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Regional geology and ore deposits: Bergslagen is a mining district in central Sweden that has been a major metal producer for well over 1000 years and which contains over 6000 ore deposits and mineral prospects (Stephens et al., 2001). The district is the intensely mineralized part of a Paleoroterozoic (mainly 1.90-1.87 Ga), felsic magmatic province in the Baltic Shield (Fig. 1). The district contains a diverse range of ore deposit types, including banded iron formation, magnetite-skarn, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and stratabound Zn-Pb-Ag-(Cu-Au) sulphide ores, REE deposits and W skarn. In addition, Bergslagen is a major exporter of dolomite products. Most of the ore deposits are associated with skarns, meta-limestones and hydrothermally altered metavolcanic rocks. Skarns are extremely common in Bergslagen and the word "skarn" originates from this region. Four polymetallic sulphide deposits are currently being mined (Zinkgruvan, Garpenberg, Garpenberg Norra and Lovisa; Fig. 1) and two of these will be visited during the excursion. However, if a longer historical perspective is considered, Bergslagen can also be regarded as a major iron ore province and has produced a major part of Europe's iron and steel from Mediaeval times to the 1800's. Dannemora, which this excursion will visit, was the last iron ore mine in operation and closed in 1992. This mine is planned to open again in the near future.

During the past few years there has been a dramatic rise in interest in Bergslagen from mineral exploration companies. This is mainly due to the potential for new discoveries of polymetallic sulphide ores such as the recent discoveries at Garpenberg by Boliden Mineral, the potential in Bergslagen for discoveries of Iron Oxide-Copper-Gold deposits (IOCG), and the high demand for metals on world markets.

The supracrustal stratigraphy in the Bergslagen region is dominated by a 1904-1891 Ma Palaeoproterozoic, mainly felsic meta-volcanic succession that locally has a stratigraphic thickness of over 7 km (Lundqvist, 1979; Allen et al., 1996; Lundström et al., 1998; Stephens et al., 2001). The meta-volcanic succession is both underlain and overlain by argillite-arenite meta-sedimentary successions. The volcanic succession is dominated by felsic volcaniclastic rocks derived from large, shallow marine to subaerial, pyroclastic caldera volcanoes. Marble units occur sporadically, interbedded with the volcaniclastic rocks, and are most abundant in the upper part of the volcanic succession. They range from thin intercalations of limited lateral extent (100s of metres to a few kilometres) to thick (100's of metres) regionally extensive (10's of kilometres) horizons. The supracrustal rocks are attributed to an intra-continental rift (Oen et al., 1982) or a back-arc extensional basin developed on continental crust (Allen et al., 1996).  The supracrustal rocks have been extensively deformed and metamorphosed, and are now exposed as isolated, mainly upper greenschist to upper amphibolite facies inliers enclosed by abundant early orogenic to post-orogenic intrusions (Fig. 1).

 Garp Ignimbrite  Photo: Rodney Allen
 Garp Ignimbrite Photo: Rodney Allen



The supracrustal stratigraphy in the Bergslagen region is dominated by a 1904-1891 Ma Palaeoproterozoic, mainly felsic meta-volcanic succession that locally has a stratigraphic thickness of over 7 km (Lundqvist, 1979; Allen et al., 1996; Lundström et al., 1998; Stephens et al., 2001). The meta-volcanic succession is both underlain and overlain by argillite-arenite meta-sedimentary successions. The volcanic succession is dominated by felsic volcaniclastic rocks derived from large, shallow marine to subaerial, pyroclastic caldera volcanoes. Marble units occur sporadically, interbedded with the volcaniclastic rocks, and are most abundant in the upper part of the volcanic succession. They range from thin intercalations of limited lateral extent (100s of metres to a few kilometres) to thick (100's of metres) regionally extensive (10's of kilometres) horizons. The supracrustal rocks are attributed to an intra-continental rift (Oen et al., 1982) or a back-arc extensional basin developed on continental crust (Allen et al., 1996).  The supracrustal rocks have been extensively deformed and metamorphosed, and are now exposed as isolated, mainly upper greenschist to upper amphibolite facies inliers enclosed by abundant early orogenic to post-orogenic intrusions (Fig. 1).

Several supracrustal inliers, and especially those in the west, follow a first order stratigraphic cycle of coarse-grained, poorly stratified felsic volcanic rocks, overlain by finer grained, more stratified felsic volcanic rocks with abundant limestone interbeds and ore deposits, in turn overlain by argillite-turbidite sedimentary rocks. This cycle has been attributed to a first order volcanotectonic evolution from intense volcanism and crustal extension, through waning volcanism and continued subsidence, to post-volcanic thermal subsidence (Allen et al., 1996).

In spite of the degree of deformation and grade of metamorphism, primary textures are locally well preserved in several of the low and medium metamorphic grade areas (Sundius, 1923; Allen et al., 1996). This has allowed interpretation of the volcanic and sedimentary processes involved in deposition of the Bergslagen succession, and has increased understanding of the facies architecture and hence palaeogeographic setting. It has also provided insight into the setting of the various ore deposit types, several of which are spatially associated with volcanic vent complexes. The Bergslagen succession is interpreted to contain a mosaic of interfingering proximal, medial and distal products of numerous rhyolitic pyroclastic caldera volcanoes (Allen et al., 1996). The vent areas were mainly shallow marine to sub-wave base and the medial to distal flanks of the volcanoes were mainly sub-wave base. Much of the succession records re-deposition of primary volcanic ejecta by sedimentary processes during and following eruptions. The volume and frequency of pyroclastic eruptions in Bergslagen was such that the whole succession is dominated by clastic rocks with a felsic pyroclastic provenance. Other subordinate but important volcanic facies include felsic and mafic subvolcanic intrusions and lavas.


Kiirunavaara Photo: Jyrki Korteniemi

The marbles are the main non-volcanic units within the Bergslagen volcanic successions. They have been variously interpreted as microbial-sedimentary (i.e. stromatolitic) (Collini, 1965; Boekschoten et al., 1988; Lager, 2001) or hydrothermal (Sundius, 1923; Gebeyehu and Vivallo, 1991; Vivallo, 1985). Allen et al. (2003) concluded that most marble units in Bergslagen represent tabular to lensoidal, microbial stromatolite limestone reefs. Most documented examples of Proterozoic carbonates occur as part of the fill of "normal" sedimentary basins (e.g. platform settings, marginal and foreland basins). The Bergslagen marbles are unique in that they formed in a basinal environment completely swamped by the products of voluminous felsic pyroclastic volcanism.

The skarns that are commonly associated with Bergslagen ore deposits range from massive, heterogeneous, Fe-Mn-rich and/or Mg-rich skarns in marble units, to banded/bedded skarn in felsic siltstone-sandstone. The former have been given diverse origins, including the synvolcanic metasomatic replacement of limestone beds (Sundius, 1923), regional metamorphism of hydrothermally (synvolcanic) altered limestones and volcanic rocks (Ripa, 1988, 1994; Allen et al., 1996, 2003), and intrusion-related skarns (Magnusson, 1948, 1960, 1970). Magnusson (1970) believed that many iron ores were deposited as sediments along with the limestones, and that skarn developed by reaction between these original sedimentary components and their host-rocks during regional metamorphism. The banded skarns were recently interpreted by Allen et al. (2003) as interbedded felsic ash-siltstone and calcareous ash-siltstone with a Mg-Fe-Mn hydrothermal component, deposited in a sub-wave base environment. The hydrothermal component is attributed to either deposition of hydrothermal sediment at the same time as the carbonate-bearing layers, or to infiltration of hydrothermal solutions into the carbonate-bearing beds after deposition. The skarn resulted from subsequent metamorphism of the calcareous ash-siltstone and hydrothermal component in a similar fashion to that envisaged by Magnusson (1970). Most of the skarns are not spatially associated with particular igneous intrusions.

The regional folds in the central part of the Bergslagen region are D2 structures (F2 folds) with an associated S2 axial plane cleavage and L2 stretching lineation that deform an earlier (S1) tectonic fabric (Stålhös, 1984; Carlon and Bleeker, 1988; Stephens and Wahlgren, 1993; Stephens et al., 2001; Allen et al., 1996, 2003). The F2 folds are steep, tight structures that commonly vary in plunge, and in some cases strike, along their axes. In the central part of Bergslagen, the D2 structures have north to northeast trends. However, Stephens and Wahlgren (1993) and Stephens et al. (2001) have shown that at the northern and southern margins of Bergslagen the D2 structures have a west-northwest orientation controlled by strong ductile D2 and/or D3 deformation zones.

F1 folds are distinct in some areas, such as Sala, where they are tight to isoclinal, WNW trending structures with steep axial surfaces and a foliation (S1) parallel to the axial surface and subparallel to bedding. However, in many other areas such as Garpenberg, F2 folding and D2 shearing have obscured any F1 folds, and their former presence is only be inferred from a relict S1 foliation and S1-S2-bedding relationships. The intensity of foliation and metamorphism varies greatly from area to area. Ductile shear zones and brittle faults are common. Peak metamorphism outlasted the main ductile deformation, resulting in strong granoblastic recrystallization in amphibolite facies areas.  Migmatites, gneisses and pegmatites are also common in the higher grade and the argillite-dominated areas. 

 
Figure 1. Excursion route map showing ore deposits and regional geology. Modified from Geological Survey of Sweden data (Stephens et al., 2001). Geology: brown = early orogenic/synvolcanic granitoids, red = syn- to late-orogenic granitoids, yellow = metavolcanic rocks, blue = metasedimentary rocks, green = mafic rocks.

Day by day outline (and local guides):

Day 0: Thursday 14 August
Fly Oslo - Stockholm (Arlanda) SAS SK882 16.30-17.30 (770 Skr = 83 €)
Arrive at Arlanda by 18.00. Minibus transport from Arlanda airport to Dannemora (1 hour)
Accommodation: Gammel Tammen, Österbybruk Herrgård

Day 1: Friday 15 August
Introduction to Bergslagen geology (Michael Stephens).
Dannemora iron ore deposits (Lennart Falk): description of the old orebody and mining operation, which continued from Medieval times to 1992, and an introduction to the planned new mining operation. The Dannemora iron ore deposits comprise limestone- and skarn-hosted Mn-rich and Mn-poor magnetite ores. Some minor but interesting polymetallic sulphide lenses also occur. The rocks are tightly folded and steeply dipping, however, original rock textures are well preserved and the metamorphic grade is probably greenschist. The iron ores have been interpreted as synvolcanic stratabound and stratiform hydrothermal deposits formed in stromatolitic limestone (Lager, 2001). We will study outcrops of well preserved felsic volcaniclastic rocks, mafic sills, limestone and stratiform iron mineralization, which are interpreted to be stratigraphically equivalent to, or lie below, the main iron ore interval (Peter Dahlin). Lunch: Dannemora. Drive to Sala. Accommodation and evening meal: Sätra Brunn (Brunnslogi).

Day 2: Saturday 16 August
Sala: Visit the Tistbrottet and Finntorpet dolomite quarries, which occur in thick sequences of interbedded stromatolitic dolomite and rhyolitic ash siltstone-sandstone. These quarries and nearby outcrops provide evidence for the microbial stromatolitic origin of the limestones, which host most of the ore deposits in Bergslagen. The felsic interbeds in the limestones were interpreted by Allen et al. (2003) as storm wave (?tsunami) reworked pyroclastic fall deposits and volcanic sandstones. These interbeds together with the limestones provide a unique record of the volcanic activity, environment and subsidence history in the basin. Sala silver mines underground tour: Sala township grew up in medieval times around the most important silver mines in Sweden. The ore deposits comprise silver-bearing galena-sphalerite veins and stratabound polymetallic sulphide-skarn ores within dolomitic marble (Allen et al., 1996, 2001; Jansson, unpublished MSc thesis 2007). We will have an underground mine tour in the Medieval workings where the stratigraphy, structure and some of the ores are still well exposed.
Lunch at Sala mines. Drive to Garpenberg: study some outcrops or drill core if time permits. Accommodation and evening meal: Garpenbergs Herrgård.

Day 3: Sunday 17 August.
This day will focus on the mines, stratigraphy and structure of the Garpenberg area. The Garpenberg mines have past production and current reserves of about 40 MT of massive, semi-massive and disseminated Zn-Pb-Ag-(Cu-Au) ore. The deposits are irregular, stratabound, multi-lens and pod-like. They are hosted in a meta-limestone unit and adjacent felsic metavolcanic rocks, are closely associated with Mg-rich tremolite- and diopside-skarns within the meta-limestones, and have intense footwall silicification, Mg-rich alteration (phlogopite-biotite-garnet-cordierite-quartz schists) and K±Mg alteration (muscovite-phlogopite-quartz schists). The ore deposits are interpreted to be essentially synvolcanic hydrothermal deposits that formed by stratabound replacement of limestone and adjacent volcanic rocks within the caldera vent of a large marine rhyolite-dacite volcano (Allen et al., 1996, 2003). The volcanic succession comprises an alternation of rhyolitic and dacitic juvenile volcaniclastic rocks including pyroclastic flow deposits, and lesser mafic extrusions, mafic sills and felsic intrusions. The ore deposits have been significantly modified by subsequent deformation and metamorphism, and the location of economic ore bodies within the mineralized system is strongly influenced by these later events. We will have an underground mine tour at Garpenberg Norra, study drill cores and visit outcrops of the volcanic host sequence, limestone and skarn.
Garpenberg  Photo: Rodney Allen
Garpenberg Photo: Rodney Allen






Ryllshyttan magnetite-zinc mine: The Ryllshyttan mine produced 1 MT of high grade Zn ore from within a magnetite-skarn deposit in a limestone bed about 800m stratigraphically below the Garpenberg horizon. The mine is closed but has good surface exposures of volcanic rocks, skarn and folds in the host sequence. Bedded skarn deposits with stratiform magnetite layers and diverse skarn compositions are particular well exposed. Lunch at Garpenberg mine. Accommodation and evening meal: Garpenbergs Herrgård.

Day 4:
Monday 18 August.
Drive to Stollberg. The Stollberg mines are located 5 km NE of the town of Ludvika and closed in the early 1980's after five centuries of mining. Several orebodies totalling 6.7 MT were mined along a 4km strike length of a major limestone bed within felsic volcanic rocks. The ores consist mainly of disseminated to semi-massive to massive sphalerite-galena (Zn-Pb-Ag) and Mn-rich magnetite bodies within skarn-altered limestone (Ripa, 1988, 1996). The Brusmalmen ore body comprised an impregnation to massive galena-sphalerite replacement of intensely silicified limestone. The orebodies are underlain by semi-conformable alteration zones characterised by albite-gedrite-quartz and biotite-muscovite-plagioclase-K-feldspar-quartz-garnet assemblages (Ripa, 1994). Ripa (1994, 1996) envisaged that the ores are synvolcanic sea floor exhalative and replacement ores. Early hydrothermal activity resulted in deposition of regionally extensive Fe oxide formation, and a second stage of hydrothermal activity produced Mg metasomatism and sulphide mineralization that overprinted the earlier Fe-oxide ores. We will inspect surface workings, magnetite skarn, the volcanic host succession, and the footwall and hanging-wall alteration. Field lunch: (take from Garpenberg Herrgård?). Drive to Ställdalen: shallow water dacitic pyroclastic fall deposits:
Bergtjärnsåsen: subaerial ignimbrite and pyroclastic fall deposit. Accommodation: Hällefors Herrgård.

Day 5: Tuesday 19 August.
The Hällefors-Grythyttan area is regarded as a type area for Bergslagen geology and mineral deposits. This area has a relatively low metamorphic grade (greenschist facies), moderate deformation, and primary rock textures are well preserved. We will study the stratigraphy and volcanology of this area and visit a couple of representative small ore deposits. Good exposures include the Sången ignimbrites, Kottbo Mg-alteration, the regional transition from the volcanic succession to the overlying argillites at Brevik, and the stromatolitic dolomite and conglomerate at Älvestorp. Drive to Viker-Älvlången: walk a traverse through the transition from a thick felsic pyroclastic sequence to overlying limestone with a horizon of stratiform/exhalative Fe-Mn-Zn-Pb mineralization. This stratiform mineralization is attributed to deposition of metaliferous hydrothermal sediment  (Hellingwerf et al., 1988) amongst laminated, distal, deeper water carbonate facies (Allen et al., 2003). Accommodation: Askersund: Hotel B&B, shared accommodation in double rooms.
Aitik open pit  Photo: Marko Holma
Aitik open pit  Photo: Marko Holma

Day 6: Wednesday 20 August.
Mine tour of the Zinkgruvan underground Zn-Pb-Ag-(Cu) mine and visit to nearby outcrops. The Zinkgruvan mines have past production and reserves of over 40 MT of massive sulphide ore. The ore bodies comprise sheet-like, bedded, stratiform Zn-Pb-Ag-rich, generally Fe and Cu sulfide-poor, massive and semi-massive sulphide. These deposits are interpreted to be hosted in rhyolitic ash-siltstone with meta-limestone, skarn and siliceous chemical sediment beds. A large potassic alteration zone (K-feldspar) underlies the deposit in footwall meta-volcanic rocks, and silicification and Mg-rich alteration occur closer to the ores (Henriques, 1964; Hedström et al., 1989; Allen et al., 1996). Although the rocks are metamorphosed to amphibolite facies and deformed, some primary textures can be observed in the less altered host rocks and bedding is preserved in the sulphide ores. The Zinkgruvan ores are interpreted as syn-sedimentary exhalative seafloor ores formed in a distal volcanosedimentary environment (Hedström et al.,1989), stratigraphically above and lateral to one or more large rhyolitic caldera volcanoes (Allen et al., 1996). Lunch: Zinkgruvan mine. Drive Zinkgruvan - Stockholm - Arlanda airport. Fly home or stay overnight at Stockholm, Arlanda, Uppsala.

Photo: Rodney Allen
Photo: Rodney Allen

References:
Allen, R.L., Bull, S., Ripa, M. and Jonsson, R. 2003. Regional Stratigraphy, Basin Evolution, and the Setting of Stratabound Zn-Pb-Cu-Ag-Au Deposits in Bergslagen, Sweden. Final report SGU-FoU project 03-1203/99, Geological Survey of Sweden.
Allen, R.L., Lunström, I., Ripa, M., Simeonov, A. and Christofferson, H. 1996.  Facies analysis of a 1.9 Ga, contintental margin, back-arc, felsic caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe-oxide deposits, Bergslagen region, Sweden.  Economic Geology v. 91. p. 979-1008.
Boekschoten, G.J., Van der Raad, A.C., Kenter, J.A.M. and Reymer, J.J.G. 1988. Note on a mid-Proterozoic stromatolite limestone, south of Grythyttan, Bergslagen, Sweden. Geologie en Mijnbouw 67. p. 467-469.
Carlon C. J., and Bleeker, W., 1988, The geology and structural setting of the Håkansboda Cu-Co-As-Sb-Bi-Au deposit and associated Pb-Zn-Cu-Ag-Sb mineralization, Bergslagen, Central Sweden: Geologie en Mijnbouw, v. 67, p. 279-292. 
Collini, B. 1965: En stromatolitförekomst i Mellansveriges urberg. Geologi 17, nos. 9-10, p. 130.
Gebeyehu, M. and Vivallo, W. 1991. Sulfide ore genesis and related dolomitization of limestone in the Garpenberg district, south central Sweden: Geochemical and C-O isotopic evidence. In: Source, Transport and Deposition of Metals, Pagel & Leroy (eds) p. 281-283.
Hedström, P., Simeonov, A., and Malmström, L., 1989, The Zinkgruvan ore deposit, south-central Sweden: A Proterozoic, proximal Zn-Pb-Ag deposit in distal volcanic facies:  Economic Geology, v. 84, p. 1235-1261. 
Hellingwerf, R.H., Lilljequist, R. and Ljung, S. 1988. Stratiform Zn-Pb-Fe-Mn mineralisation in the Älvlången-Vikern area, Bergslagen, Sweden. Geologie en Mijnbouw 67. p. 313-332.
Lager, I., 2001: The geology of the Palaeoproterozoic limestone-hosted Dannemora iron deposit, Sweden. Sveriges geologiska undersökning Rapporter och meddelanden 107, 49p.
Lundqvist, T., 1979, The Precambrian of Sweden, Geological Survey of Sweden, series C, v. 768, 87 p.
Lundström, I., Allen, R.L., Persson, P-O. and Ripa, M.  1998.  Stratigraphies and depositional ages of Svecofennian, Palaeoproterozoic metavolcanic rocks in E. Svealand and Bergslagen, south central Sweden. GFF (Geologiska Föreningens i Stockholm Förhandlingar) v. 120, p. 315-320.
Magnusson, N. H., 1970, The origin of the iron ores in central Sweden and the history of their alterations: Geological Survey of Sweden, series C, v. 643, 127 p.
Oen, I. S., Helmers, H., Verschure, R. H., and Wiklander, U., 1982, Ore deposition in a Proterozoic incipient rift zone environment: A tentative model for the Filipstad-Grythyttan-Hjulsjö region, Bergslagen, Sweden.  Geologische Rundschau v. 71, p. 182-194. 
Ripa, M., 1988, Geochemistry of wall-rock alteration and of mixed volcanic-exhalative facies at the Proterozoic Stollberg Fe-Pb-Zn-Mn(-Ag) deposit, Bergslagen, Sweden: Geologie en Mijnbouw, v. 67, p. 443-457.
Ripa, M., 1994, The mineral chemistry of hydrothermally altered and metamorphosed wall-rocks at the Stollberg Fe-Pb-Zn-Mn (-Ag) deposit, Bergslagen, Sweden. Mineralium Deposita, v. 29, p. 180-188.
Ripa, M., 1996, The Stollberg ore field - petrography, lithogeochemistry, mineral chemistry, and ore formation: PhD thesis, Lund University, Sweden.
Stephens, M.B., Lundström, I., Wahlgren, C-H., Persson, L., Bergman, T. and Lager, I.  2001. Exkursionsguide för Bergslagsprojektets slutexkursion (Excursion guide for the Bergslagen projects end-of-project field excursion, 17-21 September 2001). Geological Survey of Sweden, 54 p. plus appendices.
Stephens, M. B., and Wahlgren, C.-H., 1993, Oblique-slip, right-lateral ductile deformation zones in the Svecokarelian orogen, south-central Sweden:  Geological Survey of Sweden, report and communication number 76, p. 18-19.
Stålhös, G., 1984, Svecokarelian folding and interfering macrostructures in eastern Central Sweden, in  Kröner, A., and Greiling R., eds., Precambrian tectonics illustrated: Schweizerbartsche Verlagsbuchhandlung, Stuttgart, p. 369-379.
Sundius, N., 1923, Grythyttefältets geologi:  Geological Survey of Sweden, series C, v. 312, 354 p. (in Swedish with English summary).
Vivallo, W., 1985a, The geology and genesis of an Early Proterozoic massive sulfide deposit at Garpenberg, central Sweden: Economic Geology, v. 80, p. 17-32.