Geology and K/Ar chronology of the Malvika schee­lite skarns, Central Norwegian CaledonidesD. WYN JAMES, JOHN G. MITCHELL, P. RICHARD INESON & 0 YSTEIN NORDGULEN

James, D.W., Mitchell , J.G., Ineson, P.R. & Nordgulen, 0 . 1993: Geology and K/Ar chronolog y ofthe MAlvika scheelite skarns , Centra l Norwegian Caledon ides. Nor. geol. unders . Bull. 424, 65-74.

Skarns present on the northern shore of Tosenfjord overpr int the S2 fabric of the adjacent gneiss(which form ed during peak regional metamorphism). Together with petrographic evidence of thelate cryst allization of scheel ite, this indicates that mineralization was post-meta morph ic and epi­geneti c. K/Ar apparent ages of nine mineral separates (four post-sche eltte amphiboles from skarn,two muscovites and three biotites from intrus ions) are concordant at the 1a level, giving an Inver­se Variance Weighted Mean (IVWM) age of 402 :!: 2 Ma. The close agreement of results fromdifferent minera ls points to rapid post-o rogenic uplift in the Central Scandinav ian Caledonide s.Two other post -scneetite amphibole skarn samples gave indistinguishable (at 1a) apparent ageswhose mean is 472 Ma. If valid, this age would substantially constrain the timing of regiona lmetamorphism and mineralization. However, the possibility that the ages are spurious , due to thepresenc e of excess argon , cannot be excluded.

D. Wyn James & P. Richard Ineson, Department of Earth Sciences, Univers ity of Sheffield, Shef­field, S3 7HF, U.K.John G. Mitchell , School of Physics , The University, Newcastle upo n Tyne, NEt 7RU, U.K.0 ystein Nordgulen, Norges geolo giske undersek etse, Post boks 3006 - Lede, N-7002 Trondhe im,Norway.

IntroductionScheelite was first discovered with in the Helge­land Nappe Complex (HNC) in 1969. It waslocated in skarns and quartz veins, associatedwith marble (Nissen 1969), near Mosjeen (Fig.1). Subsequently, scheelite was detected in thegold-arsenopyrite deposits of the Kolsvik area(Skaarup 1974). A reconnaissance explorat ionprogramme for scheelite, in 1970, led to thediscovery of several additional occurrenceson either side of Tosenfjord . Brief descr iptionsof the geology and petrography of these pros­pects were prov ided by Skaarup (1974), whoconsidered them to have a syngenet ic origin.Syngenetic tungsten deposits are believed toform by the accumulat ion on the sea-floor oftungsten-rich precipitates, which crystallize asa result of the mixing of tungsten-bearingsolut ions (emitted from sea-floor smokers af­ter percolat ing through rock with a high basicigneous component) with colder sea water(Maucher 1965).

Four princ ipal hypotheses may be postu latedfor the tungsten (scheelite) mineralization atMalvika:a) Scheelite was syngenetic with the accumu­

lation of the host lithologies.b) Scheelite precipitated from tungsten-bear­

ing hydrothermal fluids which were mobil­ised by regional metamorphism.

c) Scheelite precipitated from tungsten-bear­ing hydrothermal fluids of a magmatic orig in,or tungsten was leached from the country­rocks by fluids circulating around a graniticpluton .

d) Scheelite precipitated after the cessationof magmatic activity, from tungsten-bearinghydrothermal fluids.

The aim of the present study was to esta­blish constraints on the relative and absolutetiming of mineralization, and hence test thevalidity of the above hypotheses. The K/Arradiometric ages presented here may alsohave incidental value in clarifying the tectono­metamorph ic evolut ion of the Helgeland NappeComplex.

Regional GeologyThe Helgeland Nappe Complex is the structu­rally highest unit of the Uppermost Allochthon(Gee et al. 1985). The HNC is considered toconsist of at least two tectono-stratigraphicunits (Nordgu len & Schouenborg 1990, Thors­nes & Leseth 1991):1. Mafic and ultramafic lithologies occur in the

western part of the HNC (Nordgulen &Schouenborg 1990). These rocks have beeninterpreted as the remnants of a dismem­bered ophiolite (Husmo & Nordgu len 1988,Thorsnes & t.esetn 1991).

66 D. Wyn Jame s et al. NGU - BULL. 424. 1993

• Anatect ic Gra nitoids

t.:-:-:-:ITourmali ne Granite

1-:-:-: -:1 Gran ite . GranOdiorite

o Monzodiorite. Quartz Monzon ite. Syenite

o Porphyritic Granite and Granodionte

(»>1 Diorite. Tonalite . Granodionte, Trondh jem ite

1:·:-:-:-:-:1 Gabbro. Dior ite. Monzodiorite

D Various metasupracrustal rocks

• Greenstone. Gabbro, Ultramafic Rocks

~ Leka Op hiolite Com plex

~ Rocks below the HNC and RNC

V:..~ Thrustlin ferred thrust

Oslo•

•Trondheim

R C

",

30kmo

; .; < C',-, ji.- .....

A ~ ,..?/i-: 'j'

Fig . 1. Geolog ical map of the Helgeland Nappe Complex. HNC = Helge land Nappe Complex ; R C = Roo lngs fJallet appeComp lex ; VS = Vestranden; K = Ko lsv ik : A = Ancatsnar te n Ptuton: rectang le enclo ses area o f Fig . 2. .Ioo.f,ed from oroquten& Sundvo ll 1992.

2. A supracrustal sequence of cont inenta lderivation has also been identified. It iscomposed of strong ly deformed, high-gradepelitic to semi-pelitic gneisses (partly mig­matitic) and calcareous metasedimentaryrocks.

A cafe-alkaline intrusive comp lex, the BindalBatholith, constitutes a substa ntial propo rtionof the HNC (Nordgulen & Schouenborg 1990).

Medium- to coarse-grained and megacrysticgranites and granodiorites typify the batholith(Nordgulen & Mitchell 1988, Nordgulen et al.1993). Radiometr ic age determinations indicatethat intrusive activity was syn-orogenic, span­ning the period from the Middle/Late Ordovi­cian to the Late Silurian (Nordgulen et al. 1993).

Kollung (1967) and Myrland (1972) indicatedthat peak metamorphism reached almandine-

NGU - BULL. 424, 1993

amphibolite facies throughout much of theHNC. The formation of the dominant (52) folia­tion coincided with the peak of metamorph ism(Thorsnes & t.esetn 1991). Later deformation­al events (D3 and D4) produced open, large­scale folds (Myrland 1972, Terudbakken &Mickelson 1986, Gustavson 1988). Theseevents are responsible for the present attitude(c.60° dip to ENE) of the rocks in the vicinityof the study area (Thorsnes & t.eseth 1991).

As 52 structures are present in inclusionscut by the megacrystic Andalshatten pluton(dated at 447 ± 7 Ma; Nordgulen et al. 1993),the foliation must have formed prior to orduring the Late Ordovician . There is thereforeevidence from the HNC, as from the granitoidterrane of Srnela-Hltra (Roberts 1980, Gautneb& Roberts 1989) and elsewhere in the UpperAllochthon (Kullerud et al. 1988) and Upper­most Allochthon (Claesson 1979), that a majortectono-magmatic event occurred during theOrdovician (Nordgulen & 5chouenborg 1990).Nevertheless, the final eastward thrusting ofthe HNC probably took place during the 5candi­an continent-to-continent collision.

Geology and Petrography of theMalvika skarnsTungsten-bearing skarns occur in two zonesin the vicinity of Malvika , which is situated onthe northern side of Tosenfjord (Figs. 1 & 2).The southern tip of the 100 metre-wide mainzone (5kaarup 1974) lies 100 m southwestof Malvika and extends NNW for 700 m in­land, to a height of 400 metres. The second(western) zone is exposed in a road-cut 200m to the southwest, near 5kjervikbugen. This10 m wide zone extends for 70 m north­wards, where it terminates against a promi­nent, ENE-W5W trending fault.

The skarns occur as heterogeneous hori­zons not exceeding 1.5 metres in width . Theyare enclosed by leucocratic biotite gneiss ormelanocratic, weakly foliated, amphibole-bioti­te gneiss. The two zones differ in the abundan­ce of marble. The marble in the eastern zoneoccurs as widely scattered inclusions, whereasit is present as continuous horizons enclosedby < 30 cm wide skarn horizons in tt's wes­tern zone. Anastomosing vein networks andassemblages formed by the replacement ofthe first-formed skarn assemblages occur ex-

Geology and K/Ar chronology 67

• Tungsten Skarn Zone 0 km

Cl Granodiorite ~ S2 Foliation

:0:':':0 Porphyritic Granite leR1?1 Sample localities

D Monzodiorite to Quartz Monzonite

c=J Migmatitic Gneiss, Marble, Calc-silicate Gneiss

Fig. 2. Geol og ical map of the area surrou nding the scheeli teskar ns of Malvika .

tensively in the skarns of the eastern zone,whereas in the western zone veins and replace­ment assemblages are volumetr ically minor.

Fig. 3 illustrates the relationship betweenskarn , marble and gneiss, in the western zone.Each skarn horizon consists predominantlyof two primary (Le. first-formed) lithologies,one cons isting largely of pyroxene and plagio­clase whereas the other is composed mainlyof garnet and pyroxene . The former occurs asa continuous horizon up to 15 cm in width ,adjacent to gneiss. The garnet skarn , in con­trast , generally occurs as disparate lenses (upto 5cm wide) along the marble-skarn contact(Fig. 3). Fig. 4 shows that garnet skarn probab ­ly formed by replacement of marble. The gar­net skarn was, in turn , partially replaced byan epidote + quartz (+ residual pyroxene )assemblage. This accounts for the erratic dis­tribution of garnet skarn lenses, as in Fig. 3.A < 5mm wide selvage of wollastonite (+pyroxene) skarn is typically developed at thecontact between the garnet skarn and marble.A zone of amphibole + plagioclase skarn

E:;J Litholoqical boundary

U Fraclure

D Scheehte grain

68 D. Wyn James et al.

~>: >J Calcite. marble or vein

o Biotite gneiss

~ Pyroxene + felsic minerals ska rn

~ Garnet z pyroxene skarn

o cm 30

NGU - BULL. 424. 1993

O rien tat ion of mappe d surlace

Fig. 3. Detailed sketc h map of a typ ical skarn horizon of the Skjervikbugen (western) zone.

Fig. 4. Garnet skarn (G) replacing marbl e (M). A = Amph i­bole wall -rock skarn enclos ing calcite vein (C). H = Hyd­rou s r eplacemen t assemblage (after garnet skarn) con sis­ting mainly of epidote and quartz (with minor scheelite).Boulder. roadside. western zone .

« 5mm wide) is found at the contact of thepyroxene + plaagioclase skarn with the gneiss.ldocr ase (+ pyroxene) is rarely developed inplace of the garnet skarn.

A hydrou s assemblage cons isting mainly ofepidote + quartz + amphibole is extensivelydeveloped at the expense of the garnet skarn.Scheelite is locally a significant component,accounting for up to 20% of the assemblage.To a lesser extent hydrous mineral skarn (with­out scheelite) replaced the pyro xene + plagioc­lase skarn. In places it is spatially associatedwith cross-cutt ing fractures or veins. How­ever, the replacement skarn generally occursmore extensively, with preferential develop­ment along the contacts between the primaryskarn assemblages. Another type of hydrousmineral replacement is developed exclusivelyat the peripheries of quartz-calcite veins. Itconsists primarily of blades of ferro-pargasiticamphibole and cubes of almandine-spessar­tine-rich garnet (James 1991). Variable propor­tions of pyrrhot ite, calcite, quartz and epidoteoccupy the interstices between amphibole andgarnet crystals. Coeval scheelite is absent fromthis wall-rock assemblage. Biotite is a minorconstituent of the quartz-calcite veins andscheelite is rarely present. The veins are pla-

NGU · BULl. 424. 1993

bnar, exhibit preferential orientation and arerestr icted to the skarn .

8kaarup (1974) implied that the scheelite­bearing skarns were formed by isochemicaltransformation of tungsten -rich marl protol iths,during regional metamorphism. However, Fig.5 shows pyroxene + plagioclase skarn cross­cutt ing the foliation of the adjacent gneiss ata high angle. This indicates that this type ofskarn post-dated the 82 fabric in the gneiss;hence the skarn formed after the peak of regio­nal metamorphism.

The character of the ore at Malvika is alsoinconsistent with Skaarup 's (1974) conclu­sions. Investigation of tungsten skarns wor ld­wide have shown that scheelite compr ises atleast 95% of all skarn tungsten minerals (Kwak1987). This is due to the very high negativefree energy of format ion of scheelite:

CaO + WO, = CaWO• .6Gi:' = -52,320 CallMole (Hsu & Galli 1973)

Only if the Ca activity is low, or under super­gene conditions, do other tungsten minerals(mainly wolframite) form. Hence, it is reason­able to assume that any tungsten present inskarn and Ca-rich precursors to skarn wouldoccur in scheel ite. The earliest generation ofscheelite in the Malvika skarns crystallizedlate in the primary paragenes is (minor schee­lite occurs as a late-stage infill of the intersti­ces between the well-formed primary garnetcrystals ; see Fig. 6 for the complete paragene­sls), This points to the introduction of tungsten

Geology and K/Ar chronology 69

Pegmabbc dyke

Fig. 5. Pyroxene skarn pro­jecting irregularly across thefoliation of the adjace ntarnphibole-biotite gneiss .Western zone.

PRIMARY SKARN HYDROUS SKARNMINERAL

Stage III Stage IVStage I Stage 11

Apatite f----- I-

IImenite I-

Sphene I- - --Ferro-Salite----

Garnet_.6.- -~

catcue - - - - -- --Scheelite 1- - .

........... - -Epidote

Amphibole 1- - ,--.

Quartz -- --Plagioclase --Chalcopyrite -

Pyrite -

Sphalente -

Pyrrhotite -Magnetite

Goethite

Fig. 6. Mineral paragenesis of garnet skarn. The horizo ntalaxis represen ts the relative timing of crystallisation. withtime increasing from left to right. The size of each shadedarea is indicative of the approx imate volumetric abundanceof the phase . Apatite of Stage IV is fluor-apatite. Garnetof Stage I is qrossu lar . Garnet of Stages 11 and III is Gross ­Alm-Spess . Garnet of Stage IV is Alm-Spess. Scheelite isMo-free. Epidote composition spans the epidote -clinozo isitedivide and Stage IV amphibole is ferro -pargasitic (analysesrepor ted in James 1991).

70 D. Wyn James et al.

by epigenetic hydrothermal fluids. Further ­more , the most intensive phase of scheel itemineral ization occurred later in the evolut ionof the skarn , during the replacement of theprimary garnet skarn by the hydrous mineralskarn. The concentration of scheelite presentin the primary skarn is far too small for thescheel ite in the replacement skarn to havebeen derived solely by re-distribution of prima­ry skarn scheelite. Considering this togetherwith the evidence of a post- regional metamorp­hic origin for the primary pyroxere + plagiocla­se skarn, it is concluded that it is highly impro­bable that tungsten mineralizat ion in the Malvi·ka area was syngenetic with the depositionof the host -litholog ies.

In order to constrain the timing of mineraliza­tion, and thereby test the afore-mentionedhypotheses, a K/Ar geochronological investiga­tion was undertaken.

K/Ar isotopic agedeterminationsThe amphiboles (Table 1) were chipped direct­ly from fresh roads ide exposures of the veinwall-rock skarn. Biotite chip samples R11 andR12 were obtained from pegmatitic two-micagranitic dykes located in the western zone.

NGU - BULL. 424. 1993

The dykes post-date the adjacent primaryskarn, since xenoliths of skarn were presentwithin the dykes and an amphibole reactionskarn was developed at the dyke-ska rn con­tact. Three whole-rock samples were obtainedfrom fresh roadside exposures of the two­mica granodiorite pluton located 500 m north ­east from the eastern zone.

Chip samples were purified by crushing thesample in a pest le and mortar and then pro­gressively removing the relatively magneticamphibole or biot ite with an electromagneticseparator. Purities in excess of 99 percentwere atta ined.

Whole-rock samples of the two- mica grano­diorite were crushed and sieved to obta in anoptimal size fraction of 90 to 150 urn. Puritiesgreater than 99% were atta ined for biotite.Weakly paramagnetic muscovite proved diff i­cult to separate from compound grains whichconsisted of non-magnetic quartz or feldsparin combination with magnet ic biot ite or ilmen­ite. This resulted in a purity of 95% for musco­vite separates .

Potassium analyses were carried out usinga Corning 450 flame photometer with a lithiuminternal standard. Argon isotopic analyses

Table 1. K/Ar determinations of biotite . muscovite and arnphlbole separates from the MAlvika area (see map-sheet 1825·1Tosbotn for sample localities).

Sample UTM K,o ' Radiogenic" Atrnosphenc" Age

number co-ordinates (wt.%) 40Ar (mm3g-1) con tamination (%) (Ma ::: l a )

Two-mice granodiorite. Muscovite

RIO 40125-724430 9.90 :::0.14 (1.445 :::0.013) x 10-1 3.3 404 :::7R18 40105-724420 9.51 :::0.03 (1.358 :::0.012) x 10-1 2.8 396 :::4R17 40175·724455 8.32 :::0.09 (1.194 :::0.011)x 10.1 2.8 398 :::6

Two-mica granodiorite. Biot ite

R17 40175-724455 9.24 :::0.20 (1.327 :::0.012) x 10-1 1.4 398 :::9

Two-mica granitic dykes. Biotite

Rl l 40050-724330 9.80 :::0.15 (1.400 :::0.016) x 10-1 3.3 396 :::8R12 40050·724330 9.82 :::0.10 (1.420 :::0.016) x 10" 4.0 400 :::6

wen-rock skarn. Amph ibole

R2 40070-724355 1.206 :::0.010 (1.720 :::0.020) x 10-2 5.8 395 :::6R14 40050-724330 1.046 :!:0.018 (1.514 :::0.019) x 10.2 14.2 401 :::9R9 40070·724355 1.207 :::0.007 (1.755 :::0.020) x 10-2 7.5 402 :::5R6 40070-724355 1.490 :::0.014 (2.202 :::0.024) x 10-2 4.2 408 :::6R4 40070-724355 1.084 :::0.015 (1.610 :::0.020) x 10-2 7.3 410 :::8R7 40070-724355 1.403 ± 0.017 (2.115 ± 0028) x 10-2 6.3 415 :::7RB 40070-724355 1.150 :::0.020 (1.990 :::0.020) x 10.2 4.5 470 :::9R3 40070-724355 1.310 :::0.010 (2.290 :::0.020) x 10-2 7.2 474 :::5

' Mean of three analyses. " Mean of two analyses." 'H igher of two recorded atmo sphe ric contamination values.40K/K = 1.167 x 10-2 atom per cent. Aa = 0.581 x 1O-10a-1. I.P = 4.962 x 1O.10a-1.Constants after Steiger & Jager (1977).

NGU - BULL. 424, 1993

were performed by the isotope dilution methodon a modified Kratos MS10 mass spectre­meter (Wilkinson et al. 1986).

Nine of the fourteen apparent ages (Table1) are concordant at the 1c level. This groupcomprises the two muscovites (R10 and R17),the three biotites (R11 , R12 and R17) andfour of the amphiboles (R4, R6, R9 and R14).The Inverse Variance Weighted Mean (IVWM)of the nine ages is 402 ± 2 Ma. At the 2crconfidence level, twelve of the fourteen agesare concordant with an IVWM of 399 ± 2Ma (excluding only amphiboles R3 and R8).Such concordancy in apparent age amongdifferent minerals , and among samples of thesame mineral (with varying chemistries), indica­tes that the age obta ined has not been pertur­bed by factors such as the incorporation ofinitial argon into the crystal lattices or the lossor gain of variable argon /potassium causedby weathering or metasomatism (Dalrymple &Lanphere 1969). The age obtained may thusbe regarded as representative of an event inthe tectono-thermal history of the host lithologi­es which led to setting (or total resett ing) ofthe K/Ar isotopic system in the micas andamphiboles.

The cAOO Ma age accords with other K/Arages of micas from the Central ScandinavianCaledonides. Lux (1985) reported K/Ar agesfrom the Western Gneiss Region in the range410 to 370 Ma and a whole-rock sample ofkentallenite from the Bindal area (c.30 kmsouthwest of Malvika) gave an age of 399 ± 10Ma (Nordgulen & Mitchell 1988). Ages withinthe range 420-380 Ma have been interpretedas timing uplift and cooling at the end of theCaledonian Orogeny (e.g. Sturt et al. 1967).Hence, the ages obta ined from the mineralswe have stud ied are regarded as having anequivalent significance for the Helgeland Nap­pe Complex .

The concordancy of the ages is highly signi­ficant , given that muscovite has a somewhathigher blocking temperature than biotite andthat amphibole has a significantly higher block­ing temperature than muscovite (Dalrymple &Lanphere 1969). This implies that coolingthrough the respective thresholds for argonretention in each of these minerals must haveoccurred within a time scale of less than 5Ma. In the context of a regional metamorphicterrain this implies a process of rapid uplift.This inference accords with the views of Tuck­er et al. (1987), who obtained U/Pb ages from

Geology and K/Ar chron ology 71

the area to the southwest of Trondhe imsfjordthat pointed to a " 00 . very short period of buri­al and uplift in the Western Gneiss Region".This occurred at about 395 Ma (the timing ofcooling through the c.500°C threshold for isoto­pic diffusion of lead in titanite), virtually contem­poraneous with cool ing in the Malvika area.Further evidence of rapid uplift is prov idedby Johansson et al. (1990) from their studyof the metamorphic evolution of the Roan areaof Central Vestranden, also in the WesternGneiss Region. "'Ar-l9 Ar ages for six horn­blende separates give plateau ages around400 Ma (indicating the timing of cool ing throug habout 500°C). "'Ar-'9Ar ages of six muscoviteseparates indicate subsequent coo ling throughtheir c.350°C block ing temperature at 390-395Ma. This rapid cooling is alleged to have beenaccompanied by minimal uplift (Johansson etal. 1990). However, in a regional metamorphicenvironment, it is difficult to envisage by whatprocess rapid cooling could occur other thanby rapid uplift to high crustal levels. Rb-Srdating by Piasecki & Cliff (1988) on muscov itebooks from shear-zone pegmatites from theBjugn area of Central Vestranden gave agesof 389 ± 6 and 386 ± 6 Ma. Small, matr ix­size muscovite and biotite from the samemylonite zones , on the other hand, yieldedages of 378 ± 5 and 365 ± 5 Ma; theseyounger ages were interpreted by Piasecki &Cliff to relate to post-shearing uplift and coo­ling.

In the present study , among the eight amphi­boles dated one gave an age significantly ol­der than that of the main group at the 1oconfidence level (415 ± 7 Ma). Two othersamples gave indistinguishable (at 1c) appa­rent ages whose mean is 472 Ma, which isdistinguishably older than the remaining amphi­boles.

If the cA72 Ma age accurately records atectono-thermal (or crystallization) event thenthe timing of amphibolite facies metamorphismin the HNC would also be constrained. Thehigh-grade metamorphic event must have pre­ceded the crystallization of the undeformed,unmetamorphosed, amphibole wall-rock skarn .Amphibolite facies metamorphism and thedevelopment of the S2 fabric, {which coincidedwith the peak of metamorphism (Gustavson1988, Thorsnes & Lesetn 1991) would there ­fore have to be related to tecton ism of EarlyOrdovician age, or earlier .

72 D. Wyn James et al.

If the cA72 Ma age is valid then this wouldalso constrain the timing and mechanism ofmineralization. As scheelite pre-dated the for­mation of the amphibole wall-rocks, the mainmineralization event could not be associatedwith the adjacent two-mica granodior ite plu­ton. This is because the pluton is know n tobe younger than 430 Ma since it cuts theadjacent monzodiorite (Fig. 2), which has givena U/Pb age of 430 ± 7 Ma (Nordgulen et al.1993). The cA72 Ma age would also implythat scheelite mineralization pre-dates knownigneous activity by about 25 Ma (ignoring theRb/Sr Cambrian dates obtained by Nissen(1986) which have recently been supplantedby more reliable, younger UlPb ages (Nordgu­len et al. 1993). Furthermore, the earliest scne­elite mineralization at Malvlka could not thenbe related to the Kolsvik gold mineralization,since the latter is linked to Early Devonianbritt le deformation (P. lhlen, pers . comm. 1991).

These inferences are dependant on the as­sumption that the cA72 Ma apparent age isgeologically significant. However, previousK/Ar investigations of the Uppermo st AIIoch­thon have encountered problems with excessargon (Wilson 1972, Wilson & Nicholson 1973).Wilson (1972) was of the opinion that the ex­cess argon in the Sulitjelma region was in­herited as a result of argon outgassing fromthe 1700-1800 Ma basement during regionalmetamorph ism. Given the prox imity of theSulitjelma region it is a poss ibility that radioge­nic argon, released from basement rocks, wasprese nt in fluids circulating through the Malvi­ka area at the time of coo ling through theargon blocking temperature of amphibole (ata time later than 472 Ma). However, since thetotal radiogenic argon and potass ium contentsof the two samples with the oldest apparentages differ by about 15% it would be an impro­bable coincidence that excess argon resultedin their indistinguishable apparent ages. Thedifference (c.55 Ma) between the two oldestconcordant ages and the next oldest age isalso inconsistent with the randorn tztnq effectof excess argon on apparent ages.

In view of the contradictory evidence descr i­bed above, further radiometric ages wouldneed to be obta ined, utilising other isotopicmethods (e.g "'Ar - )9Ar or Sm/Nd - see Bell etal. 1989), before the two oldest K/Ar amphiboleages obtained can be conf idently regardedas having chrono logical significance.

GU - BULL. ~2~ . 1993

ConclusionsGeological evidence shows that pyroxene +plaqioclase skarn, at Malvika, post-dated theS2 fabric in the gneiss, indicating that theskarn form ed after the pea of regional meta­morph ism. This and the petrographic evidenceof scheelite crystallizing late in the skarn para­genesis demonstrate that tungsten was deposi­ted from epigenetic hydrothermal fluids. Thisrefutes Skaarup's (1 974) suggestion that thetungsten was deposited contemporaneouslywith the accumulation of the host lithologies.

K/Ar apparent ages of biotite, muscovite andthree amphibole separates def ine the coo linghistory of the Helgeland appe Complex atthe end of the Caledonian orogeny. The indis­tinguishability (at 1a) of apparent ages. fromthese minerals implies that rapid uplift wasrespon sible for the coo ling, a conclusion whichis consistent with the findings of two geochro­nological investigations of the Western GneissRegion (Tucker et al. 1987, Johannson et al.1990).

C1l 0:2 0w a: EVENT(9 w« Q.

z-cZ0

wan-rccx skarn ? Kolsvik Gold ?>w -0A408 -

Age of 2·M,ca

Iz<: Granodionteer::::>

AT

-'A00 U-Pb Age range I438 r----- o H C lntusrves

B T

Z

~-c

wan- rock skarnUs0

? >0er: T0

T ? Main W Mineralisation?? Arnpmbohte tac res e amorphism ?

505 -z<:er:Cl:;<!U

Fig. 7. Chronological context of scnee ute mineralisation .The upper black rectangle show the IVWM (with l a errorbar) of the two oldest (amphibole wall-rock skarn) K/Arages . A and B indicate the timing of cryst allization of themon zod ior ite pluton north -east of MAlvi a (see Fig. 2) andAndatsnatten pluton respectively (U·Pb on zircon; ordqu­len et al. in press .) 'Age of 2-mica Granoc ionte' indicatesthe possible time-s pan of intrusion of the cro ss-cuttinggranodiorite that lies adjacent to the U-Pb dated rnonzooiontepluton.

NGU · BULL. 424, 1993

Two oldest amph ibole samp les gave indist­inguishable (at 1o) apparent ages whose meanis 472 Ma. These apparent ages may recordthe timing of a tectono-thermal event or crys­tallization, It follows that the amphibolite faciesmetamorph ism of the host-rocks would haveoccurred during the Early Ordov ician, or earli­er. In addition, the earliest scheelite mineraliza­tion would pre-date intrusive activity in theHNC (Fig. 7) and would be unrelated to theEarly Devorvt , i gold/arsenopyrite minera liza­tion at Kolsvik. An alternative exp lanat ion forthese two apparent ages is that they are aresult of the incorporation of excess argonfrom circulating fluids , at about 400 Ma.

AcknowledgementsD,W.J. shou ld like to thank Peter Ihlen (of NGU) and Profes­sor Frank Vokes (of NTH) for their invaluable advice priorto and during fieldwo rk. Thanks are also due to the peopleof Lande for their hospitality and friendship . D'w.J shouldalso like to express his gratitude to the University of Shef­field (for the award of a Hossein Farmy Ph.D. Scholar­ship), and the NGU (which provided financial and logisticalsupport).

Geology and K/Ar chronology 73

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Manuscript received JUly 1992; revised typescript accepted February 1993.