NGU-BULL436,2000 -PAGE67
The geology, exploration and characterisation of graphite deposits in the Jennestad area, Vesteralen, northern Norway
HAvARD GAUTNEB&EINARTVETEN
Gautneb,H. &Tveten,E.2000:The geology,explorationandcharacte risatio nof graphitedepositsintheJennestad area,Vesteralen, nort hernNorway.Nor gesgeologiskeundersekelseBulletin436,67-74.
Thispaperreviewsgraphiteexplorationin theJennestadarea,Nordland. As aresult ofhelicopteraeromagnetic surveyingand subsequentgroundgeophysics,mappin g and trenching,some30,variouslysized bodi es of graphite schistwereidenti fied.Thegraphite-b earing schist occurs associated with dolomite marbles,amphibolitesand pyroxene gneisses,allofwhich areintruded bycharnockit esandgranites.Thegraphiteis coarse,fullyordered, crystallineandflaky.Gradesupto 40%carbonwere found.Ganguemineralsintheore arequartz,plagioclase,K- feldspar,biotit eandort hopy roxene.Someof thelargestorebodiescontai nabout250,000 tonneseachwit han averagegradeof20%carbon.Bench-scalebeneficiat ion testsshown thatthe orecanbeupgradedto amaximum grade of97%C. wit ha recovery of89%.Itis believed thatthe graphite schistswereoriginallysedi mentsrichin organic matterwhichwase convertedto grap hiteduring granulite-faciesmetamo rph ism.
Havard Gautne b&finarTveten,Geol ogi cal SurveyofNorway,N-7491 Trond hei m,Norwa y.
Introduction
Norw ayhasbeen amajorEuropeanproducerof flakegraph- itefor almost acentury and is todayoneof two Europ ean producers.Threemajorgraphitemineshavebeen operating in Norwayduring thelastcentury:theRendalsvik mine in Hol and sfjord sout h of Glomfjo rd,Nordl and county;theSka- landgraph it emine on the islandof Senja,Trom scounty;and theJenn estadmine in Vesteralen,Nordland county. Norway is therefor e a country wit h good potential for graphite deposits.How ever,onlythe Skalandmine iscurrentlyactive; about 7000 tonnesof graphite concent rate are produced an nually.
In theecono mi c evaluation of graphitedeposit s,thefol- lowin g factorsare important:a)size,grade and tonnageof theorebodies,and b)thegrain sizeand distribution of the grap hite flakesin theores.Comm ercialgraphiteisa relat ively expensive industrial mineral and to obtain good quality graphite concentrates,beneficiation isessentialin orderto obtai nopt imal pricesforthefinished product.
In thispaper, weaim to (1) give areview of thegeneral geology and history ofgraphite explorat ion in Jennestad area, (2)describ ethe geologic alsett ingand thepetrography of the graphite ores,and (3)describe the result s of recent beneficiationtests and mineralcharacterisat ion of thegrap h- ite ore.
Regional geological setting
Therocks of theJennest ad areabelo ng to the Archaeanto Prot erozoic rocksof theLofot en- Vesteral en provinc e.The regional tectonom agm at ic and met amorph ic evolut ion has been described elsewhere(Griff inet al. 1978,Tvet en 1978).
The oldest rocks in the l.ofoten-Vesteralen area are migm at it ic gneissesof an intermediate,andesiticcomposi- tion. They are int ruded by granodi or ite/granite plut ons datedtoabout 2600Ma (Pb/Pbwholerock Griff in etal.1978) and metamorphosed to granulitefacies atabout 2000Ma.
Unconform ablyon the gneissesandgranit oidslies aProtero- zoic supracrust al series comprising felsic to intermediat e metavolc anicgneisses,dolom it e andcalcite marbles,quartz- ites,graphiteschists and ironformations.The enti re package underwent a granu lite-faciesmetamorphicevent,dated at 1830Ma(Rb/Sr whole rock Griffin etal1978), wit hameta- morphic peak of 900°Cand 10kbar.Carbo n and oxygeniso- topesin the marbleswerest udie d by Baker&Fallick(1988, 1989),who found them tohaveunusually heavyoBCiso- topic signat ures, withevid ence of large-scale CO2infiltration during granulite-facies metamorphism. The met amorphic maximumcoincidedwith the emplacementof largevolumes of manger itictocharnock iticintrusions,which domin atethe geologyof the area. Finally,a seriesofyou nger graniteswere intrud ed atabout1300 Ma.
History of graphite investigations
The invest igat edareaissituated in the central part of the island of Langoya intheVesteralen archi pelago(Fig.1).The graph it edepositsin theJennestad areawerefirst visited and regi ste red by B.MKeilhauin about1820.Thefirstperiod of commercialminingstart ed in 1899and endedin1914.Dur- ing this period some tens of different depo sits were exploi ted in theLofoten-Vesteralendistrict,themain acti vity being in the Jennestad area.In 1938,the graphite occur- renceswerereinv estigated by ground geoph ysics anddia- monddrilling. From 1948to 1960therewasasecond period
NGU-BU LL436, 2000-PAGE 68
Fig.1 Geologicalmapof the Jennestad graph it eoccurrences.
HA v A RD GAUTN EB&EINAR TVETEN
Geological map of the Jennestad graphite
occurrences
Legend
MIGMATlTECOMPLEX _ Stromaticmigma tite
METASUPRACRUSTALROCKS _ GlaphrteschOil
_ Marble/doIomrte _ Amphibolrte
o
Pyroxenegneiss FauMracture2km
a regional geological study, and a number of unpublished reports (Skj e- set h 1952, Vokes 1954)described the graph it e deposits. In 1987, NGU per- formeda helicopt ergeop hysical survey of the area,which included3800flight km of magnetic,electromagnetic and radio met ric measurements (Mogaard 1988). This geophysical survey indi- cated a 50% increase in the area of potent ial graphite-bearing rocks.The aero-geop hysical measurements were followed up by general mapping, trenching, drilling,ground geophysical measurements andbench scale benefi- dati on tests (Renninq 1991, 1993, 0zmerih 1991 Gautneb & Tveten 1992, Gaut neb 1992; 1993, 1995, Dalsegg 1994).
Fig.2 Photo from Goliamine entrance. The graphite oreis seenjust to the right of the mine entrance.Somewhat farther to the right, dolomite marblecan be seen.Amphibolite occurs to the leftof the entrance.
of active min ing during which a total of 770 m of und er- ground adit s anddrifts weredug, togeth erwith severallarge surface trenches. During thisperiodHeier(1960)carried out
Geology of the graphite mineralisation
Graphitic schists are part of a suite of high-grade metasu- pracrust al rocks whichalso contain marbles,iron formations, amphibolites and pyroxene gneisses. The two last men- tioned have been interpret ed as representing originallyvol-
HAvARD GAUTNEB&EINARTVETEN NGU-BULL436, 2000-PAGE 69
I
Table 1.Modalanalysisof graphiteore.
Samp le Gra1 Gra2 Gra3 Gra4 GraS LH1 LH2
Locality 2 2
Quartz 2.42 1.60 0.56 0.15 0.97 52.79 30.78
Plagioclase 13.29 1.03 3.21 2.47 3.51 5.79 27.29
K-feldspar 42.75 48.86 49.51 58.02 56.1 4 6.01 17.93
Graphite 37.46 38.33 39.33 35.49 37.04 30.04 40.16
Biotite 1.21 0 7.25 0 0 0 0
Orthopyroxene 2.87 9.95 0.14 3.70 1.95 2.14 3.89
Others 0 0.23 0.15 0.39 3.20 4.48
Localityrefersto placename in Fig.1: 1=Grzeva, 2=Lille Hornvann.
THE GOLlA MINE,JENNESTAD
Fig.3Sketch map oftheGoliamineshowing the typicalrock association for thegraphite occurrences.
~ Dolomitemarble
E-=-= - = I
Graphite schist~ Granite
[:::::;] Arnphibol ite
c=::J
Pyroxenegneissmeasurements. About 30 different graphite bodies of varia- ble size were discovered and some 20 trenches were dug for sampling.The underground extensions of the selected ore- bodieswere studied bymeans of CP (mise
a
lamasse) geo- physicalmeasurement s (Renni nq,1991,1993; Dalsegg 1994), and the mostpromisin ganomalies were drilled(total800m ofdrillcore). The graphite-bearingbodies occur as elongated lenses commonly situated en echelon and following the dom- inating folds, which trend NE-SW in the area.The greatest thickness of graphite is observed in fold-hinge areas;the graphite-bearin g units have been observed wit hathickness up to 7-8 metres,but 2-4metres is more common.Grade and tonnage modelling of some of the largest graphite lenses indicate that they each contain in the order of about 250,000 tons of graphiteorewith an averagegrade of about 20%car- bon (Gautneb 1993,1995).Petrographic characterisation of the graphite ore
Graphit e ores generally occur in strongly foliat ed rocks in which thefoliation is defi ned by theparallel orientatio nof graphite flakes.Themaingangueminerals are quartz, plagi- oclase and K-feldsparwith subordinateorthopyroxene and biotite (Table. 1,Fig,4).The graphite grains are situatedinter- stitially betw een grain boundaries of gangue silicates, and more rarelyasinclusionsinthe silicateminerals. XRD analysis ofpuregraphite flakesshowsthatthedoo2inter layerspacing is 3.40
A
which ischaracte risti c of fully ordered (crysta lline) graphite,with a temperature of formation of above 7000 C (Landis1971, Katz 1987).Carbon conten tand flake size are the main parameters controll ing thequality and price of flake graphite.Manyof the importa nt physical propert ies,e.g.thermal stabi lity,are favoured by coarser grain size.Characteri sation ofsize and morphology ofthe graphite flakes is therefore importantin ore evaluation.A representative selection of thin-sections was therefore selected for microscopic image analysis, This involvesthe acquisitionof digital images ofthe thinsections
L...- - l
()
canicrocks(Griffinet al. 1978).A good localit y illustrati ng the geologica lsettin gof the graphitemineralisation can be seen attheent ranceof the abando ned Golia mine(Figs.2 and3).
The mineadithas been driven parallel to a 3 m-wide graphite schisthorizon,with dolomite marble in the hanging wall and amphibolite in the footwall. Pyroxene gneisses and intru- sionsof youngergranites occur associatedwith these rocks.
In the area,marbl es, amp hibol itesand gneissesare always observed inthe vicinityof graphitemineralisation, although the exactcontactsand mutual relationships between these rocks can rarely be studied,due to overburden. Theoutlines of the outcropping parts of the orebodies were established by use of electromagnetic and self-potential geophysical
NGU-BULL436,2000-PAGE70 HAvARD GAUTNEB&EINAR TVETEN
Fig.4:Photomicrog raph of graphite ore. Thegraph ite grains occur along the grain boundary of the silicate minerals. gr
=
graph ite,bi=biotiteand pi=plagioclase.
phological measurementsof the ore at Lille Hornvann(Fig. 1),including thesam plesLH 1 andLH2 in Table1.
The dominat ingsizeof the graphite flakes is 0.01 mm2 and the mean lengt h ofthe longestgrain axisis 0.3 mm.Mostgrap hiteflakes are oblong and,after severalsteps of processin g,morphologicalparam- shaped,but notparticularly fibrous,and the ratios between eters such asarea,perimeter,longest and shortest axes,etc., their long andshort axes are inthe range of 2 to 4 for the of mineral grainsare recorded. These were automatically majority of theflakes.Theseresultsare typ icalforJennestad reco rd ed for each graph it egrain in the thin-sectionsexam- graphiteandare alsocharact erist ic of a coarse,high-quality, ined.Aggregate measurem ents from several thin-sect ions flake graphi teore.The result softhe graphite morphological areusuallynecessaryto give a st at isti cally significant descrip- data aquisition are important for esta blishi ng appropriate tion of the ore.An exampleof the results of such measure- procedures for crushingand liberation procedures as a part mentsis shown in Fig.5which shows aggregate grainrnor- ofthe beneficiat io ntests.
Tabl e 2.Che m ical composition ofse lecte dsa mples ofthe Jennestad graphiteores.A complete analyticaldatabase is availab le fromthe senio r autho r onrequest. All sampleswe re of1-2kgsize1=LilleHorn vann.2=Ho rnva nn.3=Golia.
Sample LH-' LH-2 LH-3 LH-4 90-76 90-7C 90-50 90-9A 90-96 90-9C 90-90
Locality 2 2 2 2 3 3 3
5i02 55.29 49.37 53.49 38.63 36.37 37.87 36.26 39.49 37.47 31.8 30.86
Al203 7.94 4.66 6.26 5.39 10.1 11.02 10.48 8.13 10.56 9.47 8.93
FeP3101 4.42 7.13 6.13 12.97 5.16 2.43 3.20 3.97 1.24 6.10 4.65
Ti02 0.68 0.22 0.31 0.16 0.55 0.57 0.45 0.36 0.37 0.48 0.58
MgO 2.07 7.23 4.75 8.65 0.93 0.45 0.81 6.07 0.80 1.28 1.53
CaO 3.50 6.01 5.18 11.72 1.38 1.41 3.71 11.36 1.99 2.47 2.78
Na20 2.23 0.87 1.21 1.37 1.60 1.51 2.75 2.01 1.63 1.93 2.29
K20 0.94 1.58 1.68 0.35 4.15 5.19 0.42 0.18 5.34 2.98 1.81
MnO 0.03 0.24 0.13 0.17 0.04 0.02 0.05 0.19 0.03 0.04 0.05
Pps 0.41 0.08 0.36 0.45 0.05 0.08 0.05 0.07 0.03 0.06 0.05
C 18.02 18.18 14.79 14.52 35.86 36.88 39.65 26.22 37.23 39.23 44.31
5 2.70 1.77 2.07 6.65 0 0 1.95 0 0 1.11 1.00
5UM 98.23 97.34 96.36 101.03 96.19 97.43 99.78 98.05 96.69 96.95 98.84
HAvARD GAUTNEB s EINAR TVETEN NGU-BULL 436, 2000-PAGE71
. u .
80 -
Beneficiation of the graphite ore
Aseriesofcrushing,milling and bench-scaleflotation tests wasperformed on a 300 kgsample of graphi teore(0 zmerih 1991).Thehead samplehadagradeof 17.4%C.Following crushing and milling, an initial step of flotation was per- formed whereabout 75%of theheadsamp lewas remove d asaroughertailing.Therougherconcentrate went throug h aseries of gentle regrindingand cleaningsteps to increase the gradeandto obtainaslarge aproporti on of coarserflakes as possib le. MISC (met ylisob ut ylcarbinol) was used as a frotherandkeroseneand FlotolB ascollectors.
The combined results from a series of flotation tests includingseveral cleaning ste ps can be sum marisedas fol- low s.To produceaconcentrate of about 90%C,therecovery wasfoun dto be arou nd 75%.For a slight lylow er concen- trate grade (88-89%)the recovery is higher (82%).A maxi- mum gradeof about 97%Cwas found in the finalconcen- trateofthe+208urn sizefraction.These result s show thatthe Jennestadorecan be readily beneficiated andthat concen- tratesof a qualitycomparable wit hcommercialgrades can be prod uced.Most likelythe recovery can beimpro ved if the beneficiatio nprocedure is optimisedon anindust rial scale.
Thevarious beneficiation tests aresum marised in Tables3 and 4.
Chemical composition of the graphite ore
Thegraphit eorewascrushed,milled and analysedformajor element s using XRF;while thecarboncontentwas analysed wit h a LECO gas flowcarbonanalyser.Representativeanaly- ses ofgrap hiteoreareshow ninTable 2.Thecarbonconte nt variesfrom 18to 44%.If the analyses are recalculatedon a 100%volatile-freebasis,theorewould havea compositional variati on comparabl etothatof arenites and mudrocks(Blat t et al. 1980).The variation in thecontent of clayminerals in theinit ial sed iment s was probably large, as seen from the variatio ninK20content.
Size ofgraphite grains
Shap e ofgraphite grains
Longestaxis ofgrap hite grains
. . .c .,..,..,.,
I I I I I I I I
3 4 5 6 7 8 9 1011 1213 Longest/shortestaxis
0.015 0.030 0.045 0.060 Area(mm)
- -
I
01 2 -
- - 0,000
40 -
20 - 60 - 100 -
120 80.00
(/)
'co
C"- OJ'0
40.00"-
.o
Q)E::J Z
(/)c
'co
"- OJ'+-
o
"-
.o
Q)E
::J Z(/)c
'co
"- 80OJ
'+-0
"-
.o
Q)E
40::J Z
o
0.00 0.60 1.20 1.80 2.40 3.00 Longest axis (mm)
Fig 5:Histogramsshow ing themor ph ologicalvariat ion of thegraphite grainsin thin-section,Datawerecollected by digitalimageanalysisusing theK5 300system.The datarepresent agg regatemeasurementsofsev- eral thin-sections,and are believedtoberepresent ative of the Jennestad gra phiteore
Graphite formation
Somewhat simplified,there arebasicallythree differentproc- essesleading to the formation of economicgraphit edeposit s (Harben & Kuzvart 1996):
1) Contact metamorphism of coal deposits:such deposit s are usually oflow quality and producelow-priced prod- ucts.
2) Epigenetic graphite deposit s. The formation of these depositsis assumed toinvolve, among others,the follow - ingreact ions:
C +Hp
=
CO +H2 (1)2CO=C+ CO2 (2)
It is believed, forexam ple,thatthis process was active during theformation ofthe SriLankan type of veingraph - ite deposit s(Weis et al. 1981,Rumble & Hoering 1986, Katz 1987, Santosh & Wada 1989, Ulmer&Luth 1991).
I
NGU-BULL436, 2000-PAGE72 HA v A RDGAU TNEB s EINAR TVETENTable3. Summaryof bench-scaleflotationtestsofJennestad graphite.Thebenfrciation test consistedof an initialste pof rougher flotatio n and several steps of cleanerflotationcombinedwit hgentle regrindin g.MIBCwas usedas a frot her,andkeroseneandFlotol B were used as collectors.The grade and recoveryresultsare show ninTable 4.(Data from (2jzmerih199 1).
Flot at ion test no.
Procedure 1 2 3 4 5 6 7 8 9 10-13 14-17
Milltime(min.) 30 30 25 35 30 35 40 25 40 35 35
Roughflat. x x x x x x x x x x x
MICHcclt 200 200 200 200 150 150 200 200
Kerosene 200 200 200 200 100 100 150 200 150 150 150
Flotol B 200 200 200
Regrind1(min.) 15 15 15 15 15 15
Cleaner flot,1 x x x x x x x x x x
MIBCcelt 25
Kerosne 25 25
FlotolB 25
Regrind2 15 15
Cleanerflat.2 x x x x x x x x x x
MICBcelt 100 50 25 15 10
Kerosene 10 50 15 15 10
Cleanerflat.3 x x x x x
MICBcelt 25 15 10
Kerosene 15 15 10 10
Flotol B 10
Cleanerflat.4 x x x
MICBcelt 15
Kerosene 15 15
FlotolB 15
Tabl e 4. Gradeand recoveryofgraphit e concentratesfrom flotationof Jennestadore.(Datafrom(2jzmerih1991).
Grain size(micron)%
Product Grade Recovery +208 208-147 147- 104- -74 Hottestno.
%Carbon 104 74
Head sample 17.40
Rough cone. 59.06 86.56 1
Cleaner cone.1 79.48 58.88 2
Cleanercone.2 60.85 72.20 3
Cleanercone.2 88.20 36.86 4
Cleanercone.1+2 84.05 62.18 5
Cleaner cone.2+3 88.29 79.1 2 10.04 32.53 57.43 6
Cleanercone.2+3+4 88.29 79.12 9.24 29.32 61.44 7
Cleanercone.2 69.24 63.83 8
Cleaner cone.3 88.38 80.06 9
Cleanercone.2+3+4 89.17 80.58 9.32 13.98 14.98 19.68 42.04 10-13
Cleaner cone.2+3+4 88.61 81.91 5.76 11.52 14.60 20.72 47.40 14-1 7
HA vARDGAUTNEB&EINARTVETEN
From an industrialperspective,thegra p h it e in theseepi- gen etic dep os its isclassifi ed as'v e in'or'lu m p'type.
3) Syngenetic flak e graph ite deposit s. The formatio n of these depositsinvolv esthe alte rati o n oforgan icmatter to grap h ite durin g metamorphism.From st u di es of coal and anthrac it es, the oper ating processes have been show n to be hig hly complex (Bon ij oly et al. 1982).Th e kinet ics of flake grap h ite forma t io n have shown to be contro l ledorin f luen cedby thefo ll ow ing facto rs:
a) Thenatu re of thehydro carbon precurso rs.Aromati c compou nds wit h exi sting C-H ringsare more easily gra p h it ised than alip ha t ic (C-H st rin gs) compounds (Bu seck&Huang 1985 ).
d) Thepartialpressures ofCO2,CO,CH4,H20 an d H2. e) The regional P-T co nditions. Die sse l et al. (1978)
show ed th at co al and grap hite coexist in the P-T range of 3k ba r/2s soC- s.5kbar/335°C. All orga n ic matter is converted to graphite before the epi dote isogradat6.3kba r/390°C.
All theNorwegian ,aswellas most oftheworl d's,ope rat - ing grap hite depositsbelongtothisthirdtype.In theJennes- tad area, webelievethat graphiteis syngeneticand we hav e see n no evidenceofep ig en et ic graphite.However,migration of COr richfluids similar to those describ edby Baker&Fallick (1988) has been sho wn to be asso ciate d with epigenitic graph iteformation elsewh ere(Galbreathet al. 1988,Dukeet al. 1990,santosh&Wad a1993).Th ein flu e n ce of CO2infiltra- tion ongraphite formation in theJennestad areawas beyond thescope of our explorationwork,but it cou ld beacha lleng- ingst u d y for the future.The coexistenceof carbonate rocks and graphiteschi st s probablyshows that the schist ini tia lly rep rese nt ed an org anic-r ich sed im e nt, and the orgasnic material was converted to graphite during the granulite- faciesmetamorphism.
Summary and conclusions
The graphite occurrenc esof theJennestad area are asso ci - ated with marbles, am p h ib o li t es and pyroxene gneisses, intruded by different charnockitic rocksand granites.The graphiteschists contain up to 40%C and occur as lenses sit - uated enechelon,follow ing the main fold structures in the area.During the inve stigation s,so m e 30 differentgraphite- bearingbodies have beendiscovered.Thelargest havebeen modelled to contain about 250,000 tonnes each, with an average grad eof 20%C. In thin-section, the ore comprises the following gangu e mineral s: quartz, plagiocla se, K-feld- sp ar,biotit e an d orthopyroxe n e.Imagean alysis of thin-sec- tion sshow sthat the ore has a do m ina n t flake size of0.01 mm2anda rati o betw eenthe longest andshorte st gra in axis in the order of 2 to 3.Bench-scal eflotationtestssho w that it is possible to obtai n a maximum grade of 97% C, with a recoveryof 89%.Webelievethat the graphit eore formed as a resul t of granulite facies met amorph ism of organic -rich sediment s.
NGU-BULL436,2000-PAGE73
Acknowledgem ents
The graphitebeneficiationtests werecarriedout under contractbysIN- TEF, Rock andMineralEngineering,and wethankLevent 0zmerihfor this. The investigations were partially funded bytheNordland county autho ritiesandNorwegianHolding A/s.NorwegianHolding presently holdsthe miningrightstothe graphite deposits.Wearegrateful toBj orn Lundand Leif Furuhaugfortheirassistance duringfieldwork andtoOla Grindvoll,headmaster oftheVikeid agro-mechanicalschool,forhis sup- portduringour time inthefield.
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