NGU - BULL 427,1995 Anne Birkeland 37

Pb-isotope constraints on the metallogeny in the MerakerNappe

ANNE BIRKELAND

Anne Birkeland, Norges Geologiske Undersokelse, Postboks 3006-Lade, N-7002 Trondheim, Norway

IntroductionThe Pb-isotope compos itions of ten sulph idedeposits in the Caledon ian Meraker Nappe arepresented and discussed. The purpose of theinvestigation has been to characterise the metalsources of the sulphide deposits and in this waydetermine whether all the deposits were part ofthe same minerali sing system or if there are vari ­ations throughout the district.

Regional geologyThe depos its are hosted by rocks with differenttectonostratigraphic positions in the SularnoGroup and the Kjelhauq Group, Together withthe tectonically overlying Fundsjo Group to thewest and the underly ing Slagan Group to theeast , these rocks constitute the Meraker Nappe(Wolff 1979) of the Upper Allochthon (Gee et al.1985). Based on one fossil discovery (Getz1890) and correlat ions with other districts in theUpper Allochthon, the Sularno Group is cons ide­red to be of Middle Ordovician age, and the

Kj01haug Group of Late Ordovician age (Wolff1967). Inversion of the lithological successionsis believed to have occured during theCaledonian orogeny (Roberts 1967),

The Sularno Group is dominated by phyll itesand a metasandstone, but also comprises twogreenstone units, referred to as the Turifoss (Fig.1) and Drivsj0fjell Formations (Siedlecka 1967) ,The greenstones are believed to be metavolcani­tes (Chaloupsky & Fediuk 1967); geochemicallythey show a somewhat depleted tholeiitic affinitysimilar to ocean floor basalts (Hardenby 1986).

The Kj01haug Group is a thick unit of phyllitesand metagreywackes (Fig. 1), interpreted asflysch and turbidite formations (Siedlecka 1967) ,which are intruded by numerous, Caledon ian,hornb lende metagabbros (Siedlecka 1967, Wolff1973, Hardenby 1980) . The gabbros in the nor­thern part of the area form large bodies (Fig, 1)which are genera lly coarse-grained and have amineralog ical layering.Those in the southern partof the area are smaller and form sheets or sills,and are usually fine-grained and without laye-

INTRUSIONS_ Gabbro

SLAGAN GROUP

I::::1 Phytlite

KJl2ll HAUG GROUP

I ~ Metagreywacke and- phyll~.

SULAMO GROUP

I ~ ~ ~ j i ~ ~ j : ~ ~m] Greens10ne

I/"~%/~] Phyllite andcaJcareous sandstone

FUNDSJl2l GROUPrn vccannes

STEINFJEl l GROUPo Quartz schist

Sulphide deposits (1-10)

Fig. 1. Simplified geological map ofthe Merflker district. Mineralisations:1. Geitberget2. Sagskjerpet3. Hammerskallen4.Skomakermoen5. Sondre Knoll Skjerp6. Duddu, 7.Ebba, 8. Lillefjell9.Gilsfl Gruver,10. Dronningen

38 Anne Birkeland

ring. The sulphide deposits are hosted by themetagreywackes in the Kjolhaug Group, com­monly at contacts with small gabbros.

The rocks in the Meraker district exhibit defor­mation structures ascribed to at least two phasesof folding (Wolff 1973). Accord ing to Tor Grenne(pers. comm., 1994) and the present author, the­re are also internal thrust and shear zones in theSularno and Kjolhaug Groups. The amount oflateral movement on these zones is not known,but the rocks affected are severely deformed andoriginal contacts may be obliterated.

Sulphide depos itsAlthough the deposits were briefly mentioned byWolff (1967), no modern account has beenpublished. Only a brief description will be givenhere. Work is in progress on a more detaileddescription (Birkeland et al. in prep.).

Based on field relations, the sulphide minerali­sations may be divided into two groups. Depositsof both groups occur in both the Sulamo and theKjolhaug Groups (Fig. 1), indicating that bothgroups have been affected by the same ore-for­ming processes . The first group comprises mas­sive, stratabound deposits, in places with a pri­mary sulphide layering. Pyrite, pyrrhotite, spha­lerite and chalcopyrite are the major componentminerals. The mineralisations exhibit extensivewall-rock alteration, including sericitisation, silici­fication, chloritisation and pyritisation. These fea­tures indicate a hydrothermal syngenetic originfor this group of deposits. Among the sampleddeposits, Sagskjerpet in the TurifossGreenstone, and Sondre Knoll Skjerp and theLillefjell deposit in the Kjolhaug Group may beassigned to this group. The Gilsa deposit(Kjolhaug Group) probably also belongs to thisgroup, but the very limited wall-rock alterationmakes this deposit enigmatic.

The second group of deposits comprises tho­se in shear zones. The mineralisations arestrongly tectonised. Pyrrhotite, chalcopyrite, pyri­te and sphalerite constitute a semi-mass ive ore,containing abundant fragments of the host rocksand eyes of clear quartz. Wall-rock alteration islimited to 1-2 cm of chloritisation of the immedia­te wall-rock. These deposits have been regardedas belonging to the same generation of depositsas the syngenetic massive sulphide deposits,and have been compared to the Roros type ofdeposits (e.g. Wolff 1967). They could representfeeder systems to stratabound deposits.However, due to the tectonisation this is hard toconfirm. The Geitberget (the TurifossGreenstone), Ebba, Duddu, Skomakermoen,Hammerskallen and Dronningen deposits (all inthe Kjolhaug Group) belong to this group .

NGU - BULL 427. 1995

The sulphides in both types of mineralisationsexhibit a metamorphic texture . The ore showingsare generally small, the largest being theLillefjellet deposit estimated to about 100,000tons of ore.

Pb-isotope results and discussionGalena is rare and the Pb abundance generallylow in the sulphide deposits in the Meraker dis­trict (Karlstrom 1993). Pb-isotope analyses werecarried out by Geospec Consultants Ltd. on com­posite sulphide mineral concentrates from ore.These sulphide concentrates were leached toproduce a residue containing lead from the crys­tal lattice, giving the initial Pb-isotope compositi­on at the time of metal deposition or metamor­phic recrystallisat ion. For all deposits, except forthe Sondre Knoll Skjerp, two samples were ana­lysed. The Sondre Knoll Skjerp is represented byone sample only. The data are available fromthe author.

The analytical results are plotted in conventio­nal Pb-isotope diagrams in Figs 2a and 2b. Asevident from the diagrams, the host rocks, andnot the type of deposit, relates to the Pb-isotopecomposit ion of the mineralisations. The twodeposits from the Turifoss Greenstone are identi­cal within errors, in spite of one being of the stra­tabound type (Sagskjerpet) and the other of thetectonised type (Geitberget). The 206Pb{204Pb­compos ition for the deposits in the TurifossGreenstone varies between 17.747 and 17.780,the 207Pb/204Pb-composition between 15.423and 15.444, and the 20BPb{204Pb-compositionbetween 37.171 and 37.249. For deposits in theKjolhaug Group the three isotope ratios are18.127 to 18.512, 15.537 to 15.590 and 37.908to 38.499, respect ively. Such Pb-isotope com­positions are normal for ScandinavianCaledonian massive sulphide deposits in theUpper Allochthon and denote a mixture of nor­mal Caledonian lead in the actual region(Bjorlykke et al. 1993, Sundblad & Stephens1983). They plot on the Upper Allochthon trendof Bjor lykke et al. (1993). The mineralisations inthe Turifoss greenstone are within the compositi­on of Caledonian depleted mantle, and the depo­sits in the Kjolhaug Group have Pb-isotope com­positions similar to deposits in the UpperAllochthon influenced by sedimentary and volca­nic metal source rocks (Bjorlykke et al. 1993).

The mineralisations in the Turifoss greenstonein the Sularno Group have homogeneous Pb-iso­tope compositions , within the standard deviation,that are less radiogenic than the deposits in theKjo lhaug Group . This lead must have been deri­ved from a source low in U, Th and radiogenicPb. The isotope values are at a level usually

NGU - BULL427,1 995 Anne Birkeland 39

lead, whereas others have homogeneous Pb­isotope composit ions. Hence, the processes lea­ding to different morphologies of the deposits inthe region did not determine the lead-isotopecomposition of the deposits. Two metal sourcescontaining slightly different lead were available,but the mineralising process (es) did not homoge­nise metals from the two sources. The presentauthor suggests that syngenetic hydrothermalmassive sulphides were deposited in the sedi­mentary basin, the main metal source being thegreywacke. The greywacke may have been inho­mogeneous ; or alternatively a different sourcerock may have been present. The convectioncells below the mineralisations may have beenisolated from one another, thus preventing acomplete mixing between mineralising fluids.Alternatively , the two sources could have actedat different stages during the mineralising event.During a later compressional event, which couldbe that related to the final thrusting of theMeraker Nappe, these syngenetic deposits weredeformed to varying degrees and some of themeven remobilised into shear zones. Lead givingrise to the spread in the 206Pbj204Pb-directionmay have been introduced at this stage. Thevariation in the 206Pb/204Pb-ratio is most easilyexplained by incorporat ion of radiogenic Pb intothe sulphides during metamorphism. Due to thelow Pb abundance in the deposits, even smallamounts of radiogenic lead may have affectedthe Pb-isotope composition and been incorpora­ted in the sulphides as a result of metamorphicgrowth.

ConclusionsIn both the Sularno and the Kjolhaug Groups inthe Meraker Nappe, the primary ore-forming pro­cess was that of syngenet ic, hydrothermal , sul­phide deposition. The Pb-isotope investigationrevealed that the metal source for deposits in theSulamo Group was a depleted rock, probably theTurifoss Greenstone in which the deposits aresituated. The deposits in the Kjolhaug Groupcontain more radiogenic lead than the depositsin the Sularno Group and many of them are iso­topically heterogeneous. The action of thehydrothermal convection cells in the KjolhaugGroup did not homogenise lead from differentsources, or from one heterogeneous source,completely. At a later stage the already deposi­ted mineralisations were deformed, some sulphi­des were remobilised into shear zones, givingsemi-massive mineralisations with a tectonisedstyle. The possibility that some of the semi-mas­sive mineralisations are, in fact, feeder zones forhydrothermal fluids is likely, but cannot be pro-

o2 <1

38.50 -"Q

a..,.QN

38.00 -:aa.<XlQN

37.50 -

#37.00

11.50

15.70 a)15.66

"Q 15.60 •a. ..~.,.*Q 15.56

l::!"Q

15.50a.....QN 15.45 t

15.400 2 <1

15.35I I I

17.50 18.00 18.60 19.0 0

39.00 - b)

seen in rocks derived from the depleted mantle(e.g. Doe & Zartman 1979). The metal source forthe sulphides is probably the Turifoss greensto­ne, which shows a depleted trace element geo­chemistry (Hardenby 1986).

The Pb-isotope compositions of some of thedeposits in the Kjolhaug Group are heterogene­ous, while others are homogeneous. The Pb-iso­tope variation giving the spread in the207Pb/204Pb_ and 208Pb/204Pb-direction for theheterogeneous deposits (Figs. 2a and 2b) is lar­ger than the analytical standard deviation.Generally speaking, the metal source must beone or other of the rocks in the Caledonian nap­pes, the dominant source probably being thethick pile of greywackes in the Kjolhaug Group.The slightly more radiogenic samples are evi­dence for an influence from a somewhat moreevolved source.

Both syngenetic and tectonic types of depositsin the Kjolhaug Group contain heterogeneous

Fig.2. Lead i.otopco ratioe for sulphide miDc:nlisatiom inthe

SuIAmo IIlldKje lhaug Group&. a) :lnPb'*Pb VI ""I>bJ*FbaDdb)~*Fb VI ""I>bJ*Fb. 0 GeitbeI'get,

• Sagskjerpct. 0 Ebb&, • Duddu, .... Sendrc Knoll,

~ Drooningen. + Slc.omaIccrmOClD, X Gilsi,

* HammcnkalJea &lid • LilIc:fjeU.

I I I18.00 18.50 19.00

2061"b/20~Pb

40 Anne Birkeland GU · BULL 427. 1995

ven due to the strong deformation. Deposits ofboth the massive stratabound and the tectonisedtype contain lead with the same isotopic charac­teristics. Slightly younger, more radiogenic leadwas introduced into the mineralisations in theKjolhaug Group and incorporated in the sulphi­des during metamorphism.

AcknowledgementsI am grateful to Arne Solli who compiled the map. The Pb-iso­tope analyses were carried out by Geospec Consultant Ltd. Iwish to thank Peter Padget for correcting the English.

References

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