mineralogical and geochemical characterization of listwaenite ... -...

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Chemie der Erde 67 (2007) 213–228www.elsevier.de/chemer

Mineralogical and geochemical characterization of listwaenite from the

Semail Ophiolite, Oman

Sobhi Nasira,�, Abdul Razak Al Sayigha, Abdulrahman Al Harthya, Salah Al-Khirbasha,Omar Al-Jaaidia, Abdullah Musllamb, Ali Al-Mishwatc, Salim Al-Bu’saidid

aDepartment of Earth Sciences, Sultan Qaboos University, Al-Khod, OmanbDepartment of Geology, United Arab Emirates University, Al-Ain, UAEcDepartment of Earth and Environmental Sciences, Kuwait University, KuwaitdDirectorate General of Mineral, Ministry of Commerce and Industry, Oman

Received 12 March 2004; accepted 5 January 2005

Abstract

The late Cretaceous-lower Tertiary hydrothermal alteration of serpentinized peridotite in the Semail ophiolite hasformed two distinct types of listwaenite. Type I is characterized by the presence of calcite (Type IA) or dolomite (TypeIB)+fuchsite7spinel. Type II is dominated by silicate minerals (quartz, chlorite, fuchsite)7calcite+dolomite7magnetite7apatite7plagioclase. Most listwaenites occur as veins along thrust fault zones within the ophiolitemelange. High Cr and Ni contents, abundant occurrence of Cr-spinel within a matrix of red-brown ferruginouscarbonates within a micro-vein network of goethite, and the relics of mesh texture indicate an ultramafic protolith.Type I and II listwaenites represent different stages of hydrothermal alteration. The mineralogical and chemicaldistinctions of both types are the response to the extent of the reactions between the protoliths and the solutionsleading to different stages of metasomatic replacement. The hydrothermal fluids involved in the formation of Type Ilistwaenite were enriched in Ca, Mg, and CO2, whereas Type II listwaenite bodies were formed from a hydrothermalfluid enriched in SiO2. REE and trace elements in both listwaenite types were extracted in part from adjoiningperidotite. No Au anomaly in the study areas has been detected.r 2005 Elsevier GmbH. All rights reserved.

1. Introduction

Listwaenite (from the Russian ‘‘listvenity’’) is a termthat, until the last two decades, has been used almostexclusively by Russian geologists to describe carbona-te7sericite7pyrite altered ophiolitic mafic and ultra-mafic rocks that are veined by hydrothermalquartz7carbonate (Rose, 1837). The term was coined

e front matter r 2005 Elsevier GmbH. All rights reserved.

emer.2005.01.003

ing author.

ess: [email protected] (S. Nasir).

to describe the type locality at Beresovsk in the UralMountains of central Russia (Halls and Zhao, 1995).The term listwaenite is currently defined in Europe andNorth America (Bates and Jackson, 1987) as; ‘‘acarbonatized and variably silicified serpentinite, occur-ring as dikes in ophiolite complexes in the Arabianshield’’.

Listwaenites are regarded strictly to be a hydrother-mal alteration product of mafic and ultramafic rocks(e.g., Buisson and Leblanc, 1987; Leblanc, 1991). Theyevolved as products of two successive stages of the sameprocess: the serpentinization of ultrabasites followed by

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ARTICLE IN PRESSS. Nasir et al. / Chemie der Erde 67 (2007) 213–228214

metasomatic alteration (Kashkai and Allakhverdiev,1965; Ucurum, 2000). Listwaenitization of serpentinitesoccurs immediately after the autometamorphism of thederivatives of ultrabasic magma. Fluid movement alongfractures produces dike like listwaenites, the length ofwhich can range in hundreds and the thickness in tens ofmeters (Ucurum, 2000). Altered mafic and ultramaficrocks considered to be listwaenites have been observedin British Columbia (Ash and Arksey, 1990), Turkey(Ucurum, 1998, 2000), Canada (Jutras and Geol, 2002),Mali, Morrocco and Saudi Arabia (Leblanc, 1986;Buisson and Leblanc, 1985). These listwaenites includemany types of silica and carbonate-bearing rocks, eachwith different phyllosilicate mineral associations;namely silica-carbonate rocks, serpentine silica-carbo-nate rocks, iron silica-carbonate rocks, chlorite silica-carbonate rocks, talc silica-carbonate rocks, and chro-mian mica silica-carbonate rocks. However, listwaeniteis typically composed of quartz, carbonate minerals(magnesite, ankerite, and dolomite), and/or fuchsite,

Fig. 1. Geological sketch map of the Oman Mountains showing l

(modified after Glennie et al., 1974).

together with sulfides and a number of other accessoryminerals (Ucurum, 2000). The term listwaenite has beenelevated in status from that of a rock type to that of anassociation of rock types, all these being geneticallyrelated to the same hydrothermal event but varying as afunction of the alteration intensity and protolithscomposition (Kashkai and Allakhverdiev, 1965).

Listwaenite research is of practical as well astheoretical importance because listwaenites host or arespatially associated worldwide with gold, arsenic,cobalt, nickel, tungsten and mercury deposits (Gonch-arenko, 1984; Buisson and Leblanc, 1986; Korobeyni-kov and Goncharenko, 1986; Leblanc and Lbouabi,1988; Leblanc and Fischer, 1990; Auclair et al., 1993;Sherlock and Logan, 1995; Halls and Zhao, 1995).

Carbonatized and silicified serpentine rocks (listwae-nite) are common with the Semail Ophiolite. Theobjective of this work is to investigate and characterizethe listwaenite suite in the Semail ophiolite of OmanMountains. The following areas were selected and

ocations of listwaenite outcrops sampled in this investigation

ARTICLE IN PRESSS. Nasir et al. / Chemie der Erde 67 (2007) 213–228 215

sampled: Washihi, Tawah, Bowah, Wadi Hawasina,Wadi Bani Omar, Asjudi, Wadi Jizi, Sumeini, WadiHatta and Dibba (Fig. 1).

2. Geological setting

During the closure of the southern Tethys a pile ofnappes was obducted onto the eastern part of theArabian continental margin. The uppermost nappe unitis the Semail Ophiolite Complex that was obducted fromthe northeast onto the Haybi Complex and theHawasina Complex (Fig. 2). The mantle sequence inthe Semail ophiolite of Oman Mountains is bounded byan upper cumulate zone that is dominated by gabbrosand by an imbricate basal thrust zone (Glennie et al.,1974). Both zones have permitted fluid circulation andintense water–rock interaction, resulting in near totalcarbonation, serpentinization and/or silicification.Stanger (1985) observed that 90% of the mantlesequence had undergone extensive serpentinization thatvaried between 55% and 85% by volume. Wilde et al.(2002) proposed a Tertiary age for the hydrothermalalteration of ultramafic rocks from the Semail Ophiolite.The term ‘Amqat’ was first used by Glennie et al. (1974)for altered ophiolite rocks displaying pronounced highrelief features that form hard resistant outcrops withinthe basal serpentinite. ‘Birbirite’ is another term used forsimilar rocks by Glennie et al. (1974) and Alleman andPeters (1972) in the northern extension of the OmanMountains. Stanger (1985) described the Amqat unitwithin the basal serpentinite and noted that it sometimesforms a part of the basal thrust packet. However, theterm listwaenite is now a common name usually used by

Fig. 2. Stratigraphic and structural stacking of allochthonous thrust

listwaenite position within them. Ls: listwaenite.

geologists for such carbonated and/or silicified maficand ultramafic rocks and will be adopted in this paper.

3. Field description

The listwaenite in the Semail ophiolite forms hardresistant masses that crop out sporadically within thehighly altered ophiolite rocks along the basal thrust ofthe Semail Nappe. In addition it more frequently formsdiscontinuous planar lenses, which are oriented sub-parallel to the basal thrust, and along several fault zonesthrough the Wadi Hawasina Complex, Haybi Complexand within highly serpentinized porous and fracturedultramafic rocks. The major listwaenite outcrops tend tobe controlled by NW–SE faults. The rocks are brown tored, yellow or gray in color. The intensity of alterationlocally ranges from minor quartz-carbonate veining andstockwork to massively replaced rock. Locally highdegrees of alteration occur along imbricate or high-anglefracture zones or lineaments that cut through the mantlesequence and throughout most of the leading edge of theSemail Nappe. The listwaenite bodies range in thicknessfrom 2 to 50m. Quartz and secondary calcite veinsmeasuring from 0.2 to 3 cm in thickness are abundant.Veins commonly exhibit crustiform layering and/ortectonic breccias. In most areas, listwaenites gradevertically into serpentinite and serpentinized harzburgitefrom which they were derived.

Sampling of listwaenite within the Oman Mountainsindicates that silicification and carbonation of theSemail ophiolite is of regional extent and extend formore than 500 km from the southern to the northernends of the Semail nappes (Fig. 1). Listwaenite mostlyoccurs along thrust faults within the same stratigraphic

sheets in the Oman Mountains (after Glennie et al., 1974) and


position at the base of the Semail ophiolite. It alsooccurs along fault zones or associated with exoticlimestone, volcanic rocks, radiolarian cherts and Hawa-sina melange in the Haybi Complex (i.e. Omar Group ofBe’chennec et al., 1988). Fig. 3 shows a schematicdrawing of listwaenite in the Asjudi, Sumeini and WadiHawasina areas, and Fig. 4 shows an example oflistwaenite outcrops at Wadi Hatta. The outcroporientation is parallel to fault zones, which havedeveloped in the ophiolite nappe as a result of Paleogeneuplift (e.g., Stanger and Neal, 1984; Stanger, 1985).

Fig. 3. Drawing of typical structural setting of listwaenite in

the Oman mountains (not to scale).

Fig. 4. Field photograph showing a listwaenite layer between

the ophiolite melange and the Hawasina sedimentary rocks at

Wadi Hatta.

4. Analytical procedures

Major and trace element concentrations of freshsamples of listwaenite were determined by XRF, ICP-AES, ICP-MS and ion chromatography at the Museumof Natural History in London. CO2 and H2O wereanalyzed using a Perkin Elmer 240 CHN ElementalAnalyzer. Loss on ignition (LOI) was determinedgravimetrically. Sample solutions were prepared accord-ing to the procedures described by Ionov et al. (1992)and analyzed with an ARL-3410 Minitorch sequentialspectrometer that was calibrated with a range of matrix-matched multi-element standards. Dilution of thesolutions was accomplished by the acid dissolutionprocedure described by Jenner et al. (1990). Solutionswere then analyzed for the rare earth elements (REE),Nb, Hf, Ta, Pb, Th, U and Y using a VG ElementalPlasma Quad 3 ICP system.

Mineral analyses were carried out utilizing a CAME-CA SX50 microprobe with three wavelength dispersivespectrometers at the Mineralogisches Institut, Universi-tat Wurzburg, Germany following standard procedures.

5. Petrography

Petrographically listwaenites belong to two rockcategories, carbonate (Type I) and silica-carbonate(Type II), that differ in mineralogy and chemicalcomposition. Type I listwaenites are carbonate richand consist mainly of calcite (Type IA) and/or dolomite(Type IB). Fuchsite, Fe–Cr chlorite, Cr-spinel, magne-tite, lizardite, barite and quartz occur as accessoryminerals. Fuchsite and chlorite occur as vein-fillingminerals or as scattered dendiritic grains (Fig. 5). TypeIA listwaenite is usually gray in color with abundantblack spots of Mn-oxide/hydroxide. Type IB is brec-ciated, red in color and includes abundant Fe-hydro-xides in the groundmass. Barite may occur as micro-vein

Fig. 5. Microphotograph showing the dendiritic form of

fuchsite in Type IA listwaenite. Fc, fuchsite, Cc, calcite.

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Fig. 6. Microphotograph showing fine-grained silica in Type

II listwaenite. Ch, chalcedony, Do, dolomite, Gth, goethite.

Fig. 7. Microphotograph showing dolomite rhombs with

associated magnetite from the Type IB listwaenite. Do,

dolomite.

S. Nasir et al. / Chemie der Erde 67 (2007) 213–228 217

filling in few samples. Type II liswanites are rich inquartz, which occurs mostly as very fine-grainedchalcedony (Fig. 6). Calcite, dolomite, magnetite, andgoethite are minor minerals. Spinel is absent from TypeII listwaenite. Apatite and relics of plagioclase werefound in a few samples. Quartz veins are abundant andcommonly cut calcite veins.

The primary minerals of the mantle sequence (olivine,ortho- and clinopyroxene, Cr-Spinel7plagioclase andamphibole) have been altered, to varying degrees, tosecondary assemblages of lizardite, chrysotile, ironoxides, Fe–Cr chlorite, Cr-mica, quartz and carbonates.Only the Cr-spinel seems to be resistant to chemicalchanges during alteration. Nonetheless, spinel is com-monly rimmed by magnetite and goethite. Extensivealterations of olivine have produced a ‘‘chicken-wire’’ ormesh texture in which a network of serpentine hasformed a mosaic array of polygonal cells enclosingcrystals of primary olivine. Most of the carbonategroundmass contains chromite grains that display amesh texture outlined by lamellae of goethite exsolution.

In most cases carbonation is complete, as in Type Ilistwaenite. Alteration calcite has crystallized as euhe-dral vug precipitates within the veins. Some calcitecrystals are tabular, and many crystals show twinningand epitaxial growth on dolomite.

Listwaenites (Type IB) from the Tawah and Bowahareas are clast-supported breccias whose clasts consist ofserpentine and chromite, and in which diagenetic calciteand dolomite rim clasts and fill fractures in thesebreccias. Clast shape is angular to subangular, suggest-ing very short transport and rapid deposition. Thecarbonate grains are arenaceous in size with ‘pseudo-porphyritic texture’. Dolomite occurs as well developedrhombs in association with sulfide and magnetite, whichfrequently form cores in these rhombs (Fig. 7). Dolomitecrystal shape varies from rhombohedral to symmetricalsaddle forms.

6. Mineral chemistry

Microprobe analyses of carbonate minerals aregiven in Table 1. All carbonates from Type IAlistwaenite are low Fe and Mg calcite which, however,contain considerable amounts of 2.0–3.4mole%MnCO3. Type IB and II listwaenites consist of dolomitewith MgCO3+CaCO3 ranging between 94.2 and96.2mole%.

Spinel is highly variable in composition (Table 2).Spinels from Type IA listawenites displays 51–63mole%of FeCr2O4 and 30–31mole% of MgAl2O4, whichcompares well with spinel from fresh harzburgite inHatta area (Nasir, 1996). By contast, spinel from TypeIB listwaenites have generally lower FeCr2O4 contentsof 37–59mole%, but has higher FeAl2O4 contents of25–50mole%; it is characterized by high ZnAl2O4

contents of 2.5–4.7mole%, similar to spinels fromextensively metasomatized ultramafic rocks (e.g., Challiset al., 1995).

Magnetite occurs as nearly stoichiometric magnetitein both Type IA and IB listwaenites (Table 2).

Chlorite is rich in Cr and Mg and is classified as TypeI Mg-chlorite according to the classification of Bayliss(1975) and Bailley (1980) (Table 3).

Mica is present as the Cr-rich muscovite varietyfuchsite (Table 3). Fuchsites analyzed from bothTypes IA and IB show similar chemistry. They arecharacterized by high Cr contents, ranging between 0.52and 0.69 atoms per formula unit and high high Sicontents of 6.73–6.79 atoms per formula unit, which iscommon for phengite and fuchsite from metasomatizedultramafic rocks (e.g., Chao et al., 1986; Argast, 1995;Challis et al., 1995; Grapes and Palmer, 1996; Devarajuet al., 1999; Brigatti et al., 2001). However, the Na+Kvalues in the analyzed fuchsites are low (1.56–1.64 atomsper formula unit), which indicates that these mica arealtered.

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Table 1. Selected microprobe analyses of carbonates from the listwaenite areas in the Semail ophiolite.

Mineral Calcite Dolomite

Type IA IB II

Sample H-4 H-7 J-1 SH-2 B-1 H-C-3 HC-7 D-1 H-1 T-4 SB-3 D-3 SB-1

Wt%

MgO 0.06 0.00 0.08 0.13 0.18 0.25 0.00 0.40 20.39 19.6 19.78 20.56 20.36

CaO 53.18 53.13 53.08 54.11 53.74 53.31 53.25 54.91 29.16 29.79 30.66 29.05 29.48

MnO 2.11 2.04 1.70 1.46 1.63 1.55 1.25 1.69 0.13 0.50 0.10 0.13 0.10

FeO 0.18 0.45 0.33 0.09 0.32 0.28 0.05 0.57 0.8 1.45 0.59 2.10 2.38

CO2 44.0 44.4 41.98 43.38 44.22 43.78 44.07 42.14 47.06 48.63 48.86 48.15 47.66

Total 99.53 100.02 97.89 99.16 100.09 99.17 98.62 99.71 98.14 99.97 99.99 99.99 99.98

Mole%

MgCO3 0.13 0.0 0.2 0.28 0.64 0.53 0.0 0.85 42.68 41.02 41.42 43.02 42.65

CaCO3 95.0 94.9 95.8 96.15 95.99 95.73 96.62 97.66 52.07 53.21 54.76 51.9 52.66

MnCO3 3.42 3.32 2.77 2.37 2.36 2.52 2.03 2.65 0.20 0.82 0.17 0.21 0.17

FeCO3 0.3 0.74 0.55 0.15 0.52 0.46 0.08 0.89 1.29 2.35 0.96 3.38 3.85

S. Nasir et al. / Chemie der Erde 67 (2007) 213–228218

7. Geochemistry

Chemical analyses of listwaenite from differentlocalities in the Semail Ophiolite are given in Table 4.The averages of chemical analyses of Type I and IIlistwaenites are given in Table 5, along with chemicalaverages of various types of listwaenites, carbonate,fresh harzburgite, gabbro, basalt and serpentinite fromthe Semail ophiolite.

The most salient feature of listwaenites is the extremevariation in the silica content. The highest SiO2 contentis observed in the Type II silica listwaenite samples SB-1and SB-2 from the Bowah region. The lowest silicacontent, on the other hand, is recorded in the Type IBdolomite-bearing listwaenite samples from the Dibbaarea (D-3) and Bowah area (SB-3,). Type I listwaeniteshows an extreme relative enrichment in Ca and coupleddepletion in Si and Mg relative to fresh harzburgite,gabbro, basalt and serpentinite from the Semailophiolite. It is represented by dark gray (Type IA) andred-brown (Type IB) varieties. The dark gray listwae-nites of Type IA are characterized by their exceptionallyhigh MnO, high CaO, low Fe2O3 and low MgOcontents. The high MnO content is related to thepresence of Mn-rich calcite in these rocks (Table 1).Black pigmentation in calcite is generally caused bymanganese oxide and/or graphite inclusions (e.g., vonHanold and Weber, 1982). By comparison, the red-colored Type IB dolomite listwaenites have lower MnO,lower CaO, higher Fe2O3 and higher MgO contents. Thered-brown coloration is mainly due to inclusions of ironoxides or iron hydroxides. Samples SB-1 and SB-2, fromthe Bowah area, are silica liswanites (Type II, yellow-brown in color) and show a similar chemistry to thesilicified serpentinite of Stanger (1985) and Wilde et al.(2002) (Table 5). Fig. 8a, a SiO2–Fe2O3–CaO+MgO

ternary diagram shows that the carbonate listwaenites(Type I) plot along and near the SiO2–CaO+MgOpseudobinary at values of 450% CaO+MgO ando50% SiO2, while silica-carbonate listwaenites (TypeII) plot very near the SiO2 corner. Type IA listwaenitesplot at values of 460% CaO, whlie Type IB listwaenitesplot at lower values in a SiO2–MgO–CaO ternarydiagram (Fig. 8b).

Similar to the large variability in the content of majorelements, the minor and trace elements also show largechanges in their contents. High and variable Cr, Ni andSr contents were observed in most listwaenite samples(Fig. 9). Low Cr contents have been recorded in Type IItwo samples (SB-3 and SB-4), which also show thehighest Sr and Ba contents (Table 4) and contain tracesof barite. All listwaenite samples contain very lowcontents of REE, Pb, Zr, Y, Nb, U, Th, F, Cl and S.Neither Type I nor Type II listwaenites carry economicconcentrations of Au in any of the study areas, althoughAu contents of up to 90 ppb in the Bowah area (sampleSB-1), 107 ppb in the Hatta area (sample HC-3) and upto 200 ppb in the Wadi Jizzi area, are sufficiently high towarrant additional exploration. Gold-bearing silicifiedand/or carbonatized serpentinite (Au40.1 ppm) havebeen recorded along major faults or thrust planes in BouAzzer, Morroco, in Saudi Arabia (Buisson and Leblanc,1986; Leblanc, 1991) and in Turkey (Ucurum, 1998).Many examples of gold mineralization in listwaenitelenses (0.1–10 ppm) have been reported in serpentinitemassifs from various ophiolitic complexes throughoutthe world (Henderson, 1969; Buisson and Leblanc, 1986,1987).

Average REE concentrations normalized to chondri-tic values are plotted in Fig. 10. The REE patterns forsedimentary carbonate (Turekian and Wedepohl, 1961),secondary vein carbonate of hydrothermal origin

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Table 2. Selected microprobe analyses of spinel and magentite from listwaenites in the Semail Ophiolite.

Mineral Spinel Magnetite

Type IA IB HZa IA IB

Sample J-1 HC7 T-4 HC-3 HH-1 HC7 HC-3 T-4

Wt%

TiO2 0.09 0.06 0.12 0.08 0.15 0.06 0.00 0.00

Al2O3 26.88 30.93 27.56 33.87 30.51 0.0 0.07 0.10

Cr2O3 40.99 37.2 36.39 30.45 38.11 0.06 0.0 0.09

Fe2O3 2.66 1.47 0.72 0.51 3.29 68.38 69.1 69.14

FeO 16.67 18.51 25.33 25.73 11.59 30.64 28.12 28.56

MgO 12.44 11.89 3.23 5.33 16.58 0.02 0.51 0.41

MnO 0.33 0.24 0.49 0.18 0.12 0.00 0.21 0.10

NiO 0.14 0.18 0.06 0.18 0.00 0.02 0.98 1.24

ZnO 0.33 0.30 6.2 3.34 0.00 0.00 0.00 0.00

CaO 0.00 0.00 0.01 0.00 0.00 0.18 0.36 0.00

Total 100.53 100.79 100.11 99.67 100.3 99.37 99.35 99.63

Mg] 0.56 0.53 0.18 0.27 0.72 0.01 0.03 0.02

Formula on the basis of four cations

Ti 0.002 0.001 0.003 0.002 0.003 0.002 0.000 0.000

Al 0.957 1.083 1.048 1.238 1.045 0.000 0.003 0.005

Cr 0.979 0.868 0.928 0.747 0.876 0.002 0.000 0.003

Fe3+b 0.061 0.033 0.018 0.012 0.072 1.995 1.997 1.993

Fe2+ 0.421 0.460 0.684 0.667 0.282 0.992 0.902 0.914

Mg 0.560 0.528 0.155 0.246 0.719 0.001 0.029 0.023

Mn 0.008 0.006 0.013 0.007 0.003 0.000 0.007 0.003

Ni 0.005 0.007 0.002 0.007 0.000 0.001 0.047 0.060

Ca 0.000 0.000 0.000 0.000 0.000 0.007 0.015 0.000

Zn 0.007 0.006 0.148 0.076 0.000 0.000 0.000 0.000

3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

End members

MgAl2O4 30.6 31.3 9.8 16.1 32.6 — 0.1 0.2

FeAl2O4 — 16.3 25.8 49.6 — — — —

FeCr2O4 62.9 51.4 58.6 37.3 54.7 0.2 — 0.3

NiFe2O4 0.3 0.4 0.1 0.3 — 0.1 4.5 6.0

MgFe2O4 5.3 — — — 12.3 0.1 0.5 2.0

ZnAl2O4 0.2 0.2 4.7 2.5 — — — —

Fe3O4 — — — — — 99.4 94.1 91.1

aHH-1: spinel from fresh harzburgite (Nasir, 1996).bFe3+/Fe2+ calculated from charge balance.


(Nelson et al., 1988) and for harzburgite from the Semailophiolite (Lippard et al., 1986) are also shown forcomparison. All Type I listwaenite samples have verylow REE concentrations and are characterized by arelatively flat REE pattern. These patterns are verysimilar to the pattern of hydrothermal carbonate, as wellas the pattern of sedimentary carbonate, which however,has negative La, Eu and Tm anomalies. In comparison,fresh harzburgite from the Semail Ophiolite shows evenlower REE concentrations with V-shaped pattern andnegative Eu anomaly. This pattern is typical for depletedperidotite from ophiolites and oceanic basement (e.g.,Pallister and Knight, 1981). Type II listwaenite showsthe lowest REE content, which is broadly similar to theperidotite REE pattern.

The metasomatic transformation of the harzburgite toform listawenite with a different chemical and mineralassemblage involves changes in the concentrations of themajor oxide and trace element components of the rock,and may also alter density and volume. Tables 4 and 5and Fig. 8 show that CaO was considerably enriched,whereas MgO, Cr and Ni were depleted in thelistwaenites relative to fresh harzburgite from the Semailophiolite. SiO2 in Type II listwaenites is highly enrichedrelative to harzburgite, whereas it is depleted in Type Ilistwaenites. Geochemical data was used to obtaininformation on element mobility and to undertake massbalance calculations. In order to estimate the absoluteamounts of gains and losses during hydrothermalalteration, the equation of Gresens (1967) (see Abu

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Table 3. Selected microprobe analyses of chlorite and fuchsite from listwaenites in the Semail Ophiolite.

Mineral Chlorite Fuchsite

Type IA IB IA IB

Sample HC-7 HC-3 HC-7 H-1 T-4 SB-3 HC-3 D-1

Wt%

SiO2 31.43 31.78 51.15 51.22 51.35 51.08 52.73 51.28

TiO2 0.03 0.03 0.17 0.08 0.20 0.15 0.21 0.20

Al2O3 15.88 16.48 25.35 25.18 26.09 25.5 25.13 25.03

Cr2O3 3.46 2.27 6.02 6.57 5.0 5.93 5.73 6.16

FeO 3.17 2.85 0.73 0.68 0.75 0.80 0.67 0.49

MnO 0.05 0.03 0.02 0.08 0.02 0.00 0.01 0.00

MgO 33.13 32.98 3.74 3.49 3.84 3.51 3.61 3.64

CaO 0.18 0.10 0.00 0.02 0.00 0.00 0.02 0.04

Na2O 0.06 0.01 0.17 0.25 0.22 0.23 0.26 0.22

K2O 0.03 0.02 9.15 9.14 9.51 8.93 9.11 9.13

Total 87.42 86.55 96.5 96.72 96.98 96.13 98.48 96.19

Cations on the basis of

28 oxygen 22 oxygen

Si 5.969 6.048 6.750 6.755 6.736 6.757 6.792 6.783

Ti 0.004 0.004 0.017 0.008 0.020 0.015 0.020 0.020

Al 3.554 3.696 3.943 3.914 4.034 3.975 3.967 3.902

Cr 0.520 0.342 0.628 0.686 0.519 0.620 0.584 0.644

Fe 0.503 0.454 0.081 0.075 0.082 0.088 0.072 0.054

Mn 0.008 0.005 0.002 0.009 0.002 0.000 0.001 0.000

Mg 9.380 9.356 0.736 0.686 0.751 0.692 0.693 0.718

Ca 0.037 0.020 0.000 0.003 0.000 0.000 0.003 0.006

Na 0.022 0.004 0.043 0.064 0.051 0.059 0.065 0.056

K 0.007 0.005 1.540 1.538 1.592 1.509 1.497 1.541

Total 20.004 19.934 13.739 13.738 13.792 13.714 13.694 13.723


El-Enen et al., 2004 for details) was applied to thelistwaenites in comparison with unaltered harzburgitefrom the Semail ophiolite (Table 6). The assumptionscommonly used in mass balance calculations are thateither the overall volume or mass of a system isconserved, or that one or more components (such asAl2O3, TiO2, the REEs, or Zr) have remained immobilethroughout the alteration process (Grant, 1986). Thisassumption has, however, been found invalid in thisstudy. Mass balance calculations based on constantAl2O3, TiO2, REE or Zr yielded variable and high valuesfor the volume factor (1.3–2.2). Therefore, we calculatedthe absolute values of gains and losses by assumingconstant oxygen. This approach seems to be justifiedsince, with constant oxygen, the resulting volume factors(fV) are close to 1 (Table 6). Based on the chemicalanalyses of fresh harzburgite from the Semail ophiolite(Lippard et al., 1986) and assuming constant oxygenduring the metasomatic process (see Abu El-Enen et al.,2004 for discussion), absolute gains of the major elementsamounted to 22.87, 13.29 and 9.23 g/100 g in type IA, IBand II listwaenites, respectively. The gain is predomi-nated by Si in type II listwaenite, by Ca in type IA and IBlistwaenites, small amounts of Na and K and subordi-

nately Mn in type IA listwaenites. In reverse, absolutelosses amounted to 46.26, 39.25 and 30.38 g/100 g in typeIA, IB and II listwaenites, respectively, predominantlySi, Mg, Fe and small amounts of Al. These elementswere lost from the metasomatized rock. Most REEwere gained, whereas about 1690–4415mg/1000 g ofCr+Ni+Pb were lost. Type II listwaenites show verylow gains in REE relative to Type I listwaenite.

Mass balance calculation for serpentinized harzbur-gite from the Semail ophiolite indicates that theserpentinite lost about 14.26 g/100 g of major elementsand 5.35mg/1000 g of Ni. The only gain was 845mg/1000 g Cr.

Gains and losses based on the average of gabbro orbasalt (not shown) will give lower results of gains andlosses of the major elements than those based onharzburgite. However Cr and Ni will be lost from thelistwaenites relative to gabbro or basalt.

8. Discussion

Field relationships, the presence of Cr-rich mica andMg- and Cr-rich chlorite, REE patterns and the high Cr

ARTICLE IN PRESS

Table 4. Major and trace elements analyses of listwaenite from the Semail Ophiolite

Location Wadi Hawasina Wadi Jizi Washihi Tawah

Sample H-1 H-2 H-4 H-7 J-1 J-5 SH-1 SH-2 T-4 T-5

Type IA IA IA IA IA IA IA IA IB IB

Color dg dg dg dg dg dg dg dg rb rb

Wt%

SiO2 5.35 15.3 11.8 5.88 13.4 11.9 7.82 3.48 22.0 27.2

TiO2 0.02 0.01 0.02 0.03 0.02 0.01 0.03 0.02 0.01 0.02

Al2O3 1.23 0.49 0.94 0.66 0.82 0.53 1.47 0.98 0.31 0.39

Fe2O3 6.59 5.18 5.79 5.09 2.26 0.86 5.91 4.22 6.57 7.43

MnO 0.12 0.39 0.63 1.85 0.44 0.71 1.2 0.97 0.14 0.12

MgO 12.37 8.2 3.89 1.83 4.2 1.2 1.7 1.55 12.3 9.89

CaO 32.7 36.8 40.8 45.2 42.0 45.1 43.2 47.0 24.1 22.3

Na2O 0.03 0.02 0.02 0.03 0.02 0.03 0.01 0.02 0.01 0.02

K2O 0.02 — — — — — — — 0.01 —

P2O5 0.09 0.11 0.06 0.13 0.1 0.01 0.09 0.11 0.06 0.13

H2O 0.9 2.99 1.54 0.89 1.85 0.6 1.25 1.08 0.25 0.74

CO2 40.1 29.3 34.0 38.3 34.3 37.4 36.5 39.0 34.35 31.6

Total 99.82 98.76 99.47 99.88 99.29 98.41 99.18 98.47 100.06 99.8

Trace elements (ppm)

Cl 37 79 55 30 63 70 37 32 62 54

S 276 122 65 41 144 198 281 234 64 78

Cr 1290 1400 1940 1550 2640 1560 2390 1980 1440 1410

Ni 460 1020 1280 540 1580 1280 630 430 510 1010

Sr 130 360 255 295 1160 665 215 265 840 520

Ba 59 22 5 — — 70 62 — 7 20

Pb 46 18 26 4 35 17 30 26 222 126

Zr 3.2 0.4 0.21 0.1 0.05 0.12 3.93 1.15 0.06 1.14

Y 3.1 0.86 2.94 18.1 0.77 4.12 4.78 8.81 5.36 3.29

Nd 0.72 0.39 1.16 7.8 0.94 4.79 5.75 5.57 1.11 0.96

La 0.79 0.45 0.57 3.18 1.93 11.9 5.39 4.62 0.32 0.8

Ce 0.96 0.75 1.21 9.42 2.51 15.1 10.9 10.0 1.01 1.78

Trace elements (ppb)

Nb 225 161 — — — — 546 79 — 288

Pr 166 109 227 1487 262 1347 1320 1259 197 255

Sm 195 91 365 2523 164 706 1234 1373 746 406

Eu 95 22 96 708 601 520 383 344 246 126

Gd 282 113 479 2827 184 905 1311 1622 996 563

Tb 54 15 57 463 15 94 156 213 170 100

Dy 401 90 282 2561 71 448 729 1099 868 522

Ho 89 17 47 444 12 69 108 189 150 94

Er 279 45 112 1091 29 151 240 433 326 221

Tm 43 7.1 15 144 5.1 14 26 50 41 28

Yb 263 50 77 807 36 67 149 255 208 151

Lu 42 7 10 115 7.6 9.7 23 38 25 21

Hf 65 11 9.8 12 5.9 5.8 104 29 7 31

Ta 18 6.9 — 1.1 — 2.1 33 8.8 1.1 19

Au 0.42 0.11 0.59 1.1 0.5 200 0.73 0.27 57 47

Th 37 14 — — 2.2 9.9 211 60 57 215

U 2050 132 257 252 64 63 131 118 73 687


ARTICLE IN PRESS

Location Bowah Wadi Bani Omar Hatta Dibba

Sample SB-1 SB-2 SB-3 SB-4 B-1 B-2 HC-3 H-C-7 D-1 D-3

Type II II IB IB IA IA IB IA IB IB

Color yb yb rb rb dg dg rb dg rb rb

Wt%

SiO2 88.03 69.3 3.33 7.34 10.0 14.8 6.75 10.24 11.33 2.14

TiO2 0.01 0.01 0.03 0.15 0.01 0.03 0.02 0.01 0.03 0.01

Al2O3 0.53 0.31 0.39 2.1 0.43 1.19 1.21 0.55 1.17 1.04

Fe2O3 6.78 3.32 11.5 9.77 4.94 2.1 4.44 4.82 4.95 7.81

MnO 0.01 0.05 0.19 0.23 0.51 0.75 0.23 1.05 0.36 0.24

MgO 1.14 8.72 16.4 15.3 4.37 4.19 17.33 1.06 16.82 17.88

CaO 1.05 3.2 25.9 24.8 41.8 40.3 28.33 44.75 27.33 29.76

Na2O 0.01 0.05 0.01 0.03 0.03 0.01 0.09 0.07 0.02 0.03

K2O — — 0.02 — — — 0.01 0.01 0.03 0.01

P2O5 0.02 0.08 0.01 0.24 0.08 0.97 0.01 0.01 0.04 0.01

H2O 0.52 0.96 1.87 2.38 0.53 0.96 0.50 0.80 0.88 0.94

CO2 1.25 13.0 39.9 37.6 37.8 34.4 41.07 36.1 36.8 39.74

Total 98.95 98.96 99.41 99.96 100.55 99.73 99.99 99.47 99.76 99.61


Cl 66 106 93 58 86 242 40 65 55 67

S 348 424 117 126 194 389 5 3 3 5

Cr 450 1170 100 40 1700 1950 1015 2010 1242 2113

Ni 233 480 440 360 410 160 571 843 335 435

Sr 220 395 1470 1135 315 215 97 567 134 164

Ba 12 39 60 225 81 68 20 32 37 32

Pb 150 373 13 25 72 55 10 62 28 35

Zr 2 0.31 5.67 19.6 0.24 2.66 10 5 15 35

Y — 0.53 16.3 22.6 14.3 27.8 6 9 12 9

Nd 0.08 0.14 2.71 5.07 2.22 8.42 8 6 3.1 4.5

La 0.07 0.11 1.31 3.01 1.56 13.1 3 10 5.2 5.8

Ce 0.14 0.22 3.08 7.22 2.97 19.8 4 6.2 4.5 3.8

Trace elements (ppb)

Nb — — 180 1162 — 491 3 4 100 146

Pr 20 30 503 1031 459 2145 810 938 740 996

Sm 35 43 1009 1793 693 1594 1200 1592 1234 945

Eu 19 22 377 648 374 402 410 391 325 428

Gd 45 66 1452 2382 970 2074 1580 1462 1598 1432

Tb 10 13 297 488 206 330 312 410 245 334

Dy 62 84 2047 3452 1457 2269 2560 2725 3452 2456

Ho 11 17 446 756 329 524 582 492 396 465

Er 42 50 1332 2294 967 1639 1140 1215 1654 1166

Tm 6.1 7.5 202 366 134 248 286 237 295 312

Yb 48 51 1185 2220 726 1331 1285 1859 2020 1943

Lu 6.5 7.6 187 346 108 208 262 183 167 201

Hf 6.6 7.8 98 449 13 45 113 121 65 76

Ta — — 19 82 2 665 25 3.5 15 21

Au 90 60 8 0.13 0.15 107 10 9 10 15

Th 6.9 7.4 109 594 65 183 615 420 220 318

U 189 230 1183 567 420 2612 790 492 554 598

Color: dg, dark gray; rb, red brown; yb, yellow brown.

Table 4. (continued )


and Ni contents in most of the listwaenites from theSemail ophiolite in the Oman Mountains indicateshydrothermally altered ultramafic (e.g., harzburgite)source rocks. Fuchsite occurs commonly in metasoma-

tized ultramafic rocks (e.g., Grapes and Palmer, 1996;Devaraju et al., 1999; Brigatti et al., 2001). Mostsamples display geochemical and petrofabric evidencereflecting a serpentinite or serpentinized peridotite

ARTICLE IN PRESS

Table 5. Averages of Type I and Type II listwaenites from the Semail ophiolite, Oman Mountains, in comparison with fresh

peridotite (HZ) (Sample OM1499, Lippard et al., 1986), average carbonate rocks (CRB) (Turekian and Wedepohl, 1961), fresh

gabbro from the Semail ophiolite (Lippard et al., 1986), Haybi basaltic extrusive from the Semail ophiolite (Robertson et al., 1990),

normal serpentinite (SPT) (Stanger, 1985), silica listwaenite (SL) (Wilde et al., 2002), silicified serpentinites (SSP) (Stanger, 1985) and

carbonate listwaenite (CL) (Wilde et al., 2002). Numbers in ( ) indicate number of samples in averages

Type IA Type IB Type II CRB HZ Gb Bs SL CL SSP SPT

(10) s.d. (8) s.d. (2) s.d. (1) (8) (3) (31) (18) (5) (2)

Wt%

SiO2 10.4673.83 10.6879.13 78.67713.24 5.13 45.04 49.7 48.7 79.05 21.79 80.94 38.7

TiO2 0.0270.01 0.0370.04 0.0170.0 0.06 0.03 0.45 1.81 0.01 0.0 0.04 0.0

Al2O3 0.8170.34 0.9870.6 0.4270.15 0.79 0.93 14.3 14.4 0.13 0.35 1.37 1.05

Fe2O3 4.1271.74 7.3872.35 5.0572.44 7.44 8.25 5.6 13.6 4.26 4.92 5.24 8.63

MnO 0.8570.44 0.270.08 0.0370.03 0.14 0.15 0.14 0.21 0.05 0.08 0.1 0.12

MgO 3.2272.22 14.7971.9 4.9375.36 7.79 43.51 10.7 5.3 3.57 25.49 3.74 37.54

CaO 42.772.99 26.973.35 2.1271.52 42.29 1.12 11.2 9.9 4.59 21.5 3.76 0.08

Na2O 0.0370.02 0.0370.02 0.0370.03 0.05 0.0 1.8 4.9 4.32 0.08 0.21 0.0

K2O 0.070.0 0.0270.01 0.070.0 0.32 0.0 0.12 0.3 0.02 0.03 0.02 0.0

P2O5 0.1770.28 0.0770.07 0.0570.04 0.09 0.0 0.03 0.26 — — 0.01 0.05


Cr 19127385 10817699 8107509 11 2193 665 59 203 1038 1939 3079

Ni 8177461 5157214 3567175 20 2766 157 44 341 1202 1283 2200

Sr 4317297 5627533 3077123 610 110 358 383 97

Ba 34733 58770 26719 10 156 65 69

Pb 34721 63774 2627158 9 3 2

Zr 1.471.8 11.2711.8 1.1571.19 19

Y 9.278.7 9.776.9 0.2670.37 30 18 22

Nd 4.372.9 3.3572.6 0.1170.04 4.7 0.026 11 100

La 5.374.7 2.5372.1 0.0970.23 1 0.026

Ce 7.976.3 3.372.1 0.1870.05 11.5 0.055

Nb 0.1370.21 0.267.38 0.070.0 0.3

Pr 0.9670.67 0.5970.36 0.02570.007 1.1

Sm 1.0370.77 0.9470.5 0.03970.006 1.3 0.051

Eu 0.3870.21 0.3370.18 0.002 7 0.002 0.2 0.001

Gd 1.270.85 1.2870.66 0.05570.015 1.3 0.007

Tb 0.270.16 0.2570.14 0.01270.002 0.2

Dy 1.1771.03 1.9771.24 0.01370.015 0.9 0.016

Ho 0.2270.21 0.3770.24 0.01470.004 0.3

Er 0.5970.58 1.0570.74 0.04670.006 0.5 0.019

Tm 0.0970.09 0.270.14 0.00770.001 0.04

Yb 0.4770.65 1.1670.86 0.05070.002 0.5 0.03

Lu 0.0770.08 0.1670.12 0.00770.001 0.2

Hf 0.0370.04 0.1170.14 0.00770.001 0.3

Ta 0.0770.21 0.0270.02 0.070.0 0.01

Th 0.170.14 0.2770.22 0.07570.0003 1.7

U 0.4570.77 0.8170.58 0.21070.029 2.2

s.d., standard deviation.


protolith. Constant volume replacement of serpenti-nized peridotite by carbonate and/or silica is implied onboth chemical and petrofabric evidence. This is bestindicated by the preservation of the serpentinite meshtexture in most samples and by the content of theelements Cr and Ni and REEs which are similar to thosein serpentinite and harzburgite from the Semail ophio-lite (Table 4, Fig. 7). Some Type II samples containrelics of plagioclase and apatite, suggesting a gabbroic

or plagioclase bearing ultramafic protolith. Someextreme high values of silica in Type II listwaeniteswere provided by samples from intensely quartz veinedlistwaenite outcrops. The high silica value probablyrepresents introduced silica by hydrothermal SiO2

veining. Differences in major and trace elementschemistry in Type I and II listwaenites suggestdifferences in alteration intensity or composition ofprotolith and/or chemistry of hydrothermal fluids

ARTICLE IN PRESS

Fig. 8. Ternary diagrams for listwaenites, serpentinite, harz-

burgite and carbonate (a) SiO2–Fe2O3–CaO+MgO and (b)

SiO2–CaO–MgO diagrams. Dashed line divides Type I and II

listwaenites. Data are from Table 4.

Fig. 9. Sr–Ni–Cr ternary diagrams for listwaenites, serpenti-

nite, harzburgite and carbonate. Symbols as in Fig. 8. Data are

from Table 4.

Fig. 10. Chondrite-normalized plots (Taylor and McLennan,

1985) of average Type I and II listwaenite in comparison to

hydrothermal carbonate (Nelson et al., 1988), average

carbonates (Turekian and Wedepohl, 1961), and fresh

peridotite (Lippard et al., 1986).


involved in their formation. The listwaenites in theSemail ophiolite are usually associated with non-silicified and/or carbonated serpentinite within the basalthrust zone of the mantle sequence. Fault zonereactivation subsequent to emplacement may cause

fracturing of the serpentinite, thereby permittingCa–Mg-rich groundwater circulation and alteration ofthe serpentinite and serpentinized peridotite. Precipita-tion and recrystallization of carbonate and/or silicaafter, or simultaneous with, dissolution will createcarbonated and/or silicified peridotite, depending onthe amount of carbonate and silica and the pH of thegroundwater (e.g., Barnes et al., 1973). The alteration ofolivine and pyroxene under high alkaline condition ofpH411 (Neal and Stanger, 1983) results in a loss ofmaterial and a creation of zones of high porosity. Slowmixing of near-surface bicarbonate-rich water withdeeper hydroxide groundwater favor the precipitationof calcite and/or dolomite(e.g., Stanger et al., 1988). Asecond possibility is that silica will dissolve fromserpentinits and or harzburgite into the hydrothermalfluid during the formation of Type I (carbonate)listwaenites and were taken from the environment bythe remainig silica-rich fluid in a high pH and low-temperature environment. This make sense for Type Ilistwaenite because silica is more soluble at high pH(49), while the solubility of Ca increase with decreasingtemperature (25–60 1C) at low partial pressure of CO2

(Faure, 1991; Ucurum, 2000). This suggest that thehydrothermal fluids for Type I listwaenite were char-acterized by moderate to high temperature and high pH.Neal and Stanger (1984, 1985) and Stanger (1985)suggest that there are two types of alteration inthe Semail Nappe: (1) a high-temperature altera-tion (150–300 1C) through serpentinization and (2) a

ARTICLE IN PRESS

Table 6. Gains and losses of major and trace elements calculated after Gresens (1967)

Unaltered sample Harzburgite Harzburgite Harzburgite Harzburgite

Altered sample Serpentinite Listwaenite Listwaenite Listwaenite

Type IA Type IB Type II

Volume factor fV 1.05 0.91 0.90 0.97

Balanced for g/100 g Oxygen Oxygen Oxygen Oxygen

Si �5.69 �17.34 �17.40 8.80

Ti �0.02 �0.01 �0.00 �0.01

Al �0.02 �0.17 �0.11 �0.31

Fetot �0.72 �3.98 �2.2 �3.23

Mn �0.04 0.38 0.00 �0.10

Mg �7.02 �24.76 �19.72 �23.83

Ca �0.01 22.41 13.24 0.43

Na 0.00 0.06 0.02 0.02

K 0.00 0.01 0.01 0.00

Ox 0.00 0.00 0.00 0.00

Gains 0.00 22.87 13.29 9.23

Losses �14.26 �46.26 �39.25 �30.38

Gains+losses �14.26 �23.39 �25.95 �21.15

mg/1000 g

Cr 845 �213 �1403 �1610

Ni �5.35 �1481 �2169 �2805

La 4.01 1.83 0.07

Ce 5.96 2.37 0.14

Nd 3.25 2.46 0.1

Sm 0.74 0.65 �0.01

Eu 0.29 0.24 0.00

Gd 0.90 0.93 0.06

Dy 0.88 1.43 0.00

Er 0.43 0.75 0.03

Yb 0.34 0.82 0.02


low-temperature (30–45 1C) alteration. The first type(e.g., Type I listwaenite) is the most intense and complete(�100%) along major thrust and fracture zones. It isrelatively uniform in its effects and it is possible that ittook place either whilst the ophiolite was a part of theoceanic lithosphere or during its detachment from it.The second type of alteration (Type II) is related tothe movement of meteoric waters through the rocks. Themost recent effects leading to the formation of listwae-nite are aqueous precipitates from bicarbonate-richgroundwaters (Neal and Stanger, 1984, 1985, Stanger,1985, Stanger and Neal, 1994). Glennie et al. (1974)suggested that the silicified serpentine of the SemailNappe resulted from a selective leaching of magnesiumunder tropical weathering conditions. However, Al-Sharhan and Nasir (1996) found that weathering of theSemail peridotites under tropical conditions formedduring the Maastrichtian and yielded Fe–Ni–Cr-richlaterites (e.g. in the Qahla Formation). Alternatively,Stanger (1985) concludes that silicification was a low-temperature chemical replacement feature and not aweathering phenomenon. Figs. 11 and 12 shows asuggested model for alteration in the Semail ophiolite.

The majority of the mantle sequence rocks of theSemail Nappe are between 50% and 80% altered(Stanger, 1985). The basal unit of Searle et al. (1980)at the base of the nappe is intensely sheared andgenerally close to 100% altered. The basal thrust of theSemail Nappe displays substantial mineralogical vari-eties. Type I and II listwaenites represent stages ofhydrothermal alteration and the mineralogical andchemical distinctions reported are the response to theextent of the reactions between the ultramafic/maficprotoliths and the solutions leading to different stages ofmetasomatic replacement.

Mass balance calculation showed that the gains andlosses, based on fresh harzburgite, are rather high inType I listwaenites, but small in Type II listwaenites.Losses in major elements of serpentinite and listwaenitesare relatively similar. Gains in Ca, (type I), Si (type II)and REE in the listwaenites can be attributed tohydrothermal supply, derived from adjacent serpentiniteand harzburgite of the Semail ophiolite. Silica was themain control that defined the concentration of REE andtrace elements in the hydrothermal fluids, because silica-carbonate (Type II) listwaenite is less enriched in most


REE and trace elements than Type I listwaenite.Figs. 11a and b show that Sm, Eu, Dy and Yb areclosely associated in Type I and sedimentary and/orhydrothermal carbonate, and in Type II listwaenite andharzburgite and were probably in equilibrium inhydrothermal fluids during alteration of the ultramaficprotolith. The REE pattern in Type II listwaenite

Fig. 11. Ternary diagrams for the averages of Type I (IA, IB)

and Type II (II) listwaenites, harzburgite (HZ), sedimentary

carbonate (SC) and hydrothermal carbonate (HC). (a)

Sm–Dy–Eu and (b) Dy–Yb–Eu ternary diagrams. Data are

from Table 5.

Fig. 12. Suggested model for listwaen

suggest a relatively immobile behavior of the REEsduring low-temperature alteration.

Tertiary extension in northern Oman commenced justafter obduction of the ophiolite and continued until theEocene (Fournier et al., 2001). The main axis of thisextension was oriented NE–SW and produced severalNW-trending structures, which were later inverted byNeogene compression (Fournier et al., 2001). Thepresence of Tertiary mafic dykes that were intrudedinto the Tertiary sedimentary rocks of the OmanMountains indicates that an elevated heat flow prevailedduring the early Tertiary (Al Harthy et al., 1990;Worthing and Wilde, 2002; Nehlig and Juteau, 1988;Goodall et al., 2001). Wilde et al. (2002) related thehydrothermal alteration to a high heat flow associatedwith post-obduction Tertiary faulting and magmatismand circulation of oxidized groundwaters similar tothose active at present. During hydrothermal processes,generally related to serpentinization, serpentinite may beprogressively transformed into carbonate-silica listwae-nite rocks. Listwaenitization occurs in the shear, thrustand faulted fractured porous zones of ultramafic rockswhich act as pathways for altering fluids. Mostinvestigations show listwaenitization to post-date ser-pentinization of ultramafic rocks and to be super-imposed on the earlier serpentinite (e.g., Spridonow,1991; Schandl and Naldrett, 1992; Ucurum, 2000). Theserpentinites of Oman were silicified and carbonatizedby post-obduction regimes with elevated geothermalgradients.

Acknowledgements

The authors would like to express their thanks to theSultan Qaboos University for financial support. Thanksto all staff member at the Museum of Natural History,London for their cooperation and provision of thechemical analyses. S. Nasir thanks the DAAD for ashort scholarship, which enabled him to stay 3 monthsat the Institute of Mineralogy, Wurzburg, Germany.Thanks to Ulrich Schussler, Wurzburg for his

ization in the Semail ophiolite.

ARTICLE IN PRESSS. Nasir et al. / Chemie der Erde 67 (2007) 213–228 227

assistant and discussion. Martin Okrusch is thanked forthorough reviews and discussions that have greatlyimproved the manuscript. Kirsten Druppel is thankedfor supplying an excel spreadsheet for mass balancecalculation.

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