nystrom and henriquez-1994

5/27/2018 Nystrom and Henriquez-1994

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Economic eologyVol. 89, 1994, pp. 82{)-839

Magmatic eatures f Iron Oresof the KirunaType n Chile andSweden:Ore TexturesandMagnetiteGeochemistryJANOLOVNYSTR/SM

Swedish useum f NaturalHistory,S-10405Stockholm,wedenANDFERNANDOENRiQUEZ

DepartamentoeMinas,Universidade Santiago,asilla 0233,Satiago, hileAbstract

Magnetite avas nd eederdikeson the flanks f the volcano l Laco n the ChileanAndesare characterized y textures emonstratingapidcrystalgrowth romsupersaturated elts.Columnarmagnetite, conspicuousorm of magnetite t El Lacowith occasionalendriticbranching, asbeen found n two other apatite ron provinces:he Cretaceousron belt inChile, a 600-km-long onealong he Pacificwith about40 deposits, nd he Early Protero-zoic Kirunaore field in Sweden.Presence f columnarmagnetite n an iron ore is suggestedto be diagnostic f a magmatic rigin.Platymagnetite, notherdendritic ormwidespread tKiruna,alsooccurs t El Laco.Moreover,manyoresof the three provinces ontainpyroxeneor pseudomorphsfter t with dendriticmorphology.he occurrence f similar apid-growthtexturesn the investigated patite ron oresdemonstratessimilaroriginwith emplacementof ore magmas t or near the surface. n fact, existence f vesicularore lava and pyroclasticore at Kiirunavaara hows hat this deposit s volcanic.A commonorigin of the ores s supported y similarcompositionsf their magnetites.Analysisof ca. 50 concentratesrom 17 deposits hows hat the magnetites re very poor inCr (


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KIRUNA-TYPE RES: EXTURES ND MAGNETITE HEMISTRY 82 ]ward gradation rom apatite iron ore into quartz-bandedore. The deuterichypothesiss mainlybasedon the stronghydrothermalalterationassociatedwith someChileanandCanadian epositsRuizet al.,1968; Hildebrand, 1986).Several ttempts avebeenmade o use he chemi-calcompositionf the Kirunaoresor magnetite epa-rated from them for geneticpurposes. andergren(1948), Hegemann nd Albrecht (1954), Frietsch(1970), Partk 1975a,b), andLobergandHorndahl(1983) compared he geochemistry f the Kirunaoresandother ypesof iron deposits, ut thesestud-ies were inconclusive ecause he magmatic efer-ence material used for the comparisonsmagnetitesegregations nd accessory r minor magnetite nigneousocks) s too different n characterrom hemagnetite onstitutinghe predominant haseofapatite ronores.The discovery f lava lowsof mas-sivemagnetite t the volcano 1Laco n the ChileanAndes (Park, 1961; Henrquezand Martin, 1978)demonstrateshat apatite ron ores can form frommelts,andprovides n opportunityo determinefthe composition f the Kiruna magnetitess consis-tent with a magmaticorigin.Here, we give the resultsof a comparativegeo-chemicalstudy of magnetite rom E1 Laco, otherwell-preservedeposits f the Kiruna ype n Chile,and the Kiruna area and describeore texturesdiag-nostic f a magmatic riginwhichshow hat he Kii-runavaara eposit s volcanic. t is not our purpose odiscussow he ore magmasormed.The geochemis-try of the associated patite will be treated byHenrquez ndNystr/Smn a forthcomingaper.Theemphasisuring he sampling f the Kirunamaterialwason ore with primary extures n order o allowacomparison ith analogousextural ypesat E1Lacoandelsewhere, nd o avoidpossiblenfluence n hechemistry y recrystallizationuringmetamorphism.Textures anprovidemuch nformation n ore for-mation: In recentyearssomuchemphasis asbeenplaced n sotopes,luids, hemistry,nddeposit ndprocessmodels hat the textural eatures avebeenignored" Barton,1991).

InvestigatedDepositsOur samplesome rom hreedifferent patiteronore provinces, ach epresented y several eposits:the PlioceneE1Laco ores Fig. 1), an extensiveCre-taceous rovince arallel o the Pacific oast f cen-tral Chile, generally eferred o as he Chilean ronbelt (Fig. 2), and the Early ProterozoicKiruna orefield (Fig.3). Samplesroma limestone-hostedtrati-form depositn the iron belt (Bandurrias;ig. 2) arealso ncluded or comparison.The E1Laco ron depositsPark,1961;Ruiz et al.,1965; Haggerty, 1970; Frutos and Oyarzfn, 1975;Henrquez ndMartin,1978;Wegner,1982;Garde-

alluvial & glacialo[]deposits'[] hydrothermallteration[] Pleistocenendesite[]iron oreJ---1 l LacoQndesite Camp/550FIG. 1. Iron depositst El Laco,a volcanon northern hilebelongingo a Pliocene-Recentolcanicrc seeFig. 2 for oca-tion).Total esourcesfore >60 wt %Fe equiv)son heorderof500 Mr. The deposits onsist f sul)horizontalava lows LacoNorte, LacoSur,andSanVicenteAlto), a domelikentrusionSanVicenteBajo),anddikes.Ore dikeswithindepositsre shownnwhite (Rodados egros ncludes boulder ield and is muchsmallerhan he mapsuggests).odified romRuiz et al. (1,9t35).

wegandRamirez, 985;Frutos t al., 1990) aresitu-ated on the flanksof an andesite-rhyodaciteolcanoat an altitude of 4,700 to 5,300 m. The ores wereemplaced t about2 Ma ago accordingo fission-trackdata or apatiten the ore (2.1 _ 0.1 Ma; Mak-saevet al., 1988) andK-Ar whole-rock atingof anandesite rom the eastern flank of Pico Laco (2.0 _0.3 Ma; Gardeweg nd Ramirez,1985). Within anareaof 30 km2 there are sevendepositsFig. 1), withtotal resources on the order of 500 million tons corre-spondingpproximatelyo one-fourthf thepremin-ing onnage f Kiirunavaaracf. Figs.1 and3). LacoNorte s he argest ody.Drillingandsurface bser-vationshere show he followingupward succession:andesiteava,pyroclasticre up o 30 m thick),mag-netite lava (up to 50 m), a secondpyroclastic nitwith fragments f massive re (about20 m), and asecond andesite flow.The ores occur as lava flows and related feederdikes LacoNorte,LacoSur,andSanVicenteAlto),pyroclastic aterial, ndsubvolcanicodies SanVi-centeBajo, a domelike ntrusion, ndRodados e-gros, Laquito, and CristalesGrandeswhich are


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8 2 2 NYSTROMNDHENR[QUEZ

E Laco;'

nSontiogCerr

70'

PacificOcean

28'

29'

Jer6n

Los Coloradc

Dorado DOR,'

50 kmi I

0 sampled apatite vein100 equivalenteI1 Pliocene-Recent.... upperretaceous-iocene lowerretaceous,;-'/.,,re- retaceous

rio. 2. Iron depositsn the Chilean ron belt (Ruiz et al., 1965)andsome ssociatedpatite eins.The country ocksof the Cre-taceous eposits onsist f basic o intermediateavas nd grani-toids. he geologyssimplifiedromSERNAGEOMIN 1982).Thelocations f the El Laco Pliocene) ndMagnetita edernalesMP;Tertiary) ron deposits re given n the inset.

dikes).Apatite s locallyabundantn the intrusionsbut is merely an accessory hase n the lavas.Theorescontainhighlyvariableamounts f pyroxenenthe form of thin elongated risms, requentlywithdendriticbranching. t has been erroneously e-portedasamphibole actinolite) n the literaturedueto the unusual abit.The pyroxenes partly o com-pletely altered o talc, opal, goethite,and smectite.The octahedral acesof magnetitecrystals ining

openspaces re oftenpitted by moldswith a squarecross ection fter pyroxene.Hematite is a ubiquitousoxidationproduct aftermagnetite n extrusive ndpyroclastic re; occasion-ally cavitiesare lined by well-developed rystals fhematite.The orebodies re surrounded y narrowmetasomaticureolesup o 1 m thick n lavabut con-siderablywider n tuff) where he andesitic ost ock

PG-K6PG-235PG-K8PG-K9 I ironoreG-KII.PG-Sal ....: quartzite.PP.p i ?:.;t]::,oU.;i:",owerauk,olcani.:*?:.,1'.*.:.'acidnterm.olconics',i ::60 wt % Fe) at Kiirunavaara,he argestndivid-ualorebody, xceed ,000 Mt. The oresdip 50 to 75 to the eastand are hosted 1)v acid and intermediate volcanic rocks of the Por-phyry roup.Je ower auki oleanicsompriseyroclasticand sedimentary rocks.


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KIRUNA-TYPE ORES: TEXTURESAND MAGNETITE CHEMISTRY 823

is transformedo pyroxene,scapolite,quartz, andgarnet.The andesiteshowsno granoblasticextureclose o the ore.The volcanic omplexncluding resisaffected y a ocallystrong ydrothermal lterationwith silicificationcristobalite,ridymite,and amor-phoussilica),development f gypsum,arosite,andalunite, and depositionof fumarolic sulfur in frac-tures (Vivalloet al., 1991).Frutos and Oyarz6n (1975), quoting Thomas(1970), reported that incompletelydigested rag-mentsof itabirite occur n the magnetiteavasat E1Laco.This s totallymistaken; o such hinghaseverbeen observed and the whole idea is based on a mis-understanding y Thomas.Whether there are sedi-mentary iron strata ("lower Paleozoic erruginousschists") r not below the volcanic omplex s m-pliedby FrutosandOyarz6n 1975) andFrutoset al.(1990) is not known.To the best of our knowledgethere is no outcropof such ocks n the E1Laco area(cf. Boricet al., 1990). The nearest edimentaryrondeposit itabirite) s situatedn Argentina bout200km northeast f E1 Laco, acrosshe generalnorth-south strike of the formations.The Chilean ron belt (Geijer, 1931; Ruiz et al.,1965, 1968; Park, 1972; Espinoza, 1984a, 1990;Oyarz6n and Frutos, 1984; Menard, 1986; Ruiz andPeebles, 988) is approximately00 km ongand25km wide. t consistsf sevenarge 100 million onsof high-gradeore) and about 40 medium-sized ndsmalldeposits f apatite ron ore (Fig. 2). The sizesrefer to premining eserves incemanyof the oreshavebeen exploited; nly Romeral,Algarrobo, osColorados, nd E1 Tofo were in productionduring1993. Three of the argedepositsBoquer6n hafiar,Cerro Negro Norte, andCristales)are unexploitedoronly partially mined. The apatite content showsalarge variationwithin and between the different de-posits;genetically elated apatite veinsoccur abun-dantly n the ore-bearing egion.The depositsof the iron belt have been dividedinto four groups ccordingo their geologic etting(Espinoza, 984a, 1990).The firstgroup,comprisinga few orebodiesituated pproximately0 km eastofthe belt axis, re stratified ndsediment osted e.g.,Bandurrias; isternas, 986; Espinoza,1986). Theylack apatite and were deposited n an Early Cre-taceous shallow-marine basin. The ores of the otherthree groupswere formed n a magmatic rc alongthe westernmarginof this basin Espinoza, 990).Some deposits re hostedby granitoids,othersbyvolcanic ocksbut mostof them--including all thelarge ones--occur associated with metavolcanicrocks n tectoniccontactwith granitoidswithin thenorth-trending tacamamegafault one (Espinoza,1984a,b, 1990; Oyarz6nandFrutos,1984). The ironbelt coincides patiallywith thiszone,and ndividualorebodies re elongated arallel o it.

The Cretaceous olcanic ocksand granitoids ost-ing the iron deposits re interpretedascomagmatic(Bookstrom, 977' Pich6n, 1981; Montecinos, 983;OyarzfinandFrutos,1984) or derived rom differentparent magmas Gonzalezand Henr[quez, 1991).The volcanicpile is dominated y basaltic o andes-itic lava flows,and mostof the granitoids re dioriticmembersof a large batholith, seeminglyyoungerthan the lavas. The ores are coeval with the volcanicrocks according o Espinoza 1984a) and OyarzfnandFrutos 1984), but Ruiz et al. (1965, 1968) con-sidered hem to be somewhat ounger.Available a-diometricagesof the rocksassociated ith the oresfall in the range 100 to 128 Ma (Zentilli, 1974; Pi-ch6n, 1981; Montecinos, 983; the range sbasedon11 K-Ar mineral and whole-rockages or four de-posits).The lavasaway from ore deposits nd granitoidsare affectedby nondeformationalegionalmetamor~phism,usually t prehnite-pumpellyiteacies Levietal., 1989). The gradeof the alteration eflectedbyextensive evelopment f amphibole s higher nearlarge orebodiesdue to contactmetamorphismromnearbygranitoids r processeselated o the forma-tion of the ores,or both (Espinoza, 984a). The am-phibolization s overprintedby hydrothermal lter-ationof ower emperature Galatztn ndHenrquez,1979).We havesampled ll the iron depositsndicated ynames n Figure 2 exceptCerro Negro Norte andBo-quer6nChafiar.Romeral Bookstrom, 977), repre-sented n this studyby severalsamples,s typicalofthe argedeposits.t iscomposed f two majorbodies(CuerpoPrincipal ndRomeralNorte), partlydelim-ited by zones of faulting and mylonitization,andthree smallbodies.Five samplesROM-59 to ROM-62, and ROM-66) were collected rom Cuerpo Prin-cipal and its envelopeof ore breccia. This body islenticular,with a lengthof 850 m, an averagewidthof 250 m, anda depthof 600 m. One sample ROM-58) representshe strongly eformedandmartitizedRomeral Norte orebody (300 m long, 50-120 mwide, and up to 240 m deep), and two come fromveinsandpatches f ore in meta-andesite ndquartz-apatite-calcite-scapoliteock near a youngerdioriteintrusion the low-gradeSicilianoore; ROM-63 andROM-64). Bookstrom (1977) regarded RomeralNorte asa replacement re hostedby schistsn a latePaleozoic asement.n addition,samplesrom twomagnetite-bearing patiteveins n the Romeralareaare included or comparisonLa Escoba= ESC andYayita -- YAY in Fig. 2).Algarrobo (Geijer, 1931; Ruiz et al., 1965;Montecinos, 1983; Espinoza, 1984a; Gonzalez andHenrquez,1991) is another arge deposit ery simi-lar to Romeral. t alsoconsists f two major bodies(AlgarroboC and Penoso); ur samples ome rom


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824 N$TROMNDHENR[Q EZthe former which is 1,100 m long, has an averagewidth of 400 m, and reaches depth of 400 m. Ojosde Agua Geijer, 1931, 1967; DobbsandHenrquez,1988), situated n the northern continuationof theAlgarrobodistrict, s composed f six steepdikelikebodiesof apatite-rieh re (the argest s 200 m long,4m wide, and 100 m deep). This smallwell-exposedandunexploited eposit s of specialnterestbecauseit showsmanyof the featuresof the larger but nowexploited ores. Two other deposits ich in apatitetreated here are E1 Dorado and Carmen. The latter isa 500-m-long, 25- to 60-m-wide, and 50- to 200-m-deep tabularbody hostedby volcanic ockswithoutknown intrusions n the vicinity (Henrlquez et al.,1991). E1Dorado and two other ores ncluded n thestudy,E1Tofo (Geijer, 1931) and Cerro Imtn (Ruizet al., 196.5), belong to the Romeral-Algarrobogroup.The Kirunaore field (Fig. 3) andsurrounding reasin northern Swedenconstitute he greatestconcen-trationof apatite ron ores n the world.The depositsoccuras abularbodieswithin a thick sequence om-posed argelyof intermediate o acid volcanic ocks(the PorphyryGroup), which contains gnimbritesand whose ower part.was nterpretedas a calderafilling by Geijer and Odman (1974). The deposits tthe top of the PorphyryGroup (Rektorn,Henry, Nu-kutusvaara, nd Haukivaara;Fig. 3), collectively e-ferred to as the Per Geijer ores,differ from Kiiruna-vaaraandLuossavaaray higherapatiteandhematitecontents nd he presence f quartzand/orcarbonatein significantmounts. adiometric atingof the hostrocksand a crosscutting ranophyrie ike indicatesthat the ores ormedat about 1880 to 1890 Ma (We-lin, 1987; Cliff et al., 1990; U-Pb zircon and Sm-Ndwhole-rockdata).Veins with gypsumand pyrite in the ore are ex-pressions f hydrothermal ctivity(Nystr/Sm, 985).The ores are locally deformedby shearing nd arereerystallized, ut relict primary exturesand struc-turescanneverthelesse seen,especially t Kiiruna-vaara (Nystr/Sm,1985; Nystr/Smand Henrquez,1989). This is consistentwith the nondeformationalnature of the greenschistaciesregionalmetamor-phism hat characterizes he country rock. Contactmetamorphic ffects n the latter, if present,are in-conspicuous reerystallization and eoarsening ofgrainsize).Detailed geologic escriptions f the Ki-runaareacanbe found n the numerous ublicationsby Geijer, Frietseh, and Partk see references nFrietseh, 1978, 1984).

Primary TexturesThe existenceof pyroelastic re (Fig. 4A and B)and ore lavaswith flow ayeringandhighlyvesicularupper parts at E1 Laco (Figs. 5A and B, and 6A) is

telling evidenceof crystallizationrom volatile-richmelts.Several f the E1Laeosamplesncludedn thisstudyare fromore of unequivocal olcanic haracter:a volcanic omb, avawith ropy surface, ndvesicu-lar lava.The bomb FHL-121) is 9 by 16 em n crosssection ndhasa regular usifbrm hapewith ongitu-dinal luting. ts inner part is a porous ggregate fca. 0.2-mm-largemagnetiteoetahedra oatedby afilm of iron phosphate. he porousore lavas lsoarecomposed f oetahedrawhich coalesce n massiveparts.The pyroelastic re at E1Laeooccursn the vicinityof feeder dikes and comprises olcanicbombsandfiner air-fall material, alone or mixed with blocksofore lava. The structuresare indistinguishableromthosenormally ound n near-ventpyroelasticmate-rial. The E1Laeo material s poorlyconsolidatedndfriable,whichcauseshe structureso be easilyde-stroyedexceptwhere the ore hasbeen sinteredbyheat roma nearbymagnetite low or dike.The pyro-elastic resconsist f iron oxidewith magnetite s heprimary constituent,and some oxide grains havefilmsof green ron phosphate. patite,usually bsentor accessory,anbe present n considerablemountsand gives ise to a fine stratificationhat can be fol-lowed up to severalmeters (Fig. 4A). Presenceofbombsags emonstrateshe falloutnatureof the ma-terial.The stratificationanbe attributed o sortingdur-ing the eruption.Magnificationeveals hat the apa-tire layersconsist f euhedralapatiteprisms,gener-ally lying with their longaxiswithin the stratificationplane, and some magnetite oetahedra Fig. 4B). Avarying number of apatite prismsalso occur n themagnetiteayers the amountof apatite s constant teach evel),but the prisms how ess egularorienta-tion here, consistent ith simultaneouseposition fthe two minerals. heir euhedralshape nd he fabricof the stratified re (Fig. 4B) dentify he materialaspyroelastic. iny platescomposed f magnetiteoeta-hedra n parallel ntergrowth el. Fig. 7D) are wide-spread.Oreswith apatitebandingare morecommonn theKirunaarea. n manyeases he banding s clearly oftectoniccharacter, ut some tructures ppear o beprimary,caused y a process f sedimentation.heyare representedn this studyby sampleswith cross-bedding PG-531; llustrated n Nystr/Sm, 985, fig.8) and fine stratification f uniform thickness Fig.4C-D; seedetaileddescription f the stratificationnGeijer, 1967). The stubbyapatiteprisms n the apa-tite layers Fig. 4D) have he sameorientation s hatdescribed bove or the pyroelastic re at E1 Laco,and iny magnetiteplatesare abundant n the magne-tite layers where octahedraalso can be discerned.The stratifications locallydisturbedby intraforma-tional oldingandbrecciation t a smallscale.


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KIRUNA-TYPE RES: EXTURES ND MAGNETITE HEMISTRY 825

Fic. 4. Stratifiedapatite-banded)yroclasticre. A. Crystal uff,LacoSur sample LS-2;black=magnetite,white = apatiteand reflecting rystal acesof magnetite), he wavystratifications onlyapparent ecause f surfacerregularities. . SEMphotomicrographf an apatite ayerandsurroundingore n the same ampleELS-2)showinghe euhedral abitof heconstituent inerals nd he owdegreeof consolidationn the loose rystalaggregate. he small ounded rowths n the apatiteprisms ndmagnetite ctahedra onsist f SiOn.C. Crystal uff, Kiirunavaarasample G-235;black= magnetite,white = apatite, he latter partly emoved y weathering). . Photomicrographf a thin section f thesamesample PG-235; crossed icols).

Numerous blocks of ore with vesiclelike holeswere found n a few placesduring a recent visit toKiruna (Fig. 6B). The blockswere dumpedduringthe open-pit tage f the miningof Kiirunavaara,ndtheir present ositionndicateshat they came romthehanging-wallideof the southern alfof the ore-body.The ore consistsolelyof magnetite xcept none dumpwhere t is stronglyoxidized o hematiteandhydrouserric oxides. here s no evidencehatthe holes epresent avities enerated y weatheringof sulfides,calcite, or other minerals.The holes areoftenelongated lthough ighly rregular n detailbecausehe ore seemso be a crystalaggregate. ev-eral samples how hat the aggregatesre composedof octahedra,which here and there may be inter-

grown o formsmall lates.Minorquartzsdepositedin manyholes.Someore texturesat El Laco constitute ndepen-dent evidence f a magmatic rigin.Henrquez ndMartin (1978) described columnarormof magne-tite that locallygradesnto spheruliticiber and/oroctahedralrystal Fig. 8D) andshowedhat hecol-umns and spherulites re rapid growth featurescaused y sudden upersaturationf degassingxidemelts.The columnarmagnetites common t El Lacowhere t typicallyoccurs suniform rrays f parallel0.5- to 15-cm-long olumns rientedperpendicularto nearbyopenspaces,ike flow topsand gasescapetubes n feederdikes Fig. 8A andD). Oftenseveralcolumnar rrays resuperimposedbove achother,


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826 NYSTROMIDHERfQUEZ

FIG. 5. Magnetite avawith contorted lowstructure.Note thedrawnout hape f the vesicles ue o flowage. . LacoNorte (out-crop).B. LacoNorte (sample LN-19,).

resultingn a banded tructure. olumnsurfaces is-playa striation t an angleof 45 to 90 to the axisofthe column; he striae are the edgesof very thinplatesstacked n eachother. Locally he magnetiteshows dendriticmorphology, ccurring s adiatingarraysof branching ibresand smallcolumns.Columnar magnetite has also been observedatKiirunavaaraGeijer, 1910, 1967; Nystr6m, 1985)ascompositeands f parallel1- to 12-cm-long olumns

(Fig. 8C) which orm enses p to a few meters ongin the ore. The columns of each band are uniform insize and orientedperpendicularo the extension fthe lens; he lenses re parallelwith the contacts fthe orebody.A detailedstudyof a few crosscutsnthe Kiirunavaaramine has shown hat lessregularsmallarraysof columnarmagnetiteare quite wide-spread ndcanbe found n ore typesof verydifferentapatitecontent Nystr6mand Henriquez, 1989).A search or columnarmagnetite n the Chileanironbelt revealed hat t ispresentocally n sixout ofeight nvestigated epositsTable 1). It wasnot seenat Bandurrias hich s a nonapatitic tratiformore orat El Tofo where he ore appearso be partlyrecrys-tallized. However, poorly preservedmagnetitecol-umnshavebeen observedn a small ron deposit ustnorth of the main El Tofo orebody.The parallelar-rangement f the columnsoundat El Laco Fig. 8A)and Kiirunavaara Fig. 8C) is rare in the iron belt,where hey tend to be orientedat random Fig. 8E).Thisnonoriented arietyof columnarmagnetite lsooccurs at El Laco.Columnarmagnetites not he only ormof magne-tite that may showdendriticmorphologyn the Ki-runa ore field. The so-called keletonore (Geijer,

FIG. 6. Vesicularmagnetiteava.A. LacoSur sample LS-20;photographed ith a coatingof ammonium hloride n order toeliminate eflexes). . Kiirunavaarasample IR-34).


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KIR UNA-TYPEORES:TEXTURESAND MAGNETITE CHEMISTRY 827

.

, .:..,..- -- .. : - ....'...' .,f.' '.. ...., ..- _. .. ... :-:%,-- .?::'. .-_-. ... ." ....

. :.": - .- .... < .. ,. . : : .. :?: .:._.- ..:.":. :: .'

..:. : ..... ; . .. -:... ;.. -- .. ...

FIG. 7. P atymagnetite.A. Skeleton re, .e., dendriticmagnetite latesembeddedn apatite,Kiirun-avaara sample G-36:lA). B. Closeup f a magnetite late n skeleton re showing arallelgrowthofoctahedra, iirunavaarasample IR-55). C. Apatite-free nalogue f skeleton re, LaeoSur (sampleELS-I). D. SEMphotomierographf a tiny magnetite latecomposedf intergrown ctahedran pyro-elastic re, LaeoSur (crystal uff, sample LS-2;cf. Fig. 4A-B).


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828 NSTROMHDHENRfQUEZ

A B

mm mmi I

D

2 mmFIG. 8. Columnarmagnetite.A. Array of parallelcolumns, aco Norte (sample HL-76). B. Artifi-ciallygrown olumnarmagnetite, egerforsronworks. ote he gas ubblesn the owerandupperpartof the sample. From he collection f C. Benedicks1875-19581,an outstandingwedish xperimentalmetallurgist. he collection f Benedicks asdonated y the Swedishnstitute or Metal Researchn1989 to the SwedishMuseumof NaturalHistory.No note or report describinghe conditions nderwhich he magnetite asgrown ccompaniedhe collection.) . Coarse rrayof parallel olumns, iirun-avaara sampleK1R-15).D. Columns erminatedby octahedral aces,Laco Sur (sampleELS-19). E.Chaotic rrayof broadcolumns, lgarrobo sample LG-24).


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KIR UNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 89.9TABLE 1. Minor andTraceElementCompositionsf Magnetite romSwedish ndChileanApatite ron Ores in ppm)

Sample Ti V Cr AI Mg Mn Ni Co Zn Cu Si Ore typeEl Laco

EL-C:24 1,460 1,380 13 1,610 3,390 486 144 128 60 17 1,730 Pyroxene-apatite-richreEL-2:6 665 2,480


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830 N'$TROMNDHENR[Q EZ1910; Park, 1975a, b; Nystr6m, 1985) is a morecommondendriticvariety. It consists f magnetiteplatesembedded n apatite.The platesare orientedat random r formsubparallelrrays,ocallywith off-shootsike branchesrom a stem Fig. 7A). Nystr6mand Henrlquez 1989) observedhat the platesaremadeup of octahedra howing arallelgrowth Fig.7B). Skeletonore has not been reported from anyChileandeposit, ut apatite-free rrays f platymag-netite dendrites nalogouso those n Figure 7A arefoundasa rare extural ypeat E1Laco Fig. 7C), andsmallmagnetiteplatesseem o be quite widespreadin the pyroclastic re (Fig. 7D).At Kiirunavaara olumnar ndplaty magnetiteareintimatelyassociated ith eachother n apatitesegre-gations ound occasionallyn the skeletonore (seeillustrationsn Nystr6mand Henrlquez,1989). Thecontact etween he about1- to 10-cm-large egrega-tionsandsurroundingre s markedby a millimeter-thickshellof subparallelmagnetite olumns rientedperpendicularo the segregation.he magnetite ut-side and nside) he shelloccurs splaty dendritesembeddedn a matrixof apatiteprisms.Someof thesegregationsisplay trachytoid exturedefinedbyslenderapatite prismsand thin magnetiteplates;othershave been interpretedas miniaturediapiricstructures y Nystr/m ndHenrlquez 1989).Amphibole actinolite)s he dominant ilicatemin-eral in apatite ron oresaccordingo the literature.However,somedeposits ontain yroxenenstead famphibole, or example, he extrusive nd intrusiveorebodies t E1Laco.The pyroxene ends o be con-centrated n layersof subparallel longatedprismsgrowing oward flow tops, cavities,and dike inte-riors,alternatingwith layersof columnarmagnetiteand ayersof conicalapatiteprisms n intrusivebod-ies (a type of comb ayering;Fig. 9A). The 0.5- to afew centimeters-longyroxenes ften form fanlikedendrites imilar o those llustratedn Figure 9B.Both monoclinic (diopside) and orthorhombic(ferroanenstatite) yroxenes re found.The pyrox-ene in the aureoles at the ore-host rock contacts oc-cursasanhedralgrainsand prismswithout dendriticbranching.

At Kiirunavaarahere are two typesof amphibole:one (normal)of a fibrous-prismaticndone of a den-dritic variety.The latter wasdiscovered y Geijer(1910) in a 150- by 10-m-large onewithin the ore-body where t characterized distinctive re typenow removedby mining ("ophitic ore"). The den-dritic amphiboles a pseudomorphfter pyroxene,andmicroprobenalysisf relictsof the original y-roxene (Wo44En44Fsl2)hows hat it was similar ncompositiono the clinopyroxenen the E1Laco ore(Wo48En4Fs; Nystr6m,1985). The arrangementfthe pseudomorphs efines a flow texture; comblayering ike that at E1Laco hasnot been described.

No pyroxenehasbeen reported rom the iron belt;however, here are dendritic pseudomorphsesem-bling the E1Laco pyroxene n manyof the deposits(Fig. 9B-C). Whether the originalmineralwaspyrox-ene, amphibole,or both is not known; n the follow-ing we refer only o a pyroxene recursoror simplic-ity. The pyroxene seudomorphsonsist f actinoliteandvaryingamounts f talc, quartz,andcalcite.Theyare common n apatite-richpartsof oresgrowingonapatiteprismsor formingcomb ayersand they areoftenassociatedithcolunnar agnetite.Analytical Procedures

Fifty-one concentrates f magnetite rom 18 irondeposits nd two apatiteveins Figs. 1-3) were ob-tained by repeated crushing,washing n destilledwater, and treatmentwith a Franz isodynamicmag-netic separator.A few sampleswere considerablymartitized (Table 1). Small amountsof apatite re-mained n samplesrom very apatite ich ore types;no attemptwasmade o remove t by selectiveeach-ing.After anXRD purity controlof the magnetite, gof each samplewas dissolved n warm 5N HC1 towhich a smallquantityof hydroxylamine asaddedas a reducingagent.Silicates mostlyactinoliteandquartz) difficult to separatecompletely rom somemagnetite oncentratesemainedassolid esidue upto 0.04 g in onesample; sually ess han 0.01 g). Thedissolvedmagnetites ere analyzed or Fe, Ti, V, Cr,A1,Mg, Mn, Ni, Co, Zn, Cu, Si, Na, K, Ca, andP withICP emission pectrometry t the Departmentof Ge-ology and Geochemistry, tockholmUniversity,byBirgittaBostr/m. he values or the elements rom Tito Si are given n Table 1. Kiruna sampleswith num-bers prefixedby PG come from Per Geijer collec-tions, samplePF-HE was donatedby Paul Forsell(Kiruna),and sampleHjL-1 is markedwith the nameHj. Lundbohm;all the other sampleswere collectedby us. The material s deposited t the SwedishMu-seumof NaturalHistory.

Magnetite GeochemistryThe minor and trace elementcompositionsf themagnetitespresented n Table i are uncorrected or

the smallamounts f apatite n somesamples. om-parisonof apatite-freeand apatite-rich amplesromthe sameorebodyshows hat the included apatitedoesnot affect he istedelements. xamples re sam-ples HjL-1, KUJ-3, and PG-530 (apatite-free)andPG-235, PG-531, and PG-K5 (apatite-rich) rom Ki-runa.Inspection f polishedhin sections t highmagni-fication eveals hat a few Kirunamagnetites ontainsmall nclusions f TiO2, probably utfie, and thereare ndicationshat somegrains oo small o analyzequantitatively onsist f i]menite an Fe-Ti phaseac-cording o SEM-EDS).Geijer (1910) reported are


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KIRUNA-TYPEORES:TEXTURESAND MAGNETITE CHEMISTRY 831

FIG. 9. Dendritic pyroxene includingpseudomorphsfter pyroxeneor amphibole, eferred to as"pyroxene").A. Comb ayeringdefinedby alternating ands f columnarmagnetite, endriticpyroxene,andapatite,Rodados egros El Laco,sample LR-3; sketch f irregularsurface). . Curvedbranching"pyroxene"n ronore,Carmen sample MN-28, thin section). . Coarse endritic pyroxene" mbed-ded n columnarmagnetite,Romeral sampleROM-66).


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8 3 2 NYSTROMNDHENR[Q EZilmenite on jointing planes,crystals f titanitc nearthe footwall,and a few grainsof allanite,all in theore.Very hin,crystallographicallyrientedex- solved) amellaeoccur n many magnetitesrom E1Laco.The argest f them 2 #m thick) s lmeniteandaroughroportionetweenhe mountf amellae 2and he Ti contentof the magnetite lsosuggestshatthese amellaeare ilmenite. Minute grainsat theirborders ook like spinel.Henrlquez nd Martin 08(1978) dentified mall uhedral rystals f rutfieoc-casionallyssociatedithmagnetiteetahedraneav- 0.6ities. lmenitewas bserveds nclusionsn relatively 0.Ti rich magnetitesrom he ironbelt.The followingconclusionsan be drawn from aninspection f Table 1: 02

1. The magnetitesorm two populations ith re-gard o V, Ni, andA1.One groupconsistsf the Ban-durriassamplesthe stratiform ron deposit n lime-stone),and the other groupcomprises ll the magne-tites from apatite iron ores and associated patiteveins Fig. 10).2. The magnetite n the apatite ron oresandapa-tite veinshas ow Cr (


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KIRUNA-TYPE ORES: TEXTURESAND MAGNETITE CHEMISTRY 833i i ippm

ooo [][] [] ApotitEl [] ores& veins o2000] *-+ o*[P o o/OOO oO800 e600

400

200 Kiruna

0 E[ Laco [] Romero[100 A[garrobo80 Ojos eAgua60 /% erromdn E[ Tofo

x Jarmen40 + ElDoradoV Apotiteeins Bandurrias20 (V=0 I I I I0 100 200 300 ppm Ni + Co

FIG. l. Vanadiums.Ni + Co ormagnetitesromapatiteronoresn theKiruna rea 3 deposits),1Laco 7 deposits),nd heChileanronbelt 8 depositsnd2 related patite eins).Note hat heV andNicontentsn thenonapatiticandurriasamplesremaximumalues. nalysesepresentedy a numberwithin circle re aken rom he iterature: -- apatiteronores, orthern weden; -- apatiteronores,Missouri;3 = stratified ron ores, northern Sweden;4 -- stratified ron ores, northernFinland; 5 --nonapatiticron ores,centralSweden; = Lahn-Dillores Co givenas


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8 3 4 NYSTROMNDHENR[Q EZMgppm7000

6000

5000

4000

3000

2000

1000

oo

Oo

Kiruna0 E[ Laco[ Romeral A[garrobo Ojos de AguQ/ Cerro Im(JnV E[ TofoX CQrmen+ E( DorQdoV Ap(3titeeins

X

+/x +2'o 3 'o 5'o do /Ni

FIG.12. Magnesiums.V/Ni formagnetitesromapatiteronores n theKiruna rea,El Laco, nd heChilean ron belt (including wo apatiteveins).

ular, often fiat, and hin cavities. t is obvious hat thiscorrespondso our vesieular re, which n all likeli-hood epresentshe top of a magnetite low; he flat-teningof the vesicles an be explained y flowage.The oxidation uggestsxposure t the surfacen anal-ogywith the conditionst E1Laeo.Abundantelictsof magnetite eveal he originalcompositionf theore, whichwith (stratigraphic)epthpassednto un-

T^BI 2. Analysesf Magnetite nd ron Ore fromE1LacoandKiruna n thisStudy nd he Literature in ppm)Ti V Cr AI Mg Mn Ni Co Zn Cu

El LacoThis studyAll samples 930 1,360


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KIRUNA-TYPE ORES: TEXTURESAND MAGNETITE CHEMISTRY 835

alteredmagnetite, irst vesicular, hen nonvesicular.The oxidized low op musthavebeena palcosurface,since the ore in the rest of the Kiirunavaara mountainridge was unoxidized.The occurrenceof vesicularore alongonly a part of the hangingwall indicatesconsiderablerosion,which s supported y the exis-tence of commonKiruna ore typesas angularandroundedragmentsn the mmediatelyverlying ol-canic ocks see llustrationsn Partk,1975a).It appears hat Geijer did not realize the nature ofthe vesicular ore, because he did not use it as evi-dence for his original opinion that Kiirunavaaraeruptedas ava Geijer, 1910). Later onhe foundve-sicularore at Nukutusvaara, ne of the Per Geijerores Fig. 3), andreported hat "A localphaseof themain ore body of this field [Nukutusvaara]s vesicu-lar, with...structure...nalogouso thatof anamygda-loid lava rock" (Geijer, 1919, p. 18). However, atthat time he had ust changedhis interpretationofthe Kirunaores, rom then on regarding hem as n-trusive sills),due to the localoccurrence f ore brec-cia at the hanging-wall ontacts.Emplacementof the ore melts at the surface n theKirunaarea s supported y a new interpretation fGeijer's (1910, 1967) "stratifiedore," i.e., the apa-tite-banded re with a fine ayeringof uniform hick-nessllustratedn Figure4C andD. We considert tobe a crystal uff, analogous ith the partly consoli-datedmagnetite-apatitealloutashat E1Laco (Fig.4A andB). In both areas he material s composed fmagnetiteand apatite of the samedimensions ndhabits,ncluding latymagnetite, rrangedn a simi-lar way. These features, aken together,cannotbegeneratedby later recrystallization r replacement.Accordingo Geijer (1967), the stratifiedore wasfound near the footwall in some sectors of the Kiirun-avaaraorebody, onsistent ith an early eruptionofgaschargedwith tiny crystals f magnetite nd apa-tite. The intraformationalolding and brecciationcouldhavebeencaused y the subsequentlyrupt-ing ore ava,whosemovements estroyedhe stratifi-cationexceptvery locally.This, in combinationwithlater shearing,probably explainswhy brokenuppiecesof apatite-poorore often are foundembeddedin apatite-rich,originallypyroclastic ?) deformedore in the lowermost art of the body.Whether theKiirunavaara eposit onsistsf a singleor more hanone ava low s understudy.The presence f columnarmagnetiten almostallthe investigated patite ron depositss remarkableconsideringhe rarity of this extural ormof magne-tite. It has o the best of our knowledge nly beendescribedromoneotherdepositn the world,Mag-netitaPedernalesFig. 2; Grez et al., 1991), which salso napatite ronore.The reasonwhy he columnarmagnetite n the iron belt usuallyoccurs n chaoticrather hanparallelarrays s notknown.Their coexis-

tence at E1 Laco and occasionallyn the iron beltshows hat they are varieties of the same generaltype.The mostconspicuousariety of columnarmagne-tite at Kiirunavaara s strikingly similar n arrange-ment and size to the magnetitecolumns ear flowtopsand n feeder dikesat E1Laco (Fig. 8C and A)and to artificiallygrowncolumnarmagnetite Fig.8B). This similarity s a strongargument or rapidcoolingof a melt at Kiirunavaara. he parallelarraysof columnarmagnetite esemblemorphologicallyhemonomineralicayersof "columnar-dendritic"eld-spar described y Petersen 1985) from a contactzoneof a lardalitic ing complex n the Osloregion,Norway. According o Petersen he layers formedunder conditionsof supercooling,an explanationwhichhasalsobeen givenby HenrquezandMartin(1978) for the crystallization f columnarmagnetiteat E1 Laco.

The magnetitedendriteswith branchlikeoffshootsat KirunaandE1Laco (Fig. 7A and C) resemble he"christmas-tree" form of dendritic chromite de-scribedby Greenbaum 1977) and Leblanc (1980)from the Troodosophiolitecomplex,Cyprus.Theyattributed he dendritic exture o rapid crystalliza-tion of a magmasupersaturated ith respect o chro-mium.Skeletal rystals f other minerals galena ndsphalerite)morphologicallyimilar o the branchingmagnetiteplates are illustratedby Barton (1991).The sulfides ormed at lower temperatures romfluids ather than from a melt, but an essential eaturein commonwith the platy magnetite s supersatura-tion during crystal growth. Existenceof dendriticmagnetiteplates,per se, doesnot prove crystalliza-tion from a melt, as shownby their occurrence nPahtohavare, a tuffRe-hosted stratiform Cu-Au de-posit n the Kirunaregion.Nystr6m ndHenrlquez 1989) suggestedhat heapatitesegregations ith shellsof columnarmagne-tite found in skeleton ore at Kiirunavaara resultedfrom ocaldifferentiationn a crystallizing re melt.The segregationsan be regardedas volatile-richpockets elative to the surrounding re, and theirshells epresent nterfaceswhere volatilesdiffusedinto the accumulating ockets.The growth ratemightbe relativelyhigh at the interface, eading ocrystallizationf columnarmagnetite; lowergrowthaway rom t produced laty dendrites. he transferof volatiless analogouso a ocaldegassingf the oremelt. Crystallizationrom a melt is supported y thetrachytoid exture; the distributionand deformationpattern of the dendriticpseudomorphsfter pyrox-ene n the ophiticore (Nystr6m,1985, fig. 7) consti-tute independent videnceof flow in a melt.Studies f basaltic ystems how hat dendriticpy-roxenesgrow at relatively high cooling ates (Lof-gren, 1980), indicating hat the dendriticpyroxene


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836 NYSTROMNDHENR[OUEZin the E1Laco ore is alsoa rapid growth eature. tscloseassociation ith columnarmagnetitehere (Fig.9A) and n the iron belt (Fig. 9C) supportshe ge-netic interpretationgiven above for the columnarmagnetite. he dendriticpyroxene t E1Lacocannotbe a metamorphicmineralbecause he ore is unaf-fectedby metamorphism--neither re the relict py-roxeneat Kiirunavaara nd the inferred pyroxene nthe iron belt metamorphicphases.The magnetitesurroundinghem shows o signof recrystallization,and the regionalmetamorphic rade n the two oreprovincesgreenschistnd prehnite-pumpellyitea-cies,respectively)s too low for pyroxene o form.Moreover, he Kiruna pyroxenediffers chemicallyfrom he moreFe rich pyroxenesn metamorphosediron-formationscf. Deer et al., 1978).Magnetitecan form in an exhalative-sedimentaryenvironment.t hasbeen reported rom the AtlantisII Deep in the Red Sea ift where t is an uncommonmineralassociated ith hematiteandpyroxene Zier-enbergandShanks, 983). This magnetite s a recrys-tallizationproductafter goethiteand hematiteanddoesnotshowdendriticdevelopment. he pyroxene,whichoccurs saggregatesf prismatic rystals r isintergrown with magnetite, ranges n compositionfrom ow A1aegirine-augiteo hedenbergite. patitehasnot been described. hus, the mineralogyhereand that in well-preserved patite ron oresbear noresemblance to each other.The upward gradation rom apatite ron ore intothe quartz-banded ematiticore described y Parilk(1975a, b) from someplaces n the Per Geijer de-positss suggestivef an exhalative-sedimentaryro-cess.However,all the examples f quartz-banded reseenby us n thesedeposits ear he imprintof defor-mationaccompanied y oxidationof magnetiteandintroduction of silica, and we consider them to beformedby eruptionof very volatile-richmeltsat thesurface. here is evenevidence or emplacement ypyroclasticlowat Nukutusvaara,neof the deposits(Fig. 3).Magnetitegeochemistry

Primarymagnetite ndsecondaryematite romE1Lacoplot similarly n Figure 11, indicating hat mar-titizationat low temperature roduced o significantchangesn the trace elementcomposition f the ironoxide.The chemistryof the magnetitealsoseems obe unmodified y recrystallization,ssuggestedy acomparison f ores with primary texturesand de-formedores romKiruna.The magnetites aveno vis-ible inclusionshat can nfluence heir composition,the only exception eingTi phases.Figure 10 illustrates he overallgeochemicale-semblance etween he investigatedmagnetitesromapatite ron ores.The similarity s notable f the com-parison s restricted o E1Laco andKiruna Figs.11

and 13; Table 2). It can alsobe seen n Table 2 thatthe dataobtainedn thisstudy or the Kirunamagne-tites are in good agreementwith the analyses fFrietsch (1970) and Parilk (1975a), and that our E1Laco data are similar to the values reported byWegner (1982) and Loberg and Horndahl (1983).However, the E1 Laco analysesgiven by Parilk(1975a) differ considerably ith regard o Cr (muchtoohigh),Ni, andCo (bothhigh).A possible xplana-tion for his strange esults s the useof differentana-lyticalmethods nd aboratoriesor the KirunaandE1Lacomaterial. n addition, he E1Laco analyses re-sentedby Frutosand Oyarzfin 1975) are somewhatlow in V andhigh n Cr.The widely varyingTi contentof the magnetitesfrombothE1LacoandKiruna Table 1; Fig. 13) em-

Tippm

4000

3OOO

2000

1000

i

Kiruno0 E[ too[] Romero[ A[gorrobo 0jos de Aguo/q Cerro Im(JnV E[ TofoX CarmenJr E[ DoradoV Apotiteeins

o []

L,)o [] j[] [] []- I I I I10 20 30 /-,.0 50 'o /NFIc. 13. Titaniumvs. V/Ni for magnetitesrom apatite ronores n the Kirunaarea,El Laco,and he Chilean ron belt (includ-ing two apatiteveins).


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KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 837

phasizeshe dangerof usingonly a few samplesnorder o characterize deposit hemically. he scat-tered high valuesof Ti in magnetitesrom E1 Lacoand the iron belt can be attributed to exsolved ilmen-itc. The relativelyhigh Ti in someKirunasamplessmore difficult o explain,becausehe mostTi rich ofthem (samplePG-K5) lacksvisible nclusions. utileand titanitc, both insoluble n HC1, cannot accountfor the Ti. Apatite ron oreswith Ti-rich magnetite(0.5-2 % TiO2) containing xsolvedlmeniteoccur nthe Bafq region, ran (F/firster t al., 1973), whichshowshat a low Ti content annot e usedasa diag-nostic eatureof thisore type.The similarityn magnetite eochemistryswell asin primaryore texturesdocumented ere for E1Lacoand Kiruna constitutesndependent vidence or asimilar origin. Parftk's 1975a) conclusionhat theKirunaandE1Lacooresare too different hemicallyto allow a geneticcomparison annotbe supportedby the datanow available.nsteadof posing prob-lem, the geochemistryf the ores avors close ela-tionship.The Romeralsamples ary more in compositionthan hoseromE1LacoandKiruna Table1), but hevariations small aking nto accounthat they com-prisesuch ontrastingypesasmassive uremagne-tite ore, ore breccias, ndpatches f ore in quartz-apatite-calcite-scapoliteock.The strongly eformedandrecrystallized ematiticore fromRomeralNorteis very similar hemicallyo the columnarmagnetitein the mainorebodyTable1), arguing gainst ook-strom's 1977) conclusionhat the two orebodies reof different rigins. atingby Munizaga t al. (1985)of the foliated "Paleozoic" host rock of RomeralNorte yieldeda normal ron belt age (110 _+3 Ma),supportinghe opinionof the geologistst Romeralthat he differenceetweenhe orebodiess merelydue to deformationHugoAguirre,pers.commun.,1983).We have been unable to find chemical data in theliterature or magnetiten oreswhichcanbe estab-lishedbeyond oubt o be exhalative-sedimentarynorigin.Frietsch 1970) analyzedmagnetite nd he-matitefrom a largenumberof mostlySwedishronores,manyof themconsideredo be metamorphosedchemical sediments related to basic volcanic rocks(the stratified ndnonapatitic res n Fig. 11). TheBandurrias re, analyzed y us, is interpretedas acontactmetamorphosedxha]ative-sedimentarye-positby Cisternas1986).The most mportantchemicaldifferencebetweenthenonapatiticres lassifiedssedimentarynd heapatite iron ores s the low V contents n the former(Fig. 11). A low V contentalsoseems o characterizeiron-formations.essler ndMilllet (1988) reportedthe followingmeanvalues or about170 iron oxideconcentrates from the banded iron-formation in

Minas Gerais, Brazil: Ti -- 114, V -- 138, Cr = 77, A1= 1,068, Mg = 40, Mn -- 170, Ni -- 167, Co = 120,andCu = 40 (all n ppm).The ranges f Davy (1983)plotted n Figure 11 refer to ores.Anotherapparentdifference s the higher Cr content n the sedimen-tary ores.Frietsch 1970) obtained n average f 90ppm for the stratified ron ores n northernSwedenwhich agreeswell with the iron-formation alueandis approximately 0 times higher than the apatiteiron ore data (Table 1).In volcanic ocks he iron oxide s usually itano-magnetitewhich is much richer in Cr, A1, and Mnthan he magnetite f our apatite ron ores commonrock valuesare 0.01-0.2 wt % Cr2Oa, 1-3% AlcOa,and0.5-1% MnO; Haggerty,1976; FrostandLinds-ley, 1991). The iron oxide n layeredmafic ntrusionsalsodiffersby high Ti, Cr, and V contents, nd ex-solved odiesof ilmeniteare typicalof the iron oxidein plutonic rocks. The differencesn compositiondemonstrate hat genetic conclusionsor iron oresbasedon chemical omparisonsf ore and ockmag-netite are meaningless.

ConclusionsThis studyof apatite ron ores from E1 Laco, theChilean iron belt, and the Kiruna ore field shows hatwell-preserveddepositshave several features incommon esides tabularshape, ssociation ith sub-aerial volcanic ocks, presenceof ore breccia, andother featuressupporting magmaticorigin. Themagnetites f the ores are similar o each other incompositionverypoor n Cr andrelatively ich n V)and morphology.Columnarmagnetite,which is arare rapid-growthormaccordingo the literature, sfound n almostall the investigated eposits.t maybe accompaniedy platymagnetite,ocallywith den-dritic branching, nd dendriticpyroxeneor pseudo-morphsafter it.The volcanic atureof the magnetiteava lows ndpyroclastic re on the flanksof the E1Laco volcanohasbeenestablishedeyonddoubt.Oreswith similartextureand structurealsoexistat Kiirunavaara, .g.,highly vesicularore and stratifiedmagnetite-apatitetuff, which show that this deposit is volcanic andformedby eruptionof lavaandair-fallmaterial.Min-erals ndicating ydrothermal ctivityoverprint heore n both provinces, nd n the iron belt, consistentwith a volcanic etting.Apatite iron ores with volcanic features are farfromunique,suggestinghatemplacementt or closeto the surfaces characteristic f this ore type. Theyhavebeenreported romE1Laeo,Cerro de Mercadoin Mexico (extrusive and pyroelastic ore; Lyons,1988), the Bafq egion n Iran (flowsandpyroelasticlayersof magnetite;F6rster and Knittel, 1979), andMagnetitaPedernalesn Chile (fragments f ore avawith flattenedvesiclesndicating low; Grez et al.,


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8 3 8 NSTflOMND HENflfOUEZ1991). To this ist now canbe added ava flowsandpyroelastic heets f magnetite ecentlydiscoveredin the ronbelt (V. Travisany, . Hendquez,andJ. O.Nystr6m,unpub.data)andat Kiirunavaara,he typelocalityof the apatite ron ores.

AcknowledgmentsWe thank U. Hlenius, B. Levi, and B. Lindqvist(Stockholm), nd two EconomicGeology eviewersfor constructive riticismof the manuscript. no Sam-uelson Stockholm) id the photographic ork andSolveig evalland nger Arnstr6m Stockholm) rewthe figures.We are grateful for economicsupportfrom the SwedishAgency or ResearchCo-operationwith DevelopingCountries SAREC,grant86/199),the Swedish Board for Industrial and Technical Devel-opment NUTEK, grant92-00224P), the FondoNa-cionalde DesarrolloCienfificoy Tecno16gicoFON-DECYT, grant89-0759), and the Departamento eInvestigacionesienfificas Tecno16gicas-Universi-

dad de Santiagode Chile (DICYT, grant 05-92-lSHB).September2, 1992;September9, 1993

REFERENCESBarton,P.B.,Jr., 1991, Ore textures: roblems ndopportunities:MineralogicalMagazine,v. 55, p. 303-315.Bookstrom, .A., 1977, The magnetite eposits f E1 Romeral,Chile: ECONOMIC EOLOGY,. 72, p. 1101-1130.Boric,R., Diaz, F., andMaksaev,V., 1990, Geologla yacimientosmetalfferos e la Rgionde Antofagasta:ervicioNationaldeGeologla Minerla [Chile]Boletin,no. 40, 246 p.Cisternas,M.E., 1986, Stratigraphische,azielle und lithogeo-chemische ntersuchungenn der Unterkreideder RegionAta-cama:Metallogenetische edeutung m Beispielder schichtge-bundenen isen-Lagerstiitteandurrias: eidelbergerGeowis-senschaftliehe bhandlungen, . 2,268 p.Cliff, R.A.,Rickard,D., andBlake,K., 1990, sotope ystematicsfthe Kirunamagnetiteores,Sweden:Pt. 1. Age of the ore: ECO-NOMICGEOLOGY, . 85, p. 1770-1776.Davy, R., 1983, A contributionon the chemicalcomposition fPrecambrian iron-formations, in Trendall, A.F., and Morris,R.C., eds., ron-formation: actsandproblems: msterdam, l-sevier, p. 325-343.Deer, W.A., Howie, R.A., and Zussman, ., 1978, Rock-formingminerals.Vol. 2A. 2nd edition.Single-chain ilicates: ondon,Longman, 668 p.Dobbs,M., andHenrlquez,F., 1988, Geologla,petrografia alter-aci6ndel yacimiento e hiefro Ojos de Agua, II Regi6n:Con-gresoGeo16gicoChileno, 5th, Santiago,1988, v. 3, p. G71-G81.Espinoza,S., 1984a, Le r61edu volcanisme u Crtac inf6rieurdans a mtallog6nsede la ceinture errifbre d'Atacama-Co-quimbo,Chili: Unpublished octoral hesis,Paris, 'UniversitPierre et Marie Curie, Paris, 153 p.--1984b, Dos perfilespor la franjamilonltica e Atacama-Co-quimbo un alcance obre a metalognesisel hierro:Comuni-caciones Chile], no. 34, p. 75-80.--1986, Sobreel origenvolc'anico-sedimentarioe los mantosferruginosos andurrias Manolete,al sur de Copiap6:RevistaGeo16gica e Chile, v. 27, p. 33-40..1990, The Atacama-Coquimbo erriferous belt, northernChile, in Fontboth, L., Amstutz, G.C., Cardozo, M., Cedillo, E.,

andFrutos,J., eds.,Stratabound re depositsn the Andes:Ber-lin, Springer-Verlag, . 353-364.F6rster,H., andKnittel, U., 1979, Petrographic bservationsn amagnetite eposit t Mishdovan, entral ran: ECONOMICEOL-OGY,v. 74, p. 1485-1489.F6rster, H., Bachtiar, ., and Borumandi,H., 1973, Petrograph-ische Detailuntersuchungenm Bereich der Eisenerzlager-stilttenvon Bafq/Zentraliran: eutschenGeologischen esell-schaftZeitschrift, . 124, p. 121-134.Frietsch, R., 1970, Trace elements n magnetite and hematitemainly from northern Sweden: Sveriges Geologiska Un-ders6kning, o. C646, 136 p.1978, On the magmatic riginof ironoresof the Kiruna ype:ECONOMIC EOLOGY, . 73, p. 478-485.--1984, On the magmatic riginof iron oresof the Kiruna ype--a reply: ECONOMIC EOLOGY,. 79, p. 1949-1951.Frost, B.R., and Lindsley,D.H., 1991, Occurrence f iron-tita-niumoxidesn igneousocks, n Lindsley,D.H., ed., Oxidemin-erals:Petrologic ndmagnetic ignificance:eviewsn Mineral-ogy, v. 25, p. 433-468.Frutos,J., and Oyarzfn, J., 1975, Tectonicand geochemical vi-denceconcerninghe genesis f El Laco magnetite ava flowdeposits,Chile: ECONOMIC EOLOGY,. 70, p. 988-990.Frutos, ., Oyarzfn,J., Shiga, ., andAlfaro,G., 1990,The E1Lacomagnetite ava flow deposits, orthernChile: An up-to-date e-view and new data, n Fontboth,L., Amstutz,G.C., Cardozo,M.,Cedillo,E., andFrutos,J., eds.,Stratabound re depositsn theAndes:Berlin, Springer-Verlag, . 681-690.Galatz'fin, ., andHenrlquez,F., 1979, Zonasde aheraci6n dya-centesa los cuerposde magnetitade Cerro Negro, area Cris-tales, III Regi6n: CongresoGeo16gicoChileno, 2nd, Atica,1979, v. 2, p. C265-C283.Gardeweg,M., and Ramirez, C.F., 1985, Hoja Rio Zapaleri, IIRegi6nde Antofagasta:ervicioNacional e Geologla Mineda[Chile],Carta Geo16gica e Chile, no. 66, 89 p.Geijer, P., 1910, Igneous ocksand iron ores of Kiirunavaara,Luossavaara nd Tuolluvaara:Stockholm,Scientificand Practi-cal Researchesn Lapland Arranged by Luossavaara-Kiiruna-vaaraAktiebolag,278 p.1919, Recentdevelopments t Kiruna: SverigesGeologiskaUndersiSkning,o. C288, 23 p.1931, The iron oresof the Kiruna ype: SverigesGeologiskaUnders6kning, o. C367, 39 p.1967, Internal eatures f the apatite-bearing agnetite res:Sverigeseol.o.iska nders6kning,o.C624,32 p.Geijer, P., and Odman,O.H., 1974, The emplacement f the Ki-tuna iron ores and related deposits: verigesGeologiska n-ders6kning, o. C700, 48 p.Gonzllez,S., andHenriquez,F., 1991, Geologla geoqumicaelas ocas el yaeimiento e hierroE1Algarrobo, egion e Ata-cama,Chile: Congreso eo16gico hileno,6th, Vifia del Mar,1991, v. 1, p. 403-405.Greenbaum,D., 1977, The chromitiferousocksof the Troodosophiolite complex, Cyprus: ECONOMIC EOLOGY, . 72, p.1175-1194.Grez, E., Aguilar, A., Hendquez, F., and NystriSm, .O., 1991,Magnetita edernales: newmagmaticrondepositn northernChile: ECONOMICGEOLOGY, . 86, p. 1346-1349.Haggerry, .E.,1970,The Lacomagnetiteava low,Chile:Carne-gie nstituteWashington earBook68, p. 329-330.--1976, Opaquemineral xidesn terrestrialgneousocks: e-views n Mineralogy,v. 3, p. Hg-101-Hg-300.Hegemann, ., andAlbrecht, ., 1954,Zur GeochemiexydischerEisenerze:Chemie der Erde, v. 17, p. 81-103.Henrlquez,F., andMartin, R.F., 1978, Crystal-growthextures nmagnetitelows nd eederdykes, 1Laco, hile:Canadian in-eralogist,v. 16, p. 581-589.Henrlquez,F., Dobbs,M., and Nystr6m,J.O., 1991, Caracteriza-ci6ngeoqulmicae magnetita, patita anfibola el yacimiento


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KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 839de hierroCarmen,RegiGn e Atacama: ongreso eolGgicohi-leno, 6th, Vifia del Mar, 1991, v. 1, p. 159-161.Hildebrand,R.S.,1986, Kiruna-type eposits:heir originand e-lationshipo intermediate ubvolcaniclutonsn the GreatBearmagmatic one,NorthwestCanada: CONOMICEOLOGY,. 81,p. 640-659.Kessler,W., and Miiller, G., 1988, Minor and trace-element dataof iron oxides rom iron-formationsf the Iron Quadrangle,MinasGerais,Brazil:Mineralogy nd Petrology, . 39, p. 245-250.Landergren, ., 1948, On the geochemistryf Swedishron oresand associatedocks:SverigesGeologiska ndersGkning,o.C496, 182 p.Leblanc,M., 1980, Chromite rowth,dissolutionnddeformationfrom a morphologicaliew point:SEM investigations: ineral-ium Deposita, v. 15, p. 201-'210.Levi, B., Aguirre,L., NystrGm, .O., Padilla,H., andVergara,M.,1989, Low-grade egionalmetamorphismn theMesozoic-Ceno-zoic volcanic equencesf the CentralAndes: ournalof Meta-morphicGeology, . 7, p. 487-495.Loberg,B.E.H.,andHorndahl,A.-K., 1983, Ferridegeochemistryof Swedish recambrianron ores:MineraliumDeposita, . 18,p. 487-504.

Lofgren,G., 1980, Experimental tudies n the dynamic rystalli-zationof silicatemelts, n Hargraves, .B., ed., Physics f mag-matic processes: rinceton, Princeton University Press,p. 487-551.Lundberg,B., andSmellie, .A.T., 1979, Painirova ndMertaineniron ores:Two deposits f the Kiruna ron ore type in northernSweden:ECONOMIC EOLOGY,. 74, p. 1131-1152.Lyons, .I., 1988, Volcanogenieron oxidedeposits, erro de Mer-eadoandvicinity,Durango,Mexico:ECONOMICEOLOGY,. 83,p. 1886-1906.Maksaev, ., Gardeweg,M., Ramirez,C.F., andZentilli,M., 1988,AplieaciGn el mGtodorazasde fisiGn fission-track) la data-ci0n de euerpos e magnetita e El Laeoe Incahuasi n el alti-planode la RegiGn e AntolZagasta:ongreso eolGgico hi-leno,5th, Santiago, 988, v. 1, p. B1-B23.MGnard, .-J.,1986, Un modulemtasomatiqueour esgisementsde la Ceinturede fer du Chili: AeadGmie esSciencesParis]Comptes endus esSGances,er. I, v. 302, p. 775-778.Montecinos, ., 1983, PGtrologie es roches ntrusives ssoeiGesau gisement e fer E1 Algarrobo Chili): Unpublished octoralthesis,Universit6 e Paris-Sud, 91 p.Munizaga,F., Huete, C., andHerv, F., 1985, Geoeronologfa -Ar y razonesnieiales rST/Sr6de a "FajaPacifica" e "desarro-11osidrotermales":ongreso eolGgico hileno, th, Antofa-gasta, 1985, v. 3, p. 4-357-4-379.NystrGm, .O., 1985, Apatite ron oresof the KirunaField, north-ern Sweden:Magmaticextures ndcarbonatiticffinity:Geolo-giskaFGreningensStockholm Grhandlingar,. 107, p. 133-141.NystrGm, .O., ar, Henrlquez,F., 1989, Dendriticmagnetite ndminiaturediapir-likeconcentrationsf apatite:Two magmaticfeatures f the Kiirunavaararonore:GeologiskaGreningensStockholm Grhandlingar,. 111, p. 53-64.OyarzGn,.,andFrutos, ., 1984,Tectonicndpetrologicalrame

of the Cretaceousron deposits f northChile:MiningGeology,v. 34, p. 21-31.Parfk, ., 1975a,The originof the Kiruna ronores:Sveriges eo-logiskaUndersGkning,o. C709, 209 p.1975b, Kiruna ron oresare not "intrusive-magmaticresofthe Kiruna type": ECONOMIC EOLOGY,. 70, p. 1242-1258.--1984, On the magmatic rigin of iron oresof the Kiruna ype--a discussion:CONOMIC EOLOGY,. 79, p. 1945-1949.--1985, Phosphorusn different ypesof ore,sulfidesn the rondeposits, nd the type and origin of oresat Kiruna:ECONOMICGEOLOGY, . 80, p. 646-665.Park, C.F., Jr., 1961, A magnetite flow" in northernChile:ECO-NOMICGEOLOGY,. 56, p. 431-436.--1972, The iron ore deposits f the Pacificbasin:ECONOMICGEOLOGY, . 67, p. 339--349.Petersen,J.S., 1985, Columnar-dendritic eldsparsn the lardaliteintrusion,Oslo Region,Norway: . Implications or unilateralsolidificationf a stagnantoundaryayer: ournal f Petrology,v. 26, p. 223-252.PichGn,R., 1981, Contribution l'tude de la ceinture du fer duChili. Lesgisements e BandurriasProv.d'Atacama) t LosCo-loradosNorte (Prov.de Huasco):Unpublished octoral hesis,Universit de Paris-Sud, 26 p.

Ruiz, C., andPeebles, ., 1988, Geologa, istribuciGn gGnesisde losyacimientos etal[feros hilenos: antiago, ditorialUni-versitaria, 334 p.Ruiz, C., Aguirre,L., Corvalan, ., Klohn,C., Klohn,E., andLevi,B., 1965, Geologia yacimientos etalfferos e Chile: nstitutode InvestigacioneseolGgicasChile],386 p.Ruiz, C., Ortiz, F., Moraga,A., andAguilar,A., 1968, Genesis ftheChileanronoredepositsf Mesozoic ge: nternational eo-logicalCongress, 3rd, Prague,1968, v. 7, p. 323-338.SERNAGEOMIN, 1982, Mapa geolGgico e Chile, 1:1.000.000:ServicioNationalde Geolog[a Minerfa Chile].Thomas,A., 1970, Beitrag ur TektonikNordchiles: eologischeRundschau, . 59, p. 1013-1027.Vivallo,W., Henrlquez,F., andEspinoza, ., 1991, AlteraciGn i-drotermalen el complejovole'nieo 1 Laco, Norte de Chile:CongresoGeolGgico hileno, 6th, Vifia del Mar, 1991, v. 1, p.44-47.Wegner,R., 1982, E1Laco/Chile:LavastrGmeusEisenerz: api-dus,v. 7, p. 15-18, 42.Weidner,J.R., 1982, Iron-oxidemagmasn the system e-C-O:CanadianMineralogist, . 20, p. 555-566.Welin, E., 1987, The depositional volutionof the Sveeofenniansupracrustalequencen FinlandandSweden: recambrian e-search, . 35, p. 95-113.Wright, S.F., 1986, On the magmatic riginof iron oresof theKiruna ype--an additionaldiscussion:CONOMIC EOLOGY,.81, p. 192-194.Zentilli, M., 1974, Geological volution nd metallogeneticela-tionships n the Andesof northern Chile between 26 and 29 South:Unpublishedoctoralhesis,Kingston, ntario,Queen'sUniversity, 446 p.Zierenberg,R.A., and Shanks,W.C., 1983, Mineralogy ndgeo-chemistry f epigenetic eatures n metalliferous ediment,At-lantis I Deep, Red Sea:ECONOMICEOIOGY,. 78, p. 57-72.

nystrom and henriquez-1994

Documents

laco n

flanks f

geochemistry f

diagnostic f

presence f columnarmagnetite

n fact

otherwellpreservedeposits

magmatic eatures f iron