Platinum-group elements geochemistry in podiform chromitites and associated peridotites of the Mawat ophiolite, northeastern Iraq

Journal of Asian Earth Sciences 37 (2010) 31–41

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Platinum-group elements geochemistry in podiform chromitites and associated peridotites of the Mawat ophiolite, northeastern Iraq Sabah A. Ismail a,*, Tola M. Mirza b, Paul F. Carr c a

Applied Geology Department, University of Kirkuk, Kirkuk, Iraq Geology Department, University of Sulaimania, Sulaimania, Iraq c School of Earth and Environmental Sciences, University of Wollongong, Wollongong, Australia b

a r t i c l e

i n f o

Article history: Received 7 October 2008 Received in revised form 3 July 2009 Accepted 9 July 2009

Keywords: PGE Chromitite Ophiolite Mawat Zagros Iraq

a b s t r a c t Small lensoidal bodies of massive and disseminated chromitites have been examined in association with ultramafic rocks of the Mawat ophiolite complex (MOC), Iraqi Zagros Thrust Zone, northeastern Iraq. The chromitites are surrounded by dunite envelopes of variable thickness, exhibiting transitional boundaries to harzburgitic host rocks. The primary chromite composition exhibits high Cr varieties; the average Cr# of chromite is 0.73, and have <0.2% TiO2 content, which may reflect the crystallization of chromite from boninitic magma. Partial melting in the upper mantle and assimilation of wall rocks by primitive melt might have played an essential role in the chromitite formation in the studied area. Unusually high platinum-group elements contents of up to 1094 ppb have been reported for the first time in these chromitites pods. Chondrite normalized patterns of platinum-group elements are typical for ophiolitic chromitite, with enrichment of the IPGE, and depletion in PPGE indicating a high degree of partial melting of the mantle source. The PGE diversity between different chromitite pods in the MOC may be attributed to the differences in chemistry of the magma involving in the chromitite formation. The mineral chemistry data and PGE geochemistry, indicates that the Mawat ophiolite was generated from an arc-related magma above a supra-subduction zone setting. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The origin of podiform chromitites, particularly their content of platinum-group elements and tectonic setting of emplacement, is a current topic of debate (Arai and Yurimoto, 1994; Zhou et al., 1998; Uysal et al., 2007; Alapieti and Latypov, 2008). The mineral chemistry of primary chromite and the associated interstitial primary silicate minerals, as well as their order of crystallization reflects the composition of the parental melt (Rollinson, 2008). Chromite compositions are generally believed to be an indicator of magma affinity (Melcher et al., 1997); high-Cr chromitite [Cr#>0.60 where Cr# is the atomic ratio Cr/(Cr + Al)] crystallizing from boninitic compositions, whereas high-Al chromitite (Cr#<0.60) indicates derivation from MORB-like tholeiitic magmas (Zhou et al. 1994). The platinum-group elements (PGE: Pt, Pd, Rh, Ir, Ru and Os) display a wide range of chemical and mineralogical similarities. The PGE are considered to be potential geochemical indicators of processes involving material transfer from the mantle to the crust (Naldrett, 1981; Garuti et al., 1997). Because of the different degrees of S-saturation and PGE contents of boninitic and tholeiitic magmas, PGE concentrations also provide useful information about * Corresponding author. Tel.: +964 7705123017; fax: +964 50218288. E-mail address: [email protected] (S.A. Ismail). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.07.005

chromitites and their genesis (Hamlyn et al., 1985). High-Cr chromitite PGE patterns are the result of interaction of S-undersaturated boninitic magma with harzburgites depleted by prior partial melting, whereas the high-Al chromitite PGE patterns reflect interaction between initially S-saturated tholeiitic magmas and depleted harzburgites (Zhou et al., 1998; Ahmed and Arai, 2002). The Mawat Ophiolite Complex (MOC), located in the Kurdistan Region 30 km northeast of Sulaimani city and 5 km north of Chwarta, is one of the best exposed fragments of oceanic lithosphere in the Iraqi Zagros Thrust Zone (IZTZ) of northeastern Iraq (Fig. 1). This rock complex contains a significant number of podiform chromitite bodies forming small lenses and irregular bodies within slightly serpentinized dunite and harzburgite located mainly in the central parts of the Ser-Shiw Valley, 2 km north of Kuradawi village. The only publication dealing with platinumgroup minerals (PGM) in Iraq is relates to the chromitite deposits of the Rayat area, IZTZ, northeastern Iraq (Ismail, 2008). The current study provides the first comprehensive description of PGE and their distribution in podiform chromitite and associated ultramafic rocks from the MOC. It has two main goals: the first one is to utilize the mineralogy and mineral chemistry of the Mawat chromitite and the associated peridotites to try determining the origin and tectonic setting of chromitites, and also to characterize the mantle section in the IZTZ. The second goal is to study the distribu-

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S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

Fig. 1. Tectonic subdivision of Iraq after Numan (2001), showing the location of study area.

tion of PGE within chromitite, dunite and harzburgite to provide further insights into the history of the mantle section in the IZTZ.

2. Geological setting The geological setting of the study area, as discussed in this paper is based on Mirza (2008) and unpublished reports. The IZTZ forms part of the larger Zagros Belt which extends 2000 km from southeastern Turkey through northern Syria and Iraq to western and southern Iran and proposed to have result from the opening and subduction of the Neo-Tethyan oceanic and sub sequent oblique collision of Afro-Arabian (Gondwana) with the Iranian microcontinent in the late Cretaceous-Early Tertiary (Jassim

and Goff, 2006). Fragments of mafic-ultramafic associations representing parts of a dismembered ophiolite complex are abundant in the IZTZ. The studied area is one of the small ophiolitic complexes including the Rayat, Qalander, Bulfat, Mawat and Penjwen bodies, emplaced as parallel bodies within the Zagros Thrust Zone. The MOC crops out over 250 km2 as a triangular, elevated area with significant topographic relief within the larger Zagros Thrust Zone (Fig. 2). The MOC consists of a thick sequence of slightly serpentinized ultramafic and predominantly mafic rocks (gabbros capped by volcanic rocks), associated with minor silicic, intermediate and mafic intrusions (Mirza and Ismail, 2007). These ultramafic rocks consist mainly of harzburgite and dunite, with less lherzolite, and minor pyroxenite and chromitite. Lens-shaped chromitite pods are enclosed in a dunite envelope and both rock types are sur-

S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

33

Fig. 2. Geological map of the Mawat ophiolite complex, northeastern Iraq, modified from Hama-Aziz (2008)

rounded by harzburgite. Cretaceous age was proposed for the MOC by stratigraphy (Al-Mehaidi, 1974) as well as radiometric age determination on its basaltic rocks indicate an age from 97 to 105 Ma (Aswad and Elias, 1988). The MOC is overlain by the volcanic and sedimentary Gimo group in the north, whereas in the east, west and south it is ‘‘sandwiched” between two main thrust sheets comprising the Walash and Naopurdan groups of Eocene age (AlMehaidi, 1974). Both the ophiolite and Gimo sequence have undergone green schist facies metamorphism (Jassim and Goff, 2006).

diffuse contacts with the enclosing dunite. Chromite grains are 0.5–3 mm in massive pods and 0.1–2 mm in disseminated types, dark brown in thin section, and subhedral to anhedral in shape. The interstitial matrix of chromite grains consists mainly of secondary chlorite and amphibole, together with rare, primary olivine. Chromite grains in these chromitite pods contain inclusions of silicate and PGE minerals. These silicate inclusions comprise chlorite and amphibole group minerals, which are usually linked to the matrix by irregular veinlets of these secondary minerals. Rare grains of primary olivine are preserved as inclusions within chromite.

3. Petrography 3.2. Peridotites 3.1. Chromitite Chromitite occurrences within the study area are mostly represented by small bodies of massive and disseminated ores with chromite content ranging from >75% (by vol.) in massive chromitites, to 40–60% in disseminated types. Dunite and harzburgite contain 4–10% and <4% of accessory chromite respectively. Chromitite pods in the MOC are lensoidal in shape (0.3–2 m across and 0.5–12 m long) with both relatively sharp, and in some cases,

The Mawat peridotites consist chiefly of harzburgite and rare occurrence of lherzolite. The primary harzburgite is plagioclasefree. Porphyroclastic textures are common with large, subhedral crystals of orthopyroxene (or bastite pseudomorphs) in a matrix of smaller (0.3–0.5 mm), anhedral olivine grains or serpentine and variable extents of serpentinization, contains ‘‘islands’’ of fresh olivine surrounded by serpentine. Clinopyroxene is partly altered to tremolite. Chromite grains are the least altered but many are

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S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

partly to completely replace by magnetite. Olivine and orthpyroxene contents in the studied harzburgites vary from 73 to 87 modal% and 10 to 26 modal%, respectively. Clinopyroxene content is in the 1–5 modal% range in the harzburgites. Chromite is ubiquitous and less than 4 modal%. 4. Analytical techniques Bulk-rock compositions were determined with a Phillips PW1400 X-ray fluorescence spectrometer. Fused glass discs were used for major oxides, whereas pressed powder pellets were used for trace element determinations. The accuracies of the XRF analyses are estimated to be ±1% (relative) for SiO2, ±2% for the other major oxides and ±5% (relative) for both minor oxides and trace elements. Detection limits for minor oxide (wt%): TiO2:0.02; MnO: 0.02; Na2O, K2O, P2O5: 0.05; detection limits for trace elements (ppm): Ni, Cr, Sc, V, Zn: 3; Cu: 7. The mineral chemistry of the studied chromitites and associated rocks was determined by Cameca SX100 electron probe

Table 1 Analytical result (ppb) of international reference standards. Standards

Os Ir Ru Rh Pt Pd

AMIS0007

HGMNEW

Control blank

Nominal

Result

Nominal

Result

Nominal

Result

67 93 450 250 2460 1510

67 97 474 274 2674 1570

1150 810 2650 900 2750 7600

1218 813 2744 885 2647 7386

<2 <2 <2 <1 <2 <2

<2 <2 <2 <1 3 4

microanalyser at the Department of Geology, Washington State University, using wavelength dispersive spectroscopy with ZAF data reduction procedure. Operating conditions were 20 kV accelerating voltage, 13 nA beam current; the beam diameter was 5 lm. The counting time was 10 s for Na, Al, Si, and Fe, 15 s for Mg, Mn, Ni, and 25 s for Ca, Ti, and Cr. The standards used were olivine (Mg, Si), albite (Al), wollastonite (Ca), ilmenite (Fe, Ti), chromite (Cr), and V, Mn, Ni and Zn metal. Chromite Fe3+ content was calculated on the basis of 24 cations using the charge balance equation of Droop (1987). A total of 11 chromitite and associated dunite and harzburgite samples were selected from the Ser-Shiw valley for PGE (Os, Ir, Ru, Rh, Pt, Pd) and Au analyses. The very low PGE and Au contents impose the use of a combined procedure of pre-concentration and matrix elimination prior to the detection with ICP-MS. This was achieved by fire assay with nickel sulfide collection (mixing the sample with mixture of soda ash, borax, silica sulphur and nickel carbonate or nickel oxide). After pre concentration the sample solution were analyzed using Perkin-Elmer Sciex ELAN 9000 ICPMS at the Genalysis Laboratory Services Pty. Ltd. at Maddington, Western Australia. Corrections are made, where applicable, for isobaric isotopic interferences and polyatomic interferences. Laboratory standards were used for instrument calibration and drift correction. Analytical accuracy and precision were routinely checked using international standards, including AMIS0007 and HGMNEW, and by analyzing blanks and duplicates. The precision for all PGEs is better than 10% (Table 1). Based on reference samples analyzed during several years in the lab recovery was estimated to be better than 85%, which is in the range of the efficiency usually obtained by NiS fire assay. Detection limits were 2 ppb for Os, Ir, Ru, Pt and Pd; 1 ppb for Rh, and 5 ppb for Au.

Table 2 Representative microprobe analysis of chromite in podiform chromitite from the Mawat ophiolite complex, northeastern Iraq. The chromite composition used as indicators of melt composition. Structural formulae are based on 24 cations and Fe3+ was calculated using the charge balance equation of Droop (1987). Cr# = Cr/ (Cr+Al), Mg# = Mg/ (Mg+Fe+2), Fe3+# = Fe3+/(Fe3++Al+Cr). Sample No. Rock type

W19 Massive

W25 Massive

W28 Massive

W30 Massive

W33 Massive

W29 Disseminate

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total

0.04 0.16 13.63 50.16 25.64 0.51 8.04 0.00 0.03 0.01 0.04 98.26

0.02 0.16 14.25 52.64 18.35 0.40 11.70 0.01 0.07 0.01 0.07 97.68

0.05 0.24 13.66 52.28 21.38 0.34 10.45 0.01 0.01 0.01 0.06 98.43

0.04 0.19 14.35 51.98 20.46 0.39 11.02 0.01 0.01 0.01 0.09 98.55

0.01 0.09 12.62 51.81 25.37 0.38 7.68 0 0.03 0.01 0.02 98.02

0.04 0.14 11.71 53.43 24.90 0.36 7.44 0.00 0.01 0.00 0.03 98.06

Recalculated to 24.0 cations Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K Ni Mg# Cr# Fe3+#

0.011 0.032 4.295 10.602 1.017 4.715 0.115 3.205 0.000 0.002 0.002 0.008 0.427 0.712 0.088

0.005 0.031 4.393 10.887 0.646 3.369 0.089 4.563 0.000 0.003 0.000 0.014 0.602 0.712 0.062

0.013 0.047 4.223 10.843 0.812 3.878 0.076 4.087 0.002 0.002 0.002 0.012 0.537 o.720 0.072

0.010 0.037 4.404 10.702 0.798 3.657 0.086 4.279 0.002 0.002 0.002 0.017 0.561 0.708 0.069

0.003 0.018 4.013 11.051 0.894 4.829 0.087 3.089 0.000 0.009 0.003 0.004 0.418 0.734 0.087

0.011 0.029 3.745 11.463 0.71273 4.938 0.083 3.010 0.000 0.003 0.000 0.006 0.415 0.754 0.085

12.81 0.26

12.59 0.36

12.85 0.30

12.18 0.17

11.79 0.23

Composition of parental melt TiO2 12.58 0.26 Al2O3

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S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

Fig. 4. Plot of Cr2O3 vs Al2O3 in chromite from the Mawat podiform chromitite. Compositional fields from Bonavia et al. (1993).

5. Mineral chemistry 5.1. Chromite Fig. 3. Plot of Cr# versus Mg# for chromite in chromitite, dunite, harzburgite and lherzolite. The Alpine-type field is from Irvine (1967) and other fields arefrom Zhou and Bai (1992).

Chromite shows little compositional variation from sample to sample within and between different chromitite pods. It has Cr2O3 and Al2O3 contents of 49.26–54.21 wt% and 9.79– 15.96 wt%, respectively and is best described as Cr-rich chromite (Table 2; Fig. 3) typical of podiform chromitite in alpine type peridotite (Fig. 4). The Cr2O3 contents equate to high Cr# values (mean

Table 3 Selected microprobe analyses of constituent minerals in chromitite, dunite and harzburgite rocks from the Mawat ophiolite complex northeastern Iraq. For chromitite spinel, see Table 1. FeO* total iron as FeO and Mg# = Mg/ (Mg+Fe). Rock

Chromitite

Minerals

Olivine

Amphibole

chlorite

Chromite

Olivine

Orthopyroxene

Amphibole

Chromite

Olivine

Orthopyroxene

Clinopyroxene

Amphibol

SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O NiO Total

40.43 0.01 0.02 0.01 8.66 0.02 50.57 0.00 0.00 0.02 0.34 100.08

57.89 0.05 0.77 0.28 0.85 0.01 23.91 13.48 0.15 0.03 0.12 97.54

36.22 0.06 21.12 2.66 2.24 0.00 37.34 0.02 0.01 0.08 0.32 100.06

0.01 0.16 14.72 48.67 26.53 0.39 7.72 0.02 0.00 0.02 0.07 98.31

40.09 0.01 0.01 0.02 7.83 0.13 50.85 0.02 0.04 0.15 0.29 99.44

49.19 0.00 0.17 0.02 3.59 0.10 46.22 0.41 0.02 0.03 0.29 100.04

57.16 0.07 0.60 0.41 1.63 0.05 23.60 13.08 0.06 0.00 0.07 96.73

0.06 0.02 21.10 44.66 23.60 0.38 8.82 0.00 0.04 0.00 0.04 98.72

40.60 0.02 0.01 0.01 9.46 0.15 48.97 0.01 0.04 0.02 0.28 99.57

49.98 0.1 0.99 0.44 7.58 0.05 40.02 0.09 0.02 0.03 0.15 99.45

53.5 0.06 1.40 0.45 2.87 0.12 17.41 24.53 0.08 0.01 0.06 100.49

58.47 0.01 0.25 0.07 6.62 0.16 30.53 0.34 0.12 0.00 0.06 96.60

Formula Si Ti Al Cr Fe Mn Mg Ca Na K Ni Total Mg#

0.984 0.000 0.001 0 0.176 0.000 1.847 0.000 0.000 0.001 0.007 3.016 0.913

Dunite

7.896 0.005 0.123 0.03 0.097 0.001 4.864 1.971 0.041 0.005 0.013 16.039 0.98

5.864 0.007 4.018 0.404 0.303 0.002 9.068 0.003 0.003 0.017 0.03 19.721 0.968

0.000 0.004 0.574 1.273 0.735 0.011 0.380 0.001 0.000 0.001 0.002 2.981 0.674

Harzburgite

0.984 0.000 0.000 0.000 0.161 0.003 1.86 0.001 0.002 0.000 0.006 3.011 0.92

1.718 0.000 0.007 0.001 0.104 0.003 2.421 0.015 0.000 0.000 0.008 4.277 0.956

7.887 0.007 0.098 0.045 0.19 0.006 4.855 1.934 0.016 0.000 0.007 14.874 0.964

0.002 0.001 0.789 1.119 0.630 0.010 0.417 0.000 0.002 0.000 0.001 2.971 0.422

0.993 0.000 0.000 0.000 0.196 0.003 1.807 0.000 0.002 0.001 0.006 3.008 0.902

1.804 0.000 0.004 0.001 0.190 0.002 2.387 0.004 0.002 0.002 0.005 4.401 0.926

1.946 0.002 0.06 0.013 0.09 0.004 0.944 0.956 0.006 0.000 0.002 3.888 0.956

7.949 0.001 0.039 0.007 0.75 0.018 6.187 0.05 0.032 0.000 0.006 14.994 0.892

2560 2790 29 15 50 44 2330 2316 36 10 39 38 1666 3883 90 35 9 62

0.73, range 0.7–0.8, n = 45), whereas Fe# [Fe3+/(Fe3+ + Al3+ + Cr3+), atomic ratio] is mostly 0.1 which is extremely low. The TiO2 content is also low in the 0.1–0.25 wt% range. Similarly low Ti-chromites are common in ophiolitic peridotites (Dick and Bullen, 1984; Arai, 1992) which are characteristic of high-Cr# podiform chromitites (Dickey, 1975). Chromite from the MOC has appreciable amounts of MnO, (up to 0.6 wt%) and low NiO contents (<0.1). The minor elements Mn and Ni correlate positively with MgO contents.

2750 1636 15 5 <7 22

2844 15562 45 4 8 82

2362 2965 31 7 <7 46

2594 3110 41 9 <7 50

2335 2768 40 10 <7 45

2526 2556 27 8 <7 44

2911 3510 32 8 32 51

2511 3003 30 15 18 48

2610 3011 44 10 17 60

2493 2833 38 9 8 47

2594 2153 21 8 4 40

1701 4231 75 18 8 72

5.2. Olivine Olivine compositions from the MOC correlate broadly with rock type ranging from Fo91 to Fo95 in chromitite, Fo90 to Fo92 in dunite, and Fo89 to Fo92 in harzburgite (Table 3). Similarly, the NiO and MnO contents of olivine correlate with Fo content and have overall ranges of 0.23–0.38 wt% and 0.02–0.17 wt%, respectively. On the basis of the Ballhaus et al. (1991) equation for Fe–Mg exchange between olivine and spinel, the compositional data for minerals from the MOC suggest re-equilibration temperature of 656–830 °C for chromitites, 639 °C and 606 °C for harzburgite and dunite, respectively. All these temperatures are substantially below mantle temperatures and indicate significant Fe–Mg equilibration during cooling and small olivine grains do not preserve their original composition. 5.3. Pyroxene Orthopyroxene is a minor interstitial phase in the dunite and harzburgite samples where most of the grains were altered to chlorite and amphibole. The Mg# values for orthopyroxene in dunite and harzburgite have a range of 0.95–0.97 and 0.90–0.93, respectively, whereas the Al2O3 content of orthopyroxene in dunite has a very limited range (0.16–0.17 wt%), but is much more variable in harzburgite (0.05–1.63 wt%). Orthopyroxene grains have low Cr-contents and plot in the enstatite field. Clinopyroxene occurs as a minor phase in dunite and harzburgite and has Mg# of 0.90–0.96. This variation in Mg# is related to the degree of serpentinization with strongly serpentinized samples having higher Mg#. All clinopyroxene grains plot in the diopside field and have high Al2O3 and low Na2O contents (Table 3). 5.4. Amphibole Amphibole as alteration product of pyroxene represents a minor interstitial phase in ultramafic rocks of MOC. All analyzed grains are Mg-rich tremolite and actinolite, with low Cr2O3 and TiO2 contents (<1 and 60.1 wt%, respectively).

6. Whole-rock

2798 3005 26 7 16 46

2575 3053 20 6 <7 30

6.1. Geochemistry

Trace elements (ppm) Ni (ppm) 3002 Cr 3211 V 35 Sc 11 Cu 17 Zn 47

43.11 <0.02 1.67 7.32 0.11 40.83 2.41 0.06 4.39 99.91 84.79 0.95 41.55 <0.02 1.55 8.21 0.13 39.59 2.46 0.07 6.55 100.12 82.82 0.95 42.88 0.03 1.2 11.52 0.16 34.92 3.33 0.07 5.55 99.68 75.19 0.81 40.53 0.02 0.91 12.01 0.16 36.87 3.96 0.1 4.52 99.09 75.42 0.91 42.27 <0.02 0.41 6.88 0.11 43.72 0.85 0.07 4.78 99.11 86.40 1.03 40.64 0.02 0.56 8.83 0.13 41.38 0.34 0.11 6.99 99.01 84.07 1.02 45.12 <0.02 0.70 7.89 0.12 39.83 1.37 0.1 4.87 100 83.46 0.88 42.93 <0.02 0.61 8.84 0.13 40.59 1.30 0.06 5.51 99.98 82.12 0.95 42.32 <0.02 0.36 7.77 0.12 41.39 1.12 0.06 5.95 99.12 84.19 0.98 41.48 <0.02 0.56 8.11 0.13 43.54 0.58 0.08 4.87 99.37 84.30 1.05 41.35 <0.02 0.66 7.34 0.12 41.34 0.81 0.08 7.54 99.25 84.92 0.99 41.73 <0.02 0.43 8.01 0.13 41.01 1.32 0.08 6.32 99.04 83.65 0.98 40.60 <0.02 0.39 7.58 0.12 40.11 1.17 0.07 8.95 99.01 84.11 0.99 38.34 <0.02 0.64 7.28 0.13 44.68 1.07 0.08 5.58 97.80 85.99 1.17 39.62 <0.02 0.12 7.71 0.12 45.32 2.06 0.08 4.12 99.16 85.46 1.14 39.27 <0.02 0.28 7.97 0.13 44.99 2.30 0.08 4.43 99.46 84.95 1.15 39.5 <0.02 0.31 6.90 0.13 45.40 0.47 0.05 6.46 99.23 86.80 1.15 Major elements (wt%) 39.21 SiO2 <0.02 TiO2 0.40 Al2O3 8.09 Fe2O3 MnO 0.14 MgO 45.34 CaO 0.54 0.09 Na2O LOI 5.33 Total 99.16 Mg# 84.85 Mg/Si 1.16

W17 Dun.

W20 Dun.

W21 Dun.

W23 Dun.

W26 Dun.

R7 Hzb.

R8 Hzb.

W12 Hzb.

W14 Hzb.

W35 Hzb.

D23 Hzb.

D34 Hzb.

K7 Hzb.

K9 Hzb.

K4 Lhe.

K5 Lhe.

D32 Lhe.

D33 Lhe.

S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

Sample No. Rock type

Table 4 Representative major and trace elements of dunite and peridotites from the Mawat ophiolite complex, northeastern Iraq. Mg# = 100 * Mg/(Mg + Fe) (using mol.%). Dun., Hzb. and Lhe. are dunite, harzburgite and lherzolite respectively.

36

Major and trace element compositions of the analyzed dunite and peridotites are listed in Table 4. The peridotites have been serpentinized to various degrees as indicated by their loss on ignition (LOI). However, despite the different degree of alteration, the major element concentrations do not appear to have been greatly modified and the major oxide contents are compatible with the mineralogical compositions. The harzburgites have the highest MgO contents (average 41.4 wt%) (Mg# = 84), but relatively low CaO (average 0.98 wt%), Al2O3 (average 0.52 wt%) and characterized by very low TiO2 (<0.02 which is below the detection limits of XRF). The lherzolite has slightly lower MgO (average 38 wt%) and Mg# (80) and relatively high CaO (average 3.0 wt%), Al2O3 (average

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S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

Table 5 Whole-rock platinum-group element contents (ppb) of representative samples from Mawat ophiolite complex, northeastern Iraq. AMIS0007 and HGMNEW standard were used to ensure accuracy. Mass. = massive chromitite rock, Diss. = disseminate chromitite rock, Dun. = dunite rock, Harz. = harzburgite rock. Sample No. Rock

W19 Mass.

W25 Mass.

W28 Mass.

W30 Mass.

W31 Mass.

W33 Mass.

W29 Diss.

W26 Dun.

W21 Dun.

W36 Harzb.

Q20 Harzb.

Os Ir Ru Rh Pt Pd Au P PGE P P IPGE/ PPGE Pd/Ir Ru/Pt P P PPGE/ IPGE

54 557 342 17 89 35 <5 1094 6.75 0.06 3.84 0.15

61 110 190 13 10 6 <5 390 12.44 0.05 19 0.08

68 91 172 19 54 46 <5 450 2.78 0.5 3.18 0.39

67 75 172 13 20 21 <5 368 5.81 0.28 8.6 0.17

25 43 106 9 6 6 <5 195 8.28 0.14 17.6 0.12

26 46 104 10 8 17 <5 211 5.03 0.37 13 0.19

10 12 38 4 5 9 <5 78 3.33 0.75 7.6 0.3

2 3 9 2 5 10 <5 31 0.82 3.33 1.8 1.21

5 6 11 2 12 14 <5 50 0.78 2.33 0.9 1.27

6 6 12 4 16 17 6 61 0.64 2.83 0.7 1.54

12 10 23 5 40 21 15 111 0.68 2.1 0.6 1.46

1.3 wt%) and TiO2 (average 0.02 wt%). The majority of the Mawat peridotites have MgO similar to those of mantle peridotites. High field strength element concentrations are all close to or below the detection limits of the XRF procedure. Low contents of highfield strength elements reveal strong analogies with peridotites from supra-subduction zone ophiolites (Orberger et al., 1995) 6.2. PGE contents PGE contents and distribution patterns are widely variable in rocks from the MOC with total PGE contents of 195–1094 ppb for massive chromitite, and 31–111 ppb in disseminated and accessory chromitite (Table 5). The chondrite normalized PGE patterns of chromitite samples, both massive and disseminated ores (Fig. 5) are characterized by enrichment in IPGE (Os, Ir, Ru) relative to PPGE (Rh, Pt, Pd) (IPGE/PPGE = 2.8–12.4). The host dunite and harzburgite samples from the MOC are highly depleted in PGE contents compared to both chondrites and associated chromitite and have relatively flat patterns (IPGE/PPGE = 0.6–0.8) at approximately 0.01 times chondrite (Fig. 6). The relative enrichment of IPGE in chromitite samples is also reflected by slightly negative slopes from Ru to Pt (Ru/Pt = 3.2–19). These patterns and the low PGE abundances are typical of ophiolitic chromitites elsewhere (Page and Talkington, 1984; Ahmed and Arai, 2002; Proenza et al., 2007; Uysal et al., 2007; Chen and Xin 2008). The Pd/Ir value which is an indicator of PGE fractionation (Naldrett et al., 1979) ranges from 0.05 to 0.75 in chromitite of MOC, and has a negative correlation with the total PGE content (Fig. 7). PGE-sulfides form the main PGM found in the MOC chromitites, comprising mainly laurite [RuS2] and rare iridium disulfide. The Au content for all chromitite and dunite samples is below the detection limit (<5 ppb), whereas Au contents of the two harzburgite samples are 6 ppb and 15 ppb.

Fig. 5. Chondrite-normalized PGE patterns for podiform chromitites samples from the Mawat ophiolite complex. Chondrite values from Naldrett and Duke (1980; cf. 514, 540, 690, 200, 1020 and 545 for Os, Ir, Ru, Rh, Pt and Pd, respectively).

Fig. 6. Chondrite-normalized PGE patterns for dunite and harzburgite from the Mawa ophiolite complex. Chondrite values from Naldrett and Duke (1980).

7. Tectonic and genetic implications The field relationships together with the petrographic and geochemical characteristics of chromitite and associated ultramafic rocks of the MOC can be used to constrain the tectonic setting of this part of the Iraqi Zagros Thrust Zone. The tectonic setting of chromitite emplacement and the mechanism of chromite concentration are still a subject of considerable debate. The island-arcs and back-arcs settings are the most widely-accepted settings for the formation of podiform chromitites (Arai and Yurimoto, 1994; Zhou et al., 1998; Ballhaus, 1998; Bédard and Hébert, 1998; Rollinson, 2005, 2008; Uysal et al., 2007). In these settings, they form by the interaction of upper mantle lithologies with migrating magmas produced at greater depth (Arai and Yurimoto, 1994; Zhou et al.,

Fig. 7. PGE content versus Pd/Ir ratio of podiform chromitite from Mawat ophiolite complex, northeastern Iraq.

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Fig. 8. The relation between 100 Cr / (Cr + Al) of chromite and Al2O3 wt% of orthopyroxen from harzburgite rocks (MOC). The field from Bonatti and Michael (1989).

Fig. 9. Relationship between (Fe3+/(Fe3+ + Al + Cr) atomic ratio and TiO2 wt% of chromite from the Mawat ophiolite complex. The discrimination boundaries of spinel compositions of MORB, Arc magma and intraplate magma are from Arai (1992).

1994; Rollinson, 2005). The MOC harzburgite is characterized by the high Cr# (>0.65) of spinel. According to the Bonatti and Michael (1989), the relation between Cr# of chromite and Al2O3 of orthopyroxen (Fig. 8) and the modal composition of harzburgite indicates the generation of MOC harzburgite from subduction tectonic environments. The chemical composition of chromite can be used as an indicator for the origin of magma and as a discriminate for different tectonic settings (Dick and Bullen, 1984; Kamenetsky et al., 2001). The high Cr# of almost all chromitites suggests crystallization from a boninitic magma (Johnson et al., 1985). Similarly, abundances of Al2O3 and TiO2 in magmatic spinel can be used to discriminate between different magma types, their tectonic affinities and mantle sources (Kamenetsky et al., 2001). The high Cr# (0.7–0.8) and low TiO2 content of chromite (<0.02 wt%) from the MOC suggest a supra-subduction tectonic setting possibly involving boninite or high-Mg arc tholeiite (Arai, 1992, 1994; Ahmed et al., 2001). In addition, the TiO2 and Fe# contents of chromite from chromitite deposits of the MOC are indicative of generation from an arc-related magma (Fig. 9) on the based on the discriminant diagram presented by Arai (1992).

Fig. 10. Plot of TiO2 versus Al2O3 in chromite from the Mawat ophiolite complex. Fields are after Kamenetsky et al. (2001). SSZ; Supra-subduction zone; LIP, large igneous province; MORB, mid-ocean ridge basalt; OIB, ocean island basalt.

Kamenetsky et al. (2001) have compiled a database of Al2O3 and TiO2 compositions of chromite and have identified fields with varying degrees of overlap that can be used to distinguish six different tectonic settings. On the basis of these parameters, chromite compositions for MOC samples plot in the arc and supra-subduction zone fields (Fig. 10). Rollinson (2008) building on the earlier observations of Kamenetsky et al. (2001) and Wasylenki et al. (2003) that the Al2O3 and TiO2 contents of chromite correlate with the contents of these components in the melt. He has computed the compositions of melts from which the chromite was crystallized. Using the Rollinson (2008) approach, the TiO2 and Al2O3 contents of the melts from which chromite was precipitated in chromitite, dunite, harzburgite and lherzolite from the MOC have been calculated. These derived melt compositions, when plotted on a Al2O3 versus TiO2 diagram together with fields for MORB (after Godard et al., 2006), melts of depleted mantle (after Schwab and Johnston, 2001; Wasylenki et al., 2003), and Oman boninites (after Ishikawa et al., 2002), provide constraints on the origin of the chromitite and related rocks (Fig. 11). As expected, the chromite compositions in dunite, harzburgite and lherzolite are indicative of precipitation from melts derived from depleted mantle. Chromitite samples from the MOC have low TiO2 and Al2O3 contents and are far-removed from the MORB field but show strong affinity with melts from depleted mantle and boninites (Fig. 11) and probably represent cumulates that formed deep in the mantle. Partial melting in the upper mantle and assimilation of wall rock by primitive melt have probably played an essential role in the chromitite formation in the MOC. The evidences for that are field relations between the chromitite and dunite envelope with associated harzburgite. Dunite containing accessory spinel intermediate in composition between those of harzburgite and chromitite (Table 3) and Cr/Al ratio of the chromite in harzburgite increased toward the chromitite bodies (Cr/Al in harzburgite 2.1, dunite 3.3 and 3.8 in chromitite). Three processes comprising partial melting, crystal fractionation and alteration usually control the content of PGE in igneous rocks (Barnes et al., 1988). Gold and Pt are more readily mobilized than the other PGE during alteration process, and Pt may be mobilized by hydrothermal fluids (Stumpfl 1986). Since there is no clear evidence for hydrothermal alteration in the study area, crystal fractionation and partial melting seem to be the main factors controlling the distribution and fractionation of PGE in the MOC. Despite the major variations in PGE concentrations in chromitites from the MOC (Table 5), all massive and disseminated samples are distinctly

S.A. Ismail et al. / Journal of Asian Earth Sciences 37 (2010) 31–41

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Fig. 11. TiO2 vs. Al2O3 plot showing the field of the main MORB array, the diagram and parental melt calculations from Rollinson (2008), black diamonds are the composition of parental melts to the Oman low Cr# chromitites and white diamonds are parental melts composition of the Oman high Cr# chromitite. Gray triangles are the calculated composition of the parental melts of Mawat chromitite. Geotimes lava and the Lasail and Alley lava (after Godard et al., 2006). Field of the Oman boninites (data from Ishikawa et al. (2002)) and melts of depleted mantle (DM) from Schwab and Johnston (2001) and Wasylenki et al. (2003).

enriched in the IPGE subgroup (Os, Ir, Ru) in preference to the PPGE subgroup (Rh, Pt, Pd). The elements of the IPGE subgroup have higher melting points than Pt and Pd, and tend to be concentrated in refractory residue and in early cumulate relative to Pt and Pd which are more incompatible and tend to be retained in the melt (Barnes et al., 1985; Edwards 1990; Prichard et al., 1996). Chromitite samples from the MOC have a wide range in Pd/Ir values (0.06– 0.75; Table 5) which reflect variations in the amounts of partial

melting rather than magmatic fractionation (Fig. 12). A high degree of partial melting in the upper mantle produced exotic PGE-rich magma at higher pressure which moved upward and precipitated chromite rich in PGEs, and inevitably interacted incongruently with the harzburgite of the mantle wall to produce dunite and SiO2-rich secondary melt. This secondary magma could blend with the next inflow of relatively primitive melt and enter the primary spinel field (Irvine, 1977). Contamination of this evolved magma by crustal material to various degrees (Bédard and Hébert 1998) or changes in the thermodynamic conditions (pressure and temperature) of magma produced PGE-depleted chromitite. 8. Conclusions

Fig. 12. Plot of Pt/Pt* (PtN/(RhN *PdN) versus Pd/Ir in chromitite for the Mawat ophiolite complex. Fractionation and partial melting trends from Garuti et al. (1997).

(1) Podiform chromitite bodies in the MOC are of alpine type, and are enclosed in dunite which, in turn, is surrounded by harzburgite. The interstitial matrix of chromitite consists mainly of secondary chlorite and amphibole, together with rare, primary olivine. Chromite grains in these chromitite pods contain inclusions of chlorite, amphibole and PGE minerals. (2) Chemical data for chromite indicates little compositional variation from sample to sample between grains and between samples both within and between chromitite pods. The high Cr# (0.73) and low TiO2 content of spinel from the MOC suggests a supra-subduction tectonic setting which possibly has genetic linkage with some boninite or highMg arc tholeiite.

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(3) The PGE patterns for all chromitite pods display negative slopes from Ru to Pt and enrichment in IPGE relative to of PPGE. This is typical of an ophiolite PGE pattern. The dunite and harzburgite samples are highly depleted in PGE contents relative to both chondrites and associated chromitite. The PGE normalized patterns of dunite and harzburgite from MOC are relatively flat. (4) The PGE diversity between different chromitite pods in the MOC, may be attributed to the differences in the chemistry of the magma involving in the formation of chromitites.

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