Utilisation of sillimanite minerals, their geology, and potential occurrences in Norway - an overview

NGU-BULL436,2000-PAGE113

Utilisation of sillimanite minerals, their geology, and potential occurrences in Norway - an overview

PETER M.IHLEN

Ihlen,P.M.2000:Utilisationof sillimanite minerals,their geology, and potentialoccurren cesin Norway-an overview.

Norg esgeolog iskeundersekelseBulletin436,113-128.

The aluminium-silicate polymorphs sillimanite,andalusiteand kyanitedecomp oseto amixture ofmullite andsilica glassduring calcinat ion.Mulliteis anessentialcomponentofhigh-aluminarefractoriesformingtheinnerlining of furnacesand high-t emperatur e vesselswidelyusedinthe production of metals,ceramics,glassand cement.

Sillimani te,andalusite and kyanite constit uted 2%,74% and 24%, respectively, of the total western world's productionofsillimanite mineralsin 1998of a total ofabout 401 000 tonnes,ofwhichabo ut 95%wasconsumedby the refractory industry.The sillimanite mineralsare mined mainlyin South Africa(62%;andalusite),USA (23%;

kyanite)and France(11%;andalusite),whereasthe major consumers are found in the major iron-and steel- producingareas of the world. Produ cti onhasdeclinedoverthelastdecade,duemainlyto cut backs inironandsteel prod uction.Most economicdeposits ofsillimanite minerals canbeclassified as metamo rp hoge nicand include contact- metamor phicandalusitedeposits, st ratabound podiformdepositsof massive(corund um)-sillimanit e and (corund um)-kyanite rocksand st rat iform depo sits of kyanite quartzites.Theformer occurwit hin metapelitic sequencesinthe contact aureolesof plutonic massifs,whereasthe two latter in mostcasesare interpreted as represent ing metamorphosedhigh-aluminasediments, derivedfromhydrothermalalte ration centresinsubaerial felsic tointermediate volcanites.Allof these deposit typeshave also beenrecognised in Norway.Themost promising occurrences are theandalusite schistswit hin the contactaureolesof theFongen-Hyllin genand 0yungenGabb ro Complexesin theCaledonidesofCentra l Norway.

Peter M.lh/en,Geological Surveyof Nor way,749 1Trondheim,Norw ay;(Peter.lhlen@ng u.no)

Introduction

The term'sillimanite minerals'hasbecomefirml y ent renched in theindustria l mineralstrade in theAnglo-Saxon part of the worldasrepresenti ng allthree natu ralpolymorp hs.Thisis mainly becausetherefractory raw-mater ialim po rte d from Indiainthe earlyyearsof the refracto ryindust rywassilliman- ite.Inasimilarway,kyanit emineralshave beco methe com- mon name for the group in the USAwhereasandalusite appearsto be oftenusedin a similar groupsense inRussia and the former Soviet republics (Varley 1965). Sillimanite minerals com p risingsillimanite,andalusite and kyanit e are nat urally occur ring anhyd rous aluminium-silicate pol y- morph s wit hthechemical form ula AI_2SiOs. They are all com- mon rock-formi ng minerals differing somewhatincrystallo- grap hic characteris t ics and thereby in physical prop erti es.

Theind ust rialuseofsilli manit e mineralsis especially related totheiruniquechemicalcom posit io n, stabilityat hightem- peratures andtransformationtomullite-ric hagg regat es uti- lised as refractorymater iaI.

The aim ofthe presentpaper istogive a review of the uti- lisation, trade, consum ption, product ion and raw material specificationsofsillimanite minerals which areessentialto explorat io ngeologistsinassessingtheeconomic viability of depositsof theseminerals.Secondly,to describethe geology of some represent ative exam ples of economic deposits together with models fortheir formatio nwhichwill beused as a basis for evaluat ing possib le explorati on targets in Norway.

Sillimanite minerals,AI_2SiOs

Alumino-silicate polymorphs with a st oichio me t ric composition of 62.93%AI₂0₃and 37.07%Si0_2.The crystal lattices are composedof chain sof AI-O octahe- dra alon g the z-axis linked sidewaysby chain sofalter- nating Si-O and AI-O tetr ahedra (sillimanite, orthorhomb ic), Si-O tetrahedra and distorted AI-O oct ahedra(andalusit e,ort horhom bic) orof Si-Otetra- hedra and the remainingSi,AIand

°

triclinic).The latti cesgive room foronlylimite d substi- tution byFe, Mn,Ti ander;mainlyin andalusiteand kyanite . The bladed, needle-shaped and long pris- maticcrystals show variablecoloursin mainlyshades of grey,pink, yellowandblue.The sillimanit e minerals show hardnes sand densityin theranges5.5-7.5and 3.13-3.65, respectiv ely, wit h andalusite having the lowest andkyanite the high est density.Chiastolit eisa variety of anda lusite which has cross-sections wit h darkcarbonaceou sinclusion sdistribut ed inacruel- form patt ern, whereas fibro/ite refers to the fine- grainedfibrousvariety ofsillimanit e(DeeretaI 1966).

NGU-BULL 436,2000-PAGE 114 PETERM. IHLEN

Mullitisation

The sillimani t e mineral sdecompose irreversi bly to mullite when burnt at high temperat ures(calcina- tio n),accordingto thereaction :

3Al_2SiOs ^~ (sillimanit e)

AI_{6Si 2013} + Si0₂ (mull it e) (t ridymit e)

types of auxiliarypouring andhandling equipment.The sec- ond mostim po rta nt ap pli cati on is as furnace lin ings inthe non-ferrou smetallu rgical andglass indust riesaswellasin ceramicand cem ent kilns.Otheruses,minor in volumeterms, include foundry-mould facings, comb ustion chamber lin- ings,burnerbod ies, pyrometertubesand weldi ngrod coat- ing s(Roskill1990).

The end product of thisreaction is an aggregatecom- prising int erlocking, acicul arcrystalsof mull ite(-88%) in a matrix of silica or tridymiteglass(- 12%), which remains stable until thetemperature is raised above 1800°C(Varley 1965,Potter1991).Complete mullitisa- tionof kyanite and andalusite occursnormallyin the temperature range 1350-1380°C and 1380-1400°C respect ively, whereas sillimanite breaks down at highertemperatures, i.e.around1550°C.Ouringcalci- nat ion,kyanite increases in volume by 16-18%,silli- manite by 6-7%and andalusiteby 4-5% (Anon. 1988, McMichael 1990).As a consequence,the mixturesof mullite and silicaprod uced have lower densitiesthan those of the originalminerals.

Utilisation

Refractory use

About 95%of the silliman iteminerals produced are used as rawmat erialfortherefract ory indust ry inthemanufacturing of non-basic,high-a luminarefractories (Roskill1990, O'Dris- coil & Harries-Rees 1993).The refractory characteristics of thesemin erals isrelat edtotheir abili ty to form the refractory mulli tephasewhichcom bi neshig h streng t hwit h resist ance to physical and chemical corrosion at hig h temperatures.

Mullite istherefore anessential component of refractories forming the innerlining of furnaces and high -t emp eratu re container s wide ly used in the manufact uring of metals,glass, ceramics and cement.

The sillima nite minerals, raworcalcinated,are especially usedin themanufacturing of refractory bricks and shapesas well as of mortars or cement s,ramming and gunning mix- tures and casta b les(refractory concretes)used in the con- struction of joint -freeandself-sup porting monolithicrefrac- torylining s.Sill imaniteand andalus it e,which only expand by 4-7volume%duringmullitisation,havetheadvan tagethat they canbeused inthei rrawst ate.Sincecalcination req uir es energy,theuseof thesemineralsrepresents subst antial cost savings in respect to kyanite which expan ds considerabl y du ring heatin g.Calcin ati on istherefore a prerequisite for kyan ite inanumberofrefract ory applicat ion s.Howev er,nat- ural kyan it eis preferentially used for monolithicsand as a com ponent(10-40%)of refractoryclaymixtures(bricks, mor- tars,castables,etc.)in order to counteract shrin kage of the clay binderduring firing(Bennet&Castle1983).

Themajor end use of sillimani teminerals isin the ironand steel industrie swhich consume60%or moreofthe mullite refract ories(Oickson 1996).They areusedin crit icalareas of furnaces,steel degassingchambers,soaking pits andmany

Refractories

Refractories are materialsthat remain physicallyand chemically stable at hig h temperatures under ext reme conditionsof heatand corrosio n(O'Driscoll

& Harries-Rees 1993).Thosederived from sill iman ite mineralsbelo ngtothegroup of hig h-alum ina refrac- tories, Le.refractories exceedi ng47.5%AI₂₀₃IO'Dris- coli &Harries-Rees 1993).They are usedin acid rat her than basic environm ents (non-basic refractory) and have high degreesof resistanceto creepdefor mation, thermalshockandslag attack. Other importantprop- ertie sare highthermalconduct ivity, low coeffici ents of expansion, low porosity and high resistance to red ucin g atmospheres,fJuxingby volatile alkaliesand spalling(McMichael 1990).

Non-refractory use

Subo rdinat eamountsof theproducedsillimanite minerals (c.5%)havenon-refractoryuses(Roskill 1990).Raw andcalci- natedmin eralsareutilised in themanufacture of high-ten - sion insulators and ot her electr ical ceramics, ceramic tile body com ponents, sanitary ware, ceramic honeycombs, blown aluminiu m-silicate high-temperature insulation, brakelining s,glass melt addit ives,spinnable mullite fibres, grind ing media andextrusio n dies.Sillimanit emineralsare alsolocally usedto produce Si-AI alloys,metallicfibres and selected alum in ium oxid es(Wu1990).High -purityandfine - grained « 200 mesh) kyanite is preferentially used by ceramicmanufacturersofwall tile and sanitaryporcelainto offset shri nkage and cracking after firing (Bennet & Castle 1983).

Consumption and production

Theconsum pt ion of sillimanite mineralsisconcentra te din the relativelyhighly industrialis edareaswhere refractories aremanufacturedandwhichin turn aretypically close tothe majoriron andsteel producing regionsin the worl d (Roskill 1990).The principal consume rs in the western world are thereforefound inthe EU,North America,the FarEast and South Africa,whereasmining oftheseminerals iscontrolled byfive major producers sit uate din SouthAfrica(andalusite), USA(kyanite)andFrance(andalusite)(Fig. 1).The majorityof sillimanitemineralsprod ucedinother countriessuch asAus- tralia,Brazil,China,Ind ia,Ukraine and Zimbabw eis mostly for domesticconsumpt ion.Asa consequence,the trade in silli- maniteminera lsis internat ion al.

PETER M/HLEN NGU-BULL 436,2000 -PAGE115

60

Present Former

producer producer Resource

Sillimanitedeposit £. • /:;

Andalusite deposit I> • 0

Kyanitedeposit iiJ • 0

Fig.1.Map showi ngthedistribut io nof high -grade aluminium-silicate depositsin the world,incl uding past andpresentprod ucersand possibleresourc- es. Com piled mainlyfrom Roskill(1987,1990)andVarley(1965). Count riesindark blue andgreenhave anannual producti on ofst eelexceeding8Mt.

Supe rge nedepositsareshow n by dividedsymbo ls.

Norwegian consumption

The annual im port of sill im anite minerals, mainly andalusit e, increased from 2650 tonnes in 1994 to 3530tonnesin 1996, whereas thatof a varietyof cal- cinatedproducts (mullit e) reached 67 tonnesin 1996.

The major part of the sillimanitemin erals is used by BorgestadFabrikkerA/S, sit uat ed near Skieninsout h- ern Norway,in the manufacturingof refractorybricks and castable swith AI₂0₃content sof 35-95%.Sinc e the ste elindust ry is small in Norway,major domestic market s for refracto ry bricks and monolithics are foundin theferro- alloy indust ry, primaryalumin ium plants andalsoin theporcelain indust ry(Harries-Rees 1993).

The refract ory industry is presently plagued by over- capacit y duetothe general downturn in iron andsteelpro- duct io n and to techn ol ogi cal impro vem ent s leading to increasedlon gevit y ofthe refractor ies and declin ein refrac- tory consum pt io n pertonn e ofstee l produ ced (Harries-Rees 1993,O'Driscoll 1999).Thus,thereisintensecompetiti onfor markets atvario uslevels of thetradeinsilli ma nite minerals and mullite ref ractories,i.e.betweenhigh -alum in a refracto- ries andot her non-basic ref racto ries,betwee n differenttyp es of high-aluminarefract ori es and betw eenmemb ersoft hesil- limanit eminerals.How ever, over the last decade develo p- mentsin the iro nandsteel prod ucti onhaveresultedinever increasing ly severe furnace condi tio ns. This has been in

favour of the sillimanit emineralsbyamoveaw ayfromfire- clay materials to those containing higheralumina contents.

Thehigh-aluminarefractorymarketis very price sensit ive and thechoiceof refractory isusually basedon cost-effec- tiveness and avai labilit y rat her than unique physical and chemical properties.Thus, whereavai lable,localrawmateri- alsand products are favoured over those imported.There- fore, anda lusite-based refractori es are preferentiallyused in the refractory industry of South Africa and France and kyanite-based refr actoriesin the USA.Thisproh ibitstheuse of altern ative and technically superior materials, except where high temperatures and high loads makethem irre- placeable (McMichaeI 1990).

Salesofsillimanit e minerals are customa rily arrang edin buyer-seller bargain ing and are based on mineral produ ct spec if icat ionssuch aschemicalcomposition (alumi na con- tent s,alkalies,etc.),physicalcharact eristics (g rainsize, den- sity,etc.)and perform ancein refractory tests andsim ulate d servi ce tests.The st rong competition with other refrac tory raw materialshasa majorimpacton keepingtheprices ata relati vely sta ble level. Guidelin esfor recomm end ed prices are quoted monthly inseveral publications, e.g. in Industral Minerals.Presently,thepricesper metri ctonnemineral con- centrat efall wit hin therange $135-230(March 2000).They increase in sympat hy with aluminacontent and grain size andare higher for anda lusite concentratesthanfor those of kyanite atsimilaralum ina levels.

Over the lastthreedecades,the weste rnworld'sproduc - tioncontinu edto increaseand reachedan all-timemaximum in 1989 atabo ut 500 000 tonnes, subsequent ly decliningto an annual produ ct ion of 360 000-370 000 tonn esin 1993-

NGU-BULL436,2000-PAGE116 PETERM. IHLEN

600,0 0 0 - t -- .L.-- -.L...---I.- -.l.----L_-11-

WESTERN WORLD'S PRODUCTION IN 1998

Fig.2.Diag ram showing the annualwesternworld'sproductionof sill i- manite,andalus iteandkyaniteaswell asthe totaltonnage overthe pe- riod 1973-1998.Produ ct io n numbers compiled from Roskill(1987,1990), Pott er(1991, 1994, 1998) and estimation sgivenbyDickson(1996).

200 0 19 90

Year

Table2.The western world's pro- ductionof sillimanite minerals in 1998.Basedon data from Potter (1998).

1980

600 100 800

50 7500 6700 45000

90000 250000

40075 0 7600 295000 98150 Metrictonnes

19 70 200,000

AUSTRALIA Silliman ite Kyanite BRAZIL Kyanite FRANCE Andalusite INDIA Silliman ite Kyanite

SOUTH AFRICA Andalusite USA Kyanite ZIMBABWE Kyanite

Totalproductio n Total,sillimanite Total,andalu site Total,kyanite

400,000

<Il l1>

c:

c:o

I-

Mineral product specificationsfor refractory use

One of the most im port ant properties of high -alu- mina refractory materials is their creep resistance which is relate d to the amount of mullite and the presenceof fluxingimpuriti es,espec iallycontai nedin the gang ue minerals ofthemineral concentra tes. The chem ical composition of com mercial refractory- graderawmaterials as showninTab le 1,is character- ised by 54-70% AI20₃ and low contents of f1uxing oxides, i.e. generally <1% Fe203' <2% Ti02, <0.5%

Na20 +K20 and <0.5% MgO+CaO (Roskill 1990).Alu- minacontents abovethe maximumcontent of 62.9%

AI203for sillimaniteminerals are related to the pres- ence of otherhigh-aluminaminerals such as corun- dum and diaspore. However,the presenceof these minerals isnotnecessarilyobjectionable;rather, they may enhance the refract ory properti es of the raw material (Potter 1985).

Anoth erim portant property is the grain size of the mineral product. Refractory bricks should contain about 60%of coarse grain-sizematerial mixed with finer grain sizes and different types of binders in order to give the bricks therequired densityand strength under load (McMichael 1990). How ever, the lower costs of produ ction and installation of monolithic refractories,whicharemadetotally from fine-grained fract ions ofsillima nite minerals, have resulted in a trend away fromtheuse of bricks to theapplicationof monolithic linings wherever possible (Harries-Rees 1993). The coarse-sized grades, previo usly highly sough t afte r,are thereforebecom ing gradually less important.

1995 befo reclimb ing to thepresentlevelofabout401 000 tonnes(Fig.2).Sill im anite,andalusiteandkyanite constit ute 2%,74% and 24%,respectively,of thetot al tonnage pro- duced in1998.South Africa accounted for62%of thistotal whereasUSAand France provided 23% and 11%, respec- tively(Table2).The rema ini ng4%isproduced mainlyinIndia wit haddi tionalminor cont ributio nsfrom Brazil,Austra liaand Zimb abw e.Mining of sillimanitehas declin ed steadily from c.41 000tonn es in 1976to 7600tonnes in1998.India isthe

Name ofdeposit Swart koppies Glomel Havercroft Halsjii berg Country SouthAfrica France SouthAfrica Sweden Sillimanite mineral Sill ima nile And alusil e Anda lusite Kyanile Qualitv Hiah-orade KerphaliteKA Grade 1 Macle High-grade

SiO, 37.50-38.60 38.20 35,80

A~03 71.00-72.00 59.00-59.50 61,40 59,80

TiO, 3,10 0.20-0 .28 0.11 0,64

Fe,03 1,30 0.90-1.10 0,61 (Fe,tot.)1.30

MgO+CaO 0.24-0.45 (CaO) 0.05 0,40

Na,O+K,O 0.23-0.45 (K,O) 0.12 0,03

Grain size 25-75mm 0.3-1.6mm >4mm 60-325mesh References Roskiil (1987) Roskill (1987) Human and Anon.(1986)

Collins(1986)

main prod ucer, whereasAust ralia provide sonly about one hund red tonnes.And alusit e product ion has gro wn stron gly, passing kyanite asthe most important polymorp h in 1980.

Today,most ofthe andalusit e isprod uced from six depo sits in SouthAfrica(250 000 tonnes)andonein France(45000 tonnes).Thewest ernworld'sproductionofkyaniteischarac- terisedby one leading producerin USAandseveral smaller.

Production has declined by 33% since 1990, to c.98 000 tonnesin1998.92%of thisis mined byKyanit e MiningCoop- eratio nin Virgin ia,USA.

Table1.Chem ical composition(weight%)and grain-size rangefor some selectedcommercialprod uct s ofsillimanit e min erals.

PETERM.IHLEN

Data acquisition

Dat a regardi ng annual produ ct ion,minin g capacity and consumption are not readily availab le for the public andare in many insta nceswit hhe ld by bot hthe individualminingcompa niesand the governme ntsof theproducingcountries.The latter isusual ly the case for Chi naand the formercommu nistcountries (East- ern Bloc)andeven for many countriesin the western world.It refl ects the crucialimpo rtanceof sillimani te minerals in the produc t io n of metals and especially iron and stee l and thereby the count ry's ability to maintain productioninperiods wit hinte rnat io nalcri- ses andmilit ary conflic ts,i.e. the sillima nite minerals are ofst rateg icimporta nce.Secondly,thecompanies wantto avoid disclosingcompetitiveadvantages.The productio nnumbersgiveninTable 2 andFig.2 for the westernworld aretherefore partlyestimates.

Natural occurrences of sillimanite minerals Silli manite min erals are commonly encou ntered in peralum inou s met amorp hic, pluton ic and volcanic rocksand their weathering prod ucts.Their stability field s aredepictedin Fig. 3.They are frequ entl yret ro- graded to muscovit e, sericite,pyrophy llite,parago- nit e and/ormargarit e by inte raction with hydrother- malflui ds.The sillimanite minerals have a relat ively hig h resistance to weat hering leadingto enric hment in residual soils and local accumula t io ns in alluvial andbeach-sanddeposits. The most typ icalhard-rock occurencesofsill imanite minerals comp rise:

Kyanite,sillimanit e andanda lusite in met apelit es and quartz it es of regiona lmetamorphic terranes.

Andalusite in contac t-meta morphic pelit ic rocks adjacent tointrusion s.

Andalusit ein the deeper partsofaluni te-kaolini te (acid-sulp hate) altera t io n centres related to pal- aeo-geotherma lsystemsin subaerialvolcanic ter- ranes.

Table 3.Classificati on schemefor depositsofsillima nite minerals.

NGU-BULL436, 2000-PAGE117

Types of economic deposits

Econ omi cdeposit s ofsillima nitemineralscan be classifiedin anumber of ways.Most of them fall wit hin three genet ic groups(Table 3):met amor ph ogenic,epit hermaland super- gene.Thefirst isthe econo m ically most impo rta ntinrelation to deposits worked solely for their conte nts of sillimanit e minerals.Thelatt ertwo typesprovide in mostcasessilli man - ite minerals as by-produ ct s or special blende d produ cts which make onlyasubordinate contri but io ntotheproduc- tion sta tist ics.

Mineddeposits,aswellas potentia llyeconomic deposits (seeFig.1), are largely represent ed by podifor m and strati- formsillmanite andkyanitedeposits,toget herwit hcontact- metamo rp hic andalusite deposits.The podifo rm type con- sistsnormallyofmassivehigh-grade ores, whereasthe strat- ifo rmtypecomprisesquartzi te horizons with20-50% ofsilli- manit e minerals. These two types of deposits may occasio nally occur togeth er in the same area orevenform transition al types.Althou gh the schist-type mineralisation globa lly conta ins vast resou rces of silliman ite min erals, explo itation has,inmostcases, founderedonhigh recovery costsor poor quality of the concentra tes due to the com- monl yintimate inte rg row t h between gangueand silliman ite minerals.It is present lyofminorecono mic impo rta nceand will notbediscussedfurth er.

Economic sillimanite deposits

Podiform sillimanite dep osits which are hoste d by mica schistsandgneissescomprisemainlyirregularpodsand len- ticul ar masses of sillima nite- rich rocks with dimensions exceeding some hund red metresin lengthandseveraltens of metr esinwidt h,i.e.ore reservesrarely above 1Mt.High - gradeores contai n usuallymorethan 80% sillimanite,which is inte rgrow nwit hvariab leamo untsofeit hercorund um or quartz.Common subo rdi nateconstituentsincluderuti le,Fe- Ti-oxid es, topaz, tourm alin e anddumo rtierite.They areusu- ally min ed as ope n pit operat io nsand intropicalareas,by excavating and blasting large boulders (up to 100tonn es) occurrin gburied in residu al soils(Iate rites)andas floatand land slid ematerial. Massivesillimani te ores areofte nsoldas coarse aggregatesinpea- tolump-size grades which necessi- tatemainly crushingandclassifyi ngof the ores.

GEN ETI C TYPE GENET IC GEO LOGICA L EX PLO ITE D EXAM PLES OF ASSUME DAGE

OF DEPOSITS SUB-C LASS TERRAN ES MINERALS DEPOSITS OF FORM ATION

Metamo rphogenic Regi on al-metam. Metamorphi c Sillim anit c Aggeneysdistr.,SouthAfr ica Palaeoproterozoic Region al-metam. Metamorph ic Kyan ite WillisMt.,Virgini a, USA PaJaeo zoic Co ntac t-meta m. Plutoni c Andalusite Havercroft,South Africa Palaeoproterozoic

Glomel,Franc e Palaeo zoi c Epithe rmal Alunite-kaolinite Volc ani c Cor undum, andalusite Semiz-Bugu district, Palaeozoic

and/orpyrophyllite Kazakh stan

Supergene Beach sands Alltypes Sillimanite Kerala,India Tertiary-Quaternary

Kyanite andsillimanite TrailRidge,Florida,USA "

Alluvial All type s Kyanite El Pino, NWSpa in "

Andalusite Transvaal,South Africa "

Eluviallresid ua l All typesin Sillima nite KhasiHills,Assam ,India "

tro p ical/sub- Kyanit e Singhbhum ,Bihar,India "

tropicalareas

NGU-BULL436,2000-PAGE 118

Temperature(0C)

PETER M. IHLEN

Fig.3.Pressure/crust al depth and temperaturedia- gram showi ngtheposition of diff erent metamorphic facies (Yardley 1989)and thestabilityfieldsfor anda- lusite,kyaniteandsillimanitewith the invariantpoint adapt ed fromHolda way(1971).Thereacti on curve for pyrophylli te(pyr.)=AI-silicate+3quartz+wate r(Ker- rick 1968)represents the lowerstabilitylim itforand a- lusit e and kyanit e inthe system AI₂0₃-SiOrH20.The lower stabili tyforbiot ite(bio- In)and almandi negarnet (g nt- in) in peliticrocksis alsoshown fo r com parison.

The wetgranite solid us curveistaken from Luth etal.

(1964).PTconditions forthe formation of theBush- manland sillimanitedeposits(B), Transvaal andalusite deposits(T)and the Appalachiankyanit edeposits(A) areaccordi ng tomineral assem blages given byJou- bert(1986),Miyano(1988)and Espenshade & Potter (1960),respect ively.

100 200

2

Conact _ metamorphic

domain 12

Regional _ metamorphic

domain 14

=:::J

assemblages

900

10

"0

»

"0 20 ~

3 '

a

a.CD

"0 30 ~

~

E.

40

50 16 ..L-_ _.l..-_ _.I...:l._ _' - - _....!Wl....-...!...----l...L.._---'-_ _--'-_ _....:...---I

The Bushmanland sillimanite deposits, South Africa

Thedep o sits intheAggeneysdistrict of weste rn So uth Africawere formerly amaj or producer of refr actory-grad e sill i- mani te.Pro d uctio nhas decreased st ron g ly over the last two decades due to exhausti on of high-g rad e reserves.They occurwithin the Namaqua Mobile Belt(Fig . 4)whichrepresentsa thrust-fold beltdeve lo p eddu ri ng the lateMesoprot- erozoic,as a consequence of accretion(Kib aran orogeny)along the southwestern marginof the Kaapvaal Crat o n(Tho - masetal. 1994,Praekelt et al. 1997).InBushm an land,the mobile belt comprises int ensely deformed Palaeoprotero zo ic volcan o-sedimentaryseque nces(2000-1650 Ma) int ersect ed by different suitesof mainlygranit icpluton ic rocks,and metam orphosedin thelate Mesoproterozoic (seeFig .3 for PTconditions) (Jo u bert1986,Praekelt et al.1997).Sill im anite dep osits are hostedbysill ima n it e-b iot itean dquart z-muscovite schist units occu rri nginterlaye redwit h both mafic and acidmetavo lcani tesofislan darc-backarcaffini ties (Haib Sub-Groupin Fig.4;Colliston &Schoch 1996)and wit hdomi- nantlymeta -areniticrocks(Agg eney s Sub-Group;Praekelt & Schoch 1997)assumedto rep resentshall ow-wat er tofluvial deposits in a retroarcforeland-basin (Praekelt et al. 1997).The lastset t ingis themostim p or t antin regar d to eco no mic sillimanite deposits,but also to stratiform exhalative base-metal deposits wit h associated bedded-b aryte (Gamsb erg, BlackMoun tain,etc.).The sillimanitedepositsoccu rinthe lowerpart of theWo rtel Formationat the baseof theAgg ene ys Su b- Groupand just above a basaltransg ressive arenitesequencerestingon fine-gra inedgranit icgneisses(Praekelt et al.

1997).Theyareinterp reted as acid metavolcan it es(Sch reyer198 7). The deposit s occurassociatedwit hlenses ofam p hi - bolite s, calc-silicaterocks and lo callydolomiticmarbl es as wellas wit h differenttypes of peraluminousrocks enrich ed in corun du m, topaz,fluor it e,tou rm alin e, gahni te, dum o rti erite,rutile, apatite,AI-phosphates,anthophyll ite,cordierit e, muscovi teand/orsillimanite(Sch reyer 1987, Willner etal. 1990).

The deposits occur associated with several kilometre-long train s of bodies forming consp icuo us clusters of (quart z) sillimaniteores in theHo st on area(Frick& Coet zee1974)and corundum-sillimaniteoresinth e Achab and Swa rt ko p p ies areas(Will ne retal.1990).lnad di tio n,top az-rich silli ma ni terocksmay occur,as in the Achabdeposit(deJager& vo n Back- strom 1961).Theindividualbodi esvary in sizefro mafewdecim et res wide and some metres long tonearly 100 mwid e and250 m long .Thelarge stbod ieshad ,prio rto mining,est ima tedreserves in therange 200-250000tonnes(d e Jager&

von Backstr o rn 1961;de Jager1963).The Swartko ppiesdeposit, wh ichwas recentlywor ked (Fig. 5),consists ofcorun- dum -d o m inat ed ores containing 60-80%sillimaniteand 40-20%corundum. Theyare lo cally enr iched inmagnetiteand rutile and maygrad e into orbe intersected by vein-likemasses composed nearly ent ire lyofsill ima nit e.Theores have a massive appe aranceand show coloursin shades ofpale blueand grey(d e Jager & vo n Backstro rn 1961).Theirgrainsize is highly variableandfreq ue ntl y fine-graine dfib roli teis foundto replace coarse-gra inedneedles andsheaves of sill ima n- ite(Schreyer 1987).

PETERM.IHLEN NGU-BULL435,

zoeo -

o

_ Intrusions andorthogneisses,undiff.

Fig.4.Geologicalmaps sh ow ing the setting of the Bushma nland silli- manite deposits.a)Locati on of the Namaqua Mobile Belt; sim plif ied from Collisto net al.(1991)and b) geological map of the Aggen eys Terraneand its borderzonewith the Pofadder Terr an e in Bushman land showing the distribution of major lit h ost rat ig raphical units, thrust faults, base-metal ores and high- grade sillima ni te ores. Simplified segme nt of Fig.2inCollistonetal.

(1989)with thelo cat ionof ind ivi du- alsillima nite dep o sits accord ingto de Jager (1963), Frick & Coetzee (1974). Will ner et al. (1990) an d Praekelt&Schoch (1997).The de- positshown in Fig. Sismarked bya circle.

Witputs Fm.:

Bandedquartzites

WortelFm.:Biotite-sillimaniteschists with sillimanitedeposits

AggeneysSub-Group:

_

KoerisFm:Amphibolite meta-arkoses andconglomerates

-

HotsonFm:1)BIF and massive sulphides, 2)Meta-arkoses,quartzites

T'hammaberg Fm:Quartz-muscovite- sillirnaniteschistsandquartzites SkelmspoortFm.:Quartz-muscovite schists

HaibSub-Group:

_

HomFm.:Quartzofeldspathic gneisses

-'----'- Basal thrustof thePofadderterrane .A-J:.. ThrustswithintheAggeneysterrane

9 Base-metalmine,adits

C2'J

enrichedinFe-Ti-oxides/covered

, 30 m

Graniticorthogneissesandpegmatites, exposed/covered

Pit margins

@-25 m

....

-...

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Fig.S.Si mplified geo logi calmapandsect io noftheSwart koppiescorun d um-sil lima nite depos it, redrawnfromdeJager & von Backstr6m(196 1).

NGU-BULL436,2000-PAGE120

Economic andalusite deposits

Andalusite isprodu ced mainly from contact-metamorphic peliti c shalesandschistsoccurring inthethermalaureole of both granitic and gabbro icplutons.Exploit able andalusite occurs as coarse-graineddisseminatedporphyroblastscon- stituti ng5-20%ofthehost rock.The Glome ldeposit in Brit- tany, France,is developed in Ordovicianschistscontaining about 15%andalusitecrystals (O'Driscoll1999). It occurs in associationwithHercynian granitemassifsalongtheArmori- can chain. Comparable granite-related deposits are also found in China (Lu 1998) and South Korea(Roskill 1990).

However,the most important type are thecontactdeposits along themargin ofthe Bushveld Igneous ComplexinSouth Africawherehig h-gradereservesand inferred resourcesare inthe order of 50 Mtand 200 Mt.respectively (Oosterhuis 1998).Theandalusiteoresin both France andSout hAfrica compriseweathered schistsand horn felses which facili tate bot h open-pit minin g and subsequent dressing, which mainly include different types of heavy-media and high- intensity magnetic separat ion. Ore reserve calculations embracemainlythe uppe rweathered section rather than the andalusite-ho rnfels protore whichsuffe rs from higher bene- ficiatio ncosts,smallergrain sizeand lower recovery.

PETERM. IHLEN

Economic kyanite deposits

Medium-to high-gra deores of kyaniteare more widespread than comp arable sillima nite deposits(seeFig. 1).Theycanbe subdivi ded intopod iform andstra t iform types (Table3). The first type, represent ing high-grade (t opaz-corundum)- kyanite ores, resembles stro ngly the podiform sillimanite deposit s andhas become well known from the mining of the LapsaBurudepositsin the Sing hbhumbelt ofthe Biharand West BengaIstat esof India(Banerji 1981).However,the most commonlyencounte red typ eis stratiformkyanitequartz ite s whichform laterally exte nsive horizons or systemsoflenses.

The quartzit es are composed of 20-50% kyanite, 80-50%

quartzandsubo rdi nateamountsof muscovite,rutile,pyrite, topaz,dumort ierite and tourmalin e.The deposits are also characterisedby the presenceof various typ esof Al-phos- phates,mostcommonlylazulit e.Stratiformkyanite deposits wit h ore reserves ofte nexceedi ngseveralmilliontonnes are normally worked as open-pi tmines.The ores are crushedand milled priorto flot ation of kyanite and high -inten sity mag- neticseparat ion.

The Transvaal andalusite deposits,South Africa

Thedeposit s areallsit uatedwit hin thethermal aureole ofthePalaeoprot erozoicBushveldIgneousComp lex (BIC)which intrudestheTransvaal Supergro up(2600-2100 Ma; St rauss&Beukes 1996).Minesare presentlyoperatingin the Lyden- burg,Groot Maricoand Thabazimb idistricts(Fig. 6a).Andalusite is developedinweakly deformedmeta-pelit ic rocksof thePret oria Grouprepresenti ng theupperm ost successionof the Supergroup.Thethermaleffect of themagmas canbe tracedup to 55kmawayfromthe exposedintrusivecontactsand caused the zonaldevelopment ofmetamorphicmin- eral assemblagesin thepeliti c rocks, including an outerandalusitezone.Thiszone fluctuatesin parallelism with thecon- tactofthemaficintrusion s of theBIC(Fig.6a) and cuts across the sedimentarylayering asin theLydenburgdistrict(Fig.

6b) (Hammerbeck1986).Inspite ofconsiderablest rike length s of the andalusitezones,onlycertainpelit icunit s wit h opt imu m chemicalcomposition and metamo rphi sm (seeFig.3for PTconditions)have developed the rightcombinat ion of highconcentra tionof coarse-grainedandalusitecrysta ls,lowconte ntsofcoarse-grained deleteriousgangueminerals (garnetand staurol it e) andappro priateamo untsofsericitisatio nand softening of the rocks byweatheringnecessary for their expl oitati on.These conditio nsaremainly encount eredinthepelitic members of the TimeballHillForm ation which carrypot ent ialandalusit e hornfelsesover st rike lengths andwidths exceeding 15km and 20 m,respect ively(Hammer- beck1986;Oosterhui s1998).The mined hornf elses containnormally 8-13%ofcoarse-grained andalusit e wit hlengths and diam eters in therange 10-50 mm and 1-4mm, respect ively.Stauroliteand garnet present in theoresmust have grain sizes in thelow est size-range of the andalusitecrystals,where bythey can be removed byscreening (Oosterhuis 1998).

The andalusite depositsin theLyd enburgdistri ctincludebothhard-rock andsupergene deposits,thelast onerepresent - ing 1-2m-th ick alluvialaccumulatio nswit h limit ed exte nt andcontaining 50- 80%andalusite. Weath eredandal usite hornfelsisthe domi nantoretype which ismin edatKrugerspost,Annesley,Havercroft and Hoogeno eg,each producing annually about50000 tonne sofandalusite(Fig.6b) (Oosterhuis1998).The Hovercroft mine,whichwas reopen edin 1998 (O'Driscoll 1999), is sit uate d in theupperpart of theTim eballHillFor mation.Thedeposit comprises ac.50 m-thick and morethan5 km-lon gzone ofbanded , carbonaceousandalusitehorn felses dippingc.15°SW(Human&Collins 1986).The orezone contains onlysubordinatebarren layers andstron glyretro graded unit sofsericite-rich hornfelsesregarded as waste material.Thelower5 mof the orezoneisterm edtheGiant CrystalZonedue tothe presenceof megacrysticchi- astolite wit h diameter andlengt h ofaround 15 mm and 100 mm, respect ively. Thiszone grades intonorm al hornfe lses cont ainingcoarse-grainedidioblastsof chiastolite(10 mm long).Most of the chiastoliteisenvelopedbya thin coati ng ofsericitealterat ion.This coat ing permitsthe crystals tobe separate dcleanly fromthe hornfelsesduring crushingand min eral separat ionwhen thethinsericite layerisremovedbyattrit ion.The end productwas a very coarse-grained and high -grade andalusit e concentratemarket ed asGrade1 MacleandMacle60(seeTable 2).

PETERM.IHLEN ^{NGU-BULL436,2000- PAGE 121}

Fig. 6. a) Simplified geologi cal map showingthedistribution of majormin- ingdistrictsand andalusite minesin rela- tionto theBushveldIgneousComplex, thePretoriaGroupand theouter bound- aryofthe andalusitezone(red line).b) Mapshowing the distribution ofanda- lusitemines and metamorph ic zonesin relati ontotheTimeballHill Form ationin the Lydenburg distr ict,eastern Trans- vaal.Mapscompiledfro mAnhaeusser &

Viljoen(1986),Hamm erbeck(1986)and Oosterhu is(1998).

B

Boshoek andhigher Formations

o

, - - , Rooihoogte Formation L--J and ChuniespoortGp.

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active/i nactive

REPUBLICOF SOUTH AFRICA

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Kyanite quartzites in the Appalachian belt, eastern USA

TheAp palachi andep osit s of kyan ite quart zit es cluster in tw o mainareasat the westernmarginof theCaro lina Terrane (Fig.7),resp ectively intheCharlotte Belt of Virg inia and in theKings Mou ntainBel t ofNort hCaroli naandSouthCarolina.

Some additiona lexamplesare alsofoundwithinthe Carolina SlateBelt to theeast.The CarolinaTerrane comprises dif- fe rent segmentsof a mature Neoproterozoic-Cambrian islandarc system(Seco r et al. 1989).The Charlotte Belt,which constitutes a volcanic-plutonic complex,is thought to represent deeplyeroded axial parts of thefo rm er arc system, whereas the su pracrusta lsof the Carolinaand East Slate Beltsconstituteoverlying intra-arcandoff-axis volcano-sedi- ment ary seque nces(Go lds m it hetal. 1989,Fei ss etal. 1993).Th e Kings Mo untain Beltiscom parabl etopart s of theCar o- lin aSlat e Belt,but atsomewhat hig hermetamorp hic grade .The timing of themediu m -grade met am or ph ism responsi- ble for the development of kyanite ores(seeFig.3for PT conditions)is poorly constrained and can be attributedto either Taconic or Acadian tectonothermal events(Drake et al. 1989, Osberg et al. 1989).The Carolina Terrane also carries differ- ent typesof volcanogenicand exhalativedeposits of base-meta lsulphides,Fe-oxides,baryt e and gold (Feiss & Slack 1989).Occurre ncesofeconomica llyimporta ntgold- py riteandanda lusite-pyrop hylli tedeposits with associatedpropyl- itic, sil icic,argill icandadva nce darg illicalte ratio n(hig h-alum inaalte rat io ncentres)areconspic uous in the Carolinaand East SlateBeltsin conjunctio nwit hsub-marineto subaerialandinterme diatetofel sic volcanism (Schmid t1985, Feiss et al. 1993).

The kyanite quartzites, according to descriptions given byEspe nshad e & Potter(1960), Conley& Marr(1980) and Horton (1989), fo rm layers and lens-shaped bodies in intermediate to felsic metavolca nites and epiclastic to volcanoclastic met ased im entary rocks. The lay er srang e in thickn ess froma few met res tomo rethan 100mandform lat er allypersist en t st rat ig rap hi cal unit s whic hcan be followedconti nuously for severalkilo me tresalo ngst rike.Theindividu alho rizon s have becometect o nicall y modifiedby tight to isoclinalfoldingand shearing.Tow ard stheir marginsthey become richer in muscovite and are commonly transformed into kyanite-muscovite-quartzschists.They may overlie or grade laterally into quartz conglomerateswit h a matrixcomposed of quartz,mica and kyanite.The quartzites contain kyaniteas rather uniformal lydistrib utedcrystalsandlenticular aggregateswith grain sizes normally in the range 3-30mm. Almost rnon- omi nera licsegregationsofvery coarse-grained andrando mlyoriente dkyan iteoccu ras scat te redirreg u larpods,asthin enve lo pesoncross-cutting quar tzveins and as ruler-s hapedbodiesalo ng dilat at io nal str uct u res, lo call ytrun cati ngthe gneissic fabric of the host-rockquartzite.Thekyanite contents of the quartzites are normally in the range 10-40%.In addition, theycontainusually less than 5%of rutile,muscovite,pyrite, magnetite, diaspore and topaz.Accessory miner- als include various aluminoussilicat esand phosphates aswell as spinel,tourmaline,corundum,barite, spha leriteand fuchsite. All of thesemineralsarealsonormallyfound invarying proportionsinthehigh-alumina alterationzonesinthe Caroli naand East Slate Belts.

NGU-BULL436,2000-PAGE122 PETERM.IHLEN

The kyanite depositsatWillisMountainandEastRidge,Virginia, whichare present lymined ,constitutetwo lens-shaped bod ies containing several tens of millions of tonnes of open-pitores withover25%kyanite. Accordingto Conley & Marr (1980). they occurin the core of two tightsynforms(Fig.8)andformedoriginallya single strat umunconformablyover- lyingmafic tofelsic metavolca nitesof theHat cherComplexwhichare possibl yequivalent to the CambrianChopa wamsic Format ion (Pa vlidesetal.1982).The kyanit equartzit es restonathin basalsequenceofquartz-mica schists with lenticula r layers of micaceous quartzite and micaceous conglomerate.Thekyanite ore bodies are composed of multi plewedge- shaped layers,each consisting of basal coarse-grainedquartz-richunitsgrading upwards intokyanite-richfine-grained quartzoserocks.Layers with relict cross-bedding and channelfillcan locallybe identi fied.Elsew hereinthearea,this st ratig raphi clevel carriesbedded ort hoq uartziteswhichareoverlainby a thicksequence of graphiticmica schists,cor- relatedwit h thefossiliferousArvoniaFormat ion of Late Ordovicianage(Drake etal. 1989).

Deposit models

Theformat ion ofandalu sitedepo sit s represents a textboo k exampleof cont act meta mor phismof highly peraluminous pelit ic rocksin relation to emplacement andcrystallisationof gabbroic togranit icmagm as.

Theforma tionofthe sillim aniteand kyani tedepositsis, in contr ast, more poorlyconstra ined and possibly comp rises several different processes.

Metamorphism of high-alumina sediments derived from altered subaerial volcanites was advocated by Schreyer (1987)and Willneret al.(1990)to explainthe stratabound and strat ifor m nature of the deposits together with their common affiliation with metapeli tic units overlying sequencestypified by felsic toint ermediate volcanites.Based onthemetamorphi cmineralsofthecrudeores,thehigh-alu- mina sedi mentswere int erpretedtobe composedmainlyof variable amounts of quartz,diaspore, kaolinite and other alteration minerals.They were deposit ed in shallow-water basins adjacentto subaerialvolcanicfields bothcoevallywit h thevolcanismand durin gsubsequentepisodesoftectonic uplift. The differences between the protoli t hsof the podi- formsilli manit e and kyanitedeposits in contrast to the strat- iform kyanitequartzitescan berelatedboth to the amount of argillaceousalteration,silicificati onand hot-sprin g silicasin- ters in the source areaand to mineral separation during transport and deposition. High-alumina clays,assumed to representthe protoliths ofthe podiform deposits, were prob- ablydeposit ed ina more offshoreenvironmen t than the alu- minousquartzsandsof thestrat iform deposits which locally grade into conglomerates.

Metamorphism of syn-volcanic and bedding-parallel quartz-kaolinite alteration of felsic tuffs has been advocated to represent the mechanism for the formati on of kyanite quartzites inte rcalated withfelsicvolcanitesin theCarolina SlateBelt(Bell et al. 1980,Schmidt 1985).The hydrothermal alteration zones were proposedto be situated below hot- spring and sea-floor exhalation centres and thereby be geneticallylinkedtothedevelopmentoflocal exhalitehori-

Fig.7.Simplifiedgeological map of the south-central part of the Appala- chian orogen depictin g the distributionof some important geotectonic units and terraneshostingkyaniteand sillimanitequartzites,high-alumi- na alt erationsyste ms andst ratifo rm Fe-(Mn),base-m et al and baritede- posits.The locationof Fig.8isindicat ed.Mapcompiledfrom Espenshade

& Potte r(1960),Feiss(1982), Schmidt(1985),Horton(1989) and Hatcher etal.(1990).

Carolina terrane:

Low-to medium-gradebelts Med ium-g rade belt Medium-to high-gradebelts Tectonic boundaries

• Kyan ite quartzites o Sillimanite and

andalusit equartzites

79' 35'

NC SC

80'

" SC

""GA

82'

, Base-metaldeposits

• Baryte deposits ..Fe-(Mn)deposits

• High-aluminaalteration centres withtopaz pyrophyllite,andalusite, and/orkyanite

PETERM.IHLEN

Fig. 8.Simp lifi edgeological mapsho w ing thestratiform natureof thekyanit equartz- itein theWillisMountainarea.Redra wn seg me ntof Fig.3 in Conley&Marr(1980) wit hadditionaldatafrom Espenshade &

Potter (1960) and Pavlid eset al.(1982).

/

/ ....

.

. .

ARVONIAFORMATION:

. . Graphitic meta-pelites . .Banded quartzites and

kyanitequartzites Quartz-mica schists,

conglomerates and kyaniteschists

~ UNCONFORMITY~

NGU-BULL436,2000-PAGE 123

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zon s.Enhanced conte nts of fluo rine, boron, barium, phos- phoru s,base-m et als an d sulph ur in the quart zites arealso commonfeature of mostkyanite andsillima nite deposit s and frequ ent ly alsoof their wall rocks.This can be ascribe d to enric hme nt in conj unct io n wit h sub-seaflco r hyd roth erm al altera tion andassociate d exhalati ve processesas wellasto subseq uent erosion of alteration centres enriched in these elements.

Shear-ind uced andsyn-metamor phi c met aso mat ism of alum ino usrocks has also beenproposed to exp lain thefre- quentlocat ion of bothtypes of dep ositsinclose proximityto regi on al shearzones(Bane rji1981,Andr easson &Dall m eyer 1995,Ihlen & Marker 1998).Such mod els are supported by the invar iab le presence of syn-metamorphic, hydrothermal veins and seg regat ions of kyanite and sillimanite in the deposits (Espenshade & Pott er 1960,deJager & von Back- st ro rn1961, Schreyer 1987,l.und eqardh 1995).Compara ble but region al systemsofsyn-meta morphic upgradin g ofalu- min ousrocks arefoundin the easte rnpartof theFenn oscan- dian shield(Khizovaara;Roskill1990).Lat erally extens ivegar-

netite zones (silicate-facies iron forma tions) are here intruded by ton alit es and both are veined and pervasively repl acedbysta uro lite , kyanite and quartzinconju nct io nwit h supe rpos it io n of Palaeoproterozoic shea rzones (Ihlenet al.

1994,Ihlen &Marker1998).

Potential areas and occurrences in Norway

Nor wayis well-en dowedwit h pluton ic an d medium -tohigh- grade met am orphicterran es contai ni ng lithol og ies andmin- eralisat io ns,com mo nlyfoundassociate dwith economicsilli- man it e-m in eraldeposits. Sofar, explora tion work has been conducted on dep osit s of mainly kyanite-rich mica schist s (e.g.S0r0Y)andsomekyani tequartzites(e.g.Salt fj ellet)inthe Caledo nides.The inve st iga ti o ns have norma lly found ered on highrecov ery costsand/o r mineraldressingprobl em s.Thus, nodepo sit shavebeenput into produ ct ion.Anatte m ptwill bemadebelo wto poi ntto some pote ntia lareas andoccur-

NGU-BULL436,2000-PAGE 124

rences which deserve atte nt io naspossibl etargetsfor explo- ration work.Inthiscontextit is important to emphasisethat Norway is sit uated close to the world's leading refractory market in central Europe, thereby providing a competitive advantage.

Potential areas for sillimanite deposits

The most widespread occurrences of sillimaniteare encoun- tered in the Proterozoic paragneisses of southern Norway which havebeen affecte d by medium- to high-grademeta- morphism during theKongsbergian (Got hian; 1.75-1.55Ga) and sveconorwegian(1.2-1.0 Ga)tectonothermal events.Si1- limanite-rich metasedimentaryrocksinterlayeredwit h felsic gneissesof possible volcanic origin are especially encoun- tered in the amphibol ite-facies gneisscompl exes flanking the Mesoproterozoic bimodal volcanic sequences of the Telemark supergroup (1.0-1.5 Ma), e.g. in the Bamble- Modum sector and neighbouring areas,and in the Agder GneissComplextothe west(Fig.9).Accordingto mapping by the author, similar sillimanite-richparagneisses with associ- ated silicate-facies iron formations arealso foundin the set- skog area,southeastern Norway,in conjunction wit h felsic metavolcanites. However,indications of podiform sillimanite deposit shave so far only been detected inside the peliti c- arenaceous metasedimentary sequences of the Bamble- Modum sect or (Jesanq 1966, Morton 1971, Starrner 1976, 1978).These are in many aspects similar to the sequences hosting the Bushmanland sillimanite deposits, and contain sillimanite-cordieriterocks in the Bamble area(Morton et al.

1970)and small bodies of massivesillimanite rocks,encoun- tered by the author in the mica schistsadjacent to the Skut- terud Co-Assulphide zone in the Modum area(Andersen &

Groru d 1998).Admittedl y, these bodi es arefartoo smallto attractany major attention,but may give indicationsof a suit- able depositional environment for the necessary alumina- rich protoliths.Thus,the best startingpointfor explorationat the present state of knowledge would be in the Bamble- Modumsector.

Potential andalusite occurrences

Andalusite is found associated wit h plutonic complexes intruding low-grade met amorphic rocks in theCaledonides and in the Oslo IgneousProvince (Fig. 9). The andalusite- cordierite hornfelsesin the latter provi nceare developed at the contact between Cambrian-Early Ordovicianblack shales (alum shales) and Permian granitic plutons (Brogger 1882, Goldschmidt 1911). The andalusitezonesin the Eiker-Sands- veerarea are c.l 0 m thick andcontain rarely more than c.7%

of coarse-grainedchiastolite crystals.They appear to be sub- economic.

The most prominant development of andalusiteinNor-

Fig. 9. Simplifiedgeological map of Norway(5igm ond 1985)outlining the sillimanite- bearingargillaceous-arenaceoussequences(b lack)in the Bamble-Modumsector and known occurrences of andalusiteschistsand kyanit e quartz it es.Abb revia tions:B

=

=

=

=

=

=

Romeriksasen,5

=

=

=

=

o o o

PETER M./HLEN

Permian,Oslo ig neous province Devoniansedimentarybasins Caledo nian NappeCom p lexes Neoproterozoicarenites Precambriansupracrustals!

Caled on ised

Precambriangneisscomplexes!

Caledonised

N