Industrial mineral quality of limestone: the effect of contact metamorphism on textural properties,

NGU-BULL436, 2000-PAGE85

Industrial mineral quality of limestone: the effect of contact metamorphism on textural properties,

brightness and geochemistry

IDUNNKJ0LLE

Kjolle,I.2000:Industrialmineralqualityoflimestone:the effectofcontact metamorphismon text uralprop erties, brightnessand geochem istry.Norges geolog iskeundersekelse Bulle tin436,85-91.

When carbonates arealtered bycontactmetamorph ism theindustrialmineralqualit y oftherock canbeaffected. Based ontwo contrast ing limestoneunit sfromthe OsloRift,Norway,a study hasbeen madeof theimpactthat contactmetamorphismhas onthe quality of limestone wit hrespect toitstextural prop erties,bright ness and geochemistry.Sampleswit hvariable metamorph ic imprints havebeen examinedand compared totheirnon- metamorphosedequivalents.The resultsindicatethat,inthecontextoflimestoneutilisationforindustrial mineral applicatio ns,contact metamorphism can haveafavourableeffectonsome properties.It can red ucethecont ent of organic matterandgivetherock alightercolour andincreased brightness values,andthecalcite canattain more evengrain-shapeand-boundariesandincreased grainsize.However,there arenoindicationsfromthepresentstudy thatcontactmetamorph ismimprovesthequality withrespecttotheoccurrence andab undanceofcontaminants.

Theselimeston esshowthattheimpuritiescanremain very fine-grainedandpartlyintergrownwiththe calcite, implyingthat contact-metamorphosedlimest onemaybeno simplertopurifythannon-meta morphosedlim estone.

Ifthemarblehasalsobeenmetasomatica llyaltered, whichisnotuncomm onincontact-met amo rphicenvironments, therock is actuallylikelyto bemore conta minated,andofcorrespondi ng lypoorerquality,thanitwas originally.

Overall,the implicationsarethatcontactmetamo rph ismneed not improve the limestone qualit y sufficient ly to produce aneconomicindustrialmineraldeposit.Itmayberequiredthatthelimeston ewasofvery highpurit y prior to metamorphismandalso hasbeenunaffected by anyaccompanying metasomatism.

Id unn Kjelle.Norgesgeologiskeundersekeise,N-7497Trondheim,Norway.

Introduction

Carbon ate rocks,especially limeston ebutalso doloston es, arevery important industr ial mineralresourcesworldwi de, both in term sof producti onvolume and marketvalue.They have alargenumber of uses.The vario usend useshavevery diversequality requirements withregard to both chem ical and physical proper tiesofthe rock.Whet heracarbonaterock can beusedand thequalit y and price which can beobtained for the ind ustrial mineral produ ct is, to a large degree, depend ent on theamount and occurrenceof impuriti esin the rock. Almost monom in eralic limestone containing ap proximately 100%calcite israre;in most deposits,lesser or greateramo untsof conta mi nati ngcompo nentsarepresent.

Comm on mineralinclusions arequartz,calc-silicates,graph- ite, and sulphides. Textural properties such as grain size, shapeand boundaries,the occur rence of contaminantsas singlegrains,exsolutio ns,or in thecrystallatti ce,thenatu re of intergrowths, etc.,are crit ical factors which dete rmine whet her the im purities can be lib erated by grinding and removedduringthebeneficiat ionprocess.

The purest deposits,which repr esent raw materialsfor fine-grade,ultra whitefillers,arecommonly localisedinareas of high metamorphic grade.It can thus be expected that metamorph ism is anim portant mechanism which contrib- utestotheformation ofcarbonatequalitie sthataresuita ble for exploit ati on. One com mo n and well-know n effe ct of

metamorph ismis an increase in the rock'sgrainsize.Another favour able effect,commo nlyobserved inlimestone unitsbut less well document ed,isbleachin gof the rock colou r.

Forindustri almineralpurp oses,itisofinterestto improve ourunderstand ing and docu ment ation of theim pactsthat metamo rphism has on the properties of carbon ate rocks.

Thispape rpresentstheresultsofast udyundertaken in this context.The investigation wasrestrict ed to contact meta- morphismandwas aimedatide nt ifyingwhat typ e of effects thisgeolog icalprocesshason the ind ust rial mineral quality of limestone wit h respect to itstexturalprop ert ies,bright- ness and geochemistry.More detailed descript ion s of the investi gat ions havebeenpresentedinKjolle(1998).

Geology and sampling

Thelimesto ne units selecte d forthe st udyare located in the OsloRiftregion ofsout hern Norway.TheOsloRiftwas anat- ural choice because itisoneof fewregions inNorwayalmost unaffectedbyregional met amorph ism.Theprim aryfeatures are well preservedin the limestonesandcanbe compa redto the metamorphic effects in the contact-meta morp hosed limest onecom monl yfound adjace nt tothemany intrusions of thisregion.

Limesto neof originalhigh purity was prefer red for the st udy. In the OsloRift,such units are uncom mon,but afew relat ively purelimestones occur. Alllimestones of the Oslo

NGU-BULL 436,2000-PAGE86 IDUNN KJ0LLE

GastropodLimestone,nodular,andlimyshale,-Kalvsje Fonmation,Ordovician,levelsa

OthersedimentaryrocksofCambnan toSilunanage(shales, limestones,sandstones)

R oa

Permiangranitoids Biotite granite

• ordmarkite and related syenites

• Gabbro

CambriantoSilurian sedimentaryrocks

• Borealis&Pentamerus Limestone,partlymassive, partly nodular,ooarse-grained, - Ryneraker Fonmation, Silurian, level 7a-b

o Samplelocation - " "Fault

N

Gran

Lunner

Fig.1,GeologyoftheHadeland area with samplelocat io ns.The mapshowsa simpli fi edgeologyandisbased onthe preli m inarybedrockmap Gran 1815-1,1:50,000(Olerud & Owen 1995).

Rift are, to some exte nt, contami nated by shale, usually occurringin the form ofnodular limestone bedswhichrap- idlyalternat ewit h thinner shale layers.Limesto neformation s sam pled were selected basedon crite ria of relatively high purity,availabi lityfor sam pling in non-metamorph osedand contact metam orph ic form, wit h suffi cient expo sure along st rike,and of massive character or as beds well separate d from shale.Accord ing to Holt edahl's(1912)descriptionsof thenature and qualit iesof limeston e in theOslo Rift,areview largelybased onKize r's(1908)work onthesedime ntary rocks oftheregion,the unit bestfulfilling these criteria is the Silu- rian Borealis-Pent amerusLimestone in Hadeland.The sec- ond form ation select ed for sam plingwastheOrdovicianGas- tropod Limestone, which isalsooneof the purerlimeston es andthemosteasily availableunitin the Hadeland area.Geol- ogyandsam p lelocat ions are shownin Fig.1.

The Palaeozoic sedimentary rocksare, in many places, int ruded byPermi anplutons.Sampleswerecollected at var- iable distances from intrusive contactsin order to com pare lim estoneswith different degrees of meta morphicinfluence.

Emphasiswas alsoplacedon followingthe strikeand sam- pling the samelevelwit hinaformatio n,forthe purposesof comparingmaterialwhichwas assimilar aspossiblepriorto metamorphism.Inseveralplaces,however,itwas problemat- ical to st rict ly foll owthesame horizonoverlargerdistances.

The samples,nevert heless, crudely originate from thesame

level and should represent fairly similar origi nal material.

Mostsample swere collected in quarries and road-cuts wit h well-exposedstratigraphy.

Sampledistancesfromthe intrusionscan be seen inFig.

1.Arou nd the small,northern gabbro int rusion,metamor- phic effectswereobserved upto about250 m from the con- tact, but dista nces vary.The contact aureolesaround the bath oliths inthe southof the area can be up to 3 km(Dons et al.1996,Svensen &Jamt veit1998).Withinan aureole,a simi- lardegree ofmetamor phic alte ration was not always to be foundat quite the samedistance,at surface level,from the int rusivecontact.It wasmostadequateto label the samples based on a comb inat ion ofdistance and visible alteration effectsand to group them int o 3 broader categories;non- metamorphosed, low-medium metamorphosed and stro nglymetamorp hosedlimestone.

TheBoreali s-Pentamerus Limestone is of Lower Silur ian age andcompriseslevel 7a and7bcxaccor di ng tothe conven- tional stratigra p hicalnomenclatureof Kieer(1908). Holtedahl (1912)and ot hers.In aformal lithostratigraphic scheme it belon gs to theRytterakerFormati on (Worsley et al. 1983).

TheBorealis Limesto neof level 7a occurs as a massivebed of 4-5mthickness.Itis a bluish-grey biosparitecharacterised by the abundance of large,thick,whit ebrachiopodshells of Pen- tamerusBorealis(HoltedahI 1912). Other fossilsofim portance arecorals andstroma tolites(Owen 1978).Itis succeeded by

Fig. 2.Text uresin thelimestones.Contaminat io n bycarbon dustisprominentin thenon-meta mo rphosedlimestone,incontrast tothe whiteand pure calciteinthe contact-meta mor phic limestone.The originally dissimilar limestoneshave comparable,recrystallised,crystalloblastictextur es after metamorphi sm.(A)Non-metam orphosed BorealisLimestone:ground mass of sparry calcite,andlargecharacterist icbrachiopodshellinleftpartofphoto . SampleP25,PPL.(B)Non-met amorphosed Gastropod Limeston e:fossils setinamatrix of veryfin e-grained,dark brown micrite.Sam ple H4,PPL.(C) Stronglymetamo rphosedBorealisLimestone,from closetotheintr usive contact.SampleP7,PPL.(D)St rongly metamorphosed Gastrop odLimesto ne, closetothe contact.Sample H30,PPL.

(5

~

&

r--r-- rn

Clz ccD

rC r .".

SI'w

No o_o

"1J :J>

Clm 00

"

NGU-BULL436,2000-PAGE88

therelated Pent amerus Limestone, level 7ba , whi chisa 5-7 m-thick zone of massive bioclastic limeston e (Holtedahl 1912, Ow en 1978).ThePent amerus Limestone diff ers from level 7a in lacking the thickshells;instead,some horizons contai nabundant thinn er andusually smaller pentamerids.

TheGastropodLimeston eofUpper Ordovicianage com- prises level Sa andisnamedthe Kalvsje Format ion in the new lith ostratig raphicalterm inol ogy(Owen 1978).It consistsof nod ular and bedded bioclastic limestone alterna ting with thin shale and silt st onelayers;limesto ne forming50-70%of theformation (Owen etal. 1990,Brait hw aite etal. 1995).In the sampledareas,theGast ropod Limestoneis composedof typi cally 10-20 cm-thicknodularlimestonebeds inte rlayered with 1-3cm-thickshale layers.The colourof the unaltered limest on eisdarkgrey.Itisrichlyfossiliferous,particul arlyin echinoderm andgastropoddeb ris. Fragmentsof calcareous algae, brachiopods,corals, bryozoans and orthocon es are alsocommon(Ow en 1978, Braithwaiteetal. 1995).

Thenature ofthegroundmassisdifferent in thetw o lime- stone formatio ns. The matrix of the non-metamorphosed Gastropod Limeston e is ent irely comp osed of very fine- grained, opaque or semi-opaque micrite (microcrystalline calcit e)wit h a dark brown colour (Fig. 2B).In contrast,the non-m etamorp hosed level 7a Borealis Limestone has a rela- tively coarse-grained sparry calcite matri x (Fig. 2A). The groun dmassin level7bais a mixtureofcoarsespar and very fine-gr ained,darkbrownmicrit e,usually wit h thelatte rdom- inating.

Contamin ating minerals in the limestones and their metamorphosed equiva lents include quartz, pyrrhotite, pyrit e,magn et it e and cale-silicateslike epido te,amp hibole, pyroxene,garnet,wollastonite,vesuviani te,andtitanite.

Svensen&Jamt veit (1998) havemade estimates of the met amorph ic cond ition sfor the southeastern part of the area based on phase pet rology. The peaktemp eratures of met amorphi sm associatedwithintr usionofthe batholit hsin theGrua area havebeencalculatedto ca.560°(,assuminga pressure of 1kbar, andat distances 2 kmfromtheintr usive cont act a temperaturedecrease of ca.170°Cis suggested.

IDUNN KJ0L LE

Results

The maineffectsthatcontact metamo rphi smhas hadonthe quality of the Borealis-Pent amerus and Gastropod lime- stoneswit h respect to their textu ral properties,brightness and geochemistry, are present ed below. More detailed descriptio ns of the petrog rap hic observations, analytical data and resultsaregivenby Kj011e(1998).

Field relations and pet rograp hic invest igations have shown thatan increasing degre e ofcontact metamorphism generallyisacco mpanied by a gradualchange in thefollow- ing textu ralparamet ers:

The grainsize increases

The rockbecomeslighterincolour

A purificatio nofthe calcit e wit h regardtothe contentof carbon dust

Grainshapeand grain bou nd aries become more even

Within thecontact aureoles,the characte rof the lime- stones variesfrom relati vely fine-grained and dark grey in colour near the outskirtsof the aureoles tomedium-grained orcoarsely cryst alline marble wit hmedium to light grey or almost wh it e colour app roach ing the int rusions.Figure 2 show ssome of thetexturalfeat ures.Thermal metamorphism hasresulte d in a penetrati ve recrystallisa tio n of the lim e- stone toacrystalloblastic texture .Thesmaller grains in par- ticu lar,but alsothe coarser grain s,form apolyg onalmosaic wit h regular grain shapeand even grain boundaries. Most common grainsizes in themost contact-affe cte dlimestone are0.2-2 mm.The greater part of the calcite in the non- metamorphosed lime stone has a brownish colour and appears im pure duetoa dusting ofcryptocrystalline inclu- sions.Thesedarkparticles are most prob ab ly carbon dust.

The dark greycolour of carbonate rocksis generallyattrib - uted to organic compou nd s,especiallyvery finely dispersed carbon dust(Vogt 1897,Don s 1977,Blatt 1982,Todd 1990).

The content ofsuch dust particles ismuch redu ced in the contact-metamorphosed limestone. The calcite grains become clearer,light er and purer withincreasingint ensity of contactmetamorphism.

Fig.3.Occurrence of the impurities after contact metamorphism, shown by a sample of medium metamorphic GastropodLimestone.Contaminants aremainly calc-silicate minerals.These are, in part, presentas scat te redsmall crystals,which can occur along grainbou ndariesbut also intergrownwiththe calcit e;and,in part,they are presentasvery fine- grain ed,turbi d massesthatinvade and replacethe calcite.Withregardto carbon dust content,the cal- cite inthis photo is relatively light and pure com- pared with much of the medium-metamorphic calcite.SampleH22, PPL.

IDUNN KJ0LLE NGU-BU LL436,2000 -PAG E89

A

B

- . -FMX -·+--FMY -o-FMZ _ _R4S7

- . -FMX -·+-FMY -o-FMZ _ _R4S7

o _ N

M M M

X X X

- - - strongly metamorphic low-medium

metamorphic

Sample no.

BRIGHTNESS -Gas tropod Limestone BRIGHTNESS -Borealis-Pen tam er us Limestone

st rongly metamorph osed samples. Inthe Gastrop od Lime- stonehowever,aweak change is seenfor Fe203'CaO,Si02, K20,Ba,Rb,Ni and V.Analmostunmodi fie dchemical com- position is inaccorda ncewit h thetexturalobservat io ns.(It should,however, be not ed that in order to best evaluate purely metamorph ic effects,sam plingof limestone which appearedmetasomatica llyalteredwasavoided).Thisresult is also inagreementwit h thegeneral view andwit hot herst ud- iesoflimest onesindicatingthatmetam orphi sm is essentially isochem ical (insig nifican t change inchemi st ry) (e.g.Einaudi etal.1981,Best 1982,Labotkaetal. 1988).

Thebrightn ess valuesmeasuredongro undsam plesare shown in Fig.4.The sam ple powders represent a similar degree of grindingwit h much thesamedistribut ion of grain size(KjQJlle 1998),indicating that theirbrigh t nessvaluescan becompared.Figur e4clearlyshowsthat contac tmetamo r- phism has increased thebright nessof thelimestones.The non-metamorph osed Borealis-Pent amerusLimeston ehas a reflect iv ityof50-70%.Thelow ervalues representlevel 7ba.,

Fig.4.Brigh t ness of non-me tamorphosedand contact-metamorp hosed limeston e,A.Bor ealis-PentamerusLim eston e, and B.GastropodLime- stone.Themeasurements ofref lected light havebeenmade wit h 4fil- ters. The bright nessis give n by R457, which is the refl ectivity of monocromaticligh tof wavelength 457nm.FMX,FMY and FMZarethe reflecti vitiesof lig ht ofwaveleng thscentre daround600 nm(red), 550 nm(g reen)and 450nm(blue),respecti vely.

0 N N

~ ^N

s

"- "- ^I;' a. ^Na. a. ^{~ a.} "- s: _"- a. t: t: ^a.

"- "-

Sampleno.

00 - - -- - - - - - - 50 - - - unmetamorphic

90 -- - - --- - - - - -- - - - -

90 - - - --- - -- .- - - -.--- - - -

100,--- - - , -- - - - --r- - - -

100, -- - - - - - - - - - ,- - - - - - - -,-- - - --,

~ ⁸⁰

~

Excepti onsto thesetextural changesdo,how ever, occur.

Contactmetamor phism usually leadsto a grain size increase in biomicrit iclimestones,but this is notnecessarily the case for carbon ates wit h spar matrix.The non -m et amor pho sed Borealis Limestone is of similar coarse grain- size as the strongly metamorphosedequivalent.There are also exam- plesof biom icri t ic limeston e which hasneith er increasedin grain sizenor beenbleachedduring themet amorph ism, as show n by somelow-to medi um-metamorphosedsamplesof the Gastropod Limestone.Thisvariet yhas an almost sim ilar text ure tothenon-met amor ph osed equivale nt,alt houghit is morerecrystallised.At somefossilsitesrecrystalli sati on can be seen tohaveresultedina finerrather than coarsergrain size; cf.Fig. 6 inKjelle(1998).

Of particular importan ce forthe ind ustr ial mineral quali ty istheamount of impurit iesand howthese occur in therock.

In these respectsthetwo investigatedlimestone formations show no improvements in quality after contact metamor- phism,wit h the except io nofthereductio ninabunda nceof carbo n dust.The amountofallot herobserva-

ble contaminantsis apparent lyunchange dor has increased.Meta morp hicrecrystallisation of the originalconstitu entsis the mostcom- mon, wit h alte rat io n of shale inclusions to calc-silicates.Insome places,an increasein theamountof impurities can be seen,where the marble has undergon e met asom ati sm (add it ionof new elements from anexte rnal fluid) and the calcite has been replaced by masses of calc-silicates(skarn)±sulphides.

The occurren ce of the contamin ant s appearsto bedirectl ycomparable inthenon - metamo rph osed and contact-me tamor- phosed lim eston e. In unalt ered as well as metamorphosed sam ples, impurities are presentboth as evenlyspreadgrainsandseg- regat ion s,common lyalong grainbound aries, but alsointergrownwith, orenvelo ped by the calcite.Even though contact metamorphism produces more even grain-shape and bounda riesand increased grain size for the calcit e, the impurit ies can be just asfine- grained and commonly intervenein the cal- cite to a similar degree in met amo rph osed as in non-metamorpho sed lim eston e.Figure 3 illustratestheoccurrence of theimpuritiesin the marble.Thecale-silicatecontamin ant s are present both asscattered smallcrystalsand as very fine-grained,turbid massesor 'veils' that invade and repl acethecalcite.

Geochem ical analyses, including XRF,

acid-soluble CaO andMgO,and total organiccarbo n(TOC), aswellasmeasurement s ofbrigh tness, were made on most of the samples(KjQJ lle 1998).With regardto chemicalcompo- sit io n, themajorit yof detect ed elementsshowno syste mat ic change with the degree of metamorphic influence.Inthe Borealis-Pent amerusLimestonenearlyallelementsoccur in comparable amounts in the non-met amorphosed and

NGU-BULL436,2000 -PAGE 90

in agreemen twith a slightlydarker colour of these samples.

The weak-tomedium-metamorph osed samplesrangefrom 60to 88% and overlap with the stro ngly metamorph osed sam ples,whichlie wit hi n the range76-88%.TheGastro pod Limestone shows more of an increasing trend with the degree of metamorphism; brightness values being 30-65% for non-metamorphosed,48-73 %for weak-medium meta- morphosed, and61-89%for strong Iy metamorphosed sam- ples.

Discussion

It is a common conception andoften true that the grain size of a rock increases with metamorphic grade,but thiseffect is not universal and need not apply to all minerals present.

Ascribing metamorphic grain size justtotem perat ure-p res- sure conditions can be too simplistic and very misleading, because gradeis onlyone factorinflu encing grainsize(Vogt 1897, Best 1982).Inmarbles,im purities of calc-silicatescan be fine-grained in some situations and coarse in others.

Metamorphic cale-silicates are typically fine-grained , whereas cale-silicates formed metasomatically commonly occur as coarse crystals (Einaud i et al.1981). Metamorphic recrystallisation of the Borealis-Pentamerus Limestone and GastropodLimestoneresulted in a grainsize increaseforthe calcite, whereas the impurit iesremained fin e-grained and partly int ergrow n withthe calcite.Even metasomat ic cale- silicatescan occur with very small grainsize(a fewum),as for examplethose formed in contact-metamorphosed carbon- atesin the Ravndalskollen-Myrerkollenarea of the Oslo Rift (Jam tvei tet al. 1992a).

For a number of limestone uses,e.g.as filler,impurities firsthaveto belib erat ed by grindi ngandrem ov edby bene- ficiati onprocesses.Theoccu rrence ofimpur itiesin thelime- stones from the presentstudydoes notindicatethatcontact- metamorphosed limestonewould be simpler to purify than unaltered limestone.Although some limestones may have experiencedgrain coarsening of the contaminatingminerals aswell as the calcite,and movement of most impurit iesto calcitegrain bounda ries,this studyillustratesthat oneshould not takefor granted that contactmeta mo rphismwill auto- matically prod ucesuchquality improvement.

With regard to thecolour ofthe limestone,there is clearly a connection wit hthe measured brightnessvalues and the amount of carbon dust observed in the caleite under the microscope.Metamorphismclearly bleaches the rock as well asincreasing the reflectivi ty values of the ground samples.

There is an excellent agreement between the relative degreesof bleachi ngof thehand specimensand therelati ve brightness values. Furth ermore, when samples of rough ly similar grainsize are compared,it can be seen that the darker the rock colour,the more contaminated are the calcite grains by dark carbon dust particles. The lowest content of carbon dust in the calcite is observed in the most stronglycontact- affected samples with the lightest colour.These relations imply that limestone with the strongest metamorphic imprintmostprobablyhasthelowest conte ntof organiccar- bon.The exact amoun t of organic carbo n is unknown;it

IDUNN KJ0LLE

could not be measured usingtheTOC method. On the other hand,the detecti on level of0.10wt.% indicatesthat the amount of organiccarbon needed to give limestoneadark colour is very small.

The colou r oflimest on es part lydepend s on the grain size.

They commo nlyhavea darkercolourthe more fine-grained theyare,inpartbecause org ani cmatter can be more finely dispersed in rocksofsmall grain size.Inaddit ion,micrite, which is analogous in hydrauli c behaviour to clay minerals, adsorbs, and usually contains greater amounts of organic matter than do coarser grained limesto nes(Blattetal. 1980, Blatt 1982).Thedark colours of carbonate rocksare mainly attributed toorga nicmatter,but sulphidesand silicatescan also contrib ut etotherockcolour to some extent,as shown e.g. by sulph ide -poo rsamp lesbeingslight ly lighterin colour than samp lesconta ining moresulphides wit hin samples of thesame metamorphicgrade.

Todd (1990) has studied bleachingof contact-metam or - phosedlimestone at Notch Peak,Utah,and relat es the colo ur alteration in thelimeston eto organic carbon. HisTOC analy- sesshowthatbleached samplescontain lessorganicC than darksamples and point to loss of organic Cduring metamor- phism. Carbon isint erpre ted to have been rem ov ed by a wat er-rich fluid.Phaserelations forC-O-H fluidsunder meta- morphicconditionsseta lim itto the mole%H₂0that canbe in equilibriumwit h graphite.IfX_H20is increasedbeyond this limit, the fluid will react wit h the graph ite and carbo n is rem ov edasCO₂and CH₄according tothe reaction:

2C+2H₂₀=CO₂+CH₄

Infl ux ofa H_20-rich flui dduringmetamorphismthus seems to be a possib le mechanism by which limestone can be cleansedof organiccarbon.This issuppo rted by the data of Labot ka et al.(1988)which indi cate thatargillac eou s layers within the same formation wereinfi lt rate d by substant ialvol- umes of H₂₀-dominantfluidsfromthe Notch Peakintrusion duringthe cont act met amorphism.

Despite substant ial fluid infil tr ationduring contact meta- morphism at Notch Peak, the limestone-shale sediments were apparent lynot metasomatised;their primarycomposi- tion being essent iallypreserved(Labot ka etal. 1988).Contact metamorphismof limestone-shalesedimentary rocks in the OsloRiftwasalsotoalargeextentdriven bythrou gh-flow of H₂₀-richflu ids(Jamtveit et al.1992b).lt iscom mon,however, both in the Oslo Rift andelsewhere,thatcont act metamor- phism islocally accom panie d by metasomaticskarn forma- tion. In the studyarea,goo d exposuresof limestone can be seenwhich are ext ensively repl aced and cont aminat ed by cale-silicat es and sul phide s, for example in road-cut s and quarriesat Grua.Therefore,one should beaware that cert ain zones of contact-metamorphic envir on men t s are likely to have had addit ion of new components from an external sourceand,consequent ly,contain limestone which is less purethaninitsprimarystate.

Finally, it has beennotedthatalsothe mechanicalprop- ertiesof the lime stone seem tovary withthe degree of con- tact metamorphi sm. The non-me tamorph osed and the weak-to medium-metamorphosed lim est one appear more solid and fresh thanthe most contact-affect ed analogue.The

IDUNNKJ0LLE

latterseems moresugary, crumb lesmore easily,andismore readily weat hered.Thisis possiblyconnecte dwith thepoly- gonalfoam texture; even grain boundaries are bestdevel- oped in the most metamorphosed limestone. It appears, therefore, that contact metamorphism has a disadvant a- geous effectif the marbleis intended for nat uralstone appli- catio ns. This is in fullagreementwit h Vogt' s(1897) conclu- sions,based onstudies of a rangeof Norw egian marbles.

Conclusions

The investigation shave show n that contact metamo r- phism can,insome respects,have a favourable effecton the industrial mineral quality of limest one. It can reduce the amount of organic matter (carbon dust),giving the rock a light er colour and increased brightn essvalues, and it can produc e more even grain-shape and -boundaries and increased grain size for the calcite. However,there are no indications from the present stu dy that contact metamor- phism improvesthequality in termsof theoccurrenceand abundanceof contamina nts.Theselimest onesshow that the impuritie s can remain very fine-grained and partly inter- grown wit h the calcit e, imp lying that contact-metamor- phosed limestone may be no simpler to purify than non- metamorph osed limestone. If the marble has also been metasomaticallyalte red,whichisnot uncommo nin contact- metamorphic environments, the rock islikely to be more contaminated and therefore of poorer quality than it was prior to metamorphism.Overall, theresults indicate thatcon- tact metamorphism maynot improve the lim eston e quality suff icient ly to produce an econo mic industrial mineral deposit.It may be requ iredthat the limesto newas of very highpurity priorto metamorphism andalso has escaped any accompanyingmetasomatism.

Ackno wledgements

The aut ho risgratefulto Pet erIhlenand Tor ArneKarlsenfor valuabledis- cussionsandcommentsduringtheproje ct, and to Nigel Cook,David Corn ell and DavidRobertsforhelpfulreviewsofthemanuscript.

NGU-BULL436,2000 -PAGE91

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