Chemical and petrographic characterization of ilmenite and magnetite in oxide-rich cumulates of the Sokndal Region, Rogaland, Norway

NGU-BULL436,20 00-PAGE 49

Chemical and petrographic characterization of ilmenite and magnetite in oxide-rich cumulates of the Sokndal Region, Rogaland, Norway

sUZANNEA.McENROE, PETER ROBINsON&PETER T.PANISH

McEnroe, 5.A.,Robin son ,P.,&Pani sh, P. T.2000:Chemicaland petr ogr aphiccharacterizat io nofilmeniteand magnet ite inoxide-richcu m ulatesoftheSokndal Region, Rogaland,No rw ay.Norges geologiske undersekelseBulletin 436,49-56

Igneo usilme niteand magnetit einsome oxide-richcum u latesof the Soknda lregio nshow chem ical features and microtexturesthatrefl ect theprogressof magm atic evolut ionandsubsolid usre-equili bra tion.Thesefeat ur es also relate tothepatt ernsofaero mag neticano ma liesin thereg ion.Themost primitive oxidesst udiedoccurin theTellnes ilmeniteno rit e.These are foll ow edby oxidesin theBje rkreim-Soknda llayered intrusion, firstin i1menitenoritesand finallyin magn etite-richno rites. The trendof magm ati c oxidecrystallizat io n in Bjerkreim -Sokn dal, asprev io usly det erminedby Wilsonetal.(1996),wasfromhem atite-ri chi1menitewit h minorend-me mber magnetit etohem at it e- poor i1menite wit h titanom agn etite, a trend indicat in g progr essive dim in ut ion of Fe203' This trend was accom pan ied bydecreasingMgO andCr203,and by grad ually increasing V203tothe pointwheremagnetite became aprimarymagm ati cprecipitat e:beyo nd thisV20 3 alsodecreased . In primitive sam p les the ilmenitehas ab u ndant hem atite exsolu t io n lam ell ae,whe reasthemagn et itehasend-me mbe rcomposit ionand isfreeornearly free of lam ellae. Highl y evolved sam p les conta in near end- member ilmenite wit ho ut exsolut ion , and titanomagn etite wit h fineexsolut ionof ulvGspinelsubseq ue ntly oxid ized toi1meniteand/oroxidatio n-exsolution lam ell ae of ilmenite. MgOismost st ronglyconcent ratedin i1menite;Cr203 and V203are stronglyconcent ratedin hem at it e exsolut io n lam ellae,buteven more stro ng ly incoexisti ng magnetit e.Themo st chemicallypurei1men ite, wit h theexcept io nof MgO,occursinsmallab unda nce inthe mo st evo lved tit an omagn eti te-rich cum u lates.

Suza nneA.McEnro e&PeterRobinson,Geolog ica lSurveyof Nor way,N-7491Trondheim,Norw ay.

PeterT.Panish,DepartmentofGeosciences,UniversityofMassachu setts,Am herst,MA 01003,USA.

Introduction

In 1995, NGU completed a detailed aeromagneticsurvey of the Sokndal region as part of a broad investigation into i1menite resources(Renninq 1995). The resulting aeromag- netic map showed a dramatic pattern of aeromagnetic highs and lows.McEnroe et al. (1996) demonstrated that the mag- netic lows are produced by rocks with a high ratioof natural remanent magnetization (NRM) to suscept ibilit y (X),com- monly referred to as the Koenigsbergerratio(Qvalue) where Q:=NRM/X • F (F:=presentlocal magnetic field strength).

Rockswith high Qvalues include ilmenite-rich no ritesand anort hosites with very fine-grainedoxides,and the anoma- liesare dominated byst rong remanentnegative inclination.

Conversely, the extreme magnetic highs are produced by rockswith a very low ratio of naturalremanent magnetiza- tion to susceptibility.Rockswith these lowQvalues are mag- netite-rich,including no rites,mangeritesand quartz manger- ites,and give risetoinduced magnetic anomalies. During the study of the rock-magneticpropertiesand paleomagnetism of theserocks,a seriesof detail ed electron-probe analyses was performed on theoxide minerals,withspecial attention toanalyt ical precision for major elementsand the economi- cally significanttrace elements er and V. These are summa- rizedhere and will be explored in greater detail elsewhere.A detailed st udy is also in progress to understand the ext reme lystrongandsta ble rem anentmagnetization in the

ilmenite-rich rocks.The stableremanenc eand highQvalues found in hematite-ilmeniterockscould haveconsequ enc es for the interpretation of long- and short -w avelengt h mag- netic anomalies.

Groups of samples

Mineral analyses are presented from five partsof the region (Fig.1).The firstgroup of samplesis from the Tellnesilme nite no rite orebody in a magnetic low dominated bythe reversed remanent magnetization of the Ana-Sira anort hosite. The remainingfour groupsarefrom the Bjerkreim-Sokndal lay- ered intrusion. Of these four, two groupsare from mor e primitive ilmenit e-bearing norites that form magnetic lowsin the lower part of theint rusion as exposed at Heskestad and Mydland, though well above the lowest levelsexposedin the Bjerkreim lobe.The remaining two groupsarefrom themore evolved magnetite-richnoritesthat form themagnetic highs in the upper part of the layered intrusion at Mydland and at Bakka.

General oxide compositions and magma evolution

The derivationand evolution of the magma from whichthe Tellnes i1menite norite was formed hasbeen much debated and is sti ll unsettled(Krauseetal. 1985, Wilmart etal. 1989,

NGU-BULL 436. 2000-PAGE50 5UZANNEA.McENROE,PETERROBINSON&PETERT.PANISH

Fig.1. Simplifiedgeologicalmapfrom NGUreport 96.059 (modifiedfrom Duchesne & Michot 1987),showi ng the sam- plinglocalit iesat Tellnes Mine,Heskestad,Mydland andBak-

N

ka.

)

00 . . .1

10 km

Jotunite Mongerite Anorthosite

Nodte

leuc onorite Chornockite Guesses.undfferentiof e d

Duchesne 1999). On the other hand,the magmaticderiva- tion andevolutio noft heBjerkreim-Soknda l layeredintrusion has beenthoroug hlydocumented(Wilsonet al.1996). Nev- erth eless,thestagesof relativemagmaticevolutionofall the samp les can be roughly work ed out using such ratios as MgO/ (MgO+FeO)in the silicates,and the relativ e cont ents of MgO,V₂0_3,andCr20 3in the oxides. These show that the oxide min erals ofthe Telln esilmenitenori te aremore prim i- tive thanthose sampledby us in thelayeredintrusion.Thus, the Tellnes deposit cannot be correlated wit hlate stages in evolution of the Bj erkreim-Soknd al intr usion, nor with the internalmangeritedikesystem in theAna-SiraAnorthosite , assuggestedby Krauseet al. (1985). Detailed geochemistry on the Tellnes ilmenite norite and the associat ed rnon- zonoriteto quartz mangerit edike(Wilmartet al.1989), indi- catesthey cannotbeco-magm atic andsuggest s theilmenite norit ecould be derivedfrom crystalliza ti onof the enclosing anor t hosit e or possibly the most prim itive partsof theBj er- kreim-Sokndalintrusio nnot covered inour sampling . More recentpreciseU-Pb geochronology(Scharer etal. 1996)indi- cates that the Tellne s ilmenite norite is about 10 m. y.

youngerthantheothe rint rusionsinthe region,so thatthere are no exposed rocksavailable for st udy rep resent in g its source magma. Furt her ext ensiveconsiderationof the orig in of the oxide-richdeposits was givenbyDuchesne(1999).

Based on the broad er st udy of theBjerkreim-Soknd al lay- eredintrusionby Wilsonetal.(1996),theHeskestad, Mydla nd and Bakka locationsof thisstudy fall eit her in or slightly belowtheirMCU-IVcycli c unitor inthe upper transition from noriteto mangerit eand quartz mangerit e. The magmatic evolutio n trend ofthe oxidesofthe layeredintr usio n(Wilson et al. 1996) involves early precip it ation of hematit e-rich ilmenit e togeth er withminor Ti-p oormagnet ite. These are followedby agradualdecreaseinhemati t e contentof ilmen- itecoupled wit h increasing Ticonte ntofmagnetite.The end

result is an appearance of progressive redu ction in the oxides.This wasprobablynot produced by a change of ext er- nalenvironment , but byextract ion of cumulates,drivi ng the magma toward more reducing compos it ion s, even in this examp le where early ilmenit e dom inated over magnetite. This trend is not to be confusedwith apparent oxidati on or reduction effects withi n individual oxide grains,that were mainly theprodu ct of localizedsubsolidus exchang ereac- tions.

The oxide composit ions and petrogra ph ic observations ofthis study illust rat everywell theconclu sions of Wilsonet al.(1996). The overall chemicalresultsare illustr ated byfour tielines inFig.2,meantto show ourbest estimate oftheequi- lib riu m tie linesofthe oxides inthetriangle FeO(+MgO)- Fe20 3-Ti02under high-temperaturecondit ions.' Theseval- ues were obtained qualitatively from typ ical analyses of oxidegrains,as much as possible includingminorexsolution lamellae,and will be followed up by more precise estima tes based on modalanalysisof oxidegrain s.Duchesne(1999), by contrast.madebulk XRF analyses of high-purityoxide min- eralseparateswhich should givea goodmeasureof the bulk compositionof hostpluslamell ae. His ilmenit e and magnet- it ecomp osit ions(his Table 4)fro mtheTellnesdepositcom- pare favorablywith ours,although theilmen ite actuallyplot s closer toour TE-41from Heskestad than to our TE-3,the most Cr-rich of ourthree samp lesfrom the Tellnes deposit.

The most oxygen-rich tie line (TE-3) in Fig.2is for the

1. Amaddeningand easilyoverloo ked numericalfeatureof Fig.2is thatthedistance ofan analysispointfrom the ilmenite towardthe hematite end-member is an indirect and non-linearfunction of hematit e content. For example,a grainof composit ionhemat iteso- Fe2

+0.5Ti⁴+0.sFe3

+103' plot sat0.33 mol.%Fe203'The ilmenite TE-3, plotting at.077 mol.%Fe203'actually contains14.5%ofthe hema- tite end-member. This problemiselim inated byplotting onaless traditio nalcation basis.

SUZANNEA.McENROE,PETER ROBINSON&PETERT.PANISH NGU-BULL436,2000 - PAGE51

TE-26BakkaHigh FeTi03 (/TE-16 MydlandHigh

~^TE-41Heskestad Low ': ( -TE-3Tellnes

Fig.2. Part ofthe syste mFeO-TiOTFe203 inmole%(wit hsubordinate oxides included) showing approximate equilibrium tielinesbetween bulk ilmenit e andmagnet ite forfou r samp les fromthe Sokndal region. Inset showsthe locat ion of thedetailed diagramin the who lesystem.

Various formsof evidence(seetext)indicate thatthesetie linesshowa prog ression in magm at ic crystallization frommostprimitive (TE-3Jto mostevolv ed (TE-26), inwhichtheoxides decreasedin their oxygen con- tent.The position sof the tie lines exp lain much about the subsolidus be- havior of the oxidesin different samp les.Word s'high'and'low'refer to fieldlocations,the Bakkaand Mydlandmagnetichighs, and theHeskes- tadand Tellnesmagnetic lows.

Tellne sdeposit. Another relative lyprimitiveoxyg en-richtie line(TE-41) is forthe ilmenite-richnorite of thelayeredintru- sion in the magnetic low at Heskestad. More evolved, oxy- gen-poor tie lines represent the more evolved magnetite- rich noritesin themagnetichighs at Mydland (TE-16)and at Bakka (TE-26).

Explanation of subsolidus equilibration

Based on thetielinesin Fig.2, it is easy to explain thenatur e ofsub solid usevoluti onat each of the locat ion s. At Telln es, and in theHeskestad magneticlow, the ilmenit eis exten- sivelyexsolved with hematite lamell ae, whereasthemagnet- ite, where present ,is a nearlyTi-freeend-memb er composi- tion wit h littl eor no exsolution. Bycontrast,insamples from the magneti chighsat Mydland and Bakka,the ilmenit e is Ti- rich with relativ ely lit tle hematit e-exsolution, whereas the magnetit e is relati vely Ti-rich wit h oxidat ion-exsoluti on lamellae of ilmeniteand/orexsolution ofulvospinel subse-

quentl y oxidizedtoilmenite.Theexplanationsbased on Fig.

2 are supportedindetail,bothby pet rogra phicobservations and by elect ron-probeanalyses.

Detailed oxide petrography

Reflected-and transmitted-lightmicroscopywereperform ed to understand bett er the magnetic phases and correlative magnetic propertie s,to esta blish if the magneti c carriers were in equilib rium wit h the mineral assemb lage,and to determinethe degree of alteratio n. In all samples theoxides are well equilibrated showingcoarse anhedral grains. Dis- crete ilmenite grains are usually >1mm. Some oxides are rimmedbypyroxene, andmany arein clusterswith orthopy- roxeneand/orc1inopyroxene.The samplesdescribed below correspo nd to thefour tie lies shown inFig.2.

The ilmenitesfrom theTelln esdepositdescrib ed in detail below (Fig.3) are insample TE-3 considered tobemostprim- it ive, based onour micropr ob e analyses.Allilmenitesarefer- rian i1menit e wit h multipl e generationsof titanoh emat it e- hemat it e exsolut ion lamellae parallelto (0001).Second - and third- generation tit anoh ematit e lamellae are observed dow n to the micron scale. Based on measured magnetic prope rties (McEnroe et al. 1996,McEnroe1997),it is pro- posed thatthese lamellae cont inuedow n to the nanometer scale,ashasbeen show nforthe ilmenite-h ematiteseries in samplesfrom the granulit e regionof southwestern Sweden (McEnroe 1996,Harrison etal. 1998).Pleonasteplates,as pre- cipitat es,arecommo n in theilmenit e.Aroundthepleonaste platesthere are diffu sion haloes,whereilmenite is free of hematite exsolutio n(Fig.3a). These arewhere the ilmenite wasdepleted in Fe³+and hencenot availablefor later hema- tit e exsolution. These areasyielded the ilmeni te composi- tions closestto FeTi0₃because theyaredeplet ed in hematite exsolution. Small sulfide 'inclusions' or precipitates are present in numerousilmenites.Coexistingmagnetite grains havemin orpleonaste exsolution. Wheremagnetite is adja- centtohemo-ilm enit e, amagn et ite-pl eonast e-ilmenit e reac- tion sympl ect ite commonl yhasform ed (Fig.3b). Very fine- grained ilmenite with hematite exsolut ion,and magn etite with ilmenite oxy-exsolution and rutile plates arepresentin clinopyroxene grains. This magn etite -ilmenite exsolut ion occurs in plates and bladesparallelto (010)in c1inopyroxene (Morse 1970,Fleet et al. 1980). These abun dant very fine- grained oxides inclinopy roxene have a strong effect on the magneti c properties and aeromagnetic signature of the Telln es intr usion. As isto be expected from their different modeof occurence, these oxides commonly have composi- tionsdistinc tfrom the discrete oxide grains. The chemical summary figures in thispaper (Figs.5 & 6) are limitedto com- position s in thediscreteoxidegrains.

An oxide assemblagesimilar to thatof the Telln esilmen- ite norite, though lower in overall abundanceof oxides,is found in the areaof amagnetic low over part of the Bje r- kreim-Soknd al layered intrusion near Heskestad. Fig. 3 shows represent ativ e microphotograph s of sample TE-41 from the Heskestadregion. Ferriani1meniteandminor mag- netite are the dominantopaquephases. All ferrian ilmenites

NGU-BULL436,2000 -PAGE 52 SUZANNEA.McENROE,PETERROBINSON

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^{PETER T.PANISH}

Fig.3.Reflected-lightphotomicrogr aphs: (a).IImenit e wit h multiple generati onsofvery fine(0001)hematite exsol utionlamellae.Internaltotheilmen- iteisaspinel plate surro unded by anarea free of hematitelamellae.Noteconcentration ofhemat it elamellaeat twin plane (Tellnesmine).Widthoffield 60~. Photographed withabluefilter. (b)Magnetitewith minor(100) spinellamellaebutfree of ilmenit e lamellae.Adjacent ilmenite grain contains hematitelamellaeandat thecontactbetween magneti teandi1menit ea magnetit e-ilmenit e-spinelsymplecti te(Tellnes mine).Widt h offield120~. Pho- tographed with ablue filter. (c)IImenit ewithmultiplegenerations offi nehemat iteexsol ution lamellae(Heskestad). Widt h of field120~.Photographed withawhit efilter.(d)IImenitewit hfinehematite exsolutionlamellaeenclosinga magnet it egrain with minor spinellamellae(Heskestad). Widthof field 330~. Photographedwit ha white filter.

contain multiple gen eration sof very fine (0001) hem atit e exsolutio n lamellae(Fig 3c). Minor aluminousspinel plates, asprecipit ates are common in the ferrian ilm enite grainsas are minor sulfide 'inclusio ns'. Magnet ite has minor alumi- nousspinelexsolution(Fig3d). Afew magnetitegrainshave rareoxid ation -exsolution lame llae of ilmenite. In all cases, spine l lam ell ae have nucleated on the ilm en ite lamell ae.

When ilmenitewas oxy-exsolved from magnetite it pushed the bulk composition towards spin el saturation and pro- duceda second generation of spinelexsolut ion .

The oxid e mineral assemblage in the magnetite-rich noritefrom thearea in theBjerkreim-Sokndallayeredintru- sio n wereferto as theMydlandmagn et ichigh isverydiffer- entfrom the sam plesdescribed above. Fig.4aand bshows the two dominant oxides, subordinate ilm eni t e and very abundant magnetite. IImeniteisfree of hematiteexsolution thoughminor spi neI exsol utionispresent. When ilmeniteis adjace nt to magnetite a minor ilmenite-spin el-m agnetite symplect it e is present. The magn etitegrain s (Fig.4a)display a very fineclot h-likepattern ofilmenite replacing exsolv ed ulvospinel on (100) and well developed trell is oxid atio n- exsol utionlamellaeofilmeniteon(111). All trellislamellaeof

ilm en it e haveenclosedand marginalsplnel, Multiplegen er- atio ns of spinel exsolution parallel to (100) are present throu ghoutthemagnetit egrain s.The first generat ionof spi- nel exsolutionis surrou nd ed by a rimof ilm enite, which in turnis sur rou ndedbymag neti tewhere neither ilm eni te nor spinellame llaearepresent. Seco nd-generat ion spine llamel - lae are also surroundedbyilmenite. Subsequ ent smaller spi- nel rods do not haveilmenite rim s. Seco nd ary alterat ion of titanomagnetiteto titanomag hemit e was observed in many grain s.

Sam plesfrom themagnetit e-rich cumulatenorite of the aero mag netichigh over theBjerkreim-So kndallayered intru- sion at Bakka co ntai n subordinate hom ogeneousilmeni te andabunda ntmagn etit ewithilmeniteoxidation-exso lutio n.

Fig.4c showsthe i1me nite and magnetite in sampleTE-26.

Although abundan ti1men ite is present in thissample,most of it exist s as veryfine oxy-exsolution lamellaein magnetite. Large,first-gen eration,aluminous spinel exsolution lame llae aresurro und edbyareas of nearly pure magne tite which in turn are surroun de d byilm enite oxidat io n-exsolutio nlame l- lae. Between these areas anoxid ation-exso lution patternis presentwhichis similarto that discussed aboveforthe Myd-

NGU-BULL436,2000-PAGE 53

I

land sam ple.Theveryfine exsolut ion patt ern is indicat ive of {l DO}ulvospin elexsolution from magn etite. Subsequently, the ulvospinel exsolution was oxidizedtoilmenit e. Locally wit hin thisregi onthere are fin eplatesofspinel. Overall,the Bakkasamplecontainsless spi nel thanthatfromMydland.

Detailed electron probe analyses

The elect ron probe analysesofilmeniteandof magn etite are presented in twocompositefigures (Figs.5& 6).Instrumen- tal condit io ns for the analyses are presentedin theAppendix.

Analyses ofspi nel exsolut io ns, locally in magn etite and in ilmenit e,that are the productof morecomplexprim ary sub- sti t ut io nsand later subso lid us reaction s, will be discussed elsew here. A more mineralogically oriente d st udy of the oxideswouldshow these analysesin cat ion propo rt ions that can berelatedmoredirectlyto mineral end members (Mc En- roe etal. in revie w). Forthisstudy,however,the result sare present edinweig ht%oxid es,a method of greaterfam iliarit y in the mineralindu stry.

Fig.5 showsallanalyses of ilmenite.Whenusingthe elec- tron prob e,Fe analysesare custom arilyreported in term s of weight%FeO. On thisbasispurehematite (Fe203)is 89.99 wt. % FeO, whereasend-member FeTi03 ilme nite conta ins 47.35%FeO.Ascanbeseenin Fig.SA, some of theilmenites containhemat iteinsolidsolut io n oras fine exsolut ion lam el- lae.Thus,asTi02decreases,total Fe asFeOincreases,trend- ing tow ardthe end-membervalu eforhematit e.Conversely, whentotalFeisadequ atelyproporti on edtoFe203and FeO, andaplot ismadeof FeOvsTi02 asin Fig.5B,the analyses showincreasing FeO with Ti02fromthe idealhematitecom - positionat the origin. Analyseshaving low erFeO thanideal ilmenite at 47.35% FeO are ilmenites wit h sig nifican t amountsof MgO subst it ut ing forFeO,parti cularlyin Telln es ilmenites. When MgO isplotted against Ti02,as in Fig.5C,it isobvious that MgOisconcentrated in Ti02-rich ilmenit e end members and islessimportant in Ti02-p oorer com posi tio ns involvinggreat erhem atite com po nent.TheMgOconte nt in Tellnes ilmenites isparticul arly prominent,reach ing valu es above 4.5 wt. %,consistent wit h the appare nt ly primit ive nature of these rocks.

When the minor oxide s V203 and Cr20 3 are plotted against Ti02 as in Figs.5D and SE,there is aninve rse correla- tion, indicat ing that these oxidestend to beconcent rate d in the hem atit e component or in hematite-rich exsolut ion lamellae.In theseplots it emergesthatCr20 3isconc entrated in the mostprimitive Mg-richsam ples from the Tellne sMine which also are the most ext ensively exsolv ed, whereas V203 is concentrated particularly in the ilme nite norites from Heskestadand Mydland that crystalliz edfrom moreevolved magma sthan Tellnes,but beforemagn etite joi ned ilmenite asaprimarymagm aticprecipitate. IImenitesfrom themag- netite-ilmenite norites at Mydland and Bakka areall high in Ti02,lacking significant hematite exsolut ion, and are also depl etedin MgO, V20 3 and Cr203'due to their mature sta ge of magmaticevolut ion.

Fig. 6 shows all of the analysesof magnetite wit h all oxides plottedinweight percent. Asin Fig.SA,all Feisplot -

· L

Fig.4.(a) Mydland. Magneti teon left,wit hoxidat io n-exsolut ion lam el- lae of ilmenit e andvery finespinel{1DO}exsolut io n.A minormagnetite- spinel-ilme nitesymplect iteatcontact wit hilme nitegrains.Not elack of hematit e exsolut io n in ilmenit e (TE16).Width of field 200J,l. Phot o- graphedwit ha bluefilte r. (b)Magneti tewit h{111}lamellaeofilme nite and{lOO}lam ellae ofspinel.Coarserspinel isrimm ed by ilme nite(TE 16).

Widt hoffield200J,l. Photog raphedwit ha blue filt er. (c) Bakka(TE26).

Magnet itewithvery fin e{lOO}cloth -lik e exsoluti onof ulvospi nel subse- quent lyoxidized toilmenit e.Not ethetransiti onal pattern from aIit-par- lit USPssto alamellarUSPss' Alsopresentare two generat io nsofspinel alo ng{lOO}planes.Widt hoffield 200J,l. Photo grap hedwit hawhitefil- ter.

a r / / -. _\ :/ _r ^{I f:} _; ^-:

/ _- . . , _{: .} \ I ,.

^/ / ~'

. . , .

/ .:. ,' "

' /' , ,,,

••~ .~t-.~..

NGU-BULL436, 2000-PAGE54 SUZANNEA.McENROE,PETER ROBINSON

s

PETER T.PANISH

5...-- - - ----,

Fig.5.Electronprobe analysesofi1menite fromoxide-richcumulatesinfive areasin the Sokndalregion,plottedin terms of weight%ofotheroxidesagainstweight

%Ti0₂. Theareas arelisted inorderfrom magmatically most primitive(Tellnes) to mostevolved(Bakka). Diagramsincoun- terclockwise order are:A)TotalFeaswt.%

FeO;Blwt.%FeO(calculated);

o

wt. % MgO;D)wt.%V203:and E)wt.%(r203' InSA.pure hematiteliesat0%Ti02and 89%FeO.

60 A

50

30 40

Wt%Ti0 2

O~ ,

o 20 o

o 0

o

o+-~--,---(..l---r-~--'CxmI

10 40

10 20 30 40 50 60

Wt%Ti0 2 All Ilmenite Data

0 Tellnes

0 Heskestad Low

I!. Mydland Low

•

^{Mydland High}

+ Bakka High

0.05

80...-0 -- - - - - - - - - - ----,

o

o 70 If

~

~

<Il 60

'"

(])

u,

g

~ ⁵⁰

0.3

00 0 E

0.25 0

Ocg 0

0.2 tn_{0 0}

'"

⁰

0^N

Cb

^ltO°

0 0.15 _{I!. I!.}

<t.

~ 0.1 0 'hI!.

60

c

30 40 50 Wt%Ti02 20

B

; ~~

00

0

00 50

4

10 10

o 3 ::;;Cl

<t.

~ ² 40

c;;o 0o

30

u,(])

~

~

20

20 30 40 50 60

Wt%Ti02 1.25

00 0 D

0 0

8

^{0.75 0 I!. I!. 0}

N 1!.4:) 0

> _{I!. CO}

<t. _~

~ 0.5

Cb 10 00 0.25

0

10 20 30 40 50 60

Wt%Ti02

ted as FeO in Fig. 6A. Inthis case,notethat magnetitesfrom magn et ic lows at Tellnes, Heskestad and Mydland plot at end-memb ermagnetite wit h 93.09wt .%FeO.Only mag net - itesfro m the magnetic highs atMydl andand Bak kacontain subst antialTi02,and hen cealsolesstot al Fe. InFig.6B,the same analysesareplotted in term s ofcalcu lat ed weigh t % FeO and Ti0 2. End -m em ber magnet it esplot at 31.03%FeO, with greaterFeO only in more ulvospinel-richcomposit ion s whic hat 18.6 wt.%Ti02corresponds to about S2%ulvosp i- nel end member. Fig.6C shows MgO plotted ag ains t Ti02.

MgOis notably low erin magnetit esthan in many ilmenites (co m pareFig.SC)and seem s to be high estin somenearend- membermagnetite sthanin titan if ero usmag net itesfro mthe sam e samplesfrom the Myd land and Bakka aero ma g netic high areas.

In magnetites,V_{20 3}is highestin Tl-pcormagnetite s(Fig . 6D),particu lar lyinsamp lesfromtheilmenitenoritesfrom the

Heskestad and Mydland magnetic lows. In these norites, ilm enit e was the dom ina nt primary precipitate oxide min- eral. Magnetite hadeitherjustjoinedilm enit e or was precip - itatedfromint ercu mulus liqu id.Once mag ne ti tebecamethe do m inant primaryoxideprecip itate,the magmawas rapid ly dep let ed in vanadi um and late Ti-rich mag netites arevery lo w in V₂₀₃. Cr20 3isonly sig nifi can t inTi-poo r magn etites (Fig.6El.and then onlyinmagnetitesintheprimitive rocksof theTelln es body.

Summary

The analytical resul t s of thepresentst ud y are su mmarized as follow s. Ilm en ite from the prim itiveTellnes i1menite norite containsup to4.8wt.%MgO substitut ingforFeO. Areasrich in hematite exso lut ion lamellae are alsoenriched in V_{20 3}(u p

sUZANNEA.McENROE,PETERROBINsON&PETERT.PANISH NGU-BULL436,2000-PAGE 55

50 95 Fig.6.Elect ronprobe analysesofmagnet ite

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to004wt.%)and (r203(up to0.27wt.%). CoexistinqTi-free magnetite cont ains about 004 wt.%MgO,0.75 wt.% V203, and 1.25 wt.%(r203' IImenitefromthe ilmenite-ric h norit e samplesat Mydl and andHeskestad in theBjerkreim-Soknd al layeredintr usion containup to1.6and1.0wt.%MgO,respec- tively. Areas rich in hematit e exsolut ion lamella e are enrichedinV203(up to0.75and1.15wt.%)and(r203(upto 0.15and0.1wt.%). LowerMgO and (r203,and higherV203 in the Heskestad sample as compared to the Tellnesand Mydla ndsamples isindicat iveof the progress ofmagmatic crystallization wit h V203enri chmen t intheliquid beforethe onsetof primaryprecipitationofmagnetit e.Ti-freemagnet- itesinthese samples fromthelayeredintrusio ncontain up to 0.75 wt.%MgO,up to lA wt.% V20 3, and up to0.3 wt.%

(r20 3'

Hematite-poorilmeni te frommag netite-richnor ite sam- ples atMydland and Bakka in thelayeredintrusion contains

up to2.3and 2.0wt.%MgO, about0.1and 0.2wt.% V203, and0.07and0.07wt.%(r203'respectiv ely.Analyses of mag- netites in thesemagnetit e-rich norit es show variableTi02 consistent wit h the observed exsolution ofulvospinel and/or oxidation-exsol uti on ofilmenite. Magnetitesfrom Mydland and Bakka showslightly increasingMgOwit hincreasingTi02 up toa maximumof004and0.6wt.%MgO,decreasingV203 withincreasing Ti02from amaximum of 0.8 and 1.0 wt.%

V203,and no obviou srelationsh ipbetween (r203and Ti02 wit h a maximum of 0.15 wt.% (r203' The magmatically evolvednature of thesemagnetite noritecumulates is sug- gestedbythelowvalues in magnetiteof MgO and (r203,and also of V203,indicating crystallization wellafte rmagneti te became a primary magmatic precip it ate. Cont rary to the consistent trendsfor magn etit e,ilmenitesinthemagn etit e- rich norit es at Myd land and Bakka showhigherMgOthanthe ilmenitesintheilmenit e-rich noritesat MydlandandHeske-

NGU-BULL436,2000 -PAGE 56

stad . This trend might reflect subsolidus re e q ui li b ra t io n of these small amounts of ilmenite.

Futurepro sp ectin g forwo r ka b lean d chemically suitable ilmenite resources will fo cu son mineral concentrations,on aeromagnetic signatures, and also on the relationsh ips between magma t ic evolution, major- an d trace-el em ent chemist ry,an dsu bso lid us re-equilibration, as outline d he re.

Appendix

Analytical method

Microprobe analyses we re made at theUniversityofMassa- ch u settswit h a CamecaSX50electronmicroprobe setatan accelerating potential of15keV, a samp le current of15nA, an d a typicalbeam diameterof111 m.Countingtime sof20 or 40 seconds per elementwe re used. Correctionsfor diffe ren - tial matrix effectswere done using the Cameca onlinePAP correc t ion rout in e. Analyt ic alpre cision is estimatedat±0.1 we igh t pe rc e nt for oxide co mponen ts present at the 1 weig htpercen tlevel.An al yt ical precision on typical valuesof 0.5we ig h t percent V203and Cr203isestimatedto be

±

0.06 wei g h t percent at the 95% confidence level. In addition , thereis believed to bea sys t e m ati c overestimateof V203in

ilme ni t e of about+0.1wei gh t percentcaused byTiK~

- v

^Ka

in terfere n c e.

Acknowledgm ents

TitaniaA/S,Ragnar Hagen and KariBerge,in particular are thankedfor help with the field arrangements,accommoda tion and wit h help in obtai ningsam plesin the Tellnesmine.J-c.Duchesne providedvaluable geolog icalinfo rmatio nanddiscussions.L.P.Nilsson collectedthe Heske- stad samples. R.J. Wilso n and H. Schiellerup provided thou ghtful reviews.This workhas beensupported by Titania A/Sand the Norwe- gian Geological Survey'sstrateg icresearchfund.Each of the above per- so nsandinst ituti ons isgratefull yacknowledg ed.

SUZANNEA.McENRO E,PETERRO BIN SO N

s

PETER T. PANISH

References

Duchesne,J.C.1999:Fe-Ti depositsin Rogaland anorthosites(So ut h Nor- way):geochem ical characteristicsand problems of interpretation . MineraliumDep osita34,182-198.

Fleet ,M. E.,Bilcox,G.A.,&Barnett,R.L.1980:Orientedmagneti teinclu- sions inpyroxenes fromthe GrenvilleProvince.Canadian Mineralo- gist18, 89-99.

Harriso n,R.J.,Golla,Ute,McEnroe,S. A.&Putni s,A.1998:The effect of ultrafine-scale exsolut ionlamellaeonthemagnetic propertiesof theilmenit e-hem atite solidsolutio n: Micro structuraland chemical investi gatio n using energy-filtere dTEM.ElectranMicra scopy 1998, Sy mp osiu mE,Volume1,233-234.

Krause, H., Gjerth,E.,&Schot t,W.1985:Fe-Tideposit s in the SouthRoga- land Igneous Complex, with special reference to the Ana-Sira Anorthosite Massif.Norgesgeologiskeund ersokelseBulletin402,25- 38.

McEnroe,S.A.1995:The aeromagneti csig natureofthe Tellne sMineand surrounding reg ion:initialinterpretationand consequencesonan explorat ionst rategy .NorgesgeologiskeundersokelseIn tern al Report, Spp.

McEnroe,S.A.1996:Magneti c stability andintensityinMid-Proterozoic rocks wit h cont rasting Fe-Ti oxideassemblagesfrom southwest Swede n,south No rway and Adiro ndacks,USA.EOS77,Abstract Sup- plem ent,585.

McEnroe,S.A.1997: IImenite mineralmagnetism:im plicati on s forgeo- physical explorationforilmenite deposit s.Norg es geolo giskeunder- sekelse Bulletin 433, 36-37.

McEnroe,S.A.,Rob inson,P.&Panish,P.T.1996:Rock-magneticproper- ties,oxidemineralogy,and mineralchemistryin relati on to aero- magnetic interpretation andthe searchforilmenite reserves.Nor ges geologiskeundersekelseReport96060,153pp.

Morse,S.A.1970:Preliminarychemical data ontheaugi t eseries of the Kig lapait layered intrusion(abst ract).AmericanMinera log ist55,303- 304.

Ronning,S.1995:Hellkopt errnalln qer over kartb lad 1311-IVSokndal.

Norg esgeol ogiske undersoke lseRepo rt 95.120,14 p.

Scharer,U.,Wilmart,E.&Duchesne,J. C.1996: Thesho rt duration and anoroge niccharacte rofanorthos itemagmatism:U-Pbdat ing of the Rog aland complex,Norway.Earth and Planeta ryScienceLetters 139, 335-350.

Wilmart,E.,Demaife,D.,&Duchesne,J.C.1989:Geochemicalconstraint s onthe genesisof the Tellnes IImenite Deposit.EconomicGeology84, 1047-1056.

Wilson,J.R.,Robins,B.,Nielsen,F.M.,Duchesne,J.C.&Vande rAuwer a,J.

1996:TheBj erkreim-SokndalLayeredInt rusion,Sout hwestNorway.

InCawt ho rn,R.G. (Ed.)LayeredIntrusions.ElsevierScience,231-255.