Economic geology research, volume 1 1999-2000

Research Paper C 833

Economic geology researchVolume 1

1999–2000

Pär Weihed, editor

SGU


Economic geology research, Volum

e1 1999–2000

Research Papers C 833

Economic geology researchVolume 1

1999–2000

Pär Weihed, editor

Sveriges Geologiska Undersökning2001

ISSN 1103-3371ISBN 91-7158-665-2

Cover: Augite oikocryst-bearing layers alternating with olivine–gabbro/troctolite layers in the Hoting mafic intrusion. Photo Birger Filén.

© Sveriges Geologiska Undersökning Geological Survey of Sweden

Layout: Agneta Ek, SGUPrint: Elanders Tofters, Östervåla 2001

Content

A review of Palaeoproterozoic intrusive hosted Cu-Au-Fe-oxide deposits in northern Sweden ................................ 4Pär Weihed

Swedish layered intrusions anomalous in PGE-Au .................................................................................................. 33Birger Filén

Palaeoproterozoic deformation zones in the Skellefte and Arvidsjaur areas, northern Sweden ................................. 46Jeanette Bergman Weihed

Geochemistry and tectonic setting of volcanic units in the northern Västerbotten county, northern Sweden .......... 69Ulf Bergström

Rock classification, magmatic affinity and hydrothermal alteration at Boliden, Skellefte district, Sweden – a desk-top approach to whole rock geochemistry .................................................................................................... 93Anders Hallberg

A review of the Fe oxide deposits of Bergslagen, Sweden and their connection to Au mineralisation .......................132Magnus Ripa

Preface

The current volume is the result of a new attempt to continuously publish data from the work within the ore documenta-tion programme at the Geological Survey of Sweden. Our aim is to publish various papers of interest to exploration and mining companies as well as the interested layman. The results within this volume stem from work carried out by scien-tists of the Geological Survey of Sweden during the years 1999 and 2000. One paper (Palaeoproterozoic deformation zones in the Skellefte and Arvidsjaur areas, northern Sweden by Jeanette Bergman Weihed) is the result of an external research project financed by the Geological Survey of Sweden.

It is our ambition to continue to publish compilations and novel research based on the vast unpublished archives of the Geological Survey of Sweden. We will also try to publish new ideas on various aspects of earth sciences that can contribute to a better understanding of ore deposit geology and ore genesis.

Dr Kjell Billström (Swedish Museum of Natural History, Stockholm) and Dr Olof Martinsson (Luleå University of Technology, Luleå) are acknowledged for thorough reviews of all papers in this volume.

Uppsala, October 2001Pär Weihed

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Weihed, P,. 2001: A review of Palaeoproterozoic intrusive hosted Cu-Au-Fe-oxide deposits in northern Sweden. In Wei-hed, P. (ed.): Economic geology research. Vol. 1, 1999–2000. Upp-sala 2001. Sveriges geologiska undersökning C 833, pp. 4–32. ISBN 91-7158-665-2.

The present study is aimed at highlighting a new group of in-trusive hosted Cu-Au-Fe oxide deposits in northern Sweden. Altogether 9 different deposits are briefly described: Tallberg, Granberg, Viterliden, Gråberget, Sarvasåive, Sadenåive, Vaiki-jaur, Iekelvare, and Sjisjka. Most deposits share common char-acteristics, such as being hosted by intermediate to felsic intru-sive rocks. They are all also Cu±Au±Fe oxide dominated depos-its with a similar ore mineralogy and they generally contain one or several of potassic, sodic and silicic alteration type assem-blages. However, the host rocks belong to several different intru-sive suites from early- to syn-orogenic suites like the Jörn-type through slightly younger GSM-type to young post deformation Lina and Revsund types. The mineralizations seem to be focused on high level or subvolcanic stocks, dykes, and sills that crystal-lised late in the evolution of the magmatic systems and hence the magmatic hydrothermal fluids possibly responsible for the formation of the deposits may have a magmatic origin. The de-posits show similarities with porphyry style as well as Fe-oxide Cu-Au deposits and are here suggested to belong to the same general group of deposits. Genetic relationship with VMS, epi-thermal, and Kiruna type deposits can be inferred for some de-posits.

Pär Weihed, Geological Survey of Sweden, Box 670. Present address: Luleå University of Technology, 971 87 Luleå, Sweden. E-mail: [email protected]

Introduction

During the 1970’s and 1980’s, exploration for base metals was carried out by both the Swedish State, through SGU (later SGAB) and LKAB, and private companies, e.g. Boliden, in northern Sweden. In many cases, promising mineralizations were found although relatively little drill-ing was done. Many of the prospects contain Cu±Au±Fe-oxides and share many features although this was not em-phasised at the time. From reports on individual prospects it is clear that many of the deposits are associated with intrusive rocks of felsic to intermediate composition and show similarities to porphyry style mineralization as well as to Proterozoic Cu-Au-Fe-oxide style deposits. Another striking observation is that the mineralizations have been considered sub-economic despite the fact that only few drillholes have been drilled, assaying of the core material

is not consistently done (many mineralized cores are not assayed at all), and the mineralised systems are not known at depth.

In this review of some of the interesting deposits a short description of each is given, the style of minerali-zation is described in macro- and microscopic scale, and a metallogenetic model is proposed for the deposits. It should be emphasised that it has only been possible to work with a very limited part of the vast material avail-able from these deposits. Drill cores from the deposits described in some detail below have been logged and all available reports have been used in this study. In Figure 1 the location of all described deposits are shown.

Deposit descriptions

Below, each deposit is described separately from a regional setting point of view. Mineralization style, a summary sec-tion with some genetic considerations, and deposit char-acteristics are presented. In the final paragraph, all depos-its are discussed in a metallogenetic framework for north-ern Sweden.

Gråberget

The Gråberget Cu-deposit is located on the mapsheet 24I, square 2d (Swedish nat. grid) c. 35 km northwest of the township of Malå (cf. Figs. 1 and 2). The first discoveries of glacial boulders in the area were made in the 1940ies during the county mapping by Gavelin (1955). During this period, some trenching was made and assays from trench samples indicated c. 3.1–3.6 % S and 0.8–1.4 % Cu which at the time was considered to be of no interest and consequently the exploration in the area ceased.

Renewed interest in the area in 1974 through the ”Mineral hunt” in the county of Norrbotten, resulted in a new discovery of a glacial boulder that contained 3.2 % Cu. At the same time, regional geochemical sampling in-dicated anomalous Cu, Pb, and Mo in till samples. The area was investigated by SGU for NSG in 1975 with boulder tracing and geochemical and geophysical surveys. During 1975 to 1977 diamond drilling was carried out by SGU during which period 47 holes were drilled. The results from the drilling have been compiled in Claesson (1979). The map presented here is modified from Claes-son (1979).

A review of Palaeoproterozoic intrusive hosted Cu-Au-Fe-oxide deposits in northern Sweden

Pär Weihed

5A REVIEW OF PALAEOPROTEROZOIC INTRUSIVE HOSTED CU-AU-FE-OXIDE DEPOSITS IN NORTHERN SWEDEN

The present study is based on the report by Claesson (1979) with some additional core logging and microscop-ic investigations of thin sections.

Regional geology

The mineralization at Gråberget is spatially associated with a red quartz feldspar porphyritic phase of the Ledfat granite related to the Sorsele type intrusions. The Sorsele granite has been dated by U-Pb on zircons at 1791±22 Ma by Skiöld (1988) while the Ledfat granite at Gråberget has been dated at 1784±62 Ma and 1772±14 Ma respectively by U-Pb ages on zircons (Skiöld 1988). The red porphy-ritic phase of the Ledfat granite is intrusive into the Led-fat conglomeratic unit. This unit contains granitoid clasts, which have been dated at 1896±50 Ma and 1866±17 Ma by Skiöld (1988). Hence the Ledfat conglomerates must have been emplaced after c. 1.87–1.88 Ga and the Ledfat

conglomerates have been paralleled with the Vargfors con-glomerates in the Skellefte district which have been dated at 1875±4 Ma by U-Pb zircons from an intercalated felsic ignimbrite (Billström & Weihed 1996).

The regional geology of the map sheet 24I is shown in Figure 2, which is compiled from the digital bedrock database at SGU. The oldest rocks in the area belong to the Skellefte Group and are found in the Adak area. Rocks are dominantly felsic to mafic volcanic rocks but are also intercalated with greywackes in higher stratigraphic po-sitions in the Adak area. The volcanic rocks of the Skel-lefte Group are coeval and considered comagmatic with the calc-alkaline I-type intrusions generally in this area re-ferred to as the Jörn type of intrusions.

In the area southwest of Gråberget (Fig. 2) extensive units of mafic volcanic rocks are intercalated with grey-wackes. These mafic volcanic rocks are younger than the

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TIB-related 1.80 Ga felsic volcanic rocks

A- to I-type, 1.80 Ga felsic intrusive rocks of Revsund type, associated with minor Sn-W mineralization

S-type, anatectic, c. 1.80 Ga granites of Skellefte, Härnö, and Lina types, associated with minor Sn-W and Mo-Cu mineralizations

Metagreywackes of the Härnö and Råneå Groups, >1.95–1.85 Ga mesothermal gold lode mineralizations

Formlines of foliation

Mainly subaerial volcanic rocks, c. 1.88 Ga, Kiruna and Arvidsjaur type. Disseminated Cu-Au deposits, Kiruna type Fe ores

Mainly submarine volcanic rocks, c. 1.89–1.88 Ga, Skellefte type. VMS, epithermal Au-VMS, mesothermal gold lode deposits

Calc-alkaline granitoids and mafic intrusions of Jörn type, 1.95–1.86 Ga, and GSM type, 1.87–1.86 Ga. Porphyry Cu-Au and mesothermal gold lode deposits, mafic and ultramafic hosted Ni

Palaeoproterozic greenstone belts, 2.5–2.0 Ga. Viscaria type Cu, disseminated Cu-Au, BIF, layered intrusion hosted Ni

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Fig. 1. Simplified geological map of northern Sweden (modified from Stephens et al. 1994). The deposits described in the text are indicated on the map as follows: 1=Tallberg, 2=Granberg, 3=Viterliden, 4=Sarasåive, 5=Gråberget, 6=Lulepotten, Sadenåive, 7=Vaikijaur, 8=Iekel-vare, and 9=Sijsjka.

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Skellefte Group but older than the Arvidsjaur Group. No age determinations exist from this rock unit. The mafic rocks, which informally have been called the Tjamstan

formation, forms part of the Malå group on the Malå map sheet to the south (Bergström & Sträng 2000).

The Arvidsjaur Group of mainly subaerial felsic with

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GSM type intrusive rock, Arvidsjaur type, c. 1.88 Ga old

Sorsele type granites (incl. Ledfat type), c. 1.80 Ga old

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Mafic volcanic rock, Arvidsjaur type, c. 1.75 Ga old

Ledfat conglomerate, c. 1.75 Ga old

Ledfat sandstone, c. 1.75 Ga old

Felsic volcanic rock, Arvidsjaur type, c. 1.75 Ga old

Felsic volcanic rock, Skellefte type, c. 1.89 Ga old

Gråberget Cu-deposit

Mafic volcanic rock, Skellefte type, c. 1.89 Ga old

Greywacke, c. 1.89 Ga old

Gabbroic intrusion, c. 1.89 Ga old

Mafic volcanic rock, c. 1.88 Ga old

Form line

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Fig. 2. Simplified geological map of the Storavan mapsheet. The geology is modified from the digital database over the Norrbot-ten County available at the Geological Survey of Sweden. The Gråberget deposit is indicated.


minor mafic volcanic rocks is generally considered to over-lie the submarine Skellefte Group of volcanic rocks. The Arvidsjaur Group volcanic rocks are also considered to un-derlie the Ledfat Group of conglomerates and sandstones. The Arvidsjaur Group has been poorly dated at 1878±2 Ma (Skiöld et al. 1993) by U-Pb on zircons (only two fractions from Skyberget) and 1876±3 Ma (pooled zircon fractions from Skyberget, Bure, and Gråberget; Skiöld et al. 1993). The Arvidsjaur Group is also considered comag-matic with the Arvidsjaur type granitoids and age deter-minations of these granitoids fall within the same time span as the Arvidsjaur Group volcanic rocks.

The structural evolution in the area is not clear and no thorough studies of the metamorphic evolution have been undertaken so far. However, in general all rocks but the Sorsele type intrusions have experienced ductile deforma-tion in greenschist to amphibolite grade metamorphism during the svecokarelian orogeny. The timing of deforma-tion and metamorphism is poorly constrained but region-ally the peak of metamorphism is considered to have oc-curred between 1.86 and 1.80 Ga (cf. Billström & Weihed 1996). The main stages of penetrative deformation is gen-erally in the same time span. However, regional ductile to brittle shear zones with a NNE strike have been active at c. 1.79 Ga and hence are coeval with the young Sorsele type intrusions. An important note in this respect is that the Gråberget area is extremely well preserved with very little penetrative deformation and lower greenschist facies metamorphism. It is possible that the Gråberget (and Led-fat area) forms part of a NNE trending downfaulted belt of supracrustal rocks that are bounded by the NNE strik-ing c. 1.79 Ga shear system (cf. Fig. 2).

Mineralization

The geology of the Gråberget area is shown in Figure 3, which is modified from Claesson (1979). The host rocks to the mineralization are both granite and conglomerates, suggesting an epigenetic origin. As mentioned above, the main mineralization is located within the quartz-feldspar porphyritic border zone of the intrusion.

The granite intrudes the conglomerate as irregularly shaped dykes of variable widths. The main dyke is c. 200–300 m wide in the mineralised area. The northern contact of the dyke is steep whereas the southern contact has a gentle dip towards south. Post mineralization dia-base dykes intrude all other rocks in the area (Fig. 3). The textural changes, from clearly porphyritic at the contact to evengrained granite in the central parts of the granite dyke, appear gradual in drillcore even if it is probable that the dyke consists of several intrusive phases. The typi-cal quartz feldspar porphyritic character of the marginal zone is shown in Figure 4 (sample 75007/8.00 m and 75007/10.30 m), whereas the more typical evengrained variety is shown in Figure 4 (sample 75001/130.50 m and 75001/155.00 m). The evengrained granite consists of microcline, oligoclase, quartz, biotite, and epidote as main minerals with accessory apatite, titanite, and opaques (Claesson 1979). The porphyritic variety is more micro-cline and quartz rich with euhedral quartz phenocrysts and tabular, zoned euhedral feldspar phenocrysts (Fig 5a). The plagioclase is variably sericitized and biotite is partly chloritized. Epidote is a common fracture mineral besides occurring as an alteration mineral after plagioclase.

The conglomerates are polymict, grey to greenish grey with a fine matrix that consists of lithic fragments of vari-

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Geology of the Gråberget Cu-depositmodified from Claesson (1979)

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Fig. 3. The geology of the Gråberget Cu-deposit (modified from Claesson 1979). Drillholes and mineralizations A and B are indicated.

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ous supracrustal rocks as well as monomineralic quartz and feldspar. Clasts are generally well rounded and up to 20–30 cm in size. Supracrustal rocks, as well as scattered granite rocks, constitute the clast material. Silicification and epidote fracture fillings are common towards the con-tact with the granite (Claesson 1979).

The mineralization consists of a dissemination and veinlets of chalcopyrite and pyrite (Fig. 4). The sulphides clearly overprint earlier magnetite veins and dissemina-tions (Fig. 4).

In Figure 5, micrographs are shown from the host rocks and mineralization. In Figure 5a, the microcline-

Fig. 4a. Photographs of drillcore specimens from the drillholes 75001 and 75007 (for location see Fig. 3). The white scale bar is approximately 35 mm long. Sample 75001/15.30 m – weak mineralization in biotized conglomerate, Sample 75001/23.60 m same as previous, Sample 75001/30.30 m – porphyritic granite typical of the rim of the major granitic dyke, Sample 75001/42.60 m – strongly altered granite with microcline, albite, quartz as major alter-ation minerals, Sample 75001/48.00 m – same as previous, Sample 75001/52.50 m – same as previous, Sample 75001/56.10 m – same as previous, Sample 75001/59.60 m – same as previous with early opaque (magnetite)-biotite vein overprinted by late disseminated sulphides, Sample 75001/119.90 m – same as 75001/42.60 m, Sam-ple 75001/130.50 m – well preserved even, medium-grained granite typical of the central parts of the granitic dyke, Sample 75001/155.00 m – same as previous but finer grained variety,


albite-quartz phyric character of the host rock is clearly seen. A few larger chloritized grains of biotite are also visible. The sulphide vein-breccia in Figures 5b–c is rep-resentative of the style of mineralization. In this micro-graph the porphyritic nature of the host rock is still seen. A slightly different style of mineralization is seen in Figures 5d–e where two generations of veins exist; a coarser grained vein composed of opaques and chloritized biotite and a parallel rim of secondary fine-grained biotite quartz-microcline. In Figures 5f–g a fine network veining of sulphides is associated with a fine-grained gangue of microcline, albite, quartz, biotite, and sericite. In mineral-ised parts of the conglomerate, a strong biotitization was

evident in thin sections.Although this brief study has not identified any clear

paragenetic zoning from the limited drill core material it seems plausible that early magnetite in veins is part of the mineralising system (Fig. 4 75001/59.60 m and Figs. 5e–g). The alteration associated with the minerali-zation is dominantly a potassic-sodic style of alteration with common bleaching (microcline-albite-quartz) or bio-titization. Silicification and albitization may be part of the alteration style, but too few samples have been studied in order to understand the regional extent of these types of alteration assemblages. Brecciation with epidote and sul-phide seems to record a late stage of mineralization.

Fig. 4b. Sample 75007/8.00 m – porphyritic phase of the granite, Sample 75007/10.30 m same as previous, Sample 75007/15 m – altered granite with very fine sulphide veinlets with fine-grained microcline, quartz, biotite, and sericite, Sample 75007/22.00 m – possible hematite stained granite with relatively strong microcline-quartz-albite alteration, Sample 75007/37.00 m – same as previous, and Sample 75007/46.50 m – late brecciation with epidote, opaques and chloritized biotite and tourmaline.

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Fig. 5. Micrographs from drillcore specimens shown in Figure 4. A) Sample 75001/30.30 m shows the typical porphyritic nature of the border of the granitic dyke. Euhedral microcline and albite phenocrysts together with anhedral subgrained quartz phenocrysts in a groundmass of similar composition. Also subhedral slightly chlori-tized biotite phenocrysts occur. Crossed polars. B) Sample 75001/48.00 m shows sulphide breccia vein in a microcline, quartz, and albite dominated groundmass. Some phenocrysts of microcline and albite are also visible. Crossed polars. C) same as B in reflected light. D) Sample 75001/59.60 m early opaque-biotite vein with fine-grained microcline-albite-quartz rims. Crossed polars. E) same as D in reflected light. F) Sample 75007/15.0 m shows disseminated sulphides and sulphide veinlets in a fine-grained matrix of microcline, albite, quartz. Sericite and biotite occur together with the sulphides in veinlets. Reflected light. G) same as F with crossed polars.


Iekelvare

The Iekelvare Cu-Au-Fe oxide deposit is located on the map sheet 27I (cf. Figs. 1 and 6), square 1i–j (nat. grid). The mineralization was discovered in 1972 when SGU found mineralised boulders during exploration activities in the area. Subsequent geochemical sampling and geo-physical ground surveys led to the drilling of 13 drillholes with a total length of 2063 m in the area between 1974 and 1977. The geophysical measurements include ground magnetics, EM, and IP. No thorough estimation of grade

and tonnage has been made, but Sundbergh et al. (1980) estimated the tonnage at 200 000–300 000 tonnes at a Cu grade slightly below 1 %. The deposit was re-assayed for gold in 1984 (Lundmark & Hålenius 1984) and high Au grades were found in several sections of the deposit. In drillhole 75002 1.3 g/t Au over 4.90 m and 1.6 g/t over 1.73 m and in drillhole 75004 7.8 g/t Au over 0.40 m was reported.

The exploration activities are summarised in Sund-bergh et al. (1980), Lundmark (1983), Lundmark &

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Granitoid, GSM-type, c. 1.87 Ga old

Granite, Lina type, c. 1.80 Ga old

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Marble, Norvijaur group, c. 1.89 Ga old

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Iekelvare deposit

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Fig. 6. Simplified geological map of the area NE of Randijaure. The geology is modified from the digital database over the Norrbotten County available at the Geological Survey of Sweden. The Iekelvare deposit is indicated.

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Hålenius (1984), and Einarsson (1985). The presentation below is based on these reports together with relogging of cores and new thin sections from the drillcore material.

Regional geology

The regional geology of the Iekelvare area is relatively poorly known and beside regional compilations such as the Nordkalott map (1986) not much has been published. Unpublished exploration reports, for example Lundmark (1983), exist, but are difficult to evaluate since nomencla-ture is not consistent with present standards. In Figure 6, a regional geological map, modified from the SGU digital database from the area, is shown.

No age determinations have been published from the

Iekelvare area. Skiöld et al. (1993) published two U-Pb zircon ages from the Norvijaur granitoid situated in the southeastern corner of Figure 6 and the granitoid west of the Norvijaur group supracrustal rocks. The Norvijaur in-trusion yielded an age of 1926+13 Ma whereas the grani-toid to the west yielded an age of 1876±6 Ma (Juoks-jokko granite). This implies that the Norvijaur granite may constitute a basement to the Svecofennian rocks in the area and, provided that the contact towards the Nor-vijaur group is intrusive, it also implies that the latter is an older sequence of supracrustal rocks.

The Iekelvare area is dominated by red medium to fine-grained, foliated to non-foliated granites (cf. Figs. 7 and 8). According to Sundbergh et al. (1980), non-foliated finer

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Fig. 7. The geology of the Iekelvare area (modified from Sundbergh et al. 1980). The location of the Iekelvare deposit is indicated.

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Fig. 8. Photographs of drillcore specimens from the drillhole 75002. The white scale bar is approximately 35 mm long. Sample 75002/55.0 m – a typical reddish Lina type granite which constitute the dominating rock type in the area, Sample 75002/70.0 m - same as previous, Sample 75002/79.85 m – foliated and mineralised rock dominated by fine-grained quartz, microcline and biotite (weakly chloritized), sulphides along grain shape foliation, Sample 75002/81.30 m – strongly albite altered rock with veins and dissemination of sulphides. Minor microcline, Sample 75002/82.90 m – same as previous, Sample 75002/88.50 m – Weakly foliated mafic rock composed of clinopyroxene, plagioclase, albite, and minor quartz with a weak dissemination of opaque minerals with associated epidote and calcite, Sample 75002/89.10 m – Mafic intrusive rock with hornblende and plagioclase as major minerals, with epidote and minor quartz, microcline and sulphides, Sample 75002/90.10 m – Foliated and mineralised mafic rock with chloritized biotite, minor hornblende, epidote as mafic minerals and fine- grained quartz, albite and microcline as felsic minerals, Sample 75002/100.90 m – veins of clinopyroxene and quartz in matrix of plagioclase cut a plagioclase and hornblende domi-nated rock. Plagioclase sericite and epidote altered, Sample 75002/126.65 m – same as previous host rock, Sample 75002/146.90 m – foliated biotite, albite, microcline, and quartz schist with minor hornblende. Dissemination and thin veins of sulphides, and Sample 75002/153.10 m – same as previous.

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grained granite is intrusive into a coarser foliated type. However, these types are not distinguished on the maps of Sundbergh et al. (1980), and on the Nordkalott map (1986) the granites are referred to as Lina type. Although no field mapping has been done within this project it is plausible that the non-foliated finer grained variety indeed can be classified as Lina type and hence has an age of c. 1.80 Ga, while the older coarser foliated granite should rather be classified as belonging to the older 1.89 Ga gen-eration of granitoids. Sundbergh et al. (1980) and Einars-son (1985) report the common occurrence of restitic su-pracrustal material in the granite, which further supports a Lina type origin for the granite. In the mineralised area, the granite is intruded by a dioritic rock of unknown age. The foliated nature of the diorite may imply that is related to the older variety of granitic intrusion rather than the younger Lina type. In drillcore, Lundmark & Hålenius (1994) identified a porphyritic border phase of the dior-ite. The granite in the Iekelvare area can be seen in Fig-ure 8, samples 75002/55.0 m and 75002/70.0 m, while the diorite is shown in Figure 8, samples 75002/88.5 m, 75002/88.5m, and 75002/89.1 m.

Mineralization

Besides the mineralization at Iekelvare, several minor oc-currences of mineralised granite exist in the area, as indi-cated in Figure 7. None of these have been drilled and only scattered analyses of rock specimens from outcrops

and from bedrock exposures from trenches exist. The re-sults are reported in Einarsson (1985).

As stated above, the mineralization at Iekelvare is as-sociated with a foliated diorite which has intruded the re-gionally common red, foliated, medium grained granite. The mineralization is very scattered and heterogeneous in character and consists mainly of mm to cm wide sulphides veins, but disseminations of sulphides are also common. The bulk of the mineralization seems to be situated within a more or less pervasively altered diorite, but mineraliza-tion also occurs in the granite. The general macroscopic appearance of the mineralization is shown in Figure 8. The main ore minerals are chalcopyrite and pyrite, but sphalerite, galena, and molybdenite are also common lo-cally. Magnetite is a ubiquitous constituent of the altera-tion assemblage and it is probable that the high magnetic areas on the ground-magnetic map (Fig. 9) can be attrib-uted to the alteration system associated with the miner-alization. Quartz veins, strong silicification, biotite, and chlorite are typical of the alteration assemblage as can be seen in Figure 8 and 10.

Lundmark & Hålenius (1984) identified zones of higher grades of gold as stated above. They reported gold as part of a paragenesis consisting of fine-grained galena, sulfosalts, and arsenopyrite. They also noted that the gold grades do not follow the Cu grades. It should be noted that gold so far has mainly been assayed from Cu-rich parts (see introduction for grades) and thus the gold

Fig. 9. Ground magnetic map of the Iekelvare area. Local grid in metres.


potential of the mineralization has not been thoroughly evaluated.

The more intensely mineralised parts of the Iekelvare deposit are characterised by a strong bleaching of the rocks (Fig. 8, samples 75002/81.30, and 82.90 and Figs. 10a, d, e). This is attributed to an addition of silica and potassium and mineralogically by a quartz-plagioclase-

biotite±hornblende alteration assemblage (Figs. 10a, b, c). In the less intense mineralised parts, like in Figure 8 sam-ple 75002/89.10, primary clinopyroxenes are preserved while in intermediate stages secondary hornblende and biotite are common (Fig. 10f ). The alteration is thus char-acterised by potassic (biotite-hornblende), sodic (albite-plagioclase), and silicic alteration styles.

Fig. 10. Micrographs of some of the specimens shown in Figure 8. A) 75002/100.90 m, Plagioclase and hornblende do-minated rock with disseminated sulphides. Crossed polars. B) 75002/153.10 m, Foliated biotite, albite, microcline, quartz schist with minor hornblende. Dissemination and thin veins of sulphides. Crossed polars. C) same as B in reflected light. D) 75002/81.30 m, Strongly albite-quartz altered rock with disseminated sulphides. Crossed polars. E) same as D in reflected light. F) 75002/89.50 m, Weakly foliated mafic rock composed of clinopyroxene, plagioclase, albite, and minor quartz with a weak dissemination of opaque minerals with associated epidote and calcite. Crossed polars.

16 P. WEIHED

Vaikijaur

The Cu-Au-Fe-oxide-(Mo) deposit at Vaikijaur (cf. Figs. 1, 11, 12) is situated on the map sheet 27J square 0e (nat. grid). It was discovered in the early 1980ies and drilled between 1981 and 1983. Totally 16 drillholes were drilled during this period. The results are summarised in Sund-

bergh & Niva (1981) and Lundmark (1982, 1983, 1984). The cores have only partly been assayed for copper and gold. Digital ground magnetic data is also available from the area (Fig. 13). No tonnage and grade calculations have been made but assays of 1.84 % Cu and 1.9 g/t Au over 2.0 m, and 2.28 % Cu and 3.0 g/t gold over 1.0 m were reported by Lundmark (1984).

Ánájávrre

Klubbuddsjön

Fatjassjön

Vájgájávrre

Ö Ållojaure

Hárrejávrre

Norr-Tjalmejaure

Soarvásj

Vajkijaur

Jokkmokk

5 km

Granitoid, c. 1.89 Ga old

Granitoid, Norvijaur type, c. 1.93 Ga old

Archaean gneissesGranitoid, GSM-type, c. 1.87 Ga old

Cu-Au-Mo mineralization

Felsic metavolcanic rock , c. 1.89 Ga old

Metagreywacke, argillite, conglomerate, amphibolite (Norvijaur fm.)

Metadiabase, amphibolite (Muddus fm.)

Gabbroid, c. 1.89 Ga old

Granite, Lina-type, c. 1.80 Ga old

Granite, Jokkmokk-type, c. 1.80 Ga old ?

Form line


Fig. 11. Simplified geological map of the Jokkmokk area. The geology is modified from the digital database over the Norrbot-ten County available at the Geological Survey of Sweden. The Vaikijaur deposit is indicated.


Regional geology

The Vaikijaur area is situated totally within the Jokkmokk granite (Figs. 11 and 12), which is interpreted to belong to the c. 1.89 Ga old intrusive rocks in northern Sweden. No age determinations have been carried out in the area, but field relationships and the penetrative deformation support this interpretation. The Jokkmokk granite is gen-erally greyish white, medium grained, and foliated and is mineralogically classified as granite s.s. to quartzmonzo-diorite. The intrusion is elliptical with the approximate dimensions 10 x 5 km and is surrounded by other granites of similar age and supracrustal rocks, which are considered to be Svecofennian in age.

Mineralization

In Figure 12, the mineralised area within the Jokkmokk granite is shown (modified from Lundmark 1984). Based

on outcrop appearance and information from drillcore, the Vaikijaur deposit displays a clear zoning from a central dissemination of dominantly pyrite through a thin shell of chalcopyrite rich Cu-mineralization which in turn is bordered by a characteristic magnetite dissemination. Mo-lybdenum mineralization seems to be a more regional fea-ture, but may constitute more distal parts in a concentri-cally zoned magmatic hydrothermal system. The miner-alised area is beautifully displayed on the ground magnet-ic map (Fig. 13) where the concentric magnetic pattern is clearly visible. The Vaikijaur deposit is located in the northeastern part of the magnetic structure. According to Lundmark (1984), mineralization in outcrop is found south of the lake, which corresponds to the southern parts of the magnetic structure. It is thus plausible that the cen-tral parts of the hydrothermal system are beneath the lake and that Cu-mineralization occurs in the outer parts of the whole magnetic anomaly. In the case of Vaikijaur, as-

LakeKlubbuddsjön

80

65

60 40

Mo

Mo

Mo

Mo

Mo

Mo

CuCu

CuCu

Cu

Cu Cu

Cu

Cu

Cu

Cu

Cu

Cu

Cu

CuCu

Cu Cu

Cu

Cu

Cu

Cu

300 m

Porphyrite

Granite, Jokkmokk-type

Pyrite dissemination

Magnetite dissemination

Chalcopyrite mineralization

Foliation

Irregular molybdenite dissemination

Irregular chalcopyrite dissemination

Outcrop

Drillhole

Mo

Cu

Vaikijaur Cu-Au-Mo deposit

Fig. 12. The geology of the Vaikijaur deposit (modified from Lundmark 1984).

18 P. WEIHED

says indicate a good correlation between Cu and Au. The mineralization is dominated by veins and veinlets

of sulphides, but with omnipresent weak disseminations. The main ore minerals are pyrite, pyrrhotite, and chalcop-yrite. A few larger massive sulphide veins also occur. The style of mineralization is shown in Figure 14. Alteration associated with mineralization in the central zone of py-rite dissemination is sericitization and chloritization with

some epidote. A potassic alteration with growth of K-feld-spar is a general feature for the whole mineralised area. The microscopic appearance and style of mineralization can be seen in Figure 15. From these micrographs (Fig. 15) it is evident that the mineralization is associated with quartz-microcline-biotite alteration, epidote-biotite, and rare calcite in sulphide veinlets.

Fig. 13. Ground magnetic map of the Vaikijaur area. Note the sub-circular zonal magnetic pattern centred on the deposit. Local grid in metres.


Tallberg

The best studied and documented porphyry style deposit in the Fennoscandian shield is the Tallberg deposit (cf. Weihed et al. 1987, Weihed & Schöberg 1991, Weihed 1993, Weihed & Fallick 1994), situated within the Jörn granitoid in the Skellefte district (map sheet 23J, square 4i–j). The deposit has been extensively drilled by Boliden Ltd and figures for tonnage and grade given by Weihed et al. (1992) are c. 44 Mt of 0.27 % Cu and 0.2 g/t Au (Weihed 1992). Substantially higher Au grade is found in shear zones that cut the mineralization. Below, a short re-view of results based on the papers referred to above is made.

Regional geology

The Skellefte district is somewhat loosely defined as a WNW trending, approximately 150 km by 50 km large (cf. Fig. 16), ore-bearing belt which is dominated by vol-

canic rocks of Palaeoproterozoic age. It is generally regard-ed as a volcanic arc, which formed between a sedimen-tary basin to the south (Bothnian Basin) and a continental landmass to the north (volcanic rocks of the Arvidsjaur Group). Modern ideas favour some kind of destructive plate margin, either an island arc or a continental arc, and invoke a subduction towards the north.

The Palaeoproterozoic intrusive rocks within and ad-jacent to the Skellefte district belongs to three main intru-sive suites: 1) a syn-volcanic phase comprising granites to gabbros, 2) post-volcanic S-type granitoids, and 3) post-volcanic A/I-type granites to gabbros. The syn-volcanic intrusions are dominated by tonalites and granodiorites and have previously been considered as coeval and comag-matic with the Svecofennian volcanic rocks which host the massive sulphide ores (cf. Weihed et al. 1992), i.e. they fall in the age range 1890–1880 Ma. Within the Skellefte district, several age determinations have been carried out on the Jörn granitoid complex, which belongs to the syn-

Fig. 14. Photographs of drillcore specimens from the drillhole 83001. The white scale bar is approximately 35 mm long. Sample 83001/20 m – Typical Jokkmokk type granitoid in the Vaikijaur area. The rock is composed of glomeroporphyritic biotite-hornblende in a matrix of albite, microcline and quartz. Weakly porphyritic, Sample 83001/23.00 m – same as previous, but hornblende dominated and more microcline rich, Sample 83001/55.75 m – same as previous, but biotite dominates the mafic mineral phases. Thin sulphide veins with biotite and minor epidote and calcite, Sample 83001/63.30 m – Strong sulphide impregnation with a recrystallized matrix of albite and biotite, Sample 83001/68.70 m – weak foliation with microcline, albite and quartz as light minerals. Biotite aligned along grain shape folia-tion, Sample 83001/71.50 m – same as previous with biotite, epidote and quartz in sulphide veinlets, and Sample 83001/126.20 m – same as previous, but with accessory sericite and minor sulphides.

20 P. WEIHED

volcanic suite. Wilson et al. (1987) dated zircons from three intrusive phases of this massif at 1888+20 Ma (oldest outer zone GI), 1874+48 Ma (GII), and 1873+18 Ma (GIII). Weihed and Schöberg (1991) dated one of the porphyries associated with the Tallberg deposit at 1886+15 Ma. Fur-thermore, a monzonite and a gabbro in the Gallejaur in-trusion (Fig. 16) have been dated at 1873±10 Ma (Skiöld 1988) and 1876±4 Ma (Skiöld et al. 1993), respectively. Post-volcanic S-type granitoids are referred to as Skellefte–Härnö-type granites south and east of the Skellefte dis-

trict. These granites are minimum melt products often associated with pegmatites and aplites in areas of strong migmatitization. The Skellefte–Härnö-type has only been dated in two places: near Örnsköldsvik, approximately 200 km south of the Skellefte district (Claesson & Lund-qvist 1995), yielding an age of 1822±5 Ma (monazite) and at Skellefteå (Weihed et al. in prep) yielding an age of 1798±4 Ma (titanite). Post-volcanic A/I-type granitoids are generally referred to as Revsund-type granites. These coarse, feldspar-phyric intrusive rocks occupy vast areas

Fig 15. Micrographs of some of the specimens shown in Figure 14. A) Strong sulphide impregnation with a recrystallized matrix of albite, biotite and minor quartz and microcline. Crossed polars. B) same as A in reflected light. C) weak foliation with microcline, albite and quartz as light minerals. Biotite aligned along grain shape foliation. Crossed polars. D) same as C in reflected light. E) weak foliation with microcline, albite and quartz as light minerals. Biotite aligned along grain shape foliation with accessory sericite and minor sulphides. Crossed polars. F) same as E in reflected light.

–6

–14

–14

–9


in central Sweden and around the Skellefte district. Geo-chemically they display a monzonitic trend and a major-ity of the intrusions has a granitic to monzogranitic com-position. Subordinate, often mingled or mixed, more ma-fic intrusions are also present. The Revsund granitoids intruded after the main phases of deformation and post-dates the regional metamorphism. However, many ductile shear zones cut these granitoids indicating that greenschist facies deformation occurred at least locally in these rocks. Several age determinations have been carried out on these rocks yielding ages from 1.80 to 1.78 Ga (Patchett et al. 1987, Skiöld 1988, Claesson & Lundqvist 1995, Geologi-cal Survey of Sweden, unpublished results).

The lowermost part of the supracrustal pile in the Skellefte district consists mainly of felsic volcanic rocks, which are included in the Skellefte Group (Allen et al. 1996). Ages of the Skellefte Group all fall within the range 1890 to 1880 Ma (Billström & Weihed 1996). The su-pracrustal rocks of the Vargfors Group overlie the volcanic rocks of the Skellefte Group with complex and variable contacts. The close spatial and chemical relationships be-

tween the Vargfors mafic volcanic rocks and the Gallejaur intrusive rocks indicate that these rocks are comagmatic and genetically linked. Consequently, the published ages of 1873±10 Ma and 1876±4 Ma for the Gallejaur intru-sive rocks (Skiöld et al. 1993) have been interpreted as indicating the age of the Vargfors Group. This was also confirmed by Billström & Weihed (1996) who dated an ignimbrite within the Vargfors Group at 1875±4 Ma. To the north of the Skellefte district, subaerial volcanic rocks of the Arvidsjaur Group have been considered to overlie the subaqueous volcanic rocks in the Skellefte Group. The Arvidsjaur volcanic rocks have been dated by Skiöld et al. (1993) and ages of 1878±2 Ma and 1876±3 Ma have been obtained. The fact that only the upper part of the Vargfors Group contains clasts of rocks of the Arvidsjaur Group in-dicates that these groups are most probably coeval.

Mineralization

The Tallberg porphyry deposit is situated in the outer and oldest GI phase of the Jörn granitoid and it is associated with high-level quartz-feldspar porphyritic dykes and in-

1)

2)

Adak

Kristineberg

Boliden

Glommersträsk

Jörn

Malå

Norsjö

Långdal

Långsele

Kankberg

V Åkulla

Renström

Petiknäs N

Petiknäs S

KedträskUdden

Svansele

Björkdal

Åkerberg

Norrliden

Maurliden

Holmtjärn

Näsliden

Rakkejaur

Rävliden

Rävlidenmyran Kimheden

7250

0072

0000

23

170000165000I J K

170000165000I J K

7200

0072

5 00

0

23

Conglomerates and sandstones, polymict (Vargfors and Ledfat Groups) c. 1.87–1.85 Ga

Mudstone, black shales, sandstone and turbidites, (Bothnian Group, Vargfors Group, Skellefte Group)c. >1.95–1.85 Ga

Post-volcanic granitoids of A- and I-type (Revsund type), c. 1.80–1.78 GaPost-volcanic granitoids of S-type (Skellefte type), c. 1.82–1.80 Ga

Gabbro and diorite

Ultramafic intrusions Subaerial to shallow water basalt–andesite (Arvidsjaur Group), c. 1.88–1.87 GaSubaerial to shallow water rhyolite, dacite and minor andesite (Arvidsjaur Group), c. 1.88–1.87 GaBasalt-andesite and minor dacite lavas and sills, mainly submarine(Skellefte Group), c. 1.89–1.87 GaRhyolite, dacite and minor andesite, mainly submarine(Skellefte Gro p), c. 1.89–1.87 Ga

Synvolcanic granitoids of I-type (Jörn III granit, Gallejaur monzonite), c. 1.87–1.85 GaSynvolcanic granitoids of I-type (Jörn II granodiorite) c. 1.87 GaSynvolcanic granitoids of I-type (Jörn I tonalite and ndivided) c. 1.89 Ga

Major VHMS deposits

Major gold deposits

Antiform with plunge

Synform with plunge

Major faults and shear zones

1) Tallberg deposit2) Granberg deposits

Basalt–andesite and minor dacite lavas and sills(Vargfors Group), c. 1.88–1.86 Ga

Fig. 16. Geology of the Skellefte district (modified from Weihed et al. 1992). Both the Tallberg and the Granberg deposits are indicated.

22 P. WEIHED

trusions (Fig. 17). The age of these porphyritic intrusions is 1886 +15 Ma (Weihed & Schöberg 1991) which is, with-in errors, the same as the GI host rock and the age of the host rocks to most massive sulphide deposits in the dis-trict. The deposit is of a low grade (0.3 % Cu) and high tonnage (>50 Mt), disseminated type. Typical alteration zoning exists with a proximal zone of phyllic alteration with quartz, sericite, and pyrite grading out into a distal propylitic alteration with abundant chlorite. Ore miner-

als occur both disseminated and in a sulphide-quartz vein stockwork which is most intense in the central part of the deposit. Metals are also zoned with Cu concentrated to the central parts while Zn and Pb are concentrated to the marginal parts. Magnetite appears to be an early phase of the alteration system. Gold grades are low (<1 g/t), but in strongly sheared sericite alteration zones cutting the de-posit the gold grades can reach >10 g/t. Stable isotope data and fluid inclusion studies indicate that the sulphides pre-

9 000 N

8 000 N

11 000 W 10 000 W

Jörn GI Tonalite

Granite porphyry

Älgliden ultramafic dyke

Mafic dykes

Porphyry type deposit

Au-mineralizations

200 m

modified from Weihed (1992)

Fig. 17. Geology of the Tallberg deposit (modified from Weihed 1992).

–12


cipitated when magmatic fluids mixed with sea water at elevated temperatures of 450 to 500° C (Weihed et al. 1992).

Other deposits

Apart from the deposits described above, several minor and less well known occurrences of Cu±Au±Fe-oxide exist in northern Sweden which are intrusion hosted or show characteristics which indicate a possible magmatic hydro-thermal origin. A few are briefly described below.

Lulepotten and Sadenåive

Although the deposits Lulepotten and Sadenåive are not strictly intrusion hosted, they are briefly described here. Both deposits are hosted by a sequence of felsic to mafic volcanic rocks of a probable, c. 1.88 Ga, Svecofennian age. Padget (1966) described the geology of the area and also briefly other minor mineralizations.

The Lulepotten deposit is situated on the map sheet 25I square 6b and was drilled between 1960 and 1971 (Fig. 18). Altogether 77 drillholes with a total lenght of 17 500 m were drilled. Both the Lulepotten and Sadenåive areas are covered by groundmagnetic and IP measure-ments, which are not in digital format and therefore not shown. The deposit is estimated at 5.1 Mt with c. 0.73 % Cu and 0.25 g/t Au. Mineralization is disseminated and grades into non-mineralised host rocks. The supracrustal rocks are strongly foliated and the mineralization follows the schistosity. The main part of the mineralization is hosted by mafic porphyritic, volcanic rocks interpreted as lavas, but it also occurs in felsic volcanic rocks. A granitic rock classified as Lina type (c. 1.80 Ga), intrusive into the supracrustal rocks, is situated immediately west of the de-posit and is also in some areas weakly mineralised (Figs. 18 and 19). The style of mineralization in the granite is vein type. Padget (1966) describes a biotite gneiss and diorite in close contact with the ore, but it is not clear whether this is a deformed part of the main granite or not. Min-eralization is composed of chalcopyrite as main ore min-eral with abundant bornite, pyrite, and chalcocite. Both Sandahl (1973) and Padget (1966) mention Fe-oxides as a common part of the mineralization. Sandahl (1973) cal-culated the Fe-oxides to 43.5 % of the total opaque phase. Magnetite dominates over hematite. Padget (1966) de-scribes a K-metasomatism as a prominent feature and al-though he attributed this to ”granitization” he considers the granite as responsible for the alkali enrichment and the growth of microcline and biotite in what he refers to as biotite gneiss. Scapolitization is a common regional altera-tion product in mafic rocks throughout the area.

The smaller Sadenåive deposit is located c. 1 km east of the Lulepotten deposit in the central part of the supra-

crustal belt (Fig. 19). Ten drillholes with a total length of 1 730 m were drilled in 1978. Only Cu was analysed and no report on the gold content is available. The Cu grade was reported by Sandahl (1980) at 0.03–0.2 % without any tonnage figures given.

The geology at Sadenåive is dominated by intermedi-ate to felsic lavas and tuffs, which are intruded by numer-ous mafic dykes parallel to the regional NNE striking foli-ation (Sandahl 1980). The mineralization is disseminated in the intermediate porphyritic lava, while ”fracture fill-ings” are more common in the felsic tuff. The main ore mineral is chalcopyrite with subordinate bornite. Minor pyrite also occurs. The ore zones are less than 2 m wide and 80–140 m in length and known to 170–200 m depth. Pervasive magnetite dissemination is reported to occur in all volcanic rocks and a zone of hematite dissemination occurs c. 80 m to the west of the sulphide mineralization. The mineralization is parallel or sub-parallel to the re-gional strong foliation in NNE. Sanddahl reports ”pinch and swell” structures and minor folds which indicate that the whole unit is situated within one of the major crustal scale shear zones with a NNE to N–S strike which exist in northern Sweden. A relatively weak sericite and chlorite alteration associated with the mineralization is reported by Sanddahl (1980).

Sjiska area

In the 1980ies, the exploration division of LKAB explored an area situated c. 20 km SW of Kiruna on the map sheet 29J SW that they named the Sjiska granite area. After re-gional geophysical and geochemical surveys together with geological mapping and boulder tracing, an area situated at the Sierkavaare hill was drilled (Hedin 1984a, 1984b, 1985a, 1985b, 1986a, 1986b, Hedin et al. 1988). The mineralization found was named Sierkavaare or Pikku-järvi. During the exploration campaign, 37 drillholes were drilled on the deposit with a total length of c. 7 200 m. The tonnage was calculated at 11.4 Mt with 0.43 % Cu, alternatively 0.5 Mt with 1.19 % Cu, with only trace amounts of Au.

The Pikkujärvi deposit is situated at the NW border of a monzonitic intrusion, which is interpreted as belong-ing to the Perthite-Monzonite suite, and thus have an age of c. 1.88–1.87 Ga (Fig. 20 and 21). No age deter-minations have, however, been performed on this intru-sion. The monzonite is greyish red to red, unfoliated and variably magnetic. The mineralised area is dominated by volcanic rocks belonging to the coeval Kiruna porphyry Group. While intermediate to mafic rocks dominate re-gionally, the mineralised area close to the monzonite con-tact is dominated by intermediate to felsic volcanic rocks. The central parts are dominated by a quartz-phyric vol-

24 P. WEIHED

canic rock which, according to Hedin et al. (1988), is iso-chemical with the monzonite. This indicates a close genet-ic relationship between the two. The porphyry in the min-eralised area is chemically a rhyolite to trachyte, greyish red to red with both white and red feldspar phenocrysts.

In the porphyry, lamphrophyric dykes are interpreted as ring dykes associated with a cauldron and a subvolcanic complex. Some of the volcanic rocks are interpreted as lavas whereas others are of ignimbritic origin. Many of the porphyritic volcanic rocks are amygdaloidal with magnet-

BåtsaBåhttsá

V Rebraur

Tjålmak

Ö Rebraur

KakelGáhkal

LullebådneGublijaureGuoblajávrre

GSM type intrusive rock, Arvidsjaur type, c. 1.88 Ga old

Lina type granite, c. 1.80 Ga old

Haparanda type intrusive rock, c. 1.89 Ga



Cu-Au mineralization

Paragneiss, c. 1.89 Ga old

Form line


5 km

Fig. 18. Simplified geological map of the Stensund area. The geology is modified from the digital database over the Norrbotten County available at the Geological Survey of Sweden. The Lulepotten and Sadenåive deposits are indicated.


85

85

70

70

80

75

70

75

85

Sadenåive

Lulepotten

LakeLullebådne

Granite

Diorite

Felsic volcanic rock

Felsic volcanic rock

Laminated felsic volcanic rock

Felsic volcanic breccia/conglomerate

Quartz-feldspar porphyritic rhyolite

Feldspar porphyritic dacite

Mafic volcanic rock

Laminated mafic volcanic rock

Foliation

Bedding

Lineation

Fault

Way up

Drillhole

Outcrop

Mafic volcanic breccia/conglomerate

Mafic lava, feldspar porphyritic

Mafic lava, trachytic

Mafic lava amygdaloidal

Cu-deposit 1000 m

85

70

80

Fig. 19. Geological map of the Lulepotten and Sadenåive areas (modified from Sandahl 1980).

26 P. WEIHED

Siergajávri

Kaitums domänreservat Nilakkajávri

Vuotnajávri

Kamasjaure

GSM-type gabbro-diorite, c. 1.88 Ga old

Quartz porphyry, Kiruna porphyry Group, c. 1.88 Ga old

Intermediate Syenite porphyry, Kiruna porphyry Group, c. 1.88 Ga old

Mafic volcanic rock, Kiruna porphyry Group, c. 1.88 Ga old

Marble, Kiruna porphyry Group, c. 1.88 Ga old

Conglomerate, Kiruna porphyry Group, c. 1.88 Ga old

Kiruna porphyry Group, undivided, c. 1.88 Ga old

Paragneiss, undivided

Felsic volcanic rock, Porphyrite Group

GSM-type granitoid rock, c. 1.88 Ga old

Lina-type granites, c. 1.80 Ga old

Pikkujärvi Cu-deposit

Form line

Deformation zone, unspecified

5 km

Fig. 20. Simplified geological map of the Sjisjka area. The geology is modified from the digital database over the Norrbotten County available at the Geological Survey of Sweden. The Pikkujärvi deposit is indicated.


ite as a typical constituent of the amygdales. According to Hedin et al. (1988), hydrothermal breccias are common in the mineralised area.

The mineralization occurs as a dissemination and in thin sulphide veins. Chalcopyrite and pyrite are common with subordinate bornite. The alteration is not well de-scribed but a strong potassic alteration seems to be re-gional based on high K2O values of all reported chemical analyses from the area. Tourmaline, epidote, possible sec-ondary biotite, and scapolite are typical alteration assem-blages.

The border zone surrounding the monzonite is highly magnetic and commonly contains quartz-bearing porphy-ries, which are similar to the central monzonite as well as the lamphrophyres. This led Hedin et al. (1988) to the conclusion that the entire Sjieska monzonite may be a subvolcanic complex with high-level intrusions and ring dykes associated with mineralization in the contact be-tween the intrusion and the country rocks. The fact that mineralization is reported from other places within the monzonite and in other parts of the contact between

the monzonite the the country rocks supports this idea. This also clearly suggests a genetic relationship between the monzonite (and the subvolcanic porphyries) and Cu±Au±Fe-oxide mineralization in the Sjieska granite area.

Viterliden

Immediately NE of the Kristineberg VMS-deposit in the eastern part of the Skellefte district, the felsic volcanic host sequence to the ore is intruded by a high level tonalitic intrusion (Fig. 22). This intrusion is part of the larger Kristineberg intrusion, which has been dated at 1907±13 Ma (Bergström et al. 1999). The Kristineberg intrusion is considered as comagmatic with the host sequence to the VMS ore and is furthermore geochemically and minera-logically very similar to the main Jörn batholith which hosts the Tallberg porphyry style deposit (see above). The high level intrusion NE of Kristineberg has been drilled both by SGU and Boliden AB. Logging of one core from this intrusion indicates weak pyrite-chalcopyrite minerali-

6000 N/8000 W

3600 N/8000 W 3600 N/4000 W

6000 N/4000 W

1000 m

GSM-type gabbro-diorite

GSM-type granitoid rock, c. 1.88 Ga old

Quartz-phyric felsic volcanic rocks (Kiruna porphyry group)

Amygdaloidal felsic volcanic rock (Kiruna porphyry group)

Dark syenitic porphyry (Kiruna porphyry group)

Dark syenite-andesite porphyry (Kiruna porphyry group)

Andesite-basalt (Kiruna porphyry group)

Metadiabase (Kiruna greenstone group)

Cu-ore

Fig. 21. Geology of the Pikkujärvi Cu-deposit (modified from Hedin 1984).

28 P. WEIHED

zation associated with quartz veins and strong propylitic alteration. Some sections of the core also indicate K-feld-spar alteration associated with this weak mineralization

(see Fig. 23 samples 101.5 m and 110.3 m). The logged core has not been analysed and the grade of Cu and Au is therefore not known from this mineralization.

72250007225000

1625

000

1645

000

Basalt–andesite, lava or sill / dyke (Skellefte Group)

Ultramafic rock (Malå Group)

Basaltic komatiite, sill (Malå Group)

Sedimentary rocks (Malå Group)

Granite–tonalite

Gabbro–quartzdiorite, (Revsund–Adak suite) Rhyolite–dacite, lava or subvolcanic intrusion, (Skellefte Group)

Rhyolite–dacite (Skellefte Group)

Fault, shear zone

Viterliden mineralization

VMS deposit

Dioritoid

Andesite–basalt, plagioclase porphyritic (Malå Group, Tjamstan Formation)

Strong hydrothermal alteration

Granite (Revsund–Adak Suite)

Dacite, subvolcanic intrusion left (Malå Group)

5 km

Fig. 22. Geology of the Kristineberg area (modified from Bergström et al. 1999).


Sarvasåive area

A weak Cu-mineralization in outcrops was described by Sjöstrand (1982) from the Sarvasåive area on map sheet 24K, square 8a (see Fig. 24). This mineralization was so weak that it did not render continued exploration and no grades have been reported, although a few analysed out-crop specimens contained between 0.3 to 0.8 % Cu. The host rock is a reddish grey, non-foliated, medium grained granite which, according to Sjöstrand (1982), grades into red, felsic volcanic rocks to the north. This implies a close relationship between granites and volcanic rocks, which in turn indicates that the granite belongs to the c. 1875 Ma Perthite-Monzonite suite. Mineralization consists of weak dissemination of chalcopyrite with minor fluorite, scheel-ite, molybdenite, and arsenopyrite.

The Laver Cu-deposit is situated c. 5 km north of the Sarvasåive mineralization. This deposit was mined be-tween 1938 and 1946 and is the only mined Cu-ore sit-

uated in southern Norrbotten. The deposit consists of both massive to semi-massive ore at the contact between felsic volcanic rocks and younger fine-grained sedimenta-ry rocks, and disseminated sulphides within the volcanic rocks. The genesis and origin of the deposit remains open since no modern study of the deposit has been made. It is however, possible that the mineralization is epigenetic in character and a structural control or magmatic relation-ship cannot be excluded.

Granberg

A mineralization identical to Tallberg exists in the north-ern part of the Jörn batholith (Fig. 16). This mineraliza-tion shows all characteristics described for the Tallberg de-posit above, but no published data exist on the deposit. The deposit was drilled during the 1970ies by Boliden AB who also evaluated the area in the late 1980ies and early 1990ies. No information is available from this work.

Fig. 23. Photographs of drillcore specimens from the drillhole 78002. The white scale bar is approximately 35 mm long. Sample 23.10 m – Typical tonalitic medium-grained intrusive rock with minor sulphide disseminations and hornblende dominating over biotite as dark minerals, Sample 48.20 m – same as previous but with characteristic quartz-sulphide veins with chloritic rims, Sample 61.30 m – foliated biotite, quartz, plagioclase, albite rock with disseminated sulphides. Possibly early dyke, Sample 101.50 m – microcline rich rock with albite phenocrysts. Glomeroporphyritic biotite and rare sulphides, and Sample 110.30 m – same as previous but with a strong foliation. The last two rock types have an unclear relationship with mineralization in the Viterliden area.

30 P. WEIHED

Piteälven

Ljusträsket

Manjärv

Lill-LaverStor-Laver

Städdjejaure

St Idträsket

Vistån

Vuotnersjön

Laukersjön

Finnträsket

Brännträsket

Ö Kikkejaure

Bänkerträsket

Lyckoträsket228

Sarvasåive

Laver

10 km

Granodiorite, Haparanda type, c. 1.89 Ga old

Granite, Lina type, c. 1.80 Ga old

Syenite, monzonite, GSM type, c. 1.87 Ga old

Gabbro, diorite, GSM type, c. 1.87 Ga old

Metasediment, biotite gneiss, c. 1.89 Ga old



Granite, Storliden type, c. 1.80 Ga old (?)

Granite, Edefors type, c. 1.80 Ga old

Cu-mineralization

Form line


Fig. 24. Simplified geological map of the Laver and Sarvasåive areas. The geology is modified from the digital database over the Norrbotten County available at the Geological Survey of Sweden. The Laver mine and the Sarvasåive prospect are indicated.


Conclusions

Most deposits described above show a strong spatial re-lationship with intrusive rocks of early Proterozoic age. They also occur in various tectonic settings throughout northernmost Sweden. As this is a first attempt to high-light this style of mineralization, no thorough, genetic model or classification has been proposed for these de-posits. Since several of the deposits are of definite econom-ic interest and deserves further attention, it is proposed here that they should be classified as a group of deposits called intrusive hosted Cu±Au±Fe-oxide deposits as a non-genetic name.

It is still poorly understood how some of these deposits genetically relate to their host rocks, whereas others, like Tallberg, have been well documented and described as a porphyry Cu-Au style mineralization in genetic terms. In deposits like Iekelvare where the mineralization occurs in deformed and metamorphosed host rocks, the direct link to the host intrusive and hence to evolved magmatic hy-drothermal style mineralization still needs to be proved. In deposits like Gråberget, where the mineralization is associ-ated with late to postorogenic granites of Sorsele-Revsund type, it is more clearly a magmatic-hydrothermal genesis for the mineralization. In the case of Vaikijaur, the age of the host intrusive is not known, but the subcircular shape of the hydrothermal alteration system, seen in the ground magnetic map from the mineralised area which is situat-ed totally within the Jokkmokk granite, strongly suggests that it was a magmatic, intrusive-centred hydrothermal system that gave rise to the mineralization. Although the mineralization is situated immediately outside the main intrusion in Sjieska (typical of many porphyry style miner-alizations), several minor mineralised systems occur with-in the intrusion which suggests a possible magmatic hy-drothermal origin for this mineralisation type. In the case of Lulepotten and Sadenåive, the magmatic connection is less clear although the area is characterised by porphyritic volcanic rocks which have been interpreted as lavas. How-ever, these could be dykes, sills, and other subvolcanic in-trusions that may have introduced both heat, fluids, and possibly metals to the volcanic pile. The weak mineraliza-tion in the granite at Lulepotten at least rules out a syn-genetic origin for these deposits.

The described deposits are hosted by intrusive rocks that in age span the entire Svecofennian orogeny in this part of Sweden. The Tallberg mineralization hosted by calc-alkaline tonalites has been dated at 1886+15 Ma (Weihed & Schöberg 1991) and is thus coeval with the VMS-deposits in the Skellefte district. The Boliden de-posit in the Skellefte district has recently been suggested to be an epithermal-VMS deposit (Bergman Weihed et al. 1996) and thus the temporal link between major mag-

matic hydrothermal porphyry style mineralization, epi-thermal and VMS deposits is possible in the Skellefte dis-trict. The deposits in Norrbotten, Iekelvare and Vaikijaur, are both characterised by early magnetite (as is Tallberg and Gråberget where hematite after magnetite is report-ed) which indicates a possible resemblance with Cu-Au-Fe-oxide style mineralization and also a possible link with Kiruna type deposits. The mineralization at Gråberget, which is associated with I- to A-type, c. 1.80 Ga granitic, sensu stricto, magmatism highlights this generation of granites as prospective for magmatic Cu and possibly Au.

A common feature to many of these deposits is the association with high level, subvolcanic and porphyritic phases of the magmatism. This indicates that the mineral-ising processes are attributed to crystallising magmas and late stage fluids evolving from oxidised magmas. The min-eralizing style is in most cases disseminations and veins, which are associated with potassic, sodic, and silicic altera-tion.

References

Allen, R.L., Weihed, P. & Svenson, S-Å., 1996: Setting of Zn-Cu-Au-Ag massive sulfide deposits in the evolution and facies architecture of a 1.9 Ga marine volcanic arc, Skellefte dis-trict, Sweden. Economic Geology 91, 1022–1053.

Bergman Weihed, J., Bergström, U., Billström, K. & Weihed, P., 1996: Geology and tectonic evolution of the Paleoprotero-zoic Boliden Au-Cu-As deposit, Skellefte district, northern Sweden. Economic Geology 91, 1073–1097.

Bergström, U. & Sträng, T., 2000: Berggrundskartorna 23I Malå. Sveriges geologiska undersökning Ai 114–117.

Bergström, U., Billström, K. & Sträng, T., 1999: Age of the Kristineberg Pluton, western Skellefte District, northern Swe-den. Sveriges geologiska undersökning C 831, 7–19.

Billström, K. & Weihed, P., 1996: Age and provenance of host rocks and ores of the Paleoproterozoic Skellefte District, northern Sweden. Economic Geology 91, 1054–1072.

Claesson, L.-Å., 1979: Gråberget kopparmineralisering. Rap-port över prospekteringsarbeten utförda för NSG under åren 1974-1977. Sveriges geologiska undersökning BRAP 79001. (In Swedish.)

Claesson, S. & Lundqvist, T., 1995: Origins and ages of Pro-terozoic granitoids in the Bothnian Basin, central Sweden; isotopic and geochemical constraints. Lithos 36, 115–140.

Einarsson, Ö., 1985: Iekelvare – Prospekteringsarbeten utförda under 1984. SGAB PRAP 85035. (In Swedish).

Gavelin, S., 1955. Beskrivning till berggrundskarta över Väster-bottens län. Urbergsområdet inom Västerbottens län. Sve-riges geologiska undersökning Ca 37, 7–99.

Hedin, J.-O., 1984a: Sjisjka granitområde 1983. Unpublished exploration report LKAB K-84-3. (In Swedish.)

Hedin, J.-O., 1984b: Sjisjka granitområde 840101-841231. Unpublished exploration report LKAB K-84-48. (In Swedish.)

Hedin, J.-O., 1985a: Sierkavare – en kopparmineralisering på kartbladet 29J Kiruna SO. Unpublished exploration report LKAB K-85-22. (In Swedish.)

–9

32 P. WEIHED

Hedin, J.-O., 1985b: Preliminär geologisk modell över Sijsjka granitoidkomplex. Unpublished exploration report LKAB K-85-25. (In Swedish.)

Hedin, J.-O., 1986a: Sjisjka granitområde 850101-851231. Un-published exploration report LKAB K-86-4. (In Swedish.).

Hedin, J.-O., 1986b: Sijsjka granitområde. Unpublished explora-tion report LKAB K-86-48. (In Swedish.)

Hedin, J.-O., Hansson, K.-E. & Holme, K., 1988: Sjisjka grani-tområde – Slutrapport stödetapp III. Unpublished exploration report LKAB K-88-01. (In Swedish.)

Lundberg, B., 1980: Aspects of the geology of the Skellefte field, northern Sweden. Geologiska Föreningens i Stockholm För-handlingar 102, 156–166.

Lundmark, C., 1982: Vaikijaur – Resultat av borrningsarbeten 1981–1982. Unpublished exploration report SGAB PRAP 82064. (In Swedish.)

Lundmark, C., 1983: Vaikijaur – Resultat av borrningsarbeten okt-dec 1982. Unpublished exploration report SGAB PRAP 83048. (In Swedish.)

Lundmark, C., 1984: Vaikijaur – Resultat av borrning 1983. Unpublished exploration report SGAB PRAP 84041. (In Swed-ish.)

Lundmark, C. & Hålenius, U., 1984: Iekelvare – Guldanalyser-ing. SGAB PRAP 84040. (In Swedish.)

Lundqvist, T., Bøe, R., Kousa, J., Lukkarinen, H., Lutro, O., Roberts, D., Solli, A., Stephens, M., & Weihed, P., 1996b: Bedrock map of Central Fennoscandia. Scale 1:1 000 000. Geological Surveys of Finland (Espoo), Norway (Trondheim) and Sweden (Uppsala).

Öhlander, B., 1986: Proterozoic mineralizations associated with granitoids in northern Sweden. Sveriges geologiska under-sökning Ca 65, 39 pp.

Öhlander, B. & Markkula, H., 1994: Alteration associated with gold-bearing quartz veins at Middagsberget, northern Swe-den. Mineralium Deposita 29, 120–127.

Öhlander, B., Skiöld, T., Elming, S-Å., BABEL Working Group, Claesson, S. & Nisca, D.H., 1993: Delineation and charac-ter of the Archaean-Proterozoic boundary in northern Swe-den. Precambrian Research 64, 67–84.

Padget, P., 1966: The geology and mineralization of the Rad-nejaure area, Norrbotten county, Sweden. Sveriges geologiska undersökning C 609, 60 pp.

Rickard, D., (ed.) 1986: The Skellefte Field. Sveriges geologiska undersökning Ca 62, 52 pp.

Rickard, D.T. & Zweifel, H., 1975: Genesis of Precambrian sulfide ores, Skellefte District, Sweden. Economic Geology 70, 255–274.

Sandahl. K.-A., 1973: Lulepotten kopparfyndighet. Rapport rörande resultaten av SGU:s undersökning under åren 1960-1971. Unpublished exploration report BRAP 585. (In Swedish.)

Sandahl, K.-A., 1980: Projekt Sadenåive. Unpublished explora-tion report. Sveriges geologiska undersökning 1980-01-23. (In Swedish.)

Sjöstrand, T., 1982: Sarvasåive. Unpublished exploration report. SGAB BRAP 82034. (In Swedish.)

Skiöld, T., 1988: Implications of new U-Pb zircon chronology to early proterozoic crustal accretion in northern Sweden. Precambrian Research 38, 147–164.

Skiöld, T., Öhlander, B., Markkula, H., Widenfalk, L. & Claes-son, L.-Å., 1993: Chronology of Proterozoic orogenic proc-esses at the Archaean continental margin in northern Swe-den. Precambrian Research 64, 225–238.

Stephens, M.B., Wahlgren, C.-H. & Weihed, P., 1994: Bedrock map of Sweden. Scale 1: 3 000 000. Sveriges geologiska under-sökning Ba 52.

Sundbergh, S. & Niva, B., 1981: Vaikijaur. Unpublished explora-tion report. Sveriges geologiska undersökning BRAP 81006. (In Swedish.)

Sundbergh, S., Persson, G. & Niva, B., 1980: Kopparminerali-seringen vid Iekelvare. Unpublished exploration report. Sveriges geologiska undersökning 1980-02-25. (In Swedish.)

Weihed, P., 1992: Lithogeochemistry, metal- and alteration zoning in the Proterozoic Tallberg porphyry type deposit northern Sweden. Journal of Geochemical Exploration 42, 301–325.

Weihed, P. & Schöberg, H., 1991: Timing of porphyry type min-eralizations in the Skellefte District, northern Sweden. Geolo-giska Föreningens i Stockholm Förhandlingar 113, 289–294.

Weihed, P. & Fallick, A., 1994: Stable isotope study of the Palae-oproterozoic Tallberg porphyry-type deposit, northern Swe-den. Mineralium Deposita 29, 128–138.

Weihed, P., Isaksson, I. & Svenson, S-Å., 1987: The Tallberg porphyry copper deposit in northern Sweden: a preliminary report. Geologiska Föreningens i Stockholm Förhandlingar 109, 47–53.

Weihed, P., Bergman, J. & Bergström, U., 1992: Metallogeny and tectonic evolution of the early Proterozoic Skellefte Dis-trict, northern Sweden. Precambrian Research 58, 143–167.

Welin, E., 1987: The Depositional evolution of the Svecofen-nian Supracrustal Sequence in Finland and Sweden. Precam-brian Research 35, 95–113.

Wilson, M.R., Claesson, L-Å., Sehlstedt, S., Smellie, J.A.T., Af-talion, M., Hamilton, P.J. & Fallick, A.E., 1987: Jörn: An early Proterozoic intrusive complex in a volcanic arc environ-ment. Precambrian Research 36, 201–225.

SWEDISH LAYERED INTRUSIONS ANOMALOUS IN PGE-AU 33

Filén, B,. 2001: Swedish layered intrusions anomalous in PGE-Au. In Weihed, P. (ed.): Economic geology research. Vol. 1, 1999–2000. Uppsala 2001. Sveriges geologiska undersökning C 833, pp. 33–45. ISBN 91-7158-665-2.

A Platinum Group Element (PGE)-Au exploration programme was carried out in Sweden between 1985 and 1990. During this period a large number of cumulus textured mafic–ultrama-fic layered intrusions were identified. Ten percent of these intru-sions have so far proved anomalous in PGE´s.

This paper provides descriptions of the known targets for PGE-Au exploration in Sweden. It gives a summary and his-tory of the exploration works. It also shows the importance of studying a mafic intrusion with respect to its emplacement, igneous stratigraphy and possible ore forming processes. A thor-ough investigation is essential in exploring mafic–ultramafic lay-ered intrusions potential as the host rock for PGE-Au, Ni-Cu, titanium, and vanadium ores.

Key Words: PGE, gold, chromium, titanium, vanadium, layer-ing, layered intrusion, mafic, ultramafic, cumulate.

Birger Filén, Geological Survey of Sweden, Mineral Resources Information Office, Skolgatan 4, SE-930 70 Malå, Sweden. E-mail: [email protected]

Introduction

Platinum Group Elements, PGE, have never been prima-rily mined in Sweden. Small amounts have, however, been produced from anode slimes in Boliden’s Rönnskär plant. Most of the PGE’s produced nowadays come from scrap, either domestic or imported.

A PGE-exploration program was carried out in Swe-den between 1984 and 1990 at the request of the State Mining Property Commission (NSG). The contractor for the work was Swedish Geological Co (SGAB). Of some 80 proved layered intrusions eight showed anomalous PGE and/or gold values. If an intrusion was found to ex-hibit prominent layering, a sampling program was carried out. The criteria for a proper sampling were the presence

of ultramafic or leucocratic layers or cumulates with or without sulphides. Both boulders and outcrops were sam-pled. Six layered intrusions (Fig. 1) with anomalous pre-cious metal tenors will be described below. Bottenbäcken (Storsjö Kapell) although no layered intrusion, has been added because of its anomalous PGE-Au values.

Swedish layered intrusions anomalous in PGE-Au

Birger Filén

Göteborg

Stockholm

Luleå

0 100 200 km

12

5

4

3

6

7

1

2

3

4

Caledonides

Phanerozoic sedimentary rocks

Proterozoic rocks

Archaean rocks

Layered mafic intrusions

Kukkola

Notträsk

Näsberg

Hoting

5 Kläppsjö

6 Bottenbäcken

7 Flinten

Fig. 1. Mafic intrusions anomalous in PGE and Au in Sweden discussed in this review.

34 B. FILÉN

Classification of Swedish PGE-anomalous layered intrusions

Based on the age of the mafic-ultramafic host intrusions, the PGE-anomalous deposits can be subdivided into two groups: 1) Deposits in Early Proterozoic layered mafic in-trusions (2440 Ma), emplaced between the Archean base-ment and overlying supracrustal rocks, and 2) Svecofen-nian synorogenic deposits, hosted by mafic–ultramafic intrusions, emplaced during the main stage of the orog-eny (1900–1860 Ma) within highly metamorphosed mica gneisses.

Kukkola (Location: Mapsheet 25N, x7338750 y1877000 )

Exploration history

The 25 km long hookshaped Kukkola intrusion (Fig. 2) was mainly studied during an intermittent chromium ex-ploration campaign between 1981 and 1984. The work started with hints from Finnish geologists concerning the similarity in the aeromagnetic patterns of the newly dis-

covered Tornio layered intrusion and the patterns on the Swedish side. The Finnish geologists also claimed to have found cubic metre big chromitite boulders close to the Swedish–Finnish border. Exploration works soon started and included boulder tracing, geological mapping, geo-chemical till sampling, ground geophysics, and altogether 4676 m diamond drillings in profiles (Lundmark 1984).

Geology

The Kukkola intrusion is the westernmost part of the Ear-ly Proterozoic mafic-ultramafic layered complex, which stretches through Finland into Russia. The intrusions were emplaced at 2440 Ma, shortly after the cratonization of the Archaean crust (Alapieti & Lahtinen 1989). Small-scale pilot mining for platinum group elements has been performed at Kirakkajuppura NE of Kemi. Beside their PGE-Au potential, these intrusions have had and still have great economic significance because of the chromi-um mining at Kemi and past mining of vanadium and titanium at Mustavaara.

Kukkola

Präntijärvi?

Finland

10 km

Late orogenic granite, 1.75–1.8 Ga

Early orogenic diorite–granodiorite, 1.85–1.9 Ga

Phyllite

Pelite, basalt, dolomite, quartzite

Greenstone

Layered mafic intrusion, 2.44 Ga

Granitic gneiss, 2.67 Ga

Fault

7350

1875

1850

Sweden

Fig. 2. Simplified geology of the Kukkola area, Claesson et al. (1982).


Compared with the Finnish layered complexes in the east, the Kukkola intrusion is very thin, only 10–200 m (Claesson et al. 1982, Lundmark 1984). It is very heterogeneous and consists of strongly altered mafic to ultramafic rocks emplaced between updomed granitic gneisses of the Archaean basement and Proterozoic schists (Fig. 2). Only in a drill profile close to the border, leu-cocratic rocks (anorthosites-leucogabbros) have been en-countered. In some profiles, up to 15 cm wide seams with chromitite occur. They are poor in chromium – the best section gives 22.8 % Cr2O3 with a Cr/Fe ratio close to 1. In the profile close to the border, one 10 cm long sec-tion of a metapyroxenitic rock with 7 % Cr2O3 holds 0.64 ppm Pt, 1.1 ppm Pd, and 0.08 ppm Au. A single 2.7 m wide metaperidotite section with disseminated chromite shows 3.6 ppm Au (Claesson et al. 1983). In the western part of the big hook, a small mafic intrusion is situated in the basement. This has been considered as a feeder for the intrusion. A grab sample from a sulphide-disseminat-ed metapyroxenitic rock gave 0.84 ppm Au.

The Präntijärvi mafic intrusion is situated 6 to 10 km to the SW. It has almost the same aeromagnetic signature as the Kukkola intrusion. No outcrops are known, but it could be an analogue to Kukkola. Except for boulder trac-ing nothing has been done in the area.

Notträsk (Location: Map sheet 25L, x7321000 y1774000)

Exploration history

The gabbroic intrusion at Notträsk east of the town of Boden has long attracted the attention of prospectors and geologists. A nickel-copper-sulphide mineralization was found at the end of the 19th century. The intrusion has thereafter at several occasions been the subject for sci-entific studies and exploration activities (Sundin 1977, Svensson 1981, Arvanitides 1982, Enmark 1982, Widen-falk et al. 1985). Most of these activities have focused on nickel and copper in the southeastern part of the intru-sion. Exploration has included geological mapping, boul-der tracing, ground geophysics, biogeochemistry, and dia-mond drillings. So far, the exploration results have not been successful. Though impressive looking in hand spec-imens, the nickel content in the massive sulphides hardly ever exceed 1 % in the sulphide phase.

Between 1986 and 1989 SGAB intermittently ex-plored the intrusion for PGE-Au. Unlike other organisa-tions and companies, SGAB concentrated its efforts main-ly on the inner parts of the intrusion. Rock sampling was carried out during the winter 1988–1989 and followed up by five diamond drill holes in a profile (Filén et al. 1989).

Geology

The Notträsk gabbro (Fig. 3) is a funnel-shaped layered intrusion, which has intruded into gneisses of metasedi-mentary origin (Arvanitidis 1982, Filén 1987, Filén et al. 1989). The intrusion has by Arvanitidis (1982) been di-vided into four major zones:

Marginal Border Group (MBG)Ferrogabbroic Series (FGS)Troctolitic-Anortositic Series (TAS)Gabbroic Series (GS)

The intrusion belongs to the Perthite monzonite suite, a suite of differentiated and synkinematic intrusions in folded supracrustal rocks of Svecofennian age. The su-pracrustal rocks are mainly metasedimentary. The gabb-roic rocks of the Haparanda type are of the same age as the synkinematic Svecofennian intrusive rocks in the Väs-terbotten County (Näsberg), Jämtland County (Hoting), Västernorrland County (Kläppsjö), and Dalarna County (Flinten).

The main intrusion is 6 x 4 km large, oval-shaped, and consists of concentric layers. At the margins, the layers are steeply dipping but gradually become shallower towards the centre, where they are flat-lying. The intrusion has a complicated structure with often very abrupt lithological changes as seen in Figure 4 (olivine–gabbro–anorthosite–olivine–gabbro–dunite–troctolite).

In his Ph.D.-thesis, Arvanitides (1982) presents proofs for the existence of two magmas at the time of emplace-ment of the intrusion. One was Al-rich and formed the TAS-GS-series, and the other was Fe-Ti-rich and tholeiitic and formed the FGS-rocks. The magmas are thought to have a common ultramafic source. The separation into dif-ferent magmas and the emplacement was caused by flow differentiation, oscillating nucleation, and filter pressing. The higher density, Fe-Ti-rich magma intruded late dur-ing the formation of the TAS-GS. Gravitational crystal-lisation differentiation played a crucial role.

Later studies (Filén 1987, Filén et al. 1989) have dis-carded the existence of the Gabbroic Series. No clearly de-fined pyroxene-rich core has been identified, and no out-ward dipping troctolitic core as proposed by Widenfalk et al. (1985) can be seen in the numerous outcrops.

When sampling an abandoned quarry in 1987, one specimen was highly anomalous in precious metals. Sample number BMNA 87488 (nat. grid. coordinates x7320160, y1773930) contains 2.74 ppm Pt, 1.33 ppm Pd, 0.21 ppm Au, and 20 ppm Ag.

A subsequent diamond drilling programme in a profile planned to go from TAS-FGS into the (non-existing) GS gave the results presented in Table 1. The anomalous val-ues are most often found in anorthositic olivine gabbro.

36 B. FILÉN

65

38

25

32

35 25

28

25

2740

26

35

18

40

5

27

4728

42

55

60

45

6253

62

75

Skogså

Not- träsket

0 2 km

177500

732500

40

Metasediment

Granite

Olivine-gabbro

Magnetite-gabbro

Norite

Ultramafic layers

Magmatic layering

FaultDiorite

Fig. 3. Geology of the Notträsk layered intrusion, Widenfalk et al. (1985), Filén et al. (1988).

Fig. 4 Layers with olivine–gabbro, anorthosite, olivine–gabbro, dunite and troctolite.


The analytical results indicate that the mineralization is clearly subeconomic. Today, aggregates are produced in a dioritic-noritic satellite intrusion to the south-west.

Näsberg (Location: Map sheets 24J, 24K, 23J, 23K, x7251000 y1699000)

Exploration history

The Näsberget mafic intrusion is 9 x 4.5 km large and is situated on both sides of the Byske River in the northern-most part of the Västerbotten County. Iron ore was found here as early as 1832 (Anon. 1986). As iron ores are scarce in Västerbotten and with a favourable location close to the river not too far from the coast, high expectations linked up with the discovery. A company was established and mining started five years later. Already in 1833 the Mine Inspector had visited the discovery and he had noticed that the ore occurred as veins which often were contami-nated with pyrite. By careful hand sorting a better ore was produced, but falling iron prices and high transport costs soon forced the company to close the mine. Several later attempts were made – but with almost the same result. The last ore was mined in 1908.

When old rock samples were rechecked, some samples from the Näsberg intrusion were found to contain cumu-lates. This resulted in field visits and rock samplings and later in small scale trenching.

Geology

The Näsberget intrusion is mainly surrounded by older Jörn granitoids. Felsic volcanic rocks, which are older than the granitoids, occur to the west and to the north (Claes-son 1980). Volcanic rocks of different ages exist in the area. The intrusion is, in the north, in many places in-truded by porphyritic dykes.

The intrusion is clearly layered, the layering normally striking NE and dipping steeply NW, but especially in the north and along the Byske River local variations oc-cur (Fig. 5). The most common type of layering is igne-ous lamination but, especially in the south and south-east, modal layering occurs. Ultramafic layers have not been encountered except for some thin altered hornblendites in the south-east (Filén 1987, Filén et al. 1988).

Altogether 95 samples have been analysed for precious metals. At one locality (nat. grid coordinates x7248680, y16974009), a 0.2 m3 sulphide-bearing pyroxene gabbro boulder with cumulus textures was found. This boulder contained 1.2 ppm Pt, 3.9 ppm Pd, and 0.2 ppm Au. A later reassaying of the boulder was performed on differ-ent modal rock types. Chemical analyses gave 1.3 ppm Pt, 4.5 ppm Pd, and 0.3 ppm Au in one half of the boulder but only 0.05 Au in the other half which indicates a strong nugget effect. Microscope studies showed small amounts of gold in the PGE-empty part but no PGM’s could be found in the other part. Chips sampling from a weekly rusty outcrop 250 m to the north-west gave 0.05 ppm Pt and 0.02 ppm Au over 0.30 m. A grab sample from the old iron workings gave 0.4 ppm Au.

Hoting (Location: Map sheet 21G, x7117500 y1523500)

Exploration History

Åhman (1967) and Lundqvist et al. (1990) have described the Hoting (Rörström) gabbro. During SGAB’s PGE-ex-ploration campaign it was studied and described by Filén (1985, 1987) and Filén et al. (1988b).

When checking old samples from Hoting it soon be-came clear that rocks with cumulus textures were abun-dant. Even Åhman in his paper of 1967 wrote about ”diffuse fluidal texture” though typically enough without mentioning layering. So when the PGE-exploration start-ed it was more or less clear that Hoting was a layered mafic intrusion. In many ways it resembled Notträsk, and Hot-ing was thus given a high priority.

Geology

The Hoting massif is one of the largest layered intrusions in Sweden, with a diameter of 10 km and an 11 km long panhandle towards the south (Fig. 6). Early orogen-

Table 1. Sections with anomalous PGE-Au values (ppm) in the Notträsk gabbroic intrusion.

Drillhole Section Pt Pd Au Ag

88001 16.20–16.85 - - 0.02 -” 16.85–17-85 - - 0.05 -” 65.40–66.03 - - 0.06 -” 115.40–116.40 0.05 - - 4” 139.70–140.63 0.05 - - ” 186.44–187.44 0.05 - - -89001 31.00–32.00 0.17 - - -” 60.17–61.17 - - 0.02 -” 61.17–62.13 - - 0.05 -” 102.33–103.05 0.05 - - -” 120.75–121.75 - - 0.15 4” 124.35–124.61 - - 0.20 -” 129.92–130.92 - - 0.17 -” 130.92–131.92 - - 0.09 -” 131.92–132.60 - - 0.36 -” 132.60–133.15 - - 0.29 -” 133.15–134.26 0.05 0.03 0.19 7” 134.26–134.45 0.05 0.04 0.41 7” 147.53–148.45 - - 0.68 8” 148.45–149.40 0.07 0.04 0.47 6” 149.40–150.10 - - 0.33 -” 160.50–161.30 - - 0.09 -” 161.30–161.80 - - 0.14 -89002 45.13–45.65 1.11 0.30 0.01 1089003 14.00–14.13 - - 0.02 889004 108.00–109.00 - - 0.02 -” 109.00–110.00 - - 0.03 -” 110.00–111.00 - 0.13 0.02 -” 114.00–114.20 - 0.08 0.03 -” 115.29–115.49 - 0.05 0.02 -

38 B. FILÉN

ic tonalitic and granodioritic gneisses in the east and in the south-west surround the intrusion. Elsewhere, the in-trusion is bordered, often with tectonic contacts, by gneis-sic granites, metasedimentary rocks, and post-orogenic Revsund granite. Zones of weakness, which commonly are seen as valleys, streams, or cliffs with faults scarps, cut the massif and have acted as feeder channels for both the granites and for pegmatites and dolerites. As in the Notträsk intrusion, the layers are concentrically developed with steeply dipping outer layers and a flat layering in the

central part. Layering can be seen all over the intrusion. Modal layering is the most common type. Oikocryst bear-ing layers alternating with gabbroic (± olivine) cumulates are quite frequent (Fig. 7). Ultramafic layers on the other hand are rare.

Sulphides such as chalcopyrite, pyrrhotite, and pent-landite occur mostly in the southern part of the main in-trusion, which also contains nickel-copper grades between 0.10–0.15 % Ni and 0.20–0.28 % Cu. One single sample contains 0.47 % Ni, 0.39 % Cu, and 0.09 % Co. As the

60

45

5060

45

85

70

Snorum

Hej

Sör-Abborrträsket

Byske

älve

n

0 2 km

Jörn granitoids and felsic volcanic rocks

Näsberg mafic intrusion

Magmatic layering

Boulder of mafic layered rock anomalous in PGE

50

7250000

1700000

The NäsbergFe-deposit

Fig. 5. Layering in the Näsberg mafic intrusion, Claesson (1980), Filén et al. (1988).


sample contains much sulphur, 14.2 %, the nickel tenor in the sulphide phase will be rather poor. This is, however, typical for sulphide mineralizations in the marginal zone of a layered intrusion. A gabbro pegmatite east of the vil-lage Hoting shows a week platinum anomaly (0.2 ppm)

and several gold anomalous boulders have been found in the area. In the central part of the intrusion, an 8 cm thick, gently dipping layer with almost massive magnetite was found. One sample contained 68.1 % Fe2O3, 21.9 % TiO2, and 0.49 % V.

Fig. 6. Simplified geology of the Hoting area, Filén et al. (1988b).

Hoting

Hotingsjön

Sund- sjön

Rensjön20

4556

6060

65

75

704020

20

35

55

60

75

75

65

60

60

30

40

40

60

55

65

60

75

75

75

70

45-60 35

Granite

Revsund granite

Granite-pegmatite

Pegmatite

Granodiorite

Hoting mafic intrusion

Metasedimentary rock

Metamafic rock

Foliation

Magmatic layering5 km

40 B. FILÉN

Kläppsjö (Location: Mapsheet 20H x7065000 y1569000)

Exploration history

The Kläppsjö intrusion was investigated already in the early days of SGU’s nickel exploration campaign, in the beginning of the 1970’s. Sulphide bearing samples were collected, but they contained only small amounts of nick-el. However, some of them were anomalous in palladi-um. When the PGE project started, the Kläppsjö intru-sion was thus a natural target. The Kläppsjö area has been the subject of sampling, mapping, trenching, airborne and ground geophysics and geochemistry. Altogether 1845 m of diamond drillings in 22 drill holes has been carried out during two drilling campaigns. The Kläppsjö layered intrusion has been described in several SGAB exploration reports: Filén (1985, 1987, 1990), Filén & Lundmark (1988a, 1988b), Filén et al. (1988a, 1988b, 1989), Ekström (1988), and also by Lundqvist et al. (1990).

Geology

The Kläppsjö massif is a 6x4 km large layered intrusion situated 15 km east of the village Junsele in the Väster-norrland County. Just like Hoting, the main part of the Kläppsjö massif forms a topographic high and is mostly relatively well exposed. The intrusion is mainly surround-ed by paragneisses, and in the north-west and south-east by the late orogenic Härnö granite. In the eastern part, remnants of felsic volcanic rocks occur.

The Kläppsjö massif (Fig. 8) shows a megacyclic in-ternal structure with ultramafic, gabbroic, leucogabbroic, and anorthositic units, less than ten to more than hun-dred meters thick. The cyclic composition is thought to be the result of multiple magma injections. A section in a 20 m thick ultramafic layer in the southern part has showed highly anomalous platinum values. Samples from

a harzburgitic layer in contact with a metapyroxenite lay-er contained between 1.1 and 21.0 ppm Pt. In Novem-ber 1987, a 14-hole diamond-drilling program started. Extensive analyses of drill core samples from the ultrama-fite only revealed four sections with weak Pt-anomalies (0.06–0.16 ppm). Low Au-values, between 0.01 and 0.17 ppm, were encountered in three drill-cores. Almost half way up in the theoretical stratigraphy, a 110 m thick ul-tramafic unit occurs, which was diamond drilled during a second campaign. Eight holes were drilled in two profiles 170 m apart. 238 Pt-Pd-Au-Ag analyses were made.

Already during the field work, some anomalous PGE sections had been recorded in outcrops. Anomalous sec-tions with PGE-Au (Ag) analysed from diamond drill-holes are presented in Table 2. The precious-metal content is sub-economic, but the gold values suggest that Kläppsjö might also be a pure gold exploration target. As in Hoting, boulders with anomalous Ni-Cu tenors (0.63–0.85 % Ni, 0.37–1.03 % Cu), but poor in the sulphide phase, have been encountered in the south-eastern part of the intru-sion where marginal zone sulphide mineralizations might occur. Ilmenite-containing boulders with approx. 10 % TiO2 have also been found.

Bottenbäcken (Storsjö Kapell) (Location: Map sheet 18D x6973500 y1359000)

Exploration history

The existence of a copper mineralization in a gabbro com-plex situated in the Storsjö Precambrian window (Fig. 9) in the Caledonides was first noticed by a private prospec-tor, but also by a SGU uranium exploration team in the mid-1970’s. Exploration started soon thereafter. Twenty boulder and outcrop samples contained an average of 1.20 % Cu and 0.5 ppm Au (Tirén 1979). The mineraliza-tion which has been called both Bottenbäcken and Storsjö Kapell has later been studied intermittently by different

Fig. 7. Augite oikocryst-bearing layers alternating with olivine–gabbro/troctolite layers in the Hoting mafic intru-sion.


organisations and companies by means of geological map-ping, ground geophysics, geochemistry, and diamond drillings (Hålenius et al. 1985, Toverud 1987, Andersson 1990).

GeologyThe copper was first thought to have been enriched in de-formation zones in a gabbro complex (Tirén 1979). The mineralized gabbroic host rocks were later reinterpreted

(Hålenius et al. 1985) as metabasaltic tuffs and lavas with-in granitic mylonites. The mineralizations showed anoma-lous Pd-values with grades between traces and 4.2 ppm and were interpreted as hydrothermal in character. Ana-lytical results from two of the best drill-holes can be seen in Table 3 (Toverud 1987). Only in a few cases has Pt been encountered. The precious metal analytical results are usu-ally hard to reproduce which could indicate a strong nug-get effect.

Kläppsjön

Rängsjön

Mjövattnet

Kläppsjö

70

70

70 70

7070

65

75

75

7060

000

1569000

Ultramafic layers

Sedimentary gneiss

Mica schist, phyllite

Gabbro

Magmatic layering

Foliation

Felsic volcanic rocks

Tonalite

Granodiorite

Granite

Dolerite

75

2 km

Fig. 8. Geology of the Kläppsjö mafic intrusion, Filén et al. (1988b).

42 B. FILÉN

Table 2. Sections with anomalous PGE-Au values (ppm) in the Kläppsjö massif.

Drill hole Section Pt Pd Au Ag88101 4.50–5.50 0.11 0.05 -” 5.50–6.50 0.15 0.07 0.01” 18.50–19.50 0.13 0.05 -” 24.50–25.50 - - 0.0488102 15.00–16.00 0.05 - -” 27.00–28.00 0.06 - -” 28.00–29.00 0.05 - -” 30.00–31.00 0.06 - -” 38.00–39.00 0.07 - -88105 11.00–12.00 - - 0.08” 14.00–15.00 0.21 0.09 0.01” 15,00–16 00 0.20 0.06 -88106 2.20–2.70 - - 0.20” 3.70–5.00 - - 0.34” 5.00–6.25 - - 0.11 6” 6.25–7.50 - - 0.39 8” 7.50–8.50 - - 1.17” 8.50–9.50 - - 0.63 6” 9.50–10.50 - - 0.86” 10.50–11.50 - - 0.0188107 36.50–38.00 - - 0.26 4” 48.50–50.00 - - 0.03” 50.00–51.10 - - 0.23” 68.00–69.00 - - 0.04 5” 69.00–70.00 - - 0.10” 70.00–71.00 - - 0.25” 71.00–72.00 - - 0.04

Ytter-Röversjön

Över-Röversjön

Öster-Rotsjön

Storsjön

Särv rocks

Phyllonitic rocks

Granitic mylonites

Mafic extrusive rocks

Thrusts 0 1 2 3 km

69710001363000

Storsjö

Fig. 9. Map of the Storsjön win-dow with the Bottenbäcken mafic extrusions (after Hålenius et al. 1985).

Table 3. Analytical results from Bottenbäcken. PGE-Au values in ppm, Cu in %.

Drill hole Section Cu Au Pd Pt

86012 34.70-37.05 0.84 0.48 0.83 -” 37.05-39.40 0.44 0.24 0.34 -” 39.40-40.40 2.30 1.15 1.38 -” 40.40-41.40 0.51 0.23 0.38 -” 48.50-50.00 0.61 0.12 0.34 -” 69.20-70.70 0.24 0.12 0.06 -” 70.70-71.70 2.88 0.83 1.35 -” 71.70-72.20 0.20 0.08 0.07 -” 73.50-76.20 0.32 0.11 0.13 -” 76.20-77.20 0.81 0.34 0.35 -86013 58.80-59.65 3.28 1.30 2.65 -” 59.65-60.55 0.64 0.34 0.45 -” 60.55 -61.45 2.71 0.77 4.18 0.15” 64.50-65.20 2.88 0.95 2.88 -” 65.20-68.05 0.43 0.20 0.26 -


Flinten (Svärdsjö) (Location: Map sheet 13G x6727000 y1510000)

Exploration History

Mafic intrusions have for long been known to exist in the central parts of the ”Svärdsjö Circular Structure”. Some geophysical work has earlier been carried out in the north-eastern part of the complex, but the mafic rocks have not generally drawn any attention. During a regional field vis-it in 1988 it was concluded that the intrusion exhibits dif-ferent kinds of beautiful layering as seen in Figure 10. Ex-cept for rock sampling, only a minor geochemical study has been performed in the south-eastern part of the intru-sion (Lindholm 1990).

Geology

The ”Svärdsjö Circular Structure” is situated in an area with a low aeromagnetic signature. The inner part, Flinten, where the patterns are more variable, has been in-terpreted as a volcanic centre and the ”stock formed” high magnetic gabbros as feeder channels for the surrounding, mainly mafic volcanic rocks (Hammergren 1986).

Faults and dislocations cut the complex, which con-sists of several separate blocks or lobes (Fig. 11). Layering is very well developed. A continuous series of ultramafic and mafic to leucocratic rocks can be found ranging from dunites, lherzolites, troctolites and pyroxenites to gabbros, leucogabbros and anorthosites (Filén et al. 1988b, Filén et al. 1989). The width of the layers, which in places can be intensly folded, varies from 2 mm to hundreds of metres. In many places, small amounts of sulphides can be seen. Some of the sulphide-bearing rocks also contain anoma-lous amounts of PGE and Au.

The most anomalous sample with 1.8 ppm Au (Table 4) was taken from a weakly sulphide-bearing meta-pyroxenite layer close to the locality shown in Figure 10. The other anomalous samples came from local troctolitic, websteritic and gabbroic boulders, also in the southeastern part of the intrusion.

Remarks

When the potential for a future PGE-exploration was evaluated in 1984, the results showed that only layered mafic intrusions could be the primary targets. At that time only a handful of layered intrusions were known to exist in Sweden. The name layered mafic intrusion was mainly restricted to very large complexes like Bushveld or Still-water or to the classical Skaergaard. During the explora-tion work it was soon concluded that probably most of the larger mafic massifs showed some kind of layering. One of the few larger intrusions that did not show any true layering is the circular Kärkejaure structure (x7584000 y1708000) in the northernmost part of Sweden. The very few outcrops (mostly in the east), gravity high, extreme highmagnetic anomalies, geochemical patterns (Geol. Surv. of Finland et al. 1986), and in places dense shrub vegetation (salix and juniperus) indicate that the in-trusion is more likely an analogue to the Sokli carbonatite in north-eastern Finland.

Fig. 10. Graded bedding in the Flinten layered intrusion,

Table 4. Analytical results from samples from the Svärdsjö area. PGE-Au values in ppm, Cu and Ni in %.

Sample Au Pd Pt Ni Cu

PGBA88664 0,18 0.04 0.05 88667 0.14 0.04 - 88668 0.20 0.10 0.06PGBA89307 0.05 0.05 0.05 89310 - - - 0.2 89311 - 0.04 - 89602 - - - 0.09 0.4 89604 0.03 0.06 0.11 0.08 0.1 89605 0.09 0.19 0.13 0.05 0.15 89608 0.02 - - 0.06 0.19 89645 1.8 - -

44 B. FILÉN

It is the hope that this short review will lead to a sys-tematic investigation of layered mafic intrusions, which in turn will give insight in petrogenesis and differentia-tion of mafic magmas and how ores are formed in these

magmas. Layered intrusions will also in the future be the natural targets in exploration for PGE’s, gold, chromium, titanium, and vanadium, and hence deserve a continued research interest also in Sweden.

Logärden

Toxen Håsjön

Hinsen

Hinsen

80

60

50

60

50

6040

60

757050

40

4040

70

45

45

20

30

6060

60

30

5050

67220001510000

Younger granite

Older granite

Flinten mafic layered intrusion

Felsic metavolcanic rocks

Mafic metavolcanic rocks

Foliation

Magmatic layering

Way up, inverted

Sulphide mineralization2 km

Fig. 11. Geology of the Flinten area, Hammergren (1986), Filén et al. (1989).


References

Åhman, E., 1967: Hoting – Rörströmgabbron i Västernorrlands län. Sveriges geologiska undersökning C 607, 3–26.

Alapieti, T.T. & Lahtinen, J.J., 1989: Early Proterozoic layered intrusions in the northeastern part of the Fennoscandian Shield. Geological Survey of Finland, Guide 29, 3–41.

Andersson, L-G. 1990: Bottenbäcken – arbeten 1989 inkl. sum-mering arbeten 1988. SGAB b 9002.

Anon., 1986: Järn i Jörn – Näsbergsgruvan. Markkontakt Nr 2 13–18.

Arvanitidis, N., 1982: The geochemistry and petrogenesis of the Notträsk mafic intrusion, northern Sweden. Meddeland-en från Stockholms Universitets Geologiska Institution, Nr 253.

Claesson, L-Å., 1980: Åselet. Prospekteringsarbeten utförda av SGU 1979–1980. Sveriges geologiska undersökning BRAP 80032.

Claesson, L-Å., Filén, B. & Ekström, M., 1982: Delrapport över krommineraliseringen vid Kukkola. Sveriges geologiska under-sökning BRAP 82100.

Claesson, L-Å., Ullberg, A., Magnusson, J., Wiberg, B. & Ek-ström, M., 1983: Prospekteringsrapport, 1981–1982, Kuk-kola. SGAB PRAP 83042.

Ekström, M., 1988: Kläppsjö – Mineralogi SGAB PRAP 88023.

Enmark, T., 1982: Development and optimization procedures for gravity and magnetic interpretation and their application to some geological structures in northern Sweden. University of Luleå, 1982:019 D.

Filén, B., 1985: PGE-prospektering Etapp II. SGAB PRAP 85111.

Filén, B., 1987: PGE-prospektering 1986 Etapp II. SGAB PRAP 87003.

Filén, B., 1990: PGE-Prospektering i Sverige 1985-1990. SGAB 90026.

Filén, B. & Lundmark, L.-G,. 1988a: Kläppsjö Borrning, Etapp I. SGAB PRAP 88022.

Filén, B. & Lundmark, L.-G., 1988b: Kläppsjö Borrning Etapp II. SGAB PRAP 88056.

Filén, B., Gerdin, P., Lundmark, L.-G. & Renberg, A., 1988a: PGE – 1987 Etapp II. SGAB PRAP 88005.

Filén, B., Gerdin, P., Lundmark, L.-G. & Renberg, A., 1988b: PGE-Ni-Prospektering 1988. SGAB PRAP 88066.

Filén, B., Ekström, M. & Lundmark, L.-G., 1989: Notträsk 1989 Diamantborrning. SGAB PRAP 89025.

Filén, B., Ekström, M., Lundmark, L.-G. & Renberg, A., 1989: PGE-Prospektering 1989. SGAB PRAP 89061.

Geological Surveys of Finland, Norway and Sweden, 1986: Geochemical Atlas of Northern Scandinavia, 1:4 milj.

Hålenius, U., Lund, L.-I. & Westerberg, S., 1985: Storsjöfön-strets koppar- och ädelmetallmineralisering. SGAB PRAP 85535.

Hammergren, P., 1986: PI Projekt. Komplexa sulfidmalmer i Falu – Hoforsområdet. SGAB PRAP 86549.

Lindholm, T., 1990: Flinten Djupmorän- och bergkaxprovtag-ning. SGAB PRAP 90007.

Lundmark, C., 1984: Kukkola Resultat av diamantborrningen 1984. SGAB PRAP 84122.

Lundqvist, T., Gee, D.G., Kumpulainen, R., Karis, L. & Krest-en, P., 1990: Beskrivning till Berggrundskartan över Väster-norrlands Län. Sveriges geologiska undersökning Ba 31, 429 pp.

Sundin, N.O., 1977: Geofysiska mätningar i Notträsk. Univer-sity of Luleå, 1977:120E.

Svensson, R., 1981: Mineralogisk undersökning av Notträsk-gabbron. University of Luleå, 1981:072E.

Tirén, S.A., 1979: Bottenbäckens kopparmineralisering. Sveriges geologiska undersökning BRAP 79519.

Toverud, Ö., 1987: Resultat från utförd diamantborrning, okto-ber-november 1986, inom objekten Kalberget, Bottenbäck-en och Nynäsberget i Jämtlands län. LKAB Prospektering s 8701.

Widenfalk, L., Elming, S.-Å. & Enmark, T., 1985: A multidis-ciplinary investigation of the Notträskgabbro, northern Swe-den. Geologiska Föreningens i Stockholm Förhandlingar107.

46 J. BERGMAN WEIHED

Bergman Weihed, J,. 2001: Palaeoproterozoic deformation zones in the Skellefte and Arvidsjaur areas, northern Sweden. In Wei-hed, P. (ed.): Economic geology research. Vol. 1, 1999–2000. Upp-sala 2001. Sveriges geologiska undersökning C 833, pp. 46–68. ISBN 91-7158-665-2.

A detailed investigation of a number of major shear zones and faults in northern Västerbotten and southern Norrbotten has resulted in new information on timing and kinematics of the deformation in this area. The observed shear zones and faults cut through the Svecofennian c. 1.95–1.80 Ga supracrustal se-quence and intrusive rocks spanning ages between 1.95 Ga and 1.78 Ga. Approximately north striking, both semi-ductile and brittle deformation zones dominate the studied area. These zones are generally characterized by retrograde greenschist facies mineral assemblages and have a reverse sense of movement. They affect all intrusive rocks and must have formed during or after the regional D3 deformation (after c. 1.80 Ga) and af-ter peak metamorphism. The dominantly dip slip reverse move-ments indicate an east-west shortening during this deforma-tion. Older shear zones were found in the central part of the studied area where they commonly parallel axial surfaces of the regional D2 folds and surround lower strain lenses. These shear zones normally show a reverse oblique slip movement, com-monly with the south side up, and they are overprinted by later S3 crenulations and display statically recrystallised textures. The shear zones formed late during the main D2 folding (between 1.87 and 1.82 Ga) in response to oblique convergence from the south-east.

Jeanette Bergman Weihed, Department of Earth Sciences, Villa-vägen 16, SE-752 36 Uppsala, Sweden. Present address: Geo-logical Survey of Sweden, Box 670, SE-751 28 Uppsala, Swe-den. E-mail: [email protected]

Introduction and background

Many shear zones have been recognised during the last ten years of mapping in research projects and by the Geo-logical Survey of Sweden in the Skellefte and Arvidsjaur areas, but the shear zones have not previously been studied in detail. They probably played an important role in the geological evolution of the area, but it has so far been un-known to what extent and during what time periods the shear zones were active. A research project was therefore initiated with the purpose to identify and describe shear zones, with special emphasis on regional zones and as-sociated deformation in an area encompassing the map sheets 22–24 H–L in the Swedish national grid, covering

large areas of the counties of Västerbotten and southern Norrbotten. Identification of metamorphic variations con-nected with the shear zones and at least the relative age of the shear zones were also aims of the study.

Regional geology

The Skellefte district, broadly coincident with the area of Skellefte Group volcanic rocks in Figure 1, contains part of the Svecofennian c. 1.95–1.80 Ga supracrustal se-quence and associated intrusive rocks in the northern part of Sweden. The rocks in the Skellefte district itself have been divided into a lower sequence dominated by sub-aqueous volcanic rocks (the Skellefte Group) and an over-lying sequence dominated by shallow-water to subaerial sedimentary and volcanic rocks (the Vargfors Group). These rocks are bordered to the south and east by a vast area of strongly metamorphosed greywackes (Both-nian Group) and to the north by subaerial volcanic rocks (Arvidsjaur Group) which are similar in age to the rocks of the Vargfors Group. The supracrustal sequence is in-truded by 1.95–1.85 Ga calc-alkaline I-type granitoids (Jörn type), by S-type anatectic granites at c. 1.82–1.80 Ga (Skellefte granites), and by younger post-volcanic A- to I-type granitoids at c. 1.80–1.78 Ga (Revsund grani-toids). For a more detailed discussion of the rocks in the area, see Weihed et al. 1992, Allen et al. 1996, Billström & Weihed 1996 and references in these papers.

Previous work

Previous structural studies have mostly concentrated on smaller areas within the Skellefte district and very little attention has focused on shear zones in particular. Edel-man (1963) presented the structural evolution in the Kris-tineberg area and this area was also studied in detail by Joseph Hull (unpubl. data). The central part of the Skel-lefte district has been structurally mapped by Bergman Weihed (unpubl.) and some results have been presented in Bergman (1989, 1991) and Weihed et al. (1992). Detailed studies of mineralizations and associated deformational structures are presented in Talbot (1988), Assefa (1990), Bergman (1992), and Bergman Weihed et al. (1996). Two major phases of folding have been proposed for most of the area. The early folds (here called D2) are tight to iso-

Palaeoproterozoic deformation zones in the Skellefte and Arvidsjaur areas, northern Sweden

Jeanette Bergman Weihed

47PALAEOPROTEROZOIC DEFORMATION ZONES IN THE SKELLEFTE AND ARVIDSJAUR AREAS, NORTHERN SWEDEN

HI

JK

L

HI

JK

L

24 23 22

24 23 22

Sor

sele

Sor

sele

Karst

räsk

Karst

räsk

Arv

idsj

aur

Arv

idsj

aur

7300000 7250000 7200000 7150000

7300000 7250000 7200000 7150000

1550

000

1600

000

1650

000

1700

000

1750

000

1800

000

1550

000

1600

000

1650

000

1700

000

1750

000

1800

000

Fig.

2

Fig.

4

Fig.

5

Fig.

7

Fig.

9

Fig.

12

Bot

hnia

nB

ay

25 k

mLule

å

Pite

å

Ske

llefte

å

Lyck

sele

Lule

å

Pite

å

Ske

llefte

å

Lyck

sele

Pos

t-vol

cani

c gr

anito

id (R

evsu

nd

type

), c.

1.8

0–1.

78 G

a

Pos

t-vol

cani

c gr

anito

id (S

kelle

fte

type

), c.

1.8

2–1.

80 G

a

Gab

bro

and

dior

ite (u

ndiv

ided

)

Con

glom

erat

e, s

ands

tone

& v

olca

nic

rock

s (V

argf

ors

Gro

up),

1.75

Ga

Mud

ston

e, s

ands

tone

& tu

rbid

ite, (

Bot

hnia

n,

Var

gfor

s an

d S

kelle

fte G

roup

s), c

. 2.0

–1.8

5 G

a

Mai

nly

suba

eria

l to

shal

low

wat

er

rhyo

lite,

(Arv

idsj

aur G

roup

), c.

1.8

8–1.

87 G

aC

aled

onid

es

Mai

nly

subm

arin

e rh

yolit

e &

da

cite

(Ske

llefte

Gro

up),

c. 1

.89–

1.88

Ga

Sup

racr

usta

l roc

ks, (

Kal

evia

n G

roup

), c

. 1.9

Ga

Mai

nly

maf

ic v

olca

nic

rock

s,(K

nafte

n G

roup

), c.

1.9

5 G

a

Arc

haea

n gr

anito

id

Low

mag

netic

line

amen

ts,

faul

ts a

nd s

hear

zon

es

Per

thite

mon

zoni

tec.

1.8

8–1.

86 G

a

Syn

-vol

cani

c gr

anito

id (J

örn

and

Hap

aran

da ty

pes)

c. 1

.89–

1.88

Ga

Cal

edon

ides

Arc

haea

n

Sve

cofe

nnia

n pr

ovin

ce

Kar

elia

n pr

ovin

ce

Neo

prot

eroz

oic

and

Fane

rozo

ic ro

cks

Sve

cono

rveg

ian

prov

ince

Tran

ssca

ndin

avia

n Ig

neou

s B

elt

Mai

nm

ap

Fig. 1. Map of the whole region with geological information from the bedrock map of central Fennoscandia (Lundqvist et al. 1996b) and major low-magnetic lineaments interpreted as shear zones and/or faults (black lines).


clinal, with upright axial surfaces and variably plunging fold axes (cf. Weihed et al. 1992). Axial surfaces strike north-east in the eastern and western parts of the Skellefte district and west-northwest in the central part of the dis-trict. An axial planar cleavage is developed and shearing along this is common. Late folds (here called D3) are open with north- to northeast-striking axial surfaces and fold axes coaxial with the early folds. Talbot (1988) and Assefa (1990) report an earlier recumbent phase of folding in the Långdal area and Allen et al. (1996) reports an early folia-tion which is subparallel to bedding. This foliation may have formed during a D1 deformation although no folds related to this foliation have been recognised.

One shear zone, which parallels the axial surfaces of early folds in the central part of the Skellefte district, was reported in Bergman (1991). This shear zone dips steeply south and stretching lineations plunge moderately south-west. An s-c fabric indicates south side up reverse move-ment. An east-striking, steep shear zone also deforms the ore in Boliden (Bergman Weihed et al. 1996) and the stretching lineation plunges about 50° east. No oriented samples could be obtained from this zone so the shear sense is unclear. Both of these shear zones are overprinted by the second regional deformation and have thus formed during, or somewhat after, the first major regional D2 phase of folding. No published structural studies exist in the areas outside of the Skellefte district which are con-sidered in the present study (Fig. 1) and, in many cases, the structures are poorly known. Some structural informa-tion is, however, reported in publications dealing with the general geology of the areas. The relevant publications are referenced in each section below.

Tectonic interpretations of the area have generally fo-cused on the Skellefte district. Hietanen (1975) proposed a subduction zone dipping north beneath the Skellefte district and after that, many similar models have been proposed (e.g. Rickard & Zweifel 1975, Lundberg 1980, Pharaoh & Pearce 1984, Berthelsen & Marker 1986, Gaál 1986, and Weihed et al. 1992). A more regional geophysi-cal study was presented by Nisca in 1995. A northward subduction is supported by a magnetotelluric survey (Ras-musen et al. 1987) which found a low-resistivity slab dip-ping north under the Skellefte district and by a seismic re-flection profile in the Bothnian Bay (BABEL group 1990) which shows a north-dipping reflector east of the Skel-lefte district. The Skellefte Group volcanic rocks are gen-erally interpreted to represent some kind of volcanic arc whereas the subaerial volcanic rocks (Arvidsjaur Group) to the north may represent a continental environment coeval with the volcanic arc. The large area of metamorphosed greywackes to the south (Bothnian Basin) may be inter-preted as a fore-arc environment (Weihed et al. 1992). Archaean detrital zircons and negative εNd values (at 1.9

Ga) in the greywackes indicate that an Archaean crust, present somewhere in the area, provided material for the sediments in the Bothnian Basin (cf. Claesson & Lund-qvist 1995, Lundqvist et al. 1998). Towards the north-east, around Luleå (Fig. 1), Archaean granitoid intrusions have recently been discovered (Lundqvist et al. 1996a). A study by Mellqvist (1997) has focused on delineating the boundary between juvenile rocks and rocks with an Ar-chaean component. Results from this study indicate that tectonic contacts may be common between Archaean and younger rocks (Mellqvist 1997 & 1999).

Methods

During the present study, a few new deformation zones were identified using a combination of aeromagnetic in-terpretation and outcrop information. Interpretation of aeromagnetic maps was done to identify regionally im-portant shear zones or faults. This information was then combined with outcrop information in order to locate the shear zones in areas which have not yet been mapped in detail. These interpreted shear zones were then studied in the field in addition to shear zones identified during map-ping by the Geological Survey of Sweden.

Aeromagnetic grey tone and relief maps on a scale of 1:250 000 were used for the geophysical interpretation of the entire area. This was complemented by interpretation of maps on a scale of 1:80 000 and 1:100 000 in areas where more detail was necessary. Low-magnetic, narrow zones can be found in most parts of the study area, but they are most easily seen where the bedrock is rela-tively high-magnetic. Therefore, very few low-magnetic zones were found e.g. on map sheets 22 H–I and 23H (Fig. 1). Other parts of the area appear complex with a large number of anastomosing low-magnetic zones (e.g. western part of 23–24 K). Apart from low-magnetic lin-eaments, magnetic boundaries (commonly representing boundaries between intrusive and supracrustal rocks) and high-magnetic narrow zones (often representing form lines of bedding in sedimentary units) were noted. All this information was correlated with geological maps of the area.

Results

The most striking feature of the interpretation of low-magnetic lineaments shown in Figure 1 is the dominance of north- and northwest-striking zones on map sheets 23–24 H–I, whereas east to northeast striking zones dom-inate the southeastern-most part of the area. Northwest-striking zones occur mainly within the central Skellefte district (23 J–K), and these are less obvious on aeromag-


netic maps of the scale used in this project. These zones are, however, very clearly seen when interactively working with the aeromagnetic data on a computer.

Kinematic indicators in the deformation zones were found in the field in approximately 20 shear zone locali-ties. Nearly all observed shear zones are subvertical and have steep stretching lineations. In general, most north-striking zones show that the eastern side has moved up-wards in relation to the western side, and most north-west-striking zones indicate that the southern side has moved upwards relative to the northern side. A few zones with dominantly horizontal movements have also been found, e.g. along the northern contact of the Karsträsk dome (Fig. 1).

Below, results from all identified and studied shear zones are presented by area. The geological maps are based on the recently published Mittnorden map (Lundqvist et al. 1996b) supplemented by more detailed information from published papers and, in a few cases, ongoing map-ping by SGU. In the discussion, an attempt will be made to integrate all the data into a tectonic interpretation of the area.

The Bure area

The Bure area (Fig. 2) is located a few kilometres east of Sorsele (Fig. 1). The rock sequence in the area has recently been described by Perdahl & Einarsson (1994) as consti-tuting an example of the boundary between submarine and subaerial depositional environments, i.e. the strati-graphical boundary between the Skellefte and Arvidsjaur volcanic arcs. The exposed rock sequence constitutes a 4–8 km wide syncline (Perdahl & Einarsson, 1994) com-posed of sedimentary and mainly mafic volcanic rocks. The supracrustal units are intruded by the Sorsele grani-toid to the west and by a Revsund granitoid to the east. Perdahl & Einarsson (1994) divide the supracrustal rocks of the area into three formations: the lowermost Stalo for-mation comprising basaltic to andesitic sedimentary units and rhyolite, the overlying Bure formation which consists of rhyolite–dacite lavas and clastic units and basaltic to andesitic lavas and clastic units, and the uppermost Loito conglomerate (Fig. 2).

Two distinct low-magnetic lineaments appear on aero-magnetic maps of the Bure area and both are interpreted as shear zones. A prominent NNE-striking lineament cuts through the centre of the proposed supracrustal syncline. Field observations show that this lineament is caused by a ductile shear zone, here called the Loito shear zone. The continuation towards the south-west through the low-magnetic granitoids is unclear but towards the north-east the shear zone continues via Jokkmokk to north of Paja-la. A subparallel low-magnetic lineament cuts the Sorsele

granitoid to the west of the supracrustal sequence. The surface expression of this lineament could not, however, be found in the field due to lack of exposure.

The Loito shear zone was observed along the eastern margin of the Loito conglomerate and also in andesite-basalt of the Bure formation immediately east of the con-glomerate (Fig. 2). Several more intensely sheared zones were observed and these were separated by areas of rela-tively well preserved rocks. Bedding surfaces were visible in an andesitic siltstone between two shear zones and east of the eastern-most shear zone almost undeformed amygdules were observed in andesite lavas or intrusions. The magnetic susceptibility is generally high in the less deformed supracrustal rocks in the area, whereas within the shear zones the susceptibility varied considerably from nearly zero to 9000x10-5 SI-units.

Clasts in the Loito conglomerate are extremely stretched within the shear zone (Fig. 3a) and are really only visible on surfaces parallel to the very strong cleav-age. The conglomerate is polymict but most clasts are fel-sic and fine-grained and probably have a volcanic origin. All clasts are both strongly flattened and stretched with a steep stretching lineation plunging north and a cleavage which dips 86° west. A later crenulation lineation is par-allel to the stretching lineation and the associated crenu-lation cleavage strikes north-east. Asymmetric s-type tails on small crystals were observed in the field and they in-dicate that the eastern side has moved upwards relative to the western side. This observation was corroborated in thin section where both asymmetric wings on small clasts and crystals and shear bands indicate this sense of move-ment (Fig. 3b). The matrix in the conglomerate is com-posed of sericite, chlorite, fine-grained feldspar, quartz, and opaque phases. Calcite is common in veinlets parallel to the cleavage and also in stretched and fractured feldspar crystals. The andesite–basalt is feldspar porphyritic with a very fine-grained matrix of feldspar, sericite, and chlorite. Most of the feldspar phenocrysts are altered to chlorite, epidote, and calcite (Fig. 3c) near the shear zones where-as the feldspar phenocrysts are better preserved at some distance from the shear zones. Chlorite is common in strain shadows on epidote. Shear bands and asymmetric s-type tails on phenocrysts indicate that the eastern side has moved up relative to the western side also in the an-desite (Fig. 3c). In the eastern splay of the shear zone a protomylonitic zone was found in a feldspar porphyritic dacite. Asymmetric tails on feldspar phenocrysts and shear bands (Fig. 3d) indicate that this splay has a west side up sense of movement.

The alteration of feldspars to epidote and calcite in the andesite–basalt and the common occurrence of sericite in the matrix in both the andesite-basalt and the conglom-erate indicates that the shearing took place under green-


85

87

85

77

Norr-Svergoträsket

Sör-Svergoträsket

Heden

Sorsele

8671

8377

7877

63

71

69

67

50

61

80

70

65 60

55 70

70

50

88

2362

72

30

85

87

85

77

70

Stalo formation

Shear zone, barb on upthrown sideSorsele and Revsund granite

RhyoliteLoito formation

Polymict conglomerateand sandstone

Bure formation

Mg-basalt

Rhyodacite/rhyolite lavaand conglomerate

Andesite tuff

Andesite lava

Schist dominating?

Schist-greywacke

Basalt/andesite lava and conglomerate

Foliation

Fold axis

Bedding

Shear fabric with stretching lineation

2 km

8378

87

88

30

7264

000

7266

000

7268

000

7270

000

7272

000

7274

000

7276

000

7278

000

7264

000

7266

000

7268

000

7270

000

7272

000

7274

000

7276

000

7278

000

1580000 1582000 1584000 1586000 1588000 1590000

1580000 1582000 1584000 1586000 1588000 1590000

Fig. 2. Map of the Bure area. Geology based on observations by Perdahl & Einarsson (1994) complemented with results from recent mapping by the Geological Survey of Sweden and observations from this study. Shear zones are interpreted in this study. Small grey squares represent observed localities.


schist facies conditions. These assemblages are also com-mon in the country rocks and indicate that the shearing took place during or somewhat after peak metamorphism.

Västra Kikkejaure to Jan-Svensamössan

The north-eastern part of the Storavan map sheet is dominated by early orogenic granitoids of different types (Fig. 4). The Arvidsjaur granite occurs in the eastern-most part of the area. These early orogenic granitoids in-trude subaerial volcanic units of the Arvidsjaur Group which consist of rhyolitic to andesitic intrusions and lavas, and rhyolitic ignimbrites. These rocks were then intruded by post-orogenic granitoids of both Adak type (medium-grained) and Revsund type (coarse-grained). The volcanic units are very well preserved and commonly contain only a weak cleavage indicating that the ductile deformation of these rocks is limited. Peak-metamorphic assemblages indicate upper greenschist to lower amphibolite facies in the southern parts of the map sheet (Bergström & Triumf 1996).

Interpretation of aeromagnetic maps shows a large number of approximately north-striking low-magnetic lineaments which cut through the rocks of the area. These are interpreted to be faults or shear zones since they dis-place lithological contacts. In the central part of the Stor-

avan map sheet these north-northeast striking faults con-tain many subeconomic uranium mineralizations (Ada-mek & Wilson 1979). A few short profiles were mapped across two of these approximately north-striking low-mag-netic lineaments in the north-eastern part of the Storavan map sheet. In this area, the lineaments cross mainly syn-orogenic granitoids.

Along Långträskälven to Stenträsket (area A in Fig. 4), an east-west profile across the eastern-most low-mag-netic lineament on the Storavan map sheet covers volcanic rocks of dacitic to andesitic composition, intruded in the east by an Arvidsjaur granite. A penetrative grain shape foliation is present in the volcanic rocks but lacking in the coarse granite. Locally, the rock is cut by numerous closely spaced vertical north-striking fractures that are filled with epidote and, in these areas, the magnetic susceptibility decreases considerably. In general, however, the volcanic rocks are very well preserved with amygdules in andesite/basalt, perlitic cracks in dacite, possible ignimbrite struc-tures, and well developed porphyritic textures, indicating a limited amount of ductile deformation. The supracrus-tal sequence in this area is interpreted by Adamek (1987) to form a syncline.

Around Tjålmak (area B in Fig. 4) a low-magnetic lin-eament crosses an area of early orogenic granitoid. This is a light grey, medium grained granitoid with a magnetic

Fig. 3. a) Outcrop in the Bure area showing extremely sheared Loito conglomerate. View towards south. b) Asymmetric wings on a small opaque clast indicating east side up. Width of view 2.7 mm. c) Feldspar phenocryst with asymmetric tails indicating east side up. Feldspar is strongly altered to epidote, chlorite, and calcite. Width of view 5.4 mm. d) Sheared feldspar porphyritic dacite with shear bands indicating west side up. Width of view 2.7 mm.


susceptibility of around 1800x10–5 SI-units. Fine-grained angular to sub-rounded dioritic fragments are present in the granitoid. The only observed deformation structures are locally closely spaced fractures in many orientations, some of which have epidote infill. In one outcrop the frac-tures are so closely spaced that they define a spaced cleav-age. The orientation of this cleavage is subparallel to the low-magnetic lineament. However, lack of outcrop did not allow observation on the interpreted lineament.

Nordanås and Jan-Svensamössan to Skogberget (areas C and D in Fig. 4, respectively) are two outcrop areas on the same low-magnetic lineament. In Nordanås (area C), the lineament crosses a flesh-coloured to pink medium-grained granitoid with high magnetic susceptibility (av.

2000x10–5 SI-units). On either side of the lineament, the granitoid is well preserved with only a few fractures and contains fragments of mafic volcanic rocks with preserved bedding. At the position of the low-magnetic lineament, however, the granitoid is strongly brecciated with closely spaced fractures and epidote as an infilling mineral. The magnetic susceptibility is substantially reduced in this area and averages 100x10–5 SI-units. A similar pattern is found at Jan-Svensamössan and Skogberget (area D in Fig. 4) where granitoid is well preserved on either side of the line-ament but strongly brecciated with epidote infill and a locally developed spaced cleavage at the position for the lineament.

These observations all indicate that the north-striking

Arvidsjaursjön

V Kikkejaure

Långträsket

Avaviken

Arvidsjaur

A

B

C

D

Major faults and/or shear zones

Form lines of tectonic banding

Post-volcanic granitoid (Revsund type), c. 1.80–1.78 Ga

Post-volcanic granitoid (Adak type), c. 1.80–1.78 Ga

Mainly subaerial to shallow water rhyolite, (Arvidsjaur Group), 1.88–1.87 Ga

Mainly subaerial to shallow water daciteand andesite (Arvidsjaur Group), 1.88–1.87 Ga

Gabbro, undivided

Synvolcanic Arvidsjaur granitoidc. 1.87–1.85 Ga

Synvolcanic granitoidc. 1.89–1.88 Ga

5 km

7275

000

7280

000

7285

000

7290

000

7295

000

1625000 1630000 1635000 1640000 1645000 1650000 1655000 1660000

7275

000

7280

000

7285

000

7290

000

7295

000

1630000 1635000 1640000 1645000 1655000 16600001625000 1650000

Fig. 4. Map of Storavan NE and parts of Arvidsjaur NW. Geology on Storavan map sheet from unpublished map by Adamek and on Arvids-jaur map sheet from Kathol & Triumf (1995). Most shear zones and all form lines interpreted in this study. Small grey squares represent observed localities.


low-magnetic lineaments in this area correspond to brit-tle faults where the rocks cut by the faults have been bre-cciated and infiltrated with fluids that deposited mainly epidote. The brecciation also led to a marked decrease of the magnetic susceptibility. Observations from north-striking low-magnetic lineaments on the western part of the Arvidsjaur map sheet show similar features (Benno Kathol, pers. comm. 1996).

The brittle nature of the faults excluded the determi-nation of sense of movement and amount of displacement on the probably steep slip surfaces (steep fracture cleav-age). However, on aeromagnetic maps of the area a sinis-tral strike component is visible from a displaced gabbro body and also from other displaced magnetic markers. No information is available on the vertical component of movement.

Deppis–Näsliden shear zone

A locally diffuse low-magnetic lineament occurs on aero-magnetic maps of the western parts of map sheets 23J and 24J (Fig. 1). Sheared rocks were observed in the field at several localities along this lineament and the shear zone will here be called the Deppis–Näsliden shear zone. The northern part of this shear zone strikes north-east to north-northeast whereas in the southern part it strikes more north-northwest (Fig. 5). The shear zone is com-posed of at least three separate splays in the northern part of the area, whereas in the central part of the area the shear may be more localised to one zone. In the southern part of the area the shear zone again divides into two splays.

The Deppis–Näsliden shear zone cuts through an area dominated by supracrustal rocks of the Arvidsjaur, Skel-lefte, and Vargfors Groups. Rocks of the Arvidsjaur Group occur in the northern part of the area where they are in-truded by synvolcanic granitoids which are also cut by the Deppis–Näsliden shear zone (Fig. 5). Farther south, the shear zone occurs along the boundary between the central and western parts of the Skellefte district where mainly felsic volcanic rocks of the Skellefte Group and sedimen-tary units of the overlying Vargfors Group appear. These rocks are in the south intruded by post-volcanic grani-toids of the Revsund and Skellefte types which may be unaffected by the Deppis–Näsliden shear zone, since the shear zone cannot be traced farther south on aeromag-netic maps. However, this may be due to the low-magnet-ic character of the Revsund and Skellefte granitoids. The rocks of the Skellefte and Vargfors Groups are folded into upright tight folds with steep axial surfaces striking north-west and variable fold axes. Late open folds have steep axial surfaces striking north to north-east and fold axes which are in general coaxial with the main upright folds. Ductile structures in the commonly well preserved Arvids-

jaur Group are less well known and only weak fabrics are present. Within the Deppis–Näsliden shear zone, the sheared rocks always have a very strong cleavage with pro-nounced stretching lineations that plunge steeply west to north-west indicating movement mainly in the vertical direction.

On the Arvidsjaur map sheet around Kilisåheden (A in Fig. 5), the Deppis–Näsliden shear zone was observed in an area with volcanic rocks belonging to the Arvidsjaur Group. These are finely flow-banded feldspar porphyritic dacites, rhyolitic very crystal-rich ignimbrites and mass flows, and siltstone units. Peperitic contacts were observed between dacite and siltstones in two localities. In this area the shear is localised into at least three zones separated by low-strain lenses. The shear fabric strikes 180–200° and has a steep dip towards the west. A strong stretching linea-tion plunges steeply west to north-west indicating mainly dip-slip movement. Primary structures like bedding and flow banding in the low-strain lenses often strike subpar-allel to the shear fabric but have a less steep dip. Flow banding in the dacites is locally intensely folded with axial surfaces parallel to the shear zone and shallowly plunging fold axes. Good shear sense indicators were observed in a few outcrops and in most thin sections. Asymmetric tails on feldspar phenocrysts (Fig. 6a) and weak shear bands indicate western side up on the westernmost shear zone, whereas east side up was indicated by asymmetric tails on feldspar phenocrysts (Fig. 6b) in the eastern-most shear zone (Fig. 5). The shear fabric is defined by bands of muscovite+chlorite alternating with bands of very fine-grained feldspar±quartz. Feldspar phenocrysts are varia-bly altered to sericite and, in the more mafic rocks, to epidote+calcite. The alteration of feldspar phenocrysts is strongest in the shear zone and decreases outwards from the zone.

Farther south, the Deppis–Näsliden shear zone was encountered in rocks around the Grytfors dam (B in Fig. 5). In this area, the shear zone cuts mainly sedimentary units of silt/mudstone and volcanogenic mass flows be-longing to the Skellefte Group. Bedding surfaces in the sedimentary units outside the shear zone are folded with axial surfaces striking north-west whereas towards the shear zone bedding is transposed into an orientation par-allel to the zone itself. Stretching lineations on the steeply west-dipping shear surfaces plunge west to north-west in-dicating dominantly dip-slip movement. Shear sense indi-cators could not be found in the silt/mudstones, neither in the field nor in thin section, but in the somewhat coars-er units, asymmetric tails on megacrysts and weak shear bands (Fig. 6c) indicate that the western side has moved up.

In the area around Småberg–Hälträsket (C in Fig. 5), the Deppis–Näsliden shear zone cuts through an area of


Abborrträsk

Arvidsjaur

Glommersträsk

Norsjö

Norrliden

Maurliden

HolmtjärnNäsliden

Rakkejaur

8063

78

78

7170

7673

86

78

83

70

69

74

7368

A

C

D75

85

B

80

10 km

Conglomerate & sandstone (Vargfors Group), 1.88–1.87 Ga

Mudstone, sandstone & turbidite, (Bothnian, Vargfors & Skellefte Groups), 2.0–1.85 Ga


Post-volcanic granitoid (Skellefte type), c. 1.82–1.80 Ga

Gabbro and diorite, undivided

Ultramafic intrusion, undivided


Mainly submarine basalt-andesite, (Skellefte Group), 1.89–1.88 Ga

Mainly submarine rhyolite & dacite(Skellefte Group), 1.89–1.88 Ga

Synvolcanic intrusion (Gallejaur monzonite) c. 1.87–1.85 Ga

Synvolcanic granitoid (Jörn &Arvidsjaur types) c. 1.89–1.88 Ga

Major VHMS deposits

Basalt & andesite (Vargfors Group),1.88–1.87 Ga

Shear zone, barbs on upthrown side

Faults and/or shear zones


Form lines of tectonic bandingand/or bedding

8063

1650000

1650000

7200

000

7250

000

7300

000

7200

000

7250

000

7300

000

Fig. 5. Map of the geology around the Deppis-Näsliden shear zone. Geology compiled from Allen et al. (1996), Kathol & Triumf (1995) and the Mittnorden map (Lundqvist et al. 1996b). Shear zones and form lines interpreted in this study. Small grey squares represent observed localities.


mainly mafic volcanic rocks. East of the shear zone, very well preserved pillow lavas can be found. The shear zone itself was observed in an outcrop of a feldspar porphyritic mafic rock of unclear origin (possibly a crystal-rich mass flow or a shallow intrusion). The very strong shear fabric in this locality strikes north-northeast with a steep dip to the west and the stretching lineation plunges steeply north. An s-c fabric observed in outcrop indicates that the western side has moved up. This shear sense was con-firmed by thin sections which all contain asymmetric tails on epidote and feldspar crystals and shear bands indicat-ing the same sense of movement (Fig. 6d). The original feldspar phenocrysts have been strongly altered to epidote and calcite (Fig. 6e). The epidote is commonly rimmed by zoisite. The matrix between epidote and feldspar mega-crysts is composed of very fine-grained feldspar, musco-vite, and chlorite.

The bedrock around Rakkejaur and Näsliden (D in Fig. 5) consists of folded volcanic and sedimentary rocks of the Skellefte and Vargfors Groups (Svenson 1982, Trepka-Bloch 1989). The volcanic units are mainly rhy-

olitic to dacitic shallow intrusions and mass flows, and the sedimentary units are finely laminated mud- and siltstones. Axial surfaces of the main isoclinal folds are steep and strike north to north-northwest with fold axes plunging steeply west (Svenson 1982). Narrow zones with strong deformation and a steep stretching lineation were observed in a few outcrops north of Näsliden in a crystal-rich volcaniclastic unit with andesitic fragments. In thin section the feldspar phenocrysts are partially altered to sericite and stretched with boudin necks infilled by cal-cite. The very fine-grained matrix is composed of feldspar, sericite, chlorite, and calcite and the cleavage is defined mainly by the alignment of micas. Slightly asymmetrical tails on some plagioclase phenocrysts may indicate east side up. The eastern splay of the Deppis–Näsliden shear zone, interpreted from aeromagnetic maps (Fig. 5), should pass close to the Rakkejaur deposit but it was not found in outcrop.

In summary, the observed shear sense indicators show that the main Deppis–Näsliden shear zone had dominant-ly dip slip movement where the western side moved up

Fig. 6. a) Asymmetric wings on feldspar phenocryst in feldspar porphyritic dacite indicating west side up. Width of view 5.4 mm. b) Asymmetric wings on feldspar phenocryst in crystal-rich mass flow indica-ting east side up. Width of view 5.4 mm. c) Weakly developed shear bands in sheared crystal-rich daci-tic mass flow indicating west side up. Width of view 5.4 mm. d) Well developed s-c fabric indicating west side up, in strongly altered feldspar porphyritic an-desite. Width of view 5.4 mm. e) Same as d) but also showing the rounded epidote crystals that overgrow feldspar phenocrysts. Width of view 5.4 mm.


relative to the eastern side. An easterly splay in the north-ern part of the area shows the opposite sense of move-ment. All observations indicate that shearing along the zone took place after the main folding of the area and probably at greenschist facies conditions. The amount of movement, however, has not been possible to determine.

Central Skellefte District

In the central part of the Skellefte district (Fig. 7) a number of shear zones have been identified during previ-ous mapping (Bergman 1991). A few of these zones were studied in more detail in this project. The central part of the Skellefte district consists of a sequence of volcanic and volcaniclastic units of mainly rhyolitic to dacitic composi-

tion which belong to the Skellefte Group. Mafic sills have intruded into the sequence at some stage before deforma-tion. These rocks are overlain by siltstones, sandstones, grits, and conglomerates of the Vargfors Group. All of these rocks are relatively well preserved and greenschist facies mineral assemblages are normal. Towards the south, however, the metamorphic grade increases and north of the Karsträsk intrusion, amphibolites and strongly recrys-tallized meta-volcanic rocks occur. The main deforma-tional structures are upright tight folds with fold axes plunging around 45° SE in most of the area. An axial pla-nar cleavage developed as a penetrative grain shape folia-tion in the volcanic units whereas it is a crenulation cleav-age in the fine-grained sedimentary units. This indicates that there, at least locally, is an earlier bedding-parallel

SkellefteRiver

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Ultramafic intrusion, undivided



Synvolcanic granitoid (Jörn type)c. 1.89–1.88 Ga

Major VHMS deposits

Basalt & andesite (Vargfors Group),1.88–1.87 Ga



Form lines of tectonic banding and/or bedding

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Fig. 7. Geological map of the central part of the Skellefte district compiled from unpublished maps by Bergman Weihed and Allen et al. (1996). Most shear zones and form lines interpreted in this study. Small grey squares represent observed localities.


foliation (cf. Allen et al. 1996) and this was also observed in a few outcrops with mudstones. Later deformation caused gentle folds with steep axial surfaces striking north to north-east and largely coaxial with the first folding. Lo-cally a spaced crenulation cleavage developed along the axial surfaces to these late folds. Revsund granitoid cuts most of the structures in the eastern and western parts of the area (Fig. 7). Shear zones occur mainly subparallel to the axial surfaces of first folds but in the eastern part of the area, north-striking shear zones are also present along the western contact between supracrustal rocks and a Revsund granitoid intrusion.

A quartz-feldspar porphyry is strongly sheared around Övre Krokforsen (area A in Fig. 7). The shear zone is at least 20 m wide and there is a 2 m wide intensely sheared zone. The shear fabric strikes east and dips around 50° south and the stretching lineation plunges 30° towards WSW. All feldspar phenocrysts have been almost entirely obliterated during the shearing due to previous sericite al-teration of the feldspars whereas the quartz phenocrysts remain, although they are strongly deformed. The shear foliation is defined by bands of very fine-grained musco-vite alternating with bands of quartz, biotite, and chlorite. Asymmetric tails on quartz phenocrysts and an s-c fabric can be seen both in the field and in thin section (Fig. 8a), and they both indicate that the southern side has moved up relative to the northern side. With the shallow plunge of the stretching lineation this also gives a sinistral strike component of the movement. The shear foliation is sub-parallel to axial surfaces of major upright folds in the re-gion. In a horizontal section, a north-striking slight crenu-lation of the shear foliation is visible. This corresponds to the second regional deformation in the area and indicates that shear along the zone occurred some time during or immediately after the main first folding event, but before the second deformation.

South of Kusfors, in a 200 m long railroad cutting (B in Fig. 7), a Jörn-type granitoid which is cut by a number of quartz-feldspar porphyritic and mafic dykes is deformed by at least five separate shear zones. The steep zones are less than 10 m wide and they strike SE to SSE. A strong stretch-ing lineation plunges about 60° north-west. Much of the deformation is brittle but incipient s-c fabric and weak shear bands in thin sections (Fig. 8b) indicate that the south-west-ern side of the shear zone has moved up. In addition, there are also shear sense indicators (shear bands, tiled feldspar crystals, and oblique foliation) on horizontal surfaces and they all indicate a sinistral sense of movement. This is in agreement with the expected horizontal displacement. The shear fabric is overprinted by crenulations formed during the second regional phase of deformation.

North of the Skellefte river in railroad cuttings (C, D & E in Fig. 7), shear zones were observed in volcanic

units. These shear zones are all steep, strike ESE, and have a strong subvertical lineation. In the southern-most of these outcrops (C in Fig. 7), there are two 4 m wide zones and several narrower zones through a chloritic rock. Narrow quartz veins are parallel to the strong foliation be-tween shear planes and fuchsite was observed in outcrop. In thin section, remains of biotite can be seen in the chlo-ritic parts. Feldspar crystals have both a strong grain shape fabric and a strong lattice preferred orientation indicating dynamic recrystallization. Calcite is common and cloudy patches in the calcite may represent ghosts of feldspar phe-nocrysts. Weak shear bands are present throughout and they indicate that the north-northeast side has moved up relative to the SSW side (Fig. 8c). The strong shear fabric was crenulated during the second regional deformation. This outcrop is separated from an outcrop with shear zones farther north by an area of rather well preserved vol-canic rocks with lithological contacts that strike 140° (D in Fig. 7). In the northern-most outcrop (E in Fig. 7), a strongly deformed 20 m wide zone and two narrower 1.5–5 m wide zones were observed in a fragmental rock of andesitic composition with angular, 1 mm to 10 cm large fragments in a very fine-grained matrix composed mainly of chlorite. These deformed zones are also steep, strike ESE, and have a pronounced subvertical stretching lineation. Shear sense indicators could not be found in these shear zones, neither in the field nor in thin section.

A pronounced shear zone is present in the Petiknäs N mine (Fig. 7) and it may be traced towards the WSW in topography to Harahukberget (area F in Fig. 7) north of the Rengård dam. In the Petiknäs mine the shear zone cuts through volcaniclastic rhyolitic rocks. The zone is several meters wide and contains a 50 cm wide clay zone. The ori-entation of this clay zone is 083/45 and the boundaries to less deformed rock are rather sharp. Both horizontal and down-dip striations have been observed on shear planes (Joseph Hull pers. comm. 1992). However, on a vertical surface in a less clay-rich part, an s-c fabric was observed which indicates reverse movement. Outside the clay zone, the volcaniclastic rocks have a strong subvertical cleavage with a pronounced subvertical stretching lineation. This strongly cleaved zone is about 50 m long. Late closely spaced subhorizontal fractures have movements of less than 1 m. At Harahukberget (F in Fig. 7) a shear zone was observed in a Jörn-type granitoid. Several strongly de-formed zones were observed, especially along contacts be-tween the granitoid and mafic and quartz-feldspar por-phyritic dykes, but the strongest deformation was found in the northernmost part of the outcrop towards the con-tact to volcanic rocks. There, a more than 5 m wide pro-tomylonite to mylonite is exposed. In the mylonitic part the rock is banded with alternating light and dark bands composed of muscovite and chlorite, feldspar, and calcite,


respectively. The east-striking mylonite is steep and has a stretching lineation plunging steeply north-east. Shear sense indicators, as shear bands (Fig. 8d) and asymmetric tails on phenocrysts, all indicate that the southern side has moved up. It is probable that the more gently dipping

brittle fault in Petiknäs is overprinting and partly follow-ing an earlier shear zone which is represented by the shear zone observed at Harahukberget.

Similar shear zones to the above were also observed be-low the Vargfors dam (G in Fig. 7) in the Skellefte river

Fig. 8. a) Asymmetric tails and weakly developed s-c fabric indicate south side up in strongly deformed quartz-feldspar porphyry. Most of the feldspar phenocrysts are completely altered to sericite. Embayments in quartz grain indicate the volcanic origin. Width of view 5.4 mm. b) Weak shear band indicates south side up in protomylonitic Jörn granitoid. Width of view 5.4 mm. c) Weak shear bands indicate north side up in strongly chlorite altered volcanic rock. Width of view 2.7 mm. d) Strong mylonitic foliation with shear bands indicating south side up in strongly sheared Jörn granitoid. Width of view 5.4 mm. e) Sandstones below the Vargfors dam which are locally strongly folded with shear zones parallel to axial surfaces of the upright folds. f) Weakly developed s-c fabric indicating dextral strike slip in granitic gneiss. Width of view 5.4 mm. g) Folia-tion defined by alternating bands of quartz+feldspar and biotite+muscovite. Width of view 5.4 mm.


in siltstones of the Vargfors Group. The siltstone beds dip 40–50° NE and are generally well preserved with ripples and cross bedding indicating stratigraphic younging to-wards north-east. In localised, up to 2 m wide zones, how-ever, the siltstones are intensely folded into tight folds and subvertical bedding surfaces are found. Along the subver-tical, southeast-striking axial surfaces a cleavage is devel-oped which have strongly sheared zones with reverse dis-placements up to 50 cm (Fig. 8e). The observed stretching lineations plunge about 45° towards 030° which is more or less perpendicular to the fold axes observed in the fold-ed siltstones.

In the Renström–Karsbäcksliden area, two shear sys-tems intersect which results in a complex pattern of shear zones. This is very obvious on aeromagnetic maps of the area. The northwest-striking shear zones, which dominate the central part of the Skellefte district, encounters an anastomosing system of north-striking shear zones imme-diately west of the Renström mine. This north-striking shear system can be traced southwards along the western contact of the Revsund intrusion and northwards towards the Jörn batholith. North-striking shear zones have been observed within the ore in the Renström mine (Duckworth & Rickard 1993) and they also cut volcanic units around the Renström mine (Wanhainen, 1997). The north-strik-ing shear zones are inferred to cut smaller Jörn-type bod-ies and amphibolites farther south although no shear zones were observed in the field in these rocks. Northwest-strik-ing shear zones were observed mainly in rocks of somewhat higher metamorphic grade than the rocks in the Skellefte district described above. One of these shear zones was ob-served in the elliptical, granitic Karsträsk intrusion, and su-pracrustal rocks along the northern edge of the dome (H in Fig. 7). This shear zone is steep with a subhorizontal stretching lineation which plunges gently north-west. A lo-cally developed s-c fabric (Fig. 8f ) and small shear zones observed in outcrop show a dextral dominantly strike-slip component of movement. In thin section all minerals are recrystallized during regional metamorphism (Fig. 8g), which indicates that shearing along this zone occurred be-fore or during peak-metamorphism. A sheared Jörn grani-toid was observed in the south-eastern part of the area (I in Fig. 7). The steep shear fabric strikes north-west and the stretching lineation plunges 50° south-east. A well devel-oped s-c fabric with asymmetric tails on feldspar porphyro-clasts indicates that the southern side has moved upwards relative to the northern side. These outcrops were located close to a long system of bogs in the same direction as the shear zone and local boulders of much more intensely sheared granitoid than observed in outcrop indicate that the shear zone is fairly wide and intense.

The north-striking shear system was observed in the Renström mine (Fig. 7) in the drift at the 800 m level

from the Renström mine towards the Petiknäs mine. Two well-defined, steep, about 20 m wide shear zones cut through feldspar porphyritic andesitic rocks which may originally have been shallow intrusions. Immediately east of the shear zones, fine-grained sedimentary units with preserved bedding structures occur. Both the shear fabric and the stretching lineations are subvertical and shear sense indicators (mainly a weak s-c fabric) show that the eastern side has moved up relative to the western side. The andesitic rocks in the shear zones are strongly altered and now consist dominantly of chlorite and calcite which al-most completely obscures the original rock texture (Wan-hainen, 1997).

The age relationships between the north-west striking shear zones and the north-striking anastomosing shear system is not firmly established. However, the north-west striking shear zones formed prior to the second deforma-tion and before or during peak-metamorphic conditions whereas the north-striking shear zones indicate retrograde conditions during shearing. This may indicate that the north-striking shear system formed after the north-west striking shear zones.

All shear zones observed in the central part of the Skel-lefte district are overprinted by crenulations of the sec-ond regional deformation and shearing must thus have occurred prior to the second folding. There are, however, no absolute constraints on the age of the deformations in the Skellefte district. The first major phase of folding occurred prior to the intrusion of Revsund granitoids at 1800–1780 Ma (Skiöld 1988) and after the intrusion of the Sikträsk granitoid at about 1880 Ma (Weihed & Vaas-joki 1993, Billström & Weihed 1996, K. Billström & P. Weihed pers. comm. 2001). The second deformation af-fects, at least locally, the Revsund granitoid and must thus have occurred after 1800–1780 Ma.

The Vidsel–Röjnoret Shear System

The western parts of map sheets 22–24 K are dominated by a sedimentary sequence composed of schists and greywackes, locally with interlayered pillow lavas. This sedimentary sequence overlies volcanic rocks of the Skellefte Group in an area around Kankberg and Boliden (Fig. 9). The volcanic rocks contain a number of min-eralizations with associated hydrothermal alteration but they are commonly well preserved and characterised by greenschist facies mineral assemblages. In the sedimenta-ry sequence, in contrast, the metamorphic grade increas-es eastwards to upper amphibolite facies, and migmatites are common in the eastern-most parts of the area. In the north-western part of the area, volcanic units of the Arvidsjaur Group occur (Fig. 9). These are commonly subaerial rhyolites and dacites. The supracrustal rocks


ll

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Röjnoretstructure

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Conglomerate &sandstone (Vargfors Group), 1.88–1.87 Ga








Perthite monzonitec. 1.88–1.86 Ga

Syn-volcanic granitoid (Jörn type)c. 1.89–1.88 Ga

Major gold deposit

Major VHMS deposit



Post-glacial fault


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Fig. 9. Geological map of the eastern part of the Skellefte district with the Vidsel–Röjnoret shear system, compiled from Allen et al. (1996) and the Mittnorden map (Lundqvist et al. 1996b). Small grey squares represent observed localities.


are intruded in the west by the syn-volcanic Jörn batho-lith and in the north by perthite-monzonites of a similar age. In the east, vast areas of post-volcanic granites of the Skellefte and Revsund types intrude the sedimentary sequence.

Many low-magnetic lineaments were observed on aero-magnetic maps of the area. These form a complex array of anastomosing and splaying lineaments which are in-terpreted as shear zones or faults. This array of shear zones will here be called the Vidsel–Röjnoret shear system (VRSS) and it forms a diffuse boundary between rocks of lower metamorphic grade to the west and rocks of high-er metamorphic grade to the east. The shear zones cut through nearly all rock types present in the area. Most zones are present in sedimentary and volcanic units but one splay of the zone cuts through the Björkdal granitoid and farther south, one shear zone forms the eastern mar-gin of the Röjnoret structure. In the north the shear zone system also affects the eastern margin of the Jörn batho-lith. It is known that at least a part of the VRSS has been tectonically active even after the last glaciation since post-glacial faults have been observed (Rodhe 1987) in an area around Röjnoret (west of B in Fig. 9).

Outcrops were mapped in areas where the shear zones were expected to appear on the surface (areas B–I on Fig. 9), and a few outcrops where shear zones had previously been observed were visited (eg. area A on Fig. 9). Sheared rocks were encountered only in a few localities, and shear

sense indicators could be determined in only three locali-ties. Observations will be described from south to north.

Around Rismyrliden (area A in Fig. 9) a Skellefte type granitoid with lath-shaped feldspar megacrysts has an almost north-striking steep protomylonitic fabric with a stretching lineation plunging 77° towards 102°. This shear zone was first noted by Nilsson & Kero (1998). An s-c fabric is developed with asymmetric tails on feldspar megacrysts but in the field, the shear sense criteria give conflicting slip directions. In thin section the deformation is semi-brittle and a shear foliation, which is defined by minute broken grains of feldspar, is slightly anastomos-ing around large, very fractured, rounded grains of feld-spar. Bands between the larger feldspar megacrysts consist of fine-grained quartz with a slight ribbon texture. Shear sense criteria are tiling of feldspar grains and an oblique foliation in fine-grained quartz and feldspar between brit-tle shears (Fig. 10a). These criteria indicate that the west-ern side has moved up relative to the eastern side. Tiling of megacrysts is considered an unreliable shear criterion whereas the oblique foliation is probably more reliable (Passchier & Trouw 1996). The regional extent of this shear zone is essentially unknown. It is shown on the Mittnorden map (Lundqvist et al. 1996b) to continue southwards for some distance and to connect northwards to the more continuous shear zone described below, but the zone is not visible on aeromagnetic maps of the scale used in this project.

Fig. 10.a) Tiling of feldspars and an oblique foliations indicating west side up in sheared Skellefte type gra-nite. Width of view 5.4 mm. b) Titanites aligned in the foliation. Width of view 1.4 mm., c) Strongly sheared pegmatite with s-type wings on a feldspar crystal indicating west side up. Width of view 5.4 mm. d) Oblique foliation and tiled feldspar phenocrysts indicating west side up in same shear zone as in c). Width of view 5.4 mm.


The eastern margin of the Röjnoret structure (area B in Fig. 9) appears to be bounded by a shear zone which continues for a considerable distance to the north (com-pare Fig. 9). The outcrops in the Röjnoret structure ob-served in this project are all rather similar with a quartz and feldspar porphyritic rock, which probably is rhyolitic in composition. The rock contains about 5 %, in average 3 mm large, subhedral to euhedral feldspar phenocrysts and <1%, rounded, 7 mm large, blue quartz phenocrysts in a fine-grained matrix composed of quartz, feldspar, and biotite. The rock is invariably strongly foliated with feld-spar phenocrysts aligned along the foliation which is also defined by the orientation of biotite aggregates. The folia-tion is folded, at least on a small scale, into open folds with moderate dips of the fold limbs and gentle fold axes. Quartz phenocrysts are stretched parallel to the fold axes and flattened along the axial plane, whereas the feld-spar phenocrysts remain aligned to the folded foliation (Fig. 11a). Axial surfaces to the folds are steep. The orien-tations of structural elements are shown in Figure 11b. In the easternmost outcrop a more than 2 m wide strongly foliated zone with extremely stretched quartz phenocrysts is present. The shear foliation in this outcrop strikes 018° and dips 57° east and the stretching lineation plunges 20° south. Narrow pegmatites occur along the foliation and deformation is concentrated to the margins of these, al-though pegmatites are also deformed. In a slightly less de-formed zone around Aftonsmyran the shear fabric strikes 017° and dips 58° east with a down-dip stretching linea-tion. Asymmetric tails on quartz phenocrysts indicate that the eastern side has moved upwards relative to the west-ern side. In thin section it is clear that the foliation is also defined by bands of euhedral to subhedral titanite and epidote with cores of allanite (Fig. 10b). Zircons are also present. Both the titanites and the zircons have been dated by Kjell Billström (pers. comm. 1997). The preliminary age of the titanites is around 1790 Ma whereas the zircons give a preliminary age closer to 1870 Ma.

Outcrops between Finnforsberget and Pultarliden (ar-eas C–F in Fig. 9) were mapped in order to find the con-tinuation northwards of the above described shear zone. Around Finnforsberget and Silvgruvberget (area C in Fig. 9) the rocks are mainly mudstones and siltstones, com-monly with a rusty appearance. No clearly sheared rocks were identified but the bedding surfaces are tectonically disturbed and commonly transposed into a steeply east-dipping, rather strong cleavage with a strike of 020°. Quartz veins are common parallel to the cleavage and en échelon tension gashes were also observed.

At Fölmyrberget (area D in Fig. 9), volcanogenic meta-sedimentary rocks are exposed. Bedding in these grits, im-pure limestones, and mass flows strikes in average east-west and it is folded with steep axial surfaces striking north-east and fold axes plunging 50° north-east. A spaced and anastomosing axial planar cleavage is developed and felsic fragments in the impure limestone are stretched par-allel to the fold axis. No evidence of shearing was found in this area.

At Björklidberget (area E in Fig. 9) feldspar porphy-ritic, probably coherent dacites were observed in the east-ern part of the area. These are well preserved and contain only a weak but penetrative northeast-striking grain shape cleavage. The western-most outcrop consists of a mass flow with volcanic clasts, some of which are pumiceous. Anastomosing, north-northwest striking zones of stronger cleavage have a weakly developed s-c fabric on surfaces parallel to the lineation which plunges 65° north. This s-c fabric indicates that the western side has moved up. Individual sheared zones are rather narrow (<20 cm) and no similar structures could be found in neighbouring out-crops so the significance of this shear zone is unknown. Immediately east of this outcrop, however, there is a pro-nounced topographical low (along Lill-Häbbersbäcken) striking in the same direction as the narrow shear zones.

At Pultarliden (area F in Fig. 9), there is a very crystal-rich mass flow with about 30 %, 1–6 mm large, euhedral

fold axis189/23

20 cm

S1

very weak S2

pegmatite

quartz phenocryst

feldspar phenocryst

Npoles to S1 (n=15)

stretching lineation and fold axes (n=8)

axial surfaces

π-axis

a) b)

Fig. 11. a) Sketch of outcrop from area B in Figure 9. Vertical surface looking north. b) Stereogram showing orientations of structural elements in area B in Figure 9.


to subhedral feldspar crystals and about 10 %, 1–5 mm large, blue quartz crystals in a fine-grained matrix com-posed mainly of sericite and chlorite. One to five centime-tre large fragments of a quartz-porphyritic rock also occur. Bands with more fragments define a faint layering which strikes 040° with an unknown dip. A strong steep cleav-age strikes 020° and is defined by bands of chlorite and sericite and also the orientation of feldspar crystals. To-wards the east the cleavage increases in intensity and a protomylonitic fabric is developed. A stretching lineation plunges down-dip in the cleavage indicating mainly dip-slip movement but the sense of shear could not be de-termined. A large number of open fractures occur in the same orientation as the shear fabric.

In the area around Furuberget (area G in Fig. 9), a number of southeast-striking lineaments are linking two north-striking lineaments. One of the southeast-striking zones outcrops along the Klintån in Jörn-type granitoid. This shear zone is more than 1 m wide and the shear fab-ric dips 60° south-west with a strong stretching lineation that plunges 57° south-east. A well defined s-c fabric is de-veloped which indicates that the southern side has moved upwards. Some less than 10 cm wide, steep, semi-brittle shear zones were observed in several orientations (mainly north-northeast and east). Stretching lineations could not be observed in these small shear zones due to outcrops without relief and therefore no shear sense could be deduced. Sinistral strike separation was, however, observed in some of these zones. In other outcrops in the area very little deformation was observed and only a weak foliation is present in the Jörn granitoid. Similar observations were made in the area around H in Figure 9. In general, the Jörn granitoid is very little deformed and only narrow dis-crete faults and fractures cut the rock in many orienta-tions. One outcrop, however, differs from the above in that it is a gneissic diorite with a strong linear fabric that plunges 5° north and a planar, NNW-striking compo-nent.

Farther north in area I on Figure 9, outcrops close to the interpreted surface location of one of the larger north-striking lineaments were mapped. The main rock-type in the area is a medium- to coarse-grained pink grani-toid with abundant fragments and cross-cut by later mafic dykes. Contacts to these dykes, which have many orienta-tions, are commonly cleaved. Apart from this, very little semi-ductile to ductile deformation was noted. One <20 cm wide, steep zone with intense cleavage strikes south-east and has a steeply plunging stretching lineation. A weakly developed s-c fabric is seen both in outcrop and in thin section and it indicates that the south-western side has moved up relative to the north-eastern side. Late frac-tures and quartz veins are common, however, and most of these strike 060° and 120°.

A more than 5 m wide shear zone was observed at J in Figure 9. This protomylonitic zone cuts across a Skel-lefte granite. The mylonitic fabric is steep and strikes north-west. A strong subhorizontal stretching lineation and a well developed s-c fabric indicates sinistral strike slip movement.

East of Åkerberg at V. Selet (area K in Fig. 9) a more than 10 m wide mylonite cutting a pegmatite was observed. The mylonitic foliation is steep and strikes 189° and the stretching lineation plunges 70° north-west. Asymmetrical tails on mantled feldspar porphyroclasts (Fig. 10c) all indicate that the west side has moved up rela-tive to the east side. The mylonitic banding is defined by alternating layers of fine-grained feldspar with some mus-covite and recrystallized quartz ribbons. Other shear sense indicators observed in thin section were an oblique fo-liation in the fine-grained layers (Fig. 10d) and tiling of feldspar porphyroclasts. All indicate the same shear sense. The continuation of this shear zone is unclear since it is not visible on aeromagnetic maps of the scale used in this study.

In summarising all information on the VRSS, very limited kinematic data is still available. Shear sense indi-cators were only found in the southernmost part of the VRSS on the north-striking zones and one observation is available on the northwest-striking zones that link the north-striking zones in the centre of the area. These ob-servations indicate east side up reverse movements on the north-striking zones and south side up reverse movements on the northwest-striking zones. Very few observations on shear zones were made also during the regional mapping of the area. This may be due either to the lack of out-crops or that the shear zones simply were not recognised as such.

Kalvträsk map sheet (22J)

The south-eastern part of the Kalvträsk map sheet is dom-inated by low-magnetic granitoids which results in an aeromagnetic map with few visible structures. During the regional mapping (Weihed & Antal 1998a, b, c, d) in the area, some shear zone localities were found. These have been studied in some more detail in this project.

Near Slipstensjön (A in Fig. 12), the western outcrop exposes a tectonic contact between Revsund and Skellefte granitoids. A steep protomylonitic foliation with a steep stretching lineation is present and a weak s-c fabric indi-cates east side up, reverse movement. In thin section, the feldspars are extremely deformed with microcracks and deformation lamellae. Dynamic recrystallization has re-duced the grain size of the quartz considerably and some strongly deformed dark brown biotite is present. The eastern outcrop exposes a serorogenic granitoid (Skellefte


type) with a strong subvertical protomylonitic fabric and a steep stretching lineation. Asymmetric pressure shad-ows on feldspars and an s-c fabric also here indicate east side up, reverse movement. The deformation in this zone is semi-brittle with bands of very fine-grained muscovite and feldspar alternating with coarser quartz-rich bands with minor biotite and muscovite and a weakly developed s-c fabric (Fig. 13a).

At area B in Figure 12, mylonitic zones were observed in Skellefte type granitoid in two localities. In the north-ern outcrop, a more than 10 m wide mylonitic zone has a well developed steep s-c fabric with a steeply plunging stretching lineation. In thin section feldspars (both plagio-

clase and K-feldspar) are strongly fractured and slightly al-tered to sericite. Quartz is dynamically recrystallized and muscovite and biotite are deformed. In the southern out-crop, a couple of narrower (about 30 cm) mylonitic zones are present. A well developed steep mylonitic fabric with a steeply plunging stretching lineation is present also here. In thin section, rounded, about 2 mm large s-type feldspar megacrysts are surrounded by a very fine-grained foliated matrix of quartz, muscovite, chlorite, and opaque phases. Shear bands are also developed locally (Fig. 13b). In all studied thin sections and outcrops the same east side up reverse movement is observed.

Sikträsket

B

A

Hällnäs

Åmsele

85

8136

81






Synvolcanic granitoid (Jörn type)c. 1.89–1.88 Ga

Faults and shear zones,barb on upthrown side

Shear fabric withstretching lineation


10 km

8378

7200

0071

7500

7150

00

7200

0071

7500

7150

00

170000167500

167500 170000

Fig. 12. Geological map of the eastern parts of the Kalvträsk map sheet. Simplified from Weihed & Antal (1998a, b, c, d). Small grey squares represent observed localities.


Discussion

In the discussion below, each area presented above will be briefly discussed with special emphasis on the relative tim-ing of shearing.

The previously unrecognised (?) Loito shear zone through the Bure area (Fig. 2) shows well developed shear sense indicators that indicate mainly east side up reverse movement. On the north-eastern splay, west side up re-verse movement was indicated in one locality. The struc-tural block between the two splays of the Loito shear zone in the northern part of the area could thus be in-terpreted as a pop-up structure. The appearance of Loito conglomerate on the western side of both splays of the fault may indicate a fault repetition of the unit. However, the amount of displacement on the Loito shear zone is un-known. The timing of shearing can only be loosely con-strained. The interpretation of aeromagnetic maps indi-cates that the shear zones cut through also the Sorsele gran-itoid which has been dated at 1766±8 Ma and 1791±22 Ma by Skiöld (1988). The latest shearing must thus have occurred some time after the intrusion of this granitoid. No other age constraints exist at this stage.

The low-magnetic lineaments on the Storavan map sheet (Fig. 4) appear to be caused by dominantly brittle faults with strong brecciation of the rock and fluid in-filtration causing alteration and infill by epidote. Sense of movement and amount of displacement on the faults could not be determined. Displaced lithological contacts indicate a sinistral strike component of movement but the vertical component is unknown. On aeromagnetic maps, the lineaments in this area cut both supracrustal rocks and older granitoids (1.89–1.87 Ga), whereas no cross-cutting relationships could be observed with the younger grani-toids. The faulting can therefore only be constrained to some time after 1.87 Ga. The brittle nature of the faults, however, indicates a much later movement, probably after the intrusion of the youngest granitoids.

Deformation on the Deppis–Näsliden shear zone (DNSZ, Fig. 5) and the north-striking zones around Ren-ström mine (central Skellefte district, Fig. 7) occurred af-ter the main, tight to isoclinal folding (D2) in the area since these fold structures are deformed by the north-striking shear zones. The mineral assemblages in these shear zones is dominated by chlorite, sericite, epidote, and calcite, depending on original rock composition, and this indicates deformation during greenschist facies condi-tions. The regional D3 folding produced open folds with steep axial surfaces striking north to north-east and many small shear zones have been observed along these axial surfaces (Bergman Weihed unpublished data). Since the north-striking shear zones show no evidence of later duc-tile deformation, it is possible that they may have initially formed during, or somewhat after, the regional D3 fold-ing. The regional D2 folding occurred after the intrusion of the Sikträsk granitoid at c. 1.87 Ga (Weihed & Vaasjoki 1993, Billström & Weihed 1996) and before the intrusion of Revsund granitoid at c. 1.80 Ga (Skiöld 1988) whereas the F3 folding episode occurred during or after the intru-sion of the Revsund granitoid at c. 1.80 Ga (Skiöld 1988). The amount of displacement on the north-striking shear zones is unknown.

The northwest-striking shear zones in the central Skel-lefte district (Fig. 7), in contrast, are overprinted by crenu-lations and a local weak cleavage which can be related to the regional D3 folding. In general, the metamorphic grade in the central Skellefte district increases from green-schist facies rocks in the northern part of Figure 7 to am-phibolite facies rocks in the south. This is reflected in the mineral assemblages observed from shear zones. The shear zone observed in area H in Figure 7 has a mineral assem-blage that indicates deformation above greenschist facies conditions. Most of the observed northwest-striking shear zones have oblique-slip south side up displacement. An effect of this is that progressively deeper parts of the crust are exposed towards the south and this is also what is

Fig. 13. a) Semi-brittle deformation zone with a weakly developed s-c fabric indicating east side up in a Skellefte type granite. Width of view 5.4 mm. b) Mylonite with shear bands and asymmetric tails on feldspar grains indicate east side up in a Skellefte type granite. Width of view 5.4 mm.


observed. In contrast to most of the northwest-striking shear zones in the northern part of the area, shear zones with the same orientation north of the Karsträsk intru-sion have subhorizontal stretching lineations indicating mainly strike-slip displacement. This may be caused by a transpressive regime in this area during deformation (see below).

Very limited kinematic data is available for the Vidsel–Röjnoret Shear System (VRSS, Fig. 9). However, in the southern-most part of the VRSS, east side up reverse dip-slip movements are indicated. This deformation is prelim-inarily dated at c. 1.79 Ga (Kjell Billström pers. comm. 1996) which is close in time to the intrusion of Revsund granitoid. The dated shear zone locality probably repre-sents a slightly deeper section of the crust since titanites have formed during the shearing. This was not observed anywhere else along the VRSS where greenschist facies assemblages are most common. The metamorphic grade increases in the metasedimentary rocks towards the east and south, and the rocks in the southern-most part of Figure 9 commonly have a gneissic structure.

Tectonic implications

A crucial point for tectonic implications of the results re-ported above is the timing of shearing and faulting in rela-tion to metamorphism and magmatism in the area. The age of the Skellefte volcanic rocks has been constrained to between 1880 and 1890 Ma whereas the overlying

Vargfors Group has been dated at 1875 Ma (Billström & Weihed 1996). These rocks are affected by both D2 and D3 structures and, consequently, both these deformation phases must be younger than c. 1.87 Ga. On the other hand Rutland et al. (1997) propose that migmatization occurred before 1.89 Ga and at c. 1860 Ma south of the Skellefte district in the Burträsk shear zone. Furthermore, Lundström et al. (1999) propose that intrusions east of the VRSS, dated at 1870 Ma, contain xenoliths with an earlier deformation fabric which therefore must predate 1870 Ma. It is thus possible that earlier deformational fab-rics exist, but must be minor in the investigated area. The pre-1890 Ma deformation of Rutland et al. (1997) could of course not affect the rocks concerned here since all rock units studied are younger than 1890 Ma. The D2 of Rut-land et al. (1997), dated at c. 1860 Ma by age of mona-zites in pegmatitic neosome granitoids intruding late dur-ing D2, for the Burträsk shear zone, may correspond to the D2 discussed in this paper. This necessitates a re-interpretation of the zircon ages of 1.85–1.86 (Weihed & Vaasjoki 1993, Billström & Weihed 1996) for the de-formed Sikträsk intrusion. The peak of metamorphism in the area has not been accurately dated but is closely related to the S-type minimum melt granitoids of Skellefte- and Härnö type, dated at 1.80 to 1.82 Ga (Weihed et al. in prep, Claesson & Lundquist 1995). It is therefore likely that the best estimate of the age of peak metamorphism comes from a diopside skarn in the Burträsk shear zone where Romer & Nisca (1995) dated titanites at c. 1825

Luleå

Lycksele

Sorsele

Skellefteå

Piteå

Luleå

amphibolitefacies

amphibolitefacies

greenschistfacies

Convergence during or after D3 (post 1.80 Ga)

Orientation of mainD2 folds

Shear zone, barbson upthrow side

Convergence during or afterD2 but before D3 (1.85–1.80 Ga)

Approximate boundary between metamorphic facies

2224

23

H I J K L

H I J K L

2423

22

Fig. 14. Simplified structure map. Brown irregular area in centre represents the Jörn batholith. Arrows indicate an early oblique convergence, probably from the south-east, and late east-west shortening.


Ma. Since the early structures discussed here seems to con-tain peak metamorphic assemblages it could be argued that the D2 must not be younger than 1825 Ma while the latest deformation in D3 structures, which affects the Revsund granitoids, must be 1.80 Ga or younger.

Summarizing the observed kinematic indicators in the whole study area (Fig. 14), north-striking shear zones gen-erally have a reverse dip slip sense of movement and have formed during or after the regional D3 deformation (after c. 1.80 Ga) and after peak metamorphism. In contrast, the northwest-striking shear zones observed in the cen-tral Skellefte district have a reverse oblique slip move-ment, commonly with the south side up, and these zones have formed prior to the regional D3 deformation and be-fore the metamorphic peak. It is therefore probable that the northwest-striking shear zones formed late during the main D2 folding (between 1.87 and 1.82 Ga).

The older shear zones may have formed late during the regional D2 deformation which caused the upright folds which are present everywhere in the Skellefte district. In the western and eastern parts of the district, axial surfaces to these folds strike north-east whereas in the central part of the district they strike north-west parallel to the south-ern contact of the large Jörn batholith north of the Skel-lefte district (Fig. 14). The folds occur in lower-strain lenses between shear zones in the central part of the dis-trict and areas to the east and west are less intensely de-formed and have less shearing parallel to axial surfaces of the folds. It is proposed here that these regional F2 folds in the whole Skellefte district and the northwest-striking shear zones in the central district formed in re-sponse to oblique convergence from the south-east and that the northwest-striking structures in the central Skel-lefte district were constrained in orientation by the pres-ence of the large Jörn batholith which acted as a large block resisting deformation. This deformation occurred between 1.87 Ga and 1.82 Ga before or during peak met-amorphism.

The north-striking shear zones (and faults) formed af-ter the D3 deformation, i.e. after 1.80 Ga and affect all in-trusive rocks that are present in the area (except maybe the diabase dykes that are present in the south-western part of the studied area). The dominantly dip slip reverse move-ments observed on these shear zones indicate an east-west shortening during this period of deformation (Fig. 14). Nironen (1996) proposes that the TIB in central Sweden and the Revsund granitoids in the north formed during east–west extension. However, no indications of east-west extension at the time immediately after the emplacement of Revsund granitoids were found in this study.

An interesting possibility that emerges from this study is that most structures discussed here, which are consid-ered to belong to the Svekocarelian orogeny, temporally

may be related to the intrusion of Skellefte-Härnö and Revsund granitoids which normally are considered as late- or post-orogenic in relation to the Svecokarelian orogen. It is possible that early Svecokarelian deformation and metamorphism, related to accretionary processes and calc-alkaline magmatism at 1.95 to 1.88 Ga, in some areas are overprinted by the younger structures observed in this study related to southeast convergence at c. 1.87–1.82 Ga and to east-west shortening at c. 1.82–1.80 Ga.

Acknowledgements

This research project has been conducted with the support of many geoscientists active in the Skellefte district and its surroundings. I would especially like to thank Ildikó Antal, Ulf Bergström, Kjell Billström, Leif Björk, Thomas Eliasson, Benno Kathol, Leif Kero, Ingmar Lundström, Thomas Sträng, and Lars Kristian Stølen for sharing their information gathered during regular mapping for the Geological Survey of Sweden. My deepest thanks go to Pär Weihed for endless support during field work and in the office. Thanks are also due to Stefan Bergman, Lena Albrecht, Kjell Billström, Olof Martinsson, and Pär Wei-hed for reviews of earlier versions of this manuscript. This research project was funded by the Geological Survey of Sweden, external research grant 03-862/93.

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69GEOCHEMISTRY AND TECTONIC SETTING OF VOLCANIC UNITS IN THE NORTHERN VÄSTERBOTTEN COUNTY, NORTHERN SWEDEN

Bergström, U,. 2001: Geochemistry and tectonic setting of vol-canic units in the northern Västerbotten county, northern Swe-den. In Weihed, P. (ed.): Economic geology research. Vol. 1, 1999–2000. Uppsala 2001. Sveriges geologiska undersökning C 833, pp. 69–92. ISBN 91-7158-665-2.

The Precambrian bedrock in the northern Västerbotten county in northern Sweden includes a number of volcanic litostrati-graphic units, which can be grouped according to their unique geochemical composition.

The Bothnian Group, exposed south of the Skellefte District includes two volcanic units: one older homogeneous basalt lava and volcaniclastic assemblage with a MORB signature, and one younger (c. 1.95 Ga) fractionated basalt to rhyolite assemblage, formed in a volcanic arc setting.

The Skellefte Group is a heterogeneous unit of basalt to rhyolite, deposited in an extensional continental margin arc at c. 1.90–1.88 Ga. The stratigraphically uppermost unit is com-posed of primitive basalts and andesites of the Tjamstan Forma-tion and mudstones–siltstones of the Elvaberg Formation, re-flecting the peak evolution of the extensional environment.

The Vargfors Group succeds the Skellefte Group and in-dicates varying depositional environments at c. 1.88–1.87 Ga. The western part is dominated by greywackes, overlain by prim-itive Mg-basalts, which indicate renewed volcanic activity. The Vargfors Group in the eastern part is also dominated by grey-wacke deposition, which is succeded by evolved MORB-type volcanic rocks of the Varuträsk Formation. The latter probably reflects the initiation of a rift basin east of the Skellefte District. Erosion of the uplifted Skellefte Group rocks characterizes the stratigraphic sequences of Vargfors Group in the central part of the Skellefte District.

The Arvidsjaur Group is concentrated to the Arvidsjaur dis-trict north of the Skellefte District, and may at least partly be stratigraphically equivalent to the Vargfors Group rocks. The Arvidsjaur Group is a heterogeneous unit of subaerial basaltic–rhyolitic volcanic rocks, formed in a mature, compressional, continental margin arc.

Ulf Bergström, Geological Survey of Sweden, Earth Science Center, Guldhedsgatan 5a, SE-413 20 Göteborg, Sweden. E-mail: [email protected]

Introduction

Large parts of the Fennoscandian Shield are composed of rocks formed in Palaeoproterozoic analogues to mod-ern destructive plate margin settings. These rock assem-blages include volcanic units of various petrological, geo-chemical, and isotopic characteristics, associated granitoid suites, and sedimentary rocks. The Skellefte District in the

northern part of the Västerbotten county, northern Swe-den, is one such area where volcanic and intrusive units formed in what has been interpreted as a destructive plate margin.

Plate tectonic interpretations of ancient metamor-phosed, hydrothermally altered, and deformed volcanic units by the means of major and trace element geochemis-try are frequently used. A number of discrimination plots based on different immobile trace elements are available as well as techniques for reconstruction of original composi-tions in altered rocks.

In the Skellefte District, Claesson (1985), Vivallo (1987), Vivallo & Claesson (1987), Vivallo & Willden (1988), and Weihed et al. (1992) have presented geo-chemical data of volcanic rocks. Other geochemical inter-pretations of different volcanic rocks in the surrounding areas include Wasström (1990, in prep.) on basaltic rocks of the Knaften area south of the Skellefte District, Berg-ström (1996) on the Mg-basalts of the western Skellefte District, and Perdahl (1993, 1995) on the volcanic rocks in the Arvidsjaur area, north of the Skellefte District.

In this paper a summary of the geochemical properties of the different volcanic units in the Skellefte District and the surrounding areas are presented including a proposal on how these volcanic units can be defined and discrimi-nated.

Regional Geology

A compilation of the regional geology of northern Väs-terbotten County is presented in Figure 1. The Skellefte District forms a c. 150 x 50 km large belt situated in the northern part of the Västerbotten County in northern Sweden. It is characterized by the presence of the Skellefte Group volcanic rocks, which host a number of important volcanogenic massive sulphide deposits, and the associ-ated Jörn Suite granitoids. The Jörn Suite is composed of the large composite Jörn pluton, which forms a con-spicuous feature in the north-central part of the Skellefte District, and a number of smaller plutons. The age of the Skellefte District is suggested by Billström & Weihed (1996) to be c. 1900–1880 Ma. The volcanic rocks of the Skellefte Group have been dated at1882±8 Ma (Welin 1987), 1884±5.5 Ma and 1889±4 Ma, (Billström & Wei-hed 1996), and the age of the Jörn G1 has been dated at 1888+20 Ma, (Wilson et al. 1987), and 1886+15 Ma (Wei-

Geochemistry and tectonic setting of volcanic units in the northern Västerbotten county, northern Sweden

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ides

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Fig. 1. Geology of the Skel-lefte District and surroun-ding areas with the different volcanic units discussed in the text indicated.


hed & Schöberg 1991). An age determination of a typical Jörn type pluton in the Kristineberg area in the western part of the Skellefte District gave an age of 1907±13 Ma (Bergström et al. 1998).

The Vargfors Group, which stratigraphically overlies the Skellefte group volcanic rocks, is a heterogeneous unit of sedimentary rocks with minor volcanic intercalations. In the central Skellefte District, the Vargfors Group is composed of coarse clastic rocks, sandstones, and fine-grained argillites, which occur at the base of the Group (Dumas 1986). The Vargfors Group also includes thick greywacke units in a number of synformal basins at the margins of the Skellefte District. The volcanic intercala-tions are volumetrically small compared to the sedimen-tary rocks, and include mainly mafic units. The age of the Vargfors Group is close to c. 1875 Ma, as suggested by an age determination at 1875±4 Ma (Billström & Weihed 1996) of a dacitic ignimbrite intercalation in conglomer-ate from the central Skellefte District. In the central Skel-lefte District, the Vargfors sedimentary and volcanic units are related to the Gallejaur intrusive complex, dated by Skiöld (1988, 1993) at c. 1876–1873 Ma.

To the north of the Skellefte District, the Arvidsjaur Group volcanic rocks and the Arvidsjaur granitoid Suite are exposed in the so-called Arvidsjaur district. The Arvids-jaur Group shows east–west compositional variations and grades into the volcanic areas further to the north in the Norrbotten County (Perdahl 1994). The Arvidsjaur gran-itoid Suite is intimately related to the Arvidsjaur Group volcanic rocks and occurs as large batholithic intrusions of heterogeneous composition, although more homo-geneous plutons like the Arvidsjaur Pluton (Muller 1980) also exist. The age of the Arvidsjaur district rocks is c. 1875–1880 Ma, according to age determinations of Arvidsjaur Group volcanic rocks at 1876±3 and 1878±2 Ma (Skiöld et al. 1993), and Arvidsjaur granitoids at 1877+8 Ma (Skiöld et al 1993) and 1879 +15 Ma (Kathol & Persson 1997).

To the south of the Skellefte District within the Both-nian Basin (Hietanen 1975, Lundqvist 1987), minor vol-canic intercalations form part of the predominantly sedi-mentary Bothnian Group. The sedimentary and volcanic rocks of the Bothnian Group are intruded by several gen-erations of granitoids, often with unclear relationship to the volcanic units. An age determination of a quartz-feld-spar-porphyritic dacite from the Barsele area southeast of Storuman (Fig. 1) gives an age of 1959±14 Ma (Eliasson & Sträng 1998), which is interpreted as a rough estimate of the age of the Bothnian Group. Similar ages for in-trusive rocks have been obtained from the Knaften area (Wasström 1993, 1996). As the Bothnian Group includes similar sedimentary rocks as the Vargfors Group to the north in the Skellefte District, the nature of the contact

zone may be graditional. The suggested position of the contact zone between the two sedimentary Bothnian and Vargfors Groups is outlined in Figure 1 and the position is discussed below. It is probable that the sedimentary depo-sition in the Bothnian Group continued beyond the mag-matic ages mentioned above, at least until the Skellefte Group volcanic episode was initiated (Fig. 2).

All the above mentioned rocks have been deformed and metamorphosed at c. 1840–1800 Ma, and intruded by a late- to postkinematic granitoid suite at c. 1780–1810 Ma (Billström & Weihed 1996). The Skellefte, Vargfors, and Arvidsjaur Groups have been metamorphosed in the greenschist facies, whereas the Bothnian Group is vari-ably metamorphosed in amphibolite facies. Veined gneiss-es, migmatites, and anatectic granites are common within the Bothnian basin.

Characterization of volcanic units

Stratigraphic, petrographic, geochemical, petrophysical, and isotopic parameters have been used to identify a number of volcanic units within the Bothnian, Skellefte, Vargfors, and Arvidsjaur Groups. In some cases these units may be correlated chrono-stratigraphically and form facies components rather than individual units. The individual volcanic units are briefly presented below.

Bothnian Group

The volcanic rocks of the Bothnian Group have been studied by Wasström (1990, in prep.) in the Knaften area and brief reports can be found in Eliasson & Sträng (1998) for Bothnian group volcanic rocks in the Stor-uman area, in Björk (1995) for the Lycksele–Vilhelmina area (map sheets 22 G–I), and in Weihed & Antal (1998) from the Kalvträsk map sheet (22J). The volcanic rocks occur as intercalations in the dominating sedimentary rocks, mainly greywacke-mudstone turbidites, which have been deformed and metamorphosed in amphibolite facies. The volcanic rocks may form volumetrically important local centers including lava flows, subvolcanic intrusions and volcaniclastic rocks, surrounded by mixed volcanic-sedimentary units. Other parts of the Bothnian Group sedimentary rocks are virtually devoid of volcanic rocks. The restricted areal extent of the volcanic intercalations and the lack of stratigraphical markers prohibit any major interpretations of the physical volcanology of these rocks and the proposed tectonic setting is highly tentative.

Wasström (1990) studied the mafic volcanic rocks of the Knaften area (Fig. 1) and identified two principal vol-canic facies: more or less homogeneous massive and pil-lowed basaltic lava flows with minor interflow volcani-

72 U. BERGSTRÖM

1,95

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Fig. 2. Stratigraphic subdivi-sion of the different volcanic rocks into the Bothnian, Skel-lefte, Vargfors, and Arvidsjaur Groups.


clastic sediments, and volcaniclastic rocks of similar com-position mixed with greywackes and with only minor lava flows. In the north-eastern part of the Knaften area, the basalts are succeded by andesites, dacites, and rhyolites, which form another volcanic unit. These two principal compositional units can also be identified further to the north-west, in the Pauliden and Barsele areas, the former unit with massive and pillowed basalt lava flows and as-sociated volcaniclastics (HBA) and the latter with more fractionated rocks with a compositional range from basalt to rhyolite (FBRA). Basaltic rocks are frequently found as more or less continuous intercalations in Bothnian Group greywackes outside the Knaften and Barsele type-areas. In the Skatan area, a pillow lava is associated with a gab-bro (Weihed & Antal 1998), and similar associations of basalts and mafic intrusions were identified by Nilson & Kero (1986) on the Vindeln map sheet (21J). Small, scat-tered and rare occurrences of felsic volcanic rocks with the fractionated basalt-andesite-dacite-rhyolite compositions may also be found in the whole area. In the Knaften area, Wasström (1994) suggests a genetic link between rhyolitic tuffites, granitoid intrusions, and porphyritic dykes.

The well-preserved HBA basaltic lavas from the Knaf-ten area are mainly ophitic and locally amygdaloidal (Wasström 1990). They are mainly pyroxene porphyritic and altered to amphibole, but plagioclase porphyritic varieties are locally abundant. Hornblende and plagio-clase with accessory ore minerals, sphene, epidote, calcite, and chlorite dominate the matrix. Chert horizons have been identified as associated with the basaltic lava flows. The volcaniclastic basaltic rocks from the Knaften area are mainly epiclastic deposits with different types of vol-canic and sedimentary clasts, including ultramafic clasts (Wasström 1990). Most volcaniclastic rocks are laminated with local graded beds and imbricate structures. Rounded quartz phenocrysts are present in some beds.

Wasström (1990) describes the felsic volcanic rocks of the Knaften area as tuffites, derived from fine-grained ash deposited in water, because of their laminated charac-ter. Quartz, plagioclase, microcline, and biotite with ac-cessory epidote, ore minerals, and chlorite dominate the rock. The tuffites are underlain by dacitic to andesitic lava flows, which are different macroscopically from the basal-tic lavas by their grey colour, higher abundance of quartz-filled fissures, and quartz phenocrysts. The FBRA frac-tionated assemblage in the Storuman area (Fig. 1) includes andesitic lava flows and dacitic volcaniclastic rocks, lavas, and dykes, similar to the porphyritic dykes of the Knaften area.

Skellefte Group

The Skellefte Group volcanic rocks are exposed along the Skellefte River in a well-defined area associated with mi-nor sedimentary intercalations and Jörn Suite granitoid plutons (Fig. 1). Allen et al. (1995, 1996) have inves-tigated the physical volcanology of the Skellefte Group, and Weihed et al. (1992) and Billström & Weihed (1996) summarized the isotopic features. Claesson (1985), Vival-lo (1987), Vivallo & Claesson (1987), Vivallo & Willdén (1988), Weihed et al. (1992), Bergman Weihed et al. (1996). and Allen et al. (1996) have published petro-graphic and geochemical data of the Skellefte Group. Due to the presence of pervasive hydrothermal alteration re-lated to numerous massive sulphide deposits in the Skel-lefte Group, textural interpretations of the volcanic rocks are locally seriously hampered. Otherwise, the rocks of the Skellefte Districts. are generally well preserved.

Felsic compositions dominate in the Skellefte Group. The most common volcanic rocks are dacitic to rhyolitic, variably porphyritic, coherent subvolcanic intrusions/lava domes or pumiceous, syn-volcanic mass flows (Allen et al. 1995, 1996). The Boliden area is one type area for dac-ites, where subvolcanic intrusions and related mass flows occupy a stratigraphically high position in the Skellefte Group (Bergman Weihed et al. 1996). A plagioclase phy-ric texture is characteristic for the dacites and the whole- rock composition is partly controlled by the phenocryst abundance. Mafic phenocrysts are rare. A stronger hydro-thermal alteration is evident in the originally more porous mass flows compared to the coherent intrusions, which is expressed as plagioclase breakdown and a high sericite content. A heterogeneous group of coherent lavas, subvol-canic intrusions, and related mass flows with a rhyolitic composition, dominate the volcanic stratigraphy in vari-ous parts of the Skellefte District. The subvolcanic in-trusions show a variable amount of quartz and plagio-clase phenocrysts, which also vary in shape and size. Sil-iceous, glassy-looking, quartz phyric lava/crypto domes and subvolcanic intrusions form a rather distinct group of rhyolites. The mineralogical composition of the Skel-lefte Group rhyolites is dominated by quartz and albite, and the amount of potassic feldspar is typically very low. The rhyolitic mass flows are commonly redeposited coarse breccias and sandstones.

Mafic volcanic rocks in the Skellefte Group are gener-ally subordinate and mainly occur as subvolcanic sills or lava flows. Thicker mafic units are found for example in the Långdal area (Vivallo 1987, Allen et al. 1996), and the Holmtjärn–Granbergsliden area (Nicolson 1993, Allen et al. 1996). The mafic volcanic rocks include substantial amounts of andesites and basaltic andesites. Stratigraphi-cal studies (Vivallo 1987, Nicolson 1993, Bergman Wei-

74 U. BERGSTRÖM

hed 1996) suggest that the basalt–andesite units normally occur high up in the stratigraphy, immediately below the contact to the younger Vargfors Group. Most basalts and andesites at lower stratigraphic levels may be regarded as feeder dykes or sills to the extrusive units further up in the stratigraphy. Generally the mafic volcanic rocks in the Skellefte group are plagioclase and pyroxene phyric. The pyroxenes are altered to chlorite and actinolite.

Sedimentary intercalations in the Skellefte Group are not uncommon, but generally very limited in thickness and strongly volcanogenic in nature. Thin intercalations of black mudstones are locally found in the upper parts of the stratigraphy. Carbonate lenses may also be found, for example in the eastern part of the Skellefte District, and adjacent to massive sulphide ores.

Tjamstan Formation

The Tjamstan Formation (Bergström & Sträng 1997, 1998) is exposed in the western part of the Skellefte Dis-trict. The unit occurs stratigraphically above the felsic vol-canic rocks of the Skellefte Group and appears to be co-eval with fine-grained black mud- and siltstones of the Elvaberg Formation (Allen et al. 1996), which defines the stratigraphic contact zone to the younger Vargfors Group (Fig. 2). The reason for referring the Tjamstan Formation to the Skellefte Group rather than to the sediment-domi-nated Vargfors Group, which would be natural consider-ing the relation to the Elvaberg Formation, is simply the similar mineralogical and geochemical composition rela-tive to the Skellefte Group volcanic rocks.

A large area with well-preserved basalts and andesites of the Tjamstan Formation occurs around Malå (Fig. 1). The rocks are characterized by abundant plagioclase phe-nocrysts and well preserved volcanic syn-eruptive graded beds, where the plagioclase phenocrysts are concentrated into the base of the bed. Lava intercalations with the same composition are common and become volumetrical-ly more important in some areas. Thick units of polymict breccias occur locally. Away from Malå (for example in the Adak area), distal facies are intercalated with sedimentary rocks. The Tjamstan Formation also includes some dacitic subvolcanic intrusions, dykes, and mass flows.

Vargfors Group

Allen et al. (1995, 1996) defined the Vargfors Group as the unit of dominantly sedimentary rocks which overlies the Skellefte Group volcanic rocks. The graphite- and sul-phide bearing black mudstones of the Elvaberg Forma-tion, which forms a magnetic and electric marker horizon in the Skellefte District, constitute the top of the Skellefte Group and thereby defines the transition into the Varg-fors Group mud-, silt- and fine-grained sandstones and

greywackes. In some areas, for example in the central Skel-lefte District, parts of this stratigraphic level are com-posed of lithological units characterized by different vol-canic and sedimentary breccias, conglomerates, and sand-stones. Along the western, southern, and eastern margins of the exposed Skellefte Group volcanic rocks, thick units of greywackes dominate the Vargfors Group. Within the Vargfors Group, there are substantial volcanic compo-nents with a local provenance. In the central Skellefte Dis-trict, basalts and andesites dominate in the Gallejaur area (Fig. 1) where the volcanic rocks belong to the Gallejaur Formation. In the western part of the Skellefte District, the thick Vargfors Group greywackes have intercalations of ultramafic–mafic volcanic rocks of the Bjurås Forma-tion. Basaltic intercalations are also found in the Vargfors Group sedimentary rocks of the eastern part of the Skel-lefte District, and these are termed the Varuträsk Forma-tion.

Gallejaur Formation

The coarse conglomerates and sandstones along the Skel-lefte River, in the Gallejaur area and in the central Skel-lefte District (Dumas 1985), were included in the original definition of the Vargfors Group. These units overlie the Skellefte Group volcanic rocks and the fine-grained sedi-mentary rocks of the lower Vargfors Group. Mainly basal-tic and andesitic volcanic rocks occur within the coarse conglomerates and sandstones in the Gallejaur area. The mafic volcanic rocks envelop the Gallejaur Complex (Fig. 1), a laccolithic gabbro with a thin central core of quartz monzonite (Enmark & Nisca 1983). Dacitic to rhyolitic volcanic intercalations are found in the coarse clastic rocks. Both lavas and volcaniclastic rocks are common in the Gallejaur Formation. The andesites are often coarsely am-phibole phyric, but plagioclase phyric types are also found. The felsic volcanic rocks are normally ignimbrites, but lavas are also found locally.

The Varuträsk Formation

Minor occurrences of mafic volcanic rocks are intercalated with Vargfors Group sedimentary rocks in the Varuträsk area east of Boliden (Fig. 1) and further to the north. These rocks are compositionally similar to the Bothnian Group basalts to the south, but the host sedimentary rocks east of Boliden are included in the Vargfors Group because they show a continuous younging direction east-ward from the exposed Skellefte Group rocks.

The basalts of the Varuträsk area are normally mas-sive and pillowed lava flows. Intercalated clastic units, as well as thin massive sheets, probably sills of similar basaltic composition, occur in the surrounding sedimen-tary rocks. The rocks of the Varuträsk Formation are nor-


mally strongly deformed and metamorphosed in amphi-bolite facies. The basaltic sills and lavas have few pheno-crysts and are dominated by metamorphic hornblende and plagioclase.

The Bjurås Formation

Thick sills of ultramafic–mafic composition intrude the greywackes in the Vindelgransele area in the western part of the Skellefte District. These sills represent feeder sys-tems to lavas further up in the stratigraphy, exposed in the

Bjurås area (Fig. 1). Similar rocks exist in the Adak area to the north. Further to the north-west in the Vinliden area, the lavas are overlain by a thick package of similar volcani-clastic high-Mg basalts and basalts. The mafic volcaniclas-tic rocks are intercalated with and overlie greywackes of the Vargfors Group. Bergström (1997) proposed the name Malå Group for this greywacke-Mg-basalt package, but it can be stratigraphically correlated with the central Skel-lefte District greywackes, conglomerates, and sandstones of the Vargfors Group. Thin intercalations of volcanic rocks, similar to the Arvidsjaur Group can also be ob-

Table 1. Geochemical analyses of representative samples of mafic volcanic rocks. References to data; TEN960101: HBA basalt, Bothnian Group, Knaften area (Eliasson et al., in press), TEN930220: FBRA andesite, Bothnian Group, Barsele area (Eliasson et al., in press), L-56: basalt, Skellefte Group, Långdal area (Vivallo 1987), UJB940001: basalt, Tjamstan Formation, Näsudden area (Bergström & Sträng, 1999), KWI990089, Basalt lava, Gallejaur Formation, Gallejaur area (Antal, Bergström & Weihed, unpublished), MGN940135: Mg-basalt, Bjurås Formation, Bjurås area (Bergström & Sträng, 1999), UJB980399: basalt, Varuträsk Formation, Klubbfors area, (Kathol et al., unpublished), CHB950168: andesite, Arvidsjaur Group, Brunmyrheden area (Kathol & Triumf, in press), AFT950183: basalt-andesite, Arvidsjaur Group, Sjnjerra area (Bergström & Triumf, in press.).

TEN960101 TEN930220 L-56 UJB940001 KW1990089 MGN940135 UJB980399 CHB950168 AFT950183

SiO2 50.5 55.1 49.73 48.9 50.9 44.2 50.5 54.2 51.6TiO2 1.135 0.786 0.89 0.34 0.659 0.53 1.68 1.48 0.742Al2O3 13.5 14.7 18.09 18.8 15.7 9.54 14.9 18.1 15.4FeO 10.9 6.4 7.21 14.3 Fe2O3 1.386 1.834 1.9 9.5 10.4 12.1 8.31 8.24MgO 7.48 6.2 4.8 3.53 7.88 18 7.11 1.59 7.84CaO 9.28 7.66 11.73 13.7 6.78 12.7 8.68 6.11 7.42Na2O 3.16 3.73 3.04 2.06 3.91 0.51 2.11 3.59 3.19K2O 0.12 1.4 0.29 0.46 0.703 0.59 0.243 4.14 2.06P2O5 0.07 0.23 0.15 0.0489 0.219 0.15 0.17 0.792 0.259LOI 1.3 0.95 1.29 2.2 1.6 0.75 0.8 2.3 3.9Σ 99.9 99.26

Ba 20 688 29 149 198 241 57.1 995 765Co 45 23 59 33.8 41.4 75 47.1 28.2 30.7Cr 160 410 121 43.3 274 1600 223 28.1 408Cu 143 49.2 10 142 28.2 63 184 121 23.8Ga 16 17 27.5 20.8 23.5 11 15.9Hf 1 3.3 4 2.4 1.87 1.3 3.39 6.86 3.65Nb 2 10 2 3.19 4 8.19 17.2 4.86Ni 80 77 28 17 75.1 571 129 37 94Pb 15Rb 5 42 7 16.6 21.1 15 12.7 133 38.5Sc 45.9 23 48 36.8 29.6 32.9 35.2 20.9 21.6Sr 120 428 617 206 624 127 143 510 671Ta 0.2 1 0.078 0.298 0.718 1.31 0.50Th 0.2 3.6 0.77 1.74 1.2 0.634 8.45 5.82U 0.1 4 0.43 1.48 2 0.234 6.02 5.13V 372 180 293 206 191 322 121 115Y 20 18 17 12.9 16 14 25.2 47.3 18.2Zn 85 81 91 49.5 678 90 113 93.9 138Zr 67 153 21 20.2 62.3 60 106 330 136La 2.5 17.6 7.22 3.9 10 10.2 8.19 57.9 22.3Ce 7.5 39.4 16.61 9.38 24.4 22 25.2 124 47Pr 1.2 5 <1.17 3.29 3.24 16.2 6.14Nd 7.1 21.4 11.88 5.34 15.2 11 17.2 66.9 25.9Sm 2.3 4.8 2.87 1.21 2.67 2.5 3.88 1.5 4.89Eu 0.82 1.25 1.10 0.443 1.15 0.7 1.67 2.2 1.29Gd 1.7 2.65 1.25 2.73 4.94 10.7 4.33Tb 0.82 0.6 0.257 0.401 0.4 1.21 1.4 0.744Dy 4.2 3.7 2.70 1.74 2.81 5.06 7.02 3.58Ho 0.82 0.72 0.257 0.613 1.36 1.38 0.661Er 2.5 2.1 1.59 0.936 1.53 3.24 3.29 1.68Tm 0.4 0.3 0.163 0.217 0.618 0.565 0.248Yb 2.6 1.9 1.39 1.1 1.6 1.3 2.77 3.78 1.89Lu 0.35 0.29 0.20 0.155 0.195 0.22 0.412 0.568 0.276

76 U. BERGSTRÖM

served in the Malå Group, at a stratigraphically higher position than the Mg-basalt lavas. The ultramafic–mafic sills and lavas are dominated by actinolite, both as pseudo-morphs after the characteristic pyroxene phenocrysts and in the glassy matrix, which reflects the upper greenschist facies metamorphism that has affected the rocks in the western Skellefte District. In well-preserved samples, pri-mary augite phenocrysts can be distinguished. The vol-caniclastic rocks commonly contain a higher amount of magnetite, which results in a distinct high magnetic signa-ture, and they also contain plagioclase both as phenocrysts

and in the matrix, which results in more basaltic and an-desitic compositions.

Arvidsjaur Group

The volcanic rocks of the Arvidsjaur Group have been studied by Perdahl (1994), who emphazised the resem-blance to the volcanic rocks further to the north in the Kiruna area, and the evident east–west compositional zon-ing. Lilljeqvist & Svenson (1974) described textural char-acteristics, which identify the majority of the rocks of the

Table 2. Geochemical analyses of representative samples of felsic volcanics. References to data; JPN940254: dacite, Bothnian Group, Barsele area (Eliasson et al. in press), LAB930128, rhyolite, Bothnian Group, Mejvankilen area (Björk & Kero, 2001), 91105: dacite, Skellefte Group, Boliden area (Bergman Weihed et al. 1996), b-37: rhyolite, Skellefte Group, Boliden area (Vivallo, 1987), UJB940036: quartz porhyry rhyolite, Skellefte Group, Storliden area, (Bergström & Sträng 1999), KWI990080: dacite, Gallejaur Formation, Gallejaur area (Antal, Berg-ström & Weihed, unpublished), HLU980119: quartz latite, Arvidsjaur Group, Hornliden area, (Bergström & Triumf, in press), KBK970062: quartz latite/rhyolite/trachyte, Arvidsjaur Group, Skarpljugaren area (Kathol & Triumf, in press), MGN950271: rhyolite, Arvidsjaur Group, Fiskträsk area, (Bergström & Triumf, in press).

JPN940254 LAB930128 91105 b-37 UJB940036 KWI990080 HLU980119 KBK970062 MGN950271

SiO2 68.1 72.6 70.1 76.41 70.0 67.4 62.4 68.8 77.0TiO2 0.509 0.129 0.49 0.2 0.388 0.639 0.916 0.598 0.192Al2O3 13.9 13.9 15.0 11.55 15.9 14.7 14.6 13.0 10.7FeO 1.15 1.97 3.58 0.44 3.25 5.06 6.91 4.73 2.84MnO 0.0247 0.05 0.09 0.0709 0.095 0.226 0.086 0.021MgO 2.5 0.556 2.07 0.69 1.83 1.13 1.34 0.362 0.06CaO 3.45 1.97 3.43 1.46 1.65 2.71 3.3 1.47 0.295Na2O 1.11 2.89 3.12 2.75 1.03 4.89 4.47 3.88 3.77K2O 3.26 5.22 1.46 2.61 4.11 2.81 4.05 4.64 4.13P2O5 0.13 0.0395 0.15 0.03 0.118 0.19 0.361 0.131LOI 2.1 1.3 1.31 1.6 0.4 0.3 1.6 0.2Σ 99.0 99.9 98.9

Ba 222 208 296 787 567 1210 1200 1150 125Co 5 59 19 9.64 6.4 2.6Cr 29 18.2 13 3 15.6 54.9 34.5 14 17.6Cu 3.5 40.6 7 6 28.4 25.3 17.1 71.9Ga 16 37 13.7 20.6 20 25.8Hf 6.9 6.75 4.74 6.16 8.28 9.33 18.9Nb 13 21.7 9 3.61 14.2 7.73 13.9 24Ni 6 7.43 10 12 12.5 26.6 14.4 5.4Pb 3 4 14Rb 43 155 34 40.8 54.5 83 115 132Sc 10 11 17 15.7 9.97 11.3 7.59Sr 87 45.5 441 134 56.8 327 212 123 18Ta 1.6 0.435 0.918 0.867 1.34 1.92Th 3.9 14.8 3 2.5 5.39 6.51 9.72 13.4U 1.8 5.16 2.1 3.31 3.97 5.55 6.26 11V 38 7.75 34 24 14.5 27.9 56.9 9.81 6.15Y 33 39.2 19 38.1 38.6 29.5 35.2 31.3 68.2Zn 51.2 8.63 70 100 107 64.8 121 81.9 37.6Zr 184 168 122 172 162 260 247 315 561La 20.2 25.3 14.2 32.08 10.5 33.8 31.7 43.3 57.5Ce 41.2 66 33.0 63.58 25.7 80.7 74.0 82.1 128Pd 4.8 6.72 3.55 10.2 9.47 10.4 15.3Nd 19.6 25.5 18.2 33.24 16.4 40.1 40.0 42 64.1Sm 5.3 5.72 4.8 7.59 3.74 6.72 6.71 7.38 12.5Eu 1.2 0.246 1.46 1.68 0.792 1.45 2.98 1.55 0.714Gd 5.4 5.26 4.06 6.42 5.28 5.57 6.85 7.09 13.8Tb 0.9 0.99 1.03 0.809 1.03 1.14 2.4Dy 6 6.66 3.8 6.04 6.27 4.78 7.26 5.72 13.7Ho 1.3 1.63 1.45 0.827 1.6 1.22 2.84Er 3.8 5.24 2.0 3.75 4.4 2.38 3.2 3.32 7.82Tm 0.6 0.814 0.72 0.287 0.408 0.471 1.5Yb 4 6.62 2.08 3.54 4.82 2.8 4.67 3.32 8.35Lu 0.63 0.9 0.53 0.776 0.322 0.695 0.491 1.4


Arvidsjaur Group as subaerial. The Arvidsjaur Group vol-canic rocks are intimately related to the Arvidsjaur Suite of granitoid intrusions and many of the exposed volcanic rocks occur adjacent to or are intruded by fine-grained subvolcanic granites.

The major part of the Arvidsjaur Group volcanic rocks are rhyolitic in composition, commonly welded ash flow tuffs. Quartz, microcline, and plagioclase phenocrysts are very abundant. Some rare lava domes and subvolcanic sills or dykes occur, and probably reflect local volcanic cent-ers. Ignimbrites normally form rather monotonous pack-ages and it is not always possible to identify individual flows. In many cases, the flows probably form very thick units. The rhyolites may grade into alkali feldspar rhyo-lite compositions with lower plagioclase content and tra-chytic compositions with lower quartz content. The col-our is normally red to violet, but may grade into reddish grey.

Volcanic rocks of dacitic composition with plagioclase phenocrysts are present, both as lava domes and volcani-clastic rocks. Silica undersaturated rocks, commonly char-acterized by microcline phenocrysts, are also found in the Arvidsjaur Group, mainly as sporadical occurrences of dark grey, trachy-andesitic or quartz-latitic lava domes or subvolcanic intrusions. Similar dacitic, trachyandesitic, and quartz latitic compositions may also be found in the Arvidsjaur Suite granitoids, probably as a result of magma mixing of coeval gabbroic and granitic magmas.

The mafic volcanic rocks of the Arvidsjaur Group are of two principal types: coarse plagioclase phyric andes-ites–dacites and basalts–andesites with variable phenoc-ryst populations. The latter type may occur as both lavas and volcaniclastic rocks. The coarse plagioclase porphy-ries occur in large complexes as lavas and subvolcanic intrusions with minor clastic components. The plagi-oclase phenocrysts are up to 2–3 cm in size, and rocks with a high phenocryst content may look almost like anorthosites. The mafic volcanic rocks probably occur at several stratigraphic positions in the volumetrically more important felsic volcanic rocks.

Geochemistry

Data sets

A number of data sets on the geochemistry of the dif-ferent rock types have been included in this study. The majority of the samples were collected and analyzed dur-ing the Geological Survey (SGU) mapping programme in northern Västerbotten County during the 1990ies. These analyses are mainly high quality data with major and trace elements including REE, analyzed by ICP-MS, with some older samples analyzed with ICP-AES. Other modern

geochemical data from the Skellefte Project area include samples from a number of research projects in the 1980ies and 90ies (Claesson 1985, Vivallo 1987, Vivallo & Claes-son 1987, Vivallo & Willden 1988, Wasström 1990, Al-len et al. 1996, Bergman Weihed et al. 1996, Bergström 1997). The various older data has been included into the reference database only if major and trace elements have been analyzed and the quality was regarded as good. Some of these sample sets have been analyzed by XRF with addi-tional INAA on selected trace elements. The aim has been directed towards quality rather than quantity, and several analyses where alteration, weathering, laboratory misfunc-tion etc. are indicated, have been omitted. Cross correla-tion with good precision of methods and different labora-tories have been achieved for a number of samples, includ-ing re-analyzing of some samples. References to all data sets are given below.

Geochemical description

Bothnian Group

The Bothnian Group data include samples from the two principal assemblages: the homogeneous basalt lavas-vol-caniclastic rocks and the fractionated basalts-andesites-dacites-rhyolites. The samples are mainly from the Knaf-ten area and from various parts of the Storuman area. Gen-erally, the Bothnian Group samples have low Na2O+K2O, but some high K2O samples exist, probably due to altera-tion (Fig. 3c). The volcanic rocks mapped as homogene-ous basalts (HBA) cluster in the basaltic and picritic fields, whereas the FBRA fractionated assemblage forms a conti-nouos trend from basaltic andesite to rhyolite. The latter group is generally peraluminous.

The HBA basalts generally have flat REE patterns (LaN/YbN=1), and the majority of them are LREE deplet-ed (Fig. 4a), although some are HREE depleted (Fig. 4b). The latter samples are mainly from south of Storuman and have high Mg-Ni content indicating accumulation of olivine. For the FBRA samples, the REE patterns are frac-tionated (LaN/YbN=4–7). The basaltic andesites (Fig. 4c) do not show any negative Eu anomaly, whereas dacitic and rhyolitic samples show an increasingly more negative Eu anomaly in the more felsic rocks (Fig. 4d).

Skellefte Group

The Skellefte Group includes samples from basalts to rhy-olites from most parts of the district. As the volcanic rocks of the Skellefte Group have been subject to widespread al-teration processes, only the least altered samples have been used in this study. The samples show a low-K trend on a SiO2–Na2O+K2O plot (Le Maitre 1989) and a limited number of samples have SiO2 contents between 57 and

78 U. BERGSTRÖM

63 wt. % (Fig. 3a). Vivallo & Claesson (1987) suggested this to be evidence of a weak bimodality in the Skellefte Group. Many rhyolite samples are extremely high in SiO2, indicated by the quartz porphyritic texture. The dacites are metaluminous–peraluminous, whereas the rhyolites are mainly peraluminous.

The REE patterns of the basalts and the basaltic an-desites of the Skellefte Group are generally weakly frac-tionated (LaN/YbN=3). The REE plot of the dacites nor-mally form a concave upward shape with no Eu anomaly (Fig. 5c), whereas the rhyolites are enriched in LREE and have a negative Eu anomaly (Fig. 5e). Both dacites and rhyolites have LaN/YbN=5–7. A limited number of highly fractionated quartz-feldspar-porphyritic subvolcanic in-

trusions show a flat REE pattern with a more distinct neg-ative Eu anomaly (Fig. 5f ).

Tjamstan Formation

Two principal compositional groups are found within the Tjamstan Formation, one with basalts–andesites and one with volumetrically subordinate dacites (Fig. 3a). All the samples form a low Na2O+K2O trend in a SiO2–Na2O+K2O diagram (Le Maitre 1989), similar to the Skellefte Group.

The total REE content of the basalts–andesites is gen-erally very low (ΣREE<100 ppm) and the REE patterns are flat (LaN/YbN=1–2, Fig. 5b). The dacite samples have more fractionated patterns (Fig. 5d), with a negative Eu

35 45 55 65 750

5

15

Pc B O1

O2 O3

U1

U2

U3

F

Ph

S1

S2S3

T

R

PcB

O1O2

O3

U1

U2

U3

F

Ph

S1

S2

S3

T

R

a) b)

SiO2 (wt-%) SiO2 (wt-%)

Na 2

O+

K2O

(w

t-%

)

35 45 55 65 750

5

10 10

15

SiO2 (wt-%)

Na 2

O+

K2O

(w

t-%

)

35 45 55 65 750

5

10

15

SiO2 (wt-%)35 45 55 65 75

0

5

10

15

Pc BO1

O2O3

U1

U2

U3

F

Ph

S1

S2

S3

T

R

Pc B

O1O2

O3

U1

U2

U3

F

Ph

S1

S2

S3

T

R

c) d)

Fig. 3. Le Maitre plot (1989) for samples of a) the Skellefte Group, b) the Arvidsjaur Group, c) the Bothnian Group, d) the Vargfors Group. Fields are: F=foidite, Pc=picrobasalt, B=basalt, O1=basaltic andesite, O2=andesite, O3=dacite, R=rhyolite, S1=trachybasalt, S2=basaltic trachyandesite, S3=trachyandesite, T=trachyte and trachydacite, U1=tephrite and basanite, U2=phonotephrite, U3=tephriphonolite, Ph=phonolite. Symbols: open circles=rhyolites, Skellefte Group, half-filled circles=dacites, Skellefte Group, filled circles=basalts–andesites, Skellefte Group, open rombs=dacites, Tjamstan Formation, filled rombs=basalts–andesites, Tjamstan Formation, open squares=rhyolites, Arvidsjaur Group, half-filled squares=dacites–trachy-andesites/quartz-latites, Arvidsjaur Group, filled squares=basalts–andesites–dacites, Arvidsjaur Group, open triangles with sharp apex down=FBRA assemblage, Bothnian Group, filled triangles with sharp apex down=HBA assemblage, Bothnian Group, open triangles=Gallejaur Formation, Vargfors Group, filled triangles=Varuträsk Formation, Vargfors Group, stars=Bjurås Formation, Vargfors Group.


anomaly, which is different from volcanic rocks of similar compositions in the Skellefte Group.

Vargfors Group

The heterogeneous character of the volcanic rocks in the Vargfors Group is evident in a SiO2–Na2O+K2O diagram (Le Maitre 1989). The basalts of the Varuträsk Formation are similar to the Bothnian Group basalts (Fig. 3d). The Bjurås Formation basalts are Mg-Cr-Ni-enriched komati-itic basalts to basalts and plot within the basalt field of the Le Maitre (1989) plot (Fig. 3d), but trend with fraction-ation into more alkali-rich compositions. The Gallejaur Formation consists of both basalt samples and a number of samples forming a trend from andesite to dacite (Fig. 3d).

The Varuträsk Formation basaltic rocks show two principal types of REE pattern (Fig. 6a): flat REE patterns and LREE-enriched patterns. The LREE-enriched pattern correponds to Ti-rich samples with higher contents of to-

tal REE. The Bjurås Formation generally has flat to mild-ly LREE-enriched patterns with LaN/YbN=1–3 (Fig. 6b). This is common for more or less all strongly Mg-Cr-Ni-rich ultramafites to basalts. Basalts from the Gallejaur For-mation have a fractionated pattern without a negative Eu anomaly (Fig. 6c), whereas the dacites show a small but distinct Eu anomaly (Fig. 6d).

Arvidsjaur Group

The Arvidsjaur Group samples have been collected from the entire Arvidsjaur district and include volcanic rocks of all compositions, with sample locations concentrated to the type areas around Arvidsjaur. The Arvidsjaur Group shows a high-K trend on the SiO2–Na2O+K2O diagram (Le Maitre, 1989) and the large data set comprising vol-canic rocks mapped as rhyolites clearly plot clustered in the rhyolite field (Fig. 3b), whereas the dacites include many samples which plot as trachydacite (T field in the TAS diagram) and quartz-latite (field S3). A K-enriched

Fig. 4. REE patterns for different representative volcanic rocks of the Bothnian Group. Normalized REE abundance in rock/primitive mantle (Sun, 1982). a) Basalt lavas of the HBA homogeneous basalt assemblage from the Knaften area. b) Basalt lavas of the HBA homoge-neous basalt assemblage from north of Storuman. c) Basalts and andesites of the FBRA fractionated basalt–rhyolite assemblage. d) Dacites and rhyolites of the FBRA fractionated basalt–rhyolite assemblage.

Roc

k/C

hond

rite

a) b)

c) d)

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu1

10

100

1000

Roc

k/C

hond

rite


10

100

1000


10

100

1000


10

100

1000

80 U. BERGSTRÖM

signature is evident also for the mafic volcanic rocks, mainly plotting in the trachyandesitic S2 and S3 fields. A majority of the rhyolites are peraluminous and mildly peralkaline.

Most basalts and andesites have weakly fractionated REE patterns (LaN/YbN=3) with a small or non-existant negative Eu anomaly (Fig. 7a). The plagioclase phyric an-desites-dacites are more fractionated. The dacites show

small negative Eu anomalies, while the majority of the rhyolites have a more pronounced negative anomaly (Figs. 7b, 7c). The total REE content for the felsic rocks is rela-tively high compared to similar rocks of the Skellefte and Bothnian Groups. Samples with a quartz-latitic composi-tion do not show an Eu anomaly, but show the wavy pat-tern (Fig. 7b), similar to the andesites and dacites.


10

100

1000


10

100

1000

Roc

k/ch

ondr

ite


10

100

1000


10

100

1000

Roc

k/ch

ondr

ite


10

100

1000


10

100

1000

Roc

k/ch

ondr

itea) b)

c) d)

e) f)

Fig. 5. REE patterns for different representative volcanic rocks of the Skellefte Group including the Tjamstan Formation. Normalized REE abundance in rock/primitive mantle (Sun 1982). a) Basalts and basaltic andesites from the Långdal-Boliden and Granbergsliden areas. b) Basalts and basaltic andesites of the Tjamstan Formation. c) Dacites (lava domes/subvolcanic intrusions) from the Boliden area. d) Dacites (lavas, intrusions, and dykes) of the Tjamstan Formation. e) Rhyolites from the Boliden-Renström area. f) Evolved rhyolites from the Storliden and Östra Högkulla areas.


Tectonic discrimination

The mafic volcanic rocks of the Bothnian, Skellefte, Varg-fors, and Arvidsjaur Groups have been plotted in differ-ent discrimination diagrams in order to determine the tec-tonic settings for the respective volcanic group. Generally, it is necessary to use several different diagrams together in order to discriminate between the different groups, as just one or two diagrams for different reasons might show am-bivalent results. Ideally the diagrams are designed for unal-tered rocks reflecting melt compositions, especially the ba-saltic rocks. This is not very easily obtained in the Bothni-an, Skellefte, Vargfors, and Arvidsjaur Groups, where phe-nocrysts are common (especially in the Arvidsjaur Group) and low-intensity alteration patterns, which mainly dis-turb LIL element patterns, frequently occur. Diagrams used in this paper are: Zr–Ti, Zr–Ti/100–Y*3 and Zr–Ti/100–Sr/2 (Pearce & Cann 1973), Ti–Cr, (Pearce 1975), Al2O3–FeO+TiO2–MgO (Jensen 1976), Zr/Y–Zr (Pearce & Norry 1979), Th–Hf/3–Ta (Wood 1980), MnO*10–Ti/100–P2O5*10 (Mullen 1983), and Zr/4–Nb*2–Y (Me-

schede 1986). The felsic rocks of the Bothnian, Skellefte, Vargfors and Arvidsjaur Groups are plotted in the Rb–Y+Nb (Pearce et al. 1984), Zr+Nb+Ce+Y–FeO*/MgO (Whalen et al. 1987), and R1–R2 diagram (De La Roche 1980, Batchelor & Bowden 1985).

Bothnian Group

In the Al2O3–FeO*+TiO2–MgO-plot (Jensen 1976) the HBA homogeneous, basaltic lavas and related volcani-clastic rocks plot in the high-Fe field, typical for tholeiitic rocks (Fig. 8a). Some samples trend towards the MgO apex, reflecting a more primitive composition or, more likely, indicates cumulate processes. The FBRA rocks typi-cally plot towards the Al2O3 apex and show a calc-alkaline trend, starting in the HMT field. A Mullen (1983) plot (Fig. 9a) gives a clear discrimination for the basaltic rocks of the two assemblages.

The HBA basaltic rocks plot within or close to the MORB or ocean floor fields in most diagrams (Figs. 10a, 11a, 12a). Some samples, mainly from the Skarvsjö area

Roc

k/C

hond

rite

a) b)

c) d)


10

100

1000


10

100

1000

Roc

k/C

hond

rite


10

100

1000


10

100

1000

Fig. 6. REE patterns for different representative volcanic rocks of the Vargfors Group. Normalized REE abundance in rock/primitive mantle (Sun 1982). a) Basalts of the Varuträsk Formation. b) Mg-basalts of the Bjurås Formation. c) Basalt–andesite of the Gallejaur Formation. d) Dacite from the Gallejaur Formation.

82 U. BERGSTRÖM

south of Storuman, plot within or close to the with-in-plate field in a Zr–Ti/100–Y*3-plot (Pearce & Cann 1973) as seen in Figure 11a. These are possibly cumulate rocks.

The FBRA basalts and andesites have a calc-alkaline basalt signature. The dacites and rhyolites plot in the

volcanic arc field of the Rb–Y+Nb plot (Pearce et al. 1984), where the rhyolites are more fractionated (higher Y+Nb) than the dacites and plot closer to the WPG field (Fig. 13b).

Skellefte Group

The basalts and andesites of the Skellefte Group plot in the island arc or calc-alkaline basalt fields in the dif-ferent discrimination plots. The Zr/Y ratio discriminates between oceanic and continental arcs (Pearce & Norry 1979) and the low ratio, 2–4 for the Skellefte Group, suggests the former setting. In the Zr–Ti/100–Y*3-plot (Pearce & Cann 1973), the Skellefte Group mafic volcan-ic rocks plot all over the A, B, and C fields (Fig. 11b), with one cluster in the low-K tholeiite field and another cluster on the line separating the ocean-floor and calcalka-line fields. This suggests several magma sources within the Skellefte Group volcanic rocks. The low-K tholeiite sam-ples are from the Långdal area (Vivallo 1987) and repre-sent a stratigraphically high unit in the easternmost part of the Skellefte District, possibly reflecting a late extensional event and influx of new primitive magma. In the Zr–Ti/100–Sr/2-diagram a number of samples plot in the OFB field (Fig. 12b). These samples are mainly from the Granbergsliden area and exhibit Sr depletion due to alter-ation processes (Nicolson 1993). The felsic volcanic rocks plot in the volcanic arc field of the Rb–Y+Nb diagram (Fig. 13b).

The Tjamstan Formation samples plot generally as volcanic arc basalts–andesites. The low Zr/Y value sug-gests an oceanic island arc setting.

Vargfors Group

Few samples (n=3) from the Varuträsk Formation are available and the results must be reviewed with some cau-tion. The Varuträsk Formation basalts are generally simi-lar to the Bothnian Group, HBA homogeneous basalts and plot within or close to the MORB/ocean floor fields in most discrimination diagrams. The Varuträsk Forma-tion samples have high Fe and Ti content (Fig. 8c). The Varuträsk Formation basalts plot as low-K tholeiites and within-plate basalts in the Zr–Ti/100–Y*3 diagram (Pearce & Cann 1973, Fig. 11c).

The Bjurås Formation Mg-basalts are strongly Mg- and Cr-enriched, which is clearly seen in the Al2O3–FeO+TiO2–MgO (Jensen 1976, Fig. 8c) and a Ti-Cr-plot (Pearce 1975) not shown here. The tectonic setting of the Bjurås Formation Mg-basalts suggested from most dis-crimination diagrams (Figs. 9c, 10c, 11c, 12c), is a calc-alkaline volcanic arc setting, where the Mg-basalts repre-sent juvenile, primitive melts.

The Gallejaur Formation basalts are calc-alkaline vol-

Roc

k/C

hond

rite


10

100

1000

Roc

k/C

hond

rite


10

100

1000

Roc

k/C

hond

rite


10

100

1000

a)

b)

c)

Fig. 7. REE patterns for different representative volcanic rocks of the Arvidsjaur Group. Normalized REE abundance in rock/primitive mantle (Sun 1982). a) Basalts and andesites. b). Dacite/quartz-latite/trachyandesites. c) Rhyolites.


canic arc basalts according to most diagrams (Figs. 9c, 10c, 11c, 12c), similar to the Bjurås Formation high-Mg basalts. Andesites and dacites from the Gallejaur Forma-tion form a calc-alkaline trend in the volcanic arc field in the Rb–Y+Nb-plot of Pearce et al. (1984), similar to the Skellefte Group (Fig. 13c).

Arvidsjaur Group

The subdivision of the mafic volcanic rocks from the Arvidsjaur Group in more primitive basalts–andesites and evolved plagioclase phyric andesites–dacites is seen in most discrimination diagrams. The basalts–andesites plot in the HMT field in the Jensen plot (Jensen 1976), where-as the plagioclase phyric andesites–dacites cluster in the CA-TD fields. The low Mg and Ca (MgO+CaO = 8–12

%) and high SiO2 content of the andesites–dacites make them unsuitable for many of the plots, and the results are to be considered as indications on magma composi-tion during fractionation. The Zr/Y ratio above 4 for most samples indicates a continental margin setting (Pearce & Norry 1979). The basalts–andesites generally plot in the calc-alkaline basalt fields in most diagrams. In the Zr–Ti/100–Y*3-plot the andesites–dacites mainly plot within and partly outside the calc-alkaline C field, whereas the basalts are restricted to the ocean floor/volcanic arc B field. The felsic volcanic rocks of the Arvidsjaur group show a volcanic arc signature in the Rb–Y+Nb-plot (Pearce et al. 1984). A strong crustal influence is indicated for the rhyo-lite samples, which trend into the within-plate field.

TR

TD

TAHFT

HMT

CR CD CA CB

BK

PK

TR

TD

TAHFT

HMT

CRCD CA CB

BK

PK

a) b)

c) d)

Al2O3 MgO

FeO*+TiO2

Al2O3 MgO

FeO*+TiO2

Al2O3 MgO

FeO*+TiO2

Al2O3 MgO

FeO*+TiO2

TR

TD

TA HFT

HMT

CRCD

CACB

BK

PK

TR

TD

TA HFT

HMT

CRCD

CACB

BK

PK

Fig. 8. Jensen plot (Jensen, 1976) for samples from a) the Bothnian Group, b) the Skellefte Group, c) the Vargfors Group, d) the Arvidsjaur Group. Fields are: PK=peridotitic komatiite, BK=basaltic komatiite, HFT=high-Fe tholeiite basalt, HMT=high-Mg tholeiite basalt, CB=calc-alkaline basalt, CA=calc-alkaline andesite, CD=calc-alkaline dacite, CR=calc-alkaline rhyolite, TA=tholeiitic andesite, TD=tholeiiitic dacite, TR=tholeiitic rhyolite. Symbols as in Figure 3.

84 U. BERGSTRÖM

Discussion

Tectonic setting

The HBA homogeneous basalts of the Bothnian Group are interpreted as MORB basalts formed on the ocean floor, based on their flat REE to mildly LREE depleted pattern, and their behaviour in most discrimination plots. The FBRA assemblage is interpreted as volcanic arc type rocks. The interpreted tectonic setting of the Bothnian Group is speculative, but the overall setting with mixed MORB-type and arc-type volcanic rocks and sedimentary rocks is interpreted as an arc setting, where the HBA rocks formed on the ocean floor in marginal basins, surrounded by arc crust. The FBRA blocks constitute younger arc seg-ments.

The Skellefte Group basalts to rhyolites are interpret-ed as calc-alkaline rocks formed in a marine volcanic arc. The mafic volcanic rocks have both island arc tholeiite

and calc-alkaline components. The felsic volcanic rocks include dacites and rhyolites, which are quartz-enriched and K-depleted. The most fractionated end members are coarse quartz-porphyritic subvolcanic stocks with flat REE pattern and a negative Eu anomaly. They are mildly bimo-dal, possibly reflecting an arc under extension. As felsic compositions predominate, one would suspect that a sig-nificant proportion of continental crust has been present in the source area and that AFC (Assimilation-Fractiona-tion-Crystallization, DePaolo 1981) was the main process that generated the more felsic rocks. Allen et al. (1996) suggested an extensional continental margin arc setting rather than an oceanic arc for the Skellefte Group, based on the physical volcanology of the Skellefte Group volcan-ic rocks. This is in accordance with the geochemical data, with the mixed, but predominantly felsic compositions, the subordinate but present primitive island arc magmas, and the mild bimodality. The Tjamstan Formation mafic

OIT

OIA

CAB

IAT

MORB

OIT

OIA

CAB

IAT

MORB

OIT

OIA

CAB

IAT

MORB

OIT

OIA

CAB

IAT

MORB

a)

MnO*10 P2O5*10

TiO2 b)

MnO*10 P2O5*10

TiO2

c)

MnO*10 P2O5*10

TiO2 d)

MnO*10 P2O5*10

TiO2

Fig. 9. Mn*10–TiO2–P2O5*10 tectonic discrimination-plot for basaltic rocks (SiO2=45–54 wt. %) according to Mullen et al. (1983) for samples from a) the Bothnian Group b) the Skellefte Group, c) the Vargfors Group, and d) the Arvidsjaur Group. Fields are: OIT=ocean island tholeiite basalt, OIA=ocean island alkali basalt, MORB=mid ocean ridge basalt, IAT=island arc tholeiite basalt, CAB=calc-alkaline basalt. Symbols as in Figure 3.


volcanic rocks are similar to the mafic volcanic rocks of the Skellefte Group, indicating a new generation of rather primitive island arc magmas in the arc environment.

The Vargfors Group includes three principal units of mafic volcanic rocks, each with a distinct tectonic setting, but on the same principal stratigraphical level. The Varu-träsk Formation consists of MORB-type basalts similar to the Bothnian Group basalts, maybe with a somewhat stronger crustal affinity, which is interpreted from the higher LREE content, the increasing levels of titanomag-netite crystallization, and the tendency to plot in within- plate fields in many discrimination diagrams. The in-terpreted tectonic setting for these rocks is a rift basin, possibly a back-arc marginal basin. The Mg-basalts of the Bjurås Formation are strongly Mg-Cr-Ni-rich, highly primitive volcanic arc rocks, indicated from several dis-

crimination diagrams. The Gallejaur Formation volcanic rocks are calc-alkaline and intermediate between the Skel-lefte and Arvidsjaur Groups.

The Arvidsjaur Group is a heterogeneous, strongly fractionated volcanic unit with a substantial crustal com-ponent, emplaced in a continental volcanic arc environ-ment. The volcanic rocks are medium- to high-K in char-acter, also evident from abundant microcline phenocrysts. The geochemical signature suggests significant arc ma-turity. Perdahl (1993, 1994, 1996) emphasized that the Arvidsjaur Group rocks from the the western part of the Arvidsjaur District, the Arjeplog volcanics, are bimodal, suggesting emplacement in a continental rift, whereas the central part (which dominates the data set in this study) are more arc like.

A

B

C

D

A

B

C

D

a) b)

c) d)

Ti (

ppm

)

0 25 50 75 100 125 150 175 200 225 2500

3000

6000

9000

12000

15000

18000

0 25 50 75 100 125 150 175 200 225 2500

3000

6000

9000

12000

15000

18000

Ti (

ppm

)

0 25 50 75 100 125 150 175 200 225 2500

3000

6000

9000

12000

15000

18000

0 25 50 75 100 125 150 175 200 225 2500

3000

6000

9000

12000

15000

18000

A

B C

D

A

B

C

D

Fig. 10. Tectonic discrimination Zr–Ti-plot for basaltic rocks according to Pearce and Cann (1973) for basalts–andesites from a) the Both-nian Group, b) the Skellefte Group, c) the Vargfors Group, and d) the Arvidsjaur Group. Ocean floor basalts plot in fields D and B, low-K tholeiite basalts plot in fields A and B, and calc-alkali basalts plot in fields A and C. Symbols as in Figure 3.

86 U. BERGSTRÖM

Discrimination

The Skellefte and Arvidsjaur Groups probably reflect two discrete episodes within the same destructive plate mar-gin setting, separated by 5–10 million years (Billström & Weihed 1996). The Skellefte Group was deposited in a marine, extensional environment with influx of primi-tive island arc melts, whereas the Arvidsjaur Group was formed in a subaerial, compressional, continental margin arc. The geochemical differences between the two groups are mainly related to the much higher degree of crustally derived magmas in the Arvidsjaur Group, with higher K, Rb, and U-Th contents and a REE pattern with a negative Eu anomaly, especially in the more felsic rocks. This is readily seen in spectrometric surveys, where the K-U-Th components are much higher in the Arvidsjaur Group volcanic rocks compared to Skellefte Group vol-canic rocks. The felsic components of the Skellefte Group normally lack microcline phenocrysts and fractionate to-wards a SiO2-rich quartz porphyritic end member. Sam-ples from the Skellefte Group plotted in a R1–R2 dia-

gram (Fig. 16) suggest a strong mantle component for the Skellefte Group, whereas the Arvidsjaur Group sam-ples have a late-orogenic signature. The mafic rocks in the Skellefte Group include island arc tholeiites, contrary to corresponding Arvidsjaur Group volcanic rocks, which are typically calc-alkaline. In Figure 14b, the Skellefte Group mafic volcanic rocks plot close to the TiO2 apex and the oceanic arc field and grade towards the Arvidsjaur Group volcanic rocks, which are restricted to the continental arc field and consequently plot close to the K2O apex. The Tjamstan Formation volcanic rocks are compositionally similar to the Skellefte Group volcanic rocks and is in-cluded as the uppermost unit in the Skellefte Group.

The time interval between the development of the Skellefte and Arvidsjaur arcs is dominated by the dep-osition of greywackes, now exposed in the eastern and western margins of the Skellefte District. Mafic volcanic rocks intercalated with the sedimentary rocks exist in both areas, but the chemical compositions are different. The Mg-basalts of the western Skellefte District are highly

AB

C

D

A

B

C

D

A

B

C

D

A

B

C

D

a)

Zr Y*3

Ti / 100 b)

Zr Y*3

Ti / 100

c)

Zr Y*3

Ti / 100 d)

Zr Y*3

Ti / 100

Fig. 11. Tectonic discrimination Zr–Ti/100–Y*3-plot for basaltic rocks according to Pearce and Cann (1973) for basalts–andesites from a) the Bothnian Group, b) the Skellefte Group, c) the Vargfors Group, and d) the Arvidsjaur Group. Within-plate basalts plot in field D, ocean floor basalts plot in field B, low-K tholeiite basalts in fields A and B, and calc-alkali basalts in fields B and C. Symbols as in Figure 3.


primitive volcanic arc rocks, whereas the Varuträsk For-mation basalts in the eastern district have MORB and within-plate characteristics, probably reflecting a rift envi-ronment. The coeval Gallejaur Formation volcanic rocks erupted around the Gallejaur intrusive complex in the central Skellefte District and were deposited on the base-ment of the older Skellefte Group volcanic rocks. The vol-canic rocks of the Gallejaur Formation are similar to the Skellefte and Arvidsjaur Group rocks, and may be regard-ed as a variety of Arvidsjaur arc volcanism on top of the Skellefte Arc.

The Skellefte, Vargfors, and Arvidsjaur Groups repre-sent different tectonic facies within the same destructive plate margin setting. The Skellefte Group was deposited in an early, mainly marine, extensional arc which was fol-lowed by the subaerial, compressional, mature Arvidsjaur arc. The Vargfors Group reflects the dominantly sedimen-tary stages between the two volcanic arc episodes. To the west, an intra-arc basin formed as a deep-water delta, suc-ceded by primitive, Mg-rich, mafic volcanism, which may

correspond to the initial stage of the Arvidsjaur volcan-ism. To the east, a rift basin was formed including new oceanic crust. This basin partly separates the Skellefte arc from poorly known felsic and mafic volcanic rocks in the Burträsk area, that possibly form an arc remnant.

The fractionated basalt to rhyolite assemblage (FBRA) of the Bothnian Group south of the Skellefte District is geochemically similar to the volcanic rocks of the Skellefte Group. The FBRA volcanic rocks probably represent rem-nants of old volcanic arcs. The limited exposures of this unit prevent stratigraphic correlations, but age determi-nations clearly indicate that this unit is at least 50 mil-lions years older than the Skellefte Group volcanic rocks. The boundary between similar greywackes of the Bothni-an and the Vargfors Groups south of the Skellefte District is somewhat arbitrary since the subdivision between the Bothnian and Vargfors Groups is geographical as well as stratigraphical. The boundary which is outlined in Figure 1, is one interpretation. Critical areas for possible contact rela-tions are the Bure, Pauliden, Njuggträskliden, and Burträsk

OFB

CAB

IAB

OFB

CAB

IAB

OFB

CAB

IAB

OFB

CAB

IAB

a)

Zr Sr / 2

TiO / 100 b)

Zr Sr / 2

TiO / 100

c)

Zr Sr / 2

TiO / 100 d)

Zr Sr / 2

TiO / 100

Fig. 12. Zr–Ti/100–Sr/2 plot for basaltic rocks according to Pearce & Cann 1973 for basalts-andesites of a) the Bothnian Group, b) the Skellefte Group, c) the Vargfors Group, and d) the Arvidsjaur Group. Fields are: OFB=ocean floor basalts, IAB=island arc tholeiite basalts and CAB=calc-alkali basalts. Symbols as in Figure 3.

88 U. BERGSTRÖM

areas (Fig. 1). In the two latter areas, the contact relations are probably strongly overprinted by both deformation and processes related to Ni mineralization. In Figure 14a, basaltic–andesitic volcanic rocks from the FBRA of the Bothnian Group and the Skellefte Group have been plot-ted in a La/Yb–Sc/Ni-plot of Bailey (1981). The rocks may be discriminated mainly due to the low Ni content of the Skellefte Group rocks, which is a general feature of island-arc basalts (Wilson 1989) and suggests impor-tant olivine fractionation in the evolution of the Skellefte Group magmas. The MORB type basalts of the homo-geneous basalt assemblage (HBA) of the Bothnian Group

is easy to discrimate from the Skellefte Group (Figs. 15a and b), mainly due to the strong Fe-enriched tholeiitic character of the rocks. The HBA basalts of the Bothnian Group are very similar to the basalts of the Varuträsk For-mation in the Vargfors Group, and the units cannot be dis-criminated from each other with the available database in the Whalen plot (Whalen et al. 1987), where they plot close to the A-granite field, contrary to the Skellefte Group rhy-olites (Fig. 16b). Figure 16c shows that a large part of the Arvidsjaur Group felsic volcanic rocks tend to plot in the A-granite field, contrary to the corresponding Gallejaur volcanic rocks which plot in the OGT field.

Rb

(ppm

)

1 10 100 10001

10

100

1000

ORGVAG

WPGsyn-COLG

a)

Rb

(ppm

)

1 10 100 10001

10

100

1000

ORGVAG

WPGsyn-COLG

b)

Rb

(ppm

)

1 10 100 10001

10

100

1000

ORGVAG

WPGsyn-COLG

c)

Rb

(ppm

)

1 10 100 10001

10

100

1000

ORGVAG

WPGsyn-COLG

d)

Y+Nb (ppm) Y+Nb (ppm)

Y+Nb (ppm) Y+Nb (ppm)

Fig. 13. Tectonic discrimination Rb–Y+Nb-plot (Pearce et al. 1984) for felsic volcanic rocks from a) the fractionated basalt–rhyolite assemblage (FBRA) of the Bothnian Group, b) the Skellefte Group, c) the Gallejaur Formation of the Vargfors Group, and d) the Arvidsjaur Group. Fields are: VAG=volcanic arc granitoids, ORG=ocean ridge granitoids, syn-COLG=syn-collision granitoids and WPG=within-plate granitoids. Symbols as in Figure 3.


Conclusions

The Skellefte District and surrounding areas include a number of litostratigraphic units which are geochemically distinguishable.

The Bothnian Group, exposed south of the Skellefte District, includes two volcanic units: one older homo-geneous basalt lava–volcaniclastic rock assemblage with MORB signature, and one younger (c. 1.95 Ga) fraction-ated basalt to rhyolite assemblage, formed in a volcanic arc setting.

The Skellefte Group is a heterogeneous unit of basalt to rhyolite, deposited in an extensional continental mar-

gin arc at c. 1.90–1.88 Ga. The stratigraphically upper-most unit is composed of primitive basalts and andesites of the Tjamstan Formation and mudstones, reflecting the extensional environment.

The Vargfors Group succeeds the Skellefte Group and indicates varying depositional environments at c. 1.88–1.87 Ga. The western part is dominated by grey-wackes, overlain by primitive Mg-basalts, which are the first sign of a renewed volcanic activity. The Vargfors Group in the eastern part also started with greywacke deposition, succeeded by evolved MORB-type volcanic rocks of the Varuträsk Formation. The latter probably re-

Sc/Ni

La/Y

b

a)

b)

0 2 4 6 80

5

10

15

20

Andean arc

Continental island-arc

Other oceanic arc

Low-K oceanic arc

K2O P2O5

TiO2

Oceanic

Continental

a)

b)

Cr (ppm)

Ti (

ppm

)

10 100 10001000

10000

100000

LKT

OFB

Al2O3 (wt-%)

Fe 2

O3+

FeO

(w

t-%

)

6 9 12 15 18 21 24

22

4

7

10

13

16

19

14. Discrimination plots. a) Basalts and andesites of the fractiona-ted basalt–rhyolite assemblage (FBRA) of the Bothnian Group ver-sus basalt–andesites of the Skellefte Group in a La/Yb–Sc/Ni-plot (Bailey 1981). b) Basalt–andesites of the Skellefte and Arvidsjaur Groups in a K2O–TiO2–P2O5-plot of Pearce et al. (1975). Symbols as in Figure 3.

Fig. 15. Discrimination plots. a) Basalts of the HBA homogeneous basalt assemblage of the Bothnian Group and basalts–andesites of the Skellefte Group in a Cr–Ti-plot according to Pearce (1975). Fields are: LKT=low-K tholeiite basalt, OFB=ocean floor basalt. b) Discrimination between basalts of the HBA homogeneous ba-salt assemblage of the Bothnian Group and basalts–andesites of the Skellefte Group in a Fe2O3+FeO–Al2O3-plot. Symbols as in Figure 3.

90 U. BERGSTRÖM

flects the initiation of a rift basin east of the Skellefte Dis-trict. Subaerial environments and erosion of the uplifted Skellefte Group rocks characterize the central part of the Vargfors Group. The Arvidsjaur Group is deposited in the Arvidsjaur district north of the Skellefte District and may, at least partly be stratigraphically equivalent to the Varg-fors Group rocks. The Arvidsjaur Group is a heterogene-ous unit of subaerial basaltic to rhyolitic volcanic rocks, formed in a mature, compressional, continental margin arc.

R1 = 4Si – 11(Na + K) – 2(Fe + Ti)

0 500 1000 1500 2000 2500 30000

500

1000

1500

2000

2500

R2

= 6

Ca

+ 2

Mg

+ A

l

1

2

3

4

5

6

7

a)

b)

c)

100 1000 50001

10

100

Zr + Nb + Ce + Y (ppm)

100 1000 5000

Zr + Nb + Ce + Y (ppm)

FeO

* / M

gOF

eO*

/ MgO

FG

OGT

1

10

100

FG

OGT

Fig. 16. Discrimination plots for felsic volcanic rocks. a) R1–R2-plot (De la Roche 1980, Batchelor & Bowden 1985) for felsic vol-canic rocks of the Skellefte and Arvidsjaur Groups. Fields are: 1=mantle fractionates, 2=pre-plate collision, 3=post-collision uplift, 4=late orogenic, 5=anorogenic, 6=syn-collision, 7=post-orogenic. b) Tectonic discrimination for granitic rocks according to Whalen et al. (1987) for samples of the Skellefte Group and the fractionated basalt–rhyolite assemblage of the Bothnian Group. c) Tectonic dis-crimination for granitic rocks according to Whalen et al. (1987) for felsic volcanic samples of the Gallejaur Formation and Arvidsjaur Group. Symbols as in Figure 3.


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93ROCK CLASSIFICATION, MAGMATIC AFFINITY, A HYDROTHERMAL ALTERATION AT BOLIDEN, SKELLEFTE DISTRICT, SWEDEN ...

Hallberg, A., 2001: Rock classification, magmatic affinity, and hydrothermal alteration at Boliden, Skellefte district, Sweden – a desk-top approach to whole rock geochemistry. In Weihed, P. (ed.): Economic geology research. Vol. 1, 1999–2000. Uppsala 2001. Sveriges geologiska undersökning C 833, pp. 93–131. ISBN 91-7158-665-2.

Unambiguous mapping of rocks, geological contacts, and altera-tion patterns is vital in geological surveys as well as in the ex-ploration for mineral deposits. In many Precambrian ore-bear-ing regions in Sweden and other parts of the world, hydro-thermal alteration, penetrative deformation, and overprinting regional metamorphism have destroyed all diagnostic features in the rock. Correct identification of precursor rocks and char-acterisation of alteration in such areas are extremely difficult. Modern lithogeochemical techniques then provide a powerful and cost effective tool to decipher origin and history of the rock.

The main aim with this paper is to explore the use of litho-geochemical techniques on the rocks that make up host and wall rocks to the Boliden massive sulphide deposit, both strongly al-tered and least altered rocks have been studied.

The results of the study are only valid for the rocks in the studied area and direct application of the results on rocks from other areas might be misleading. More important, however, is the approach to whole rock geochemistry presented in this paper, an approach that can be used to identify alteration and indicate reliable less mobile elements from any rock and from any geological environment.

Anders Hallberg, Geological Survey of Sweden, Mineral Resources Information Office, Skolgatan 4, SE-930 70 Malå, Sweden. E-mail: [email protected]

The Boliden deposit

The Boliden Cu-Au-As massive sulphide deposit is situ-ated in the most gold-rich area in Europe – the eastern Skellefte district, northern Sweden (Fig. 1). Within a dis-tance of less than 26 km from Boliden, two gold deposits and eleven gold-rich massive sulphide deposits have been mined. The combined gold production from these mines is in excess of 220 metric tons of gold. Around sixty per-cent, or 128 metric tons, of the gold came from the Boli-den deposit.

The Boliden deposit has been the subject of numer-ous studies since the discovery of the deposit in 1924, e.g. Ödman (1941), Gavelin (1955), Nilsson (1968), Grip & Wirstam (1970), Isaksson (1973), Rickard & Zweifel

(1975), Vivallo (1987), and most recently by Bergman Weihed et al. (1996). The following is a brief summary of the main characteristics of the deposit.

Base metal and gold mineralization at Boliden consists of massive arsenopyrite ore, massive pyrite + pyrrhotite ore, and veins and disseminations below the massive sul-phides. The total tonnage of the deposit was 8.4 Mt with an average grade of 15.5 g/t Au, 1.43 % Cu, and 6.8 % As.

The deposit is hosted by a large zone of strongly altered rocks including chlorite schists, quartz-sericite schists, and rutile-bearing andalusite-sericite rocks. The rocks outside the altered area consist of volcanic rocks of intermediate to felsic composition.

About the data

Almost 90 whole rock geochemical analyses from the Boli-den area are available from publications (Ödman 1941, Nilsson 1968, Grip & Ödman 1942, Bergman Weihed et al. 1996). In addition, more than 200 analyses come from unpublished research reports (Hallberg 1994, Bergström 1994) and from other unpublished sources. A lot of the unpublished data have been published as averages or as di-agrams without primary analyses in e.g. Vivallo (1987) and Allen et al. (1996). For several of the analyses there are very little information on sampling method, sample prepara-tion method, analytical method, or precision in analyses. All available data are found in Appendix A–C. Informa-tion on analytical procedures, analytical methods, and laboratories are, when available, also given in Appendix A–C. The effect of crushing and milling methods are dis-cussed in the text.

In order to avoid any obstacles due to uncertainties in location, analytical quality, or sampling procedure, only modern analyses with sufficient control of the analytical procedures are used in this study. In practise this means that analyses done before 1985 are not used. Thus, most of the data used come from two unpublished research studies on the Boliden Cu-Au-As deposit made by the au-thor and by U. Bergström in 1993–1994 (Hallberg 1994, Bergström 1994). Some of the data have been published in Bergman Weihed et al. (1996), all other data are given in Appendix A and B.

Most of the samples in series B (B001–B205, Hallberg

Rock classification, magmatic affinity, and hydrothermal alteration at Boliden, Skellefte district, Sweden – a desk-top approach

to whole rock geochemistry

Anders Hallberg

94 A. HALLBERG

1994) come from altered rocks within the Boliden altera-tion zone and are sampled with the purpose to investigate internal zoning patterns. The sampling, with the excep-tion for outcrops and archive hand specimen samples, was done by picking rock chips from drill core in order to get a representative sample. Sectioning has in most cases been guided by geological logging of the drill core. Outcrop and archive samples were cut by diamond saw and ana-lysed. All samples were analysed at XRAL, Canada, using XRF, ICP, and INAA on crushed and milled powder. The first batch of samples (B001–B073) was pulverised using a tungsten carbide ring mill whereas the second batch (B151–B205) was milled in a Cr-steel ring mill. The Co-contamination from tungsten carbide and the Cr-contam-ination from the Cr-steel ring mill is estimated at 60–80 ppm for each element, and can be seen in Appendix A.

Samples 91125 to 951517 (Bergström 1994) come

from less altered rocks from the vicinity of the Boliden deposit. Sampling procedures, sample preparation, and analytical work differ somewhat between the samples. In most cases these samples consist of hand specimen sam-ples from outcrops and approximately 15 cm drill core sections from underground samples. The samples have been analysed at XRAL using XRF and NA and at SGAB, Luleå using ICP, see Appendix B.

The approach

The data treatment reported in this paper chiefly follows the technique described by MacLean & Barrett (1993) and references therein.

Most of the chemical data used in this study relate to rocks that have been mapped as altered or strongly altered. It has not been possible to determine the precursor to

Renström

Petiknäs

Åkulla

Kankberg

Boliden

Långdal

Björkdal

Åkerberg

Långsele

Kedträsk

Åsen

7200000

1700

000

5 km

Younger granits

Early orogenic mafic intrusions

Early orogenic granitoid intrusions

Mafic metavolcanic rocks

Felsic metavolcanic rocks

Metasedimentary rocks

Fig. 1. Geological map of map sheet 23K Boliden SV (Lundström & Antal 2000) with airborne magnetics (200 m line distance, 30 m ground clearance, data from the Geological Survey of Sweden) in the background. Large red and yellow dots show the position of sulphide and gold mines, respectively. Small black dots show the position of sulphide and gold prospects. Data from the Geological Survey of Sweden, Mineral deposit database.


these rocks in hand specimen with a few exceptions. Even among rocks mapped as least altered, and thus considered to have a primary composition, a more detailed analysis reveals evidence of alteration. For example, only four out of the eight analyses of ”least altered rocks near Boliden” presented in Bergman Weihed et al. (1996) plot within the igneous spectrum of Hughes (1973), which means that they have been affected by alkali alteration.

The initial steps in the procedure described by MacLean and Barrett (1993), e.g. mapping and core log-ging, have turned out to be difficult and equivocal. There-fore the techniques presented by MacLean and Barrett (1993) have been used in a somewhat different order and some modifications of the methods have been done. The modified work plan for single precursor and multiple pre-cursor systems are outlined in Table 1.

Altered rocks that host the Boliden deposit consist of andalusite schist, sericite schist, chlorite schist, and mix-tures between two or more types. The occurrence of anda-lusite rocks and locally also rutile rocks indicates that al-teration at Boliden was more intense than elsewhere in the Skellefte district. Due to pervasive alteration and penetra-tive deformation, the precursor rocks within the alteration zone are impossible to identify, with one important excep-tion, the quartz porphyritic rock that makes up a signifi-cant part of the north-western wallrock.

Table 1. Work plan for single and multiple precursor systems.

Single precursor system1. If possible, identify a single precursor system, that is a single

rock unit that can be followed from the least altered state into strongly altered states.

2. Check the single precursor chemical data for immobile and in-compatible elements.

3. Identify precursor composition of single rock and calculate mass change during alteration.

Multiple precursor system4. Search the database for least altered rocks.5. Search the chemical data of all rocks for immobile and incompa-

tible elements.6. Construct a fractionation curve using immobile incompatible-

compatible element pair.7. Use fractionation curve and least altered rock compositions to

calculate mass changes during alteration.

Single precursor system – the quartz porphyries around Boliden

The rock

Quartz porphyritic rocks (qz-p) in the Boliden area occur as two major stocks, the Boliden stock and the northern stock, and several narrow quartz porphyritic dykes (Berg-man Weihed et al. 1996). The major characteristic of the rock type, already indicated by its name, is bluish, opal-escent quartz phenocrysts that make up a significant part of the rock. In less altered rocks, the phenocrysts are host-

ed by a fine-grained, greyish matrix of quartz, feldspar, and mica, mainly biotite (Nilsson 1968). What makes the qz-p so unique and so useful for geochemical studies is the fact that the quartz phenocrysts remained unaffected during hydrothermal alteration. Even in strongly altered quartz-sericite schist the qz-p precursor can be identified by the quartz phenocrysts. This was recognised more than 50 years ago (Ödman 1941) and quartz phenocrysts in strongly altered rocks have since then been used to map the Boliden qz-p stock in underground exposures and in drill cores down to 600 m depth.

The Boliden qz-p stock makes up a significant part of the northern wall rock to the Boliden Cu-Au-As ore. At the surface, the rock occurs in a weakly altered state (Bergman Weihed et al. 1996) while at depth, and within the alteration zone, it consists of chlorite-sericite schist, sericite-quartz schist, and locally andalusite-bearing seric-ite schist (Nilsson 1968). By extrapolating the southern contact of the qz-p stock from mine maps (Ödman 1941) towards the surface, it can be shown that surface expo-sures of qz-p occurring immediately north of the Boliden mine are part of the same stock. Outcrops of the northern stock, on the other hand, show no visual evidence of al-teration (Bergman Weihed et al. 1996). The fact that qz-p rocks in different states of alteration are exposed and avail-able for sampling makes it highly suitable for alteration studies.

Based on visual examination of the qz-p rocks in outcrop and in drill core, they have been divided into least altered, altered, chlorite±sericite altered, and sericite±andalusite altered. The chemistry of the 22 quartz porphyry rock samples is shown in Table 2. Least altered rocks are the well preserved rocks from the northern stock, altered rocks are found in outcrops of the Boliden stock, whereas chlorite±sericite and sericite±andalusite altered rocks occur at depth.

Alteration

Figure 2 shows all qz-p data plotted in Hughes igneous spectrum (Hughes 1973). This graph is used here to rec-ognise samples with an anomalous alkali distribution, i.e. altered samples. All samples that plot to the right of the ig-neous spectrum by Hughes are considered to be altered in some way whereas the samples that plot within the spectrum are classified as least altered samples. As can be seen from the diagram in Figure 2, only one (sample 92026, Table 2) of the samples from the northern stock falls within the igneous spectrum. The other sample from the northern stock (sample 92027, Table 2) shows a weak K-alteration. In the altered samples from outcrops of the Boliden qz-p stock, most of the Na has been leached and the K is enriched. Total alkali is, however, nearly the same

96 A. HALLBERG

as in the least altered sample. The alteration could be due to a replacement of Na by K in the fine-grained matrix of the qz-p rock – a K-feldspar alteration. At least some of the K-enrichment is, however, due to sericite alteration

since sericite-filled veins can be seen in the rock. Whether this K-alteration is related to the hydrothermal alteration beneath the Boliden deposit is not known.

For a distance of about 500 m from outcropping Boli-

Table 2. Whole rock geochemistry of the quartz porphyries in the Boliden area. Data from Hallberg (1994) and Bergström (1994). More information on analytical methods and laboratories used in Appendix A–B.

sample # B016 B022 B033 B034 B035 B036 B038 B039 B049 B050 B051 B052 B053 B054 B075 B157 91124 92028 92026 91125 92027 951504

bh 75 99 100 100 100 100 100 320 322 322 322 322 322 322 outcrop outcrop 30 outcrop outcrop 30 outcrop 97core (m) 47,0–51,3 0,0–15,0 0,0–15,7 15,7–34,0 34,0–50,9 50,9–66,5 68,7–87,4 0,0–15,7 0,0–18,3 18,7–33,8 33,8–41,7 41,7–44,2 44,2–50,1 50,1–54,4 573,6–574,0 585,1–585,4 69,9–70,1

level (m) 330 410 410 410 410 410 410 570 570 570 570 570 570 570 424 429 90N (RT90) 7204331 7204297 7204318 7204335 7204353 7204369 7204388 7204282 7204303 7204320 7204332 7204337 7204341 7204346 7204650 7204650 7204337 7204650 7205140 7204344 7205150 -

E (RT90) 1716531 1716552 1716551 1716550 1716549 1716549 1716548 1716585 1716584 1716583 1716583 1716582 1716582 1716582 1716200 1716200 1716523 1716330 1715670 1716516 1715730 -

SiO2 79,9 80,7 77,5 82,4 77,4 74,0 76,2 81,2 78,5 80,8 78,7 84,2 77,5 75,5 74,9 72,7 72,7 75,3 73,0 73,6 71,9 70.8

Al2O3 11,5 15,2 16,8 13,9 11,1 11,9 12,2 14,8 17,6 13,4 13,0 11,1 12,5 11,9 12,3 12,4 11,7 11,4 12,7 12,8 13,0 12.3

CaO 0,1 0,1 0,1 0,0 0,6 0,1 0,4 0,0 0,1 0,1 0,1 0,1 0,1 0,1 2,6 3,2 0,1 3,0 2,4 0,2 2,6 0.1

MgO 0,3 0,2 0,2 0,1 2,1 2,8 1,7 0,2 0,2 0,3 1,6 0,2 1,6 2,8 1,8 1,7 3,7 1,7 1,8 2,7 1,8 5,4

Na2O 0,50 0,29 0,38 0,26 0,18 0,24 0,33 0,17 0,07 0,31 0,29 0,25 0,23 0,18 0,22 0,29 0,16 0,24 2,80 0,23 1,85 0.21

K2O 2,77 1,43 1,31 0,96 2,04 2,08 2,73 0,96 0,31 2,14 2,88 2,83 2,81 2,07 4,05 4,09 1,69 3,15 2,12 2,24 2,75 1.66

Fe2O3 2,3 <0,01 <0,01 <0,01 4,0 4,8 3,8 <0,01 <0,01 0,1 1,5 0,2 2,0 5,0 3,0 3,4 6,2 2,9 3,7 5,0 3,4 5,0

MnO <0,01 <0,01 <0,01 <0,01 0,04 0,02 0,02 <0,01 <0,01 <0,01 0,01 <0,01 0,01 0,04 0,06 0,07 0,06 0,11 0,08 0,05 0,12 0.13

TiO2 0,23 0,32 0,37 0,26 0,24 0,30 0,27 0,34 0,32 0,28 0,23 0,21 0,26 0,24 0,22 0,24 0,22 0,25 0,28 0,26 0,30 0,25

P2O5 0,05 0,07 0,08 0,04 0,05 0,05 0,06 0,06 0,05 0,07 0,06 0,04 0,06 0,06 0,05 0,06 0,10 0,06 0,07 0,09 0,07

LOI 2,50 1,90 3,45 2,30 2,60 2,60 2,75 2,40 2,95 2,50 0,47 1,40 1,75 2,35 0,85 0,95 3,42 1,30 1,20 2,99 1,15 3.4

SUM 100,2 100,1 100,2 100,2 100,4 99,0 100,4 100,1 100,1 100,0 98,9 100,5 98,8 100,3 100,0 99,0 100,0 99,4 100,1 100,2 99,0 99,2

Cu 142 3,2 1,1 1,0 1,4 4,8 76,5 1,2 1,3 0,5 16,2 0,6 2,4 3,3 8,4 12,1 1

Zn 78,2 7,1 6,7 6,2 30,2 58,4 274 6,3 4,2 4,3 36,2 3,1 13,7 21,8 48,5 32,7 32 149 68 31 58 134

Pb 21 <2 <2 <2 <2 <2 32 <2 <2 <2 <2 <2 <2 <2 <2 3 11 19

Cd <1 <1 <1 <1 <1 <1 1 <1 <1 <1 <1 <1 <1 <1 <1 <1

Au 22 23 23 16 <5 <5 <5 150 <5 <5 7 <5 6 5 11 14

Ag 1,6 0,3 0,6 <0,1 0,3 0,6 0,6 0,6 0,8 0,6 0,7 0,6 0,3 1,0 0,6 0,1

As 31 2 11 340 3 3 11 <2 <2 2 2 3 <2 2 7 6 6 12

Sb 0,7 0,3 0,2 0,5 0,2 0,5 0,8 0,2 0,2 0,3 0,3 <0,2 0,3 <0,2 0,6 0,7

Bi 6 <3 4 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 5 <3 <3

Br <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 1 <1

Co 38 37 28 53 29 25 17 34 30 36 35 31 30 26 24 <5 5 4 5

Ni <1 1 3 1 <1 <1 3 <1 5 1 1 <1 2 1 1 5 7 15

Cr 8 2 <1 <1 4 3 4 1 3 1 1 <1 1 3 2 116 5 11

Mo 1 <1 <1 <1 2 <1 <1 2 <1 <1 <1 <1 <1 <1 <1 1

V 11 19 18 6 6 23 14 20 14 19 8 6 7 7 8 8 5

Ba 402 92 128 111 557 506 606 70 <50 250 550 599 662 597 796 845 554 385 387 831 506 442

Cs <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3

Rb 49 30 28 17 32 27 44 22 21 36 40 48 43 29 53 32 11 30 36 20 43 29

Sc 12,2 11,2 12,0 13,8 9,1 12,5 11,9 12,9 10,5 16,4 14,8 14,2 13,2 11,4 10,9 10,3 8,3 9,3 11,0 8,6 11,0

Sr 26,3 18,8 30,6 19,4 15,1 14,5 29,8 17,7 11,1 24,7 16,6 15,6 16,4 11,7 133,0 143,0 11 65 65 19 79 13

Be 1,0 <0,5 <0,5 <0,5 0,9 1,0 1,1 <0,5 <0,5 <0,5 0,6 <0,5 0,7 1,1 1,2 0,9

Hf 4 5 5 4 4 4 4 5 6 3 4 3 4 4 4 4 5,0 5,0 5,0

Sn <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10

Ta <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1

Nb <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 10 12 11

Th 4,3 5,5 5,9 4,1 3,5 3,8 4,2 5,2 5,3 4,4 4,2 3,9 4,5 4,1 4,9 4,5 3,3 4,1 4,1

U 2,8 3,5 3,0 2,6 2,1 2,2 2,9 3,0 3,0 2,6 2,6 2,1 2,4 2,4 2,4 2,6 2,3 2,0 2,2

Y 6,5 5,6 7,3 7,2 7,9 6,9 7,3 7,1 6,7 7,4 8,9 6,9 6,4 6,5 18,3 20,1 26 21 24 34 19 27

Zr 165 197 211 189 154 156 164 202 230 189 174 159 176 158 162 159 148 158 171 159 178 170

La 23 38 39 27 21 24 26 34 35 31 29 23 32 23 26 26 21 24,0 26 19,8 28,0

Ce 47 76 80 54 45 50 52 70 73 64 61 50 66 52 54 54 50 52 46 53

Nd 20 30 30 20 20 20 20 30 30 30 30 20 30 20 30 20 25 26 29 27

Sm 4,1 6,1 7,4 4,9 4,1 4,5 5,1 6,4 6,6 5,8 5,9 4,7 5,8 4,7 5,0 5,0 4,4 4,9 6,67 4,9

Eu 0,9 1,0 1,8 0,9 0,6 0,7 0,5 1,1 1,3 1,2 1,4 1,1 1,1 0,7 1,5 1,1 1,4 0,8 0,95 1,1

Tb <0,5 0,7 0,7 0,5 0,5 0,5 0,6 0,7 0,8 0,6 0,9 0,7 0,7 0,5 0,7 0,5 0,6 0,6 0,8

Yb 2,4 1,8 2,3 2,6 2,6 2,3 2,6 2,4 2,3 2,0 4,0 3,7 3,3 2,6 2,6 2,7 3,2 2,3 2,9 3,72 2,7

Lu 0,34 0,26 0,34 0,38 0,37 0,33 0,36 0,34 0,34 0,32 0,51 0,47 0,44 0,37 0,40 0,42 0,34 0,35 0,34

stock BS BS BS BS BS BS BS BS BS BS BS BS BS BS BS BS BS BS NS NS NS BS

Alt. SA SA SA SA CS CS CS SA SA SA CS SA CS CS A A CS A LA CS LA CS

BS: Boliden stock, NS: Northern stock, SA: sericite±andalusite altered, NS: chlorite±sericite altered, A: altered, LA: least altered


den qz-p to the closest underground drill core sample of qz-p rock, sample B038 from the 410 m level, there is a large area from which there are no data.

The samples from underground drill cores differ sig-nificantly from the relatively weakly altered surface expo-sures. They consist of chlorite rocks, chlorite-sericite rocks and sericite rocks, locally andalusite-bearing. Since these rocks are part of the alteration zone under the Boliden de-posit, they have most likely been formed by an ore-relat-ed alteration. The K/Na ratio is broadly the same as for the less altered outcrops but the total alkali has decreased significantly. This means that also K is leached from the rock which suggests that there is a tendency towards total destabilisation of K-bearing silicates and a total removal of alkali from the system. In the most altered qz-p sample in this study, sample B049 from the central parts of the alteration zone, the K2O content is as low as 0,3 % and Na2O is close to the detection limit. This confirms previ-

ous observations of strong hydrothermal leaching in the more central parts of the alteration zone (Nilsson 1968, Bergman Weihed et al. 1996).

The effect of alteration on the other major elements is shown in the spider diagram in Figure 3. Data for altered samples have been normalised to the least altered sample (sample 92026, Table 2). Both chlorite±sericite schist and sericite±andalusite schist show a strong depletion in Ca and a higher loss of ignition (LOI).

The chlorite±sericite schist is strongly enriched in Mg and Fe due to the high chlorite content whereas the relatively chlorite-poor sericite±andalusite schist shows a strong depletion in those elements. Fe and Mg vary with the chlorite content in the samples indicating that chlorite is the major host mineral for these elements. The strong correlation between Mg and Fe (Table 3) is also due to the strong affinity to chlorite. With increasing chlorite content the Fe and Mg increases proportionally. Evident-

0 20 40 60 80 1000

2

4

6

8

10

sericite±andalusite alteredchlorite±sericite alteredaltered (outcrop)

weakly alteredleast altered

Na 2

O+

K2O

(w

t%)

K2O/(Na2O+K2O)x100

Na-altered

K-altered

unaltered

Northern qz-p stock Boliden qz-p stock

Fig. 2. Data for quartz-porphyries from the Boliden area plotted in the igneous spectrum, Hughes (1973).

-2,0

-1,5

-1,0

-0,5

0,0

0,5

LOIP2O5TiO2MnOFe2O3K2ONa2OMgOCaOAl2O3SiO2

log10

(alte

red

/leas

t alte

red)

Fig. 3. Spider diagram showing the major element com-position of altered quartz-porphyry rocks normalised to the least altered quartz-porphyry (sample 92026, Table 2). Same colours as for the dots in Figure 2.

98 A. HALLBERG

ly, chlorite was not a stable mineral in the more central parts of the alteration zone. Note also the small but sig-nificant Al-increase for sericite±andalusite altered samples (Fig. 3).

Less mobile and mobile elements

Of particular interest is the strong correlation between a class of elements often referred to as immobile or less mo-bile, e.g. Al2O3, TiO2, V, Th, U, LREE, and Zr shown in Table 3. Most of these elements are high field strength (HFS) elements and their resistance to hydrothermal al-teration has been well documented (MacLean & Barrett 1993 and references therein). Al and Ti are often reported as enriched or depleted in hydrothermally altered rocks (e.g. Nilsson 1968), but these changes are almost always due to residual enrichment or dilution. It is noteworthy that some HFS elements are strongly affected by hydro-thermal alteration at Boliden. The content of Nb, for ex-ample, drops below the detection limit (10 ppm) and the Y content is reduced by 70 percent in the most altered qz-p samples from Boliden. The HREE seem also to have been affected by the alteration. Thus, there is no reliable ”rule of thumb” to identify immobile elements. This must be carefully checked for every new set of data.

For the qz-p at Boliden it seems, however, that the relative proportion of Al2O3, TiO2, V, Th, U, LREE, and Zr remains constant during alteration. The content of se-lected less mobile and mobile elements in least altered to strongly altered qz-p is shown in Figure 4. Al2O3 and Zr show a nearly identical pattern independent of the degree of alteration, in agreement with the high correlation coef-ficient of 0,95 between the elements (Table 3). Th, TiO2 and some LREE show a similar pattern, and a good cor-relation with Al2O3 and Zr. The behaviour of more mobile elements is illustrated by K, Mg, and the HFS-elements Y and HREE, exemplified by Yb (Fig. 4). The fact that some elements are unaffected by hydrothermal alteration make them suitable for rock classification and mass change cal-culation.

Mass changes during alteration

Among the demonstrated less mobile or immobile ele-ments, Al, Ti, and Zr are most useful for mass change cal-culations, mainly because the content of each of these ele-ments is well above the detection limit. In the Al2O3 vs. Zr diagram (Fig. 5) the variation in Al and Zr content in different qz-p samples is illustrated. All samples plot along a line with the least altered samples in the middle of the population. Together, the plotted data form an alteration line (MacLean & Barrett 1993) with a nearly constant Al2O3/Zr-ratio at around 745±60. Samples with higher

Table 3. Matrix of correlation coefficient for the quartz porphyries. Calculated from the data in Table 2.

n=22 SiO2 Al2O3 MgO Na2O Fe2O3 MnO TiO2 V Nb Th Y Zr La Ce SmSiO2 - Al2O3 0.27 - MgO -0.80 -0.49 - Na2O -0.33 -0.05 0.01 - Fe2O3 -0.83 -0.63 0.88 0.13 - MnO -0.82 -0.39 0.70 0.40 0.64 - TiO2 0.09 0.81 -0.37 0.17 -0.44 -0.24 - V 0.11 0.50 -0.33 0.17 -0.32 -0.46 0.81 - Nb -0.53 -0.17 0.26 0.81 0.30 0.81 0.07 o - Th 0.21 0.84 -0.55 -0.15 -0.59 -0.45 0.66 0.46 -0.36 - Y -0.74 -0.27 0.63 0.29 0.62 0.78 -0.26 -0.41 0.78 -0.27 - Zr 0.42 0.95 -0.59 -0.02 -0.77 -0.43 0.78 0.49 -0.15 0.78 -0.41 - La 0.36 0.85 -0.65 -0.03 -0.75 -0.44 0.77 0.58 -0.18 0.88 -0.47 0.88 - Ce 0.34 0.86 -0.59 -0.17 -0.73 -0.48 0.73 0.51 -0.27 0.88 -0.42 0.87 0.98 - Sm 0.15 0.82 -0.34 -0.18 -0.48 -0.38 0.66 0.46 -0.26 0.86 0.01 0.73 0.72 0.79 -

B049

B039

B034

B033

B022

B052

B050

B016

B053

B051

B038

#91125

B054

B036

B035

#951504#91124B

157

B075

#92028

#92027#92026

10 ppm

100 ppm

1000 ppm

1 %

10 %

Yb

La

Zr

YTh

TiO2

K2O

MgO

Al2O3

Fig. 4. Spider diagram showing the content of less mobile elements in least altered and altered quartz-porphyries. K2O and MgO are in-cluded to illustrate the behaviour of typically mobile elements. Note the mobile behaviour for Y in most of the chlorite+/-sericite samp-les and all of the sericite+/-andalusite samples. Colour bar at the base of the diagram indicates type of alteration and uses the same colours as in Figure 2 and 3.


Al and Zr contents than the least altered samples have been affected by residual enrichment of immobile ele-ments when several of the mobile major elements have been leached from the rock. The rock has been affected by net mass loss. In other samples, the Al and Zr contents have decreased due to dilution by added elements. Here the rock has experienced a net mass gain. The purpose with mass change calculations on the chemical data is to quantify the net mass gain or loss and to determine the magnitude of change for each mobile element.

The mass change is the change in content of a mobile element, expressed in %-units, in an altered sample rela-tive to the unaltered precursor. Since the precursor com-position of the altered samples is not known, the composi-tion of the least altered sample is used. The first step in mass change calculations is to determine the reconstruct-ed compositions (RC). The reconstructed compositions for each altered sample is the composition where the im-mobile element content in the altered sample is the same as in the least altered sample (MacLean 1990). The re-constructed compositions should not be confused with a chemical analysis of the rock samples. It is simply a math-ematical construction that can, depending on degree of alteration, deviate considerably from 100 %. The recon-structed composition is here reported as %-units in order not to be mixed-up with the common way of reporting chemical data.

RC= Xalt.rock. (IMMleast alt./IMMalt)

RC is the reconstructed composition, Xalt.rock is the content of an element in the altered rock, and (IMMleast alt./IMMalt ) is the ratio between less mobile element content in least altered and altered rock respectively

Mass changes during alteration are the differences be-tween the reconstructed composition and the composi-tion of the precursor. The composition of the least altered qz-p sample represents the precursor composition.

Mass change=RC – precursor composition

Mass change is also reported as %-units for the same reasons as for the reconstructed composition.

Figure 6a shows the major element chemistry for the least altered sample from the Northern stock, for the mildly altered sample from the same stock, from the outcropping Boliden qz-p, and for chlorite±sericite and sericite±andalusite rocks at depth. The reconstructed com-position is shown in Figure 6b. In the calculation, Al has been used as immobile element but the use of Zr or Ti as immobile element does not result in any significant differ-ences. Finally, the calculated mass changes are shown in Figure 6c.

For the majority of the samples, it is the SiO2-content that controls the total mass changes during alteration. The net mass change of Si, expressed as SiO2 %-units, varies between –16 %-units and +23 %-units in sericite±andalusite altered rocks. It is also obvious from the dia-gram in Figure 6c that the samples mapped as sericite± andalusite altered could be subdivided into samples with a net gain and a net loss of Si. Samples with a net addition of Si are also enriched in K. Mass changes of Si seem thus to be related to changes in K.

It should be noted here that Nilsson (1968), by meas-uring the amount of quartz phenocrysts in variously al-tered qz-p, argued that there had been no major change of volume during alteration. MacLean and Barrett (1993) also noted that a large mass change not necessarily results in any drastic change in volume.

Classification diagrams

A useful feature concerning less mobile or immobile ele-ments is that they are used to classify rocks. Classification diagrams by Winchester and Floyd (1977) utilise the ra-tios between Zr, Y, Nb, and TiO2. Other rock classifica-tion diagrams are partly or fully based on major elements, several of them known to be mobile during hydrothermal alteration, e.g. diagrams by LeMaitre et al. (1989), Jensen (1976), and De la Roche et al. (1980). All of these dia-grams are frequently used for rock classification.

For further geochemical discussion it is useful to in-vestigate how the qz-p rocks, least altered to strongly al-tered, plot in various classification diagrams. For this pur-pose the qz-p samples are plotted in five different classifi-cation diagrams in Figure 7a–e. The igneous spectrum by Hughes (1973), also a form of rock classification diagram, has been discussed previously. It should be noted, how-ever, that these diagrams are designed for unaltered and non-porphyritic igneous rocks. The results gained from plotting data for qz-p rocks, most of them strongly al-tered, should be interpreted with uttermost precaution. It can, nevertheless, be informative to see how altered and

0 150 200 250

0

12

16

20

Al 2

O3

(%)

Zr (ppm)

leastaltered

mass gain mass loss

Fig. 5. Al2O3 vs. Zr diagram to illustrate the positive correlation bet-ween these elements. Equation for the line is Al2O3 (%)=0.0788 Zr (ppm) - 0.7318 and R=0.95. The largest mass loss is experienced by sericite±andalusite samples while most of the chlorite±sericite is affected by mass gain. Same symbols as in Figure 1.

100 A. HALLBERG

less altered samples behave in these frequently used dia-grams.

The name quartz porphyry simply indicates that the rock contains quartz phenocrysts, it does not say anything about the rock composition, except that it is a fairly felsic rock since it contains free quartz. When plotted in di-agrams using SiO2 as a variable, the least altered qz-p sample plots close to or inside the rhyolite field. Graphs not using SiO2 as a variable rather suggest that the qz-p has a dacitic to rhyodacitic composition. Apparently, the quartz phenocrysts give the rock a higher SiO2-content than would be expected if the rock was non-porphyritic and illustrates why porphyritic rocks easily give erratic

results when plotted in classification diagrams (see i.e. Hughes 1973). In the diagrams not using SiO2 the rock has consistently a dacitic to rhyodacitic composition.

The altered samples show a large scatter when plotted in the classification diagrams. Beside the disturbance due to quartz phenocrysts, also residual enrichment of Si, di-lution of the Si-content, or a net addition or removal of Si strongly affects graphs with SiO2 as variable. In most cases, the altered samples will appear to be more felsic in composition. Absence or presence of chlorite, with net re-moval or addition of Mg and Fe, will indicate a too felsic or too mafic composition, respectively, in the AFM dia-gram (Jensen 1976). The Mg and Fe contents seem also

70

80

90

100

B049

B039

B034

B033

B022

B052

B050

B016

B053

B051

B038

#91125B

054B

036B

035#951504#91124B

157B

075#92028#92027#92026

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B049

B039

B034

B033

B022

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#91125B

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036B

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157B

075#92028#92027#92026

-25

-15

-5

5

15

25

-25

-15

-5

5

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25

LOIMnOFe2O3

K2ONa2O

MgOCaOAl2O3

SiO2

B049

B039

B034

B033

B022

B052

B050

B016

B053

B051

B038

#91125B

054B

036B

035#951504#91124B

157B

075#92028#92027#92026

a) Original composition (%) b) Reconstructed composition (%-units)

c) Mass change (%-units)

60

mass gainmass loss

Fig. 6. Columnar diagrams to illustrate the mass change calculations discussed in the text. a) Original whole rock geochemistry of the 22 qz-p samples. Average sum of major element oxides is 99.8+0.6/-1.0. The samples have not been normalised to 100 % before mass change calculations.b) Reconstructed composition (RC) of qz-p samples. To calculate the RC, the whole rock geochemical data for all altered samples have been multiplied with a factor so that the immobile element content is the same as for the least altered sample. For the calculations, Al has been used as immobile element. This is shown in the diagram as an identical Al2O3 content of all samples. Samples with a RC larger than 100 %-units have experienced mass gain whereas samples with RC less than 100 %-units show mass loss, compare Figure 5. Note that the RC is expressed in %-units in order to avoid confusion with ordinary whole rock geochemical data.c) Mass change during alteration. The mass change is the difference between the RC and the composition of the precursor rock. Here the least altered qz-p is used to represent precursor composition. The mass changes for the least altered samples are, by definition, zero. Note that several samples showing total mass gain have experienced a large mass loss of Na and Ca.


SiO

2 (w

t.%)

SiO2 (wt.%)

50

60

70

80

Zr/TiO2

Zr/

TiO

2

subalkalinebasalt

andesite

rhyodacitedacite

rhyolite

trachyte

trachy-andesite

0,01 0,10

40 50 60 70 800

2

4

6

8

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14

Na 2

O+

K2O

(w

t.%)

basalt

basalticandesite

andesitedacite

Fetot+Ti

MgAl

THOLEIITE

CALC-ALKALINE

KOMATIITE

rhyolitedacite

andesite

basalt

0 1000 2000 30000

1000

trachy-andesite

dacite

rhyodacite

alkali rhyolite

andesite

basalt

rhyolite

latiandesitelatite

2000

a) b)

c) d)

e)

n=22 n=3

Nb/Y

1,00

rhyolite

rhyodacitedacite

andesite

andesite-basalt

subalkalinebasalt

trachy-andesite

alkali-basalt

trachyte

0,01

0,10

0,1 1,0 10

rhyolite

R2

R1

R1=4Si–11(Na+K)–2(Fe+Ti)R2=6Ca+2Mg+Al

Fig. 7. Data for the qz-p rocks at Boliden plotted in different rock classification diagrams. Same symbols as in Figure 2. a) SiO2 vs. Zr/TiO2 (Winchester and Floyd 1977). b) Zr/TiO2 vs. Y/Nb (Winchester and Floyd 1977). c) The total alkali vs. SiO2 (TAS) (LeMaitre et al. 1989). d) AFM ternary diagram (Jensen 1976) and modified by Rickwood (1989). e) R1R2 diagram in which R1=4Si-11(Na+K)-2(Fe+Ti), and R2=6Ca+2Mg+Al, all elements as cation proportion (De la Roche et al. 1980).

102 A. HALLBERG

to affect how the rock plots in an R1–R2 diagram (De la Roche et al. 1980). Notable is that even the weakly al-tered samples can result in false rock classifications. This is important to consider for anyone using classification dia-grams.

The fact that altered samples, also weakly altered ones, result in false rock classification makes it possible to use classification diagrams to distinguish least altered samples. Altered samples usually give different results depending on which classification diagrams that are used, whereas the least altered sample should, with the exception of phe-nocryst-bearing rocks, result in identical rock classifica-tions.

Multiple precursor systems – rocks from the Boliden area

The first part of the study was focused on one well-de-fined rock type, the quartz porphyries around Boliden. Due to the occurrence of quartz phenocrysts in the rock, a mineral that remains stable despite strong hydrothermal alteration, the precursor rock can be easily identified even in strongly altered rocks. The element changes that take place during alteration were determined, qualitatively as well as quantitatively, but also immobility for some ele-ments were demonstrated. Various rock classification dia-grams indicate that the qz-p has a dacitic to rhyodacitic composition. Altered qz-p samples plotted in the diagrams yield varying rock compositions, an effect that can be used to discriminate altered samples and to identify the least altered samples.

For the other country rocks and rocks in the alteration zone around the Boliden deposit, no simple trace mineral like quartz-phenocrysts exists. However, with the knowl-edge gained from the qz-p data it is possible to expand the study to other rocks in the Boliden area, rocks for which the precursor is unknown.

Least altered rocks

In this study, most of the samples come from the altera-tion zone beneath the Boliden ore. None of these samples, mapped as mineralised rocks, sericite rocks, quartz rocks, chlorite-bearing rocks, andalusite-bearing rocks etc., can be considered as least altered samples. Several of the sam-pled country-rocks around the Boliden deposit have been mapped as altered (Bergman Weihed et al. 1996) and these samples must also be excluded in the search for least altered samples. Among the 267 analysed samples in the database (Appendix A–C), about 135 show visible evi-dence of alteration, according to the descriptions. Addi-tionally c. 20 samples are excluded on geological reasons. These rocks include the so-called lamprophyre dykes,

which are younger mafic dykes that intrude ores and rocks at Boliden. The age of these dykes and the genetic rela-tionship to mineralization and alteration at Boliden is un-clear. In addition, they do not make up any significant portion of the rocks at Boliden. Metasedimentary rocks, chemical sediments, tourmaline veins, and ore samples cannot be used since they do not follow igneous fractiona-tion trends on which the discussion below is based.

A second step in the search for least altered rocks is to exclude all samples that show chemical evidence of al-teration. For that purpose, the remaining samples have been plotted in Hughes igneous spectrum (Hughes 1973). There are 52 samples that show K-alteration and 16 sam-ples that are Na-enriched. This procedure thus eliminates another 68 samples from the database and leaves 44 sam-ples as possibly least altered samples.

There is no simple way, like the Hughes igneous plot, to identify disturbances among the other major elements. However, by plotting the data in the different rock clas-sification diagrams and comparing the result from the dia-grams it is possible to distinguish samples with disturbed major element contents, i.e. altered samples. It turns out that only eight samples give a reasonably similar result in-dependent of classification diagram used. These samples, shown in Table 4, are considered to be the least altered rocks in the Boliden area. They will be used to define fractionation curves and precursor rocks, and to calculate mass changes in the altered rocks. Note that the least al-tered qz-p samples are not included in these eight samples due to the occurrence of quartz phenocrysts in the rock.

Fractionation curve and precursor rocks

A large part of the rocks in this investigation are altered in such a way that precursor rocks cannot be visually iden-tified. Lithogeochemistry provides a tool to circumvent these obstacles, but before applying geochemical tech-niques, the immobility of used elements must be investi-gated. It has already been shown, for the qz-p rocks, that several elements are immobile during alteration and it is assumed that these elements behave in a similar way also when hosted in other rock types around Boliden. A ma-trix of correlation coefficients for all igneous rock, felsic to mafic, around Boliden is shown in Table 5. The matrix does not, however, show the same good correlations as the matrix for the qz-p rocks (Table 3). The most likely reason for the weak correlation between for example Zr and Ti is that these elements, beside being less mobile, also behave differently in fractionation processes. Due to the highly incompatible behavior for some trace elements, e.g. Zr, Th, and REE, they are strongly enriched in the melt dur-ing igneous fractionation and are thus enriched in late stage fractionation rocks, i.e. felsic rocks. Other elements,


e.g. Ti, behave in a more compatible manner during frac-tionation and will be enriched in the solid phase and de-pleted in the melt, i.e. in felsic rocks. Therefore, for a mul-tiple precursor system like the one at Boliden, which in-cludes mafic to felsic rocks, a good correlation between incompatible and less incompatible elements is not ex-pected. Of importance, however, is that Zr shows a rath-er good correlation with other highly incompatible, HFS elements, e.g. Th and LREE. This indicates that the ele-ments used to calculate mass changes etc. for the qz-p can be used also for other rocks around Boliden.

In Figure 8, the contents of Zr and TiO2 in all igneous samples from the Boliden area are plotted. The previous-ly discussed qz-p rocks show a distinct trend with high Zr contents and low TiO2 contents but with a Zr/ TiO2 ratio at 0.065±0.013. The qz-p rocks around Boliden can thus be identified on their unique Zr/ TiO2.

In the diagram (Fig. 8), the eight least altered sam-ples, together with variously altered samples, have been plotted. The least altered samples make up a trend that resembles a fractionation trend for igneous rocks. Ba-saltic andesites, with high TiO2 content and low Zr content, plot in the upper left part of the fractionation trend, more intermediate samples further down along the trend towards the most felsic end member, the qz-p. It should be noted that this does not necessarily mean that the rocks share magmatic affinity or that they are part of the same fractionation series. It simply indicates that they are following fractionation trends.

Altered samples have been divided into three groups: weakly altered samples (with no visible signs of al-teration and that plot within the igneous spectrum of Hughes 1973), samples that are K- or Na-altered according to Hughes (1973), and samples that are mapped as altered. In the diagram, the altered samples together with the least altered samples form trends or clusters, similar to the qz-p trend, but at different Zr/ TiO2 ratios. It is suggested that the Zr/ TiO2 ratio is unique for every igneous rock type in the Boliden area, in analogy with the qz-p samples. This is corroborated by the fact that each of the least altered samples has a unique Zr/ TiO2 ratio, low ratios for mafic rocks and high ratios for felsic rocks.

From this it is possible to determine the composi-tion of the precursor rock for every altered sample in the data set. The compositions of the least altered sam-ples can then be used to approximate the primary com-positions in mass change calculations. Note that the sampling from strongly altered rocks in most cases has been guided by degree of alteration. Therefore the sam-ples might contain a mixture of more than one precur-sor rock composition.

Chemical mapping

Different rock types, deduced from their Zr/TiO2 ratio, have been plotted on the geological map in Figure 9 and the geological profile in Figure 10. The division according to the Zr/TiO2 ratio is found in Table 6. The geology de-duced from whole rock geochemistry broadly agrees with the geological map and profile presented in Bergman Wei-hed et al. (1996, Figs. 3 and 4) and apparently the method of using the Zr/TiO2 ratio to classify rocks works well in this area.

Table 4. Whole rock geochemistry of the least altered samples found from the Boliden area. See text for explanation.

sample # 91101 91103 91104 92005 92022 92033 931008 Bo39

bh 686 686 686 30 Giller 428

vattnet 2

core 262,5- 262,9 352,6- 353,0 390,3- 390,7

253,9-254,2 217,0 129,0- 129,4

local n -154 -5096 -235

local e 66 -1634 7

z 188 188 170

N (RT90) 7204892 7204914 7204923 7203950 7204144 7204893 7204103 7209260

E (RT90) 1717035 1716999 1716984 1715270 1716615 1714480 1716543 1718200

SiO2 56,1 55,2 56,6 59,7 61,1 68,7 69,4 73,2

Al2O3 17,0 17,3 17,6 15,1 15,4 13,6 14,6 13,2

CaO 5,2 7,2 6,7 6,8 6,6 2,5 3,6 3,1

MgO 5,2 5,0 4,4 2,4 3,4 1,2 1,2 0,8

Na2O 3,31 2,64 2,78 2,87 2,14 3,86 4,47 3,81

K2O 0,73 1,10 1,03 1,39 1,43 1,86 1,77 1,25

Fe2O3 9,7 10,3 9,7 8,0 8,5 4,1 4,2 3,1

MnO 0,09 0,16 0,14 0,16 0,10 0,12 0,10 0,09

TiO2 1,07 1,12 1,04 0,79 0,72 0,45 0,48 0,19

P2O5 0,28 0,12 0,17 0,23 0,28 0,09 0,15 0,05

LOI 2,75 2,02 1,85 2,70 0,75 1,30 0,50 1,25

SUM 101,5 102,2 102,0 100,1 100,5 97,9 100,4 100,0

Cu 75 190 47 29 31 1 17 <2

Zn 1125 115 123 87 96 92 60 71

Pb 10 <10 3 8 <2

As <3 7 2 12 6

Co 19 23 6 10

Ni 9 176 9 11 5 5 <2

Cr 42 6 12 150 180 10 82

V 302 345 311

Ba 151 283 202 543 344 508 365 310

Rb 43 27 40 26 25 32 28 25

Sc 27,0 32,0 30,0 25,5 24,5 16,0 16,5

Sr 341 328 390 555 414 81 176 116

Hf 2,0 2,0 3,0 3,2

Nb 9 10 11 9 10

Th 1,4 1,9 4,1 3,9 18

U 1,4 0,7 1,6 2,9 <2

Y 17 11 11 19 18 24 25 31

Zr 75 64 67 109 122 136 150 174

La 11,7 9,7 19 18,0 22,0 32,7

Ce 26 23 40 39 44 64

Nd 17,3 15,1 20 20 21 29

Sm 3,65 3,92 4,0 4,0 4,5 5,4

Eu 1,90 1,49 1,7 1,6 1,4 1,1

Tb 0,6 0,5 0,6 0,5

Yb 2,36 1,85 2,3 2,1 2,4 2,1

Lu 0,35 0,30 0,35

method-lab. ICP-Lul ICP-Lul ICP-Lul XRAL-92/XRF/NA XRF/NA XRF/NA XRF/NAXRAL-93-95/XRFRock Andesite Andesite Andesite Dacite, Dacite Dacite Dacite Rhyolite,

feldspar porphyritic porphyry volcaniclastic porphyry feldspar porphyritic

Alter., akt actinolite Preserved

struct.,

minerali-

sation

104 A. HALLBERG

The dacite unit in the uppermost part of the volcanic sequence on the southern side of the ore (Fig. 11) seems to be thinner compared to the geological profile in Bergman Weihed et al. (1996). The chemistry also indicates an an-desitic unit within the dacites. A c. 250 m thick sequence dominated by andesite with some basaltic andesite rocks occurs stratigraphically below the dacites. The chemistry shows a rather heterogeneous composition varying from dacite to basaltic andesite, with an andesitic average com-position. The chemistry thus suggests that this unit is somewhat more intermediate in composition compared to Bergman Weihed et al. (1996) who mapped the unit as basaltic andesite. The lowest exposed unit on the

southern side of the Boliden ore is a thick sequence of dacitic rocks with some andesitic intercalations. Accord-ing to the chemistry, this sequence is thicker than sug-gested in Bergman Weihed et al. (1996). This stratigraph-

0 100 200 300 400 5000,0

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weak alteration

least altered

basalt

rhyolite

dacite

andesite

basalticandesite

qz-p rocks

TiO

2 (%

)

Zr (ppm)

Fig. 8. TiO2 vs. Zr diagram with all igneous samples from the Boliden area. Altered rocks divided according to the stepwise elimination of altered rocks discussed in the text. Open red ellipses: samples mapped as altered Filled red ellipses: samples identified as altered in Hughes (1973) igneous spectrum.Filled blue ellipses: samples identified as altered by comparing different classification diagrams. Black ellipses: least altered samples. The basalt to rhyolite sectors defined from the least altered samples. Qz-p sam-ples within hatched line. The least altered samples define a trend resembling a fractionation trend.

Table 5. Matrix of correlation coefficient for all igneous rocks from the Boliden area. Chemical data from Appendix A–C.

n=239 Al2O3 TiO2 V Sc Th Zr La Ce Nd Sm Tb Yb LuAl2O3 - TiO2 0,81 - V 0,38 0,69 - Sc 0,39 0,68 0,82 - Th 0,36 0,10 -0,27 -0,25 - Zr 0,55 0,22 -0,24 -0,14 0,79 - La 0,58 0,34 -0,06 -0,05 0,68 0,60 - Ce 0,60 0,37 -0,05 -0,04 0,71 0,64 1,00 - Nd 0,65 0,45 -0,03 -0,01 0,74 0,69 0,95 0,97 - Sm 0,65 0,49 -0,03 0,01 0,76 0,72 0,90 0,93 0,98 - Tb 0,46 0,44 0,08 0,18 0,69 0,71 0,54 0,59 0,68 0,78 - Yb 0,11 0,01 -0,21 -0,05 0,34 0,37 0,25 0,27 0,29 0,35 0,59 - Lu 0,19 0,08 -0,20 -0,01 0,42 0,45 0,31 0,34 0,36 0,43 0,66 0,97 -

Table 6. Rock classification according to Zr/TiO2-ratio.

Rock classification log10(Zr/TiO2) Zr/ TiO2

rhyolite (incl. rhyodacite) >-1,40 >0,040(quartz porphyry) (-1,12 to -1,24) (0,075 to 0,057)dacite -1,41 to -1,59 0,039 to 0,026andesite -1,60 to -1,94 0,025-0,011basaltic andesite -1,95 to -2,24 0,011 to 0,006basalt <-2,34 <0,005


ic unit can be correlated with rocks in a drill hole about 5 km southwest of the deposit. The stratigraphical col-umn C in Allen et al. (1996) is partly based on that drill hole.

The rocks on the northern side of the deposit are more mafic in character, with the exception of the intrusive quartz-porphyry, which has a dacitic to rhyodacitic com-position. The uppermost part of the observed stratigraphy consists of andesites. These overlie a nearly 100 m thick sequence of basaltic andesites. The mapped dyke-like ba-salt-andesite that occurs north of the deposit (Bergman Weihed et al. 1996, Fig. 4 and Ödman 1941) cannot be distinguished by geochemistry. Evidently the rock is some kind of dyke but isochemical with its country rocks. Be-low the basaltic andesite, the few samples in the database indicate a unit with dacites and andesites. This unit can

be traced into the alteration zone. Further down in the stratigraphy the chemistry indicates an andesitic basalt composition of the rocks. A stratigraphy nearly identical to the one observed immediately north of the Boliden de-posit is found in a drill core about 1 km northwest of the deposit.

The 16 samples previously identified as Na-altered by plotting in Hughes igneous spectrum (Hughes 1973) all seem to occur in the upper part of the volcanic sequence at Boliden, about 60–80 meters below the contact to the overlying metasedimentary rocks. Na-altered samples at this stratigraphic position are found in a drill core (ddh 686) about 900 m from the Boliden deposit and in an outcrop a further 1100 m to the north-east. South-west of the deposit, Na-altered samples are found in drill core about 1 km from the deposit (ddh 678), in another drill

ddh 678

ddh 686

Profile

rhyolitic–daciticlavas, breccias etc.

dacitic porphyriticlavas and domes

metasedimentaryrocks

basalt-andesite

Northernquartz-

porphyry

Bolidenquartz-

porphyryshear zone

Bolidenalteration

zone

Bolidenore

sediment

basaltic andesite

andesite

dacite

quartz porphyry

rhyolite

1714000 1715000 1716000 1717000 17180007203000

7204000

7205000

7206000

Fig. 9. Geological map of the Boliden area from Bergman Weihed et al. (1996). Position of ore and alteration zone from Nilsson (1968) and Ödman (1941). Dots show sample location and the chemical rock classification. The surface geology has not been reinterpreted on the basis of the lithogeochemistry due to the low amount of analysed rock samples. For details see text. Coordinate system is Swedish National Grid (RT 90).

106 A. HALLBERG

core about 1.4 km from the deposit (ddh 679), and in an outcrop nearly 5 km away. In the mine area, Na-altered samples are found within the upper dacite unit south of the deposit and in andesitic basalt north of the deposit. Towards the alteration zone beneath the Boliden ore, the Na-anomalous character of the rocks disappears, and close to the alteration zone the rocks become K-altered instead. This is best seen among the samples from the basaltic an-desite in the northern hanging wall (Fig. 10). A set of samples that shows the transition of a basaltic andesite from Na-altered to a near normal alkali distribution, to K-altered, and to strongly altered sericite±andalusite schist can be found in appendix A (samples B011, B176, B186–B187, B189–B194). The three samples with a nor-mal alkali distribution occur between the Na-altered rock to the north and the K-altered rock to the south. Most likely these samples have been affected by a weak K-alter-ation that erases the Na-alteration and gives the rocks an apparent least altered composition. The Na-alteration is thus overprinted by the alteration system that envelops the Boliden deposit indicating that the Na-alteration is

older. The Na-anomalous rocks are outlined in the geo-logical profile in Figure 10.

The trace element chemistry shows that the host rocks to all Na-altered samples are dacites to basaltic andesites. The alteration is thus not confined to any particular rock type. Possibly the Na-alteration represents a spilitized sequence, but it could also be remnants of an earlier hydrothermal event responsible for the syn-volcanic sul-phide deposits in the region.

The apparent offset of Na-altered rocks at Boliden (Fig. 10) is most likely caused by the east–west striking sinistral, north-side-up, shear zone that cuts the ore and alteration zone (Bergman Weihed et al. 1996).

Vivallo (1987) used the Alteration Index (A.I.=(K2O +MgO)/(CaO+MgO+Na2O+K2O)x100, Hashighusi et al. 1983) to distinguish Na- and K-altered samples from least altered samples. He found that a large part of the rocks in the Boliden area showed Na-enrichment. However, the results from using the A.I. in defining Na-alteration are not compatible with the results from the Hughes igneous spectrum (Hughes 1973). The reason for the differences

Na-alt

Na-alt

Na-alt

Na-alt

ddh 686700 m north-east

of the Boliden mine

ddh 6781.4 km south-west of the Boliden mine

ddh 6792.5 km south-west of the Boliden mine

Bolidenore

Bolidenalteration

zone zone with most intense alteration

North South

andesite

dacite

dacite

dacite

metasedimentary

rocks

quartz-porphyry

basaltic

andesite andesite

andesite

Zn-Pb

mineralisation

quartz porphyrydaciteandesitebasaltic andesitemetasediments

Na-altered

100 m

Fig. 10. Geological profile of the Boliden deposit. Modified and reinterpreted from Bergman Weihed et al. (1996) and Ödman (1941). Coloured bars and dot show position and classified rock composition from chemistry for whole rock samples. Areas with light blue stripes indicate position of rock samples that show Na-alteration. Light grey area below and south of the Boliden ore indicates the zone of most intense alteration. Position of profile and ddh 686 and 678 are shown in Figure 9.


is that Vivallo (1987) considered all samples with an A.I. of less than 50 to be Na-enriched. An A.I. smaller than 50 can, however, also be caused by other element changes, e.g. increase in Ca. Several of the samples used by Vivallo (1987) show anomalous Ca values, a feature that will re-sult in a low A.I. Thus, the A.I. is less reliable than Hughes igneous spectrum to identify Na- and K-altered samples.

An important advantage with geochemical mapping is that it is independent of hydrothermal alteration, at least in this case. Geological units can, thus, be followed into the alteration zone. The east–west striking shear zone that cuts the area (Bergman Weihed et al. 1996) and the Na-enriched rocks on both sides of the deposit suggest that the basaltic andesites on the northern side correlate with the thin, but eastward thickening, and more mafic an-desite unit within the dacites. In the geological profile in Figure 10 this results in an apparent offset of nearly 200 m.

Mass changes

Knowledge of the precursor to altered rocks and the pos-sibility to estimate the unaltered composition of that rock makes it possible to calculate mass changes during altera-tion. Firstly, the composition of the unaltered precursor to the altered rock is calculated. This is achieved by calculat-ing the regression equations on the element content and immobile element ratio in the least altered rocks. Here equations of the second order are used with the Zr/Ti-ratio defining the rock type (Table 7).

From the resulting equations, the composition of the unaltered precursor of every sample is calculated. Then the calculated precursor composition is adjusted to the same Al, Ti, and/or Zr content as the analysed sample. The difference between the adjusted composition and the actual composition of the rock sample is the mass change, or ∆-value, and is expressed in %-units. The ∆-value can be either negative, for mass loss, or positive, for mass gain.

The procedure described here is the same as for the single precursor system, the qz-p rocks, with the exception that the precursor composition is represented by a regression equation of several least altered samples and not a single chemical analysis of a least altered sample. Since the meth-od is based on igneous fractionation and associated ele-ment changes of the volcanic sequence around Boliden, non-igneous rocks and rocks of a different magmatic affinity cannot be treated.

Mass change calculation in the way described above is a fast method to treat large amounts of geochemical data. It gives, however, crude results due to uncertainties in the Zr, Ti, and Al determinations. Also the composition of the least altered rocks is based on very few samples. Further-more, the method is not independent of precursor rock type since one type of rock can lose several tens of percents of an element that only occurs in minor amount in an-other rock. Compare, for example, the Mg content in least altered basalt and quartz porphyry in this study. Neverthe-less, the calculated mass changes give reasonable results. In Figure 11, plots for Si, Mg, K, and Y show the mass changes among those elements on a profile across the ore and alteration zone.

It can be noted that silicification is not common among the altered rocks at Boliden. There are very few samples that display a positive mass change for Si. De-silicification, on the other hand, is very common and it seems that almost every altered rock has lost a significant portion of its Si. The most intense desilification occurs be-low the sulphide ore. The mass changes of K follow the opposite pattern compared to Si: where Si is strongly de-pleted, K is strongly enriched. At surfac,e the mass change calculations reveal both K-enriched and K-depleted areas (Fig. 12). Whether these element changes have any rele-vance for exploration remains to be checked. The changes in Mg show a more complex pattern. There is a large de-pletion beneath the sulphide ore, but outside this deplet-ed area, Mg shows both enrichment and depletion. Mass change calculations can also be applied to minor or trace elements as shown for Y in Figure 11d. In fact, Y is the best element to outline the alteration zone. None of the samples from outside of the mapped alteration zone show any significant mass changes for Y. Figure 11d also illus-trates the mobile nature of Y.

For the rock classification and mass change calcula-tions, only Zr and Ti have been used. Al has been avoid-ed despite the positive correlation between all three ele-ments. The reason for this is that Al sometimes acts in a mobile manner during hydrothermal alteration in the Boliden area.

Table 7. Regression equations for major element compositions. Based on data from Table 4.

SiO2 = -2,3235x2 + 8,4064x + 85,273Al2O3 = 2,2768x2 + 3,8501x + 14,743CaO = 0,2276x2 - 2,7972x - 0,5438MgO = 2,7419x2 + 5,2542x + 3,1736Na2O = -0,0381x2 + 1,001x + 5,1494K2O = 0,8272x2 + 4,3131x + 6,4602Fe2O3 = 1,6737x2 - 0,9906x - 0,1558

Where x=log10(Zr/TiO2)

108 A. HALLBERG

Simass change

-100%-units

+50%-units

0

Kmass change

-5%-units

+6%-units

0

Ymass change

–40ppm-units

0

Mgmass change

–9%-units

+12%-units

0

a) b)

c) d)

North South

North South

North South

North South

0 100

-600

-500

-400

-300

-200

-100

00 100

-600

-500

-400

-300

-200

-100

0

0 100

-600

-500

-400

-300

-200

-100

00 100

-600

-500

-400

-300

-200

-100

0

Fig. 11 a–d. The calculated mass change for Si, K, Mg, and Y plotted on a black and white version of the geological profile found in Figure 10.


Conclusions

A single rock type that occurs in different states of altera-tion and contains some kind of alteration-resistant marker is highly appreciated by anyone investigating the lithogeo-chemistry of hydrothermal systems. The quartz porphyrit-ic rocks, with alteration resistant quartz phenocrysts, that occur as stocks and dykes around and beneath the Boli-den deposit fulfil all of these criteria. In the database of 22 high-quality whole-rock analyses from less altered to strongly altered samples, several immobile elements can be identified, i.e. Al2O3, TiO2, V, Th, U, LREE, and Zr. Most of the major, minor, and trace elements are, how-ever, mobile during alteration, even some elements that were expected to behave less mobile, i.e. Y and HREE. A plot in different rock classification diagrams helped to identify one least altered sample. By multiplying the ele-ment content of each altered sample with the immobile element ratio of the least altered and the altered sample and then comparing the result with the composition of

the least altered sample, the mass change for every sample and every mobile element could be calculated.

Other rocks around the Boliden deposit lack alter-ation-resistant markers. However, by assuming that the elements shown to be immobile during alteration of the quartz porphyries also behave in an immobile manner during alteration of other rocks, and by confirming this assumption on some incompatible trace elements, e.g. Zr, Th, and LREE, it is possible to apply mass change calcula-tion to the whole set of igneous rocks around Boliden. By excluding altered samples on geological and/or chemical grounds a set of least altered samples were identified. The least altered samples, plotted in a TiO2 vs. Zr diagram, de-fine a trend that resembles a fractionation trend for igne-ous rocks. Data for altered samples deviate considerably from the trend of least altered samples due to mass chang-es during alteration and residual enrichment or depletion of immobile elements caused by the mass change. How-ever, since the immobile element ratios are believed to be constant during alteration, it is possible to identify precur-

Kmass

change

+5 %-units

–2 %-units

rhyolitic-daciticlavas, breccias etc

dacitic porphyriticlavas and domes

metasediments

basalt-andesite

Northernquartz-

porphyry

Bolidenquartz-

porphyry

ddh 678

ddh 686

Fig. 12. The calculated mass change for K in surface samples plotted on a black and white version of the geological map of the Boliden area found in Figure 9.

110 A. HALLBERG

sor rocks to every igneous sample in the database. This has been used to lithogeochemically map the wall rock types in the Boliden deposit and to trace geological con-tacts into the alteration zone. The results are generally in agreement with previous results based on core logging. The mass change during alteration has been calculated in the same way as for the quartz porphyries, with the excep-tion that the composition of least altered rocks has been replaced by a regression equation describing the composi-tion at different immobile element ratios, e.g. Zr/TiO2.

Acknowledgements

Critical comments from Magnus Ripa, Pär Weihed, and Jan-Anders Perdahl improved the manuscript and forced me to strengthen my arguments. Two anonymous review-ers further contributed to the manuscript. Boliden Min-eral AB are thanked for providing access to drill cores. Fi-nancial support from the Swedish National Board for In-dustrial and Technical Development (NUTEK) and the Geological Survey of Sweden (SGU) is acknowledged.

References

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MacLean, W.H., 1990: Mass change calculations in altered rock series. Mineralium Deposita 25, 44–49.

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Appendix A

sample # B001 B002 B003 B004 B005 B006 B007 B008 B009 B010 B011 B012 B013bh 6 6 11 11 10 11 11 52 52 52 52 52 75

core 32,6-32,9 60,5-60,7 111,5-111,6 56,1-56,3 59,1-59,3 64,9-65,1 109,8-110,1 19,8-20,6 0,0-10,0 10,2-13,3 13,3-19,5 20,6-32,0 1,2-7,1

local n -82 -65 -71 -98 -48 -94 -72 -111 -96 -103 -107 -117 38local e 2 -3 18 25 243 24 18 -25 -25 -25 -25 -25 -2z 28 49 100 52 51 60 99 250 250 250 250 250 330N (RT90) 7204256 7204272 7204267 7204241 7204302 7204245 7204267 7204225 7204241 7204234 7204229 7204219 7204376E (RT90) 1716538 1716533 1716554 1716563 1716777 1716561 1716554 1716513 1716512 1716513 1716513 1716513 1716528SiO2 44,4 67,0 67,8 67,1 50,7 79,6 62,8 42,8 63,6 44,9 54,4 47,6 53,2Al2O3 8,0 11,3 21,6 15,0 14,9 13,6 23,7 32,8 24,8 39,0 30,7 17,0 24,7CaO 10,4 0,8 0,7 0,5 8,1 0,1 1,7 1,0 0,4 0,2 0,3 0,9 0,4MgO 13,4 0,9 0,5 3,4 6,1 0,3 1,0 6,1 0,5 0,3 0,4 15,5 0,8Na2O 0,26 1,34 1,86 0,40 0,90 0,55 2,25 1,49 0,93 1,27 1,03 0,36 0,89K2O 0,05 1,75 3,69 3,20 1,63 3,34 3,76 2,26 5,63 7,75 7,22 0,49 6,29Fe2O3 9,7 9,9 0,3 5,0 13,9 0,2 0,5 2,6 0,1 0,0 0,2 9,8 6,0MnO 0,33 <0,01 <0,01 0,02 0,20 <0,01 <0,01 <0,01 <0,01 <0,01 <0,01 0,06 <0,01TiO2 0,38 0,49 1,04 0,68 0,88 0,51 1,04 1,62 0,99 1,27 1,80 1,00 1,54P2O5 0,12 0,11 0,24 0,16 0,09 0,02 0,27 0,52 0,25 0,14 0,16 0,23 0,22LOI 1,80 6,50 2,75 3,60 2,15 1,90 2,75 2,90 3,35 5,10 3,95 7,20 4,65SUM 88,9 100,1 100,4 99,0 99,6 100,1 99,7 94,1 100,5 99,9 100,2 100,2 98,7Cu 4840 79,3 27,4 46,3 116 14,8 16,5 9,5 2,5 1,8 3,2 44,4 75,6Zn 406 1640 33,9 322 171 15,4 34,8 32,3 13,1 9,7 6,9 187 2560Pb 13800 227 84 186 24 15 48 80 5 <2 5 5 54Cd 2 10 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 21Au 2800 67 47 38 <5 550 79 370 80 240 600 21 14Ag 59 1,8 1,2 2,7 1,7 1,4 0,5 6,9 1,1 1,2 2,2 1,5 0,7As 2000 130 13 46 3 15 20 290 12 2 30 11 40Sb 8300 110 19 14 2,5 6,5 2,8 1,8 1,6 2,5 4,0 2,0 2,9Bi inf 87 133 7 25 13 115 619 20 38 527 28 25Br <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1Co 54 52 43 36 37 33 42 130 18 11 30 25 31Ni 157 2 2 8 3 <1 9 102 2 <1 <1 <1 5Cr 1220 16 8 17 15 5 6 35 8 5 4 6 6Mo <1 <1 <1 <1 <1 1 1 <1 1 12 2 <1 1V 113 82 115 116 363 76 113 150 142 116 263 112 324Ba 76 267 324 189 279 256 509 250 412 519 491 70 1210Cs <3 <3 4 <3 8 <3 <3 <3 <3 <3 <3 3 3Rb 140 30 54 64 40 59 59 42 85 107 102 15 81Sc 23,0 17,7 20,7 22,1 42,6 15,5 19,9 18,5 23,1 25,8 37,7 29,9 57,6Sr 86,7 121,0 178,0 34,5 188,0 82,4 180,0 54,0 56,2 90,6 102,0 27,8 61,7Be 1,8 1,6 0,6 1,5 2,5 <0,5 1,8 1,1 <0,5 0,5 0,7 2,0 1,8Hf 10 <1 <1 2 <1 <1 <1 2 7 13 6 3 <1Sn <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10Ta <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1Nb 15 14 <10 <10 <10 <10 <10 <10 <10 <10 <10 11 <10Th 1,2 1,4 7,7 2,3 1,0 2,0 5,9 1,7 5,4 12 4,8 2,1 1,9U 9,1 1,0 2,4 1,5 0,6 2,3 3,2 1,5 2,9 6,5 2,5 1,1 1,3Y 12,4 6,5 13,6 9,4 9,8 10,0 12,2 25,4 8,9 16,5 8,1 8,7 6,0Zr 148 89 213 124 30 124 206 220 201 298 186 126 113La 7 19 68 18 6 12 39 8 35 326 39 18 9Ce 14 40 140 36 14 30 80 18 71 580 85 37 18Nd <10 20 60 20 <10 10 30 10 30 210 40 20 10Sm 1,9 3,5 12,8 4,0 1,8 3,8 7,1 3,3 7,8 38,6 11,2 4,2 2,5Eu 4,0 1,8 2,9 1,0 1,4 0,5 2,1 0,4 1,1 5,4 1,4 0,9 0,9Tb <0,5 <0,5 1,4 0,5 <0,5 0,8 0,9 0,6 1,0 1,9 1,1 0,5 <0,5Yb 3,0 1,7 2,3 1,6 1,0 2,6 2,2 1,5 1,8 4,7 1,8 2,0 3,0Lu 0,36 0,26 0,34 0,26 0,14 0,38 0,33 0,22 0,37 0,71 0,32 0,33 0,39Ref. 1 1 1 1 1 1 1 1 1 1 1 1 1 Lab.-year XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94

method XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/

NA NA NA NA NA NA NA NA NA NA NA NA NA

Rock Lampro- Sericite- Sericite- Sericite- dark fine- Sericite Sericite- Tourma- Sericite- Sericite- Sericite- Chlorite- Chloritee-

phyre quartz quartz chlorite grained schist quartz line vein quartz quartz quartz sericite sericite

schist rock rock rock schist schist schist schist schist schist

Alter., struct. minerali sericite- sericite sericite pyrrhotite sericite sericite Cr-mica sericite sericite sericite chlorite chlorite

sation pyrite chlorite quartz sericite sericite

porphyrite

112 A. HALLBERG

Appendix A, cont.

B014 B015 B016 B017 B018 B019 B020 B021 B022 B023 B024 B025 B026 B02775 75 75 75 75 75 75 75 99 99 99 99 99 99

13,0-19,6 41,5-47,0 47,0-51,3 51,3-58,9 59,0-61,9 69,2-80,5 80,5-84,6 85,6-99,2 0,0-15,0 15,0-17,6 17,6-25,0 25,0-27,2 27,4-29,7 29,4-40,9

26 -2 -7 -13 -18 -33 -40 -50 -41 -50 -55 -60 -62 -69-2 -2 -2 -2 -2 -2 -2 -2 18 18 18 18 18 18330 330 330 330 330 330 330 330 410 410 410 410 410 4107204363 7204336 7204331 7204325 7204319 7204305 7204297 7204288 7204297 7204289 7204279 7204276 72042701716529 1716531 1716531 1716531 1716531 1716532 1716533 1716533 1716552 1716553 1716553 1716554 171655446,5 67,6 79,9 78,6 60,2 72,2 47,0 67,3 80,7 74,4 60,5 73,9 65,3 83,619,8 14,3 11,5 10,3 25,9 17,5 18,6 11,4 15,2 18,9 21,5 17,3 15,4 11,20,3 0,5 0,1 <0,01 0,3 0,4 0,4 0,4 0,1 0,1 0,4 0,4 0,4 0,111,0 2,2 0,3 1,1 0,5 0,3 10,0 5,1 0,2 0,2 5,6 0,3 4,5 0,20,26 0,73 0,50 0,42 1,32 1,11 0,38 0,38 0,29 0,48 1,29 0,63 0,33 0,341,75 2,97 2,77 1,65 4,80 2,83 2,18 1,45 1,43 2,83 0,03 4,11 2,30 2,4913,3 7,1 2,3 5,5 2,3 0,2 13,9 8,1 <0,01 <0,01 2,4 0,0 7,2 0,00,10 0,04 <0,01 0,02 <0,01 <0,01 0,06 0,02 <0,01 <0,01 0,00 <0,01 0,02 <0,010,71 0,53 0,23 0,39 1,24 0,94 0,99 0,54 0,32 0,65 0,40 0,86 0,70 0,420,15 0,10 0,05 0,05 0,13 0,22 0,18 0,14 0,07 0,10 0,10 0,31 0,22 0,126,27 3,15 2,50 2,20 3,40 2,90 6,80 5,20 1,90 2,50 2,60 2,25 3,95 1,55100,1 99,2 100,2 100,3 100,1 98,6 100,4 100,0 100,1 100,1 94,8 100,1 100,3 100,1419 3420 142 1270 261 4,0 255 72,2 3,2 0,9 11,4 8,3 176 5,3550 10500 78,2 884 20,6 7,3 260 74,9 7,1 7,1 24,5 7,2 41,5 5,625 65 21 488 18 9 11 9 <2 <2 <2 <2 <2 <2<1 218 <1 6 <1 <1 <1 <1 <1 <1 <1 <1 <1 <152 1100 22 1400 110 140 27 23 23 33 <5 <5 <5 <52,9 13,3 1,6 44,2 2,0 1,1 0,6 0,6 0,3 0,4 1,0 0,5 0,8 0,56 3 31 6200 90 8 6 20 2 2 <2 4 3 22,0 0,7 0,7 23 1,8 0,9 1,2 1,9 0,3 0,3 0,3 0,9 0,8 0,619 inf 6 996 36 18 17 4 <3 6 <3 5 <3 7<1 <1 <1 <1 <1 <1 1 <1 <1 <1 <1 <1 <1 <117 59 38 4500 97 35 120 19 37 29 19 28 27 301 5 <1 112 8 1 4 8 1 2 4 1 2 57 6 8 7 5 4 9 4 2 2 3 1 4 4<1 27 1 <1 1 <1 <1 2 <1 1 <1 <1 <1 <156 76 11 56 171 154 231 50 19 64 23 51 49 31346 531 402 262 657 280 344 236 92 295 57 451 287 259<3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <332 50 49 46 68 39 31 31 30 45 <10 50 34 4318,6 17,6 12,2 17,0 32,3 20,7 29,4 12,2 11,2 10,9 5,5 14,8 19,4 11,614,8 20,7 26,3 19,2 80,3 79,8 29,0 20,3 18,8 28,1 21,8 36,1 19,0 20,42,5 1,4 1,0 1,3 0,9 0,6 2,3 1,4 <0,5 <0,5 0,5 0,5 1,1 <0,55 5 4 <1 <1 4 <1 2 5 5 <1 4 4 4<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 1 <1 <1 <1 <1 <1 <113 <10 <10 42 <10 <10 <10 <10 <10 <10 <10 <10 <10 <105,7 3,8 4,3 3,4 5,6 4,9 1,2 1,2 5,5 5,8 1,5 3,5 2,4 3,03,8 1,8 2,8 2,6 3,1 2,0 0,9 1,1 3,5 3,6 0,9 1,9 1,8 1,88,5 7,0 6,5 20,7 10,7 8,2 5,5 7,3 5,6 6,4 12,2 7,5 7,2 4,0187 144 165 130 194 154 88 106 197 215 118 168 137 1208 24 23 23 43 25 10 11 38 35 7 24 23 1717 50 47 54 90 51 22 25 76 71 15 54 50 37<10 20 20 30 50 20 10 10 30 30 10 30 20 201,7 4,8 4,1 7,7 10,9 4,8 2,6 2,7 6,1 6,9 1,8 6,5 5,2 3,80,5 0,9 0,9 4,6 3,7 1,1 1,1 1,1 1,0 1,6 0,3 1,3 1,5 1,1<0,5 <0,5 <0,5 2,0 1,2 0,5 <0,5 <0,5 0,7 0,8 <0,5 0,8 0,7 0,52,4 2,3 2,4 17,8 3,5 2,2 1,2 1,4 1,8 1,9 0,7 2,8 2,3 1,60,38 0,29 0,34 2,03 0,51 0,36 0,27 0,18 0,26 0,30 0,09 0,50 0,36 0,271 1 1 1 1 1 1 1 1 1 1 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94

XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/

NA NA NA NA NA NA NA NA NA NA NA NA NA NA

Chlorite Chlorite- Sericite Sericite- Sericite- Sericite- Chlorite- Chlorite- Sericite- Sericite- Tourma- Sericite- Chlorite- Sericite-schist sericite schist chloritequartz quartz sericite sericite quartz quartz line vein quartz sericite quartz schist quartz schist schist schist schist rock schist schist schist schist

chlorite chlorite- sericite sericite- sericite sericite- chlorite- chlorite- sericite sericite quartz sericite chlorite- sericite sericite chlorite chlorite sericite sericite brecciated sericite


Appendix A, cont.

B028 B029 B030 B031 B032 B033 B034 B035 B036 B037 B038 B039 B040 B04199 99 99 99 99 100 100 100 100 100 100 320 320 320

40,9-49,1 49,1-61,5 61,5-70,0 70,0-89,4 89,4-92,4 0,0-15,7 15,7-34,0 34,0-50,9 50,9-66,5 66,5-68,7 68,7-87,4 0,0-15,7 15,7-27,3 27,3-28,2

-79 -89 -100 -113 -125 -20 -3 14 30 39 50 -59 -72 -7918 18 18 18 18 18 18 18 18 18 18 50 49 49410 410 410 410 410 410 410 410 410 410 410 570 570 5707204260 7204250 7204239 7204225 7204214 7204318 7204335 7204353 7204369 7204378 7204388 7204282 7204268 72042621716554 1716555 1716555 1716556 1716557 1716551 1716550 1716549 1716549 1716548 1716548 1716585 1716585 171658575,4 52,1 53,9 64,1 66,8 77,5 82,4 77,4 74,0 53,5 76,2 81,2 66,9 67,617,2 36,6 32,8 13,7 13,2 16,8 13,9 11,1 11,9 16,3 12,2 14,8 23,1 15,10,1 0,1 0,2 0,3 2,0 0,1 0,0 0,6 0,1 0,5 0,4 0,0 0,1 0,30,2 0,2 0,3 9,6 5,3 0,2 0,1 2,1 2,8 9,0 1,7 0,2 0,4 2,90,44 0,65 0,76 0,12 0,27 0,38 0,26 0,18 0,24 0,21 0,33 0,17 0,45 0,412,89 4,55 7,17 1,53 2,32 1,31 0,96 2,04 2,08 1,20 2,73 0,96 2,80 3,040,0 0,3 0,1 4,8 4,9 <0,01 <0,01 4,0 4,8 12,4 3,8 <0,01 0,4 5,2<0,01 <0,01 <0,01 0,03 0,07 <0,01 <0,01 0,04 0,02 0,09 0,02 <0,01 <0,01 0,010,59 1,32 1,18 0,50 0,43 0,37 0,26 0,24 0,30 0,75 0,27 0,34 0,75 0,830,07 0,04 0,15 0,11 0,11 0,08 0,04 0,05 0,05 0,30 0,06 0,06 0,10 0,213,35 4,60 4,25 4,70 3,15 3,45 2,30 2,60 2,60 5,30 2,75 2,40 5,05 3,35100,3 100,4 100,7 99,4 98,6 100,2 100,2 100,4 99,0 99,5 100,4 100,1 100,1 99,01,5 2,0 2,5 32,3 8,9 1,1 1,0 1,4 4,8 53,7 76,5 1,2 10,7 2133,3 9,3 10,4 114 70,3 6,7 6,2 30,2 58,4 185 274 6,3 4,5 18,5<2 <2 <2 <2 <2 <2 <2 <2 <2 <2 32 <2 <2 <2<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 1 <1 <1 <154 390 42 21 <5 23 16 <5 <5 <5 <5 150 <5 70,4 0,7 0,5 0,5 0,6 0,6 <0,1 0,3 0,6 0,8 0,6 0,6 0,5 0,94 2 <2 <2 2 11 340 3 3 <2 11 <2 2 190,5 1,4 0,5 0,5 0,5 0,2 0,5 0,2 0,5 3,0 0,8 0,2 0,3 1,7<3 111 5 <3 <3 4 <3 <3 <3 5 <3 <3 <3 <3<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 1 <126 13 9 21 15 28 53 29 25 20 17 34 13 301 1 1 4 3 3 1 <1 <1 1 3 <1 1 5<1 3 3 3 3 <1 <1 4 3 4 4 1 2 51 2 <1 <1 <1 <1 <1 2 <1 <1 <1 2 2 <147 122 134 44 36 18 6 6 23 90 14 20 52 74228 358 421 152 253 128 111 557 506 301 606 70 223 264<3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <344 61 87 20 51 28 17 32 27 26 44 22 35 4414,7 20,2 23,3 12,6 12,5 12,0 13,8 9,1 12,5 23,4 11,9 12,9 12,7 21,424,5 47,0 57,8 11,9 48,0 30,6 19,4 15,1 14,5 10,2 29,8 17,7 33,9 30,0<0,5 <0,5 <0,5 0,9 1,3 <0,5 <0,5 0,9 1,0 1,6 1,1 <0,5 <0,5 0,84 6 6 3 3 5 4 4 4 4 4 5 7 4<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1<10 <10 <10 <10 <10 <10 <10 <10 <10 14 <10 <10 10 <105,5 8,1 5,9 2,9 3,7 5,9 4,1 3,5 3,8 2,9 4,2 5,2 2,8 2,54,6 4,8 3,5 2,1 2,2 3,0 2,6 2,1 2,2 1,8 2,9 3,0 3,3 1,87,2 8,0 8,9 8,3 10,9 7,3 7,2 7,9 6,9 7,0 7,3 7,1 8,4 6,0191 270 253 130 127 211 189 154 156 146 164 202 238 12532 63 47 19 19 39 27 21 24 32 26 34 9 2468 127 100 41 41 80 54 45 50 66 52 70 18 5230 60 40 20 20 30 20 20 20 30 20 30 10 206,8 12,1 9,6 3,9 3,6 7,4 4,9 4,1 4,5 6,7 5,1 6,4 2,3 5,40,9 1,9 2,1 0,9 1,2 1,8 0,9 0,6 0,7 1,1 0,5 1,1 0,6 1,00,9 1,3 1,3 <0,5 <0,5 0,7 0,5 0,5 0,5 1,0 0,6 0,7 <0,5 0,62,9 2,1 2,1 1,9 2,0 2,3 2,6 2,6 2,3 1,8 2,6 2,4 2,5 2,50,38 0,34 0,37 0,31 0,26 0,34 0,38 0,37 0,33 0,32 0,36 0,34 0,36 0,351 1 1 1 1 1 1 1 1 1 1 1 1, 3 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94

XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/


Sericite- Sericite- Sericite- Chlorite Chlorite Sericite- Sericite- Chlorite Chlorite- Chlorite- Chlorite- Sericite- Sericite- Chlorite-quartz quartz quartz schist schist quartz quartz schist sericite sericite sericite quartz quartz sericiteschist schist schist schist rock schist schist schist rock rock schist

sericite sericite- sericite chlorite chlorite sericite- sericite chlorite chlorite- chlorite- chlorite- sericite sericite chlorite- andalusite andalusite sericite sericite sericite sericite

114 A. HALLBERG

Appendix A, cont.

B042 B043 B044 B045 B046 B047 B048 B049 B050 B051 B052 B053 B054 B055320 320 320 320 320 320 320 322 322 322 322 322 322 394

28,2-31,2 31,2-52,7 52,7-62,9 62,9-66,3 66,3-72,3 72,3-88,5 88,5-94,2 0,0-18,3 18,7-33,8 33,8-41,7 41,7-44,2 44,2-50,1 50,1-54,4 0,0-5,0

-81 -93 -109 -116 -120 -131 -142 -37 -20 -9 -4 1 6 -7149 49 48 48 48 47 47 50 50 50 50 50 50 -4570 570 570 570 570 570 570 570 570 570 570 570 570 2707204260 7204248 7204232 7204225 7204220 7204209 7204198 7204303 7204320 7204332 7204337 7204341 7204346 72042671716585 1716586 1716586 1716586 1716586 1716586 1716587 1716584 1716583 1716583 1716582 1716582 1716582 171653271,9 54,4 53,9 56,8 65,8 70,1 48,4 78,5 80,8 78,7 84,2 77,5 75,5 72,718,0 15,8 29,6 16,8 14,3 13,7 5,8 17,6 13,4 13,0 11,1 12,5 11,9 18,80,6 0,3 0,1 0,3 4,3 5,0 11,5 0,1 0,1 0,1 0,1 0,1 0,1 <0,010,3 14,4 0,4 12,3 3,9 2,4 17,4 0,2 0,3 1,6 0,2 1,6 2,8 0,40,60 0,09 0,77 0,13 0,53 0,89 0,17 0,07 0,31 0,29 0,25 0,23 0,18 0,694,49 0,68 7,37 1,49 2,95 3,03 1,16 0,31 2,14 2,88 2,83 2,81 2,07 4,600,3 7,1 0,1 5,8 4,5 4,3 9,1 <0,01 0,1 1,5 0,2 2,0 5,0 0,1<0,01 0,08 <0,01 0,04 0,07 0,07 0,27 <0,01 <0,01 0,01 <0,01 0,01 0,04 <0,010,91 0,66 1,63 0,62 0,55 0,47 0,30 0,32 0,28 0,23 0,21 0,26 0,24 1,090,40 0,13 0,07 0,15 0,14 0,12 0,05 0,05 0,07 0,06 0,04 0,06 0,06 0,052,55 6,45 4,00 5,65 1,40 0,35 3,80 2,95 2,50 0,47 1,40 1,75 2,35 0,60100,0 100,0 98,0 100,1 98,3 100,5 97,9 100,1 100,0 98,9 100,5 98,8 100,3 99,0449 21,4 1,0 76,1 15,1 22,6 18,3 1,3 0,5 16,2 0,6 2,4 3,3 <0,55,3 82,6 4,2 124 68,8 74,6 143 4,2 4,3 36,2 3,1 13,7 21,8 3,5<2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2<1 <1 <1 <1 <1 <1 2 <1 <1 <1 <1 <1 <1 <188 9 340 10 <5 <5 15 <5 <5 7 <5 6 5 312,6 0,4 1,2 0,8 0,7 0,6 0,5 0,8 0,6 0,7 0,6 0,3 1,0 <0,116 130 11 <2 <2 2 17 <2 2 2 3 <2 2 90,8 0,7 1,9 0,3 0,2 0,4 0,6 0,2 0,3 0,3 <0,2 0,3 <0,2 1,316 <3 362 7 4 <3 3 <3 <3 <3 <3 <3 5 <31 <1 <1 <1 1 <1 <1 <1 <1 <1 <1 <1 <1 <125 37 9 28 22 30 54 30 36 35 31 30 26 201 73 6 7 2 2 375 5 1 1 <1 2 1 13 182 4 4 3 5 739 3 1 1 <1 1 3 <1<1 <1 <1 2 2 <1 1 <1 <1 <1 <1 <1 <1 <162 113 120 62 49 50 135 14 19 8 6 7 7 206459 145 398 192 437 425 140 <50 250 550 599 662 597 450<3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <366 21 91 23 47 52 35 21 36 40 48 43 29 6425,1 23,6 22,4 17,8 16,4 15,2 32,0 10,5 16,4 14,8 14,2 13,2 11,4 16,956,6 8,8 70,0 15,7 77,4 148,0 48,6 11,1 24,7 16,6 15,6 16,4 11,7 37,90,7 1,0 <0,5 0,8 1,5 1,4 1,1 <0,5 <0,5 0,6 <0,5 0,7 1,1 <0,54 3 8 4 4 3 <1 6 3 4 3 4 4 3<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 1<10 <10 <10 10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <102,9 3,1 14 4,2 3,6 3,8 1,3 5,3 4,4 4,2 3,9 4,5 4,1 3,12,4 2,0 6,0 2,6 2,3 2,1 0,8 3,0 2,6 2,6 2,1 2,4 2,4 1,68,2 6,6 11,8 9,2 16,2 17,5 8,2 6,7 7,4 8,9 6,9 6,4 6,5 5,1172 120 326 162 137 132 43 230 189 174 159 176 158 12525 17 91 23 22 21 5 35 31 29 23 32 23 3454 36 190 50 44 44 13 73 64 61 50 66 52 7020 20 80 20 20 20 <10 30 30 30 20 30 20 305,9 3,6 16,4 5,2 4,3 4,0 1,4 6,6 5,8 5,9 4,7 5,8 4,7 7,31,8 1,2 2,3 0,9 0,9 1,2 0,6 1,3 1,2 1,4 1,1 1,1 0,7 1,21,0 0,5 1,2 0,6 <0,5 <0,5 <0,5 0,8 0,6 0,9 0,7 0,7 0,5 0,73,8 1,8 2,9 2,7 2,5 2,2 1,1 2,3 2,0 4,0 3,7 3,3 2,6 2,00,50 0,29 0,43 0,41 0,39 0,33 0,10 0,34 0,32 0,51 0,47 0,44 0,37 0,311 1 1 1 1 1 1 1 1 1 1, 3 1 1 1

XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94


NA NA NA NA NA NA NA NA NA NA NA NA NA NA Sericite- Chlorite- Sericite Chlorite- black fine black fine Mafic Sericite- Sericite- Seicite/ Sericite Seicite/ Chlorite- Sericite-quartz sericite schist sericite grained grained dike quartz quartz chlorite schist chlorite sericite quartzschist schist schist rock rock rock schist sericite sericite schist rock schist schist

sericite, chlorite- sericite chlorite- pyrrhotite sericite sericite sericite- chlorite- sericite chlorite- chlorite sericitequartz sericite sericite andalusite sericite sericite sericitevein


Appendix A, cont.

B056 B057 B058 B059 B060 B061 B062 B063 B064 B065 B066 B067 B068 B069394 394 394 394 394 394 395 395 395 395 395 395 395 395

5,0-8,0 8,1-10,6 13,4-23,3 24,6-34,1 36,2-41,9 43,4-45,2 0,0-6,2 6,2-15,3 16,0-17,1 17,1-23,5 23,5-28,7 30,2-38,0 38,0-39,7 39,7-41,6

-67 -64 -55 -44 -34 -29 -81 -88 -94 -98 -104 -112 -116 -118-4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4270 270 270 270 270 270 270 270 270 270 270 270 270 2707204271 7204273 7204282 7204293 7204303 7204308 7204257 7204249 7204244 7204240 7204234 7204226 7204221 72042201716532 1716532 1716531 1716531 1716530 1716530 1716533 1716533 1716533 1716533 1716534 1716534 1716534 171653546,5 38,3 60,4 57,6 72,4 52,6 48,9 49,6 44,8 37,6 56,4 67,4 54,6 44,735,8 28,1 17,3 14,2 19,0 16,7 34,2 33,3 37,5 50,7 28,3 21,0 23,8 24,50,1 0,3 0,3 0,4 0,5 0,6 0,1 0,1 0,1 0,0 0,2 0,4 0,2 0,40,5 10,1 6,0 9,9 0,3 3,6 0,4 0,3 0,4 0,3 0,4 0,6 7,4 11,31,23 0,63 0,42 0,27 2,46 1,17 1,33 1,33 1,50 0,84 1,08 0,68 0,45 0,338,83 4,53 2,87 0,95 1,67 2,66 8,32 7,86 8,69 4,66 6,81 5,27 4,48 3,980,3 9,3 6,1 10,1 0,1 13,7 0,1 0,1 0,2 <0,01 0,1 0,3 4,6 6,4<0,01 0,03 0,02 0,06 <0,01 0,02 <0,01 <0,01 <0,01 <0,01 <0,01 <0,01 0,01 0,031,94 1,79 1,02 0,75 0,86 1,10 1,31 2,24 2,03 2,00 1,72 0,89 1,21 1,350,07 0,20 0,14 0,17 0,24 0,14 0,06 0,03 0,05 0,06 0,13 0,27 0,20 0,284,40 6,40 4,30 5,70 2,10 7,85 4,00 4,00 4,30 4,05 3,15 2,10 2,50 6,4599,7 99,7 98,9 100,1 99,7 100,1 98,8 98,8 99,5 100,2 98,3 98,9 99,4 99,8<0,5 14,5 12,0 334 4,2 107 <0,5 0,8 <0,5 <0,5 1,4 12,7 13,4 20,24,2 104 119 164 4,0 42,4 2,8 5,6 7,9 5,4 3,9 9,9 144 276<2 <2 55 15 46 33 <2 <2 4 <2 <2 <2 25 23<1 <1 <1 2 <1 1 <1 <1 <1 <1 <1 <1 <1 <1<5 <5 <5 36 2000 87 120 48 97 150 260 39 <5 <5<0,1 <0,1 0,5 1,8 2,5 1,4 0,8 <0,1 0,6 0,6 0,7 0,2 0,4 0,32 2300 36 17 <2 50 3 4 2 5 <2 3 4 <21,9 2,7 2,0 2,2 0,8 2,2 1,6 1,4 2,3 1,8 3,2 0,6 1,5 1,710 9 3 <3 11 9 23 19 104 44 202 4 5 8<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <17 27 25 32 18 43 6 <5 <5 8 14 18 20 17<1 3 5 2 1 1 1 2 1 1 1 2 3 21 4 2 5 4 3 4 2 4 3 1 2 3 42 1 <1 <1 1 <1 <1 2 4 4 13 2 <1 <1371 362 153 99 84 252 107 320 203 88 258 120 155 172946 527 359 181 99 298 747 490 582 307 379 408 368 273<3 <3 <3 <3 <3 <3 <3 <3 4 <3 3 <3 <3 <3130 63 48 20 26 43 117 108 114 59 105 81 69 6235,4 63,1 33,4 23,4 12,3 30,2 25,1 26,7 26,0 17,7 28,1 27,8 41,9 53,069,8 39,7 22,3 13,5 121,0 51,4 55,4 79,5 93,8 64,3 60,5 58,8 50,0 52,3<0,5 1,2 0,9 1,3 <0,5 1,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 1,0 1,45 4 4 2 4 <1 6 6 6 5 5 2 4 6<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <102 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 133,1 1,7 1,3 1,6 5,3 1,8 7,7 4,0 4,1 4,6 3,4 1,5 2,1 3,12,3 <0,9 0,7 1,2 2,8 0,9 5,4 3,1 4,2 4,2 2,9 1,6 1,6 2,85,6 7,2 5,9 6,1 6,5 5,3 8,6 6,6 7,6 6,8 5,7 5,8 6,5 9,8169 140 124 107 184 91 280 218 219 214 195 127 152 17836 13 11 17 44 9 56 38 106 98 74 11 12 2876 27 24 34 82 20 121 79 208 203 140 25 26 6140 10 10 20 30 <10 60 30 100 110 50 10 10 308,1 3,5 2,9 3,9 5,8 1,8 14,5 7,6 19,3 22,4 9,2 3,4 3,4 6,41,5 1,0 0,9 1,5 2,3 0,6 1,9 1,4 2,6 3,1 2,5 1,3 1,7 1,90,9 0,6 <0,5 <0,5 0,5 <0,5 1,6 0,9 1,0 0,8 0,6 0,6 0,7 1,01,6 2,0 1,6 1,6 1,1 1,3 4,6 2,4 2,7 2,4 2,4 2,3 2,7 3,50,23 0,28 0,32 0,31 0,16 0,13 0,65 0,43 0,42 0,39 0,34 0,30 0,42 0,451 1 1 1 1 1 1 1 1 1, 3 1, 3 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94



Sericite- Chlorite- dark grey dark grey Sericite dark grey Sericite Sericite Sericite Andalusite Sericite Sericite- Chlorite- Chloritequartz sericite rock rock schist rock schist schist schist rock schist quartz sericite schistschist schist schist schist

sericite chlorite- chlorite, chlorite, sericite- chlorite- sericite- sericite sericite andalusite sericite sericite chlorite- chlorite sericite foliated foliated andalusite, chalcopyrite andalusite sericite foliated

116 A. HALLBERG

Appendix A, cont.

B070 B071 B072 B073 B074 B075 B076 B077 B078 B079 B151 B152 B153 B154395 395 395 428 428 Gruvstugan 11 11 stuff 270 stuff 760 576 576 577 577

41,6-46,8 46,8-48,6 48,6-50,0 1,8-2,0 10,8-11,0 71,0-71,2 93,9-94,2 1,7-4,7 13,4-18,1 34,5-39,3 45,3-47,2

-122 -125 -127 -108 -117 -91 -80 -57 -44 -26 -17-4 -4 -4 7 7 23 20 35 36 54 54270 270 270 170 170 65 85 250 250 250 2507204216 7204213 7204211 7204230 7204221 7204650 7204248 7204259 7204283 7204295 7204314 72043241716535 1716535 1716535 1716544 1716545 1716200 1716560 1716557 1716570 1716570 1716588 171658746,7 53,6 70,6 63,9 54,9 74,9 inf 10,6 8,0 13,4 69,7 49,0 51,6 54,317,7 14,8 14,6 22,7 18,1 12,3 inf 0,0 0,2 9,5 17,1 14,1 14,9 15,31,3 2,2 0,3 0,2 0,3 2,6 inf 2,4 0,3 3,4 0,6 1,1 4,9 7,911,0 9,4 1,0 0,3 11,0 1,8 inf 0,4 0,2 0,2 1,3 3,1 8,1 6,50,32 0,52 0,27 1,00 0,24 0,22 inf 1,03 0,12 0,41 0,85 1,46 1,08 1,152,41 1,42 4,11 5,25 2,20 4,05 inf <0,01 <0,01 2,51 3,45 1,40 1,38 1,2711,0 10,1 4,9 0,2 5,7 3,0 inf 58,3 72,2 32,0 2,2 18,0 12,4 9,00,08 0,14 <0,01 <0,01 0,03 0,06 inf 0,17 nd nd <0,01 0,05 0,10 0,120,94 0,83 0,60 2,13 0,75 0,22 inf 0,02 1,26 5,25 0,92 0,94 0,99 1,030,18 0,14 0,15 0,11 0,19 0,05 inf 0,10 0,12 2,24 0,42 0,16 0,14 0,188,35 6,65 3,60 2,65 5,05 0,85 inf 27,60 29,90 31,80 2,98 11,00 3,85 2,00100,0 99,7 100,1 98,4 98,5 100,0 100,6 112,4 100,7 99,6 100,3 99,4 98,858,7 14,7 29,8 2,5 5,0 8,4 inf inf nd nd 46,2 108 18,3 23,1300 134 56,1 18,6 336 48,5 inf inf nd nd 27,0 104 119 10773 18 15 <2 5 <2 inf inf nd nd 20 49 2 42 1 1 <1 <1 <1 inf inf nd nd <1 <1 <1 <158 340 43 96 <5 11 inf 1300 inf inf 140 140 <5 <51,3 1,7 0,6 1,6 0,6 0,6 inf inf nd nd 0,3 1,0 0,4 0,754 44 46 9 13 7 inf 5700 inf inf 110 100 6 34,2 3,0 1,9 7,2 1,1 0,6 inf 250 inf inf 0,8 2,6 2,5 1,24 3 <3 159 5 <3 inf inf nd nd 165 21 <3 4<1 <1 <1 <1 <1 1 inf <1 inf inf <1 <1 <1 <129 53 25 20 17 24 inf 92 inf inf 91 33 11 123 8 4 1 2 1 inf inf nd nd 4 2 2 36 9 8 5 7 2 inf inf nd nd 48 162 107 732 <1 <1 2 <1 <1 inf inf nd nd 1 <1 <1 <1213 172 97 132 85 8 inf inf nd nd 92 179 190 199269 122 397 269 182 796 inf 715 147 338 441 287 168 231<3 <3 <3 <3 <3 <3 inf <3 inf inf <3 <3 <3 440 48 65 71 34 53 inf 44 37 26 52 26 17 1335,7 26,1 20,9 33,6 25,9 10,9 inf 0,8 inf inf 24,0 28,7 32,3 33,335,7 58,8 36,2 268,0 34,1 133,0 inf inf nd nd 44,8 93,6 106,0 193,01,8 1,9 1,1 <0,5 1,0 1,2 inf inf nd nd <0,5 0,5 0,6 0,92 2 3 14 4 4 inf <2 inf inf 3 2 1 2<10 <10 <10 <10 <10 <10 inf inf nd nd <10 <10 <10 <10<1 <1 <1 2 <1 <1 inf <1 inf inf <1 <1 <1 <1<10 <10 <10 <10 <10 <10 inf 326 19 31 10 <10 <10 <102,4 2,2 3,5 14 3,6 4,9 inf <0,5 inf inf 2,4 1,2 1,2 0,90,9 1,3 1,8 10,9 2,2 2,4 inf <0,5 inf inf 1,5 0,5 0,9 0,68,1 7,2 6,9 23,4 9,8 18,3 inf nd nd nd 6,0 3,9 8,5 9,9103 88 125 448 150 162 inf 12 74 633 120 86 71 7415 11 17 85 21 26 inf <1 inf inf 24 9 7 929 25 38 199 47 54 inf <3 inf inf 51 20 14 2010 10 20 120 20 30 inf <10 inf inf 20 10 10 103,6 2,5 3,8 32,5 4,4 5,0 inf <0,5 inf inf 5,9 2,5 1,8 2,21,6 1,1 1,1 5,0 1,5 1,5 inf 1,6 inf inf 1,9 1,4 0,7 0,70,5 <0,5 <0,5 3,4 0,5 0,7 inf <0,5 inf inf 0,6 <0,5 <0,5 <0,51,5 1,3 1,7 5,9 2,1 2,6 inf 0,6 inf inf 1,9 1,3 1,0 1,10,26 0,24 0,28 0,95 0,32 0,40 inf 0,05 inf inf 0,29 0,19 0,18 0,191, 3 1 1 1 1 1 1 1 1 1 1 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94



Chlorite Sericite- Chlorite- Sericite Chlorite- Quartz Arsonopyrite Pyrite ore Arsonopyrite Arsonopyrite Sericite- Sericite- dark grey dark greyschist quartz sericite schist sericite porphyry ore ore ore quartz quartz rock rock schist schist schist, schist schist quartz vein

chlorite sericite chlorite- sericite chlorite- pyrite chalcopyrite sericite sericite- pyrite veinlets sericite, sericite pyrite chalcopyrite


Appendix A, cont.

B155 B156 B157 B158 B159 B160 B161 B162 B163 B164 B165 B166 B167 B168577 577 Gruvstugan 464 464 464 464 464 464 464 464 464 464 600

49,7-49,8 58,9-65,4 4,2-8,7 8,7-13,0 13,0-23,3 23,3-30,7 30,7-37,2 37,2-42,9 42,9-52,9 52,9-62,0 62,0-66,5 66,5-70,1 12,5-15,6

-13 -1 -78 -83 -90 -99 -106 -112 -120 -129 -136 -140 -11754 54 -26 -25 -25 -24 -24 -23 -23 -22 -22 -21 -17250 250 330 330 330 330 330 330 330 330 330 330 2107204327 7204340 7204650 7204258 7204254 7204246 7204238 7204231 7204225 7204217 7204207 7204201 7204197 72042201716587 1716586 1716200 1716511 1716511 1716512 1716513 1716514 1716515 1716516 1716517 1716518 1716518 171652149,4 58,3 72,7 41,8 72,0 53,9 53,3 53,6 59,0 65,6 60,8 58,0 59,3 58,311,2 14,1 12,4 20,1 16,2 31,4 34,7 30,9 20,9 15,1 13,9 15,2 14,7 15,910,9 6,5 3,2 0,3 0,5 0,3 0,3 0,2 0,3 0,3 1,4 4,1 4,6 0,99,3 5,2 1,7 14,3 1,3 0,3 0,2 0,4 4,4 5,2 7,7 6,5 5,0 8,80,56 1,27 0,29 0,22 0,56 0,75 0,29 0,98 0,55 0,35 0,35 0,30 0,31 0,420,53 0,98 4,09 1,17 3,71 4,72 1,67 7,20 4,50 2,57 2,43 3,16 3,53 1,8014,1 9,7 3,4 13,7 1,4 0,4 0,2 0,2 3,4 5,7 7,7 8,4 8,2 7,10,22 0,12 0,07 0,09 <0,01 <0,01 <0,01 <0,01 0,01 0,03 0,08 0,10 0,12 0,050,55 0,74 0,24 1,06 0,82 2,28 1,67 1,57 1,02 0,59 0,55 0,75 0,75 0,840,13 0,17 0,06 0,15 0,19 0,10 0,19 0,15 0,15 0,14 0,14 0,18 0,21 0,241,35 1,85 0,95 7,60 2,50 5,10 6,30 4,15 4,45 4,35 4,25 2,05 1,55 5,3098,2 98,9 99,0 100,5 99,2 99,3 98,9 99,4 98,7 100,0 99,3 98,7 98,3 99,799,7 56,8 12,1 108 32,3 3,6 3,1 3,0 23,3 39,6 37,4 14,2 15,8 53,9161 82,2 32,7 152 14,4 2,6 2,7 0,8 52,7 149 120 127 138 172<2 <2 3 <2 16 <2 <2 <2 <2 2 5 <2 7 10<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <112 <5 14 8 88 82 24 81 6 19 16 <5 <5 150,8 0,4 0,1 0,1 <0,1 <0,1 0,1 0,1 <0,1 <0,1 0,4 <0,1 <0,1 0,315 5 6 3 6 3 2 19 <2 4 8 2 4 21,0 1,2 0,7 1,7 0,6 1,7 0,7 0,8 0,6 0,6 0,9 0,2 0,4 1,4<3 <3 <3 5 <3 83 28 27 6 <3 <3 <3 <3 9<1 1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 171 11 <5 37 7 <5 <5 5 12 10 20 14 12 13482 7 5 24 8 1 2 10 5 7 24 4 4 41060 137 116 84 44 33 25 15 50 67 152 137 146 433 <1 1 <1 <1 3 3 1 2 <1 3 <1 <1 <1171 93 8 352 201 121 100 172 226 74 103 150 93 77118 232 845 164 310 245 163 452 404 244 305 384 438 227<3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 3 4 <316 13 32 16 59 73 26 113 69 32 48 48 51 3230,0 23,6 10,3 41,6 26,3 20,6 17,0 23,3 31,4 18,3 17,8 23,7 21,8 24,464,8 217,0 143,0 12,2 32,1 36,8 19,6 46,5 31,1 26,2 36,4 76,2 92,5 37,3<0,5 0,9 0,9 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 0,8 0,9 0,9 0,72 2 4 2 1 7 5 7 2 3 3 2 2 3<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1<10 <10 <10 <10 <10 14 12 14 136 <10 <10 <10 <10 <102,1 1,5 4,5 1,4 1,8 8,6 4,9 5,8 2,1 2,7 2,4 1,1 1,5 1,91,8 1,0 2,6 0,7 0,7 5,7 3,6 3,0 1,2 2,3 1,8 0,7 1,4 1,312,9 16,3 20,1 4,4 4,0 7,2 6,0 6,9 3,9 6,5 9,8 10,9 13,6 9,760 99 159 75 86 271 220 228 112 128 112 91 106 1188 21 26 11 13 87 48 50 17 21 14 12 14 1717 43 54 26 28 183 104 102 36 44 31 28 31 3710 20 20 10 10 90 50 50 20 20 10 10 10 201,9 4,3 5,0 3,0 3,4 19,8 10,8 9,6 4,0 4,3 3,1 3,1 3,5 3,90,9 1,5 1,1 1,1 1,5 3,1 2,0 2,0 0,8 1,2 0,8 1,2 1,3 1,3<0,5 0,5 0,5 <0,5 <0,5 1,0 0,9 1,0 0,6 0,5 <0,5 0,5 <0,5 0,51,3 2,0 2,7 1,3 1,3 3,0 2,3 3,3 1,6 1,9 1,8 1,5 1,9 2,40,19 0,35 0,42 0,20 0,24 0,44 0,33 0,47 0,31 0,29 0,27 0,21 0,27 0,321 1 1 1 1 1 1 1 1 1 1 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94



dark grey dark grey Quartz dark grey Sericite- Sericite- Sericite- Sericite- Sericite- Sericite- Sericite- dark grey grey fine- grey fine- rock rock porphyry ser-qz quartz quartz quartz quartz quartz quartz quartz fine-grained grained grained schist schist rock rock rock schist schist schist rock rock rock sericite sericite sericite sericite sericite sericite sericite sericite

118 A. HALLBERG

Appendix A, cont.

B169 B170 B171 B172 B173 B174 B175 B176 B177 B178 B179 B180 B181 B182600 11 11 428 428 428 428 stuff 138 stuff 296 stuff 297 stuff 428 stuff 436 stuff 462 stuff 505

15,6-20,6 23,0-46,7 46,7-55,0 0,0-20,0 20,0-50,0 60,0-90,0 90,0-119,0 -120 -108 -100 -116 -141 -181 -211 -60 30 12 -52 -103 96 -52-19 28 26 7 7 7 7 -36 -1 -1 1 14 9 33210 34 48 170 170 170 170 210 250 250 170 907204217 7204231 7204239 7204222 7204197 7204157 7204128 7204276 7204368 7204349 7204286 7204236 7204434 72042871716519 1716566 1716564 1716545 1716546 1716548 1716550 1716499 1716529 1716531 1716536 1716551 1716536 171656859,2 63,3 57,8 58,4 54,9 62,8 65,9 69,0 61,2 44,5 46,1 63,7 65,6 40,014,9 15,6 16,5 20,1 15,0 16,3 13,5 12,4 15,3 16,4 22,2 14,1 15,2 24,00,6 5,5 6,4 0,8 6,2 5,5 6,6 0,4 0,5 0,7 2,3 4,9 6,0 0,58,8 2,4 4,8 5,6 5,8 2,0 1,5 5,8 4,8 3,2 2,8 4,5 1,4 8,90,37 3,17 0,86 0,67 3,08 3,18 2,94 0,48 0,50 0,95 1,20 0,65 1,61 0,651,92 1,63 2,66 4,06 1,13 1,50 1,55 1,54 2,48 2,64 1,28 2,50 1,01 3,288,0 4,7 6,5 4,3 9,9 5,3 4,1 5,6 9,2 19,6 15,9 5,9 4,0 14,60,05 0,08 0,12 0,03 0,14 0,10 0,13 0,03 0,12 0,05 0,01 0,11 0,12 0,060,79 0,63 0,70 0,93 0,88 0,65 0,53 0,83 0,81 0,89 0,96 0,62 0,97 1,060,21 0,18 0,19 0,16 0,20 0,18 0,15 0,15 0,19 0,11 <,01 0,17 0,19 0,265,45 0,95 1,45 4,45 1,55 0,95 1,50 3,85 4,80 11,30 5,55 1,40 1,90 6,70100,2 98,2 98,0 99,5 98,7 98,4 98,4 100,1 99,9 100,3 98,2 98,5 98,1 99,926,2 39,8 74,7 42,5 51,4 34,0 41,3 31,0 108 50,3 2520 61,4 40,0 135128 74,8 113 116 113 80,9 70,9 92,9 343 154 440 95,3 53,9 6608 3 10 18 6 <2 <2 7 12 94 150 7 6 72<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <128 <5 7 130 14 <5 13 12 51 120 47 8 31 48<0,1 <0,1 0,1 0,7 0,5 <0,1 0,2 <0,1 0,4 2,4 1,8 <0,1 <0,1 0,412 14 30 550 6 9 45 3 25 170 160 29 12 23001,4 0,7 3,2 9,8 0,5 0,4 0,5 3,9 3,5 8,4 17 2,8 2,0 115 <3 <3 13 <3 <3 <3 4 3 11 inf <3 <3 6<1 <1 <1 <1 <1 <1 <1 1 <1 <1 <1 <1 <1 <112 18 18 18 22 15 13 8 18 36 17 20 21 133 9 10 10 3 8 7 7 3 10 12 11 12 945 72 69 49 47 150 91 100 71 149 204 90 113 121<1 <1 <1 3 <1 <1 <1 <1 <1 <1 2 <1 1 297 97 119 115 150 100 64 154 128 191 148 116 219 169178 330 274 283 225 297 272 211 511 362 281 236 308 202<3 <3 3 <3 <3 <3 <3 <3 <3 <3 <3 3 <3 <328 31 68 69 20 50 37 21 41 28 27 62 22 6023,6 24,2 27,7 24,6 30,0 23,6 18,7 24,3 29,5 24,2 25,2 25,9 29,8 32,825,2 256,0 144,0 65,0 238,0 402,0 199,0 22,3 28,3 88,7 193,0 118,0 345,0 50,40,7 1,1 1,0 0,6 0,9 1,2 0,9 <0,5 0,7 1,0 1,1 0,8 1,1 0,73 3 3 4 2 4 3 2 2 1 2 3 2 4<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 17 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1<10 <10 <10 10 <10 <10 <10 <10 <10 <10 10 <10 <10 <102,1 3,6 3,0 4,3 2,1 3,5 2,5 1,2 1,3 0,6 1,4 2,0 2,5 3,51,1 2,2 1,3 3,6 0,7 1,6 1,4 1,1 0,7 0,7 2,0 1,9 1,0 <0,98,3 16,6 14,0 9,8 16,0 13,0 13,8 4,2 6,2 1,7 7,9 12,6 14,6 8,4103 125 128 148 90 133 112 77 96 66 132 104 91 18115 19 18 29 15 20 21 3 29 2 3 14 23 2432 39 39 62 32 41 44 6 61 6 8 31 52 5110 20 20 30 10 20 20 <10 30 <10 <10 10 20 203,6 4,3 4,1 6,8 3,7 4,5 4,3 1,3 6,3 0,7 2,0 3,6 6,1 5,31,2 1,5 1,3 1,1 1,5 1,2 1,8 0,7 2,3 1,1 1,0 1,3 2,2 2,40,5 0,6 0,5 0,9 0,5 0,6 <0,5 <0,5 0,8 <0,5 0,8 0,6 0,7 0,61,7 2,3 2,3 2,9 2,2 2,2 1,8 0,9 1,4 0,8 2,2 1,8 2,0 3,10,26 0,34 0,33 0,41 0,32 0,35 0,28 0,19 0,25 0,15 0,33 0,27 0,31 0,481 1 1 1 1 1 1 1 1 1 1 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94



grey fine- dark grey dark grey Sericite- grey fine- grey fine- dark grey black dark grey black rock Dacite fine- light grey black fine-grained fine-grained fine-grained quartz grained grained fine-grained schist siliceous grained siliceous grainedrock rock rock schist rock rock rock rock black rock rock rock sericite pyrite filled pyrite pyrite amygdules


Appendix A, cont.

B183 B184 B185 B186 B187 B188 B189 B190 B191 B192 B193 B194 B195 B196stuff 506 stuff 509 stuff 511 stuff 615 stuff 616 stuff 626 stuff 676 stuff 677 stuff 678 stuff 687 stuff 688 stuff 689 stuff 1109 30

144,0-- 160,0

127 118 112 -26 -17 38 -41 17 29 -27 29 54 17 -2071 -33 -23 -7 -3 -2 8 2 -20 -9 -7 2 -21 9390 90 90 170 170 170 130 130 130 90 90 90 570 967204465 7204453 7204448 7204311 7204321 7204376 7204297 7204354 7204365 7204310 7204366 7204391 7204354 72041031716526 1716493 1716503 1716527 1716530 1716528 1716542 1716533 1716511 1716524 1716523 1716531 1716510 171664957,6 57,7 74,2 59,2 53,8 55,3 58,9 61,0 55,2 69,0 48,6 57,2 40,3 65,213,7 21,3 13,0 17,3 17,3 17,3 17,2 13,7 16,6 10,1 17,5 15,7 27,8 15,46,3 3,6 3,8 5,3 6,9 12,7 6,1 6,4 5,0 1,4 7,8 6,4 10,0 2,74,9 1,3 1,0 3,9 3,2 2,7 2,8 3,4 4,8 0,7 5,1 3,8 5,8 2,21,67 6,54 2,86 2,59 6,04 0,89 2,42 3,01 3,81 1,87 3,30 3,91 1,49 2,071,05 1,77 0,79 0,78 0,12 0,18 0,88 0,37 0,24 0,80 0,29 0,29 0,24 3,1810,9 2,4 2,3 6,1 9,0 6,3 7,7 9,2 10,4 9,9 13,1 9,8 8,5 5,00,20 0,03 0,03 0,09 0,12 0,15 0,07 0,14 0,11 0,03 0,20 0,17 0,12 0,040,70 1,42 0,40 1,09 1,13 0,77 1,07 0,78 1,00 0,66 1,13 0,95 0,60 0,550,13 0,23 0,10 0,24 0,25 0,24 0,19 0,14 0,11 0,07 0,12 0,26 0,14 0,131,55 2,50 0,70 2,10 0,30 1,85 2,10 0,80 1,70 5,40 1,20 0,60 3,45 2,3098,7 98,8 99,2 98,8 98,1 98,4 99,5 98,9 99,0 99,9 98,3 99,1 98,3 98,871,3 79,4 12,6 19,1 54,9 5,1 46,3 15,7 48,8 27,5 41,2 22,5 25,8 33,7138 148 35,1 109 110 78,4 71,8 94,2 131 21,0 125 100 65,3 91,33 8 6 5 8 15 13 <2 2 10 <2 <2 4 3<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <130 17 <5 <5 <5 <5 11 8 <5 40 <5 12 55 <50,1 <0,1 <0,1 <0,1 0,1 <0,1 0,5 <0,1 0,1 <0,1 <0,1 <0,1 1,7 0,522 12 5 3 10 25 13 4 3 85 4 2 15 171,3 0,9 0,6 0,7 1,3 2,4 1,9 1,4 1,4 2,1 1,5 1,0 0,5 2,3<3 <3 <3 <3 <3 <3 4 <3 <3 6 <3 <3 16 3<1 2 <1 1 3 <1 <1 <1 <1 <1 <1 <1 <1 <123 14 10 18 50 12 20 21 26 20 36 27 15 106 5 5 2 2 3 4 4 3 3 3 2 12 4191 65 72 67 68 74 88 165 125 118 73 71 42 149<1 <1 3 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 1176 423 57 257 276 147 196 182 266 132 334 222 69 56200 325 137 201 <50 <50 211 148 156 173 140 188 79 4993 <3 <3 <3 4 <3 <3 <3 <3 <3 <3 <3 <3 <323 29 10 14 <10 <10 29 <10 <10 <10 <10 <10 12 4429,1 44,4 16,7 40,2 40,3 32,7 33,8 25,3 33,5 38,4 38,7 28,2 27,7 15,9178,0 416,0 470,0 396,0 271,0 350,0 313,0 263,0 197,0 211,0 367,0 351,0 299,0 126,0<0,5 0,9 1,2 1,1 0,9 0,8 1,0 0,9 0,9 0,7 0,8 0,7 3,9 1,12 2 5 3 3 3 1 2 2 1 2 1 12 3<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1<10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 12 <101,4 2,9 4,6 2,2 0,8 2,5 0,9 1,5 1,4 0,8 0,9 1,7 8,8 4,40,8 1,9 1,8 1,1 <0,7 1,4 0,6 0,7 1,6 1,1 1,6 0,9 5,0 1,913,9 6,9 10,3 15,6 18,0 19,1 8,5 12,7 14,7 2,9 12,7 15,9 19,2 14,467 105 143 99 101 104 96 72 87 64 95 88 383 14211 15 23 18 18 16 8 12 16 3 12 13 35 2824 34 50 41 38 39 18 26 33 7 24 27 71 5410 20 20 20 20 20 <10 10 10 <10 10 10 30 202,9 4,3 5,0 5,2 4,7 4,5 2,0 2,7 3,2 1,2 2,9 3,0 6,7 4,61,3 1,5 2,0 1,9 2,1 1,8 1,6 1,4 1,4 0,9 1,1 1,1 1,3 1,9<0,5 0,6 0,6 0,8 0,7 0,6 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 1,1 0,52,0 1,1 2,0 2,4 2,1 3,4 2,1 1,4 1,6 1,0 1,5 1,5 4,4 2,60,31 0,25 0,35 0,43 0,38 0,50 0,30 0,22 0,25 0,15 0,27 0,24 0,68 0,421 1 1 1 1 1 1 1 1 1 1 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94



black dark grey light grey dark grey black fine- light grey black rock dark grey black rock light grey black rock black rock coarse- light greyschist rock fine- fine- grained rock siliceous siliceous grained rock grained grained rock rock rock rock rock rock pyrite sulfides pyrite

120 A. HALLBERG

Appendix A, cont.

B197 B198 B199 B200 B201 B202 B203 B204 B20530 30 30 30 30 30 30 30 30

210,0-240,0 265,0-295,0 320,0-336,0 336,0-357,0 357,0-384,0 400,0-420,0 420,0-440,0 450,0-474,0 474,0-498,0

-176 -149 -120 -111 -99 -73 -61 -42 -2677 64 51 48 44 35 31 24 18152 198 243 256 272 306 322 344 3627204136 7204163 7204188 7204199 7204212 7204236 7204248 7204268 72042831716629 1716614 1716602 1716598 1716592 1716583 1716578 1716570 171656269,6 58,9 59,9 62,8 60,7 73,4 69,7 63,3 75,913,8 15,7 15,9 13,6 13,6 17,0 20,7 22,9 13,84,0 7,2 3,3 0,9 1,1 0,2 0,2 0,2 0,20,7 2,3 2,5 3,2 7,4 0,5 0,3 0,6 1,13,54 3,27 0,33 0,32 0,32 0,57 0,65 0,70 0,491,69 1,74 4,90 4,78 2,66 4,33 4,00 4,52 3,143,0 5,7 8,4 8,7 7,8 0,9 0,3 2,2 1,60,06 0,13 0,16 0,37 0,25 <0,01 <0,01 <0,01 <0,010,50 0,68 0,84 0,74 0,55 0,72 0,93 1,11 0,570,13 0,21 0,21 0,21 0,13 0,13 0,09 0,14 0,131,50 2,50 2,80 2,85 4,85 2,65 3,35 3,70 2,3598,6 98,3 99,2 98,5 99,3 100,5 100,1 99,4 99,317,6 43,4 73,4 114 101 50,1 5,3 18,7 39,461,4 93,7 94,4 1980 1080 26,6 4,9 6,6 7,48 3 9 989 695 17 <2 4 5<1 <1 <1 <1 <1 <1 <1 <1 <1<5 <5 24 82 100 320 460 100 440,1 <0,1 0,3 2,6 1,6 0,1 <0,1 <0,1 0,16 5 33 52 22 5 2 22 160,9 0,6 1,0 5,1 3,7 1,1 0,8 0,9 0,5<3 <3 <3 <3 <3 5 14 10 8<1 <1 <1 <1 <1 <1 1 <1 18 25 19 19 20 10 <5 14 124 9 2 4 10 6 2 7 476 66 127 90 133 33 24 56 521 <1 <1 3 <1 1 <1 1 <145 107 108 104 97 126 148 144 41412 390 1240 1800 503 282 220 388 410<3 3 3 <3 3 <3 <3 <3 <323 40 76 80 51 68 64 73 3813,0 26,8 29,8 27,7 20,9 21,5 23,2 28,5 17,8221,0 307,0 73,4 34,0 21,5 28,4 23,0 25,6 19,31,1 1,0 0,9 0,5 0,7 <0,5 <0,5 <0,5 <0,53 4 3 4 3 3 6 5 3<10 <10 <10 <10 <10 <10 <10 <10 <10<1 <1 <1 <1 <1 <1 <1 <1 <1<10 <10 <10 <10 <10 16 13 <10 <103,3 2,6 2,5 2,2 2,9 2,9 5,6 4,4 2,71,9 2,2 1,5 1,3 2,4 2,0 3,4 2,9 2,813,2 14,1 15,6 10,4 8,7 3,6 3,6 4,8 4,1129 118 107 97 107 112 155 151 11821 26 22 17 20 24 46 37 2143 52 45 36 41 47 95 74 4220 20 20 20 20 20 40 30 203,8 4,8 4,9 3,6 3,7 4,8 8,8 7,7 4,20,9 3,0 2,2 1,0 1,4 1,6 2,2 2,1 0,9<0,5 0,5 0,6 <0,5 0,5 0,5 <0,5 1,0 0,62,0 2,6 2,5 1,9 1,8 2,2 2,1 4,0 2,50,30 0,42 0,36 0,27 0,31 0,30 0,44 0,59 0,361 1 1 1 1 1 1 1 1 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94 XRAL-94

XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP XRF/ICP/ XRF/ICP/ XRF/ICP/ XRF/ICP/

NA NA NA NA NA NA NA NA NA

light grey- dark dark grey rock Sericite- Sericite- Sericite- Sericite- Sericite-brown rock brown- brown- quartz chlorite quartz quartz- quartz grey rock grey rock rock rock rock andalusite rock schist

sericite sericite- sericite- sericite sericite chlorite andalusite


Appendix B

sample # 91101 91102 91103 91104 91105 91106 91107 91108 91109 91110 91111 91112 91121bh 686 686 686 686 686 686 678 678 678 678 678 678 30core 262,5-262,9 296,0-296,3 352,6-353,0 390,3-390,7 458,9-459,2 492,7-493,0 126,1-126,5 247,2-247,6 402,9-403,2 413,0-413,2 475,5-476,0 532,3-532,6 56,2-56,5local n -6759 -6721 -6671 -6667 -6647 -6629 -247local e -248 -295 -354 -358 -382 -404 107z 109 214 349 358 412 461 19N (RT90) 7204892 7204900 7204914 7204923 7204940 7204948 7203148 7203190 7203243 7203247 7203268 7203288 7204051E (RT90) 1717035 1717022 1716999 1716984 1716956 1716943 1715763 1715719 1715662 1715659 1715636 1715615 1716657SiO2 56,1 58,4 55,2 56,6 70,1 74,5 61,1 72,3 51,0 50,8 68,4 64,2 63,1Al2O3 17,0 17,0 17,3 17,6 15,0 13,7 19,5 12,9 24,7 19,3 15,2 13,9 16,0CaO 5,2 4,5 7,2 6,7 3,4 3,2 7,2 3,3 7,5 9,4 4,1 6,4 3,9MgO 5,2 4,2 5,0 4,4 2,1 0,5 2,0 1,3 2,8 6,1 0,6 1,9 2,6Na2O 3,31 3,21 2,64 2,78 3,12 3,94 3,70 1,32 3,76 1,74 5,99 1,44 3,12K2O 0,73 1,20 1,10 1,03 1,46 1,09 0,78 2,17 2,83 1,57 1,08 3,51 1,46Fe2O3 9,7 9,0 10,3 9,7 3,6 1,3 5,0 4,7 5,8 9,4 1,3 4,8 8,0MnO 0,09 0,08 0,16 0,14 0,05 0,05 0,08 0,08 0,07 0,20 0,05 0,09 0,08TiO2 1,07 1,13 1,12 1,04 0,49 0,45 0,43 0,53 0,87 0,66 0,56 0,52 0,73P2O5 0,28 0,22 0,12 0,17 0,15 0,14 0,10 0,12 0,06 0,13 0,20 0,17 0,17LOI 2,75 2,52 2,02 1,85 1,31 0,75 1,12 1,48 1,27 1,94 1,99 2,77 1,78SUM 101,5 101,5 102,2 102,0 100,8 99,6 101,0 100,2 100,5 101,2 99,5 99,8 100,8Cu 75 26 190 47 7 8 37 34 5 54 8 9 27Zn 1125 367 115 123 70 28 89 10 82 102 109 75 112Pb 10 10 As Sb Co 47 62 61 Ni 9 8 176 9 10 8 11 9 14 40 7 6 12Cr 42 13 6 12 13 5 18 9 24 10 9 5 10V 302 255 345 311 34 30 92 24 332 247 52 54 66Ba 151 358 283 202 296 414 289 420 370 479 194 1136 230Rb 43 49 27 40 34 17 26 130 87 32 18 64 45Sc 27,0 27,0 32,0 30,0 11,0 6,8 9,0 11,0 33,0 27,0 12,0 12,0 18,0Sr 341 260 328 390 441 216 472 143 379 237 241 164 338Hf Nb 10 Th U Y 17 15 11 11 19 13 8 18 13 7 21 18 17Zr 75 86 64 67 122 112 61 133 29 24 122 119 89La 11,7 9,7 14,2 14,0 Ce 26 23 33 16 30 11Nd 17,3 15,1 19,2 18,3 Sm 3,65 3,92 4,07 3,59 Eu 1,90 1,49 1,46 1,53 Tb Yb 2,36 1,85 2,08 2,1 2,44 2,5Lu method-lab. ICP-Lul ICP-Lul ICP-Lul ICP-Lul ICP-Lul ICP-Lul ICP-Lul ICP-Lul ICP ICP ICP-Lul ICP-Lul ICP-Lul

Rock Andesite Andesite Andesite Andesite Dacite Dacite Volcanic Volcanic Siltstone Siltstone Dacite Dacite Volcanic porphyry porphyry flow in sandstone porphyry porphyry sandstone sediment

Alter., struct., minerali-sation actinolite actinolite actinolite

122 A. HALLBERG

Appendix B, cont.

91122 91123 91124 91125 91141 91142 91143 92005 92006 92007 92008 92009 92010 9201130 30 30 30 29 29 29 70,7-71,7 118,3-118,6 573,6-574,0 585,1-585,4 80,1-80,4 140,6-140,9 187,8-188,0 -240 -218 37 45 -216 -191 -171 106 99 -21 -28 -113 -114 -115 32 74 424 429 89 143 186 7204058 7204080 7204337 7204344 7204119 7204145 7204165 7203950 7203990 7203780 7203620 7203280 7203290 72035701716655 1716648 1716523 1716516 1716424 1716422 1716420 1715270 1715180 1714290 1714050 1714260 1714190 171546062,6 63,7 72,7 73,6 56,5 73,3 57,2 59,7 67,0 64,7 57,6 55,0 66,8 68,320,5 12,8 11,7 12,8 23,4 13,7 15,6 15,1 14,9 14,4 17,0 18,7 15,0 16,44,6 1,0 0,1 0,2 3,9 2,0 5,0 6,8 4,6 6,4 5,4 3,2 2,8 3,32,0 5,1 3,7 2,7 1,2 1,4 4,0 2,4 1,2 1,4 3,3 4,2 2,1 0,82,75 0,22 0,16 0,23 6,34 3,03 0,72 2,87 2,11 1,99 2,38 3,40 2,67 4,373,15 2,49 1,69 2,24 2,58 2,28 2,80 1,39 2,50 2,08 2,69 3,06 2,28 2,092,7 9,5 6,2 5,0 3,3 2,3 11,2 8,0 5,9 6,3 9,4 9,9 5,3 2,20,04 0,63 0,06 0,05 0,11 0,10 0,15 0,16 0,10 0,13 0,16 0,12 0,08 0,060,37 0,70 0,22 0,26 1,06 0,49 0,98 0,79 0,62 0,74 0,82 0,73 0,56 0,620,11 0,09 0,10 0,09 0,24 0,10 0,16 0,23 0,18 0,25 0,24 0,18 0,15 0,161,46 3,96 3,42 2,99 1,16 1,08 1,87 2,70 1,20 2,25 1,10 1,85 1,20 1,45100,2 100,2 100,0 100,2 99,8 99,8 99,7 100,1 100,4 100,6 100,0 100,4 99,0 99,817 82 18 69 29 31 12 8 63 261 32 31 59 54 120 87 72 75 98 114 74 69 27 23 9 10 <10 4 19 12 <3 11 2 3 19 11 8 16 12 10 125 13 7 15 7 1 1 5 22 5 11 60 20 21 150 10 38 286 5 173 53 244 670 1249 554 831 982 362 499 543 741 803 803 501 579 603160 34 11 20 22 24 33 26 36 29 43 40 49 2710,0 28,0 8,3 8,6 25,5 16,6 19,1 23,5 20,2 14,7 17,0701 48 11 19 362 155 233 555 307 281 414 285 296 307 2,0 2,0 3,0 3,0 5,0 3,0 4,010 10 5 4 9 9 9 9 10 11 11 4,0 4,0 6,0 1,4 2,6 1,9 2,6 4,4 2,6 3,3 2,0 1,4 1,5 1,4 1,2 2,1 1,5 1,99 5 26 34 18 11 13 19 12 16 22 24 13 1991 23 148 159 177 135 77 109 134 131 124 179 153 15910,7 21 19,8 19 19,0 17,0 21,0 26 21,0 21,024 46 34 11 40 38 36 42 53 41 4114,5 29 20 19 18 20 27 20 202,85 6,67 4,0 3,9 4,0 4,4 4,9 3,9 4,01,44 0,95 1,7 1,4 1,3 1,3 1,3 1,2 1,1 0,6 0,5 0,6 0,6 0,7 0,5 0,51,40 3,2 3,72 2,3 1,8 2,0 2,3 2,6 2,1 2,0 0,35 0,24 0,28 0,27 ICP-Lul ICP-Lul ICP-Lul XRF-Not. XRF-Not. XRF-Not. XRAL- XRF/NA XRF/NA XRF/NA XRAL- XRF/NA XRF/NA 92/XRF/NA 92/XRF/NA A A

Volcanic Biotite Quartz Quartz Dacite Dacite Andesite Dacite, Dacite Dacite Dacite Dacite Dacite Dacite flow in siltstone porphyry porphyry porphyry porphyry feldspar porphyry porphyry porphyry porphyry porphyry porphyrysediment porphyritic

chl/garnet biotite- biotite- Preserved ”granite” biotite akt actinolite actinolite chlorite chlorite texture


Appendix B, cont.

92012 92013 92014 92015 92016 92017 92018 92019 92020 92021 92022 92023 92024 92025 30 30 30 30 30 176,3-176,7 192,8-193,0 253,9-254,2 289,9-290,1 244,9-245,2 -192 -184 -154 -136 -159 86 82 66 58 69 124 138 188 218 181 7205750 7203050 7203040 7205630 7204740 7206500 7206120 7206720 7204106 7204114 7204144 7204163 7204139 72052901716800 1714550 1714570 1717520 1716420 1717600 1717500 1716990 1716635 1716630 1716615 1716606 1716617 171534070,6 79,7 70,9 71,0 69,5 61,7 68,6 71,2 63,5 68,7 61,1 52,8 66,4 71,013,6 10,2 11,8 15,4 13,3 19,8 14,3 13,0 19,1 16,3 15,4 23,3 13,6 12,24,2 1,2 6,3 1,6 4,1 3,3 2,5 3,0 2,4 2,4 6,6 5,8 6,1 5,00,6 0,3 0,7 0,8 1,2 1,5 1,1 1,4 2,1 1,5 3,4 0,1 0,9 2,42,33 4,87 2,74 5,57 3,31 3,89 2,40 1,96 4,86 5,50 2,14 5,13 3,08 0,632,06 0,77 1,37 1,54 1,51 2,95 4,36 3,74 2,66 1,56 1,43 2,34 1,82 2,393,7 1,7 2,1 2,3 3,8 5,3 4,6 4,1 3,1 2,2 8,5 5,4 4,1 3,80,11 0,04 0,10 0,04 0,09 0,13 0,14 0,18 0,03 0,03 0,10 0,16 0,10 0,130,46 0,35 0,30 0,55 0,72 0,59 0,49 0,37 0,71 0,61 0,72 1,18 0,52 0,440,12 0,07 0,07 0,14 0,19 0,14 0,13 0,06 0,16 0,15 0,28 0,26 0,14 0,101,60 1,30 3,75 1,10 1,65 1,30 0,75 1,30 1,60 0,80 0,75 1,20 2,80 1,2099,4 100,5 100,1 100,1 99,4 100,6 99,3 100,4 100,2 99,8 100,5 97,7 99,6 99,312 27 12 13 5 18 17 31 27 16 544 46 43 55 72 73 72 105 87 87 96 320 58 49 11 20 6 5 7 6 6 16 6 7 8 118 2 1,0 1,111 5 9 8 8 8 5 14 12 23 24 11 6 5 5 11 5 5 190 10 120 180 80 150 12 436 302 617 245 456 1430 950 731 423 283 344 531 391 39132 12 26 33 26 43 58 50 36 29 25 45 30 4611,6 7,7 8,6 16,2 18,4 18,5 13,3 14,0 19,4 15,2 24,5 32,8 13,5 11,0266 161 305 182 195 307 218 102 281 247 414 390 193 1043,0 3,0 3,0 3,0 2,0 5,0 3,0 3,0 4,0 3,0 2,0 3,0 2,0 5,012 11 9 10 9 13 10 12 12 11 10 11 9 93,3 2,7 3,8 3,8 1,7 4,9 3,3 3,9 2,7 3,3 1,9 4,4 2,5 5,01,9 1,8 1,7 2,1 1,1 1,3 1,8 2,0 2,7 1,8 0,7 1,3 1,1 1,813 17 18 13 12 28 17 24 21 14 18 25 16 30127 164 137 137 105 192 131 153 183 158 122 185 135 17320,0 24,0 27,0 22,0 13,0 36,0 21,0 25,0 26,0 22,0 18,0 25,0 20,0 27,041 46 48 44 27 68 40 49 54 44 39 51 38 5420 22 22 21 13 33 20 20 28 23 20 28 20 203,6 3,7 3,7 3,9 3,3 5,7 3,7 4,8 5,4 4,5 4,0 5,6 3,7 5,10,9 0,6 1,4 1,3 0,7 1,1 1,0 1,5 2,2 1,2 1,6 1,8 1,1 0,90,5 0,5 0,5 0,5 0,6 0,7 0,5 0,6 0,7 0,6 0,5 0,7 0,5 0,61,9 2,2 1,9 1,9 1,4 2,9 1,8 2,5 2,1 1,6 2,1 3,3 1,9 2,80,25 0,36 0,28 0,28 0,15 0,25 0,44 0,23 0,30 0,43 0,30 0,39 XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA Dacite Dacite Volcanic Dacite Dacite Dacite Dacite Ryolite Dacite Dacite Dacite Dacite Dacite Ryolite porphyry porphyry sandstone porphyry porphyry porphyry porphyry porphyry porphyry porphyry porphyry porphyry silicified actinolite biotite- actinolite

124 A. HALLBERG

Appendix B, cont.

92026 92027 92028 92029 92033 92034 93035 931001 931002 931003 931004 931005 931006 931007 Gillervattnet 2 69 69 24 24 24 24 428 428 428 217,0 163,3 263,5 54,8-55,0 74,9-75,1 77,8-78,1 114,0-114,3 13,0-13,2 22,9-23,2 28,8-29,1 -5096 -431 -531 -103 -95 -94 -79 -119 -129 -135 -1634 35 39 90 87 87 83 7 7 7 188 250 250 66 84 87 119 170 170 1707205140 7205150 7204650 7204550 7204893 7203906 7203806 7204233 7204241 7204242 7204257 7204219 7204209 72042031715670 1715730 1716330 1714800 1714480 1716576 1716581 1716624 1716621 1716621 1716616 1716541 1716541 171654173,0 71,9 75,3 63,3 68,7 71,9 57,1 46,5 53,7 41,4 54,0 64,9 66,4 54,612,7 13,0 11,4 14,3 13,6 14,6 21,3 14,6 17,8 16,0 12,5 14,2 13,9 15,92,4 2,6 3,0 2,9 2,5 2,1 6,1 8,0 0,5 0,6 1,5 0,2 4,1 6,91,8 1,8 1,7 3,6 1,2 1,3 1,9 13,5 9,3 8,3 14,2 8,9 3,6 5,52,80 1,85 0,24 2,23 3,86 6,48 4,17 0,40 0,69 0,84 0,12 0,19 0,52 2,272,12 2,75 3,15 2,37 1,86 0,58 1,74 0,56 2,17 1,76 1,43 1,97 2,75 2,543,7 3,4 2,9 8,3 4,1 2,2 5,2 11,9 9,2 19,4 9,9 4,7 6,1 10,40,08 0,12 0,11 0,08 0,12 0,04 0,08 0,20 0,07 0,09 0,20 0,05 0,08 0,170,28 0,30 0,25 0,82 0,45 0,54 0,44 0,84 1,21 1,12 0,50 0,48 0,74 0,880,07 0,07 0,06 0,25 0,09 0,15 0,10 0,18 0,25 0,22 0,23 0,15 0,22 0,221,20 1,15 1,30 1,70 1,30 0,85 1,00 3,65 5,80 10,20 4,95 4,55 1,70 0,65100,1 99,0 99,4 99,8 97,9 100,7 99,1 100,4 100,6 99,9 99,6 100,3 100,2 100,0 10 1 1 8 1 4 269 328 1 23 1768 58 149 96 92 82 74 119 375 301 518 117 69 96 11 5 3 5 11 3 47 59 84 13 9 2 6 1 2 12 30 7 35 130 240 1 3 2 4 5 5 16 6 12 10 26 22 41 22 12 10 22 5 5 10 3 4 1 86 7 5 3 10 20 20 26 26 33 700 54 70 43 387 506 385 664 508 153 393 199 256 253 187 214 582 94736 43 30 30 32 13 43 17 32 29 25 29 49 5211,0 11,0 9,3 21,0 16,0 14,0 15,0 31,8 39,9 30,2 25,8 19,3 21,3 31,165 79 65 82 81 214 544 66 45 56 1 15 90 2485,0 5,0 5,0 1,0 3,0 3,0 2,0 1,6 2,7 2,1 2,5 3,2 2,2 2,512 11 10 10 11 10 9 6 8 7 10 9 6 34,1 4,1 3,3 2,0 4,1 3,4 3,4 1,5 2,0 1,8 4,4 3,6 2,1 1,72,0 2,2 2,3 0,9 1,6 1,9 1,4 5,8 2,1 2,0 3,8 3,2 1,7 2,124 19 21 14 24 21 15 19 25 10 10 21 19 19171 178 158 124 136 136 122 83 103 95 87 127 107 10226 28,0 24,0 18,0 22,0 17,0 16,0 16,7 0,38 14,5 14,5 20,1 17,1 14,552 53 50 39 44 38 31 36 23 32 32 42 36 3126 27 25 20 21 21 16 19 13 19 16 21 19 164,9 4,9 4,4 4,0 4,5 5,1 2,9 4,3 2,9 4,3 3,5 4,1 4,1 3,50,8 1,1 1,4 1,2 1,4 1,4 0,9 1,2 1,0 1,5 1,5 1,2 1,3 1,10,6 0,8 0,6 0,5 0,6 0,8 0,4 0,5 0,5 0,6 0,4 0,5 0,5 0,52,9 2,7 2,3 1,8 2,4 2,5 1,3 1,6 1,7 1,6 0,6 2,0 1,8 1,80,35 0,34 0,34 0,41 0,35 0,23 0,24 0,24 0,23 0,09 0,32 0,27 0,26 XRAL- XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA92/XRF/NA A Quartz Quartz Quartz Andesite Dacite Dacite Volcanic flow Andesite Andesite Andesite Ultramafic Biotite Biotite Biotite porphyry porphyry porphyry dyke volcaniclastic porphyry flow in dyke in ore sand/ sand/ sand/ sediment siltstone siltstone siltstone

sulphides, fractures Preserved chlorite chlorite- altered biotite- actinolite pyrite chlorite


Appendix B, cont.

931008 931009 931010 931011 931012 931013 931014 931015 931016 931017 931018 931019 931020 931021428 428 428 428 428 428 428 428 320 320 320 30 583 583129,0-129,4 156,2-156,6 168,0-168,3 172,7-172,9 194,6-194,8 200,0-200,3 204,9-205,2 224,0-224,3 64,4-64,7 76,3-76,5 91,2-91,4 397,9-398,0 114,6-114,9 120,4-120,7-235 -262 -274 -279 -301 -306 -311 -330 -116 -127 -142 -74 -114 -1127 7 7 7 7 7 7 7 48 47 47 36 733 738170 170 170 170 170 170 170 170 570 570 570 305 410 4107204103 7204075 7204064 7204059 7204037 7204032 7204027 7204008 7204221 7204210 7204195 7204225 7204210 72042121716543 1716544 1716544 1716544 1716545 1716545 1716545 1716545 1716582 1716582 1716582 1716582 1717267 171727269,4 71,2 47,5 72,0 69,4 61,4 68,6 72,8 64,0 67,3 50,1 67,0 47,6 48,214,6 14,9 23,6 14,5 14,9 15,5 12,9 11,4 15,2 14,5 6,6 14,4 16,0 18,33,6 2,6 6,2 1,1 1,4 2,4 3,7 2,9 0,4 4,0 10,1 0,3 10,7 4,01,2 0,8 5,3 1,2 0,8 2,5 1,9 2,9 7,9 2,4 19,0 6,2 8,8 8,54,47 5,40 1,43 0,65 4,97 4,14 3,30 1,90 0,23 2,25 0,24 0,32 1,04 0,481,77 1,77 4,97 4,29 1,83 2,02 1,09 0,55 2,20 2,86 1,71 2,16 1,90 3,094,2 2,6 7,4 3,7 4,8 9,2 6,9 5,2 5,2 5,4 10,2 5,7 12,2 13,50,10 0,05 0,13 0,03 0,03 0,05 0,05 0,05 0,04 0,09 0,21 0,05 0,26 0,170,48 0,50 1,28 0,68 0,50 0,60 0,49 0,38 0,52 0,56 0,30 0,58 0,70 0,850,15 0,16 0,35 0,24 0,16 0,14 0,13 0,12 0,16 0,15 0,09 0,15 0,11 0,160,50 0,60 2,30 2,20 1,40 2,25 1,30 1,85 4,40 0,65 1,30 3,80 0,85 3,10100,4 100,5 100,5 100,6 100,2 100,2 100,3 100,0 100,2 100,1 99,8 100,7 100,2 100,417 5 33 12 22 60 42 7 120 1 40 1 17 4960 75 111 134 49 63 56 69 98 82 98 208 93 1638 18 17 13 7 13 10 10 1 11 5 11 2 412 23 60 1400 92 3 5 34 1 2 26 1 5 42 10 6 11 12 6 14 10 18 13 8 72 11 29 365 1 8 5 4 8 12 8 5 3 438 10 3 982 130 40 85 84 88 110 130 33 92 2000 64 30 36 365 395 778 598 609 677 441 144 309 555 102 269 589 86828 23 81 48 21 30 19 6 27 38 32 27 26 4716,5 19,7 41,6 15,3 13,8 19,3 15,1 14,5 16,6 15,8 33,9 19,1 46,5 48,2176 193 177 57 186 294 333 285 20 147 22 23 168 1433,2 3,9 4,5 3,1 3,5 4,2 3,1 3,1 4,7 3,7 0,6 2,6 0,5 1,09 7 7 6 9 11 8 7 10 10 4 8 2 53,9 3,4 3,2 1,9 4,0 3,6 3,8 2,9 3,6 3,9 1,6 3,7 0,4 0,52,9 5,2 3,0 1,9 5,2 24,2 10,9 4,2 4,1 3,1 1,4 3,7 12,4 1,225 21 31 23 21 11 18 16 24 21 7 28 10 11150 158 168 105 153 156 140 134 152 151 45 127 45 4832,7 24,1 42,1 12,2 22,5 25,5 23,9 18,6 21,2 23,1 6,6 23,6 6,9 7,664 50 80 26 48 53 50 39 44 46 16 50 16 2029 25 36 15 22 23 25 19 21 22 7 24 10 105,4 4,9 6,7 3,7 4,9 4,7 5,0 3,8 4,4 4,2 1,8 4,8 2,6 2,41,1 2,0 2,8 1,9 1,2 1,4 1,6 1,2 1,0 1,1 0,8 1,1 1,0 0,90,5 0,6 0,7 0,5 0,6 0,4 0,5 0,5 0,6 0,5 0,3 0,7 0,4 0,32,1 2,0 2,9 1,6 1,9 1,2 1,3 1,7 2,1 2,1 0,7 2,5 1,5 0,90,35 0,30 0,45 0,23 0,27 0,19 0,25 0,32 0,35 0,12 0,36 0,23 0,13 XRAL- XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA92/XRF/NA A Dacite Dacite Dacite Dacite Dacite Dacite Dacite Volcanic Dacite Dacite Ultramafic Andesite Biotite Biotite porphyry porphyry porphyry porphyry porphyry porphyry porphyry sandstone dyke in sandstone sandstone dacite

Preserved Preserved, chlorite- sericite- Preserved, chlorite- Preserved, biotite- biotite- akt bleached pyrite pyrite bleached pyrite chlorite chlorite

126 A. HALLBERG

Appendix B, cont.

931022 931023 931024 931025 931026 941001 941002 941003 941004 941005 941006 941007 942251 942252647 647 647 647 647 Strömfors 6 Strömfors 6 Strömfors 6 Strömfors 6 69 Strömfors 6 24 30 24221,1-221,5 285,6-286,0 388,1-388,5 418,3-418,6 461,5-461,8 578,0 514,0 434,0 379,0 158,0 529,0 58,3 334,0-334,7 51,9-239 -255 -279 -287 -297 -4111 -4143 -4183 -4211 -426 -4136 -102 -112 -105634 649 674 681 692 2600 2600 2600 2600 35 2600 89 49 90618 679 775 803 844 501 445 376 328 250 458 69 254 637204086 7204071 7204045 7204038 7204027 7205934 7205902 7205862 7205835 7203911 7205910 7204234 7204187 72042321717170 1717186 1717211 1717219 1717230 1718766 1718764 1718762 1718760 1716576 1718765 1716623 1716596 171662467,4 68,4 60,8 59,9 62,6 70,4 45,8 47,4 46,2 48,1 49,7 54,6 61,3 49,814,1 14,4 11,8 16,0 13,7 15,4 9,2 8,3 8,3 7,5 12,9 15,0 16,2 16,34,0 5,7 1,5 0,4 6,2 4,4 9,3 8,4 8,2 9,9 10,2 1,8 2,6 6,92,1 1,6 2,6 2,6 2,1 1,6 19,8 21,1 21,8 21,0 13,2 9,9 1,6 9,31,41 0,75 0,24 0,16 0,42 3,08 0,30 0,23 0,14 0,22 1,37 1,39 0,28 0,532,88 2,66 1,46 3,39 4,84 1,09 0,02 0,03 0,12 0,03 0,10 0,38 4,99 3,354,5 5,5 18,6 13,6 7,4 2,6 11,2 10,6 10,8 9,6 9,7 10,6 8,9 10,20,09 0,16 0,12 0,26 0,86 0,04 0,17 0,23 0,20 0,20 0,16 0,08 0,10 0,110,40 0,47 0,52 0,60 0,42 0,63 0,52 0,38 0,34 0,30 0,67 1,00 0,86 1,010,13 0,12 0,11 0,14 0,11 0,19 0,13 0,10 0,11 0,13 0,20 0,20 0,20 0,200,90 0,55 2,40 3,05 1,45 0,55 3,25 3,35 3,75 2,70 1,75 5,20 3,05 1,5098,0 100,2 100,1 100,1 100,1 99,9 99,6 100,1 99,9 99,8 99,9 100,2 100,0 99,29 7 77 145 5 194 1571 85 88 519 79 50 140 96 1127 4 1 54 14 2 1 170 810 17 8 14 14 9 8 8 17 12 8 5 69 59 73 67 37 16 4 7 7 4 8 600 300 600 700 300 81 130 66 60 65 61 2300 1800 2200 2000 1500 20 767 578 204 384 1040 263 54 1640 31441 46 52 59 76 23 10 85 8214,1 19,2 16,6 22,5 18,2 19,2 25,5 23,8 12,1 20,9 25,4 29,2 188 108 20 16 121 328 55 68 48 38 547 109 55 1033,7 3,7 3,3 3,1 2,9 3,0 1,2 0,7 0,8 0,6 1,6 1,9 7 10 6 9 7 4,1 3,5 2,9 2,9 3,8 3,2 2,4 1,4 1,7 1,7 3,9 1,8 2,0 4,3 3,3 2,7 3,1 3,1 2,0 1,8 1,5 1,2 1,0 2,3 1,0 1,1 17 20 25 23 24 23 17 17 25 19136 128 101 110 117 122 42 29 27 18 101 75 109 9423,2 23,1 15,2 18,4 20,8 17,2 12,4 6,3 7,1 5,9 16,5 12,7 14,4 48 47 33 38 45 38 26 15 15 15 37 26 32 21 21 17 20 20 18 12 6 7 8 19 13 19 4,3 4,4 3,6 4,0 4,3 4,3 2,9 1,5 1,6 1,6 4,5 2,8 5,4 1,0 1,6 1,1 0,7 1,4 1,3 1,0 0,5 0,5 0,7 0,9 0,9 1,9 0,6 0,5 0,6 0,5 0,6 0,6 0,4 0,3 0,3 0,3 0,4 0,4 0,8 2,0 2,2 2,4 2,1 2,0 2,5 1,1 0,9 1,1 0,9 1,5 1,5 2,2 0,31 0,36 0,37 0,32 0,31 0,39 0,15 0,13 0,14 0,11 0,23 0,23 0,33 XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA XRF/NA

Dacite Dacite Dacite Dacite Dacite Dacite UM in Ultramafic Ultramafic Ultramafic UM in Andesite Andesite Andesiteporphyry porphyry porphyry porphyry porphyry porphyry volcanic/sedi- dyke in dyke in dyke in volcanic/sediment contact sediment sediment dacite ment contact

breccia? breccia? chlorite- chlorite- chlorite- biotite-actinolite garnet garnet garnet


Appendix B, cont.

942253 942254 942255 942256 951501 951502 951503 951505 951504 951506 951507 951508 951509 95151024 24 30 30 99 69 23 343 97 4 4 4 4 1067,3 85,3 106,6 85,7 23,1-23,3 42,4-42,6 44,1-44,3 8,6-8,8 69,85-70,00 48,6-48,7 56,8-57,0 64,8-65,0 80,0-80,2 34,9-35,0-98 -91 -224 -233 -57 -310 -170 -166 -162 -155 -6188 86 101 104 18 31 -405 -406 -407 -409 24377 93 64 46 410 250 90 42 49 56 69 357204238 7204245 7204074 7204065 7204280 7204027 7204160 7204164 7204168 7204175 72042721716622 1716620 1716650 1716654 1716551 1716569 1716131 1716130 1716129 1716126 171677656,8 83,5 55,8 41,9 55,3 66,5 72 51,9 70,8 41,4 36,7 66,1 66,5 65,814,7 6,0 11,4 20,8 20,8 17,7 14 15,9 12,3 14,9 22,6 13,8 14,4 15,20,4 0,2 10,2 2,4 0,3 0,5 1,61 0,28 0,1 0,8 1,2 0,3 0,3 0,66,8 0,3 4,1 10,9 7,2 1,9 1,89 12,5 5,38 5,2 1,1 2,1 3,3 3,90,43 0,41 0,16 0,51 0,48 0,33 2,31 0,16 0,21 0,48 0,91 0,32 0,32 1,112,19 1,52 2,87 3,77 3,51 5,75 2,57 0,73 1,66 2,12 4,87 2,03 2,38 2,0510,9 5,0 9,5 10,5 6,1 2,1 3,34 10,5 4,98 22,0 19,4 11,4 7,8 6,60,04 0,01 1,03 0,38 0,03 0,02 0,02 0,06 0,13 0,05 0,01 0,06 0,07 0,040,94 0,23 0,57 1,02 0,64 0,61 0,411 0,776 0,251 1,04 1,54 0,84 0,72 0,590,21 0,11 0,07 0,14 0,10 0,01 0,14 0,01 0,27 0,17 0,20 0,156,80 2,60 2,30 5,80 4,95 3,20 1,55 5,95 3,4 12,30 11,70 2,90 3,35 3,90100,2 99,8 98,0 98,1 98,7 99,711 98,896 99,211 100,2 100,3 100,0 99,4 99,937 1170 95 1160 1 1 16 1 1 34 16 323 41 38287 4240 121 2090 58 106 66 75 134 144 47 204 59 130 7 28 6 12 19 27 45 18 6 4795 6660 40 583 6 17 5 24 12 74 69 12 5 25 1000 100 1400 100 100 219 171 1010 276 867 736 595 151 442 319 740 388 669 28840 43 52 81 55 76 43 17 29 38 86 31 36 43 49 27 200 49 25 49 219 11 13 63 78 18 15 126 1,3 5,9 5,3 2,2 0,8 3,7 3 1,2 24 2 9 8 37 24 27 7 27 13 24 17 22 2588 76 26 44 214 167 200 85 170 86 118 82 116 13813,2 31,4 22 17,5 29 67 50,4 37 17 34 25,1 19 4,4 8,2 6,2 4,9 1,2 1,2 1,05 0,9 0,7 1,1 0,8 0,7 1,9 3,7 2,9 2,2 0,30 0,55 0,39 0,32 XRF/NA XRF/NA Andesite Quartz Biotite Biotite Mafic rock Dacite Dacite Mafic dyke Quartz Sericite- Sericite- Sericite- Volcaniclastic Andesite porphyry siltstone siltstone volcaniclastic porphyry pyrite rock pyrite rock pyrite rock rock

chlorite sericite- chl/garnet chl/garnet quartz biotite- banded banded banded quartz altered pyrite phenocrysts chlorite phenocrysts

128 A. HALLBERG

Appendix B, cont.

951511 951512 951513 951514 951515 951516 95151710 18 20 22 679 679 19653,5-53,7 50,0-51,0 93,8-94,0 65,2-65,4 484,0-484,2 468,9-469,3 156,8-157,0-51 -38 -71 -58 -7344 -7349 -341243 200 -103 -220 -1435 -1430 64451 64 101 66 419 406 4107204282 7204295 7204265 7204276 7202636 7202631 72039841716776 1716733 1716431 1716313 1714542 1714548 171718350,2 58,2 48,4 56,5 71,1 66,7 53,517,0 18,1 16,2 14,8 14,5 15,3 12,12,0 0,8 6,4 1,6 2,2 0,5 0,56,4 3,4 9,1 6,2 1,9 4,0 9,13,11 3,50 1,03 1,45 0,88 0,20 0,031,23 2,34 0,39 1,65 4,38 4,97 0,1714,3 8,9 12,1 11,3 3,0 5,1 18,00,07 0,03 0,12 0,07 0,02 0,02 0,790,99 0,74 1,05 0,98 0,43 0,48 0,720,12 0,17 0,15 0,15 0,09 0,10 0,084,60 3,95 3,20 4,20 1,25 2,60 4,80100,0 100,1 98,1 98,9 99,7 99,9 99,7132 148 54 76 18 49 25247 521 111 534 61 117 18634 177 1 228 1 15 122 67 8 79 7 7 2540 100 100 300 341 243 169 473 871 666 12528 35 8 37 66 93 7 168 301 139 63 110 33 7 1,4 4,9 0,9 2,6 11 23 16 5 27 21 342 169 84 90 186 193 33 9,9 26,8 23 57 13 28 3,3 6,3 1,1 1,1 0,5 0,8 1,5 2,7 0,23 0,44 Andesite Andesite Andesite Andesite Pumice tuff Pumice tuff Volcaniclastic rock

altered altered quartz quartz garnet- phenocrysts phenocrysts chlorite


Appendix C

sample # Bo161 Bo39 Bo686-436 Bo686-450 Bo686-483 Bol679 Bol679 Bol679 Bol679 MHA930026 MHA930141 OIL940007 OIL971001A 6 1 Abh 161 686 686 686 679 679 679 679 678 core 436,0 450,0 483,0 local n -99 -4992 -4988 -4978 -7500 -7500 -7500 -7500 local e 85 821 814 798 -1250 -1250 -1250 -1250 z 250 dagen 357 369 396 N (RT90) 7204237 7209260 7204946 7204950 7204959 7202469 7202469 7202469 7202469 7202420 7208370 7202300 7203064 E (RT90) 1716619 1718200 1716937 1716931 1716915 1714718 1714718 1714718 1714718 1713250 1718420 1711850 1715814 SiO2 65,3 73,2 63,9 54,0 67,6 66,4 70,6 69,4 71,8 65,3 63,4 56,0 67,7 Al2O3 15,5 13,2 15,6 20,5 15,1 15,0 13,9 17,1 13,9 15,5 15,7 16,6 13,2 CaO 5,2 3,1 5,5 6,0 2,4 2,8 3,3 2,5 2,9 5,2 3,4 7,0 4,3 MgO 2,3 0,8 0,9 1,7 2,1 2,2 2,7 0,5 2,7 2,3 1,7 3,9 1,3 Na2O 2,30 3,81 4,40 5,00 3,60 4,24 2,42 6,27 2,14 2,30 3,36 3,40 3,59 K2O 2,16 1,25 1,40 1,80 1,50 0,92 2,10 1,11 2,35 2,16 1,90 0,23 0,85 Fe2O3 5,8 3,1 4,5 5,9 5,2 6,4 4,3 1,9 3,5 5,8 6,6 10,5 5,5 MnO 0,11 0,09 0,10 0,10 0,10 0,06 0,08 0,01 0,04 0,11 0,08 0,14 0,11 TiO2 0,52 0,19 0,81 1,29 0,52 0,60 0,51 0,64 0,43 0,52 0,67 0,91 0,42 P2O5 0,15 0,05 0,20 0,30 0,10 0,14 0,13 0,17 0,08 0,15 0,17 0,17 0,12 LOI 1,00 1,25 1,50 1,60 1,60 1,30 0,55 0,65 0,35 1,00 1,20 1,20 2,60 SUM 100,3 100,0 98,8 98,2 99,8 100,0 100,5 100,3 100,0 100,3 98,2 100,0 99,7 Cu 7 <2 4 11 2 15 4 18 <2 7 38 Zn 74 71 75 109 76 165 79 86 66 74 130 <50 263 Pb 26 <2 3 7 13 35 <2 25 4 26 Cd Au <5 170 Ag <5 <5 As <3 6 41 6 79 14 3 <2 <2 Sb 0,5 1,5 Bi Br <1 <1 Co 22 39 34 Ni 4 <2 3 <2 3 <2 Cr 10 20 57 Mo <5 <5 2 V 15,1 Ba 467 310 358 444 426 276 293 364 281 467 735 80 205 Cs <3 <3 Rb 34 25 22 29 23 18 35 17 36 34 56 12,0 13,7 Sc 17,8 31,3 20,1 Sr 250 116 244 386 371 311 256 252 253 250 303 287 237 Be <0,5 Hf 5,0 <1 5,3 Sn 1,16 Ta <2 <2 <2 <2 <1 <1 0,56 Nb 8 10 10 7 9 8 9 10 10 8 9 3 4 Th 18 7 5 5 2,3 <0,5 3,3 U <2 <2 <2 <2 1,3 <0,5 2,3 Y 27 31 <1 7 4 19 22 21 25 27 29,0 15,0 20,7 Zr 164 174 121 109 138 126 139 162 182 164 147 80 105 La 13,0 7,0 17,6 Ce 31,0 12,0 35,9 Nd 10,0 <10 18,6 Sm 3,1 2 3,65 Eu 1,3 1 0,95 Tb <0,5 <0,5 0,69 Yb 2,4 1,5 2,36 Lu 0,38 0,25 0,50 Ref. ref 4 4 4 4 4 4 4 4 4 4 5 5 5

method-lab. XRF- XRAL-93-95/XRF XRF, ICP-ms XRF, ICP-ms XRF, ICP-ms XRF, ICP-ms XRAL-93- XRAL- XRAL- SGAB-97/ICP- F,ICP,NA 95/XRF 96/XRF+IN 96/XRF+IN AES,MS AA AA

Rock Andesite, Rhyolite, Andesite Andesite- Dacite, Mafic mass Dacite Dacite Mass flow Dacite, Feldspar Andesite Volcanic intrusive, feldspar basalt basalt feldspar flow porphyry porphyry intrusive, porphyritic mass flow feldspar porphyritic porphyritic feldspar lava porphyritic porphyritic

Alter., struct., minerali-sation fine grained, fine grained, amygdules amygdules

130 A. HALLBERG

Appendix C, cont.

B-01 B-04 B-06 B-07 B-08 B-20 B-22 B-23 B-25 B-27 B-30 B-31 B-32 7206550 7207050 7207050 7207100 7207250 7205980 7206360 7206380 7205400 7205450 7204550 7204600 72046001717210 1716730 1716730 1716690 1716480 1716600 1716430 1715590 1717120 1717130 1714770 1714850 171485073,4 66,1 77,7 70,3 72,6 73,1 76,3 74,6 48,2 67,7 70,6 77,4 66,012,2 15,3 9,5 10,9 12,8 11,9 10,6 12,0 11,7 16,4 12,8 10,1 15,42,3 2,1 2,2 2,8 3,0 4,1 1,4 0,7 11,8 3,0 3,9 2,7 1,71,6 1,5 0,9 1,6 1,1 1,6 2,7 1,1 10,7 1,1 3,0 1,6 2,30,49 1,44 2,39 0,27 1,84 0,54 0,91 1,64 1,21 4,23 0,36 0,75 0,613,66 6,80 2,61 5,86 3,27 3,02 2,64 3,76 0,43 1,92 3,58 3,22 6,444,4 4,4 2,6 4,1 3,2 4,4 3,5 3,6 12,4 2,7 4,2 3,0 4,30,07 0,10 0,05 0,15 0,07 0,17 0,08 0,11 0,26 0,07 0,04 0,05 0,050,33 0,38 0,24 0,29 0,37 0,39 0,26 0,31 0,66 0,72 0,36 0,20 0,370,03 0,02 0,02 0,01 0,02 0,09 0,02 0,01 0,25 0,07 0,071,38 1,29 2,65 3,74 1,02 1,13 1,70 1,42 3,56 1,30 1,16 0,75 1,8199,9 99,5 100,8 100,1 99,2 100,3 100,0 99,2 101,0 99,4 99,9 99,7 99,05 4 5 4 4 6 22 4 3 16 5 5 384 77 58 63 56 219 112 59 173 43 75 33 7316 26 14 25 11 19 52 11 15 19 16 17 22 31 9 33 14 32 26 35 35 14 165 3 3 4 10 10 15 19 357 6 15 12 1220 4 5 5 16 15 5 18 1679 10 6 6 6 77 51 15 35 76 75 25 66 260 64 8 18 29641 803 322 890 437 1209 493 1551 121 353 224 357 541 78 80 36 80 43 36 10 65 5 8 67 7916 19 13 14 16 15 13 14 35 19 14 12 12140 100 90 135 102 134 79 60 137 261 91 95 78 14 15 11 12 12 10 11 29 32 21 27 22 22 13 26 13 20 19 15 22148 182 112 133 127 107 125 152 56 124 116 88 13426,21 27,59 22,26 20,98 17,84 9,44 19,97 47,64 56,73 47,14 40,54 38,61 22,55 41,44 24,81 27,22 21,11 19,35 18,44 12,31 20,24 5,35 6,12 4,68 4,43 4,39 3,27 4,48 1,09 1,23 1,10 1,14 0,98 0,87 1,24 2,72 3,10 2,20 2,21 2,62 1,11 1,83 0,19 6 6 6 6 6 6 6 6 6 6 6 6 6 ICP-Nan ICP-Nan ICP-Nan ICP-Nan ICP-Nan ICP-Nan

Ryolite Ryolite Ryolite Ryolite Ryolite Dacite Ryolite Ryolite Ultramafic Dacite Volcaniclastic Volcaniclastic Volcaniclastic porphyry, dyke in dacite porphyry rock rock rock volcaniclastic


Appendix C, cont.

B-33 B-34 B-35 7204620 7203950 72035501714900 1715130 171462062,5 58,2 67,115,5 14,8 12,93,6 7,7 2,82,5 2,4 2,42,25 3,89 3,703,07 1,49 0,968,6 8,1 8,00,09 0,15 0,110,66 0,65 0,730,20 0,16 0,232,11 3,73 1,30101,0 101,1 100,211 28 2297 100 11011 15 12 22 21 11 125 34 11 59 132 58384 344 148 21 25 823 27 28110 310 324 20 18 27114 92 129 18,26 40,91 21,68 5,18 1,63 2,43 6 6 6ICP-Nan. ICP-Nan ICP-Nan

Andesite Dacite Dacite porphyrydyke porphyry

actinolite? actinolite

References(6) Vivallo (1987)(4) Allen et al. (1996)(5) Lundström & Antal (2000)

132 M. RIPA

Ripa, M,. 2001: A review of the Fe oxide deposits of Bergsla-gen, Sweden and their connection to Au mineralisation. In Wei-hed, P. (ed.): Economic geology research. Vol. 1, 1999–2000. Upp-sala 2001. Sveriges geologiska undersökning C 833, pp. 132–136. ISBN 91-7158-665-2.

This presentation results from a review of a selection of pros-pecting reports and published papers on gold in the iron ores of Bergslagen, Sweden. It will describe the different types of iron ore deposits, their geological and volcanic setting, and their rela-tion to gold mineralisation.

The Bergslagen region has had a long history of mining base and precious metals (Cu, Zn, Ag), as well as iron. The country rocks to the ore deposits belong to a Palaeoproterozoic, meta-morphosed volcano-sedimentary succession (the ”leptite forma-tion”). Most mineral occurrences are hosted by skarn-altered car-bonate rocks interlayered with volcanogenic ash-siltstone strata. There is a genetical relationship between more or less coeval subvolcanic intrusions emplaced into the host rocks and miner-alisation. The supracrustals were also intruded by early-orogenic ultramafic to granitic plutons, mafic dykes, late-orogenic gran-ites, post-orogenic plutons, and later dolerites. The supracrus-tal rocks, the early-orogenic plutonic rocks, and the mafic dykes were deformed and metamorphosed at varying grades during the Svecokarelian orogeny.

In at least 39 iron ore deposits in the Bergslagen area gold occurrences have been noted. Gold occurs only in a few geo-graphically restricted areas, which are spatially close to some of the younger plutonic rocks. From this preliminary study it seems certain that most, if not all, gold in the area formed in re-lation to regional Svecokarelian metamorphism and coeval plu-tonic activity. The connection to Fe mineralisation is coinciden-tal, and is probably due to the fact that the host rocks were fa-vourable horizons for both mineralising events.

Magnus Ripa, Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden. e-mail [email protected]

Introduction

This presentation results from an initial study on the geo-logically interesting and genetically diversified iron ores of the intensely mineralised ore district of Bergslagen, south central Sweden. The study has this far involved reviewing of a selection of prospecting reports available at the SGU Mineral Information Office at Malå and published pa-pers. The presentation will describe the different types of iron ore deposits, their geological and volcanic setting, and their relation to gold mineralisation.

The Bergslagen region has had a long history, since medieval days, of mining base and precious metals (Cu,

A review of the Fe oxide deposits of Bergslagen, Sweden and their connection to Au mineralisation

Magnus Ripa

Zn, Ag), as well as iron. During the last century, tungsten has also been mined intermittently. Today, three mines for base metals (mainly Zn) are in operation in the area. The total tonnage of the region is dominated by iron ore pro-duction, which amounts to more than 420 million tonnes of ore.

Regional geology and volcanic setting of the Bergslagen iron ores

The country rocks (Fig. 1) to the Bergslagen iron ore deposits belong to a Palaeoproterozoic, c. 1900 Ma old (Lundström et al. 1998), metamorphosed volcano-sedi-mentary succession, informally known as the ”leptite for-mation” (e.g. Magnusson 1936). The metasupracrustal rocks consist dominantly of rhyolitic volcanic, subvolcan-ic, and volcaniclastic rocks deposited in a submarine envi-ronment (Oen et al. 1982, Van der Welden et al. 1982, Lundström 1987, Allen et al. 1996). Subordinate inter-mediate and mafic volcanic rocks together with chemical, epiclastic, and organogenic sedimentary rocks occur at different stratigraphic levels of the volcanic pile.

Most mineral occurrences are hosted by skarn-altered carbonate rocks interlayered with volcanogenic ash-silt-stone strata interpreted to represent distal volcanic facies (Allen et al. 1996). There is, however, a spatial and prob-ably genetical relationship between more or less coeval subvolcanic intrusions emplaced into the distal facies and mineralisation. In fact, one type (type c below) of iron ore is partly hosted by subvolcanic rocks. Thus, the ores are hosted by distal facies but are genetically related to this somewhat later phase of igneous activity (Allen et al. 1996). Overlying (and locally also underlying) the vol-canic rocks are argillites, greywackes, quartzites, and con-glomerates (Lundström 1995).

The supracrustal rocks were intruded by early-orogenic ultramafic to granitic plutons, mafic dykes, late-orogenic granites, post-orogenic plutons, and later dolerites (Lund-qvist 1979). The supracrustal rocks, the early-orogenic plutonic rocks, and the mafic dykes were deformed and metamorphosed at varying grades during the Svecokare-lian orogeny (Lundqvist 1979). Mesoproterozoic and Phanerozoic sedimentary rocks overlay the metamorphic rocks in some areas.

133A REVIEW OF THE FE OXIDE DEPOSITS OF BERGSLAGEN, SWEDEN AND THEIR CONNECTION TO AU MINERALISATION

Classification and description of the iron ores

Traditionally, the iron ores of the Bergslagen area have, for both geological and metallurgical reasons, been divided into two major groups (Geijer & Magnusson 1944). The discriminant factor in this primary division is the phos-phorus (or apatite) content of the ores. One group con-

tains (considerably) less than 0.2 wt-% P and the other 0.2 wt-% or more. Based on their style of occurrence, the ores are also subdivided (Geijer & Magnusson 1944):

a) banded quartz-hematite(±magnetite) oreThis type of deposit occurs as thin, mm to cm scale,

alternating layers of quartz and hematite. The layers are parallel to bedding in the surrounding country rocks, and may amount to c. 20 m in thickness. The hematite is

Stockholm

Garpenberg

Falun

Grängesberg

Stollberg

Yxsjöberg

Zinkgruvan

Diabase

Fe-oxide deposit

Au-bearing Fe-oxide deposit

Young sedimentary rocks

Young plutonic rocks (c. 1.85–1.65 Ga and 1.5 Ga)

Svecofennian metasupracrustal rocks

Old plutonic rocks (c. 1.89–1.85 Ga)

SWED

EN

50 km

Fig. 1. Schematical geological map of the Bergslagen area. Fe oxide deposits (n=5955) and Au-bearing Fe oxide deposits (n=39) are shown.

134 M. RIPA

locally reduced to magnetite. Ore grades are c. 50 % Fe.b) skarn-limestone magnetite oreThe gangue of the skarn-iron ores has given rise to the

expression skarn, now in international usage in a slightly more restricted way than in the original sense. The ores occur as beds or massive to disseminated replacements in variably skarn-altered carbonate rocks. Iron mineralisation was locally accompanied by manganese and/or base metal enrichment. Ore grades are 43–62 % Fe and 0–5 % Mn. Ore grade base metals occur in several deposits.

c) massive apatite-rich magnetite (±hematite) oreThe apatite-rich magnetite ores occur as massive re-

placements in extrusive volcanic host rocks. They are also to some extent hosted by subvolcanic intrusions emplaced into the latter. As the subvolcanic rocks most likely are genetically related to mineralisation, this (Kiruna) type of iron ore can be considered to belong to the group of por-phyry-style deposits. A deposit of this type, at Gränges-berg (Fig. 1), was the single most productive in the re-gion (c. 150 million tonnes). Grades are 56–63 % Fe and 0.9–1.3 % P. Locally, apatite contents were high enough (27–43 %) to be mined as a by-product.

d) disseminated apatite-bearing magnetite oreThe apatite-rich magnetite ores grade laterally and

stratigraphically into disseminated deposits. Due to the more dispersed character of these, only a few were eco-nomic.

Type a) and bedded varieties of type b) most likely formed as exhalites interbedded with the surrounding vol-canic rocks (Geijer & Magnusson 1944). The other types are interpreted to have formed slightly later during the emplacement of subvolcanic intrusions into the volcanic rocks (Allen et al. 1996), or during the intrusion of syn-genetic plutonic rocks (Geijer & Magnusson 1944). It is known that some iron mineralisation occurred in connec-tion with the intrusions of late-orogenic magmas and con-temporary deformational events (Bergman et al. 2001). Especially ores of types c) and d) may totally or in part have formed this way.

Other types of deposits in the Bergslagen area

Base metal and tungsten deposits are also found in the region. The former occur as volcanic-hosted massive sul-phide ores or as disseminated to massive sulphide replace-ments, locally in association with some of the skarn-iron ores. The VHMS-type ores are exemplified by the famous Falun (Fig. 1) Cu-Zn deposit and the still producing Zinkgruvan (Fig. 1) Zn deposit (Zinkgruvan Mining AB). Some sulphide deposits occur in skarn or limestone as-sociations, but without an immediate connection to iron

ore. Examples of this type are the two producing Garpen-berg mines (Boliden AB; Fig. 1) of mainly zinc. A mined sulphide deposit associated with iron ore is the Stollberg (Fig. 1) Fe-Pb-Zn-Mn(-Ag) ores (Ripa 1988, 1994). The tungsten deposits are also hosted by limestone or skarn-bearing rocks, locally overprinting iron ore formations. An example is the Yxsjöberg (Fig. 1) scheelite deposit (Lind-roth 1922, Ohlsson 1979).

The base metal mineralisation occurred at the same time as some of the skarn-iron ores formed; i.e. more or less coeval with the volcanic rocks (Ripa 1994), whereas the tungsten ores formed c. 100 million years later in re-lation to, but in a late stage of, Svecokarelian metamor-phism (Ohlsson 1979, Romer & Öhlander 1994).

Gold in the iron ores

Gold occurrences in the Fennoscandian Shield and in the Bergslagen region have been reviewed by Gaal & Sund-blad (1990), Bergman (1990), and Bergman & Sundblad (1991). According to those studies, gold may occur in all of the ore types noted above, and do so in at least five iron ore deposits. In these, a positive correlation between Au and Bi is noted.

This study shows, that in at least 39 iron ore deposits in the Bergslagen area gold occurrences have been noted (Fig. 1). It must be remembered that in most of these cases the method of gold detection and the amount of gold have not been stated. Furthermore, in deposits where gold has not been noted, it is uncertain whether a proper investigation for gold has been undertaken or not. Gold grades as high as 45 g/ton are locally reported, but tonnage has not been evaluated (Geijer & Magnusson 1944).

The gold mineralisation may have occurred at four different stages:

1) synsedimentary with the iron ore-types a) and part-ly b) above,

2) at the same time as the other iron ores and the sul-phide ores,

3) during regional (Svecokarelian) metamorphism (as the tungsten deposits), or

4) during local (Sveconorvegian) deformation at c. 1.0 to 0.9 Ga.

If gold mineralisation occurred at stage 1 or 2 (and possibly 3; see above), there should be a genetical connec-tion between iron and gold (and base metal) precipita-tion, i.e. the ore-forming process was the same for both metals. If gold mineralisation occurred at stage 3 or 4, the connection between iron and gold may be that both formed in a favourable horizon, but the ore-forming proc-esses were different. If valid, the latter implies that gold may be found laterally away from known iron deposits.

135A REVIEW OF THE FE OXIDE DEPOSITS OF BERGSLAGEN, SWEDEN AND THEIR CONNECTION TO AU MINERALISATION

According to Gaal & Sundblad (1990) and Bergman & Sundblad (1991) it is likely that gold mineralisation oc-curred at stages 2 and 3, because it is related to late quartz veins and skarn formations.

In this preliminary investigation, a selection (n=59; those containing a thorough geologic description) from 109 available prospecting reports and papers on gold in Bergslagen at the SGU Mineral Information Office in Malå has been studied in order to try to evaluate the rela-tion between gold and other metals. Judging from these reports and papers (including those cited above), it is fairly evident that Au in all cases formed, or at least could have formed, at stage 3 or 4 because it is related to quartz veins and skarn formations, which are at the oldest synmeta-

morphic. Stage 4 gold is, however, this far, only known from rocks situated to the west of the area indicated in Figure 1. Thus, with a few possible exceptions (e.g. Berg-man & Sundblad 1991), most Bergslagen gold apparently formed at stage 3.

In Figure 2, the position of all known gold occur-rences in Bergslagen are plotted on a simplified version of the map of Figure 1. In this version, the younger plu-tonic rocks are high-lighted (in red). It can be noted that gold only occurs in a few geographically restricted areas of Bergslagen, which are spatially close to, but never ac-tually within, some of the younger plutonic rocks. A ge-netical connection between Au and younger granites have been suggested in some of the prospecting reports men-

Gold occurrencesYoung plutonic rocks

All other rocks

50 km

Fig. 2. Simplified geological map of the area shown in Figure 1. Younger plutonic rocks are high-lighted in red. Yellow dots indicate all known gold occurrences (n=145) in Bergslagen.

136 M. RIPA

tioned above and by Gaal & Sundblad (1990) and Berg-man (1994).

Conclusions

From this preliminary study it seems certain that most, if not all, gold in the Bergslagen area formed in relation to regional Svecokarelian metamorphism and coeval plu-tonic activity. The connection to iron mineralisation is coincidental, and is probably due to the fact that the host rocks were favourable horizons for both mineralising events. The magnetite-bearing, favourable horizons are easily followed by magnetic geophysical methods.

Acknowledgements

This paper and a preliminary version of it (the latter pre-sented as a poster at the joint SGA-IAGOD meeting in London 1999; Ripa 1999) have benefited by reviews by K. Billström, D.J. Blundell, O. Martinsson, and P. Wei-hed.

References

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Economic geology research, volume 1 1999-2000