ARTICLE IN PRESS

(1)

Diversity of platinum-group minerals in podiform chromitites of 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 2006

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 or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very 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 Proterozoic Pan–African ophiolite of the Eastern Desert of Egypt. The populations of platinum-group minerals (PGM) in the studied CED and SED 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 sulfur fugacity and temperature conditions are the main controller of PGE mineralogy in the host chromitite at the initial stage within the upper mantle. Os-rich laurite is stable at high sulfur fugacity and low temperature conditions, whereas Os–Ir alloy is stable at lower sulfur fugacity and higher temperature conditions. The diversity of PGE mineralogy combined with the differences in petrological characteristics of chromian spinels from CED chromitites to SED ones suggests different degrees of partial melting of the mantle rocks 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.

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

1. Introduction

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

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

Ore Geology Reviews 32 (2007) 1 - 19

www.elsevier.com/locate/oregeorev

E-mail address:[email protected].

doi:10.1016/j.oregeorev.2006.05.008

ARTICLE IN PRESS

(2)

P

(e.g.,Augé, 1988; Tarkian et al., 1991, 1992; Garuti and Zaccarini, 1997; Garuti et al., 1999a; Ahmed and Arai, 2003). The IPGE mineral phases, e.g., Ru–Os–Ir sulfides and alloys, therefore, are the most common PGM 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–ultramafic system is mainly controlled by the degree of partial melting, temperature and sulfur fugacity (Tredoux et al., 1995; Brenan and Andrews, 2001; Andrews and Brenan, 2002). The paragenesis of PGM, combined with characteristics of chromian spinel, is therefore very useful to constrain the early magmatic conditions and tectonic setting of chromitite formation (e.g.,Stockman and Hlava, 1984; Brenan and Andrews, 2001). It can also

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

In general, there are very few studies available on PGE concentration and distribution of the late Pre- cambrian ophiolitic chromitites in the Eastern Desert of Egypt (Ahmed, 2001; Ahmed and Arai, 2002). Very few descriptions also have been given of PGE mineralogy in podiform chromitites of the Pan–African ophiolite of Egypt (El Haddad, 1996; Styles et al., 1996; Zaccarini and Garuti, 2001). In this study, the description and geochemical analysis of the identified PGM in podiform chromitites from the Eastern Desert of Egypt is comprehensively presented. This may provide insights into the genesis of PGM and host chromitite.

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

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

ARTICLE IN PRESS

(3)

2. Geological background and sample locations

The basement complex of the Eastern Desert of Egypt constitutes the western side of the Arabian–

Nubian Shield, which was cratonized in the late Precambrian (e.g., El-Gaby et al., 1990). In general, three categories of basement rocks are found in the Eastern Desert of Egypt (Fig. 1): (1) Pre-Pan–African rocks, composed of high-grade metamorphic rocks, i.e., gneiss; (2) Pan–African rocks, comprising ophiolites and island arc associations and; (3) Phanerozoic alkaline rocks. The late Precambrian Pan–African rocks, occupy ca. 10% of the Egyptian land surface, covering a huge area in the Eastern Desert, and small areas in southern Sinai and southern parts of the Western Desert (Fig. 1).

Several ophiolite suites have been reported from the

Eastern Desert of Egypt generally located to the south of latitude 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), from El-Rubshi area (Khudeir, 1983), and from the southern Eastern Desert at Wadi Bitan, Wadi Rahabah and Wadi Naam (Ashmawy, 1987). Ophiolite complexes in the Eastern Desert of Egypt are usually found as highly dismembered mafic to ultramafic bodies; the most complete ophiolite sequence is described from the Wadi Ghadir area (El Sharkawy and El Bayoumi, 1979).

Chromitite pods of sub-economic value are common in the central and southern parts of the Eastern Desert of Egypt (e.g., Khudeir et al., 1992; Ahmed et al., 2001).

They occur mainly as lenticular bodies of variable

Fig. 2. Distribution of late Precambrian ophiolitic fragments and the studied chromitite localities in the central Eastern Desert (CED) and southern Eastern Desert (SED) of Egypt (afterShackleton et al., 1980).

3

ARTICLE IN PRESS

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

(4)

dimensions; however the southern Eastern Desert (SED) chromitites, in most cases, are larger than the central Eastern Desert (CED) examples. Podiform chromitites, in most cases, are hosted by fully serpentinized peridotite, mainly harzburgite host and subordinate dunite which forms an envelope around chromitite pods. The mantle harzburgite and dunite envelopes are completely serpentinized in the CED; no primary silicate minerals have survived alteration, whereas they are relatively fresh at some localities in the SED (e.g., Abu Dahr and Abu Syeil areas;Fig. 2), containing fresh primary olivine and orthopyroxene.

Six chromitite localities in the CED of Egypt along the Qift–Quseir and Idfu–Marsa Alam roads (Fig. 2) have been examined; all of them have found to include PGM 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 been investigated for their PGE mineralogy.

3. Spinel chemistry and PGE concentrations

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

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 examples showing the alteration trend (Fig. 3). The spinels with low Fe³⁺# are often intact and represent the primary compositions (e.g.,Quick, 1990; Ahmed et al., 2001).

This compositional trend is common in chromian spinel altered to various degrees at low-temperature conditions (e.g.,Roeder, 1994). The chromian spinel of both CED and SED chromitites is generally low in TiO2content, resembling ophiolitic chromitite.

Consistent with chromian spinel chemistry, the SED chromitites show uniform chondrite-normalized PGE patterns with general negative slope from Ru to Pt as the ophiolitic chromitites (Fig. 4) (e.g.,Page et al., 1982;

Page and Talkington, 1984). The total PGE contents of the SED chromitites range from 140 up to 320 ppb (Ahmed and Arai, 2002). The CED chromitites, on the other hand, exhibit variable PGE contents and distribution patterns from locality to locality. Some chromitite pods in the CED show unusually high PGE concentrations reaching up to 3.2 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 negative slope of PGE pattern (Fig. 4), being enriched in both IPGE (Os, Ir, Ru) and PPGE (Rh, Pt, Pd), while the associated dunites exhibit U-shaped patterns showing an enrichment in both Os, Pt and Pd (unpublished data). The PGE-poor chromitites from other CED localities show low to

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

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

4

ARTICLE IN PRESS

(5)

intermediate PGE concentrations ranging from 58 to 365 ppb and generally display negative slopes (Fig. 4) as in almost all ophiolitic chromitites.

4. Analytical techniques

One hundred and thirty six polished thin sections (70 from the CED and 66 from the SED) of chromitites and associated ultramafic rocks were carefully examined at high magnifications using the optical microscope for identification of various PGM grains with different sizes.

Quantitative analysis of PGM grains was carried out using a JEOL JXA-8800 electron probe microanalyzer at the Center for Cooperative Research of Kanazawa University, Japan. Analytical conditions were 25 kV accelerating voltage and 20 nA probe current. Standards used were pure metals for the elements Os, Ir, Ru, Rh, Pt, Pd, Cu, and Cr, gallium arsenide for As, and pentlandite for S, Fe and Ni. The count time was 10 s for each element, except S and Fe (20 s). The KαX-ray lines were used for Ni, S, Fe, and Cr, Lαlines for Os, Ir, Ru, Rh, Pt and Cu, and Lβlines for Pd and As.

5. PGM mineralogy

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

were petrographically and geochemically examined in both the CED and SED chromitites; 141 grains from the former and 67 grains from the latter. The domains of PGM grains in the CED chromitites were observed in Wadi El-Lawi area (95 grains from 20 polished thin sections of three chromitite pods); up to 9 PGM grains per thin section. Other chromitite localities in both CED and SED contained comparable numbers of PGM grains, ranging from nil up to 3 PGM grains per thin section. Os-rich laurite, the most common PGM in the CED chromitites, comprises 54% of the total PGM grains found in all studied areas (Fig. 5), whereas the least abundant PGM are (Pt–Pd)–Fe alloys (Fig. 5). In general, three PGM groups have been found in the podiform chromitites from the Proterozoic ophiolite, Eastern Desert of Egypt: sulfides, alloys and sulfarsenides–arsenides in decreasing order of abundance. In most cases the PGM grains are predominantly mono- phase and subordinately show polyphase PGM associations. The most common associations are laurite + osarsite and Os–Ir alloy + laurite.

5.1. PGM in the CED chromitites 5.1.1. PGE sulfides

PGE-sulfides form the main PGM found in the CED chromitites. 108 PGM grains, out of 141 grains, are laurite; 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

Fig. 5. Frequency distribution histograms of different PGM in the central Eastern Desert (CED) and southern Eastern Desert (SED) chromitites of late Precambrian ophiolite of Egypt. Note the common abundance of laurite in the former and Os–Ir alloy in the latter.

5

ARTICLE IN PRESS

(6)

localities (Fig. 5). The grain size of laurite occasionally exceeds 5μm, up to 40μm across. In most cases Os-rich laurite is found exclusively as solitary euhedral crystals embedded within chemically fresh, but commonly cracked, chromian spinel (Fig. 6A–C). There is no systematic distribution of Os-rich laurite grains within chromian spinel, in the center (Fig. 6A) or in the periphery of chromian spinel grains (Fig. 6C). Some Os-

rich laurite grains are associated along the chromian spinel fractures with other PGM such as irarsite and Ir– Rh alloy (Fig. 6D). Upon weathering, Os-rich laurite in the altered part of chromian spinel has been changed completely 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)

Fig. 6. Back-scattered electron images of different PGM in the CED chromitites of Egypt. (A), (B) and (C) are solitary perfect euhedral Os-rich laurite grains within chemically fresh, but sometimes cracked, chromian spinel. Samples A and C are from Barramiya, B is from Wadi El-Lawi. (D) Os-rich laurite intergrown with irarsite (I) and native Ir within cracked chromian spinel (Wadi El-Lawi). (E) Porous Os-poor laurite within slightly altered chromian spinel (Wadi Bezah). (F) Os-rich and Os-poor laurites surrounded by osarsite (O) within altered chromian spinel (Wadi El-Lawi).

(G) Unnamed (Ir–Cu–Fe–Pt–Rh)S associated with CuS in fresh-cracked chromian spinel (Wadi El-Lawi). (H) Composite PGM grains containing Pt–Fe alloy, Pd–Fe alloy, sperrylite (Pt–As), native Pd and hollingworthite (H) within altered chromian spinel and along cracks (Wadi El-Lawi).

(I) Irarsite (I)–hollingworthite (H) solid solution series in the interstitial silicates (Wadi El-Lawi).

6

ARTICLE IN PRESS

(7)

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

5.1.2. PGE alloys

Despite careful observation, Os–Ir alloy was not found in the CED chromitites. One grain of Ru–Os–Ir alloy with low analytical total was found in a cracked chromian spinel grain, resembling the mode of occurrence of PGE oxides reported from the Oman ophiolite (Ahmed et al., 2002).

Although some of the CED chromitites are notably enriched

in Pt and Pd, very few grains of Pt and Pd phases have been found in the interstitial silicate matrix and chromian spinel fractures. Pt–Fe, Pd–Fe alloys and native Pd are the main Pt and Pd phases found as small aggregates, usuallyb10μm across, of composite grains in the altered chromian spinel parts and along spinel cracks (Fig. 6H). A small grain, N5μm across, of native Ir is found associated with Os-rich laurite and irarsite in a fractured chromian spinel (Fig. 6D).

5.1.3. PGE sulfarsenides–arsenides

Irarsite (I) and hollingworthite (H), with wide range of solid solution between the two end-members, are the main PGE sulfarsenides in the CED chromitites of Egypt. Large

Fig. 7. Back-scattered electron images of different PGM (A–G) and BMS (H and I) in the SED chromitites of Egypt. (A) and (B) Solitary grains of perfect euhedral Os–Ir alloy within fresh chromian spinel. Sample A is from Belmhandite and B from Abu Syeil areas. (C) Os–Ir alloy associated with small Os-rich laurite and native Cu (Abu Dahr area). (D) Ru–Os–Ir alloy within fresh chromian spinel (Belmhandite area). (E) Osarsite (O)– irarsite (I) solid solution series associated with millerite (M) within altered part of spinel (Um-Thagr area). (F) Os-poor laurite intergrown with unnamed (Rh–Ir–Cu–Fe)S within fresh chromian spinel (Abu Dahr area). (G) Composite grain of Os-poor, Os-free and Os-rich laurites within interstitial silicates of chromitite (Abu Dahr area). (H) Euhedral millerite (M) crystal within slightly altered chromian spinel (Um-Thagr area).

(I) Composite inclusion from millerite (M) and pyrite (Py) within slightly altered chromian spinel (Belmhandite area).

7

ARTICLE IN PRESS

(8)

patches and clots of I–H solid solutionN50μm across are found mainly in the interstitial silicate matrix of chromitite spinel (Fig. 6I). Small grains and blebs of I and H are also found associated with Os-rich laurite and along with composite Pt–Fe, Pd–Fe alloys, and native Pd in the cracked and altered chromian spinel (Fig. 6D, H). In addition to the alteration product of Os-rich laurite (Fig. 6F), solitary small grains, ca. 5μm across, of osarsite are also found in the interstitial matrix of chromitite spinel.

The PtAs (sperrylite) and unnamed (Pd–Pt)As are found as a part of composite PGM grains in the altered parts of chromian spinel (Fig. 6H).

5.2. PGM in SED chromitites

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

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 alloys

Fifty PGM grains, out of 67, are Os–Ir alloy accounting for 75% of the total PGM grains found in the SED chromitites (Fig. 5). In most cases, Os–Ir alloy was 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 is associated with small grains of Os-rich laurite (Fig. 7C).

Os–Ir alloy in the SED chromitites is commonly associated with voids (Fig. 7A, B), similar to those reported 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 chromian spinel (Fig. 7D). This Ru–Os–Ir alloy is quite different

Table 1

Representative microprobe analyses of PGE sulfides in the CED chromitites, late Precambrian ophiolite, Egypt

S. No. Laurite Unnamed

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

LW9 LW10 BR50 LW10 BZ37 BZ42 LW10 UH17 LW9 LW9

wt.%

S 33.15 32.47 33.78 32.52 37.09 35.46 38.08 37.15 21.73 22.66

Os 27.87 25.24 14.74 9.36 3.79 6.86 0.59 n.d n.d n.d

Ir 5.61 6.80 7.31 8.99 3.03 1.34 1.79 2.66 29.60 39.45

Ru 29.92 29.34 36.13 38.22 50.31 50.82 50.94 54.79 4.39 0.15

Rh 0.72 0.49 0.81 1.21 1.10 0.84 1.42 1.15 3.90 5.47

Pt n.d n.d n.d n.d n.d n.d 0.25 n.d 10.84 14.77

Pd 0.26 0.31 n.d 0.21 n.d n.d 0.12 0.11 0.07 0.21

As n.d 0.92 n.d 5.82 3.75 n.d 3.58 0.71 n.d n.d

Ni 0.01 0.05 0.04 0.67 0.44 n.d 0.33 0.05 0.12 0.10

Cu n.d n.d n.d n.d n.d 0.03 n.d n.d 19.19 10.37

Fe 0.57 0.89 1.01 0.59 0.64 1.50 0.46 1.13 6.65 1.78

Cr 1.54 1.78 3.83 1.08 1.17 4.59 1.01 3.94 3.65 3.60

Sb 0.37 n.d 0.01 n.d

Bi 0.30 0.82 0.28 0.23

Total 99.65 98.96 97.65 99.49 101.61 101.44 98.80 101.69 100.14 98.56

at.%

S 67.78 66.96 67.83 63.17 65.31 65.55 66.61 65.94 48.68 56.95

Os 9.61 8.78 5.00 3.07 1.13 2.14 0.17 – – –

Ir 1.91 2.34 2.45 2.91 0.89 0.41 0.52 0.79 11.06 16.54

Ru 19.40 19.19 23.01 23.55 28.10 29.79 28.26 30.84 3.12 0.12

Rh 0.46 0.32 0.51 0.73 0.60 0.48 0.78 0.64 2.72 4.28

Pt – – – – – – 0.07 – 3.99 6.10

Pd 0.16 0.19 – 0.12 – – 0.06 0.06 0.05 0.16

As – 0.81 – 4.84 2.83 – 2.68 0.54 – –

Ni 0.01 0.05 0.05 0.71 0.42 – 0.31 0.05 0.14 0.13

Cu – – – – – 0.03 – – 21.69 13.15

Fe 0.67 1.06 1.16 0.65 0.64 1.59 0.47 1.15 8.55 2.57

Sb 0.20 – – –

Bi 0.09 0.25 0.08 0.06

CED: central Eastern Desert. S. No.: sample number. n.d: not detected. Samples LW9 and LW10 are from Wadi El-Lawi, sample UH17 is from Um- Huitate area, samples BZ37 and BZ42 are from Wadi Bezah, and sample BR50 is from El-Barramiya area (seeFig. 2for the locations).

8

ARTICLE IN PRESS

(9)

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

5.2.2. PGE Sulfarsenides

Sulfarsenide grains are occasionally zoned, with osarsite in the core and irarsite in the rim. This is possibly an alteration product of a precursor primary PGM, i.e., Os–Ir alloy (Fig. 7E). The osarsite–irarsite association is usually located in the altered parts of

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

5.2.3. PGE sulfides

A few small grains of Os-free, Os-poor and, to a lesser extent, Os-rich laurite, b2 μm across, are usually associated with Os–Ir alloy (Fig. 7C) or with other PGM 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 are never found in the SED chromitites. However, large aggregates of Os-poor, Os-free and Os-rich laurite are found to be located in the interstitial matrix of chromitite spinel (Fig. 7G). Only a single grain of unnamed Cu-rich (Rh–Ir)S, b5 μm across, is associated with Os-free to Os-poor laurite within fresh chromian spinel (Fig. 7F).

The mineral has a greenish grey color which is less reflective than the associated laurite grain.

5.3. Base metal sulfides (BMS)

Two modes of occurrence are found for the base metal sulfides (BMS) in the Eastern Desert chromitites of Egypt; small inclusions within chromian spinel and large aggregates within the interstitial silicates and along cracks of chromian spinel. The BMS in the CED chromitites consist of the following minerals, in decreasing order of abundance: millerite, heazlewoodite, pentlandite and chalcopyrite. Millerite is also very common in the SED chromitites as small euhedral grains ca. 10μm across within slightly altered chromian spinel

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) and southern Eastern Desert (SED) chromitites of the late Precambrian ophiolite of Egypt. Os–Ir alloys from Vourinos ophiolite (Augé, 1988), Ray–Iz ultramafic complex (Garuti et al., 1999b), and Oman ophiolite (Ahmed and Arai, 2003) are presented here for comparison.

Fig. 9. Variation of Ru versus Os (atomic ratios) of laurite in the central Eastern Desert (CED) and southern Eastern Desert (SED) chromitites of the late Precambrian ophiolite of Egypt. Note the perfect negative correlation.

9

ARTICLE IN PRESS

(10)

(Fig. 7H), or associated with osarsite–irarsite minerals (Fig. 7E) in the altered chromian spinel. It is also sometimes associated with pyrite in the same inclusion within chromian spinel (Fig. 7I). Pyrite is usually found as relatively large grains, ca. 10μm, either as single or composite grains with millerite enclosed by chromian spinel (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 sulfides are 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 an average of 0.72. The Os content of laurite in the CED chromitites varies from nil up to 27.9 wt.%, with an average of 16.2 wt.% (Table 1,Fig. 8). The Os-poor and Os-free compositions are usually found in the altered

laurite grains located in the interstitial matrix and along the fractures of chromian spinel (seeFig. 6E, F). The Os content in laurite displays a perfect negative correlation with Ru content (Fig. 9), indicating substantial substitution of Ru by Os. The Ir content of laurite varies from 1.19 up to 14.44 wt.%, with an average of 7.36 wt.%, but does not exhibit any systematic relationship with Os or Ru contents (Table 1, Fig. 8). Laurite composition varies from a pure RuS2in the altered, Os-free laurite, up to (Ru0.59Os0.29Ir0.06)_Σ= 0.94S2.06in the most Os-rich variety. Relatively high amounts of As and Rh are detected in the Os-poor and Os-free varieties of the altered laurite grains (Table 1). Two grains of unnamed (Ir, Pt, Rh)(Cu, Fe) sulfide are found associated with CuS within relatively fresh chromian spinel of CED chromitite. The optical properties of this mineral are similar 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 to 19.19 wt.%), and its PGE:BM (base metals) ratio is approximately one (Table 1). This phase is characterized

Table 2

Representative microprobe analyses of PGE alloys and PGE sulfides in the SED chromitites of the late Precambrian ophiolite, Egypt.

S. No. Alloys Sulfides

Os–Ir alloy "osmium" Ru–Os–Ir Laurite Rh–IrS

GL11 AD122 AS50 BL52 GL1 GL1 AD129 AD122 AD122 AD122

wt.%

S n.d n.d 0.01 n.d n.d n.d 36.16 38.77 30.49 26.03

Os 70.77 60.36 62.57 58.21 17.42 17.82 1.87 0.25 34.79 3.58

Ir 21.89 33.03 30.72 23.46 14.07 14.53 3.65 1.52 6.10 23.50

Ru 4.18 4.52 3.89 9.26 62.57 61.98 52.75 54.68 23.62 2.74

Rh 0.21 0.06 0.33 0.59 3.59 3.69 1.42 2.45 0.76 24.62

Pt n.d n.d n.d n.d n.d n.d n.d n.d n.d 0.62

Pd 0.31 0.04 0.04 n.d n.d n.d 0.28 0.29 n.d 0.52

As 0.13 n.d n.d n.d 0.02 n.d 1.24 1.95 n.d 0.07

Ni n.d 0.04 n.d n.d 0.03 0.03 0.13 0.08 n.d 1.42

Cu n.d n.d n.d n.d n.d n.d n.d n.d n.d 8.54

Fe 0.59 0.38 0.45 1.33 0.52 0.46 0.18 0.52 0.54 3.06

Cr 2.09 1.34 1.76 6.80 1.77 1.68 0.73 1.31 2.23 3.12

Total 100.17 99.77 99.77 99.65 99.99 100.19 98.41 101.81 98.53 97.82

at.%

S – – 0.03 – – – 65.69 66.33 67.14 56.31

Os 68.34 58.56 61.05 55.75 11.06 11.34 0.57 0.07 12.93 1.31

Ir 20.90 31.68 29.63 22.21 8.83 9.14 1.11 0.43 2.24 8.48

Ru 7.59 8.23 7.15 16.67 74.67 74.14 30.39 29.67 16.5 1.88

Rh 0.37 0.10 0.59 1.04 4.21 4.34 0.81 1.30 0.52 16.60

Pt – – – – – – – – – 0.22

Pd 0.53 0.07 0.07 – – – 0.15 0.15 – 0.34

As 0.33 – – – 0.04 – 0.96 1.43 – 0.06

Ni – 0.13 – – 0.05 0.06 0.13 0.08 – 1.68

Cu – – – – – – – – – 9.32

Fe 1.95 1.24 1.48 4.33 1.13 0.99 0.19 0.52 0.68 3.80

SED: southern Eastern Desert. n.d: not detected. Samples GL1 and GL11 are from El-Gallala area, AD122 and AD129 from Abu Dahr area, BL52 from Belmhandite area, and sample AS50 is from Abu Syeil area (seeFig. 2for the locations).

10

ARTICLE IN PRESS

(11)

by monosulfide stoichiometry (XS) giving an average composition of (Ir0.29Cu0.38Pt0.11Fe0.12Rh0.08)_Σ0.98S1.02, which is very close to the composition of xingzhongite reported by Cabri (1981), and Ferrario and Garuti (1990).

PGE-sulfides in the SED chromitite are represented mainly by Os-poor to Os-free laurite and rarely by small grains of Os-rich laurite. Laurite composition varies from pure RuS2 to (Ru0.50Os0.39Ir0.07)_Σ= 0.96S2.04 for Os-rich varieties which is close to the laurite–erlichmanite bound- ary (Table 2,Fig. 8). Laurite from the SED chromitites represents the most Ru-rich variety in the analyzed PGM grains (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). The PGE:BM ratio is approximately 2:1 giving the average 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 fresh chromian spinel, suggests that it is an unnamed mineral phase rather than an alteration product.

6.2. PGE alloys

The Pt–Fe and Pd–Fe alloys are found mainly as composite grains in the altered parts and along cracks of chromian spinel grains of the CED chromitites.

The Pt–Fe alloys display variable compositions from tetraferroplatinum (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(Fe2.99Ni0.12

Bi0.03)_Σ= 3.14. The Pt–Fe alloy contains minor amounts of Pd, Ni and Bi; 2.36, 1.29 and 2.08 wt.% on average, respectively (Table 3). According toCabri (2002), there is no Pd–Fe alloy mineral phase. The Pd–Fe alloy in the CED chromitites which is a part of composite PGM grains has a general composition of

“isoferroplatinum” type (X3Fe), giving the average composition Pd2.90Fe1.10(Table 3). One grain of Ru–

Table 3

Representative microprobe analyses of PGE alloys and native metals in 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.d

Os n.d n.d 0.08 n.d n.d n.d n.d 20.33

Ir n.d 0.04 n.d n.d 86.17 87.10 n.d 12.39

Ru n.d 0.11 n.d 0.04 0.44 0.42 n.d 50.06

Rh 0.39 0.42 0.05 n.d 5.38 5.05 8.20 3.16 Pt 74.45 42.05 0.34 0.22 1.30 1.25 4.42 n.d Pd 3.01 1.67 78.77 80.99 0.09 0.07 75.48 n.d

As 0.08 0.20 0.33 0.22 n.d n.d 6.18 0.02

Ni 0.64 1.93 0.62 0.33 0.43 0.41 0.09 0.03

Cu n.d n.d n.d n.d n.d 0.01 n.d n.d

Fe 13.89 44.74 17.03 12.05 2.61 2.33 1.21 0.52 Cr 2.23 2.32 1.56 3.77 3.04 2.78 1.16 1.77

Sb n.d n.d n.d 0.03 0.03

Bi 2.71 1.44 n.d 0.31 0.13

Total 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 – – – – 15.13

Ir – 0.02 – – 79.14 80.61 – 9.11

Ru – 0.10 – 0.04 0.78 0.74 – 69.99

Rh 0.56 0.37 0.04 – 9.22 8.72 7.98 4.34

Pt 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 0.04

Ni 1.58 3.02 0.99 0.57 1.28 1.24 0.17 0.07

Cu – – – – – 0.01 – –

Fe 35.88 73.81 28.69 21.80 8.25 7.43 2.16 1.31

Sb – – – 0.02 0.02

Bi 1.87 0.64 – 0.15 0.06

Samples LW1 and LW10 are from Wadi El-Lawi, and sample BR53 is from El-Barramiya area (seeFig. 2for the locations). n.d: not detected.

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

11

ARTICLE IN PRESS

(12)

Os–Ir alloy, redefined as“Ruthenium”byCabri (2002), ca. 7 μm across, displays less reflectivity and lower analytical total than the associated laurite (Table 3). It gives a composition of (Ru0.70Os0.16Ir0.09Rh0.04Fe0.01).

The petrographical and geochemical properties of this mineral suggest that it is PGE oxide/hydroxide like those reported from the Vourinos (Garuti and Zaccarini, 1997) and Oman ophiolite (Ahmed et al., 2002).

Palladium and iridium are the native metals found associated with composite PGM grains in the CED chromitite. Native palladium contains appreciable amounts of Rh, Pt, As and S; 8.20, 4.42, 6.18 and 2.59 wt.%, respectively (Table 3). It has an average stoichiometry of (Pd0.73Rh0.08Pt0.02As0.09S0.08), comparable to native palladium reported byCabri (2002).

Native iridium also contains appreciable amounts of

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

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

Table 4

Representative 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 1.27

Os 0.06 0.05 n.d 0.06 n.d 42.49 46.00 n.d n.d n.d

Ir 53.59 53.09 0.02 43.49 34.01 1.49 1.50 n.d n.d n.d

Ru 3.03 3.33 0.02 2.16 3.58 15.81 13.39 n.d n.d n.d

Rh 3.10 3.46 39.26 12.40 21.27 0.29 0.20 1.93 0.84 3.63

Pt 0.60 0.45 5.53 0.18 0.25 n.d n.d 47.40 30.05 52.57

Pd n.d 0.19 13.69 n.d n.d n.d 0.12 11.54 38.38 0.74

As 18.65 19.44 25.15 21.79 21.88 24.58 22.49 30.09 24.70 33.13

Ni n.d 0.09 0.16 0.05 n.d 0.32 0.25 0.04 0.07 0.02

Cu 0.13 n.d n.d n.d n.d n.d n.d n.d n.d n.d

Fe 0.42 0.55 0.88 0.54 0.39 0.67 0.68 1.13 1.65 0.96

Cr 0.69 1.29 1.26 1.09 0.54 1.45 1.38 1.72 1.62 1.65

Sb 0.04 0.06 1.22 n.d 0.10 0.05 0.50 2.58 n.d 2.45

Bi 4.99 4.45 0.10 3.08 2.86 n.d n.d 2.55 1.46 2.61

Total 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 4.68

Os 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 4.18

Pt 0.31 0.22 2.19 0.08 0.11 – – 28.72 17.10 31.95

Pd 0.00 0.17 9.95 0.00 0.00 – 0.10 12.82 40.05 0.83

As 25.04 24.84 25.95 26.32 24.54 28.76 26.87 47.47 36.61 52.44

Ni – 0.15 0.21 0.08 – 0.47 0.39 0.07 0.12 0.03

Cu 0.20 – – – – – – – – –

Fe 0.75 0.95 1.21 0.88 0.59 1.06 1.08 2.39 3.28 2.03

Sb 0.03 0.05 0.77 – 0.01 0.36 0.48 2.50 0.00 2.38

Bi 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 from Wadi El-Lawi area (seeFig. 2for the location).

12

ARTICLE IN PRESS

(13)

4.24 wt.% (Table 2,Fig. 8). The chemical composition of “Osmium” in the SED chromitites ranges from (Os0.54Ir0.39Ru0.07) to (Os0.71Ir0.21Ru0.08); the average stoichiometry is (Os0.60Ir0.33Ru0.07). The “Ruthenium”

in the SED chromitite contains a relatively high amount of Rh (3.64 wt.% on average;Table 2), with an average stoichiometry of (Ru0.74Os0.11Ir0.09Rh0.04). The Ru content of “Ruthenium” in the SED chromitites is remarkably 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 sulfarsenides found in the CED chromitites. Irarsite and hollingworthite have a relatively wide compositional range along the IrAsS–RhAsS join of the RhAsS–OsAsS–IrAsS triangle (Fig. 10). The irarsite richest in Ir contains ca. 1.36 wt.%

of Rh. Irarsite has appreciable amounts of Bi and Ru, up to 5.34 and 13.22 wt.%, respectively (Table 4,Fig. 10). It has the general formula (Ir0.93 Ru0.06Rh0.04Fe0.06)_Σ= 1.09

(As0.76Bi0.08)_Σ= 0.84S1.06. Substitution of Ir by Rh in irarsite reaches 21.27 wt.% Rh, and the mineral contains ca. 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 contents and low Ir content (13.69, 5.53 and 0.02 wt.%, respectively;Table 4). It contains minor amounts of Sb, Fe and Bi (ca. 1.22, 0.88 and 0.09 wt.%, respectively; Table 4), giving the average formula (Rh0.90

Pd0.29Pt0.07Fe0.04)_Σ= 1.30As0.79S0.91. Osarsite in the CED chromitite is found as alteration products of Os-rich laurite. It has appreciable amount of Ru (14.16 wt.% on average;Table 4,Fig. 9), giving the general formula of (Os0. 6 3Ru0 . 3 9Fe0 . 03Ir0 . 02Ni0 .0 1)_Σ= 1 . 08As0 . 84S1 .0 8. Arsenides in the CED chromitites are represented by unnamed Pt–PdAs and Pd–PtAs minerals and sperrylite (PtAs2), as parts of composite PGM grains. There is a solid solution series in the (Pt–Pd)–As composition from Pt-rich to Pd-rich varieties (Table 4). The Pt content varies from 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 and Rh ( Table 4). The Pt-rich arsenide has the general chemical formula (Pt0.59Pd0.26Rh0.05Fe0.05)_Σ= 0.95

(As0.97Sb0.05Bi0.03)_Σ= 1.05. The most Pd-rich arsenide 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, Bi and 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 reported from various localities (Cabri, 2002).

No PGE arsenide phases were found in the SED chromitites, and the PGE sulfarsenides are entirely located along the OsAsS–IrAsS join of the RhAsS–

OsAsS–IrAsS–RuAsS tetrahedron (Fig. 10). There is a complete solid solution series between osarsite and irarsite end members (Fig. 10). The most Os-rich osarsite has ca. 1.59 wt.% Ir, with the general chemical formula (Os0.96Ir0.02)_Σ= 0.98As0.91S1.11, whereas the irarsite richest in Ir contains ca. 4.27 wt.% Os and minor amounts of Ni and Fe (Table 5), giving the average chemical formula (Ir0.77Ni0.12Fe0.09Os0.06)_Σ= 1.06As0.72S1.22. Except for a few analyses, the osarsite–irarsite series of the SED chromitites have a remarkably low Ru content, ca.

1.37 wt.% on average, compared with sulfarsenides in the CED chromitites which have much higher Ru contents (ca. 3.35 wt.% on average (Fig. 10). This may

Table 5

Representative microprobe analyses of PGE sulfarsenides in the SED chromitites 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 14.07 Os 28.28 36.13 50.89 60.40 29.07 12.32 6.57 4.27 Ir 23.55 14.17 8.67 1.59 30.2 46.12 52.10 52.60

Ru 8.28 8.46 0.82 n.d 0.57 0.48 1.74 0.88

Rh 0.21 0.17 0.11 0.03 0.28 0.20 0.45 0.46

Pt n.d n.d n.d n.d n.d n.d 0.03 0.24

Pd n.d n.d n.d 0.12 n.d 0.60 n.d 0.01

As 24.39 24.57 23.16 22.92 21.88 22.70 20.84 19.47 Ni 0.13 0.18 0.14 0.04 0.32 0.32 0.48 2.56

Cu n.d n.d n.d n.d n.d n.d n.d 0.14

Fe 0.95 0.79 0.90 0.92 0.86 1.11 0.99 1.70 Cr 1.46 1.13 1.51 1.41 1.15 2.57 2.11 1.64 Total 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 40.42 Os 13.37 17.30 26.14 31.04 14.98 6.13 3.30 2.07 Ir 11.01 6.71 4.40 0.81 15.39 22.68 25.83 25.21

Ru 7.36 7.61 0.79 – 0.55 0.45 1.64 0.80

Rh 0.18 0.15 0.11 0.03 0.27 0.18 0.42 0.41

Pt – – – – – – 0.01 0.11

Pd – – – 0.11 – 0.54 – 0.01

As 29.26 29.83 30.17 29.87 28.59 28.63 26.51 23.94 Ni 0.20 0.28 0.23 0.07 0.53 0.51 0.77 4.02

Cu – – – – – – – 0.20

Fe 1.52 1.28 1.58 1.61 1.51 1.88 1.69 2.81 Sample UT65 is from Um Thagar area (seeFig. 2for the location). n.d:

not detected.

13

ARTICLE IN PRESS

(14)

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

6.4. Base metal sulfides

Millerite (NiS) and heazlewoodite (Ni3S2) are the main base metal sulfides (BMS) identified in the Proterozoic ophiolitic chromitites of Egypt. Subordinate amounts of pentlandite (Fe,Ni)9S8, pyrite and chalcopyrite are also found associated with composite PGM grains. The BMS, in general, have very low PGE contents, usually b2 wt.% PGE ( Table 6). Millerite has variable Fe contents ranging from 0.82 up to 8.01 wt.%, with an average of 2.27 wt.% and an average chemical composition of (Ni0.91Fe0.04)_Σ= 0.95S1.05. The Fe content of heazlewoodite is usually b1 wt.%

(Table 6), giving the general chemical formula (Ni2.93

Fe0.03)_Σ= 2.96S2.04. Pentlandite has up to 18.21 wt.% Fe (Table 6), with the average chemical formula (Ni5.93

Fe2.75)_Σ= 8.68S8.32. Pyrite has up to 4.50 wt.% Ni

(Table 6), giving the average chemical composition (Fe0.91Ni0.10)_Σ= 1.01S1.99.

7. Discussion

7.1. Laurite vs. IPGE alloys: stability conditions In general, the highly siderophile elements (PGE, Au and Re) show good correlations with Cr and Ni which, in turn, mainly controlled by spinel and/or olivine in many of the 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 is considered as one of the main controllers of behavior of the highly siderophile elements; there is a strong positive correlation between Cr content of spinel and PGE contents of the rock (e.g., Crocket, 1979; Economou, 1986; Leblanc, 1991; Ahmed and Arai, 2003). This suggests co-precipitation of chromian spinel and PGM in

Table 6

Representative 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 34.01

Os 1.52 n.d n.d n.d n.d n.d n.d

Ir 0.16 n.d n.d 0.06 n.d n.d n.d

Ru n.d n.d n.d 0.12 n.d n.d 0.03

Rh n.d n.d n.d n.d n.d 0.07 0.09

Pt n.d 0.04 n.d n.d 0.01 n.d n.d

Pd 0.12 0.01 0.15 0.01 n.d n.d 0.11

As 0.06 0.02 n.d n.d 0.03 0.12 n.d

Ni 49.64 58.35 68.67 69.86 41.29 4.31 0.01

Cu 6.08 0.10 0.34 n.d 0.20 0.19 28.86

Fe 1.44 1.72 0.62 0.67 18.21 37.58 29.57

Cr 3.28 4.22 1.97 2.30 4.90 6.18 2.75

Total 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 51.83

Os 0.36 – – – – – –

Ir 0.04 – – 0.02 – – –

Ru – – – 0.06 – – 0.01

Rh – – – – – 0.03 0.04

Pt – 0.01 – – – – –

Pd 0.05 – 0.07 – – – 0.05

As 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 22.19

Fe 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 (seeFig. 2 for the locations). n.d: not detected.

14