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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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