The Moyale graphite deposit, southern Ethiopia

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NGU-BULL436, 2000-PAGE 169

The Moyale graphite deposit, southern Ethiopia

HAILE-MICHAELFENTAW, SEIDMOHAMMED,NESIBUSEB HAT&HAvARD GAUTNEB

Fent aw,H.M.,Mohammed,S.,Sebh at,N&Gaut neb,H.2000:TheMoyalegraphite deposit southernEthiopia.Norge s geo lo giske undersekelseBulle tin436,169-173.

Precambr ian metamo rphicrocks ofsout hernEt hio pia,belongingtothePan-Afr ican(Mozambiq ue Belt).hostthe Moyalegraphite deposit.Theprinci pal rock units are amphibo leschists,quartz-feldspar-micaschists,granodiorites, quartzit es andgraphiteschists. Graphite-bearingunitsare hostedbyquartzitesand quartz-feldspar-micaschistsand generallyformcontinuo us bodies extending for hundredsof metres.A Regi onalvariatio n in grade and flakesize of thegraphiteisnote d.From itslateralextentandmod e ofoccurrence,thegraphite isbelieved to besediment ary in orig in. Accord ingto imageanalysis,theaveragevolume percentageof graphiteis7.9%;theave ragelengthsofthe shortestand longe staxesofthe graph itegrainsare 155and 407 IJm,respectively.Stat istica l analyses indicate that about60%ofgraphitegrainshavetheir longestaxesbetween 100and300 IJm,the remaining40%fallingin the>300 IJm sizefracti on.Prelimi naryflotation trialsto concent rate thegraphitesinthe<400IJmfractio nresulted in a concent rategradeof71%wit hareco ver yin excessof90%.

Hai le-M ichaelFentaw,Seid Mohammed&NesibuSebhot,Ethiopi anInstitu te of GeologicalSurveys,P.O.Box2302,Addis Abob o,Ethiopia.

HcivardGautneb,GeologicalSurvey of Nor way,N-7491Trondh eim,Nor wa y.

Introduction

Graphit e occurs inmo st of the metamorphic rocks of Ethio- pia. Some ofthe knownoccurrences (e.g.the Kibre Mengist area)are character ised byvery fin eflakes and contain sul- phide minerals. Veintyp e graphite depositshavenot sofar been reported in Ethiopia. The present st udy concerns graphite from the Moyale area, located 750 km sout h of Addis Ababa, on the Kenyan border (Fig. 1).The graphite occurrenceis abo ut6 km westof Moyaletown.Investigat ions of theMoyale graphite have com prisedmap ping thegraph- itebod yby geologicaland geoph ysicalmeth od s, analysisof thegraph it e carbo nconten t,and determ ination of thesize and texture of the graphitegrains. Prelimi nary effortshave also been made to concentrate the graphite by flotation techniques.

Geological setting

The nort heastern branch of the Mozamb ique Belt,a major Proterozoic structuraland metamorphic unit ofEastAfrica, exte nds from Kenya throu gh Et hiopiaand the horn of Africa into sout hern Arabi a (Warde n&Horke1984).In theMozam- biqu e BeltofsouthernEth iopi a, th reemajordivisions(Lower, Midd leand Up per Complexes)have been diffe rentiated by characteristic contrasts in lithology, metamorphism and structural style(Kazminetal. 1978).Metasedimentary rocks (graphitic phyllites, biotite schistsandmetacalcareou srocks) in the Adolaarea formtheuppermostunitof theUpperCom- plexof southern Ethiopia.On thebasisofthisscheme,the rocks of the Moyale region are correlatedwit h thelower part of the Upper Complex, and are probablyNeoproterozoic in age(Alene&Barker1993).According to Hussien(1999), the rock associatio nin the Moyale areaistypical ofa subductio n

complex consist ing of metamorphosed mafic-ultramafic rocks, fore-arc and accretiona l wedge-derived metasedi- mentsand associated rocks.Hussien further suggestedthat the lit hological association inthe Moyaledomain indi cates the existenceofoceaniccrust prior to subd uction.

The majo r orogen icbelt s of theHorn of Africa,Arabian Shiel dandMozambique Beltintersectin the Moyale region (Kazmin et al. 1975,Vailet al. 1986).The local geolog y com- prises polydeformed and metamorphosedmaficand ultramafic rocks,granodiorite s and subordinateamounts of meta-

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Fig.1.Locati onmapof theMoyalegraphite deposits.

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NGU-BULL436,2000- PAGE170 HAILE-M ICHAEL FENTAW,SEID MOHAMMED,NESIBUSEBHAT&HA v A RDGAUTNEB

sediment ary rocks.The dominant rock unitsof the Moyale area are amphibole schist, quartz-feld spar-micaschist,granodiorite,quartzite and graph it e schist.

The graphite occurrence s

Graphite occurrences in the Moyale area are hosted by quartzite,quartz-feldsp ar-m icaschistand rarelyby am phib- oleschist.Pegmatiticgraphitebodies,thoug hlesscommon, usually occurdiscordant to the main schist osity.Occurrences of grap hite inthe Moyaleregi on can be divided intothree areas.Theseshowsome differences inmode of occurrence as wellas in the gradeand flakesize of thegraphi te.

AreasI and III(Fig.2)containnearlycontinuou sgraphit e bands extendi ng for severalkm.Such bands aredislocated byfaultsin the centralregionand showatendency topinch out towardsthewest. AreaI consists ofrelati vely thickgraph- itebands separated by feldspar-qu artz-mica schists.Allthe graphiteschistin thisareadips toward sNE atabout45°and hasarelat ively uniform graph it econtent(Fig.3).Area1Icon- sists ofsho rt,discontinuousgraphit ic sch ist s,host ed mainly by quartzites and pegmatite s. Flake size of the graphite dimin ishes towards the east (Area Ill); very fine grained graphit edom inatesfarthereastfro mArea Ill.The association of graphit e wit h rocksofmedium metamorphicgrade,their wide lateralextent and characte rinoutcrop,as well asthe grade and flakesizeof the graphite,suggests thatthey are, mostprob ably,sedimen taryinorigin.

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Materia ls and exper imen tal methods

Detailed geolog ical mapp ing of the Moyale grap hite was facilit atedby a linesurvey spacedat100 m andlabell ed every 20 m.Theprofile stret ched NE-SW,nearly perpendicularto the regionalschistosity.Trenches wheredug almostparallel to the profile lines.Thelocati on andaverage graphitegrades ofthe trenches areshown inFig.3. These grid lines werealso usedforcollecting ground geophysicaldata.Scint rexTM-2 GENIE/hor izontal loop portab le electroma gnetic transm it- tersand EGS-2/EM-4 receiverswere used in the EMGENIE Survey.

Petrogr aph icstudy wascarried outbyimageanalysis of represent ativethin-sections from graphite -bearingsamp les collect ed from trenc hes.By recording and processing the digitalimages,the volumepercent age of graph iteand the morphological feat uresofgraphitegrains were determined.

Graphite carbo n conten t was determined by aloss of igni- tion (LOI) test.Thematerial is ground tolessthan 500 urn and 1g of the ground samp leis heated in an oven(100°C) overn ight andin a furnacefor 2hoursatapeaktemperat ure of1000°C.

Graphiteconcentrationwascarried outinalabor at ory- scale flotation cell using collectors and frothersincludi ng PineOil,MIBCand EKOFOL and combinationsthereof.Fine

« 400 urn) fract ion s weresubjected to flotatio n.Tw o to threedropsofcollecto rswereusedfor abou t500g ofmate- rial.Concentrates werecollecte d afte ra 3 minu tespindle agitatio n.Aninflowofair from aninletatthesideof the flotation cell facilitate sflot ati on.

Results and discuss ion

Geophysics

Integratedgeoph ysical explorationsurvey s confirmed geologically mapped graphitezones in theMoyale area.The

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HAILE-MICHAELFENTAW,SEIDMOHAMMED,NESIBU SEBHAT&HAvARD GAUTNEB NGU-BULL 436,20 0 0- PAGE171

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graphite -richzonesgave high-lP,low-resistivity, st rong neg- ativeGENIEEMvalues anda moderatelycontrasting radiom- ete ric respon se,incompa risonwit h the surro unding barren rocks.EMGENIE stacked profi lesusingcoilseparat ionsof50 m and 100 m and three frequency pairs,show clearanoma- lies trendingNW-SE in theTinishu Gedem ssaarea(Fig.4).The

trends of the anoma lies closely follow the geologically map ped graphite unit s, separate d by quartz-m ica schists.

How ever,EM failedtopickout the graphite units of Area11.

Thisis probably because the graphites in such an area are host ed by low-lyingmetasedimentaryrocks.All geophysica l anomaliescontinuetowards the sout heast,but as the graph -

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NGU-BULL436,2000 - PAG E172 HAILE-MICHAELFENTAW,SEIDMOHA M MED,NESIBUSEBHA T& HAvAR D GAUTNEB

Table1.Aggregatestatisticaldata obtained fromimage analysisofseveralrepresentative thinsectionsofgraphite,indicatingselected morphologicalfeatures.

Area Area Perim feretmin feretmax ellipsea ellipseb

(lJ m²) (mm²) (IJm) (IJ m) (u rn) (IJ m) (IJm)

Average 36147.8 0.036 1471.9 155.8 407 172.4 57.8

Maximum 1382548 1.383 19290.7 1982.3 3836.5 1919.8 518.7

Minimum 3808.7 0.004 267.4 17.7 85.9 38 5.5

Note:Area =Areaof grains,perim = lengt hofgrain perimeter;feretmin,feretmax = lengt hs ofshortestandlongest axes,ellipsea/ellipseb=

lengt hs of longestandshortestaxesoftheoretical ellipsessurroundingthe grains.

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st at ist icalrepresentativedata aspossibl e,agg rega t evalue s from several thin sectio nswere recorded.The average volume percentag e of graphite wasfo u ndto be 7.9%.Statistical values fo rthe sizeanddimensions of graphiteare shown in Table 1. Av erag egrain size in thin-sect ion is 0.036 mm²;the lo ngestand sho rtes t grain axesare407 and 155 IJm,respectively(Table 1,Gaut neb1997).Moreover,statisticalanalyses indi cate that abo ut60%of graphitegrains have their longest axisbet w een 100 and300 IJm;the remaining40%falling in the>300 IJmsize fraction (Fig .5).

Graphitecarbo n analyses

Grap hit e carbo n conten t was determined for both who le rock sam p lesand the differ ent size fract io ns. The average gradeof thegrap hi te for the who lero ck ran g esfro m 7to 10%

wit ha cumula tiveaverage of 9.01%(Fentaw & Mohammed 1998). The highest graphite concentration is confined bet w een400 and 63 IJm, with the coarser and finer fractions havinggraphit ecarbon conte ntsco m para b lewit h that of the who lerock. As can be seeninFig.3, samples fro m Area11 are ch aract erised by relatively higher graphite contents while tho se in AreaI have nearly uniform graphitecarbon contents.

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it eis finer grainedinthisarea thisdoesnotenco ura g e further exp loration. The Geophysical evid ence strongly indica tes that thegraph it e contentof therocks is the solecau seforthe GENIEEMand lP/Resistivit yrespon ses(Seb hat 1998).

Petrographical analysis

The mainpurposeof petrographical st udywas to determin e thevo lu m e percentage, grain size and mor ph ol o gy of the grap hite.Sucha studyisview edas a pre-requisiteforevalua- tion of graphitebeneficiationtests.Duringsieving,thesho rt- est axiswill be regarded as themaximum size that passes thro ug h the sieve.In other words,theshortest axis corre- spo nds approxima t elyto the libera t io nsize of the graphite grains.Fromthin-sectio n imageanalysis,volume%graphite and morphological data werereco rd ed. To obtain as good Fig. 5. Carboncontentvs.size fractionofsieved samples.

Graphitebeneficiation

Grap h it econ cent rat io n isdependenton the graphite mor- pho lo g y andlibera tion size.Grap hit ewas concentrated by twostagesof flota tion.The first stage of flota tion produced con cent ratesandtailing s.A seco nd flot at io niscarried out to clean the first grap hi te concent rat es and results in th ree product s:a concen trate,middling(m ateri al that sunkduring thesecon dflotation)andtails.Asind icat ed in Table2,itwas po ssibl etoup g rad ethegrap h it efro m 7 to 71%.The grade of the tailingis belo w 1%,suggestingthatnearlyallgraphitein thehead materia liscont ained intheconcentra tesand that the co llect o rs are particularlyeffec tivefor the Moyale samples. This results in a high recovery of the grap hit e, even though the conce nt rates have on ly a moderate grade. The main reason fo rthis is that the head mate ria l consists of coarse material (400 IJm), whith a high incid ence of inter- lo cked quartz-graphite grains.Such grainsare frothed along wit h the pure graphi te and hence belong to the concentrates. Hig h-g rade graph it e concent rat es can best be achi eve d by st epwi se grindi ng and flotation until all non- graphiticmaterialsin theconcentra teare liberated.

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HAILE-MICHAELFENTAW,SEID MOHAMMED,NESIBUSEBHA T&HA VARD GAUTNEB

Tab le 2.Resultsfromthe flotation testsofthe Moyale graphite.

NGU-BULL436,2000 -PAGE173

Sample Product Wtprod. Weight Grade Wt.Grp Recovery

Collector

No. typ e (gm) (%) (%) (gm) (%)

Gt5-8 Concentrate 45.30 14.74 71.4 32.22 95.75 EKOFOL

Gt5-8 Tailing 261 85.25 0.55 1.43 4.25

306.13 100 33.65 33.65 100

Gt 13-4 Concentrate 34.6 11.51 70.2 24.28 95.82 EKOFOL

G[13-4 Tailing 266.00 88.48 0.4 1.06 4.18

300.6 100 25.34 25.34 100

Gt13-4 Concentrate 39.74 14.18 66.5 26.42 95.48 MIBCt EKOFOL

Gt 13-4 Tailing 240.4 85.81 0.52 1.25 4.52

280.14 100 27.67 27.67 100

Gt 12-3 Concentrate 69.39 17.7 52.2 36.22 94.47 MIBCt EKOFOL

Gt 12-3 Tailing 322.6 32.29 0.66 2.12 5.53

391.99 100 38.34 38.34 100

Gt12-3 Concentrate 5.8 1.89 42.4 2.45 9.04 MIBC

GL 12-3 Concentrate 4.7 1.53 57.2 2.68 9.88 MIBCtPINE

G[12-3 Tailing 296 96.57 7.43 21.99 81.08

306.5 100 27.12 27.1 2 100

Composite Concentrate 118.55 11.08 67.1 79.54 93.25 EKOFOL

Composite Tailing (1st) 890 83.21 0.34 3.02 3.54

Composite Tailing(2nd) 61 5.7 4.5 2.74 3.21

1069.55 100 85.3 100

Composite Concentrate 132 11.61 60.8 8.25 92.8 MIBCtEKOFOL

Composite Tailing(lst) 983 84.56 0.41 4.03 4.66

Composite Tailing(2nd) 44.5 3.82 4.94 2.19 2.54

1162.5 100 86.47 86.47 100

Not e:Tailin g (2nd)

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Middlin g;Compo sit e

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Mixedsamples; Head grade7-11%;Wt.Product

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Weightofthe product;Wt. Grade(%)

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Weight percent of grap hite

Conclusions

The economic potential of a graphite-bearingrock depends mainl y onthe graphitecarbon conte nt andflake size, since the marketprice forgraphitereflects thesetwo characteris- tics.The aver agegrade ofthe Moyalegraphiteschist ranges between 7 and 11% (mean 9.1%). The Moyale graphite schiststherefore havemoderatelylow,graphite carbon con- tents. How ever,the graphitegrains have theirlongest axes greater than 100 IJm, whichsuggests that flakesizeisoptimal and that it is technically feasibleto produce graphiteconcen- trates usingthe availabletechnology.TheMoyalearea con- tains an indicated mineralresource of about 450 000 tonnes of graphite.Withoneor two exseptionsthe geomorphologi- calcondition sarealsofavourable for open pit mining.

Ackno wledg em ents

We aregratefu l totheEthi opianInstitut e of GeologicalSurveysforpro- vidingall thenecessaryfinancial support to carryout thisproject.Our special thanksgo to NTNU,Trondheim,foraccessto the facilitiesfor graphi tebeneficiationtests.This workispart of the ETHINORco-opera- tionprogrambetw eenEIGSandNGU.

References

Alene,M.&Barker,A.J.1993:Tectonometamorphi cevolutionofMoyale regionPrecambria nResearch,62,272-283.

Fentaw,H.M.&Mohammed, S.1998:Geologyand EconomicAspectsof Moyale graphite,Unpubli shedreport,EthiopianInsti tu te ofGeologi- calSurveys.

Gautn eb,H.1997.Briefpetrographic stu dy of Moyale area, Sout hern Ethiopia,NorgesgeologiskeundetsekelseReport97.005.

Hussien,B. 1999:TheGeology, Structu reand Geoch em istry of the Crystal- line Rocks of theMoyaleArea,Southern Ethiopia:Implicat ion for Tee- tonogen esis of thePrecambrianBasem ent,Ph.Dthesis,Universityof Tubinqen, Band50,102pp.

Kazmin,V.,Shiferaw, A.&Balcha,T. 1978:The Ethiopian Basement:

St ratigraphyandpossiblemannerof evolution,Geol ogischeRund- schau67,531-546.

Nesibu,S.andseveralco-aut hors.1998:Integrated Geophysical Survey of Moyale Graphite, UnpublishedreportEthiopi anInst itu te of Geo- logical Surveys.

Vail,J.R.,1986.Geology ofSout hern Blue NileProvince,Sudan,Bull. Geol.

Res.Autho ritySuda n,32pp.

Warden,A.J.&Horke,AD.1984:The Geological Evolut ionofNE-Branch oftheMozambiqueBelt(Kenya, Somalia,Ethiopia),Mitteilungender OesterreichischenGeolagischenGesellschaft77,161-184.

The Moyale graphite deposit, southern Ethiopia