Granasen, a dolomite-brucite deposit with potential for industrial development

NGU-BU LL436, 20 00-PAGE75

Granasen, a dolomite-brucite deposit with potential for industrial development

ODD0VERENG

0vereng,O.2000:Granasen,a dolom ite-b rucitedepositwithpote nt ial forind ust rial develo pm ent.Nor ges geolog iske undersekelseBulletin436, 75-84.

Granasen,situat edclosetoMosjoenin northernNorw ay,is adepositcontainingseveral milli ontonnesofdolomite and brucite. Thedep osit has been investigated and characte rised in prep arati on for future developme nt and exp loitation.Theore consistsof dolomiteandbrucite.Brucite hasformed as aresult of hydrothermal alteration associate dwit h intru sionofgab b ro.Testshavebeencarriedout to evaluatethe comme rcialvalueof the dolomite resources. A variet y of differentbeneficiationmet hods wereinvestigate d to achiev ecom mercialconcent ratesof brucite.Themo stsuccessful resu ltwas obtained by select iveflocculation,which up-gradestheoretoamaxim um gradeof 95.5%bruci te,wit harecov ery of80%.As analte rnative,or by-product of brucite concent rate produ ct ion, thedep ositcouldalsobeworked fordolomite.

Odd0vereng,Norgesgeol ogi ske undersekelse,N-7491 Trondh eim,Norw ay.

Introduction

Since the beginningof the 1970s, the GeologicalSurveyof Norway(NGU) hasinvestigat ed alargenumber of industrial mineraloccurrence s.In the early stageof this programme,a large deposit of high-qualit y dolo mite marbl e was found some 13 km northof thetown Mosjoen,in Nordlandcounty (Fig.1).Thedep osithas afavourablelocation,as itis sit uat ed only 3 km from Vefsnfjord,wit h railway and good harb our facilities,and is close to a well-devel oped infrast ruct ure at Mosjoen.The location isalso favourablefrom an environ- ment alpointofview.This paper reviewstheexplorat ionhis- tory of theGranasendolomite-brucit edeposit.

History of investigations

TheGranasendolomitemarbledeposit was firstdescribedby 0vereng(1972).Sincethen,muchwork hasbeencarried out wit h theprimarygoalof obtaining reliableinformationabout the size,geomet ryand qualit y ofthe deposit.In 1974, the centra l part was map ped at a scale of 1:5,000, and several short drillholes were financedbyNorcemA/Stotestthequal- ity of the ore (0 vereng 1974).Norcem A/S signed royalty ag reementswit h thelandow ners.

In1975 aco-operat iveproject between NGUandSINTEF was initi atedtoevaluate tech nicalaspectsfor use of Norwe- giandolomitemarb leinproduction of basicrefractoryprod- ucts.Dolomite marbl esfrom a number of localities were tested forthispurpose(Seltveit et al. 1977).The dolomite from Granasengaveexcellent result s.Materialwasalso sent toDolo mitwerkeGm bH,WQlfrath,Germany,one ofthe lead - ing producersof refractor y bricks.Thetestsgave resultscom- parabletothose obtainedat SINTEFandenco uragedNGUto continu ecoredrilling at Granasenin1978. Additionaltesting ofthesecores confirmed theearlierresults(0 vereng 1978).In 1979,the mineral brucite(Mg(OHb lwas discoveredin parts

of theGranasen dolom it emarble (Faye & 0vereng 1979).

Extensive mineralogical and geological invest igations, including diamond drilling, were carried out in 1979 and 1980(0vereng 1981),andconfirmed the inte rpretation that thebrucit emineralisati onisrelatedtothe contact-metamor- phiceffects of the Mosjoen gabb ro.During thisprogramm e, a varietyof different tests werecarriedout to esti matethe likely commercia lvalueofthedeposit(0vereng 1995).Simi- lar brucite-dol omitedepositsaredescrib edfromCanada,e.g.

byGoudge (1957) and Amb rose(1943), andfrom the USA (Burn ham 1959).

Brucit e isanindust rialmine ralwith a farhigher contentof MgO(69.1wt.%)orelemental Mg(41.6 wt.%)than anyother natu rally occurring magn esium compo und, except the ratherraremineralpericlase (MgO). In comparison,the main currently exploited sou rces ofMg-co mp oun ds,magnesite, magne sium chloride and dolomite,cont ain 28.8,25.5and 12.6wt.%Mg, respect ively.Brucit e is therefor ea mineral of con siderableinterest as analte rnat ive raw mate rialfor pro- duction of MgO andeleme nta l Mg.

Geological setting

TheGranasen area islocated in theElsfj ord-Mosj oen tect on- ost ratigrap hic unit (Riis & Ramb erg 1979)in theHelge land region of Nordland.The rocksofthe area belongtothe east- ern part oftheHelgeland NappeComplex (HNC), theupper- mostunit ofaseries of nappesin theCentralScandinavian Caledonid es andascribed totheUpp ermo stAllocht hon(Gee

&Zachrisson 1979).The UppermostAllochth onin thisregion consists of three tecton ic com plexes,the HNC,the Beiarn Nappe and the Rod inqfjallet Nappe Complex.These com- plexes broadly consist of comparabl e lith olo gies: mica schists, calcareo us micaschists,marbles,am phibo lites,ser- penti nites, gneissesand granit oid int rusions(Gavelin&Kull- ing1955,Ramberg 1965,1967,Gust avson 1981).

NGU-BULL 436,2000-PAGE76 ODD0VERENG

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Dolomitemarble Calcitemarble

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Acid inrusions(granite, diorite) Greenschist

Gabbro/Amphibolite Certainbounda ry Uncertain bounda ry Strike/d ip

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Fig.1.Geological mapofthe Granasendolo m it edeposit (0vereng 1981).

ODD0VERENG NGU-BULL436,2000 -PAGE77

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Dolomite marble Calcitemarble

Mica schist/Garnet-micaschist Quartzite/Calc-silicate rock Mica gneisswith lensesof acid igneousrock

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Gabbro/Amphibolite Certainboundary Uncertain boundary Drillhole

LEGEND

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Fig. 2.Geological profilesfrom theGranasen dolom it edeposit (0vereng 1981).

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Traditionally,the Uppermost Allochthon had beenconsid- ered as essentially comprising Cambro-Silurian rocks, deformed and metamorphosed during Caledonian moun- tain building (e.g., Strand 1960). No fossilshave yet been found in sedimentary rocks from theUppermo stAllochthon, but radiometricages indicat e that partsof thegneissicunit s are of Precambr ian age.Intrusiverocks,however,areofboth Late Precambrian and Cambro-Silur ian to Late Siluri an age (Priem et al.1975,Claesson1979).Rock units in thearea are strongly deformed and the observed mineral assembla ges

aretheresult of the majorphaseof deform ation,metamor- phism and nappe translationthat tookplaceduringtheScan- dian phase of the Caledon ian Orogeny in LateSilu rian to Early Devonian tim e. In contrast to other nappes in the Uppermo stAllochthon,the HNChostssizeable bodiesofsyn- orogenic intr usions which crop out overmorethan 30%of thearea.In accordancewiththeregion altrend ,therockunit s inthe Granasen area have acomplexand prolong eddefor- mationhistory(Riis&Ramberg1979).In the HNC,early folds haveN-S axesandalmostuprightaxial planes.Gabrielsen&

NGU-BULL436,2 00 0 -PAGE 78 ODD0VERENG

Thefinal produ ctsof the carbonate rocks aftercontact metamorphism depend on a numberof factors.The most importantarethe chemical composition and mine ralog yof the sedimentary sequenceand the temperature and pres- sure prevailing at the time of intrusion .

content is,on average,about 17 wt. %;individua l sam ples may contain upto23wt.%brucite.

Exp loratory core drilling in the deposit(5000m of core) revealed that bruciteoccursin distinct layers within the dol- omite marble. Distribution of brucite wit h in the layers appea rsto be concentrated in a very complex networkof veinlets,with a gradual decrease in brucite concentration away fromthe gabbro contact. Layers of brucite-b earingdol- omitealternatewithlayers of pure dolomiteor dolomite with granulesof olivine (forsterite), or of oliv inedisplayingvarying gradesof alteration to serpentine and brucite.Since brucite is more soluble than carbonates,the brucite content can be roug hly est imated from the naturally-etched weat hering sur- faces.Thereis always some olivine and/orserpentine in the brucit e-bearinglayers, but only a minor amount of brucite in the olivine/serpentine-richlayers.The brucite-richlayers are spaced atintervals from a few cm up to ca.20m.Fieldobser- vatio ns,together with examinationof drillcore,suggest that thelayersenrichedin bruciteare notalwaysconcordant with the primarybedding in the dolomite marble.

M inera logy

Thedolomite marblesare mostly granular,and vary from fine- to coarse-grained(0.1-1.5mm,mean0.6mm).reflectingthe grade ofthe metamorphism. Most of the dolomitemarbles are white,butin certainparts of the deposit the colourispale blu ish-grey, due to minor amounts offinely dispersedgraph- ite.The mostim port antimpuritiesarecalcite,forsterite,ser- pent ine,mica andtremolite(Table 1).Tremolite,as irregular masses ordissem inated grains,only occurs in specific layers in the dolomite marble,but is practicallyabsent in the bru- cite-rich zones.Most common accessoriesare quartz,feld - spar,mica,graphite,diopside,zircon,rutile,apatite,titan it e and variousoxidesand sulphides.

Based on the observations frommappi ng,diamonddrill- ing and geophysics,the most promising area for quarrying puredol omite marble would be in the central part of the deposit(Tables2&3).Geochem ical investigationshave also been carried outin thispart of the deposit.

The occurren ce of brucite in marble is quite common (Table 4).The classicalmechanism for brucite formation is as follow s:when dolomite marble or Mg-bear ing limestoneis exposed to contact metamorphism,the mineral periclase (MgO)isformed and immediately hydrates to brucitein the presenceofwater(Turner1954):

MgO + H20 - > Mg(OH)2 periclase brucit e

(1)

(2) - > CaC0₃ + MgO + CO2

calcite periclase CaMg(C0₃)2

dolomite Ramberg (19 79)suggested two phases of thrusting for the

HNC, both older than the latest folding phases inthe area.

Metamorph ism in the Mosjoen unitof the HNC isapparentl y linked to the second,main deformation phase (02). which overprints older magmatic mineral assembl ages.The P-T conditions of metamorphism are assumedto be about500 - 600°C,at2.0- 3.5kb, wit h increasingtemperaturestoward s the contact wit h the Mosjoen Igneous Complex (Theisen Juell1985).

The Granasen dolomite-brucite deposit

Thelens-shapeddeposit, which cropsout between 115and 180 m abovesealevel,is1200minlengt hwit h a maximu m width of900m.In total, it covers an area ofabout 1.3 km2, with a thickness of more than400 m, as provenbycoredrill- ing.The area has been mapped at the scaleof1:5,000(Fig.l).

The central part ofthedeposit isaflat-lying swamp y area.

Core drilling indicatesthat thethickne ssof theoverb urd en varies from 1 to 3 m.

The deposit is builtup ofasedim enta ry sequ ence(Fig.2) consisting of dolomite marble,underlain byagrey,impure calcitemarble that gradually,with anincreasing content of silicates, grades into calcite-mica schist and mica schist.

Locally,the mica schistis mixedwith thin bandsof quartzite/ quart zschist. Assuming thatthesuccession ofsedi mentary rocks is not inverted,thedolomite marble is the youngest unitin the sequence.The contact betweenthe dolomit emar- ble and the underlying calcit emarb le is sharpandconcord- ant.The metasedimentary units arest rongly folded,and now dip steeply to the west at about 70°. The major fold axis strikesNNW-SSE,paralle l to the reg ion al stri ke,wit h a SW plu nge.

The MosjoenGabbro Massifintrudes the metasedimen- tarystrata.The contact between the gabbromassifand the metasedimentary rocks is very irregular and sharp, and mostly concordant.There are numerousapoph ysesof gabb - roic material inthe form of dykes,sills and irregular masses wit hin the surround ing strata.Some of theseoccur ata con- siderabledistance from themaincontact.Isolated , irreg ular bodies of gabbro distributed withinthe sequenceare con sid- ered to be stringer int rusions from thegabbro massif.The youngest geological event in the deposit is recorded by a series of minor lensesof cross-cutting pegmatitesofgranitic to granodioritc composit ion. Geophysical invest igati ons (Eidsvik 1979,Dalsegg 1981)and exploratory core drilling s indicatethat the gabbro under liesagreat er partof the sedi- mentary sequence.

Along the main contact wit h the gabb ro massif, and around minorbodies of gabbrowit hin the depo sit,thedolo- mite marblehas been contact metamorphosed,and brucit e, Mg(OH)2, is one of the finalproduct s.Brucit emineralisat ion occurs exclu sively in the dolomite marble adjacent to the gabbro and has not been foundinthe contact zonearo und granite-granod iorite pegmatite bodies. The average thick- ness ofthe mineralised zone along themaincontact is esti- mated to be about40 m overa length of800m. Thebrucit e

ODD0VERENG NGU-BULL436,2000 - PAGE79

Table1.Mod al ana lyses of puredolomitemarble.Drillcore sam p lesare representati veof the cent ral part of thedep o sit.

Mineral1sample 2-79/1 2-79 /3 2-79/1 4 2-79/19 2-79/22 2-79 /24 2-79/ 26

Brucite * * * * * * *

Dolomite 95.0 95.4 96.9 98.8 99.0 95.5 95.6

Calcite 3.5 3.1 2.1 * 0.6 3.7 1.3

Olivine/5erpentine 1.4 1.0 0.5 0.3 nd nd nd

Otherminerals 0.1 0.5 0.5 1.8 0.4 0.8 3.1

* not measured nd notdet ect ed

Table 2. Major ele mentana lyses (XRF) of pur edolom it emarbleinwt.%.The sam p lesarecollecte d from 6 drill ho lesinthe centra l part of the deposit. Each sam ple rep resent s a coreleng t hof 30m.Measurem ent sbyPhilipsPW1480 x-ray fluor escen cespect ro meteron glassdiscs made byfusingthe sam pleandLi₂B₄0₇in therat io1:7(analyst: B.Nilsen ).

Oxid e1Sampl e Bh.l/77 Bh.2/77 Bh.3/77 BhAI77 Bh.S/77 Bh.6/77

5i02 7.19 <0.1 4.31 <0.1 4.34 2.6

AI203 <0.1 <0.1 <0.1 <0.1 0.38 <0.1

Fe203 0,21 0.15 0,40 0.14 0.27 0.25

Ti02 <0.01 <0.01 <0.01 <0.01 0.02 <0.01

MgO 22.62 21.46 21.7 21.36 21.52 21.24

CaO 31.66 31.44 31.23 31.54 32.53 31.43

Na20 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1

K p

MnO <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

P20S <0.01 0.14 0.02 0.02 0.02 0.02

Tab le3.Minimum and maximum valu es (wt.%)for acid-solubleCaO an d MgO in puredolomit emarbl e.CaO an dMgOwere det erm in ed by dissolvingthepowdered sam ple indil ute HCI(1:4)du ringheatingfollowedby titration wit h EDTA(an alyst:J.Roste).

Sample CaO MgO

Min. Max. Min. Max.

1n7 32.68 33.46 16.4 7 21.69

2n 7 30.71 31.52 17.05 20.73

3n7 30.71 33.02 17 .43 21.31

4n7 30.44 32.0 6 18 .01 21.50

5n7 31.25 32.87 18.98 21.11

6n7 31.52 32.33 16.85 21.31

Table4.Mod alanalysesof brucite-b earin gdolom it emarble.

Mine ral1Sampl e 2A-79 2B-79 2C-79 2D-79 2E-79 2F-97 2G-79 2H-79 21-79

Brucite 21.1 20.5 19.3 20.4 20.6 22.4 21.4 18.9 18.5

Dolomite 49.1 45.1 45.3 41 43.4 43.3 43.0 43.7 38.1

Calcite 28.8 33.8 35.0 37.8 35.9 33.9 34.6 36.7 38.4

Olivine/5erpentine 1.0 0.6 0.4 0.8 0.1 0.4 1.0 1.0 5.0

NGU-BU LL436,2000 -PAGE 80 ODD0VERENG

BRliCl T EDOI.O,\IITE GRA. ASE .:\ORWi\Y

Mineral resource assessmen t

Diamond drillin g show sthat brucite mineralisation of potential eco no mi cvalueoccurs not onlyinthe borderzone alongthe maincontact,butalso in the contac tzone around Carhondioxide

10

se

1.2 Mtonnes 3.2 M m³ 8.64Mtonnes 1.50 M tonnes Calcination

2hours I{j; 800^cC Rawore Grinding

T

Leaching - 4minutes

±35Calcine ·350ce ILO

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Dc-gassing- - - -

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such cases, occur as extremely fine-grained intergrowths wit h chrysotile.In spite of careful examinatio n of thin -sec- tionsfrom a large numberof repr esentative specimens, no periclasehasbeenfound. At Gran asen,spi nel occurring as cores in brucite hasbeen proven by elect ron microprobe examination.Tabl e 5 shows the chem ical composition of brucit ic dolom it e marble from different places along the main contact of thegabbro massif.Tabl e 6 shows element composit ion sof brucite,dolomite,calciteandforsteritefrom different placesalong the contact zone.One of the holes drilledin the contact zonehasapart icularlyhigh brucite con- tent, varyi ng from20to 30 wt .%,over athickness ofabout 120 m.

Filtering

~ t ---"

Rcsidue Precip itation

bv boiling Fig. 3.Flow sheetfor select ive calcin at io n andleaching (Jepsen1981l,

Basedon exist ingdrillcore inte rsectionsand geologicalmap- ping,theavailab leton nage of puredolomite marbledow n to sea level isestimatedto bebetw een 80 and 100 M tonnes.

Thetonnage est imation of the brucit e ore,however, is less certa in.A tent at ive estimateintw o restrictedareas(Finnhau- gen, Stuv rem ma)inthe maincont act zonetothe gabbro,1 km in length and40 met resin thickn essand with an indi- cated average of 17 wt.%bruc it e,gave the followingresults:

(RyssdaI 1984) Total volume:

Totalrocktonn age(S.G

=

Total tonnageof brucit e(17wt.%):

Recoverab lebrucite(assum ing80% recovery):

In theGranasen area, brucitemineralisation occurs exclu- sively inthedolom it e marble adja cent togabbro.This sug- gests that the brucite is genet ically relate d to the intru ding gabb ro,and that the intrusion supplied the heatnecessary to form hydroth erm al so luti onswhic h decomp osed dolomite to form bruciteandaccompanyi ngminerals.The major bru- citemin eralisation inGranasen area occursalo ng the main contact with thegab b romassif, which indicates a directcor- relati on betw eenthe sizeof intrusionandthedegree ofdol- omite breakd own into brucit e and calcit e. Brucite content typ icallydecreasesaw ayfrom thecontact.

In individu alsamplesof brucit e-b earin gdolomitemarb le, the dolomite/calcite ratio is widely variable,depending on theinte nsit y of the alt erat ionof dolo mitetobrucit e,calcit e and other minerals.The calculated dolomite/calciteratio in the areaof brucit e-bearing dolomi te varieswithi n a wide interva l(0.8- 1.3), butthemolecula rratio MgO/CaOfor dolo- mite marb lewit hor wit houtbrucit eonlyvarieswit hi na nar- row range(0.62 - 0.68)andis close to thatofpuredolom it e (0.72).These resultsindicat e that the original strataat the borderwere pure dolomite marbles priortothe int rusion of gabbro.

Mostof thebrucit eintheGranasenarea occursasirregu- lar androundedgrains ranging in size from<0.01mm up to 1mm; on average,around 0.35 mm.The granules appear milky-wh it e in colou r inafreshhand specimenand colour- lessorfaintlybrownunderthe microscope.This granularbru- cit etexture occurs in dolomit e,calciteandolivi ne/serpe ntine assemblages.Diff erences in the shape ofbrucit e granules are noted to dependon the dominant mineralogy of thematrix, calciteordolomit e.Generally,rounded brucite grains occur wit hi n calcit e, whereas brucite wit hin dol om it e showsangu- largrain bounda ries,whichinmany casesconform to the car- bonate cleavage. Proj ect ing tongues or veinlets of brucite in some cases extend from both angularand round edbrucite granules into the surrounding carbonat es, along cleavages or grain bound aries. Some of the brucite grains have the well-known, concentri c, onion-skin texture (Hunt & Faust 1937), with flakesor fibresin differin g orientations in each successive layer.Inplane-polarised lig ht the grains appear colourless;the complexfoliate dtexture isoptimallyrevealed undercrossednicol s(Fig. 3). Vein sof brucit e are rare,though they are occasionallyseen in drill cores and are composed of pale green, translucent,flaky brucit e with a pearly lustre.

Othermineralsare calciteandminoramounts of quartz,feld- spar,spinel,epidot e,tita nite,apati te,actinolite,forsterite and diop sid e. In addition,opaque minerals such aspyrit e and hematite are sporadicallypresent.

In theGran asen area,some ofthe brucitegrains occur as altera tionproducts offorst erite.The distributionand general appea ranceof theforsterit e,whichisdistribu ted throughout the dolomite andcalciteas round small grains0.01-0.5mm in size,issimilar in many respects to those ofthe granular brucite.There are many examples of progressive replace- mentof forsterite bybrucit e.Inmanythin-sect ion s,polycrys- tallin e agg regatesofbrucit ehave replaced wholegrainsof forsteri te,retaining the original crystal shape.Forsterite has also in varyingdegrees alteredto chrysotile .Brucite may,in

ODD0VERENG NGU-BULL436 ,2000-PAGE81

Table5. Majoreleme ntana lyses(XRF) ofdolomite marble with differentcontentsof bruciteinwt.%.MeasurementsbyPhilipsPW1480X-ray fluorescen ce spe ct ro mete r(analyst: B.Nilsen ).Brucite conte nt isdet ermin edbythePhenfieldmeth od (Graff 1983).

Sample Si02 AI203 Fe203 Ti02 MgO CaO Na20 K20 MnO P20S Brucite

Pr.1 0.28 <0.10 0.08 <0.01 23.3 35.4 <0.1 0.02 0.05 <0.01 23.3

Pr.2 0.35 <0.10 0.12 <0.01 23.0 33.8 <0.1 0.02 0.06 <0.01 16.2

Pr.3 <0.10 <0.10 0.12 <0.01 22.0 32.0 <0.1 0.02 0.06 <0.01 2.5

Table 6.Elem ent com positionsofbrucite, dolomite,ca lcite and forsterite(wt.%oxides). Minera lswereanalysedwithaJe ol 733X5uper probe instrum ent (acceleratingvoltag e=15kV,probe cu rre nt=15nA,cou nt time=lOs,rastermodeana lysis.Rawdatawerecorrecte d usingthe JeolZAF-cor rection.

Si02 AI203 Ti0 2 FeO MgO CaO Na20 K20 MnO NiO

Brucite:

Dhl.l/1 0.26 0.03 0.00 0.29 74.31 0.20 0.07 0.09 0.01 0.03

Dhl.l/2 0.09 0.08 0.07 2.71 67.43 0.28 0.03 0.00 0.00 0.00

Dolomite

Dh.5/1a 0.04 0.05 0.02 0.00 23.18 32.22 0.011 0.13 0.00 0.05

Dh.5/1b 0.03 0.03 0.00 0.09 24.07 34.95 0.000 0.04 0.00 0.02

Dh.5/1c 0.02 0.00 0.00 0.15 21.89 32.54 0.000 0.02 0.02 0.02

Calcite

Dh.5/1a 0.00 0.06 0.03 0.04 1.82 55.07 0.075 0.44 0.03 0.07

Dh.5/1b 0.00 0.05 0.00 0.07 3.22 67.41 0.000 0.17 0.00 0.01

Forsterite

Dh.6/1a 38.87 0.07 0.02 0.31 55.07 0.09 0.034 0.00 0.03 0.01

Dh.6/lb 36.35 1.51 0.00 0.32 41.88 0.05 0.000 0.05 0.05 0.00

Dh.6/1c 41.81 0.04 0.05 0.36 55.11 0.08 0.026 0.00 0.03 0.01

Dh.6/1d 39.48 0.47 0.04 0.18 40.22 0.07 0.031 0.07 0.00 0.03

the largest bodies of gabbro wit hi n the dolom it e marbl e.

Becauseof the complexity of themin eralisation it isneces- sary to undertake ext ensive additio naldiamond drilling to obtain sufficientdatato provide areli abletonnage est imate for thebrucit eore.

Brucite concentration

Thebruciteoresare relati velyfine-grained and requiremill- ing to30-40%minus40-150urn,toobtain 80-90%total lib- erati on of brucit e.Crushing and mill ing tests show thatthe bruciteoredid not fit into existi ng liberationmodelsbecause of the different milling propertiesof bruciteversusthecar- bonateminerals.Thedom inant liberat io n mechani smislib- erat ionof sing legrainsalongthe contact.A characte ristic of the milling productsisthe high content of brucite in the coarsefract ion and in the40-150pm fraction.Brucitehasa layered crystal str uct ure,which manifests itself asa mica- ceous foliation and aperfectbasalcleavage.Thus,crushing, im pact or vigorous attrit ion of ind ividual brucite grains should be avoide dinthe process.Oth erwise,the brucitemay cleav einto thin plates whichwould bediff icult toseparat e in a sizingappa rat us.Thetestscarriedoutat NTNU,Trondh eim,

indi cate thatcrushing /milling meth od s witha low abrasive effect give the lowest selective milling of brucite and,as a result,reduced contentsof bruciteinthe finerfractions(Mal- vik1980,1982).A range of processesfor developing abrucite concent rate have been tested.Thisarticle reviews onlythe most fruitfulmethodsand the results.

Flotation

The Ore-treatmentLaboratoryat theUniversityofTrondh eim (NTNU) carried out a preliminary test flotation, wit hout obtain ing an accept abl ebrucit e concentra te(DybdahI 1980).

The best result from the test programm e wasa concentr ate wit h 40-50wt.%brucitewitha recovery of30-40wt.%.

Tests carried out atBasic Inc.Gabbs, Nevada,USA (Jepsen 1981)

Material sent to BasicIne. Gabbs,wastestedusing (i) heavy liquid s, (ii) direct leaching and (iii) select ive calcinat ion and leaching.The last methodgavethebestresult s.Samplesvar- ying from alowsilicacontent of 0.33%to a high of 16.57% Si0₂were tested.Thelow-silicasamples,afterselect ive calci- nation, producedprecipitatesassaying morethan 94%MgO at recoveriesover64% of theavailable magnesia,leaving

Fig.4.Bruciteasa seriesof concentricswirls of foliatedaggregates. The bluish/blackenclosedgrains are fragments of unalteredforsterite.Pho- tomicrographundercrossednicols.

resid u es contai ni ng more than 75%CaO.The resu lt s from the test in gwit h selectivecalcination an d leach ingare given inTable7 and a flowsheetfo rthe process isshownin Fig.4.

o

NGU -BULL 436,2000-PAGE82

2mm

ODD0VERENG

Direct leaching

TheInstitu t e for sil icateand high-temperaturechemistryat NTNU/SINTEFcarriedou ta leachingtest on a sample of rep- resentative brucite ore.The test procedure was a modified versio nof the so-called Sulmag-proces sand resu lted in150 kg bru cite perto nn e of ore. The raw mater ialhad acontent of bruc it eofabout15 wt.%(M o n sen&Selt veit 1984).

Gravity separation

Basic Ine. Gabbs, Nevada, USA used a patented sink-f loat methodwithheavy liquid media(m et hylen e brom ide,2.48 kq/drrr' ).Th e resultwasa concentratewith 93.3wt.%of bru- cit e (Jep sen 1981).Tests carried out at Purdue Unive rsit y, USA,with Wolframat dissolved inwate r as mediu m,gave a concen tr at e of85wt%bru cit ewit h a reco ver yof about80% (0 v ere ng 1987a).

Selective flocculation

Bruciteisa very brittlemineraland,asa result, itwill usually be enrichedin the finer fractionsin thecrushingandmilling pro cess.Preli m in arytest s indicated that about 40%of the bru cit e of the feed ore goes into the fines (less than 30-50 urn)du ring the crushing and mill ing processandcan n o t be treated by thetrad it io n al separation methods.Bruc ite ore from the Granasen deposit has been tested by trad itiona l sep arat io n met h ods withoutobtainingasat isfact o ry bruc ite co n ce nt rat e.

At Purdu e Universi ty,USA,the late Prof. Kull er ud and his groupdevelo p ed a separation method which is very effi cien t forfiner grain fractions.The group had earlierused th istech - niq u eto sepa ratealunite from quartzin alunite ore.Results sho w thatthe techniqueis very effic ientfor particlesize<40 pm,The methodofselec tive f1occulat ion is also dependent onan efficienttechniqueto separate the bruc it efloes fro m the ot hermineralsin the slurry.Research wo rk hasbeen car- riedoutat Pu rd u e University,Indiana,USA,and a flow sheet using th e new techniquewas dev eloped .Testing at bench scale gave acon cent rat ewith",87wt.%brucite atarecove ry

of",76%(0 v eren g 1987b)These resu lt salso in dica te d that if

Table 7.Results fromthetest wit hselective calcination andleaching.Brucitecontentisdeterm inedbythePhenfield method.

Sample no. Total Total Residue Precipitated MgO Rawmaterials

%MgO %Si0₂ %CaO %MgO %Recovery %Brucite

G-3 22.0 0.33 89.98 98.18 85.49 2.5

G-10 22.2 0.53 88.38 97.80 86.90 1.1

G-4 23.3 0.72 94.26 97.1 6 94.60 23.95

G-2 23.0 1.68 75.08 94.58 64.12 16.23

G-9 23.5 3.43 74.18 91.78 70.84 11.79

G-1 20.9 4.22 64.99 92.47 31.84 23.3

G-7 24.3 7.90 67.63 89.18 59.72 15.13

G-5 22.6 8.37 70.83 85.53 60.21 13.32

G-6 23.1 13.30 59.81 84.91 45.35 6.61

G-8 21.9 16.57 56.99 74.45 22.67 7.91

0000VERENG

Tabl e 8.Physicaldataof dead -burn edsamplestested at

Do lomittwerkeGmbH,WOlfr ath,Germa nyan dSINTEF,Trondh eim .

Dolom it w erke SINTEF

Density,g/cm^{3 2.86 -}

Volumeweight,g/cm^{3 2.78 2.79- 2.84}

Openporosity,% 2.80 1.24-1.90

Totalporosity,% 2.80 -

Tab le 9.Physical datafrom acalcin at ion/dead -burnedtestof sam p lesof puredol omitemarbl e.

Burned

1650°C Burned,2h. oxidising

at 1800^0C at m.

Split Briquet-

Briqu ettes Split tes

5-10 0=50mm 5-10 mm 0=10

mm mm

Density, g/cm^{3 3.43} - ^-

Volumeweight, 2.08 2.83 3.05 3.18

g/cm³

Openporosity,% 39.0 15.8 9.2 2.2

Totalporosity,% 39.4 17.5 11.1 7.3

not developed

Tabl e 10.Volum e weightand po ro sit y ofdol omitemar b le,sintered in Ar-atrn .at 20 00QC fordifferentlengthsof time,in minutes (SINTEF).

Sinteredat 2000^0C Volu m e weig hting/cm³ Open porosity

30 min. 2.89 13.6

2.78 16.4

60min. 3.01 10.3

3.05 10.7

120 min. 3.01 10.9

3.05 9.5

180 min. 3.11 8.2

3.00 11.4

Tabl e11.Repr esentat iv ereflectancevalu esforthepuredolomite marbl e.

Filter Collectsample*

R457Tappi(Brightness) 92.3

FMXlCRed(Amber) 93.7

FMY/CGreen 93.3

FMZ/C Blue 92.3

R46 1.50

*fractions

<

NGU-BULL436,2000- PAGE83

eachste p in the flowsheet could be opt imised, the process could sti ll beimproved.

Product development

In 1975,a co-operat iveeffo rt betweenNGUand SINTEFwas initiatedto evaluatetechnic al aspect s for theuseof Norwe- giandolom itemarblein theproduction of basicrefractor ies.

Dolomitemarbl efrom anumberoflocalitieswas tested for thispurpose.Thedolomitemarblefrom Granasen,tested at SINTEF, gaveexcelle nt results(Seltveit et al. 1977).Material from Granasenwasalsosent to one of theleading producers of refractor ybrick,Dolom it werkeGmbH,Wulfr ath,Germ any.

Result swere comparableto those obtai nedat SINTEF(Table 8).Thisenco uragedanaddi t io nalcore-d rilling progr amm e in 1978.Materialfrom thecoredrilling was tested by SINTEF.

These testsconfirmed thepreviou sresult s(0 vereng 1977, 1978).Resultsfrom different testsindicatethatthedolomi te marbl e is suita b le asa raw materialforthe produ ction of basicrefractories (Seltveit etal. 1977).

Melted dolomite

Theinterest10 usemelteddolomit emarble is growinganddif- ferent institution s and com panies have tested Norw egian dolomitemarbl e for production ofmelted dolom it e. Results from different tests indicate that dol omite marbl e from Granasen may be of inte rest for theproducti on of melt ed dolomite (Tables9,10).

Filler/ coating

Dolomitemarble for fillersandcoati ng isagrowing market and high-qualitydolomite marblefromtheGranasendeposit should be of interest for thismarket.Chemi calanalysis and othertestdataindicatethat asignificant portion of the dolo- mite marble fulfils criteria for the mineral filler industry.

Brightnessisanim portant parameterof filler/coatingmateri- alsand averagevaluesof a repr esentative samp le from the central partof the deposit aregiven in Tabl e 11.

Conclusions

The Granasen deposit contai nsaround 100 M tonnes of high-qualitydolomite marbleand 10-15M tonnes of brucit e- bearing dolom ite contain ing around 17wt .%brucit e.The brucite mineralisationoccursin the contactaureole to a gab- bromassifand formed bycontact met amo rphismassociated wit h the intrusion.The result sfrom different testsindicate that the existing mineralresources at theGranasendeposit canpotentially beused inalargerang eof products.Puredol- omite marbl efrom theGranasendeposit seems to be a suit- able raw materialfor theproduction of magnesium met al, basic refractories, glass, filler, insulation, variou s build ing materials,mineralwool,special cements, and alsoin various ap plicat io ns inagricult uraland environmentaluses.

A variety of different separat ion techniqueshavebeen testedfor theproduction of brucite concentrate.Themost promising methodsgavethe following result s:

NGU -BU LL436,2000 -PAGE84

Selecti veflocculation:brucite conce ntra te(brucite87 wt.

%).recovery76%.

Gravity separatio n: bruciteconcent rate (brucite 85 wt.

%).recovery 80%.

Leaching (Sulm ag): brucite concentrate: '50 kg/t o nne (raw ore' 5wt.%bruci te).

Brucitecon centratesfrom Granasenwouldbeespeciall y suita ble fortheprodu ct ionof high -qu alit ybasicrefracto- ries,MgO,Mg-metal and different fillers.The deposit of Granasen is therefore a min eral reso urce with great pot ential.Future investigat ionsofthedeposit needto be focused on atotal evaluation ofit s possib ilit ies.

Acknowledgements

Thein vesti g at ion programmehasbeen carr iedoutasajointproject be- tw eenNor ges geologiskeundersokelseand Vefsn UtbyggingsselskapN S,Mosjoen.Selectiveflocculatinq testshave been carriedout at Purdue University, Indiana, USA.The authorexpresseshisthankstoth e lat eProf.

G. Kullerud and hisstaffwho introduced himtothat technique.Thanks are also duetomy colleagues,TA Karlsen and thelat e BA Sturt forthei r valuable criticismand advice in a earlyst age of preparing this manu- scrip tand to NigelJ.Coo kfo r correctingthe manuscriptandim p rovi ng theEng li sh language.David Harrison(BGS)andYryo Pekkola(GSF)are than ked fortheir helpfulrevie w s.

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