Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt

30 Pages·2016·6.79 MB·English

by  Arman Boskabadi

International Geology Review
ISSN: 0020-6814 (Print) 1938-2839 (Online) Journal homepage: http://www.tandfonline.com/loi/tigr20
Carbonate alteration of ophiolitic rocks in the
Arabian–Nubian Shield of Egypt: sources and
compositions of the carbonating fluid and
implications for the formation of Au deposits
Arman Boskabadi, Iain K. Pitcairn, Curt Broman, Adrian Boyce, Damon A.
H. Teagle, Matthew J. Cooper, Mokhles K. Azer, Robert J. Stern, Fathy H.
Mohamed & Jaroslaw Majka
To cite this article: Arman Boskabadi, Iain K. Pitcairn, Curt Broman, Adrian Boyce, Damon A.
H. Teagle, Matthew J. Cooper, Mokhles K. Azer, Robert J. Stern, Fathy H. Mohamed & Jaroslaw
Majka (2016): Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt:
sources and compositions of the carbonating fluid and implications for the formation of Au
deposits, International Geology Review, DOI: 10.1080/00206814.2016.1227281
To link to this article: http://dx.doi.org/10.1080/00206814.2016.1227281
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INTERNATIONALGEOLOGYREVIEW,2016
http://dx.doi.org/10.1080/00206814.2016.1227281
Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt:
sources and compositions of the carbonating fluid and implications for the
formation of Au deposits
Arman Boskabadia,b, Iain K. Pitcairnb, Curt Bromanb, Adrian Boycec, Damon A. H. Teagled, Matthew J. Cooperd,
Mokhles K. Azere, Robert J. Sterna, Fathy H. Mohamedf and Jaroslaw Majka g,h
aGeosciencesDepartment,University ofTexasatDallas,Richardson,TX,USA;bDepartmentofGeologicalSciences,Stockholm University,
Stockholm,Sweden;cScottishUniversitiesEnvironmentalResearchCentre,EastKilbride,UK;dNationalOceanographyCentreSouthampton,
UniversityofSouthampton,Southampton,UK;eGeologyDepartment,NationalResearchCentre,Cairo,Egypt;fGeologyDepartment,Faculty
ofScience,AlexandriaUniversity,Alexandria,Egypt; gDepartmentofEarthSciences,UppsalaUniversity,Uppsala,Sweden;hFacultyof
Geology,GeophysicsandEnvironmentalProtection,AGH–UniversityofScience andTechnology,Kraków,Poland
ABSTRACT ARTICLEHISTORY
Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive Received22January2016
carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Accepted17August2016
Notwithstanding the extent of the alteration, little is known about the processes that caused it,
KEYWORDS
the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, Arabian–NubianShield;
stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably serpentinite;listvenite;
carbonatealtered ultramaficsfrom theMeatiqareaof theCentralEastern Desert(CED)of Egypt. carbonation;gold;
Thesamplesinvestigatedincludeleast-alteredlizardite(Lz)serpentinites,antigorite(Atg)serpen- fluid-mobileelements;stable
tinitesandlistveniticrockswithassociatedcarbonateandquartzveins.TheC,OandSrisotopesof isotopes;Srisotopes
theveinsamplesclusterbetween−8.1‰and−6.8‰forδ13C,+6.4‰and+10.5‰forδ18O,and
87Sr/86Srof0.7028–0.70344,andplotwithinthedepletedmantlecompositionalfield.Theserpen-
tinitesisotopiccompositionsplotonamixingtrendbetweenthedepleted-mantleandsedimen-
tary carbonate fields. The carbonate veins contain abundant carbonic (CO ±CH ±N ) and
2 4 2
aqueous-carbonic (H O-NaCl-CO ±CH ±N ) low salinity fluid, with trapping conditions of 270–
2 2 4 2
300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile
elementsrelativetoprimitiveanddepletedmantle.TheextensivelycarbonatedAtg-serpentinites
containsignificantlylowerconcentrationsoftheseelementsthantheLz-serpentinitessuggesting
thattheyweredepletedduringcarbonatealteration.Fluidinclusionandstableisotopecomposi-
tionsofAudepositsintheCEDaresimilartothosefromthecarbonateveinsinvestigatedinthe
study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived
CO -bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au
2
andStothehydrothermalfluidsthatlaterformedtheAu-deposits.Thisisthefirsttimethatgold
hasbeenobservedtobe remobilizedfromrocksduring thelizardite–antigoritetransition.
1. Introduction carbonation due to abundant olivine [(Mg,Fe) SiO ] and
2 4
pyroxene [(Ca,Mg,Fe) Si O ], which react with H O and
Carbonationofmaficandultramaficrocksisanimportant 2 2 6 2
CO to form hydrous silicates such as serpentine, Fe-oxi-
geological process with implications that include carbon 2
des (magnetite) and carbonates (Kelemen and Matter
sequestration (Seifritz 1990; Lackner etal. 1995; Kelemen
2008).Carbonationofmaficrockshasbeenusedtocalcu-
andMatter2008),thecarboncycleandfluxingofCO to
2 latefluxesofCO totheEarth’satmosphereinthegeolo-
theEarth’satmosphere(KerrickandCaldeira1998;Lasaga 2
gicalpast(Skelton2011),andmaybelinkedtoformation
et al. 2001; Skelton 2011), and formation of ore deposits
ofAudepositsworldwide(Grovesetal.1998).
(Barnes et al. 1973; Groves et al. 1998; Phillips and Evans
Ophiolites of Arabian–Nubian Shield (ANS) are frag-
2004). Carbonation occurs when susceptible rocks in the
mentsoftheMozambiqueOceaniccrustgeneratedina
crustandmantleinteractwithCO -richfluidsresultingin
2 supra-subduction zone setting during Neoproterozoic
alterationandprecipitationofcarbonateandotherminer-
time (Stern et al. 2004; Azer and Stern 2007). The
als. Mafic and ultramafic rocks are particularly prone to
CONTACTArmanBoskabadi [email protected] DepartmentofGeosciences,TheUniversityofTexasatDallas,ROC21,800West
CampbellRoad,Richardson,TX75080-3021
Supplementaldataforthisarticlecanbeaccessedhere.
©2016InformaUKLimited,tradingasTaylor&FrancisGroup
2 A.BOSKABADIETAL.
Pharaonictimes(Harraz2000;KlemmandKlemm2013).
A significant proportion of these deposits occur in or
near carbonated ultramafic rocks (Osman 1995; Botros
2002, 2004; Zoheir and Lehmann 2011; Abd El-Rahman
et al. 2012). Despite this clear spatial relationship, the
genetic relationship among carbonation, deformation
and Au deposition remains poorly understood.
Thisstudyinvestigatesthesourcesandcompositions
of carbonate altered serpentinites in the Meatiq area of
the CED. We use stable (C, O) and radiogenic (Sr) iso-
topes to determine the source of CO -rich fluids, and
2
fluidinclusionmicrothermometryandmineralchemistry
toconstrain the conditions ofalteration. Thisstudy also
investigates the element mobility that has occurred
duringthecarbonatealterationanddiscussestheimpli-
cationsoftheseresultsfortheformationofAudeposits
in the CED.
2. Geological setting
The Arabian–Nubian Shield represents the northern part
ofEastAfricanOrogen(EAO),whichformedinTonianand
Cryogeniantimeaccompanyingterraneaccretionaround
theMozambiqueOcean(Stern1994,2002).TheEgyptian
segmentoftheANSisexposedintheEasternDesert(ED)
andcanbesubdividedintofourmainlithotectonicunits:
(i) a lower unit ‘infrastructure’ composed of gneiss and
deformed granitoids, (ii) a structurally overlying ‘supras-
tructure’ composed of metamorphosed ophiolitic and
islandarcassemblages,(iii) a sequenceofnon-metamor-
Figure 1. Simplified geological map showing the extent of the
phosed to weakly metamorphosed and deformed volca-
ANS(inset),thedistributionofophioliticbodiesandselectedAu
deposits in the Egyptian segment of ANS. nic (Dokhan Volcanics) and sedimentary rocks
(Hammamat Group) that unconformably overlie the
suprastructure in places, and (iv) post-orogenic granitic
plutons that commonly intrude the other units
ophiolites constitute one of the most distinctive rock (Andresen et al. 2009, 2010). Over 30 different locations
sequences among ANS basement rocks (Figure 1). The ofophioliteshavebeenmappedintheEgyptiansegment
ophiolitic rocks are commonly altered due in part to of the ANS. (Figure 1; Stern et al. 2004). These consist of
interaction with migrating carbonate-rich solutions tectonically disrupted fragments ofmantleserpentinized
(Stern and Gwinn 1990). Carbonation of the ophiolite ultramafic rocks and crustal gabbro, sheeted dykes and
units is extensive and indicates a large flux of CO - pillow basalts (Stern etal. 2004). CEDophiolitic rocksare
2
bearing fluid. However, there have been few investiga- stronglydeformedandmetamorphosedto conditions of
tions into the sources and composition of carbonating lower greenschist and amphibolite facies (Johnson et al.
fluids, or the conditions at which this alteration 2004;Sternetal.2004).RadiometricagesofANSophiolites
occurred. The carbonation has been focused along range between 880 and 740 Ma (Ries et al. 1983; Kröner
faults and shear zones and has resulted in formation 1985; Kröner et al. 1992, 1994; Stern 1994; Loizenbauer
of variably foliated talc-rich rocks previously referred to et al. 2001), with two age maxima from about 850 to
as ‘talc-carbonate schists’, ‘Barramiya rocks’, ‘listvenite- 780Maandfrom750to700Ma(Stern1994).
like rocks’ (Azer 2013) or ‘talc-rich rocks’ (Stern et al. The study area is known as Meatiq Dome, which is
2004; Ali-Bik et al. 2012). located about 40 km west of the Red Sea (Figures 1 and
TheophioliticrocksinsomepartsoftheANSsuchas 2). The Meatiq Dome is a large, ca. 500 km2 quartzofelds-
theCentralEasternDesert(CED)ofEgyptcontainabun- pathicgneisscomplexsurroundedbylow-gradeophiolitic
dantAu deposits(Figure1)thathave beenminedsince melange (Andresen et al. 2010). The eastern and western
INTERNATIONALGEOLOGYREVIEW 3
Figure2. (a)GeneralgeologicalmapofNeoproterozoicbasementintheMeatiqarea(afterAndresenetal.2009;El-Gabyetal.1984;
Loizenbaueretal.2001;AbdEl-Rahmanetal.2010,2010),showingimportantlithologies,samplelocalitiesandstrike-slipstructures.
(b) Inset showsthe locationof theMeatiq areainthe CED,Egypt.(c) Schematic cross-section across MeatiqGneiss Dome showing
therelationshipbetweentheMeatiqDomelithologies,thesurroundingophioliticrocks,HammamatclasticsandDokhanVolcanics.
The ages are from Andresen et al. (2009).
dome margins are bounded by sinistral strike-slip shear The structurally overlying ophiolitic and island arc
zones that belong to the Najd Fault System whereas the assemblages are considered to be ‘suprastructure’
northernandsouthernmarginsaredefinedbyprominent (Habib et al. 1985; Neumayr et al. 1996, 1998), Pan-
normalfaults(Figure2,Wallbrecheretal.1993).UmBa’anib African nappe complex (El-Gaby et al. 1988; Bregar
Orthogneiss that comprises the core of the Meatiq Dome et al. 2002) or eugeoclinal allochthon (Andresen et al.
consists of coarse-grained, foliated orthogneisses that 2009, 2010), and are characterized by greenschist- to
become gradually more mylonitized and fine-grained at lower amphibole facies mineral assemblages (Neumayr
shallower levels, forming a garnet-bearing mylonitic cara- et al. 1998). A high-strain zone separates the amphibo-
pace (Andresen et al. 2009). The Um Ba’anib Orthogneiss lite-facies orthogneisses in the core of the high-grade
andtheuppermetasedimentsareoftenconsideredtobe metamorphic domes from the surrounding lower grade
‘infrastructure’(Habibetal.1985;Neumayretal.1996,1998). ophiolitic rocks (Ries et al. 1983; Sturchio et al. 1983;
TheprotolithageoftheUmBa’anibOrthogneissis631Ma, Habib et al. 1985). Ophiolitic rocks of the Meatiq area
whereas the undeformed Arieki granite intruded the suc- comprise mainly serpentinites and their altered pro-
cessionat590Ma(Andresenetal.2009). ducts such as listvenitic rocks, metagabbros and meta-
A variably mylonitized and metamorphosed package basalts. Strictly speaking, the term listvenite (also
of ortho- and paragneisses, overlies the Um Ba’anib spelled listvanite, listwanite, listwaenite; Kelemen et al.
Orthogneiss. These are grouped as mylonitized 2011) refers to fuchsite-quartz-carbonate lithologies
gneisses and called Abu Fananni Thrust Sheet by derivedfromultramaficrocksbypotassicandcarbonate
Habib et al. (1985). A detailed description of the meta- metasomatism(HallsandZhao1995).Fuchsiteisabsent
morphic evolution of the Abu Fannani Thrust Sheet is inthesampleswestudied,andtheyarealsocarbonate-
given by Neumayr et al. (1996, 1998) and Loizenbauer poor, and therefore we prefer to use the terms talc-rich
et al. (2001). The mylonitized paragneisses are intruded and/or listvenitic rocks instead of talc carbonates or
in south by the syn-tectonic Abu Ziran diorite, which listvenites.
yields a U–Pb zircon and titanite age of 606 Ma The contact of serpentinites with country rocks is
(Andresen et al. 2009). The Abu Ziran diorite is gener- highly sheared and deformed with the development of
ally undeformed, but the western margin of the pluton mylonitized metasediments (Loizenbauer et al. 2001).
contains a >50 m wide ductile shear zone (Andresen Serpentinites constitute up to 10% of the total area of
et al. 2009). suprastructure around the gneiss dome and form
4 A.BOSKABADIETAL.
elongated folded tabular bodies or sheets (Figure 2). rocks represent the least altered serpentinites in the
Ophiolitic serpentinites underlie and are intercalated studyarea.LocalityBontheNEflankofthedomeshows
with the meta-volcano-sedimentary rocks. Andresen a complex exposure of greenish grey serpentinite and
et al. (2009) reported a crystallization age of shearzonehostedtalc-richrocks(Figures3(c)and4).The
736.5 ± 1.2 Ma for zircons from an ophiolitic gabbro of serpentinitesfromlocalityBaresofteranddonotshowthe
thewesternpartofMeatiq(Fawakhirarea),whichwetake chrysotileveinletsseenatlocalityA.Theserocksareheav-
toapproximatetheageofophiolitesinthestudyarea.A ily sheared in places with yellowish-white to brownish-
detailed age constraints on the evolution of the cream coloured talc-rich rocks occurring in the shear
Neoproterozoic Meatiq Dome has been given by zones(Figure3(d,e)).Veins,nodulesandirregularpockets
Andresenetal.(2009). of magnesite and dolomite with quartz occur within the
Asimpleinterpretationofthestructuralevolutionofthe talc-rich bodies (Figure 3(e)). The quartz and carbonate
Meatiq Dome study area is that ophiolitic suprastructure veins generallyhavea NW-SEtrendconcordantwiththe
originally rested roughly horizontally above gneiss infra- foliationinthetalc-richbodies(Figures3(e)and4).
structure.Someoftheshearsinthesuprastructureformed The talc-rich rocks are mostly very easily weathered
during ophiolite emplacement sometime after 736 Ma. and so despite the excellent exposures in this area,
Highheatflowaccompanyingsyntectonicgranitoidempla- sampling of these rocks was limited. Most samples
cement weakened the infrastructure at roughly the same from locality B were collected from a talc quarry
timethattheNajddeformationoccurredat~600Ma.The (Figures 3(f) and 4). The contacts between the talc-rich
heating combined with the Najd deformation caused rocks and the serpentinite range from gradational to
upwelling of the infrastructure forming the Meatiq Dome sharp (Figure 3(d)), and some talc-rich bodies were
(Andresenetal.2009).Ifthisinterpretationisapproximately observed to have a sharp contact on one side and a
true,thenstudyareasA,BandCareallcomponentsofthe gradational contact on the other (Figure 4).
lowersuprastructure,andcarbonatealterationoccurredat Locality C, which occurs SE of locality B, shows a
oneormoretimesduringthissequence. shear zone hosted exposure of talc-rich rocks within
The Fawakhir Au mine (Au = 1.5–29.7 g/ton; Hussein serpentinites(Figure 3(g–j)).Thearea exhibits thestruc-
1990) occurs in the western part of the study area, tural controls on listvenitization with the majority of
associated with a 15 m wide zone of graphitic schist in talc-rich bodies showing sharp fault contacts (Figures 3
the contact of serpentinites, metagabbros and grani- (g,h)). These rocks in shear zones experienced intensive
toids. It is one of the several Au mines associated with mylonitization and are characterized by talcose and
CED ophiolitic rocks that have been extensively worked schistose fabrics making talc-schists (Figure 3(i)). At
since Pharaonic and Roman times (Harraz 2000). one area the talc-rich rocks have been partially exca-
vated allowing sampling of relatively fresh rocks. These
bodies extend up to several hundred meters in length
3. Sampling and are up to 25 meters wide (Figure 3(j)).
Inordertoinvestigatethesourcesoffluids,theconditions
of alteration, and chemical changes during alteration of 4. Analytical methods
the ophioliticfragments intheMeatiqarea, a suiteof17
4.1. SEM
samples of serpentinites, talc-rich rocks and associated
quartzandcarbonateveinswerecollected(Table 1).The Polished thin-sections of selected serpentinites, talc-rich
sampleswerecollectedfromthreemainlocalitieswiththe rocks and carbonate veins were examined with a Philips
choiceofsampleareacontrolledbythestyleofserpenti- XL30 FEG environmental scanning electron microscope
nization,theextentofcarbonation,andcrucially,theavail- (ESEM) at Stockholm University, operating at 20 kV and
abilityoffresh,unweatheredmaterial.Talc-richsamplesin equippedwithOXFORDenergydispersiveanalyticalX-ray
particular are extremely easily weathered, and therefore spectrometer. The spectrometer detects elements with
most of these samples were collected from quarries. atomicnumber>4(Bandheavierelements).
LocalityAconsistsoflargeroadsideexposuresofserpenti-
nite SW of the Meatiq Dome (Figure 2). The serpentinite
4.2. EPMA
outcropsarebrownishgreyincolourandoccurasmassive
unitswithbrecciationalongfaultswithclearlydeveloped In situ analyses of Cr-spinels of Lz-serpentinites were
macroscopic mesh texture and cross-cutting chrysotile carried out using a field emission electron probe micro-
veinlets in hand specimen scale (Figure 3(a,b)). These analyser (FE-EPMA), JXA-8530F JEOL HYPERPROBE at
INTERNATIONALGEOLOGYREVIEW 5
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Me28 n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d._n.d.3.30.083.39.533.60.001236518.10.02n.d.n.d.n.d.n.d.n.d.5346.515732358771513247088248995632355073026577148219511591568.32.849085.90.441.0 nued
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Me17 57.921.577.980.1223.366.550.09<0.010.033<0.013.13100.82471183060371b.d.l.8.00.9b.d.l.b.d.l.b.d.l.100.24n.d.n.d.b.d.l.b.d.l.3406708024040381002011030901410017b.d.l.b.d.l.4000b.d.l.301.8
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study Lz-ser Me22 39.290.57.280.0837.990.76<0.01<0.010.001<0.0113.5599.466.936.71924101.522550.0918.50.710.030.7134.528010.312.4502732.31512589127418071311928.811816.496.918.545.15.735.74.43.73.997.25.73750.74
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C %)
Table1. Lithologysample (wt.SiO2OAl23FeO(T)MnOMgOCaOONa2OK2TiO2OP25L.O.I.TotalSc(ppm)VCrCoNiRbSrYNbSbAsSAu(ppb)Au**LiCsBaLaCePrNdSmEuGdTbDyHoErTmYbLuHfTaPbThUEu/Eu*
6 A.BOSKABADIETAL.
Department of Earth Sciences, Uppsala University,
8
2 1 3
Me 0.61.51.62.032. Sweden. Analytical conditions were 15 kV accelerating
ns voltage, 10 nA probe current and 1 μm beam diameter,
ei
v and the raw data were corrected with an PAP routine.
onate Me12 0.620.911.01.469.1 Counting time was 10 s on peak and 5 s on ± back-
arb ground. Kα spectral lines were measured. Analytical
C
standards were natural and synthetic silicates and oxi-
2
e 36 3
M 2.711.12.2.288. des. The compositions of chromite are recalculated to
cationproportionsusingtheFe3+calculationschemeof
7 Droop (1987). The results are reported in
Me1 5.52.42.10.911.9 Supplementary Table.
5
ocks Me1 _2.01.5_0.96 4.3. Gold analyses
r
Talc-rich Me14 _5.75.7_0.54 pentinites. GUthonelidveurlstaritnay-a,lloyuwsseinsdgetwTehecerteiromnocalXirmrSieeitrdiemsoe2tuhtIoCdPa-tMdeSSsctfoorilcblkoehwdoinlmign
ser Pitcairn et al. (2006, 2014). The 3σ method detection
e
m limit is 0.033 ppb Au. Analytical precision for Au
1 a
e s
M 3.74.16.11.51.1 he analyses were controlled through analyses of
t
of CANMET reference material TDB1 and USGS refer-
p
Me24 _3.21.90.070.15 dgrou erenpcreesemnattaetriviaelsAWuMcSo-n1,ceCnHtr-a4t.ionIns oinrdesrerptoentoinbittaeisn,
n
o nine hand specimen samples of serpentinite were
c
e
s divided into two groups, and were powdered sepa-
3 e
nites Me2 3.12.42.70.643.5 orth rately (total of 18 rock powder) using a hardened
enti esf steel tema. The Au concentrations of these two
p u
Lz-ser Me22 3.211.814.42.13.2 Au**val ganrodupasntaimreonreypoarnteadlysseespawraetreelyailnsoTacbalrerie1d. Aorustenaict
mit), SfltuoocrkehsocelmnceUnsivpeercstirtoy,mbeytryhyd(HriGd-eAFgSe)neurasitniogn-aatoPmSiAc
Me21 3.325.822.14.52.9 ctionli 10.055 Millennium Excalibur instrument following
e the methods described in Pitcairn et al. (2006,
et
d
2014). Analyses were carried out on the same acid
w
Me9 7732707 elo digests as those used for Au analyses. Method detec-
9.1.1.0.0. d.l.(b tion limits, using acid digested blanks for 3σ, are
b. 0.043 ppb As and 0.079 ppb Sb. Reference materials
erpentinites Me7ME8 2464.30.441.00.220.560.030.230.030.08 (notdetermined), TtiniDcaBTla-1pb,rleeWc1Mis.iSo-n1aannddaCcHc-u4rawcye.reThueserdesutoltscoanretrorelpaonratelyd-
Atg-s b,n.d. 4.4. Major and trace element analyses
Me6 394603511 npp Sampleswereanalysedformajorandtraceelementsby
3.0.0.0.0. i
U ICP-MS following lithium metaborate/tetraborate fusion
o
ut and total acid digestion at Activation Laboratories Ltd.
5 A
Me 41773112 m (Actlabs),Canada.AnalyseswerecarriedoutonaVarian
ued). 2.1.0.0.0. m,fro Vista 735 ICP-MS and calibration was performed using
n p seven prepared USGS and CANMET reference materials.
onti pm) inp Loss on ignition (LOI) is determined by weight differ-
Table1.(C Lithologysample (La/Sm)N(La/Yb)N(La/Lu)N(Tb/Yb)NTotalREE(p FromSctoS ebbneecttwweeeaeefnnter00i.0g.00n05i1ti%oanndaatn5d10p00p0.m0°C1.%fTohrfeotrrdaectmeecaetjoiloermneelinlmetmsit,seanantrsde,
INTERNATIONALGEOLOGYREVIEW 7
Figure3. Field photographs of serpentinites and talc-rich rocks in the study area. (a) Mesh-textured Lz-serpentinite from locality A
with visible chrysotile veinlets. (b) Lz-serpentinite exposures in locality A showing cross-cutting veinlets of fibrous chrysotile (Ctl).
(c) Atg-serpentiniteshowingrelict meshtexturesfromlocalityB.(d)ExposuresofalteredAtg-serpentinitefromlocalityB.Thepale
layers are highly sheared talc-rich rocks. Location of photo is shown in Figure 4. (e) A carbonate (Cb) and quartz (Qz) vein hosted
within sheared talc-rich rocks from locality B. Location of photo is shown in Figure 4. (f) The talc quarry from locality B showing
cut surfaces of Atg-serpentinite. Location of photo is shown in Figure 4 (g) talc-rich rocks in a shear zone with sharp contacts to
altered serpentinites in locality C. (h) An exposure of serpentinite with pale coloured shear-hosted talc-rich rocks from locality C.
Sharp contact with shear zone to altered serpentinite control the location of the talc-rich rocks. (i) Sheared talc-schist between
more coherent bodies of Atg-serpentinite from Locality C. (j) The view north from locality C showing extensive outcrop of shear-
hosted listvenitic rocks within serpentinite.
between 0.5 ppm and 0.001% for base metals. Selected Analyses were carried out on a Thermo Fisher
traceelementswereanalysedbyICP-MSattheNational Scientific XSeries 2 ICP-MS with accuracy and precision
Oceanography Centre Southampton (NOCS) following determined from repeated measurements of JA-2. The
hydrofluoric, perchloric and nitric acid digestion. results are reported in Table 1.
8 A.BOSKABADIETAL.
Figure4. SimplifiedgeologicalmapoflocalityBshowingpatchesoftalc-richrockswithinantigoriteserpentinite,thelocationofthe
talcquarry,samplinglocalities,andthepositionwherephotosinFigure3weretaken.Thedottedlineleadingtothequarryisaroad.
4.5. Sr isotope analysis 4.6. Stable isotope (C and O)
Sr isotope analysis was carried out at the National Carbon and oxygen isotope analyses were carried out
OceanographyCentreSouthampton(NOCS).Samplepre- on the carbonate fraction of serpentinite whole rock
paration procedure is the same as explained for trace samples and three carbonate veins. The serpentinite
elements analyses at NOCS. The mother solutions were samples were prepared by micro-milling of carbonate
subsampled to give approximately 1μg Sr and the Sr bearing areas within the serpentines to produce a car-
isolated using 50 μl Sr-Spec resin columns, the column bonate rich powder. Isotope analyses were undertaken
blankswere<0.1ng.Thedriedsampleswereloadedonto using an automated triple-collector gas source mass
asingleTafilamentwithaTaactivatorsolution.87Sr/86Sr spectrometer (Analytical Precision AP2003) linked to an
wasanalysedusingmultidynamicpeakjumpingroutines automated gas preparation device at the Scottish
onThermoFisherScientificTritonPlusThermalIonisation Universities Environmental Research Centre (SUERC),
MassSpectrometerswithabeamsizeof88Sr=2Vnormal- East Kilbride. For the C isotope analyses, 2 mg of the
ized to 86Sr/88Sr = 0.1194. The long-term average for powdered whole-rocks and carbonate veins samples
NBS987 on the instrument is 0.710244 ± 0.000019 (2sd) were reacted with 103% phosphoric acid to produce
on138analyses.TheresultsarereportedinTable2. carbondioxide,whichwasthenpurifiedbeforeanalysis.
Table 2. Geochemical and isotopic data, Meatiq study area.
Concentrations(ppm)a δ18O δ13C
Sample Rb Sr 87Rb/86Sr 87Sr/86Sr (87Sr/86Sr) b (SMOW) (PDB)
600
Atg-serpentinites
Me05 0.028 0.23 0.352 0.70697±17 0.70396 11.0 −5.4
Me06 0.025 0.42 10.6 −5.9
Me07 0.022 0.31 11.0 −5.6
Me08 0.031 0.40 0.226 0.70779±15 0.70585 10.3 −5.6
Me09 0.009 0.45 11.5 −5.3
Lz-serpentinites
Me21 0.11 5.82 0.054 0.70669±16 0.70622 15.1 −4.2
Me22 0.093 18.49 0.014 0.70461±13 0.70448 14.0 −4.1
Me23 0.117 69.75 0.005 0.70432±16 0.70428 12.3 −4.2
Me24 0.033 56.37 0.002 0.70422±14 0.70421 13.7 −4.2
Carbonateveins
Me02 0.04 1135 0.0001 0.70344±12 0.70344 6.4 −8.1
Me12 0.005 503 0.00003 0.70281±13 0.7028 9.4 −6.8
Me28 0.001 2365 0.000001 0.70293±13 0.70293 10.5 −6.8
aBlank-correctedconcentrations.
bIsotopiccompositionat600Ma.
INTERNATIONALGEOLOGYREVIEW 9
Samples were reacted for 96 h at a constant tempera- carbonate veins prepared for microthermometry study
tureof70°C. Gasmineralfractionation fordolomite was was selected for determination of fluid composition
calculated following Rosenbaum and Sheppard (1986). trapped in inclusions. In addition, serpentinite samples
Magnesite and mixtures of magnesite and dolomite fromlocalities A andB wereanalysedtoidentify serpen-
were treated as dolomite. Precision and accuracy were tineminerals.AnAr-ionlaser(λ=514nm)wasusedasthe
monitored by reference to long-term analysis of labora- excitationsourcewithanoutputpoweratthesampleof
toryandinternationalstandards.Precisionisbetterthan 8 mW. The instrument was integrated with an Olympus
0.2‰at1σforcarbonandoxygen.Resultsarereported microscopeandthelaserbeamwasfocusedtoaspotof
as δ‰ values relative to the V-PDB and V-SMOW scales 1 µm with a 100× objective. The spectral resolution is
for C- and O-isotopes, respectively (Table 2). about 0.3 cm−1. The instrument was calibrated using a
neon lamp and the Raman line of a silicon wafer
(520.7 cm−1). Instrument control and data acquisition
4.7. Microthermometry wasmadewithLabSpec5software.
Three dolomite samples(Me2, Me12, Me28) andmagne-
site veinlets in serpentinite sample Me9, and one quartz
5. Results
sample (Me3) were selected for detailed fluid inclusion
studies.Thesampleswerepreparedas150µmthickdou- 5.1. Petrography and mineralogy
bly polished sections. After careful documentation and
Serpentinites from locality A (referred to from this point
selection of fluid inclusions, microthermometric analyses
onwards as Lz-serpentinite) are composed essentially of
werecarried out at Stockholm University usinga Linkam
lizarditeandlatechrysotileveinletswithminorcarbonate,
THM600heatingandcoolingstagemountedonaNikon
opaque minerals and traces of chlorite, bastite pseudo-
microscope utilizing a 40× long working-distance objec-
morphsandtalc.Inplaces,serpentinemineralsretainthe
tive. The reproducibility was ±0.1°C for temperatures
crystal habit of the original mafic minerals. Chrysotile
below +40°C and ±0.5°C for temperatures above 40°C.
occurs as long fibrous veinlets cross-cutting the lizardite
The stage was calibrated with synthetic fluid inclusion
matrix,indicatingprotractedserpentinization(Figure5(a,
standards (SynFlinc®) and well-defined natural inclusions
b)). Carbonates are stained with iron-oxide and occur as
inAlpinequartz.TheresultsarelistedinTable3.
scatteredblockyaggregatesorfillingthemeshtextureof
original mafic mineral or secondary chrysotile veinlets
(Figure 5(b)). The opaque minerals in the serpentinites
4.8. Raman spectrometry
are Cr-spinel, pentlandite and magnetite. The Cr-spinels
Raman spectrometry analyses were performed using a (1–3%byvolume)areslightlyalteredandoccurasdeep-
laser Raman confocal spectrometer (Horiba instrument red grains with sub-rounded outlines. Locally the Cr-spi-
LabRAMHR800)equippedwithamultichannelaircooled nels show slight alteration at grain boundaries forming
CCD detector at Stockholm University. The same set of narrow rim of Cr-magnetite with large unaltered core
Table 3. Microthermometric data of fluid inclusions from the Meatiq study area.
Sample Fluidinclusiontype TmCO (°C) Tmice(°C) Tm,cla(°C) ThCO (°C) Salinity(mass%NaCl) Thtotal(°C)
2 2
type-I
Dol-Me2 Mg-Na-Cl-H0 −2.6to0.0 0.0to4.3 239to265
2
Dol-Me2 Ca-Na-Cl-HO −16.4to−4.0 6.4to16.9* 225to383
2
Dol-Me2 Mg-Na-Cl-H0secondary −1.3to0.0 0.0to2.2 161to255
2
Qz-Me3 Mg-Na-Cl-H0 −3.5to−0.1 0.2to5.7* 225to266
2
Qz-Me3 Mg-Na-Cl-H0secondary −3.5to−0.1 0.2to5.7 177to234
2
Mgs-Me9 Mg-Na-Cl-H0 −2.4to−0.5 0.9to5.0 231to335
2
Dol-Me12 Mg-Na-Cl-H0 −2.9to−0.8 1.4to4.8 253to302
2
Dol-Me28 Mg-Na-Cl-H0 −3.3to−1.0 1.7to5.5 262to315
2
type-II
Qz-Me3 CO±(CH/N)-HO-NaCl −57.7to−56.6 7.7to9.5 26.8to28.0 1.0to4.7* 305to331
2 4 2 2
324to365(g)
Dol-Me12 CO±(CH/N)-HO-NaCl n.d. 7.5to9.8 0.2to4.9 252to276
2 4 2 2
Dol-Me28 CO±(CH/N)-HO-NaCl −57.3to−56.6 6.6to10.0 26.8to29.2 0.0to6.8 242to331
2 4 2 2
type-III
Qz-Me3 CO±(CH/N) −57.3to−56.6 26.8to29.5
2 4 2
Tm,CO,meltingoftheCO;Tm,cla,meltingofCO hydrate;Th,CO,homogenizationoftheCO phasestoliquid(l);Tm,ice,finalmeltingofice;Salinity,wt.%
2 2 2 2 2
NaCl calculated from Tm,ice or Tm,cla; Th,total, total homogenization to the liquid phase or gas phase (g); Tc, critical homogenization temperature;
Dol,dolomite;Qz,quartz;Mgs,magnesite;n.d.(notdetected).*mass%(NaCl+CaCl)eq.
2