Geology of quartz

RUNEB.LARSEN, MIREILLE POLVE &GUNNARJUVE NGU-BULL436,2000-PAGE 57

Granite pegmatite quartz from Evje-Iveland:

trace element chemistry and implications for the formation of high-purity quartz

RUNE B.LARSEN,MIREILLEPOLVE&GUNNARJUVE

Larsen, R.B.,Polve,M.,&Juve, G. 2000: Granit e pegmatite quartzfromEvje-Iveland:trace elementchemistryand implicat ions fortheformation of high-puri ty quartz.Norges geologiskeundersogelse Bullet in 436, 57-65.

Previous studies imply that granite pegm atit es and hyd roth ermalquartzveins are themostpromising igneous repositor ies ofhigh-purity quartz. Thisis because quartzfromhigh ertemp eratur e geolog icalsettin gs(granit es, monzonites,diorites,etc.)accommo dates higher concentrationsofimpurit iesin its atomicstr uct ure. Systematic studiesofgranitepegmatites fromEvje-Iveland, SouthNorway,showamarkedrelation shipbetwe en petrogenesis and thedistributi on of st ructu ralimpurities. Accordingly,lessfract ionated pegmatit es featu re relatively high concentrationsof Ti,Mg, Ca andCrwhereasmore fractionatedpegm atit es contain higher concent ratio nsofFe,Li and B,and thetot alconcentrati onof struct ural impuriti esrises wit hthedegree of diff erent iat ion.Therefore,detailed know ledgeofapegm atitefield, ifcombinedwit hquartzanalysesfroma fewcarefu lly selectedlocalit ies,may drasticallyreducethe area in whichprospect ing forhigh-purity quartz resour ces isfeasible.

RuneB.Larsen,GeologicalSurveyof Norway,N-7491Trondh eim,Nor way.

MireillePolve,UMR5563.UniversitePaulSabatier,38Ruedes36Ponts,31400Toulouse,France.

Gunnar Juve,GeologicalSurveyof Norway, OsloOffice,P.O.Box5348Majo rstuen,N-0304 Oslo,Norway.

Introduction

High-purityquartzis common quartzthat is characterisedby exceptionallylowconcentrations of elementsother than sili- con and oxyg en. Untreated,naturally occurring quartzwith lessthan SO ppm ofimpurit ies qualifies as high-purityquartz;

however,quartzwit h as much as 500 ppm totalimpuriti es may suff ice if industrially feasible dressing technique ssuc- ceed in lowering the impurity level to lessthan 50 ppm. Given thesespecifications,prices in excessof 1000 US$/t on may be obtained in a market which,according to a 1992 Roskill report 'T he Economicsof Quartz", is st ipulated to increaseby 5-8%perannum.The USGS Min eral sInformation (ref.: http:/ /minerals.usgs.gov/minerals/pubs/commodity/ ) also forecastssolid growth in the demand for high-purity quartz, notleastasaresultof continued expansionin the pro- duction of silicon oxidewafersfor semi-conduct or technol- ogy. High-purityquartzis largely used to manufacture silica glass, which is formed by melting processed crystalline quartz at temperatures between1750and 2000°C.The final product contains > 99.995%Si0₂ and,because of its out- standi ngchemi caland physicalprop erties,silica glassis the only single-compon ent glass that has wide commercial applicat ions(Fa nderlik 1991).Themost important propertie s ofsilica glass areresista nce toext reme fluctuations in tem- peratur e,chemicaldurabil ityinacidicenvi ronment sand its abilit y of transmitting light from near ultravioletto inf rared parts of thespectrum.Therefore,silica glasshasfoundwide applicati onsin themetallurgical,chemical and optical indus- tries,as wellas incommunicatio n technologyfor themanu- factureof opticalwave-guid es.Anexcit ingapplicati on is asa

raw material in thedevelopmentof high -performance solar panelsfor energyproduct ion.

Because the trace-element chemistry of quartz is only cursorily evaluated in most liter ature ,thefirst part of this communication summarisesthe appearance andcharacter of impuritiesin quartzand definestheprincipl efeat ures of high-purityquartz.This sect ionisfollowedbya case history from Evje-Ivelandin South Norway where the qualityof igne- ousquart zin a closely relatedsuite of granitepegmatiteshas been st ud ied. Thiscase historydoes not report onthedis- covery of new high-purity quartzoccurrences inNorway;in fact,givenour presentknowledgeit is unlikely that theEvje- Ivelandareacontainsany promisingtargets.Rathe r,werefer to this areaasprovidingan illustrationof theprincipl esand st rat egies that may be applied in future prospecting for industrially feasibl e quartz deposits in granit e pegmatite provinces.

Geology of quartz

The follow ing outline is primarily extracted from work by Dennen (1964, 1967),Dennen etal.(1970),Lehm ann &Bam- bauer (1973), Fanderlik (199 1), Jung (1992), Perny et al.

(1992),Deer et al.(199 7)and Wattet al.(1997).Othersources are cit ed in thetext.

In evaluat ing the quality of quartz, disting uishi ng betw een st ruct ural impurities,solidand liquid inclusions(Fig 1),is imperat ive. Solid and liquid inclusions areevaluated only briefly at the end of this secti on because,to a large extent, they are removed during the processingof quartz unlesstheyaresmall and/orvery abundant.Structuralimpu- rity elements,on thecontrary,can only bepartiall yremo ved

NGU-BULL 436,20 00-PAGE58 RUNE8.LARSEN,MIREILL EPOLVE& GUNNAR JUVE

atomic latti ceand these elements are classifiedasstruct ural impurit ies. Elem ent sthatare most commonly identifiedas structu ralimp urit iesencom pass AI,B,Ca, Cr,Cu, Fe,Ge,H,K, Li,Mg, Mn,Na, P, Pb,Rb,Ti and U. Notallstudiesagree wit h this list of elements.Jung(1992)suggested that onlyAI,B,Ge, Fe,H, K, Li,Na,P and Ti may be regarded as true structural impurities,whereasCa,Cr,Cu,Mg,Mn,Pb,Rb andUarethe results of contam ination by microscopic solid and liquid inclusion s whichwere notentirelyremoved before analysis.

Anot herreason for thisapparentcontroversy may be the fact that certain element s,e.g.the alkali metals,tend to form minute atomi c clusters adsorbed at specific growth direc- tions Le.the 0001 surface(Brouard et al. 1995). Forming adsorbed clusters,they hard ly classifyas conventionalstruc- tural impurities. However, being strictly confined to one growth direct ion, the incorporation of atomic clusters is depen den t on the physicsand hence on the atomic lat ti ce prop ert ies ofspecificcrystallographicorientations . As they arenot sostronglyarreste din the quart z structureas com- pared wit h con vention al st ruct ural impurities,atomic clus- tersmay bemore exposedto acidleach and,therefore,may bepart ially or fully removed duringprocessing of the quartz rawmaterial.

Sub stit ut ional impurities compete with Si⁴+in the Si-O tetrahedron compo sing the quartz lattice (Fig.1),whereas int ersti t ialim purities mostly includesmallmonovalent ions thatfitintost ructural channelsrunning parallel to the c-axis and function as charge compensators balancing substitu- tion al impurities(Fig 1). AI3+,for example,is a common sub- stitutionalimpurity whereasLi+or Na",in thest ruct ural chan- nels, is balancing the missing posit ive charge. Next to aluminium,Tl,Fe(1I 1) and Ge arecommon substitutionsforSi whereas H,l.i,Na and also K(e.g. Watt et al. 1997)include most chargecom pensators. However,His rare in magmatic quartz,whereas it dominates over the other common charge com pensators in quartz that formed from dilute aqueous solutions,e.g.inalpine-ty pequartzveins. This lattertype of quartz,occasion allyfeat uring lamellar growth structures, is alsoknow n for highlyasym met ric distr ibutionsof structural impuritiesgiving rise to sectoral or concentriczonation pat- ternsbroug ht about by selectiveaccumulation alongspecific gro wth orientati ons of certai n elements (e.g. Paquett e &

Reeder 1995). In singl ecrystalsof some lamellar quartz,for example, theconcentr ation ofAI may exper iencean abrupt increasefrom 20ppmin one growth sector to 620 ppmin a neighbouringgrowthsector ofthe same crystal.

Highly uneven distribut ions of im p urit ies arelim it ed to lamellar hydr otherm al veinquartz, whereasstructural impu- ritiesinigneou sand metamo rp hic quartz are more evenly distributedthroug houtindividual crystals.

It is challenging totry to forecast the specific geolog ical envi ron ment that most favours the genesis of high -purity quartz. Traditionally, high-puri ty quartzis recovered from quartz veinsand granit e pegmati te s.BrazilandMadagascar, inpart icular, were leadin gprodu cers of vein-typehigh-purity quartzwhereas,today, theSp ruce Pinepeg matitedist rict in Nort h Americais almostthe solitary world supplier. Recently, theprod uct ion ofhigh-purityquartz from Drag,North Nor- Structural impurities

Fluidinclusions

Structuralimpurities Typesof impurities

Fiq.l,This figure illustratesthe types ofimpuritiesthat are common in quartz,The left partof the figure illustratesdifferent types of impurities, Le, solid inclusions,fluidinclusionsand structural impuriti es, The right hand figureshows types ofst ructuralimpurities, Location s ofst ruct ural impuri ties (fig,to the right)are shownfora-q uartz which isthe most common type of quartzattheEart h's surface and isthe only type of quartz fo und in the Evje-Iveland area,Also to the right,theatomic con- fig uration s ofsilico nand oxygenare viewedin a sectionperpendicularto the crystallographic c-axis. al and a2denotes crystallographic axes.

Basement geology afterPedersen(1981).Falkum (1982)and Padget (1994).

by time-co nsuming and expensive dressing techniques.

Therefo re, itistheconcent rationofthese elementsthatulti- mately distinguisheshigh -puri ty quartz deposits frominfe- rior qualityoccurre nces.

The speciat io nand character ofimpuritiesinquartzare non-obvious featu res which complicate prospecting for high-pur ity quartzresources. For exam ple,quartz, wh ich at first glance ap pears clear and inclusion-free, may contain thousandsofppm ofstructural im purities. Onthecontrary, dark smoky quartzwit h manysolidandliquid inclusion s may provide an excellent raw mate rial for certain ap plicationsif the inclusio ns canbe removedthrough affordableindust rial dressing met hods. This is because the smoky col our of quartzis causedby low levelsofionisingradiati on induced bythedecay ofradio activ e elementsin neig hb ouring miner- als(e.g.4°Kinalkali feldspar),whereasthequartz itself may contain onlyafewppm of struct urallyboundim puritie sthat, however, areconfinedto colour cent resin theatomiclat tice, hence inducing the smoky colour. Other colour variations mayindeed be signsofabund ant structu ral impurities, such as inamet hyst,for example,that owesitscolour to structur- ally boundFe(e.g. Cohen &Hassan 1974,Cohen 1985,Aines

&Rossmann 1986,Ad ekeye &Coh en1986);and rosequartz col ou rswhich, accord ingto some studies,aredue to high concentra tionsof struct urallyboundFeand Ti(e.g.Hassan &

Cohen 1974,Cohen&Makar 1984, 1985),AI-Psubsti t uti ons (Maschmeyer&Lehmann 1983)and/or,accord ing torecent st udies,are causedby inclu sions of submicrosco picdurnor- tierit e[AI₇₍B0₃₎₍Si0₄₎03]fibre s(JuliaGoreva,unpublished).

Quartz hasan exceptionally st rong at omicconfig uration ofSi-O bondsthatallo wsonlya minimumof otherelements into itsstr uct ure. However, min ut e amo unts of substitu- tional and interstit ial im pu rit ies maybeincorpor atedintothe

RUNE8.LARSEN,MIRElLLE POLVE&GUNNARJUVE

way, recommencedafter a near 1a-year periodofinactivity and quartzproduction fromtheDraggranitepegmatite shas now been inope rati on for more than 3 years.

In most minerals, there is a crude posit ive correlati on bet w een temp erature and the concentration of st ruct ural im puri ties, and this relationship also applies to structural im purit ies in quartz.Accordingly,ifsilica-ov ersat urated igne- ous rocksand their derivati ves from granodi orit es throu gh granit es to pegm atites and to hydroth erm alveinsarecon sid- ered, thebestqualit ies aremostl y present in peg matit es and hydr oth erm al vein s. Indeed, experience has shown that high -temp eratur e igneo us rocks produce poor-q ualit y quartz in term s ofstructuralim pu rities,whereasmany peg m- at ites andsome hydroth ermalveindepo sits,in general,pro- ducebett erqualit ies. How ever, it is alsoclear that the con- cent ration of impurities in ig neous, hydrotherm al and pegm atitequart z variesover several ordersof magnit ude and, therefo re,otherparam etersthantempe rature also influ- ence the incorporation of trace elements into thequartz- crystal st ruct ure. These ot her parameters are rarely addressed in thelit eratur e;however,the incorpo rat ion of Fe, for example,partially depend s uponthe oxid ation stateand hencetheoxyg enfugacity of the quartz-forming environ- ment.Theacti vity of otherminerals mayalso inf luence the availabilit y ofsomeeleme nts that may potentially be incor- porated into thequartz-cryst alstructure. Finally,thetrace

NGU- BULL436,2000 -PAGE59

eleme nt distrib ut ion inigneo usquartzmay foll owthepetr o- genetic historyofthe quartz-forming melt,as isdocumented in thepresent study.

Composition of pegmatitic quartz at Evje-Iveland

Inan approach to evaluati ng the paramete rs that infl uence theincorp orati on ofim purit ies intoigneousquartz,granit e pegmatites were sampl ed fromtheEvje-Iveland , Glamsland and Froland areas in South Norw ay (Fig 2).Thesepart icular areas were chosenbecausedet ailed mineralogicalinvestiga- tion sthroug houtthe20^{t h}century have shown thisto be the most wellst udiedgranite pegmatiteprovin ceinNorwayand, arguably,inthe entire FennoscandianShield(e.g.,Bj erlykke 1935,1937,1939,Pedersen 1973,1975,1981 ,1988,Juve&

Berq st e t1990,Fought 1993,Stockmarr 1994,Hansen et al.

1996,Bingen &vanBreemen 1998).

Althou gh pegmat iteswere sampledin all th reeareas(Fig 2),the presentpreliminarystudywillconcent rate on Evje -Ive- land whereearlieracti vity haddocument ed acloselyrelated suite of granit es and pegm atites that prob ablyform edfrom alim it edparent al source during aprotracted igneo useven t intheUpperProt erozoic (Bjerlykke1935, 1937;Frig stad 1968, 1999;Fought 1993,Pedersenand Konnerup -Madsen, 1994, Stockmarr1994).

• Monzonite

Biotit e graniteand Coarse -gral ne granite

• Iliorite

• Fennefos

auge ngneiss

• Gjer stad suite Augengneiss

D

Agder base- ment complex

rno

^{Bambl ebase-}

ment complex

O

Pegm atit c

fields

- ^Elm

^{DJ]] Monzonite}Biotlte granite ^_~ Handed gneiss ~

Diorite&ano rt h.• Gahbro&am p h._ Pegmatites b.

Granit icgneiss Augengneiss

Sam ples

Fig.2.Geol og y andsam plesitesfor pegmati t icquartzadd ressedinthepresent st udy.The figure totheleftis a genera lgeolog icaloutlineof Southwest Norwayandshow s the mostimp ortant pegmatitefieldsinthe region.The right-handfigureis a detailed map ofthefram ed area to the left and shows pegmatites,sample localit iesandthe geologica lsetting of the studiedarea.Seelegend and maintext for moredetails.Geology afterPedersen(1981) and Pedersen and Konnerup-Mad sen (1994).

NGU-BULL436, 2000-PAGE60

Geological outline of the Evje-Iveland area

The granite pegmatites of the Evje-Iveland area are located in the southwestern part of the Mesoproterozoic Sveconorwe- gian/Grenvillian province of South Norway(Ped ersen 1981).

In contrast to many other partsof Norway,the Early Palaeo- zoic Caledonian orogenic event had a minimal effect on the basement lithologies;hence,the Sveconorwegian orogeny comprises the youngest pervasive deformation recorded in the region. The Evje-lvelandarea is dominated bya suite of graniticand mafic plutonic rocks(Fig 2)that formed over a period of 450-500 million years beginn ing withint rusion of coarse-grained,granitic augen gneisses yielding whole-rock Rb/Sr maximum ages of ca.1290 Ma(e.g. Pedersen & Kon- nerup-Madsen 1994). This suite of igneous rocks is also known as the Gjesdal Suite Augen Gneisses,forwhich U/Pb sphenedat ing provided a min imumage of 1166+66/ -2 1 Ma (Bingen & van Bremen 1998). Recent U/Pb zircon geo- chronologyof the prominent lveland-GautestadNorit e(Fig 2)gave an age of 1278+/-2 Ma and, accordingly,suggests that mafic igneous rocks alsocharacterisedtheinit ial mag- matic activity in the Evje-Ivelandarea(Ped ersen 1981,S.Ped- ersen,Pers.comm. 1997).Howeve r,large-scaleigneous crus- tal amalgamation did not begin before 1150- 1100 Ma(or even later) when acid plutonism gave riseto widesp read emplacement of granites(Fig 2). At 1034+/-2 Ma (Bingen andvan Bremen,1998)the large Fennefoss augen gneiss was generated and,at 985+/-1 Ma(Ped ersen 1981,S.Pedersen, perscomm.1998)was follo wedbyemplacem entof theEvj e diorite.

Thislong,discont inuous,plutonichistorycome to anend wit h the emplacement of the Hevrinqsvatnet granite -mon- zonite complexat 950-900 Ma(Fig 2) (Pedersen 1981),a body that may becompared to the inferred parent magma of the granite pegmatites addressed in the present st udy(Fought 1993,Pedersen & Konnerup-Madsen 1994,Stockmarr1994).

The Hevrinqsvatnet bimodal complex comprises ring- shaped massesof monzonite and granitethatareintersecte d by a cone-sheet system of later monzonites and granites (Ped ersen 1981, Pedersen& Konnerup-Madsen 1994). All granitic int rusions are characterised by relativelyhig h con- centration sof K,Ti,Ba,Sr,Zr,Pand REE,a smallor absent Eu anomaly and a fairlyjuvenile87Sr/86Sr ratio of 0.7040-0.7045 (Pedersen1988).

Granite pegmatites occur in a 5 km-widebelt extending southward sfrom the Hevrinqsvatnetcomplex(Fig 2). Along this belt, several thousand pegmatites are scattered overa distance of ca.25 kilometres. Recent studies ofthe peg rna- tites in the northern part of the area have shown that they were probably derived from the same source as the rnon - zonites in the Hovrinqsvatnet complex (Fought 1993,Stock- marr 1994). In agreement wit h thisorig in, the pegm atit es were considered to have formed from relatively primitive granitic melt sand inherentl y have a low 87Sr/86Sr ratio of 0.7063. As previously emphasised by Bjerlykke (1935,1947) and Barth(1947),most of thepegmatites wereemplacedint o the Iveland-Gautestad norite along subvertical dykesthat

RUNE8.LARSEN,MIRElLL EPOLvI:&GUNNARJUVE

pass into subhorizont al sillsand solidified as stronglyzoned chamb er pegmatites. Well-exposed bodies display both wall-,inte rm ediate-and core-zones andsomeexam ples may also develop arelati vely fine-grained contact- zone. In gen- eral,the geo met ry and distr ib ut ion ofthe zonesare symmet- rical with the wall-zoneenveloping the interme diate- and core-zon es.However,several cor e-zones may develop inside the inte rmediate- zonein differentpart s of thesamepegma- tite.Asit wasconcl udedin earlier works(Bj erlykke 1935,Frig- stad 1968, 1999, Fought 1993, St ockmarr 1994) and sup- portedbythe stric tzon alevolution,mostpeg mat it es formed from a singlebatchof ign eousmelt.Some pegmati tes,how- ever,displaymagmaticero sionfeatures and a repetitiveevo- lution of theintermediate zone,i.e.featuresthat may imply replenish ment of sili cate melt before the pegmatite was com p letelysoli dified.

Where present , the contact-zone is an assemblageof rel- ati vely fine-g rained,perth it ic K-feldspar,plagioclase,quartz and biot it e.The wall-zonecontainsdecimetresize crystalsof plagioclase,perth it ic feldspar,quartz,biotiteand whitemica andin placesshow s exam plesof graphic feldspar and quartz intergrowth. Spect acular examples of graphic granite are commonin the inter medi ate -zonewhere metre-size crystals of quartzand feldsparare intergrownwith somebiotite and muscovite int erspe rsed throughout the zone. Finally,the core-zone may be comp osed entirely of quartz but mostly contains afew rafts of plagioclaseandperthiticfeldsparfloat- ing in thequartz matrix. Biot ite isthe dominant micaand generally occurs asbladesradiating from a common nuclea- tio n point. Otherthan themajor mineralspreviously men- tioned,thepeg matit es contain a wealth of accessoryphases that aresummarisedinBj erlykke(1935,1937). the mostcom- mon being mag netite, spessartite garnet,monazite,beryl, xenot imeandgadolinit e.

Analytical methods

Samp les of quartz were collected from the margins of the int ermediatezone from 10pegmatitesin a profile extend ing over a distanceof 12 kilometres(Fig 2). They were crushed and sieved to the 150-250urn fraction,then subjected to magnetic separati onfollowedby acid leach and,finally,were hand-picked andinspect edfor mineralimpuriti es by binocu- lar micro scopy. Traceimpurities,whichwere measured after thesepreparation s,willprimarily representstructuralim p uri- ties, adsorb ed atomic clusters or elements associated with dislocati ons,or ot heratomic latticedefects or elementsdis- solved influid inclusions. Grainswere again cleaned with acetoneanddeionized waterin a clean room,thendissolved by ultr a-p ureconc ent ratedHFusingthe combined efficiency ofanult rasonicbat h and a microwaveoven. After evapora - tion to dryness,whichallows Sito escape as volatile SiF6'res- idue wasrecover edwith5m l of 2%distilledHN0_3. Solutions wereanalysedby ICP-M Sat Universite Paul Sabatier (France) on a Perkin-Elmer ELAN 5000,using In and Re asintern al standard s and SARM49forqualitycontrol. Thisstandard is onlycert ified for a fewofthe elementswe analysed,whereas most eleme ntsare listed as recommendedvalues; however,

RUNEB.LARSEN,MIREILLEPOL

ve

^&GUNNARJUVE

at the time of analysis,certified trace element standardsfor quartz were difficult to obtain. A suite of 33 elements com- prising

u .

Be, B,Mg,Ca,Sc,Ti,V,Cr,Fe,

eo.

Ni, Cu,Zn,Ga, Rb, Sr,Y,

zr.

^{Nb,Mo,Cs, Ba,La, Ce,Nd,Sm, Eu,}W,Au, Pb, Th and U were included in the analytical programme. Resultsofthe finalanalysisarelist ed in Table 1;however,elementsgiving insignificantconcentrationsareomitted.

Habit of quartz

Quartzis common througho utthe pegmatites and,inquan- tity ,isinferior only to K-feldspar. Most of the quartzis trans- parent althoug hin placesitmay be milky-whitedueto high abundances of microfractur esor, in rare cases, to highdensi- tiesof fluid inclusions. Pale grey smoky quartz is particularly common throu ghoutthe walland inte rmediate zonesand always occurs in 10-15 cm-wide rims enclosing feldspar.

Quartz thatoccurs atdistances greaterthan15cmfrom feld- spar ismilky-wh it e ortransparent. Accordingly,it isim p lied that the smoky colour ofquartzis a result ofy-radiat ion from decayi ng radi oactive eleme nts in feld spars,rath erthanbeing

NGU-BULL436, 2000-PAGE 61

a consequence of radiogenic elements from st ruct ural or solid inclusionsincorporate din thequartz.

Microscopic exami nationof quartzshows thatsolidinclu- sions other than rutile are rare, alt hough magnetit e maybe presentin some ofthe pegmatites.Rut ileis common in inter- mediate-zonequartz but rarein thecore-zone and occurs as minute micrometre to sub-micrometre thick needlesthatare even lydispersedthro ug houtthequartz.Primaryfluidinclu- sions arecommon and varyst ronglyin proportion and size but are subhedralandconsistentlyclassifyas HP-COz-NaCI type inclusions. The interm ediate-zon eis charact erisedby low-to medium-salinity (in NaCl equivalent s)HP-COz-NaCI inclusions with 10-15 vol% COz whereas the core-zone is characterisedby low-salinityHzO-COz-NaCIinclu sions wit h 5 -10 vol%COz.From themicrotherm om etricbehaviourof ice (Larsenet al. 1998a &b)itis impliedthatthe dominantelec- trolytesin the aq ueous phase wereCI- and Na", Secon dary incl usio ns mostly have the same composition as primary inclusionsin theinspected samples,but in places showevi- dence of liquidimmiscibility with phaseseparation ofacar- bonicandanaqueousphase,respectively.Aseparate manu-

Table1.Compositionof quartzfromthe intermediatezon eof thegranit epegm atites. Concent rat io nsin ppb.Presum ed str uct uralimp urities inbold lett ers.UTM-N and UTM-E:Locat ionofsam plesinmetr es and accord in gtotheUTMsystem.

RBL96004 RBL96006 RBL96008 RBL96010 RBL96012 RBL96014 RBL96016 RBL96017 RBL960 21 RBL96022 UTM-E

UTM-N

B Ba Be Ca Ce Cr Cs Cu Fe Ga La Li Mg Nb Pb

Rb Sr

n

U V W y

Zn

436400 6494700

225 60 56 4198

1591 10 71 3344 254

1053 942 3 59 106 130 26131

15 12 2 16

436300 6494700

50 14 141 12073 2 69 45 102 2461 263

774 1651 5 279 168 479 25197 2 7 4 7 35

437050 6496000

163 31

129 6379 4 2288 64 23 3978 222 2 644 331 9 58 359 257 25111

6 12 9

27

436500 6489950

570 300 223 4252 6 3276 39 18 5079 286 3 6652 374 4 44 285 148 261 70

17 11

11 43

433275 6488275

734 23 85 3200 2 391 391 15 3075 328

5062 963

8 114

437 133 23131 3 2 11 4 32

433500 6483975

2984 403 374 13375 10 1279 290 157 9339 243 5 852 1188 65 351 3779

618 12334 4

93 22 7 269

433500 6484250

369 64 154 4786

679 781

49 5982 409

o

4252 355 15 25 1129 137 3348 17

12 3 17 43

433600 6485500

2164 157 237 31230

22 567 263 81 6956 556 10 6471 1874 22 50 782 791 31906 6 51 19 21 83

430750 6496650

243 307 182 2798

312 126 26 2447 145

o

2702 1451 6 22 215 76 8772

3 11 3 33

433925 6495100

95 762 84 8074 10 1113 37 64 8840 314

5 811 901 61 282 1025 750 23557 27 15 12 11 77

NGU-BULL 436,2000-PAGE62 RUNEB.LARSEN, MIREILLEPOL

vt s

GUNNARJUVE

Fig.3.Concentrationsofstructuralim puri- tiesin quart zfromtheintermedi ate-zone ofgranitepegmatitesatEvje-Ivelandina profilerunning from sout hto north inthe studiedarea.

Rb,ppb

tunn

10000

1000 lOO

0

Nor th, metres

®

^{North, metres}

lOO ⁾⁽⁾

6480000 6485000 6490000 6495000 6500000 6480000 64850011 64911111111 649511(KI 65I1tK)(KI

1000 Ga,ppb ^)()OOO Ba,ppb

1000

lOO

10

©

^{Nort h,met res}

®

^North,metres

lOO I

6480000 6485000 6490000 6495000 65000011 648110011 64851100 64900011 64951100 6500000

Fe,ppb

IOO(Hl 100000

script, which addre sses the chem ist ry, composition and distributionof fluidinclusions,is in preparat ion.

Amongthe elementsanalysedby ICP-MS(Tab le 1),only B, Ca,Cr,Fe, Ga,

u.

^{Mg,Pb, Rb,Sr,Ti and Uare commonly}

accepted as true st ruct ural impurities(e.g.,Fanderlik 1991, Perny et al. 1992,Hemming et al. 1994,Watt et al.1997).

Indeed, these element s constitute more than 95% of the trace im purities (Table 1) detect ed in analysing the quartz samples.Because Ti is occasionally represented in minute ruti leneedles,the exactamount ofthis elemen tcan only be quantifiedin rutile-free specimens.

Indecreasingorder of concentration Ti,Ca,Fe andLi are the most abundant elementsand mostlyare present at the pp m level. Cr,Mg,Rb,B,Sr,Ga and Pbconsistentlyyieldhun- dred s ofppb and in a few casesexceed one ppm in concen- tration. Finally,the Uconcent rat ion is lower than 20ppb.

Among the other analysed elements thatnormally are not regarded as stru ctu ral impurities, only Ba, Cs and Beare presentinany ab unda ncealthoughthe concentrationsvary inconsistently fro mtens to hundreds of ppb. On the average, quartz shows thefollow ingdistribu t ion of structuralimpuri- ties (in ppm and in decreasing order of abundance),Ti(2l}, Ca(9.0}, Fe(5.2}, Li(2.9), Cr(1.2}, Mg(1.0), Rb(0.83}, B(0.76), Sr(0.35),Ga(0.30),Pb(0.13},U(0.007) (See alsoTable 1).

Onaregion alscale,thetotal concent rat ionofim p urities varies conside rablyfrom only 20 ppmin RBL96021 to 84 ppm in RBL960 17(Table1). These ab undancesare onlyadvisory as AI,K,and Na are not included andin some cases(in part ic- ularAI)consti t utealargepart ofthest ructura limpurities.

Certain elements show conspicuous regional trends in their concentrat ions throughout the studied area. Seen together,the concentration of many elements exper iencesa gradua lsout hwa rdincrease as exemplifi ed byFe,Rb, Ga and B(Fig 3). Not all structural impurities show such regular trendsbut theoverallpatternis thatquartzin the sout h con-

tainshigh erconcent rations of impurities.Fig.4,whichshows the regionaldistribution patt ern ofLi and Cr,illustrat esthis trend.

Discussion and conclusions

Studiesof quartz from granit e pegmatites at Evj e-Iveland, South Norw ay,have show nthatthequartz has a relat ively high purity,althoug hthe absenceof AI,Na and K analyses precludesany firm conclu sion s.Noneof the granitepeg ma- tites so farstu die din theEvj e-Ivelandprovidefeasib letargets for indust rial exploitation, and in part icular Ti, Fe and Li exceed theconcent rations preferred for high-purity quartz.

With this conclus ion in mind , it shouldbest ressedthat the presentstudyembraces onlyafraction of thepegmatites and the geochemical patte rns imply that bet ter qualit ies of quartz may befound in the northern part sof the stu died area.

Structuralimpuriti es that normally would beexpecte din quartz alsoconstituted most oftheimpuriti esfound inthe present study. However,Ba,Cs and Be alsoyielded signifi- cant concent rat ionsduringICP-MSanalysisanditisnotclear if these elements occur as sub-microscopic inclusions,are confinedtodislocati onsor ot herareas with significant latt ice defects,or iftheymaybedissolved in the aqueoussolutions forming the fluid inclusions. Feldspar wouldbe the prime suspect if Baand Cscamefrom sub-microscopicinclusions but thelack of correlationbetween Ba,Cs,Rb andSr rulesout thispossib ility. However,giventhehighlyirregular distribu - tion oftheseelementswhen com paring different pegma- rites.eith er of these optionsora combin ation of the three may explaintheorig inof the Ba,Csand Be concentration s.

Regarding petrogeneticpatt erns,a systematic distribu- tion of trace element sin thequartzimpli es that thereisa strong connection betweengranit e pegmatitegenesisand

NGU-BULL436,2000-PAGE63 RUNEB.LARSEN,MIREILLEPOLVE& GUNNARJUVE

Fig.4.Areal dist rib ut io nof struct ural impurities in quartz. In the two uppe r figures,sizesof triang lesare proport ional tothe concen- tration s ofLiandCs,respec- tively. In the two lowe r diagrams,sizesoftriang les are proport ional to the MgO/FeOandSr/Rb ratios, respecti vely.Thelegendto the geolog yisshown in Fig.

2.

the distribution of structural impuritiesin quartz. Petroge- netically significant elem ent rat ios demonstrate aconspicu - ouspatte rnwhenevaluatedon a regional scale. Accordingly, the MgO/FeOand the Sr/Rbratiosof quartz showconsiste nt nort hwardincreases(Fig 4) whereas thetotalconc ent ration of Nb+Y (Fig 5) is falling toward the north. In whole-ro ck analysis, and in the analysis ofspecific igneousmineralsin graniticrocks,theseratioswould indicatethe degreeof mag- maticdifferent iation , and,atEvje-Iveland ,thedistributio nof these elements implies that the pegmatites tow ards the sout h formed from progressively more differenti ated, i.e.

evolved, granitemelts. Although quartz hasnot previously been usedinthis context, thisconclu sion is supporte d by the analysis of K-feldspar from the intermedi ate zone of the pegmatite in which the Sr/Rb (Fig 5) and the CaO/

(K₂0 + Nap) rat ios of the K-feld spar show a consistent increasetowardsthe nort h(Larsen,in review).

In conclu sion,thepresent st udyshow s thatthe total con- centrat io n oftrace elementsin pegm atit icquartzcorrelates with the degree of fractionation of thepegmatite-forming

melts.Thisrelati on shipisin fine agreemen twit h thefact that anigneo us melt becom esprogressivelymore saturatedwith incompatibl e elementsasdifferent iati on proceeds, and the ratio between the compatible and the incom patible ele- ment sdecreases. From the present work,it may be con- cludedthat thechemistryof quartz reflects the chemicalevo- lution of a granit e melt asit different iatestoward s more evolvedcomposition s.It may seemcont radictorythatquartz that form ed from more diff erent iated pegm at it ic melts, hencesolid ified atalower temp erature , showshig herimpu- rity levelsthanquartzfrom pegmat iteswhichcrystallised at high er temp eratur esfrom mor e pristi ne pegm atitic melts.

Apparently, the observed chemist ry of quartz contradicts what isexpected from the crysta l chemistry of quart z that wasoutlined inthe first sectionof thisarticle. Accordingly, the concentration of impurit iesin quart z sho uld be propor- tion alto thecrystallisationtemperatu re, i.e.,quartz that crys- tallised from more differenti ated graniti c melt s should inclu de fewer st ructuralimpurities becausethey formed at lowertemp eratur es. A possibl e exp lanation forthis contra-

NGU-BULL436,2000-PAGE64 RUNE B. LARSEN,MIREILLE POL

ve

^&GUNNARJUVE

Fig.5. Lo werfigure -the concentration ofNb+Yin quartzasafunctio n of distance(in met res)towardsnort halon g a N-s profi lethroug hthe st ud- ied area. Upperfigure-theSr/Rbrat ioof pota ssicfeldsparfrom thein- term ediate-zoneof granitepegmatit es alo ng thesame profil e as in the low erfigure.

diction may be that crystallisat io n of granit e pegmatites occurs under conditions of strong disequilibrium, i.e., far below the liquidus tem peratu re (e.g. Jahns 1959, London 1992). Therefore,as with most ofthe othermajor mineral speciescomposing a granite pegmatite,thepegmatitic melt rapidlyreaches a levelofstrongoversat urat ion. The crystalli- sation of quartz then becomes afu nct ion of diffusion rates andthe availabi lity of nutrients for crystal grow t h, rather than strictlya function of temperature. Follow ing this argument, pegmatiticquart z inthe south,although form ing from more differentiated melts,could haveformed at nearly the same temp eratu res asquart z in thenorth ern area;andtheconcen- tration of structural impurities becomes a function of the tot alconcentratio n of impuritiesinthepeg mat it ic melts dur- ing quartz growth(see alsoLarsen et al. 1998a & b,1999).

Inadvocati ng for specificstra tegies infut ure prospecting for pegmatit ichigh-purity quartz in Norway,more data are requ iredbefo re we may reachanyfirmconclu sions.Det ailed studies of well zoned pegmatiteswhich weresampled under the auspi cesof thepresentproje ct(bythe third authorand F.

Fontan)may provide important informationon the chemical evolutio n of quartz from contactto corezone and may fur- ther elucidatehow the chemistry of quartz depends on the fracti onal crysta llisation of granite pegm at it es. A parallel development of in situanalysis of quartz by LA-HR-ICP-MS

UTl\I-Nor th

(LaserAblation High Resolution InductivelyCoupled Plasma MassSpestrometry) at NGUwill also be conduciveto effi- ciently characterising the spatial distribution of structural impuritiesin quartz,andwill int roducea rapid method that may partially repla ce con ventional andvery tim e-consumin g quartz analysis techn iqu es.How ever.for themoment. itmay be concluded that the regi onal sampling of a pegmatite province has indeed prov ided valuab le information abo ut the overall distr ibution of impu rit iesin quartz and, more im port ant ly. demonstrates the relationsh ip between the purity of quartz and the histo ry of pegmat ite genesis. In doing so,det ailed know ledge about thegenetichisto ry of a particularpegm at it eprovinc e.comb ined wit h the analysisof a few carefully selected quartz samples, may drastically reduce thesize oftheareawhere detailedprospectingis fea- sible.

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Acknowledgements

We are inde bte d to NorthCapeMinerals,Lillesand. for generous financial supportof previous and ongo ing research in the project High-purity Quartz in Norwa y(projectnum ber:2728.00).In addition,the authors pay specialthan ksto Svend Pedersen,Jens Konnerup-Madsen, Ole F.Friqs- tad andAnd reasKorneliussenwho kindly assistedin introducing the au- thorstothe geology of Evj e-Iveland and its pegmatite fields,and to F.

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6495()()() 6500000 UTM· orth,metres

11

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6498000 6496000

6490000 6494000 6492000

6482000 6488000

6484000 (,486000 0.35 Sr/Rb 0.30

0.25 0.20 0.15 0.10 0.05 0.00

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