Canadian Mineralogist Vol. 17, pp.287-298 (1979)

THE MINERALIZATION OF THE RANA MAFIC INTRUSION, ,

ROGNVALD BOYD ANDCARL O. MATHIESEN Norges Geologiske Unders/kelse, Postboks 3006, 7001 , Norway

ABSTRACT d6formation, les diss6minationsinterstitielles passent i des sulfures massifs remobilis6s dont la teneur Le gisement possbdedes r6serves The R&na synorogenic Caledonide intrusion in atteint 5Vo Ni. 100t de sulfuresd 0.33/o Ni. 0.087oOu et north Norway contains one known nickel deposit de 43 x e O.0lSVo Co. L'intrusion comprend uno zone de of significaoce, Bruvann. The mineralization consists lits et des lentilles de p6ridotite et of pyrrhotite*pentlandite+chalcopyrite*. norito avec des oc- pyrox6nite, autour d'un noyau de norite quart- curing interstitially to and orthopyroxene in de zifdre. Stratieraphiquement,elle ressemble gtosso peridotite and grading up to O.8Vo sulfide nickel. modo d nombre de complexes stratiformeg mais Locally, associated with certain deformation zones, en beaucoup d'endroits, les structures primaires ont interstitial sulfide dissemination passes into massive 6t6 d6rang6espar les plissementset les chovauche- mobilized sulfide with ap to 5/o nickel. Reserves ments Cal6doniens.Des lambeaux de gueiss semi- are 43 million metric tons with 0.337o sulfide p6litiques I calc-silicates et des schistesnoirs pris nickel, 0.087o and approximately O.|lSVo dans ces plissements,ont 6t6 enfouis au sein de cobalt. peripheral Tho RAna intrusion consists of a I'intrusion. Le massif se pr6sente sous forme d'un zone of norite containing bands and lenses of c6ne renvers6,peut-Otre tronqu6, dont I'axe s'enfon- peridotite pyroxenite, and and a core mainly of ce l6gdrement vers le nord-ouest. Liintrusion do quartz norite. The R&na mass thus has a gross RAna n'est pas facile i classifier dans les sch6mas s8atigraphy that conforms to the pattern of many courants. ks p6ridotites De motrtrent aucune varia- layered intrusions, but, over much of the intrusion, tion min6ralogique 6vidente, ni en sulfures ni en primary structures have been disturbed by the later silicates, perpendiculairementau [tage. Irs schistes phases Caledonian fold which also involved local noirs i sulfures observ6sau contact de I'intrusion overthrusting; these movements resulted in an in- ou i I'int6rieur de celle-ci le long de zones de folding and thrusting of units of semipelitic and cisaillement, ainsi que le graphite d6cel6 dans let calc-silicato gneiss and black schist into the in- diss6minationsde sulfures, semblent indiquer une tnrsion. The My has the form of an inverted, assimilation de soufre des roches encaissantes. Dossibly truncated cone with its axis plunging north- D'aprds une 6tude des isotopes du soufre, pareil westward at a moderate angle. There are problems apport do soufre d'une sour@ externe n'a 6t6 gu'un in placing the Rina intrusion within existing classi- ph6nomdneloca.l insignifiant. peridotites fications of mafic intrusions. The show par la Rddaction) tro obvious systematic variation of sulfide or si.licate Cfraduit mineralogy across strike. The presence of sulfide- bearing black schists on or close to ttre contacts INtt'ooucrroN of the intrusion and emplaced within it along shear zones and the occurrence of graphite within sulfide disseminated in peridotite suggest assimila- The RAna mafic intrusion lies at approxi- tion of sulfur from the country rocks. Sulfur mately 68o30t{, 20 km southwest of the - isotope studies do not, however, offer confirmation ore port of in (Fig. 1). of the hypothesis that atr external source of sulfur This paper will give a brief description of the has had more than very local siguificance at R6na. general geology of the complex, with a more detailed consideration of the sulfide-bearing area (Fig. Sourvrenr at Bruvann 2), the only known major deposit in the intrusion, and of the genetic im- plications involved. The information presented L'intrusion synorog6niqueCal6donienne de R6na here results from an investigation of the Rina (Norvdge septentrionale) ne contient qu'un seul mass and its nickel mineralization begun in girement nickelifBre important, celui de Bruvann. 1971, The investigation has been financed by a Ics sulfures pyrrhotine + pentlandite 4 chalco- by pyrite + pyrite, associ6sinterstitiellement b l'olivine steel company, Stavanger Staal A/S, and the et lorthopyroxbne de la p€ridotite, contiennent jus- Norwegian state, and carried out mainly by qu'd, 0,8Vo de nickel. Dans certaines zones de the Geological Survey of Norway. 287

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genic belt, in two- gneisses classified as the Narvik Group (Gustavson 1972 afr. eadier papers). These gneisses include thin units of variably hornfelsed calc-silicate gneiss up to several hundred m thick and commonly occur- ring on or slose to the outer contact of the in- trusion. These units consist mainly of plagro- clase * clinozoisite * hornblende { sphene S -F diopside f microcline { garnet quartz:! sar- bonates, and also contain bands of black schist rich in graphite and pyrrhotite. The country rocks have been affected by at least four phases of folding inchding nappe emplacement at an early stage. The RAna in- trusion appears to have been emplaced before the third phase of folding under garnet-am- phibolite facies conditions that prevailed through- out the solidification ,period of the mass. Tliis is indicated by the plesence of orthopyroxenet amphibole*amphibole/spinel symplectite coro- uas in olivine-plagioclase bearing rocks of the intrusion and by the assemblage garnet*horn- blende*plagioslase in rorckswithin shear zones in the norite. Roddick (L977) dated the in- trusion at 4OOm.y. on the basis of Rb/Sr whole rock and mineral isochrons; tle country rocks give a metamorphic age of 4@'t-16 m.y. Mafic and ultramafic rocks Fro. 1. Map of Norway showing the location of the RAna intrusion. The surface geology of the intrusion has a crudely concentric arran'gement, with a peri- The intrusion has .been described by Foslie pheral zone of norite and a core of quartz (1921), and the general geology of the sur- norite, both,locally amphibole-bearing and both rounding area by Gustavson (1972),:, see abo locally gabbroic, particularly the quartz norite. references therein. The area lies within tle re- The norite zone contains irregrlar bands and cently published 1 :25Q000 geological map-sheet lenses of peridotite (the main host-rock for the Narvik (Gustavson L974). The mass contains nickel .mineralization at Bruvann) and pyroxe- the only nickel deposits discovered in the nite. These rocks are not evenly disposed around Norwegian Caledonides with;n u radius of 100 the peripheral zone but, where common, tend to km, though the region does contain many sub- occur towards the outer margin of the mass. At economic deposits of Cu, Fb, 7s, and Fe. least locally the norite can be shoqrn to intrude The Bruvann deposit is reetricted to an area the ultramafic rocks; whether this is due to of approximately half a square km near the synmagmatic disturbance or to emplacement northwestern periphery of tle intrusion, where in separate magma pulses is not yet knorrn. it outcrops between 40O and 5@ m above sea South of the main intrusion are two outliers, one level (Figs. 2,3), A total of 7,00Om had beeu on Tverrfjell east of RAndal, and the other on drilled in tlis area up to 1960. The present the north side of Kvan6.kertind west of the investigation, in addition to geological and geo- (Fig. 2). The Tverrfjell outlier is the physical matrrping, has entailed 28,500 m of only part of the intrusion in which regular band- drining. ing on scales below 1@ m is common; these structures, wh.ich include cross- and graded bedding, are always 'right way up', with the GSNSRAL Gror.ooy oF THE fNtx,ustoN exception of a few dubious cases. Elsevrhere in aND rrs BxvrnoNs the iutrusion banding other than the gross zona- tion already Country rock described is rare. The northern and northwestern contacts of The intrusion lies within the Caledonian oro- the intrusions generally dip outward at moderate

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cAcL(srr'|(Art E i;fl't?ifi[.$9[fi'n''a,'., llllll Hffir';P*ii.i*t*,^..,, llffi crrrss : ffiffij ironrTt. GABBRo AilPrrBofrrr neoscntsr ESS]igi ISIRONGLYOEFORIIED LOCATIY} lii]l]l eeorerrrt lT::-l FIIII

----- col{lA(l tAPPnox[{ArEt A FAULT Frc. 2. Geologic map of tle Rina intrusion.

to steep angles, whereas the southern and eastern the complex and often contrasting nature of contacts dip inward under the mass at angles the Rina strustures and because of published that are shallorp to moderate in the east and reseryations on the classical cumulus theory southeast. The sontact between the norite peri- (Ca;mpbell 1978), judgnent is reserved as to phery and the quartz-norite core is distinct only the exact processes iovolved in producing the where it is tectonic, in which case its dip is internal variations of the RAna body. usually similar to that of the outer contact. The most @tnmon variety of peridotite foand Ifowever, in the eastern part of the intrusion in the intrusion is harzburgite with 4V6OTo the nority'quartz-norite contact dips outward. olivine (Foee-ezbased on nine microprobe anal- The geometry of the contacts and .gravity data yses of samples from, Bruvann), though lher- (Sindre & Boyd 1977) suggest that the intrusion zofitic and dunitis varieties also ocour; localln has the approximate form of an inverted, possi- small a,rnounts of intentitial plagioclase may be bly truncated cone with an axis plunging north- present. trn the usage of Wager et oJ. (1960), westward and with exposure nearing the roof the bulk of the peridotite is olivine ortho- or in the east. The distribution of rock types within mesocumulate, though olivine-hypersthene or- this framework and the nature of the magoatic thocumulate and locally olivine adcumulate are banding, where not affected by later deforma- present. Poikilitic intercumulus ortrhopyroxene, tion, suggest a gr6s stratigraphy similar to that less commonly augite and phlogopite, and rarely of many wElldesc,ribed layered intrusions em- plagioclase are present. The remaining primary placed in cratonic environments. Because of phases,in addition to the sulfides described later,

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Fro. 3. Geological map of the northwestern part of the R&na intrusion.

are picotite spinel, hernatite, ilmenite and mag- plagioclase heteradcumulate (adcumulus plagro- netite, all in small amounts. Where olivine has clase), commonly rrith strong larnination of the originally abutted against plagioclase, coro{tas oumulus laths of orthopyroxene and plagioolase. are developed as mentioned earlier. Olivine The hypersthene may be zoned, and only rarely shows a greater tendency to idiomorphism contains exsolved lamellae of clinopyroxene. against sulfides than against silicates; being Hypersthene and plagioclase may contain inclu- ofteu rounded and internally corroded in the sions, one of the other. Clinopyroxene, where latter case. Talc and anthophyllite are common present, has a mode of occurrence sirnilar to in deformation zones. that of hypersthene. Primary amphibole is a T\e pyroxenite is generally an ortho- or meso- feature of some of tle norites in certain areas. cumulate with up to about 80% hypersthene; Secondary amphibole and clinozoisite are com- adcu'nulate textures occur locally. The inter- mon, and, where deformed, the rock is generally sumulus material consists of augite, phlogopite, recrystallized to amphibolesfplagioclase*clino- plagioclase and ocsasionally, opaq.ue minerals. zoisite*mica. Where olivine-bearing, the norite Olivine-bearing varieties occur, but usually as a may contain ss6s disseminatednickeliferous sul- transition to peridotite. The hypersthene shows fide. Olivine-free norite may contain some sul- slight marginal zoning and may contain augite fide, but essentially nickel-free. Locally non- exsolution lamellae. nickeliferous sulfide @curs together with gra- Norite is used here to cover a wide range phite as small blebs or streaks in deformed of rock Epes, including limited amounts of norite. both primary olivine-bearing and gabbroic T1ne quartz-norite core of the intrusion ap- varieties. Alteration, deformation and recrys- pears to be less well-defined than suggested by tallization impose further complexities. The Foslie (1921), and to be more variable inter- rock is, where fres\ generally a hypersthene- nally. Plagioclase is more abundant than in

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the norite, and is commonly idiomorphic against of Bruvann and south of Sepmolfjell and the quartz, present in only small amounts. Clino- development of the Tverrfjell synform seem to pyroxene increasesrelative to orthopyroxene, and date from this period. The climax of this phase primary a,mphibole and biotite are present in of defor'mation involved disruption of the mass addition to rare orthoclase. Superimpoced on along two major zones: (1) The overthrusting these variations are the effects of deformation of the part of the intrusion known as the and recrystallization, especially marked in the Arneshesten block northwest of a line from Bru- area of Eiterdal. vann to R0,na (Fig. 3) torrards the southeasl The thrust is defined by a strongly deformed Srnucruner, Gsolocy arcuate zone that dips steeply northwest and becomes less steep at deeper levels. The Arnes- Though the RA,na intrusion may postdate tle hesten block has a complex internal structure stongest phases of regional deformation, the due to folding and faulting, some of which is effects of the later phases have locally been related to the overthrusting. In addition to very pronounced. This applies particularly to the disrupting the primary structures of the block, area north of a line from Bruvann to Saltvik- these movements resutrted in the emplacement vann and to a belt from Kvan&kertind to Eiterdal within the block of lenses and slabs of country (Fig. 2). Deformation in these areas, corrc rock including black schist (Figs. 3, 4). Relative sponding to the regional F movements, probably to most other parts of .the norite periphery, this began with the development of segregation band- area is distinguished by its higb proportion of ing in norite/.metanorite, with subsequent folding ultramafic rock to norite and by the presence of the bands. Folding of the contact southeast of large volumes of peridotite with nickel-bear-

L_9.c9!g @ ourin" beoringmck3, mointt p.ridotit€ ffil Py.or"nttr ffiffi lorita srophifo/sulridebeorinq mrya .c|s|r,llll ! ni"tg-r of slfids nirket7o t4.0mm '1%l --- lnhrprrtqtin gaotogicqtboundory L Driuiol.

gllt t:S

coordinatrs in natnrs Fra. 4. Geological interpretation of profile 2600 E in the Bruvann area; tle coordinates refer to tho system also used in Figures 2 & 3.

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ing sulfide dissemination. This is not surprising vine norite. A simplified stratigraphy for the in an upthrust block, if the model of the intru- eastern area is, from top to bottom: 5O-1@ m sion suggested earlier is correct. (2) The south- sf nn,minslalized peridotite with thin, erratic ern part of the intrusion was affected by the mineralized zones and pyroxenite bands; 2G-50 formation of synclinal structures both east and m of mineralized peridotite (the main zone) west of RAndal, those at Tverrfjell and north of qriffo thin pyroxenite bands; O-2@ m of unmine- Kvan0,kertind becoming separated from the rest ralized peridoti.te, G-70 m of pyroxenite and of the intrusion by deJormation zones contain- 150 m of norite (to explored depth). ing bands of country rock. Much of the area is complicated by deforma- The Fa trfiase of folding seen in the sur- tion zones and by bodies of other rock types rounding gneissesseems to correspond to north- including gneiss, whose geometric relationships south faults common in well-exposed areas on are often not amenable to interpretation (Fig. the south side of the mass, and also on Sep- 4). The soutlern part of the area is more com- molfjell and in Arneshesten block. Subhori- plex; mineralized horizons weaken and disappear zontal pegmatites emplaced late in the history and the peridoti,te units finger out and pass into of the mass occur in many parts of the intrusion. generally noritic rocks of rapidly and irregutrar- Iy varying character. Close Ons Gsor.ocy to the base of the mineralized perido- tite in this area occur acoumulations of mas$ive The Bruvann area epigenetic sulfide with up to SVo nickel. This type of mineralization, first found at Bruvann The deposit contains a calculated 43.6 milliou r\ 1974, is now known from several localities metric tons of sulfide-bearing peridotite aver- along an east-west line (125O N) close to the aging O.337o sulfide nickel, O.O8Vocopper and southern contact of the deposit. These inter- ca. O.O1,SVocobalt (cutoff was taken as O.LSVo sections usually have in colnmon a proxirnity to sulfide nickel). Restricted parts of ,this tonnage peridotite, or sther olivine-bearing rocks with average as much as O.6Vo, and individual anal- disseminated sulfide (Fig. 4), and location in yses reach O.8Vo. \\ese figures are based on zones of deforma.tion. analysis of the sulfide content (bromine soluble) West of the fault that dissects the Bruv,ann of 2 m core sections. The deposit has approxi- area (Fig. 3) the situation is more com,trrlex. mate dimensionsof 9@ m east-west and 500 m This part of the deposit has a gTeater tonnage north-south. In the middle of the area it is cut than the eastern area. It is nowhere exposed and by a northeast-southwest hinge fault (down- was not discovered!\ttl 1972. The western area throw to the west), the tlrow of which in- contains two major mineralized peridotite units creasesnortheastward from about.200 m close that lie rather fla,t in the south and genemlly in- to Bruvann. crease in dip to the northwest. The present strati- East of this fault the peridotite is underlain graphy, partly the result of complex deforma- by pyroxenite which in turn lies on norite. The tion, is, in simplified form, from top to bottom: base of the peridotite is flat lying in north- 50-250 m of complex, irregularly interbanded south section and rises to the east. In the north norite, peridotite and pyroxenite; 5O-10O rn of the peridotite stops abruptly along a steep east- upper mineralized peridotite with thin pyroxe- west contact against a ridge of norite, and it is nite bands; 40-100 m of interbanded peridoti,te cyt off in the east by faulting with upthrow to and pyroxenite with sub'sidiary norite; 20-100 the east. Thus, the norite underlying the Bru- m of lower mineralized peridotite; 6O-120 m vann peridotite is thought to be equivalent to the of unmineralized peridotite; 0*50 m of pyroxe- largely noritic rocks on Arneshesten.The main nite, and norite (to depth drilled, approximately mineralized zone, which lies in peridotite, strikes 400 m below sea level). east-west parallel to the rock contasts. Where The western area has several features in com- deformation is leas! as in profile 26@E (Fig. mon witl the eastern area. fn both cases the 4), it has a dip of about 45o near the surface, peridotites terminate abrupdy to the north flattening out in depth towards the sourth.Above against a ridge of norite (under coun'try rock this main zone occurs a series of thinner. less in the western area). In both casesthe dissemi- continuous zones of mineralization. pvroxenite nation tends to be richer and more homogeneous interbanded with mineralized peridotite com- in tle north and weaker or absent in the south. monly contains interstitial sulfide of lower grade The major minsl6li2sd peridotite in the eastern than the average for the minera:lized peridbtite. part of the Bruvann area and the lower mine- Spane mineralization also ocsurs locallv in oli- ralized peridotite in the west were probably

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originaily continuous; the upper peridotite in Other qeas of mineralization the west may be a tectonic repetition of the Other areas of the intrusion contain rnine- lower one. ralization of lesser significance. In Rinbogen Typical sulfide dissemination in the peridg- (Figs. 2, 3), sulfides are present in smaller titesit Bruvann contains pyrrhotite (SV7OVo), amounts as blebs and as interstitial dissemina- pentlandite (10-35Vo), chalcopyrite (5-l5Vo\ tion in peridotite, as interstitial dissemination and.minute amounts of pyrite, 1O5Vo according in norite and as massive sulfide wi,th graphite to a detailed point-count analysis by Malvik in dilations along shear zones. Common to all (t977). The sulfide minslalegy of olivine norite these mineralizations are low Ni:S ratios. and pyroxenite in the area is similar. The mine- The Eiterdal deposit (Fig. 2) contains mine' ralization occurs discontinuously and intersti- ralization in olivine norite along the underlying tially to olivine and hypersthene but, in richer in con- 'net' sheared contact of the intrusion, here portions, has tle texture described by tact with calc-silicate rock containing black Naldrett (1973). Pentlandite ocsurs mainly as schist. The nsrils ssafa'ins lenses of country discrste, irregular grains close to or at the rock and several types of mineralization includ- margins of pyrrhotite and in cracks tlrough 'blebt 'Flanes' ing sulfide and interstitial mineralization pyrrhotite grains. and lamellae of transitional into more massive sulfide. pentlandite are present in the pyrrhotite near fractures and associated with grain boundaries, but only in minor amounts, of the order of lTo ANer,Yrrcer, oere' of the total pentlandite (Malvik t977). Chalco- grains to pyrite occurs mainly as free marginal Composition of sulfid.e minerals pyrrhotite, but also as crack fi[ings cutting across other sulfides and locally penetra'ting Preliminary microprobe determinations sug- adjacent silicates. Pyrite forms isolated idio- gest that the pentlandites have compositions the morphic grains in pynhotite. Small amounts of fa[ing within the range characteristic for magnetite ocsur, coulmonly near the margins assemlhge pyrrhotite*pentlandite+chalcopy- & of olivine grains; intergtown hematite and il- ritelpyrite (Harris & Nickel 1972, Graterol pyr' menite occur interstitially. In cornmon with Naldrett L97I). Pentlandite'flarnes' in the than Ramdohr (1969), the writers have found minor rhotite contain several percent less nickel cobalt. amounts of graphite within sulfide disseminated discrete grains and also considerably less in peridotite. The microprobe was also used to search for As noted eatlier, accumulations of massive variations in olivine or sulfide compositions grain no ore occur locally along an east-west-trending relative to olivine-sulfide boundaries; and the tex' zone 5O-100 m north of the southern border such variations were found. This greater for idio- of the Bruvann area, and their precise location tural evidence (a tendency than is apparently to some extent testonicatlV c9n- morphism in olivine against sulfide Sgatlst sulfurization trolled. In most cases, this mineralization lies othei silicates) suggest that direct 1965, Naldrett beneath interstitial sulfide in peridotite and of olivine (Kullerud & Yoder important process above a zone of rock with variable amounts of 1966, 1969) has not been an peridotites. sulfide and fragments of altered, foliated mafic in the RA.na in perido- rock. In certain drillhols the massive sulfide The Ni:S ratio for disseminations 1:2 downward but may be several rn thick; in others it has been tite at Bruvann varies from and 1:5-6, with a injeoted as veins from a ferr cm to several dm is olustered between l:2 This is typical of across. The massive ore seems to have ac- median at approximately 1:4. deposits in perido- cumulated subsequent to the solidification of the values for nickel sulfide (1959). The the peridotite and along a 7'ane of disruption tites given by Wilson & Anderson ratio of less than betwlen the more homogeneous thicknesses of massii" sulfide has a Ni:S Bruvann, O.2, peridotite to the north and the more variable 1:5. The Cu/(Cu+Ni) ratio at in deposits in ultra- rock types to the south. The presence of the lies within the range found 1976, Rajamani & sulfide-rich sheared rock beneath the massive ma,fic rocls (Naldrett & Cabri of depoaits for mineralization may reflect the accumulation of Naldrett 1978). The majority cited as charactet- the massive snrlfide in dilations within or ir- which s,uch metal ratios are that as a whole regularities on a deformation zone. Note that istic occur in host complexes the Rtna mass this nni"eraliza,tion has been intersected at three are much more mafic than Manitoba belts) ' localities only and that these differ to an extent (e.g.,' deposits of the Abitibi and seelns to be that makes generalization difficult. bitt"itt metal:sulfur ratios

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sharacteristic of parts different of the intrusion. of sulfur for the sulfides in certain mafic in- At Bruvann, where the dominant mineralization !Tyy" complexes (Godlevskii & Grinenko 1963, is in_peridotite,the Ni:S ratio is 1:4; in Eiterdal, Jlafdrett 1966, Liebenberg 1969, Mainwaring & in olivine norite it is 1:10 and in RA.nbogen, Naldrett 1977). c&ere the mineralization is predominantly in Table I shorps the results of sulfur isotope norite, the ratio is l:14 or less. A similar corre- analyses,perforrned on R&na samples. On the 1,"9o:.*E tock type has been shown by Hiikli assumption tlat investigations of the origin of (197L). This trend sugg$ts that the'sulfide sulfur in ore deposits should be based on the in 9agma occurring various parts of the intru- sulfur- isotope ratio for the total sulfide content, sion attained- a degree of local homogeneitn not that of the individual minerals (Rye & the composition being dependent at liast in Ohmoto 1974), the sulfide minerals weri not part on the nature of the silicate magma present. separated individually. The samples consisted o! Sulfur isotope study -mole than 95% pure bulk suftiae fractious, with the- rnineralogy for individual rock types The presence - of small amounts of graphite as described earlier. The writers have not found in various rocks in the intrusion, the juitaposi- published sulfur isotope data for separated tion of sulfide-rich black schist with mineraiized pentlandite, but data on separated mineral frac- peridotite and massive sulfide, and variable Ni:S tions of coexist:ng pyrite, pyrrhotite and chal_ ratios led the writers to suspect an external 1974, Rye & Ohmoto 1974, source for llPYnte .(MltceH some of the sulfur in the deposits. Mainwaring & Naldrett L977, and others) sug- Graphite has been described from otheruufia"- gest that the degree of isotope fractionation bearing mafic intrusive 6odies: the Bushveld expected ig this paragenesis and at high temper- co-nplel _(Uebenberg L970), Kotalahti (Haa- atures of deposi,tion is very limited. In addition, pala 1969), Hitura (papunen 1970), the the changes in sulfur isotope ratio so far de- Harriman and Warren Aeposits in Maine'(Ra,in- monstrated as due to wal.lrock contamination ville & ,Park 1976) and from the Watei Hen 1.969,Godlevskii & Grinenko 1963, in _(Liebenberg lntrusion the Duluth complex (Mainwaring & Mainwaring & Naldrett 1977) are almost an Naldrett 1977). fn a number of these cie" order of magnitude greater than those which country-rock xenoliths are prominent. Sulfur would be expeoted from isotope fractionation. sS/ i19t9ne studies based on the-difference in results for samplesfrom R6na peridotites sS between .-The sulfur in blask schists and other (Table 1) resem-ble those cited bv Stanton metasedimentary rocks aud sulfur from normal (L972) for a number of deposits in mafic and uncontaminated mantle-derived magma have ultramafic rocks and interpreted as indicating been used to indicate a partly exteria{ ,orrr"" uncontaminated magma from a deep-seated s_ource.. .I. This applies equally to the samples of IABU SIJI.II.IARYoF SULFURIsoToPE DATA disseminationin norite; the samplesfrom-pyrox- enite and fineral ization type l,lo. of Ave. massive sulfide contain ,trgnUy samples o34s Range of 6345 heavier sulfur but the difference is small and Perl does not seem related to variations in pro- dotite dlsseminstlon ll + 0.3 -2.5 to +'1.6 the portions of sulfide miuerals. Pyroxenlte dlssemination I r 2.7 The sulfur in the bands of black schist, both Nor'lte dlssenlnatlon -0.1 6 r 0.7 to + 1.6 within and outside the intrusion, is on the aver- florlte d.lssemlnatlonrlth markedly lighter than meteoritic sulfur, aud graphite - lge 5.9 -3.7 to - 7.6 is clearly lighter than the values obtained fo, Uassive sulfide l0 ! 2.1 r0.4 to + 4.2 disseminated sulfide in peridotite. The values Uasslv€ sulfid€ rlth fall within the range cited by Stanron (Lg7Z) graphl te -12.9 -3.2 to -17.6 for ancient sedimentary rockJ with no volcanic Sulflde from blaclschlst component. These resul,ts suggest that external rlthln the lntfusion - 7,4 -3.6 to -13.6 sulfur was not significant in the formation of Sulfida from blackschlst mineralization in the peridotite at Bruvann, outslde the.lntruslon 7 - 9.0 +1.0 to -13.5 or in peridotites elsewhere in the intrusion. Onlv very locally (and only in samples in which gra- The analyses r,ere performed by GeochronLaboratorles phite is clearly visible megasCopically) has-ex- lnc., Boston. 634s i. rua.rr"d relatlve to 345732, ternal sulfur had an obvious influence. A corol- for CaftonDlablo trott.lte. lary of this conclusion is that some of the gra- phite (e.9., the flakes of graphite found in interstitial sulfide in peridotite) must clearly

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be of an early or primary magmatic nature, the peridotite bodies lends support to this whereas locally it has probably been derived model: they are not common on the eastern from the country rock with some sulfur. rim but are mostly found'along the northern and northwestern contact. Further confirmation of this model is given by the nature of the DrscussroN Arneshesten block which, having been upthrust represents the deepest level exposed in the mass A slassification of ultramafic and associated and contains not only peridotites of substantial mafic rocks was proposed by Naldrett & Gas- thickness, but also the only ones in the intrusion parrini (L97I) and later modified (Naldrett known to be significantly mineralized over large 1973, Naldrett & Cabri L976). All three papen volumes. The individual peridotites, however, emphasize the implications of the classification may be subparallel to,the outer contact (eastern for metal deposits (especially of nickel) asso. R&nbogen,TVerrfjell) (Fig. 2) or discordant to ciated wi,th these rocks. With more general it (western RAnbogen, Bruvann) (Figs. 3, 4)' geological considerations in view, ctrassifications some bodies apparently changrng along their of mafic and ultramafic rocks have been pub- tengfh fro,rn concordant to discordant. The lished by Jackson & Thayer (1972), Thayer & writers have failed to construct a model ac- Jackson (L972), Moores (1973) and Wyllie counting for all these complexities; however, (1969). The RA,na intrusion and a number of these must be related to the extensive syn- and others in the Caledonides, including those of postintrusive deformation of the area. the Seiland province (Robins & Gardner 1974), The nickeliferous sulfides at Bruvann (and Ireland (Leake 1970, Kanaris-Sotiriou & Angus elsewhere at R&na) are invariably within or L976) and northeast Scotland (Ashcroft & closely associated with olivine-bearing rocks. Munro 1978, Boyd & Munro L978), do not However, the mineralization does not occur fall easily into any of the published classifica- systematically at or near the u,pper or lower tions. This problem and many of ,the difficulties margins of the peridotites. In addition, the in interpreting the Rlna mass mineraliza- rnassive nickel-bearing mineralizatron is gener- and its 'nef tion arise from the fact that, though it has ally close to texture sulfide, bu,t it consti- many of the features of the stratiform or funnel- tutis only a small fraction of the total minerali- shaped intrusions that occur in stable areas, it zation. Thus it seems likely that the magma lies in an orogenic belt and also, at least locally, ' from which the peridotites began crystallizing has many of the features of the intrusions classi- was not saturated in sulfur, that sulfur satura- fied as alpine (Jackson & Thayer L972), e,9., tron was attained while olivine and locally a tectonite fabrics, metamorphirc and igneous smaller a.mount of hypersthene were accumulat- layers, discordant and irregular banding, poor ing, and that the sul'fide subsequently accumu- c.ontinuity of banding and dykes of mafic rock lated as part of the material interstitial to the (at RAna, norite intruding earlier formed rock cumulus phases. The further obtervation that types). In this context the use of the terms olivine sho'wsa tendency to idiomorphism where stratiform and concentric, descriptive of the it abuts against interstitial sulfide while being form of the intrusion, as opposed to alpine, more rounded or even internally resorbed against descriptive of present geological environment interstitial silicate suggests that olivine accumu- (Jaclson & Thayer 1,972), seems unfortunate lated rapidly (possibly having nucleated close to as is the tendency to equate alpine ultramalic the floor of the magma chamber) and was rocks with ophiolite (Naldrett & Cabrj 1976, resorbed to form orthopyroxene, mainly after Garson & Plant 1973). Some related reservations accumulation, through reaction with intercumu' on existing classifications of mafic and trltra- lus liquid (the Bowen-Anderson reaction). mafic rocks have been expressed by Chaltis the direct cause of the attainment of sulfur (1969), Challis & Lauder (1966), Nesbitt er a/. saturation is not clear. The isotope data tend (1970), Moore (L973), Robins & Gardner to discount any 'major role 'for country-rock (1974) and others. sulfur. Irvine (1975, 1977) has suggestedthe The form of the contacts of the R&na in- existence of a mechanism that may explain trusion suggests,the shape of an inverted cone, several features of the Bruvann mineralization: possibly truncated, with its axis plunging north- an increase in silisa or alkalis in 'magmas of westward and with the highest section of the certain compositions (either by assimilation or cone, possibly near the roof of the intnrsion, by magpamixing processes) reduces -sulfur in the east and the deepest section in the north- s6tuUitity (resulting in formation of an immis- west. In a very general sense the distribution of cible suifide magma) and causes a polymeriza-

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tion of the silicate magma (which would prod- be affected. This tends to suggest tbat the tec- uce orthopyroxene instead of olivine). Ions such tonic juxtaposition of black schist and mineral- as Ni2+ with high octahedral-site-preference ized rock seen in the Bruvann area occurrd too energies are then preferentially expelled from la.te to have had any influence, even on the the silicate liquid and partition into the sulfide massive sulfide. Conversely, to have had more magma. Some evidence for multiple infiluxes than local inflsejss, sush contamination pro- of magma at RAna may lie in the reclurence cesseswould seem to require the incorpomtion of thick peridotite layers and in the iatrusive of country rock into the magma. very early in relationship, at least localln of norite to the its crystallization history. ultramafic roeks. Late- and postmagmatic de- formation complicates the reconstruction of events. That the bulk of the mineralization at ACKNowl,BDGEMENTs Bruvann lies in peridotite and not in pyroxenite could be taken as evidense against the theory The writers thank Stavanger Staal A/S for but, on the other hand, once sulfur saturation permission to publish the Rana material, is attaiued, it can perhaps be assumed that the BArd Tltdal for assistance with microprobc sulfide ,magma would accumulate more rapidly ana;lyses,Astrid Hemrming, Lars Holipkk and than orthopyroxene; the mineralized periditi.re, Tore Skauge for help in preparing the figures particularly in its upper reaohes, is interbanded and Magne Gustavson, Gunnar Juve, Ingvar with pyroxenite which is also mineralized to Lindahl, David Roberts, John Thom,pson and some extent. Frank Vokes for their constructive critioism of The Bowen-Anderson reaction probably the text. In the cou$e of the exploration work caused partial resorbtion of cu,mulus olivine to at Rflna many people at the Geological Survey produce intercumulus orthopyroxene. That this of Norway have been involved; their help is 13ctio1 may take place between cumulus crys- gatefully acknowledged. tals and intercumulus liquid has been advocaied in t}re case of the Stillwater complex by Jackson (1961) and in the case of rocki of the Seiland RsreRENcEs province by Gardner & Robins (1974). The Asncnorr, W.A. (197E): partition coefficienr Ol"'/Hf, (HekU & Murno, M. The 1971) is structure of the eastern part of the Insch such that tlis process, mafic though affecting only a intrusion,Aberdeenshire. Scot. l. Geol, t4, 55-79, portion of the original amount of olivine crvs- Bovo, R. & MuNno, (1978): t4ljzed, would recycle quantities M. Deformation of substantial of the Belhelvie mass, nickel the Aberdeenshire.Jcot. l. Geol. to silicate magma because it woild L4, 2944. affect a large volume of olivine. Where sulf,ide CeMrnEr.L,IJI. (1978): Some problems rya'gmarilas present interstitially, it with the would absorb cumulus theory. Lithos L1., 3Il-323. this nickel preferentially with respect to crystal- Cnenrs, (1969): Iizing orthopyroxene. The effect bf this pro,cess G-4,. Discussionon the paper "The origin of ultramafic and would be enhanced if: (1) there was ultrabasic rocls" expulsion by P.J. Wyllie. Tectonophys.7, 495-505. of intercumulus liquid from olivine cuirulates Leuonn, (1966): formed before sulfur saturation, and (2) inter- & W.R. The geneticposi- tion of cumulus sulfide magma had, "Alpine" tygn ultramafic rocks, Bull. because of its Volc.29, 283-306. gxeater density, intimate contact witl a greater Fosr.rg S. (f921): volume of olivine than that against wnicn it Field observationsin Northern Norway bearing on ultimately solidified, i.e., if sulfide magmatic differentiation. J. masma-oumirlus Der- Geol. 29, 7Ol-719. colated downward through a pile of GeroNrn, P.M. (t974): olivine crystals displacing intercumulus silicate & Rornrs, B, The olivine- plagroclase reaction: geological magma. These processes may have evidence from contributed the Seiland petrographic province, to the relatively high nortlern nickel content of the Norway. Contr. Mineral, Petrology R6na sulfides. 44, 149-156. Gensor.r,M.S. & PL$tr. I. (1973): Alpine A number of factors suggest tha,t part of the type ultramafio rocks and episodic mountain building sulfur in the R0,na mineral deposit was derived in the Scottish Hiehlands. Nature Phys. Sct. from black schist lenses wirhin and around tle 242,3+38. intrusion. The sul.f,ur isotope data seem to indi- Gorr.Bvsrn, M.N. & Gnnnxro, L.N. (1963): cate tlat this process may have been active, but Some data on the isotopic compositioa of sulfur only_locally and apparently not at a stage at in the sulfides of the Norilrsk deposit. Geochem., which the major mineralization at Bruvann-could 354r.

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(t977)'. Gnernnor-, M. & Nnr-oxetr, A.J. (1971): Mine- MAr.rwARrNG,P.R. & Nerunrrr, AJ. genesis ralogy of the Marbridge No. 3 and No. 4 nicke!- Country-rock assimilation and the of Cu- iron sulfide deposits.Econ. Geol- 66, 866'900. Ni sulfides in the Water Hen intrusion, Duluth complex,Minnesota. Econ. Geol.72, 126%L2E4- GusrevsoN, M, (L972): The Caledonian mountain m chain of the southern and areas. wtlirsilt M. (1974)t A study of zulfur isotopes III. Structuresand structural history. Norg. GeoI. the Outokumpu ore deposits,Finland. Geol. Surv. Unders. 288. FinlandBull.267. (L974): Geologisk kart over Norge, berg- MALvtK, T. (1977): Mineralogisk underslkelse av grunnsgeoloeiskkart - Narvik' 1:250 000. Norg. bulkkonsentrat av RA,naNi-malm. Unpubl. Rep., Geol. Unders. Norges Tekniske Hdgskole, Trondheim. llalpar.l, P.S. (1969): Fennoscandiannickel de- Moone, A.c. (1973): Studiesof igneousand tec' posits. Iz Magmatic Ore Deposits (II.D.B. Wit' tonic textures and layering in the rocks of the son, ed.). Econ. Geol. Mon. 4,262-275, Gosse Pile intrusio& Central Australia. J. Pe- trology 14 49-E0. Hiiru, T.A. (1971): Silicate niskel and its appli' cation to tle exploration of nickel ores. Geol. MooREs, E.M. (1973): Geotectonic significance Soc. Finland Bull. 48' 247'263. of ultramafic roc,ks.Earth'Sci. Rev. 9, U1'255. Ilmrug D.C. & NtcxEL, EJI. (1972): Pentlandite NALDRETT,AJ. (1966): The role of sulturization compositions and associationsin some mineral in the genesis of iron-nickel sulfide deposits deposits.Can, Mineral. 11, 861'878. of the Porcupine Distric! Ontario. Can. Inst. Invnw, T.N. (1975): Crystallization sequencesio Mining Met. Trans.69, L47-155. the Muskox intrusion and other layered intrusions. (1969): Discussionof papers concerned II. Origin of chromitite layers and similar de' with sulfide delpsits. In MagmaJ;icOre Deposits posits of other magmatic orq. Geochim. Cos- (H.D.B.'lVilson,ed.), Econ. Geol.Mon. 4,359' mochim.Acta 39, 991'1020. 365. in the - (t977): Origin of chromitite layers (1973)z Nickel sulfide deposits -their Muskox intrusion and other stratiform intrusions: classifiiation and genesis,with special emphasis a new interpretatiot Geol. 6, 273'277. on deposits of volcanic association. Can. Iwt' fAcKsoN,E.D. (1961): Primary texturesand mine' Mining Met. Traw. 76, I83'2nI. ral associationsin the tjltramafic Zone of tle & CesRr, L.J. (1976): Ijltramafic and re- Stillwater complex, Montana. US. Geol. Sum. lated rocks: their classification and genesiswith Prof. Pap.858. special reference to the concentration of nickel & TtrevsR, T.P. (1972): Some criteria for suffidesand platinum-group elements.Econ. GeoI' disting;uishingbetween stratiform, concentric and 71,1131-1158. Int' Geol' abinJ peridotite-gabbro complexes' & GlsrannrNr,E.L. (1971): Archeannickel Congr. 24th, 2, 289-296. sulfide deposits in Canada: their classification, (1976): Klxanrs.sorrnrou, R. & ANcus, N.S. gpological setting and genesiswitl some iuqces' The Currywongaun-Doughruaghsyntectonic in- tions as to exploration' Iz Symposiumon Ardlean trusion, Connemara, Ireland, t. Geol. Soc, Inn' Rocks (J.E. Glover, ed,), Geol. Soc.Aust. Spec' don L82,485-508. Publ. 3, 201-226. Kulunuo, G. &YopsR, H.S., Jn. (1965): Sulfide- Nesrrrr, R.W., Gooos, A.D.T., Moonn, A.C. & silicate reactions and their bearing on ore forma- Horwoon, T.P. (1970): The Giles Complex, don under magmatic, postmagnatic and meta- central Australia: a stratified sequenceof mafic morphic conditions. Iz Symposium on Problems and ultr,amafic intrusions. tz Symposium on the of PostmagrnaticOre Deltosition n', Geol, Surv. Bushveld Igneous Complex and other I-ayered Czechoslovakia, Prague. Intrusiong (DJ.L. Visser & G. von Gruenewaldt' LsAKE, B.E. (1970): The ftaguentation of tle eds.), Geol. Soc. S' Alr, Spec.Publ. t, 547-564' intrusions. In Connemara basic and ultrabasic PlpuxsN, H. (1970): Sulfide mineralogy of the (G. Newall & Mechanism of lpeous Intrusion Kotalahti and Hitura nickel'copper ores, Finland' N. Rast. eds.), Geol. l. Spec,Issue 2, I0l'L22. Ann. Acad. Sci.Fennicae Ser. A. III' 109. LrrssNBERg L. (1969): The Sulphides in the & Pem, W.C. (1976): Nickelife- Inyered Sequenceof the Bushveld lgneoas Com' I{ANVrLLB,G.D. pyrrhotite Knox County' south' plex. Ph. D. thesis, Univ. Pretoria, Pretoria' rous delnsits, /z Studiesin New England Geol' South Africa. easteni-Maine. ogy (P.C. Lyons & A.H. Brownlow, eds-), Geol' (1970): layered se- The sulphidesin the Soc,Amer. Mem. 146' 3t9'347. quenceof the Bushveldigneous complex. la Sym' posium on the Bushveld Igneous Complex and RAJAIaANT,V. & Ner.onrtr, A.J. (1978): Parti' other Layered Intrusions (D.JI. Visser & G. von tioning of Fe Co, Ni and Cu between sulfide of Gruenewaldt, e&.), Geol. Soc. 5. Atr. Spec. liquid-and basaltic melts and the composition Publ. l, LO8-207. Ni-Cu zuUide deposits. Econ. Geol. 73, 8?'93'

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RAUDoHR,P. (1969): The Ore Minerals and their SraNroN, R.L. (1972): Ore Petrology. McGraw- Intergrowths. PergamonnOxford. HilI, New York. RoBrhrs,B. & GenoNrn, P.M. (1974): Synorogenic TrtrAyER,T.P. & JAcKsoN,B.D. (L972): A slassifi- layered basic intrusions in the Seiland p€trogra- cation of igneousrocks by their history of crystal- phic province, . Norg, Geol. unders. lization and emplacemenl US. Geol. $u.rv.Prof. 312, 91-130. Pop. 800.8,79-83. WAGER,L.R., Bnowx, G.M., & Weoswonrn, WJ. Rooorcr, LC.. (L977): Age of the RAna massif, (1960): Typesof igneouscumulates. J. Petrology north Norway. Fifth Eur. Colloq, Geochron. 1, 73-85. Cosmochron,& Isotope Geol. Pisa (Abstr.). Vy'u,soN,H.D.B. & ANDERSo\ D.T. (1959): The RYF"R.O. & Omamo, II. (t974): Sulfur and carbon composition of Canadian sulfide ore deposits. isotopes and ore genesis: a review. Econ. Geol. Can. Inst, Mining Met. Trans.62,327-339. 69,82G842. Wyr-r,re,P.J. (1969): The origin of ultra^maficand ulta,basic tooks. Tectottophys.7, 437455. Snrons, d & Boxo, R. (1927): TyngdemAlinger, RAna, , Nordland. Norg. Geol. (Jnders. Received July 1978, revised manuscrip accepted Rap. 1588 (Unpubl.). December 1978.

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