Geology and K/Ar chronology of the Malvika schee­ lite skarns, Central Norwegian Caledonides

D. 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 of the MAlvika scheelite skarns , Centra l Norwegian Caledon ides. Nor. geol. unders . Bull. 424, 65-74.

Skarns present on the north ern shore of Tosenfjord overpr int the S2 fabric of the adjacent gneiss (which form ed during peak regional metamorphism). Together with petrographic evidence of the late cryst allization of scheel ite, this indicates that mineralization was post-meta morph ic and epi­ geneti 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 Inver­ se Variance Weighted Mean (IVWM) age of 402 :!: 2 Ma. The close agreement of results from different minerals points to rapid post-orogenic uplift in the Central Scandinavian Caledonide s. Two other post -scneetite amphibole skarn samples gave indistinguishable (at 1a) apparent ages whose mean is 472 Ma. If valid, this age would substantially constrain the timing of regional metamorphism and mineralization. However, the possibility that the ages are spurious , due to the presenc e of excess argon , cannot be excluded.

D. Wyn James & P. Richard Ineson, Department of Earth Sciences, University of Sheffield, Shef­ field, 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.

Introduction c) Scheelite precipitated from tungsten-bear­ Scheelite was first discovered within the Helge­ ing hydrothermal fluids of a magmatic origin, land Nappe Complex (HNC) in 1969. It was or tungsten was leached from the country­ located in skarns and quartz veins, associated rocks by fluids circulating around a granitic with marble (Nissen 1969), near Mosjeen (Fig. pluton . 1). Subsequently, scheelite was detected in the d) Scheelite precipitated after the cessation gold-arsenopyrite deposits of the Kolsvik area of magmatic activity, from tungsten-bearing (Skaarup 1974). A reconnaissance explorat ion hydrothermal fluids. programme for scheelite, in 1970, led to the The aim of the present study was to esta­ discovery of several additional occurrences blish constraints on the relative and absolute on either side of Tosenfjord. Brief descriptions timing of mineralization, and hence test the of the geology and petrography of these pros­ validity of the above hypotheses. The K/Ar pects were provided by Skaarup (1974), who radiometric ages presented here may also considered them to have a syngenetic origin. have incidental value in clarifying the tectono­ Syngenetic tungsten deposits are believed to metamorph ic evolution of the Helgeland Nappe form by the accumulation on the sea-floor of Complex. tungsten-rich precipitates, which crystallize as a result of the mixing of tungsten-bearing Regional Geology solutions (emitted from sea-floor smokers af­ The Helgeland Nappe Complex is the structu­ ter percolat ing through rock with a high basic rally highest unit of the Uppermost Allochthon igneous component) with colder sea water (Gee et al. 1985). The HNC is considered to (Maucher 1965). consist of at least two tectono-stratigraphic Four principal hypotheses may be postu lated units (Nordgulen & Schouenborg 1990, Thors­ for the tungsten (scheelite) mineralization at nes & Leseth 1991): Malvika: 1. Mafic and ultramafic lithologies occur in the a) Scheelite was syngenetic with the accumu­ western part of the HNC (Nordgulen & lation of the host lithologies. Schouenborg 1990). These rocks have been b) Scheelite precipitated from tungsten-bear­ interpreted as the remnants of a dismem­ ing hydrothermal fluids which were mobil­ bered ophiolite (Husmo & Nordgu len 1988, ised by regional metamorphism. Thorsnes & t.esetn 1991). 66 D. Wyn Jame s et al. NGU - BULL. 424. 1993

• Anatectic Gra nitoids

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

1-:-:-: -:1 Gran ite. GranOdiorite o Mon zodiorite. Quartz Mon zon ite. Syenite RC o Porphyritic Granite and Granodionte (» >1 Diorite. Tonalite . Granodionte, Trondh jem ite

1:·:-:-:-:-:1 Gabbro. Diorite. Monzodiorite

D Various metasupracrustal rocks

• Greenstone. Gabbro, Ultramafic Rocks

~ Leka Op hiolite Com plex

~ Rocks below the HNC and RNC

V:..~ Thrustlin ferred thrust

o km 30

Trondhe• im

Oslo • ",

; .

; < C',-, ji.-..... A ~ ,..?/i-: 'j'

Fig . 1. Geolog ical m ap of the Helgela nd Nappe Complex. HNC = Helge land Nappe Complex ; RC = Roo lngs fJallet appe Comp lex ; VS = Vestranden; K = Ko lsv ik : A = Ancatsnar te n Ptuton: rectangle enclo ses area o f Fig . 2. .Ioo.f,ed from oroquten & Sundvoll 1992.

2. A supracrustal sequence of cont inental Medium- to coarse-grained and megacrystic derivation has also been identified. It is granites and granodiorites typify the batholith composed of strong ly deformed, high-grade (Nordgulen & Mitchell 1988, Nordgulen et al. pelitic to semi-pelitic gneisses (partly mig­ 1993). Radiometric age determinations indicate matitic) and calcareous metasedimentary that intrusive activity was syn-orogenic, span­ rocks. ning the period from the Middle/Late Ordovi­ A cafe-alkaline intrusive comp lex, the Bindal cian to the Late Silurian (Nordgulen et al. 1993). Batholith, constitutes a substa ntial propo rtion Kollung (1967) and Myrland (1972) indicated of the HNC (Nordgulen & Schouenborg 1990). that peak metamorphism reached almandine- NGU - BULL. 424, 1993 Geology and K/Ar chronology 67 amphibolite facies throughout much of the HNC. The formation of the dominant (52) folia­ tion coincided with the peak of metamorph ism (Thorsnes & t.esetn 1991). Later deformation­ al events (D3 and D4) produced open, large­ scale folds (Myrland 1972, Terudbakken & Mickelson 1986, Gustavson 1988). These events are responsible for the present attitude (c.60° dip to ENE) of the rocks in the vicinity of the study area (Thorsnes & t.eseth 1991). As 52 structures are present in inclusions cut by the megacrystic Andalshatten pluton (dated at 447 ± 7 Ma; Nordgulen et al. 1993), the foliation must have formed prior to or during the Late Ordovician . There is therefore evidence from the HNC, as from the granitoid terrane of Srnela-Hltra (Roberts 1980, Gautneb & Roberts 1989) and elsewhere in the Upper Allochthon (Kullerud et al. 1988) and Upper­ most Allochthon (Claesson 1979), that a major • Tungsten Skarn Zone 0 km tectono-magmatic event occurred during the Granodiorite ~ S2 Foliation Ordovician (Nordgulen & 5chouenborg 1990). Cl Nevertheless, the final eastward thrusting of :0:':':0 Porphyritic Granite leR1?1 Sample localities the HNC probably took place during the 5candi­ an continent-to-continent collision. D Monzodiorite to Quartz Monzonite c=J Migmatitic Gneiss, Marble, Calc-silicate Gneiss

Fig. 2. Geol ogical map of the area surrou nding the scheelite Geology and Petrography of the skar ns of Malvika . Malvika skarns Tungsten-bearing skarns occur in two zones tensively in the skarns of the eastern zone, in the vicinity of Malvika, which is situated on whereas in the western zone veins and replace­ the northern side of Tosenfjord (Figs. 1 & 2). ment assemblages are volumetr ically minor. The southern tip of the 100 metre-wide main Fig. 3 illustrates the relationship between zone (5kaarup 1974) lies 100 m southwest skarn, marble and gneiss, in the western zone. of Malvika and extends NNW for 700 m in­ Each skarn horizon consists predominantly land, to a height of 400 metres. The second of two primary (Le. first-formed) lithologies, (western) zone is exposed in a road-cut 200 one consisting largely of pyroxene and plagio­ m to the southwest, near 5kjervikbugen. This clase whereas the other is composed mainly 10 m wide zone extends for 70 m north­ of garnet and pyroxene . The former occurs as wards, where it terminates against a promi­ a continuous horizon up to 15 cm in width , nent, ENE-W5W trending fault. adjacent to gneiss. The garnet skarn, in con­ The skarns occur as heterogeneous hori­ trast , generally occurs as disparate lenses (up zons not exceeding 1.5 metres in width . They to 5cm wide) along the marble-skarn contact are enclosed by leucocratic biotite gneiss or (Fig. 3). Fig. 4 shows that garnet skarn probab ­ melanocratic, weakly foliated, amphibole-bioti­ ly formed by replacement of marble. The gar­ te gneiss. The two zones differ in the abundan­ net skarn was, in turn , partially replaced by ce of marble. The marble in the eastern zone an epidote + quartz (+ residual pyroxene ) occurs as widely scattered inclusions, whereas assemblage. This accounts for the erratic dis­ it is present as continuous horizons enclosed tribution of garnet skarn lenses, as in Fig. 3. by < 30 cm wide skarn horizons in tt's wes­ A < 5mm wide selvage of wollastonite (+ tern zone. Anastomosing vein networks and pyroxene) skarn is typically developed at the assemblages formed by the replacement of contact between the garnet skarn and marble. the first-formed skarn assemblages occur ex- A zone of amphibole + plagioclase skarn 68 D. Wyn James et al. NGU - BULL. 424. 1993

~>: >J Calcite. marble or vein E:;J Litholoqical boundary O rien tation of mappe d s urlace o Biotite gneiss U Fraclure ~ Pyroxene + felsic minerals ska rn D Scheehte grain

~ Garnet z pyroxene skarn

o cm 30

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

« 5mm wide) is found at the contact of the pyroxene + plaagioclase skarn with the gneiss. ldocr ase (+ pyroxene) is rarely developed in place of the garnet skarn. A hydrou s assemblage cons isting mainly of epidote + quartz + amphibole is extensively developed 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 (with­ out scheelite) replaced the pyro xene + plagioc­ lase skarn. In places it is spatially associated with cross-cutting fractures or veins. How­ ever, the replacement skarn generally occurs more extensively, with preferential develop­ ment along the contacts between the primary skarn assemblages. Another type of hydrous mineral replacement is developed exclusively at the peripheries of quartz-calcite veins. It consists primarily of blades of ferro-pargasitic amphibole and cubes of almandine-spessar­ tine-rich garnet (James 1991). Variable propor­ tions of pyrrhotite, calcite, quartz and epidote occupy the interstices between amphibole and Fig. 4. Garnet skarn (G) replacing marbl e (M). A = Amph i­ garnet crystals. Coeval scheelite is absent from bole wall -rock skarn enclos ing calcite vein (C). H = Hyd­ this wall-rock assemblage. Biotite is a minor rou s r eplacemen t assemblage (after garnet skarn) con sis­ ting mainly of epidote and quartz (with minor scheelite). constituent of the quartz-calcite veins and Boulder. roadside. western zone . scheelite is rarely present. The veins are pla- NGU · BULl. 424. 1993 Geology and K/Ar chronology 69

Pegmabbc dyke

Fig. 5. Pyroxene skarn pro­ jecting irregularly across the foliation of the adjace nt arnphibole-biotite gneiss . b Western zone. nar, exhibit preferential orientation and are PRIMARY SKARN HYDROUS SKARN restr icted to the skarn. MINERAL Stage IV 8kaarup (1974) implied that the scheelite­ Stage I Stage 11 Stage III bearing skarns were formed by isochemical f----- I- transformation of tungsten -rich marl protol iths, Apatite during regional metamorphism. However, Fig. IImenite I- 5 shows pyroxene + plagioclase skarn cross­ Sphene I- - -- cutting the foliation of the adjacent gneiss at Ferro-Salite a high angle. This indicates that this type of - ~ Garnet -_---.6.- skarn post-dated the 82 fabric in the gneiss; hence the skarn formed after the peak of regio­ catcue ------nal metamorphism. 1- - . -- Scheelite The character of the ore at Malvika is also ...... Epidote - - inconsistent with Skaarup's (1974) conclu­ ,--. sions. Investigation of tungsten skarns wor ld­ Amphibole 1- - wide have shown that scheelite compr ises at Quartz -- -- least 95% of all skarn tungsten minerals (Kwak Plagioclase -- 1987). This is due to the very high negative Chalcopyrite - free energy of format ion of scheelite: - CaO + WO, = CaWO• .6Gi:' = -52,320 Call Pyrite Mole (Hsu & Galli 1973) Sphalente - Only if the Ca activity is low, or under super­ Pyrrhotite - gene conditions, do other tungsten minerals (mainly wolframite) form. Hence, it is reason­ Magnetite able to assume that any tungsten present in Goethite skarn and Ca-rich precursors to skarn would Fig. 6. Mineral paragenesis of garnet skarn. The horizo ntal occur in scheelite. The earliest generation of axis represen ts the relative timing of crystallisation. with scheelite in the Malvika skarns crystallized time increasing from left to right. The size of each shaded late in the primary paragenesis (minor schee­ area is indicative of the approx imate volumetric abundance of the phase. Apatite of Stage IV is fluor-apatite. Garnet lite occurs as a late-stage infill of the intersti­ of Stage I is qrossu lar. Garnet of Stages 11 and III is Gross ­ ces between the well-formed primary garnet Alm-Spess . Garnet of Stage IV is Alm-Spess. Scheelite is Mo-free. Epidote composition spans the epidote -clinozo isite crystals ; see Fig. 6 for the complete paragene­ divide and Stage IV amphibole is ferro -pargasitic (analyses sls), This points to the introduction of tungsten repor ted in James 1991). 70 D. Wyn James et al. NGU - BULL. 424. 1993

by epigenetic hydrothermal fluids. Further­ The dykes post-date the adjacent primary more , the most intensive phase of scheelite skarn, since xenoliths of skarn were present mineralization occurred later in the evolut ion within the dykes and an amphibole reaction of the skarn , during the replacement of the skarn was developed at the dyke-ska rn con­ primary garnet skarn by the hydrous mineral tact. Three whole-rock samples were obtained skarn. The conce ntration of scheelite present from fresh roadside expos ures of the two ­ in the primary skarn is far too small for the mica granodiorite pluton located 500 m north ­ scheelite in the replacement skarn to have east from the eastern zone. been derived solely by re-distribution of prima­ Chip samples were purified by crushing the ry skarn scheelite. Considering this toget her sample in a pestle and mortar and then pro­ with the evidence of a post- regional metamorp­ gressively removing the relatively magnetic hic origin for the primary pyroxere + plagiocla­ amphibole or biotite with an electromagnetic se skarn, it is concluded that it is highly impro­ separator. Purities in excess of 99 percent bable that tungsten mineralization in the Malvi· were attained. ka area was syngenetic with the deposition Whole-rock samples of the two- mica grano­ of the host-litholog ies. diorite were crushed and sieved to obtain an In order to constrain the timing of mineraliza­ optimal size fraction of 90 to 150 urn. Purities tion, and thereby test the afore-mentioned greater than 99% were attained for biotite. hypotheses, a K/Ar geochronological investiga­ Weakly paramagnetic muscovite proved diffi­ tion was undertaken. cult to separate from compound grains which consisted of non-magnetic quartz or feldspar K/Ar isotopic age determinations in combination with magnetic biotite or ilmen­ The amphiboles (Table 1) were chipped direct­ ite. This resulted in a purity of 95% for musco­ ly from fresh roads ide exposures of the vein vite separates. wall-rock skarn. Biotite chip samples R11 and Potassium analyses were carried out using R12 were obtained from pegmatitic two-mica a Corning 450 flame photometer with a lithium granitic dykes located in the western zone. internal standard. Argon isotopic analyses

Table 1. K/Ar determinations of biotite . muscovite and arnphlbole separates from the MAlvika area (see map-sheet 1825·1 Tosbotn 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 :::7 R18 40105-724420 9.51 :::0.03 (1.358 :::0.012) x 10-1 2.8 396 :::4 R17 40175·724455 8.32 :::0.09 (1.194 :::0.011)x 10.1 2.8 398 :::6

Two-mica granodiorite. Biotite 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 :::8 R12 40050·724330 9.82 :::0.10 (1.420 :::0.016) x 10" 4.0 400 :::6

wen-rock skarn. Amphibole R2 40070-724355 1.206 :::0.010 (1.720 :::0.020) x 10-2 5.8 395 :::6 R14 40050-724330 1.046 :!:0.018 (1.514 :::0.019) x 10.2 14.2 401 :::9 R9 40070·724355 1.207 :::0.007 (1.755 :::0.020) x 10-2 7.5 402 :::5 R6 40070-724355 1.490 :::0.014 (2.202 :::0.024) x 10-2 4.2 408 :::6 R4 40070-724355 1.084 :::0.015 (1.610 :::0.020) x 10-2 7.3 410 :::8 R7 40070-724355 1.403 ± 0.017 (2.115 ± 0028) x 10-2 6.3 415 :::7 RB 40070-724355 1.150 :::0.020 (1.990 :::0.020) x 10.2 4.5 470 :::9 R3 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 Geology and K/Ar chron ology 71 were performed by the isotope dilution method the area to the southwest of Trondhe imsfjord on a modified Kratos MS10 mass spectre­ that pointed to a " 00 . very short period of buri­ meter (Wilkinson et al. 1986). al and uplift in the Western Gneiss Region". Nine of the fourteen apparent ages (Table This occurred at about 395 Ma (the timing of 1) are concordant at the 1c level. This group cooling through the c.500°C threshold for isoto­ comprises the two muscovites (R10 and R17), pic diffusion of lead in titanite), virtually contem­ the three biotites (R11 , R12 and R17) and poraneous with cooling in the Malvika area. four of the amphiboles (R4, R6, R9 and R14). Further evidence of rapid uplift is provided The Inverse Variance Weighted Mean (IVWM) by Johansson et al. (1990) from their study of the nine ages is 402 ± 2 Ma. At the 2cr of the metamorphic evolution of the Roan area confidence level, twelve of the fourteen ages of Central Vestranden, also in the Western are concordant with an IVWM of 399 ± 2 Gneiss Region. "'Ar-l9 Ar ages for six horn­ Ma (excluding only amphiboles R3 and R8). blende separates give plateau ages around Such concordancy in apparent age among 400 Ma (indicating the timing of cool ing throug h different minerals , and among samples of the about 500°C). "'Ar-'9Ar ages of six muscovite same mineral (with varying chemistries), indica­ separates indicate subsequent cooling through tes that the age obtained has not been pertur­ their c.350°C block ing temperature at 390-395 bed by factors such as the incorporation of Ma. This rapid cooling is alleged to have been initial argon into the crystal lattices or the loss accompanied by minimal uplift (Johansson et or gain of variable argon/potassium caused al. 1990). However, in a regional metamorphic by weathering or metasomatism (Dalrymple & environment, it is difficult to envisage by what Lanphere 1969). The age obtained may thus process rapid cooling could occur other than be regarded as representative of an event in by rapid uplift to high crustal levels. Rb-Sr the tectono-thermal history of the host lithologi­ dating by Piasecki & Cliff (1988) on muscov ite es which led to setting (or total resett ing) of books from shear-zone pegmatites from the the K/Ar isotopic system in the micas and Bjugn area of Central Vestranden gave ages amphiboles. of 389 ± 6 and 386 ± 6 Ma. Small, matrix­ The cAOO Ma age accords with other K/Ar size muscovite and biotite from the same ages of micas from the Central Scandinavian mylonite zones , on the other hand, yielded Caledonides. Lux (1985) reported K/Ar ages ages of 378 ± 5 and 365 ± 5 Ma; these from the Western Gneiss Region in the range younger ages were interpreted by Piasecki & 410 to 370 Ma and a whole-rock sample of Cliff to relate to post-shearing uplift and coo­ kentallenite from the Bindal area (c.30 km ling. southwest of Malvika) gave an age of 399 ± 10 In the present study , among the eight amphi­ Ma (Nordgulen & Mitchell 1988). Ages within boles dated one gave an age significantly ol­ the range 420-380 Ma have been interpreted der than that of the main group at the 1o as timing uplift and cooling at the end of the confidence level (415 ± 7 Ma). Two other Caledonian Orogeny (e.g. Sturt et al. 1967). samples gave indistinguishable (at 1c) appa­ Hence, the ages obtained from the minerals rent ages whose mean is 472 Ma, which is we have studied are regarded as having an distinguishably older than the remaining amphi­ equivalent significance for the Helgeland Nap­ boles. pe Complex . The concordancy of the ages is highly signi­ If the cA72 Ma age accurately records a ficant, given that muscovite has a somewhat tectono-thermal (or crystallization) event then higher blocking temperature than biotite and the timing of amphibolite facies metamorphism that amphibole has a significantly higher block­ in the HNC would also be constrained. The ing temperature than muscovite (Dalrymple & high-grade metamorphic event must have pre­ Lanphere 1969). This implies that cooling ceded the crystallization of the undeformed, through the respective thresholds for argon unmetamorphosed, amphibole wall-rock skarn . retention in each of these minerals must have Amphibolite facies metamorphism and the occurred within a time scale of less than 5 development of the S2 fabric, {which coincided Ma. In the context of a regional metamorphic with the peak of metamorphism (Gustavson terrain this implies a process of rapid uplift. 1988, Thorsnes & Lesetn 1991) would there ­ This inference accords with the views of Tuck­ fore have to be related to tecton ism of Early er et al. (1987), who obtained U/Pb ages from Ordovician age, or earlier . 72 D. Wyn James et al. GU- BULL. ~ 2 ~ . 1993

If the cA72 Ma age is valid then this would Conclusions also constrain the timing and mechanism of Geological evidence shows that pyroxene + mineralization. As scheelite pre-dated the for­ plaqioclase skarn, at Malvika, post-dated the mation of the amphibole wall-rocks, the main S2 fabric in the gneiss, indicating that the mineralization event could not be associated skarn form ed after the pea of regional meta­ with the adjacent two-mica granodiorite plu­ morph ism. This and the petrographic evidence ton. This is because the pluton is know n to of scheelite crystallizing late in the skarn para­ be younger than 430 Ma since it cuts the genesis demonstrate that tungsten was deposi­ adjacent monzodiorite (Fig. 2), which has given ted from epigenetic hydrothermal fluids. This a U/Pb age of 430 ± 7 Ma (Nordgulen et al. refutes Skaarup's (1 974) suggestion that the 1993). The cA72 Ma age would also imply tungsten was deposited contemporaneously that scheelite mineralization pre-dates known with the accumulation of the host lithologies. igneous activity by about 25 Ma (ignoring the K/Ar apparent ages of biotite, muscovite and Rb/Sr Cambrian dates obtained by Nissen three amphibole separates define the cooling (1986) which have recently been supplanted history of the Helgeland appe Complex at by more reliable, younger UlPb ages (Nordgu­ the end of the Caledonian orogeny. The indis­ len et al. 1993). Furthermore, the earliest scne­ tinguishability (at 1a) of apparent ages. from elite mineralization at Malvlka could not then these minerals implies that rapid uplift was be related to the Kolsvik gold mineralization, respon sible for the cooling, a conclusion which since the latter is linked to Early Devonian is consistent with the findings of two geochro­ brittle deformation (P. lhlen, pers. comm. 1991). nological investigations of the Western Gneiss These inferences are dependant on the as­ Region (Tucker et al. 1987, Johannson et al. sumption that the cA72 Ma apparent age is 1990). geologically significant. However, previous C1l 0 K/Ar investigations of the Uppermo st AIIoch­ :2 0 w a: EVENT thon have encountered problems with excess (9 w argon (Wilson 1972, Wilson & Nicholson 1973). « Q. z Wilson (1972) was of the opinion that the ex­ -c Z cess argon in the Sulitjelma region was in­ 0 wan-rccx skarn ? Kolsvik Gold ? herited as a result of argon outgassing from w> 0 - the 1700-1800 Ma basement during regional 408 - A z Age of 2·M,ca metamorph ism. Given the prox imity of the <: er: Granodionte Sulitjelma region it is a poss ibility that radioge­ :::> TI -' A nic argon, released from basement rocks, was A 00 U-Pb Age range I prese nt in fluids circulating through the Malvi­ 438 r----- o H C lntusrves ka area at the time of cooling through the B T argon blocking temperature of amphibole (at Z -c a time later than 472 Ma). However, since the wan- rock skarn Us total radiogenic argon and potass ium contents 0 0 of the two samples with the oldest apparent er: > 0 ? T ~ ages differ by about 15% it would be an impro­ T ? Main W Mineralisation? bable coincidence that excess argon resulted ? Arnpmbohte tac res e amorphism ? in their indistinguishable apparent ages. The 505 - z difference (c.55 Ma) betwee n the two oldest <: er: concordant ages and the next oldest age is Cl :; also inconsistent with the randorntztnq effect

Two oldest amph ibole samp les gave indist­ References inguishable (at 1o) apparent ages whose mean Bell, K.. Anglin, C.D. & Franklin, J.M. 1989: Srn-Nd and is 472 Ma. These apparent ages may record Rb-Sr isotope systematics of scheelites : Possible impli· the timing of a tectono-thermal event or crys­ cations for the age and genes is of vein-hosted gold depos rts. Geology 17. 500-504 . tallization, It follows that the amphibolite facies Claesson, S. 1979: Pre-Silurian orogenic deformat ion in the metamorphism of the host-rocks would have north -centr al Scandinavian Caledo nides. Geot. Foren. occurred during the Early Ordov ician, or earli­ Stockn . F6rh. 101. 353-356 . er. In addition, the earliest scheelite mineraliza­ Dalrymple, G.B. & Lanphere, MA 1969: Potassium-Argon tion would pre-date intrusive activity in the Dating: Principles. Techniques and App lications to Geo­ chronology.W.H. Freeman , San Franc isco . HNC (Fig. 7) and would be unrelated to the Gautneb. H. & Roberts, D. 1989: Geology and petrochemis­ Early Devorvt , i gold/arsenopyrite mineraliza­ try of the smeia-xnra Batholith, Centra l Norway . Nor. tion at Kolsvik. An alternative explanation for qeot.unaers. Bull. 416. 1-24. these two apparent ages is that they are a Gee, D.G., Kumpu lainen, R.. Roberts , D.. Stephe ns, M.B. result of the incorporation of excess argon Thon, A. & Zachr isson , E. 1985: Tectonostraligraphic from circulating fluids , at about 400 Ma. map, scale 1:2 000 000. In: Gee. D.G. & Sturt , BA (eos) The ceteaoniae Orogen -Scandinavia and Related Areas . John WHey & Sons , Chichester. Acknowledgements Gustavson, M. 1988: Mosj0en. Description of the 1:250,000 D,W.J. should like to thank Peter Ihlen (of NGU) and Profes­ bedrock geolog ical map. Nor. geol. unders . Skr. 87. sor Frank Vokes (of NTH) for their invaluable advice prior 42pp . to and during fieldwo rk. Thanks are also due to the people Hsu , L.C. & Galli, P.E. 1973: Origin of the scheeli te-powelli­ of Lande for their hospitality and friendship. D'w.J should also like to express his gratitude to the University of Shef­ te series of minerals. Economic Geology 68. 681-696. field (for the award of a Hossein Farmy Ph.D. Scholar­ Husrno. T. & Nordgulen, 0 . 1988: Struc tural relations along ship), and the NGU (which provided financial and logistical the western boundary of the Helgeland Nappe Com p­ support). lex, North -Centra l Norway.(extended abstract). Inst. Geol. Oslo, Intern skr. ser. nr. 54 (unpubl.), 21-23. James . D'w. 1991: The Geology and Geochem istry of the MtJlvika Tungsten Skarn Deposit. Central Norway. Un­ published Ph.D thesis . Univers ity of Sheff ield. Johansson , L., Mo ller , C. & Dallmeyer. D. 1990: Timing of Caledonian metam orph ism , uplift and cooling in Cent ral Vestranden. Geonytt 1. 62 (abstract). Kollung, S. 1967: Geologiske underseketser i det se rnqe Helqe land og nord lige Namdal. Nor. geol. enders . 254, 95pp . Kullerud , K., Stephens, M.B. & Claesson, S. 1988: Age constraints on exot ic arc-basin comp lexes and tect onic implications, centra l Scandinavian Caledonides. Geot. Foren. Stockn. Forn. lID, 402-403 . Kwak , TAP. 1987: W-Sn skern depos its and related meta­ morphic skarns and qren itoids. Elsevier, Amsterdam. Lux, D.R. 1985: K/Ar ages from the Basal Gneiss Region, Stadlandet area , Western Norway . Nor. Geol. Tiosskr. 65. 277-286. Mauch er, A. 1965: Die Antimon-W olfram -Quecksilber­ Format ion uno ihre Beziehungen zu Magmatismus und Geotektonik. Freiburger Forscnunqstiette C 186. 173-188. Myrland . R. 1972: Velfjord. Beskrivelse til det bergg runns ­ qeoloqiske gradteigskart I 18 - 1:100,000. Nor. geol. unaers. 274, 30pp. Nissen, A.L. 1969 A new Norweg ian occurrence of scnee­ lite. Nor. geol. unders. 258. 116. Nissen. A.L. 1986: Rb/Sr age determinati on of intrusive rocks in the southeas tern part of the Bindal Massif. Nord-Trondelaq. Norway . Nor. geol. unders . Bull. 406. 83- 92. Nordgulen, 0 . & Mitcnelt, J.G. 1988: Kentallenite (olivine­ monzon ite) in Bindal, Central Norwegian Caledon ides . Nor. geo l. unders. Bull. 413, 51-60. Nordqulen , 0. & Schouenborg. B. 1990: The Caledonian Heilhornet Pluto n, north-central Norway : geolog ical set­ ting. radiometric age and implications for the Scand inav­ ian Catedonides. J. geol. Soc. London 147. 439-450. ordgulen, 0 . & suncvon. B. 1992: Strontium isotope compo- 74 D. Wyn James et al. GU - BULL. 424. 1993

sition of the Bindal Batholith, central Scandinavian Velfjord-Tosen region, southweste rn part of the Helge­ Caledonides . Nor.qeot.unoers. Bull. 423, 19-39. land Nappe Complex, Centra l orweg ian Caledonides: Nordgulen, 0 .. Bickford, M.E., Nissen, A.L. & Wortman , Nor. geo l. unders . 42 1, 1-18. G.L. 1993: U-Pb ages from the Bindal Batholith and the Tucker, R.D., Rtlheim, A.• Krog h. T.E. & Corfu, F. 1987: tectonic history of the Helgeland Nappe Complex, Scan­ Uranium-lead zircon and titanite ages from the northern dinavian Caledonides. J.geol.Soe. Londo n. port ion of the Western Gneiss Region. south-central Piasecki, MAJ. & Cliff, RA 1988: Rb-Sr dating of strain­ Norw ay. Earth Planet. Sci. Lert. 81, 203 -211. induced mineral growth in two ductile shear zones in Terudbakken. B.O. & Mickelson, M. 1986: ARb/Sr stud y the Western Gneiss Region of Nord-Trondelag. Nor. from the Mosjeen unit, Helgeland appe Complex, and geol. unders. Bull. 413, 33-50. its bearing on the timing of the tectonometamo rphic Roberts, D. 1980: Petrochemistry and palaeogeographic events within the Uppermost Allochthon, Central Scandi­ setting of the Ordov ician volcanic rocks of Sm01a , Cen­ navian Caldedonides , Norway . Nor. Geol. Tiasskr. 66, tral Norway. Nor. geol. unoers. 359. 4';-60. 263-270 . Skaarup, P. 1974: Strata-bound scheelite mineralization in Wilkinson , P., Mitche ll, J.G., Cattermole, P.J. & Downie, skarns and gneisses from the Bindal area, northe rn C. 1986: Volcanic chronology of the Meru-Kilimanjaro Norway. Mineralium Deposita 9, 299-308. region, Northe rn Tanzania. J. geol. Soc. London 143, Steiger, R.H. & Jii ger, E. 1977: Subcommision on Geochro­ 601-605. nology: conven tion on the use of decay constants in Wilson, M.R. 1972: Excess radiogenic argo n in metamorp­ geo - and cosmochronology . Earth Planet. Sei. Lert. 36, hic amphibotes and biotites from the Sulitjelma Region, 359 -362 . Central Norweg ian Caledonides. Earth Planet. Sei. Lert. Sturt , BA. Miller, J.A. & Fitch , F.J. 1967: The age of alkali­ 14, 403 -412. ne rocks from West Finnmark. northern Norway, and Wilson, M.R. & Nicnotson, R. 1973: The structural setting their bearing on the dating of the Caledonian orogeny. and geoch ronology of the basal granitic gneisses in the Nor. Geol. Tidsskr. 47, 255-273 . Caledonides of part of Nordland. orway. J. geol. Soc. Thorsnes. T. & Lesetn, H. 1991: Tectonostratigraphy in the London 129, 365 -87.

Manuscript received JUly 1992; revised typescript accepted February 1993.