south-western Oregon chromitites wereformed by desulfuration of primary laurite withsignificant addition of Ni and Fe during serpentiniza-tion. The presence of Os-poor and Os-free laurite onlyalong the cracks and altered parts of chromian spinel inthe CED chromitites indicates a low temperature post-magmatic process. On the other hand, the very small Os-poor and Os-free laurite grains in the SED chromititesare mainly found as small inclusions within freshchromian spinel which reflect their primary magmaticorigin under low f (S2) and high temperature conditions.

PGE sulfarsenides are well known in ophioliticchromitites either as primary or secondary PGM (e.g.,Tarkian and Prichard, 1987; Prichard and Tarkian, 1988;Nilsson, 1990; Torres-Ruiz et al., 1996). They representone of the main secondary PGM phases in the studiedCED and SED chromitites; however they are rarelyenclosed by fresh chromian spinel. Except one smallgrain of irarsite associated with primary Os-rich laurite,minerals of the irarsite–hollingworthite series aremainly found independently and, to a lesser extent, asa part of composite PGM grains in the interstitialsilicates and altered chromian spinel in the CEDchromitites. This is due to secondary deposition ofPGE rather than primary crystallization from magma.Osarsite, on the other hand, which is occasionallysurrounded or intergrown with Os-poor and/or Os-freelaurite in the CED chromitites is interpreted to beformed by the introduction of As in the altered chromianspinel parts via low-temperature serpentinization pro-cess. PGE sulfarsenides in the SED chromitites, on theother hand, are represented mainly by osarsite–irarsiteseries which occasionally display irarsite rim aroundosarsite core in the altered chromian spinel parts. Thistextural and compositional association may also beproduced due to the high activity of As during the low-temperature serpentinization process. It is noticed thatthe Ru content of sulfarsenides is generally much higherin the CED chromitites than in the SED ones (Fig. 10).This may be due to the availability of Ru from thealtered laurite in the former type, whereas Os–Ir alloy ismuch more common in the latter type. Hence, thepresence of PGE sulfarsenides, Pt- and Pd-arsenides and


alloys associated with altered chromian spinel andserpentinite matrix suggests that they were formedduring the serpentinization process at relatively lowtemperature conditions after the emplacement of thePan–African ophiolite complexes.

7.3. Genetic implications

The PGE abundances, along with the chromian spinelchemistry, are a useful indicator of degree of partialmelting and the saturation of sulfur in the primary melt.The paragenesis of PGM in chromitite is also useful todetermine the temperature and f (S2) of the magmaticsystem during the formation of the host chromian spinel(e.g., Nakagawa and Franco, 1997; Garuti et al., 1999b).

The formation of primary Os–Ir alloy and Ru-richlaurite is possible only if they are well encapsulatedwithin chromian spinel at high temperature and lowf (S2) conditions to prevent any exchange of S withthe residual melt. The presence solely of perfecteuhedral Os–Ir alloy and the absence of well crys-tallized Os-rich laurite as inclusions in the SED chro-mitites suggests a melt with high temperature and verylow f (S2) conditions was involved in the SED chro-mitite formation. Such high temperature and low f (S2)conditions might be obtained in high-degree partialmelts which may be linked to a supra-subduction zoneenvironment. This can be also deduced from the highCr# and low Ti character of chromian spinel in thechromitite–dunite–harzburgite complexes of the SEDof Egypt (Ahmed et al., 2001; and unpublished data ofthe author).

In contrast, the presence solely of Os-rich laurite asthe primary PGM inclusions in the CED chromitites isconsistent with the narrow range of slightly high f (S2) inthe mantle melt. The degree of partial melting in theCED localities might be not so high compared with theSED ones allowing a relatively higher f (S2) in theparent magma. This is also consistent with the widecompositional range of chromian spinel of the CEDchromitites (e.g., Ahmed et al., 2001).

It can be concluded, therefore, that if the CEDchromitites with primary Os-rich laurite inclusionsrepresent the primary composition of the mantle in thelate Proterozoic ophiolite of Egypt, the chromitites in theSED with the primary Os–Ir alloy inclusions might berepresentative subduction components along the sub-duction zone setting. The diversity of primary PGMinclusions in chromian spinel from the CED to the SEDchromitites, combined with their petrological character-istics suggests that the mantle rocks of the lateProterozoic ophiolite of Egypt are representative of late

Proterozoic equivalent to the depleted mantle MORB,lithosphere produced or extensively modified in a supra-subduction setting.

8. Summary and conclusions

1. In the CED chromitites, Os-rich laurite comprises thesole primary PGM inclusions within fresh chromianspinel. This reflects the relatively high f (S2) and lowtemperature conditions for the formation of hostchromian spinel. The composition of laurite itselfis controlled by f (S2); high f (S2) will favor the high(Os+ Ir) content in laurite. Subordinate amountsof secondary sulfarsenides, Pt–Pd arsenides and(Pt–Pd)–Fe alloys are also found along cracks andin the altered chromian spinel parts.

2. In the SED chromitites, the Os–Ir alloy is the mainprimary PGM inclusions with very few small grainsof Ru-rich laurite. This clearly demonstrates that thehost chromian spinel was crystallized at hightemperature and very low availability of sulfur inthe parent magma. Such conditions were available inthe high-degree partial melts formed at a supra-subduction zone environment. Sulfarsenides, mainlyirarsite–osarsite series, are found in the altered part ofspinel confirming their secondary origin afterprimary Os–Ir alloy.

3. The mantle rocks of the late Proterozoic ophiolite ofEgypt were initially formed at a mid-ocean ridgeand had been later modified at a subduction-zonesetting.

4. A low-temperature alteration process was responsi-ble for the common association of Os-poor–Os-freelaurite, sulfarsenides and the remnants of primaryOs-rich laurite and alloy in the altered part of spinel.The process responsible for Os removal from theprimary laurite was linked with the addition of As.The PGE, therefore, can be in situ remobilized andredistributed during low-temperature serpentiniza-tion processes resulting in the formation of secondaryPGM assemblages.

Acknowledgements

I am greatly indebted to Dr. S. Arai for his support forthis study, critical review of the manuscript and fruitfuldiscussions. Special thanks are due to Dr. M. Econo-mou-Eliopoulos for her handling of this manuscript. Thecomments of Dr. J. Mungall and anonymous reviewerwere very useful to improve the earlier version of themanuscript. This study was completed at KanazawaUniversity under the support of the Japan Society for the


Promotion of Science (JSPS) through the post-doctorfellowship of the author.

References

Ahmed, A.H., 2001. Platinum-group elements and chromitite depositsin two ophiolite suites: Proterozoic ophiolite, Eastern Desert,Egypt, and Phanerozoic ophiolite, northern Oman. UnpublishedPh.D. Thesis, Kanazawa University, Japan, 170 pp.

Ahmed, A.H., Arai, S., Attia, A.K., 2001. Petrological characteristicsof the Pan African podiform chromitites and associated peridotitesof the Proterozoic ophiolite complexes, Egypt. MineraliumDeposita 36, 72–84.

Ahmed, A.H., Arai, S., 2002. Platinum-group element geochemistry inpodiform chromitites and associated peridotites of the Precambrianophiolite, Eastern Desert, Egypt. Proceedings, 9th InternationalPlatinum Symposium, Billings, Montana, pp. 1–4.

Ahmed, A.H., Arai, S., 2003. Platinum-group minerals in podiformchromitites of the Oman ophiolite. Canadian Mineralogist 41,597–616.

Ahmed, A.H., Arai, S., Kadoshima, K., 2002. Possible platinum-groupelement (PGE) oxides in the PGE-mineralized chromitite from thenorthern Oman ophiolite. Journal of Mineralogical and Petrolog-ical Sciences 97, 190–198.

Andrews, D.R.A., Brenan, J.M., 2002. Phase-equilibrium constraintson the magmatic origin of laurite+Ru–Os–Ir alloy. CanadianMineralogist 40, 1705–1716.

Arai, S., Prichard, H.M., Matsumoto, I., Fisher, P.C., 1999. Platinum-group minerals in podiform chromitite from the Kamuikotan Zone,Hokkaido, northern Japan. Resource Geology 49, 39–47.

Ashmawy, M.H., 1987. The ophiolite mélange of the south EasternDesert of Egypt: remote sensing, field work and petrographicinvestigations. Berliner Geowissenschaftliche Abhandlungen84 (A), 1–135.

Augé, T., 1985. Platinum-group mineral inclusions in ophioliticchromitite from the Vourinos complex, Greece. Canadian Miner-alogist 23, 163–171.

Augé, T., 1988. Platinum-group minerals in the Tiebaghi and Vourinosophiolitic complexes: genetic implications. Canadian Mineralogist26, 177–192.

Bai, W., Robinson, P.T., Fang, Q., Yang, J., Yan, B., Zhang, Z., Hu,X.-F., Zhou, M.-F., Malpas, J., 2000. The PGE and base-metalalloys in the podiform chromitites of the Luobusa ophiolite,southern Tibet. Canadian Mineralogist 38, 585–598.

Barnes, S.-J., Naldrett, A.J., Gorton, M.P., 1985. The origin of thefractionation of the platinum group elements in terrestrial magmas.Chemical Geology 53, 303–323.

Bowles, J.F.W., 1986. The development of platinum-group minerals inlaterites. Economic Geology 81, 1278–1285.

Bowles, J.F.W., 1987. Further studies of the development of platinum-group minerals in the laterites of the Freetown Layered Complex,Sierra Leone. In: Prichard,H.M., Potts, P.J., Bowles, J.F.W,Cribb, L.J.(Eds.), Geo-Platinum, vol. 87. Elsevier, London, U.K., pp. 273–280.

Bowles, J.F.W., Gize, A.P., Vaughan, D.J., Norris, S.J., 1994.Development of platinum-group minerals in laterites — initialcomparison of organic and inorganic controls. Transactions,Institution of Mining and Metallurgy (Section B, Applied EarthScience), vol. 103, pp. 53–56.

Brenan, J.M., Andrews, D., 2001. High-temperature stability of lauriteand Ru–Os–Ir alloys and their role in PGE fractionation in maficmagmas. Canadian Mineralogist 39, 341–360.

Brügman, G.E., Arndt, N.T., Hofmann, A.W., Tobschall, H.J., 1987.Noble metal abundance in komatiite suites from Alexo, Ontario,and Gorgona Island, Colombia. Geochimica et CosmochimicaActa 51, 2159–2169.

Cabri, L.J., 1981. Platinum-group elements: mineralogy, geology,recovery. CIM Special 23 (267 pp.).

Cabri, L.J., 2002. The platinum-group minerals. In: Cabri, L.J.(Ed.), The Geology, Geochemistry, Mineralogy and MineralBeneficiation of Platinum-Group Elements. CIM Special,vol. 54, pp. 13–129.

Corrivaux, L., Laflamme, J.H.G., 1990. Minéralogie des elements dugroupe du platine dans les chromitites de l'ophiolite de ThetfordMines, Québec. Canadian Mineralogist 28, 579–595.

Crocket, J.H., 1979. Platinum-group elements in mafic and ultramaficrocks, a survey. Canadian Mineralogist 17, 391–402.

Crocket, J.H., 1981. Geochemistry of the platinum-group elements. In:Cabri, L.J. (Ed.), Platinum-Group Elements: Mineralogy, Geology,Recovery. CIM Special, vol. 24, pp. 47–64.

Economou, M.I., 1986. Platinum-group elements (PGE) in chromiteand sulfide ores within the ultramafic zone of some Greek ophiolitecomplexes. In: Gallagher, M.J., Ixer, R.A., Neary, C.R., Prichard,H.M. (Eds.), Metallogeny of Basic and Ultrabasic Rocks.Institution of Mining and Metallurgy, London, U.K., pp. 441–454.

El-Gaby, S., List, F.K., Tehrani, R., 1990. The basement complex ofthe Eastern Desert and Sinai. In: Said, R. (Ed.), The Geology ofEgypt. Balkema, Rotterdam, pp. 175–184.

El Haddad, M.A., 1996. The first occurrence of platinum groupminerals (PGM) in a chromitite deposit in the Eastern Desert,Egypt. Mineralium Deposita 31, 439–445.

El Sharkawy, M.A., El Bayoumi, R.M., 1979. The ophiolites of WadiGhadir, E.D., Egypt. Annals of the Geological Survey of Egypt 9,125–135.

Feather, C.E., 1976. Mineralogy of platinum-group minerals in theWitwatersrand, South Africa. Economic Geology 71, 1399–1428.

Ferrario, A., Garuti, G., 1990. Platinum-group mineral inclusions inchromitites of the Finero mafic–ultramafic complex (Ivrea-Zone,Italy). Mineralogy and Petrology 41, 125–143.

Garuti, G., Zaccarini, F., 1997. In situ alteration of platinum-groupminerals at low temperature: evidence from serpentinized andweathered chromitite of the Vourinos complex, Greece. CanadianMineralogist 35, 611–626.

Garuti, G., Zaccarini, F., Economou-Eliopoulos, M., 1999a. Paragen-esis and composition of laurite from chromitites of Othrys(Greece): implications for Os–Ru fractionation in ophioliticupper mantle of the Balkan peninsula. Mineralium Deposita 34,312–319.

Garuti, G., Zaccarini, F., Moloshag, V., Alimov, V., 1999b. Platinum-group minerals as indicators of sulfur fugacity in ophioliticupper mantle: an example from chromitites of the Ray–Iz ultra-mafic complex, Polar Urals, Russia. Canadian Mineralogist 37,1099–1115.

Irving, A.J., 1978. A review of experimental studies of crystal/liquidtrace element partitioning. Geochimica et Cosmochimica Acta 42,743–770.

Khudeir, A.A., 1983. Geology of the ophiolite suite of El-Rubshi area,E.D., Egypt. Unpublished Ph.D. thesis, Assiut University, Egypt,170 pp.

Khudeir, A.A., El-Haddad, M.A., Leake, B., 1992. Compositionalvariation in chromite from the Eastern Desert, Egypt. Mineralog-ical Magazine 56, 567–574.

Leblanc, M., 1991. Platinum-group elements and gold in ophioliticcomplexes: distribution and fractionation from mantle to oceanic


floor. In: Peters, Tj., et al. (Ed.), Ophiolite Genesis and Evolution ofthe Oceanic Lithosphere, Oman. Kluwer, Dordrecht, pp. 231–260.

Legendre, O., Augé, T., 1986. Mineralogy of platinum-group mineralinclusions in chromitites from different ophiolitic complexes. In:Gallagher, M.J., Ixer, R.A., Neary, C.R., Prichard, H.M. (Eds.),Metallogeny of Basic and Ultrabasic Rocks. Institution of Miningand Metallurgy, London, U.K., pp. 361–372.

Maier, W.D., Prichard, H.M., Barnes, S.-J., Fisher, P.C., 1999.Compositional variation of laurite at Union Section in the WesternBushveld Complex. South African Journal of Geology 102,286–292.

Malitch, K.N., Augé, T., Badanina, I.Yu., Goncharov, M.M., Junk,S.A., Pernicka, E., 2002. Os-rich nuggets from Au-PGE placersof the Maimecha-Kotui Province, Russia. A multi disciplinarystudy. Mineralogy and Petrology 76, 121–148.

Malitch, K.N., Thalhammer, O.A.R., Knauf, V.V., Melcher, F., 2003.Diversity of platinum-group mineral assemblages in banded andpodiform chromitites from Kraubath ultramafic massif, Austria:evidence for an ophiolitic transition zone? Mineralium Deposita38, 282–297.

McElduff, B., Stumpfl, E.F., 1990. Platinum-group minerals from theTroodos ophiolite complex, Cyprus. Mineralogy and Petrology 42,211–232.

Melcher, F., Grum, W., Simon, G., Thalhammer, T.V., Stumpfl, E.F.,1997. Petrogenesis of giant chromite deposits of Kempirsai,Kazakhstan: a study of solid and fluid inclusions in chromite.Journal of Petrology 38, 1419–1458.

Nakagawa, M., Franco, H.E.A., 1997. Placer Ru–Os–Ir alloys andsulfides: indicators of sulfur fugacity in an ophiolite? CanadianMineralogist 35, 1441–1452.

Nasseef, A.O., Bakor, A.R., Hashad, A.H., 1980. Petrography ofpossible ophiolitic rocks along Qift–Quseir road, E.D., Egypt.Institute of Applied Geology, Jeddah, Bulletin 3, vol. 4, 77–82.

Nilsson, L.P., 1990. Platinum-group mineral inclusions in chromititefrom Osthammeren ultramafic tectonite body, south centralNorway. Mineralogy and Petrology 42, 249–263.

Page, N.J., Cassard, D., Haffity, J., 1982. Palladium, platinum,rhodium, ruthenium, and iridium in chromitites from the Massif duSud and Tiebaghi Massif, New Caledonia. Economic Geology 77,1571–1577.

Page, N.J., Talkington, R.W., 1984. Palladium, platinum, rhodium,ruthenium, and iridium in peridotites and chromitites fromophiolite complexes in Newfoundland. Canadian Mineralogist22, 137–149.

Prichard, H.M., Tarkian, M., 1988. Platinum and palladium mineralsfrom two PGE-rich localities in the Shetland ophiolite complex.Canadian Mineralogist 26, 979–990.

Puchtel, I., Humayun, M., 2000. Platinum group elements inkostomuksha komatiites and basalts: implications for oceaniccrust recycling and core–mantle interaction. Geochimica etCosmochimica Acta 64, 4227–4242.

Puchtel, I., Humayun, M., 2001. Platinum group elements fraction-ation in a komatiitic basalt lava lake. Geochimica et CosmochimicaActa 65, 2979–2993.

Quick, J.E., 1990. Geology and origin of late Proterozoic DarbZubaydah ophiolite, Kingdom of Saudi Arabia. Geological Societyof America Bulletin 102, 1007–1020.

Ries, A.C., Shackleton, R.M., Graham, R.H., Fitches, W.R., 1983.Pan–African structures, ophiolites and mélanges in the E.D. ofEgypt: a traverse at 26°N. Journal Geological Society of London140, 75–95.

Righter, K., Campbell, A.J., Humayun, M., Hervig, R.L., 2004.Partitioning of Ru, Rh, Pd, Re, Ir, and Au between Cr-bearingspinel, olivine, pyroxene and silicate melts. Geochimica etCosmochimica Acta 68, 867–880.

Roeder, P.L., 1994. Chromite: from the Fiery rain of chondrules to theKilauea iki lava lake. Canadian Mineralogist 32, 729–746.

Shackleton, R.M., Ries, A.C., Graham, R.H., Fitches, W.R., 1980.Late Precambrian ophiolite mélange in the Eastern Desert ofEgypt. Nature 285, 472–474.

Stockman, H.W., Hlava, P.F., 1984. Platinum-group minerals in Alpinechromitites from southwestern Oregon. Economic Geology 79,491–508.

Stumpfl, E.F., Tarkian, M., 1976. Platinum genesis: new mineralogicalevidence. Economic Geology 71, 1451–1460.

Styles, M.T., Nasr, B.B., Connor, E.A.O., 1996. Several occurrences ofplatinum group minerals in the Nubian Shield of Eastern Egypt.Internal Report, Geological Survey of Egypt, pp. 793–808.

Stern, R.J., 1981. Petrogenesis and tectonic setting of late Precambrianensimatic volcanic rocks, CED of Egypt. Precambrian Research16, 195–230.

Takla, M.A., El Sharkawi, M.A., Basta, F.F., 1982. Petrology of thebasement rocks of Gebel Mohagara-Ghadir area, Eastern Desert,Egypt. Annals of Geological Survey of Egypt 12, 121–140.

Talkington, R.W., Lipin, B.R., 1986. Platinum-group minerals inchromite seams of the Stillwater complex, Montana. EconomicGeology 81, 1179–1186.

Talkington, R.W., Watkinson, D.H., 1986. Whole rock platinum-groupelement trends in chromite-rich rocks in ophiolitic and stratiformigneous complexes. In: Gallagher, M.J., Ixer, R.A., Neary, C.R.,Prichard, H.M. (Eds.), Metallogeny of Basic and Ultrabasic Rocks.Institution of Mining and Metallurgy, London, U.K., pp. 427–440.

Tarkian, M., Prichard, H.M., 1987. Irarsite–hollingworthite solid-solution series and other associated Ru-, Os-, Ir-, and Rh-bearingPGM's from the Shetland ophiolite complex. Mineralium Deposita22, 178–184.

Tarkian, M., Naidenova, E., Zhelyaskova-Panayotova, M., 1991.Platinum-group minerals in chromitites from the Eastern Rhodopeultramafic complex, Bulgaria. Mineralogy and Petrology 44,73–87.

Tarkian, M., Economou-Eliopoulos, M., Eliopoulos, D.G., 1992.Platinum-group minerals and tetraauricupridein ophiolitic rocks ofSkyros Island, Greece. Mineralogy and Petrology 47, 55–66.

Torres-Ruiz, J., Garuti, G., Gazzotti, M., Gervilla, F., Fenoll Hach-Alí,P., 1996. Platinum-group minerals in chromitites from the Ojenlherzolite massif (Serranía de Ronda, Betic Cordillera, SouthernSpain). Mineralogy and Petrology 56, 25–50.

Tredoux, M., Lindsay, N.M., Davies, G., McDonald, L., 1995. Thefractionation of platinum-group elements in magmatic system,with the suggestion of a novel causal mechanism. South AfricanJournal of Geology 98, 157–167.

Westland, A.D., 1981. Inorganic chemistry of platinum-groupelements. In: Cabri, L.J. (Ed.), Platinum-group Elements: Miner-alogy, Geology, Recovery. CIM Special, vol. 23, pp. 7–17.

Zaccarini, F., Garuti, G., 2001. Platinum-Group Minerals and chromitecomposition in Mesozoic, Paleozoic and Precambrian podiformchromitites: examples from the Mediterranean area, the Ural chainand the Egyptian Eastern Desert. In: Piezstrznski, A., et al. (Eds.),Mineral Deposits at the Beginning of the 21st Century. Balkema,Rotterdam, pp. 685–688.

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