and limestone- hosted Zn-Pb-Ag-(Cu-Au)

33 IGC excursion No 12, August 14 – 20, 2008
Palaeoproterozoic volcanic- and limestonehosted Zn-Pb-Ag-(Cu-Au) massive sulphide
deposits and Fe oxide deposits in Bergslagen,
Sweden
Rodney Allen, Boliden Mineral AB and Luleå University of Technology, Sweden
Magnus Ripa, Geological Survey of Sweden
Nils Jansson, Luleå University of Technology, Sweden
Excursion No. 12
Bergslagen
Table of Contents
Bergslagen geology and field excursion route.......................................................................3
Abstract..................................................................................................................................4
Logistics.................................................................................................................................4
Dates and location ...................................................................................................................................4
Travel arrangements...............................................................................................................................4
Accommodation .......................................................................................................................................4
Field logistics ............................................................................................................................................5
General Introduction .............................................................................................................5
Regional Geology and Ore Deposits ......................................................................................5
Overview ...................................................................................................................................................5
Other major rock types in Bergslagen ...............................................................................................13
Svecofennian metasedimentary rocks................................................................................................................13
Granitoid-Dioritoid-Gabbroid (GDG) meta-intrusive rocks ............................................................................14
Metadolerite.........................................................................................................................................................14
Granite-syenitoid-dioritoid-gabbroid (GSDG) rocks........................................................................................14
Granite-pegmatite (GP) intrusive rocks .............................................................................................................15
Alkaline intrusive rocks of uncertain age ..........................................................................................................15
Mesoproterozoic and Neoproterozoic rocks......................................................................................................16
Bergslagen metallic mineral deposits .................................................................................................16
Types of mineral deposits...................................................................................................................................16
Fe, Mn and W oxide deposits .............................................................................................................................18
Base metal, iron and other sulphide deposits ....................................................................................................23
Excursion Route and Road Log...........................................................................................25
Excursion Stops...................................................................................................................25
Day 1: Thursday 14 August .................................................................................................................25
Introduction .........................................................................................................................................................25
Day 2: Friday 15 August, Dannemora Mine area ............................................................................26
Introduction .........................................................................................................................................................26
Day 3: Saturday 16 August, Sala Silver Mines .................................................................................33
Introduction .........................................................................................................................................................33
Day 4: Sunday 17 August, Garpenberg mines and region..............................................................46
Introduction .........................................................................................................................................................46
Day 5: Monday 18 August, Stollberg Mines .....................................................................................64
Introduction .........................................................................................................................................................64
Ställdalen ................................................................................................................................................69
Introduction .........................................................................................................................................................69
Day 6: Tuesday 19 August, Hällefors and Viker-Älvlången areas ................................................72
Introduction .........................................................................................................................................................72
Day 7: Wednesday 20 August, Zinkgruvan mine area....................................................................79
Introduction .........................................................................................................................................................79
References and Bibliography...............................................................................................83
2
Bergslagen geology and field excursion route
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 = felsic metavolcanic rocks, blue =
metasedimentary rocks, green = mafic rocks.
3
Abstract
The purpose of this field excursion is to discuss the stratigraphy, structure, regional geological
setting and origin of volcanic- and limestone-hosted Zn-Pb-Ag-(Cu-Au) massive sulphide
deposits in Bergslagen, and their relationships to Fe oxide deposits. The excursion is a
collaboration between the 33rd IGC and the IGCP-502 project “Global comparisons of
volcanic-hosted massive sulphide districts”.
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. The
Bergslagen district is the intensely mineralized part of a Palaeoproterozoic felsic magmatic
province in the Baltic Shield and contains a diverse range of polymetallic sulphide and iron
oxide deposits. The supracrustal successions are dominated by moderate to shallow water
volcaniclastic deposits that define large felsic caldera volcanoes. Most of the polymetallic
sulphide deposits and many of the iron ore deposits are associated with limestones and skarns
in the upper part of, and between, major caldera eruption cycles. This excursion will
demonstrate the latest results and foster debate about whether the sulphide and iron ores are
volcanogenic exhalative, volcanogenic replacement, synvolcanic intrusion-related, or synmetamorphic in origin. The excursion will visit the Garpenberg and Sala limestone-, skarnand volcanic-hosted stratabound Zn-Pb-Ag-(Cu-Au) sulphide deposits, the Zinkgruvan
volcanic-hosted stratiform Zn-Pb-Ag-(Cu-Au) deposit, the Viker-Älvlången limestone-hosted
stratiform Zn-Fe-Mn mineralization and the Dannemora and Stollberg limestone-hosted
manganiferous magnetite-skarn iron ore deposits. Outcrops of greenschist to amphibolite
facies volcanic rocks, stromatolitic limestones and skarns that host the ore deposits will also
be studied.
Logistics
Dates and location
Timing:
Start location:
End location:
Thursday 14 August – Wednesday 20 August 2008
Participants will be picked up from outside the arrivals lounge at the
international terminal (terminal 5) of Stockholm Arlanda airport at
18.30 on 14 August.
Participants will be taken to Stockholm Arlanda airport and to Uppsala
during the late afternoon on 20 August.
Travel arrangements
Participants should arrive at Stockholm (Arlanda) airport no later than 18.00 on Thursday 14
August. The excursion will leave Arlanda between 18.30 and 19.00. Transport during the
excursion will be by mini-bus. Space for luggage in the mini-busses is limited so some
luggage may need to be stored in safe lockers at Arlanda airport. Arlanda airport is easily
accessible by bus, train and car from Stockholm and Uppsala.
Accommodation
Accommodation will be in hotels and guesthouses (mainly Swedish manor houses).
Accommodation will vary from single rooms to shared double rooms depending on the
availability of rooms at each hotel and guesthouse.
4
Field logistics
Weather is expected to be pleasant and warm (15-28 C) however participants should also be
prepared for cool rainy weather. In addition to underground mine visits, walks of up to 1 km
in forest are planned. Light field boots will be sufficient. For the mine visits, participants will
be divided into two groups of about 13 each. One group will have a mine tour while the other
group studies outcrops and/or drill cores of the mine area, and then the groups will swap.
General Introduction
The purpose of this field excursion is to discuss the stratigraphy, structure, regional geological
setting and origin of volcanic- and limestone-hosted Zn-Pb-Ag-(Cu-Au) massive sulphide
deposits in Bergslagen, and their relationships to Fe oxide deposits. The excursion is a
collaboration between the 33rd IGC and the IGCP-502 project “Global comparisons of
volcanic-hosted massive sulphide districts”. The excursion will complement the IGCP-502
symposium on “Volcanic-hosted massive sulphide deposits: controls on distribution and
timing” at the IGC.
Regional Geology and Ore Deposits
Overview
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 Bergslagen 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.
Three polymetallic sulphide deposits are currently being mined (Garpenberg, Zinkgruvan 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 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.
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 scientific appraisal of the potential relationship between Fe oxide and Cu-Au
mineralisation (Iron Oxide-Copper-Gold deposits - IOCG), and the high demand for metals
on world markets.
5
Figure 2. Regional structure of Bergslagen. Geological Survey of Sweden data (modified from
Stephens et al., 2001). Geology: pink = late-orogenic granitoids, pale yellow = metavolcanic rocks and
early orogenic granitoids, dark yellow = mainly metasedimentary rocks, green = post-orogenic
sedimentary rocks.
The supracrustal stratigraphy in the Bergslagen region is dominated by 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 (Allen et
al., 1996). 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 whole package has been extensively
deformed and metamorphosed, and is now exposed as isolated, mainly upper greenschist to
upper amphibolite facies inliers enclosed by abundant early orogenic to post-orogenic
intrusions (Fig. 1). According to Geijer and Magnusson (1944) and Magnusson (1970), peak
metamorphism in the central parts of Bergslagen was caused by contact metamorphism due to
6
the intrusion of early-orogenic granitoids and mafic rocks at c. 1890 Ma, whereas peak
metamorphism outside and along the southern and northern borders of Bergslagen was
regional (tectonic) at c. 1800 Ma. The relative importance of each event is however unclear in
many areas.
Figure 3. Regional stratigraphic columns through various supracrustal successions in the Bergslagen
region (after Allen et al., 1996).
7
Figure 4. The percentage of rhyolite, dacite, andesite and basalt in Bergslagen compared to the Andes
continental margin volcanic arcs, the southwest Pacific oceanic arcs, and the Taupo Volcanic Zone.
Bergslagen is much more dominated by rhyolitic rocks than well studied volcanic arc regions, but is
similar in volcanic composition to the Taupo Volcanic Zone, which is a back arc extensional region on
continental crust (modified after Allen et al., 1996).
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 the deposition of the Bergslagen succession, and has increased understanding of
the facies architecture and hence paleogeographic 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. In summary, the Bergslagen succession records 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. However, the volume and frequency of pyroclastic eruptions in
8
Bergslagen was such that the whole succession is dominated by clastic rocks with a felsic
pyroclastic provenance. Other subordinate but important “volcanic” rocks include felsic and
mafic subvolcanic intrusions and lavas.
Figure 5. Facies model for the volcanic succession and two main polymetallic sulphide ore deposit
types in the Bergslagen region (after Allen et al., 1996).
The marbles are the main non-volcanic units within the Bergslagen volcanic successions. The
origin of the marbles has been variously interpreted in the literature as microbial-sedimentary
(i.e. stromatolitic) (Collini, 1965; Boekschoten et al., 1988; Lager, 1986, 2001) and
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 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. 13C and
18O isotope values of microbially laminated carbonate samples provide an accurate
measurement of the primary C and O isotopic signature of the Bergslagen carbonates (18.8-
9
21.7 ‰ 18O, 0.1-1.1 ‰ 13C) and confirm their marine sedimentary origin (Allen et al.,
2003).
The dominance of below wave base depositional environments over subaerial and current
affected shallow marine environments in the Bergslagen rock record is interpreted to reflect
both the rapid semi-continous subsidence that characterised basin tectonics in the Bergslagen
region, and the low preservation potential of unconsolidated pyroclastic ejecta in subaerial to
shallow marine environments, leading to its ultimate accumulation in sub-wave base
depocentres (Allen et al., 1996, 2003). The combination of semi-continuous basin subsidence
and rapid continuous resedimenation of unconsolidated volcaniclastic debris, resulted in low
regional basin gradients and large areas of relatively shallow marine basinal environments.
Stromatolite reefs developed in the relatively shallow sub-wave base environment wherever
and whenever there was a significant reduction in the influx of volcaniclastic detritus.
The skarns that are commonly associated with Bergslagen ore deposits range from massive,
heterogeneous, mainly Mg-skarns in marble units, to banded/bedded skarn in felsic siltstonesandstone. The former have been given diverse origins, including the synvolcanic
metasomatic replacement of limestone beds (Sundius, 1923), regional metamorphism of
hydrothermally (synvolcanic) altered limestones (Allen et al., 1996, 2003), regional
metamorphism of interbedded hydrothermal exhalative ores and limestone (Magnusson,
1970), and intrusion-related skarns (Magnusson, 1948, 1960, 1970). 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. Most of the
skarns can not be spatially associated with particular 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, NW to
NNW trending structures with steep axial surfaces and a foliation (S1) parallel to the axial
surface and subparallel to bedding. However, in other areas such as Garpenberg, F2 folding
and D2 shearing have obliterated any F1 folds, and their former presence can 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.
10
Figure 6. U-Pb radiometric ages of volcanic rocks and intrusions in the Bergslagen region, modified
after Stephens et al., 2001. (Geological survey of Sweden data).
Figure 7. Regional geology (left) and regional metamorphic facies map (right) of Bergslagen.
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.
11
Figure 8. Regional aeromagnetic map of Bergslagen. Geological survey of Sweden data.
12
Figure 9. Regional gravity map of Bergslagen. Geological survey of Sweden data.
Other major rock types in Bergslagen
The following text on the other Berglagen rock types and mineral deposits is a slightly
modified extract from the description to the Bergslagen project maps (Stephens et al. in prep.
Geological Survey of Sweden). References are omitted.
Svecofennian metasedimentary rocks
Palaeoproterozoic metasedimentary rocks, including metagreywacke, meta-argillite,
feldspathic metasandstone, quartzite and marble, form an important bedrock component in the
Bergslagen region and surroundings. They comprise both well-preserved as well as strongly
altered, veined and migmatitic varieties. These rocks belong to the Svecofennian supracrustal
sequence in the western part of the Fennoscandian Shield. They underlay, interfinger with and
overlay the metavolcanic rocks.
13
Granitoid-Dioritoid-Gabbroid (GDG) meta-intrusive rocks
The Svecofennian supracrustal rocks in the Bergslagen region and its surroundings were
intruded by large volumes of quartz-rich, plutonic rocks that range in composition from
tonalite to granodiorite and granite. Plutonic rocks that are intermediate, mafic and even
ultramafic in composition are spatially associated with these quartz-rich rocks and all are
affected by metamorphism, predominantly under amphibolite facies conditions. Together
these rocks form a prominent intrusive suite, referred to as the Granitoid-Dioritoid-Gabbbroid
suite. U-Pb zircon geochronological data indicate that the bulk of these rocks formed between
1.91 and 1.87 Ga. However, plutonic and hypabyssal GDG rocks with younger ages between
1.87 and 1.85 Ga have been documented in both the north-eastern and south-western parts of
the area. Thus, two separate age suites of GDG rocks are considered to be present in the
Bergslagen region and its surroundings.
Enclaves of mafic or intermediate rock are particularly conspicuous in the tonalitic to
granodioritic rocks and illustrate the mingling and close temporal relationships between the
different melt fractions. Metagranitoids are commonly even-grained, but porphyritic varieties
are also present, especially in the granodioritic or granitic varieties.
The rocks within the GDG suite show a wide range of compositions with I-type
characteristics, they straddle the boundary between peraluminous and metaluminous
compositions and show a distinctive calc-alkaline to calcic trend.
Metadolerite
Metadolerites, which commonly occur as amphibolite, constitute a significant bedrock
component in many parts of the Bergslagen area. Six different generations of metadolerite
exist, and the relationship between their intrusion, the subsequent deformation and
metamorphism, and the structural development in the country rock is complex. Presumably
they are hypabyssal varieties of the mafic plutonic rocks that belong to the GDG intrusive
suite.
Granite-syenitoid-dioritoid-gabbroid (GSDG) rocks
The Bergslagen region is bordered along its southern and western parts by predominantly
intrusive rocks that range in composition from gabbro and diorite via syenitoid to granite.
Ultramafic rocks are also present. These intrusive rocks are locally associated with
synmagmatic volcanic and subvolcanic rocks, and contemporaneous sedimentary rocks. On
the basis of solely their compositional variation, these intrusive rocks are referred to here as
GSDG rocks. Such rocks intruded the Svecofennian supracrustal rocks and most of the
intrusive rocks of the GDG suite. At several places, granitic end-members are difficult to
differentiate from the more homogeneous granites in the granite-pegmatite (GP) suite (see
below).
Based on the intrusion-deformation relationships, four different suites of GSDG rocks are
present in the Bergslagen region. The oldest rocks with GSDG composition occur north of
Stockholm, in the Åkersberga area. They are 1.88-1.87 Ga in age and are pre-tectonic with
respect to Svecokarelian ductile deformation and metamorphism. Two younger suites of
GSDG rocks intruded during later phases of the Svecokarelian orogeny. These are syn- to
post-tectonic in character and range in age from 1.87-1.84 Ga and 1.81-1.76 Ga. The fourth
and youngest suite is entirely post-tectonic with respect to this complex, polyphase orogenic
system and formed during the time interval 1.71-1.67 Ga.
14
The volumetrically more important felsic rocks are coarse- to medium-grained and both
coarsely K-feldspar-phyric and equigranular variants are present. The rocks belonging to the
1.87-1.84 Ga suite are generally coarse-grained, while the rocks in the younger 1.81-1.76 Ga
suite are medium- or medium- to coarse-grained. K-feldspar phenocrysts are locally rimmed
by plagioclase feldspar. Furthermore, rounded, fine-grained mafic enclaves are conspicuous
in these rocks. It is apparent that the more felsic rocks have hybridised with gabbroic and
noritic rocks to form quartz monzodiorite, monzonite and quartz monzonite.
The rocks within the GSDG suites show a wide range of compositions with I-type
characteristics. In contrast to the GDG rocks, they show a trend from granite through quartz
monzonite and quartz monzodiorite to gabbro, predominantly metaluminous composition and
alkali-calcic to shoshonitic affinity.
In the north-western part of the Bergslagen region, the 1.81-1.76 Ga suite of GSDG intrusive
rocks is spatially associated with supracrustal rocks.
Granite-pegmatite (GP) intrusive rocks
Granites sensu stricto and associated pegmatite, aplite and granite dykes and veins frequently
occur in most parts of the Bergslagen region. In some of the north-central parts, however, they
are less common. They are generally considered to have formed through partial melting of
crustal rocks during the culmination of the Svecokarelian orogeny and form both S- and Itype rocks.
The GP intrusive suite mainly cross-cuts, but locally conforms with, the tectonic structures of
the Svecokarelian orogeny. It has thus been denoted late-orogenic in relation to the latter.
The granites of the GP intrusive suite normally are red to grey, fine- to coarse-grained and
equigranular to microcline phyric. In places, the country rocks in direct contact with the GP
intrusive rocks show signs of coarsening and recrystallisation. The granites and their
associated veins and dykes locally grade through areas of intensely veined country rock into
migmatites.
The radiometric data for the GP intrusive suite show a wide range of ages. Maximum
obtained age is c. 1854 Ma, minimum c. 1750 Ma. Most analysis results, however, cluster in
two intervals at c. 1820-1800 and 1790-1780 Ma, respectively.
The silica content is high in the GP suite with an average of 72.4 wt %, and the rocks plot as
adamellites and granites. They are dominantly peraluminous and show an alkali-calcic trend.
Alkaline intrusive rocks of uncertain age
One alkaline complex occurs within the Bergslagen region. It is dominated by syenite with
subordinate amounts of nepheline syenite, nepheline gabbro/diorite and quartz syenite. The
surrounding country rocks of GDG-type show fenitisation. Although older K-Ar work
suggested a late Palaeoproterozoic or Mesoproterozoic age for the alkaline complex, field
relationships and the present interpretation of U-Pb (titanite) radiometric age data suggests
that the complex is considerably older and similar in age to the surrounding GDG rocks.
15
Mesoproterozoic and Neoproterozoic rocks
Dolerites, mainly occurring as dykes, are frequent in the Bergslagen region and they cross-cut
all of the rocks mentioned above. Locally, they formed flatlying sills and possibly even lavas,
and dolerite and porphyry dykes coexist, both separately and as composite dykes.
Field data and radiometric dating results suggest that the dolerites formed during four periods
at c. 1560, 1470, 1250 and 1000-950 Ma.
Two occurrences of rapakivi-type intrusive rocks, at Noran and Strömsbro, have been
documented in the Bergslagen region. U-Pb data from zircons suggests that the rocks intruded
at c. 1469 and 1500 Ma, respectively. The Noran intrusion is described as ranging from Atype alkali granite to quartz syenite. The Strömsbro granite is megacrystic with perthitic Kfeldspar lacking plagioclase rims, and it is also classified as A-type.
Bergslagen metallic mineral deposits
Mining of metallic ore deposits has a long history in Bergslagen. Mining of copper is
documented back to the 13th century and mining of iron to the 14th century. Archaeological
data suggests that mining of copper at Falun may have started already c. 500 A.D. and that
blast furnaces for iron production existed at Norberg in the 13th century.
In total, 9 major base metal sulphide ore fields (defined as ore production >1 million metric
tonnes = 1 Mt) in Bergslagen have produced c. 100 Mt of ore up to 2005. The Falun deposit
(28 Mt) is the largest known copper mineralisation in Bergslagen. It was initially known as
Tiskasjöberget or Stora Kopparberget. For a long period, the annual production was c. 200
000 t at 6 % Zn, 2 % Pb and 0.5 % Cu. In addition, Ag (50 ppm) and Au (0.4 ppm) were
important by-products. Beside metals, the Falun deposit has been a major producer of raw
material for sulphuric acid and the nationally famous red paint used on wooden houses in
Sweden and in the other Nordic countries. Mining ceased during 1992. At the present time,
three base metal sulphide mines are in operation in Bergslagen; the Garpenberg Zn-Pb-CuAg(-Au) deposits, the Zinkgruvan Zn-Pb-Ag deposit and the Lovisa Zn-Pb deposit,.
Up to and including 1992, when the last iron ore mine closed, 31 major Fe oxide ore fields
(>1 Mt) in Bergslagen had together produced c. 420 Mt ore. The largest deposit was the
Grängesberg ore field with a production of c. 150 Mt between 1500 and 1989, the second
largest was Dannemora with a production of c. 24 Mt between 1481 and 1992.
Types of mineral deposits
By far the most common type of metallic mineral deposit in Bergslagen consists of Fe oxide
with variable amounts of Mn in associated skarn and crystalline carbonate rocks. More than
2000 deposits are known, most of which are small and, since the middle of the 19th century,
without any economic significance. The economically important Fe oxide deposits in
Bergslagen may be subdivided into the following categories; Fe oxide deposits in Mn-poor
and Mn-rich skarn and carbonate rocks, quartz-rich Fe oxide deposits including banded iron
formations (BIF), and apatite-bearing Fe oxide deposits. The term “skarn” is also used nongenetically and refers to calc-silicate or Mg-silicate mineral assemblages. In addition, Fe
oxide deposits associated with high contents of base metal sulphides are also present.
16
Figure 10. Regional alteration map of Bergslagen with the field trip route and geology for
comparison. Alteration: Red = K-rich (K-spar), blue = Na-rich (albite), green = Mg-rich (chlorite-talc,
phlog-cord-anth). Geological Survey of Sweden data.
17
The Fe oxide categories above may be considered as end members, and characteristics
corresponding to more than one cataegory are locally found in different parts of, or along
strike, in the same deposit. For example, some Fe oxide skarn deposits appear to grade into
quartz-rich Fe deposits.
Mn oxide deposits comprise both stratiform ”Långban type” deposits associated with Fe oxide
skarn mineralisation and epigenetic deposits associated with fault breccia. W oxide deposits
composed of scheelite and wolframite are also a significant component in the Bergslagen
region.
A dominance of Zn and Pb is typical for the base metal sulphide deposits in the Bergslagen
region. The mineralogical and textural features are highly variable, which may reflect
different origins and/or different deformational and metamorphic histories. Mo sulphide
deposits (“climax type”), as well as other minor sulphide occurrences (e.g. Ni-Cu (-Co)
deposits), rare PGE deposits and greisen deposits with both oxides and sulphides, are also
present in the region.
Fe, Mn and W oxide deposits
The majority of oxide deposits in Bergslagen are dominated by Fe and contain variable
contents of Mn in the associated skarn and crystalline carbonate (marble) rocks. Several of
these deposits also contain Fe sulphides and base metal sulphides. Locally, more exotic metal
mineralisation, including Au, Bi, U, Th, W and REE, is present in these oxide deposits.
Gold has been noted in anomalous amounts in some Fe oxide mineralisations, e.g. the
Malsjöberg and Nordmark deposits. Discoveries of this type of gold occurrence have also
been made in the Riddarhyttan area and at Boviksgruvan north-east of Falun. At these sites,
gold is intimately intergrown with bismuth minerals. Locally, U and Th form minor
components in the Fe oxide skarn deposits but have never had any economic significance.
Examples include Digerbergsgruvan in northern Bergslagen and Håkanstorp in southern
Bergslagen. W is present in anomalous amounts as a scheelite impregnation in several Fe
oxide skarn deposits, However, W has never been extracted from these Fe oxide deposits. At
the Ceritgruvan deposit in the Riddarhyttan area in western Bergslagen and at the Östra
Gyttorpsgruvan deposit in central Bergslagen, REE-minerals occur together with Fe oxide
skarn deposits.
The different types of Fe oxide (including sulphide-bearing variety), Mn oxide and W oxide
deposits are described in more detail below.
Fe oxide deposits in Mn-poor skarn or crystalline carbonate rock
Fe oxide deposits hosted by Mn-poor skarn or crystalline carbonate rock (marble) are the
most common type of Fe oxide mineralisation in Bergslagen. Such deposits constitute nearly
50 % of all known metallic deposits in the region. They are present in all parts of Bergslagen
where supracrustal rocks occur, but are more frequent in the central-western parts. The
historically most important ore field is at Persberg, the “birthplace” of the skarn concept
(Törnebohm 1875). Other ore fields of importance are Finnshytteberg, Nordmark,
Riddarhyttan, Norberg (Åsgruvan), Herräng and Kantorp. Fe oxide mineralization in a host
rock dominated by crystalline carbonate rock occur in several of the typical skarn ore fields,
but are most frequent in the Tuna-Hästberg ore field, in the area between Fagersta and
Norberg, and in some deposits in the Hofors ore field.
18
The Fe oxide deposits associated with Mn-poor skarn or crystalline carbonate rock are
generally thin (10-50 m). However, they can be laterally extensive. They are stratabound to
stratiform, and concordant with the host 1.91-1.89 Ga metavolcanic and metasedimentary
rocks. The Fe content is variable along strike and only the richest parts have been mined. The
Fe content in mined parts is generally between 30 and 50 % and the estimated tonnage of
most of the individual older deposits is <1 Mt. However, when tonnage is calculated for
production areas or entire ore fields, which consist of several deposits, 26 sites in the
Bergslagen region show a tonnage that lies in the interval 1-10 Mt and a few in the interval
>10 Mt, e.g. the Riddarhyttan ore field with c. 14 Mt.
The Fe oxide deposits associated with Mn-poor skarn or crystalline carbonate rock commonly
contain magnetite, actinolite, hedenbergite, andradite garnet and epidote. The Mn-poor skarn
parageneses have <1 % MnO and all the Mn is accommodated in the skarn or carbonate
minerals. Magnetite and calc-silcate minerals are irregularly distributed inside the deposits,
and the volumetric proportion of calc-silicate minerals varies from deposit to deposit and even
within individual deposits. Amphibole and garnet are generally the most abundant calcsilicate minerals in the skarn. Quartz content is generally low (<20 % SiO2) in the majority of
these Fe oxide deposits. However, locally, the quartz content is higher and the deposit grades
into quartz-dominated and banded iron formations. One example is the Striberg deposit at
Nora. As skarn-rich layers occur in some banded iron formations in Bergslagen, e.g. the Utö
deposit, the classification in some cases is difficult.
Some Fe oxide mineralisations associated with Mg-rich silicates are included in the group of
Fe oxide skarn deposits discussed here. One example is the Källfallsgruvan in the
Riddarhyttan ore field. The calc-silicate minerals in the skarn in this deposit are dominated by
anthophyllite-cummingtonite-talc parageneses. They have been interpreted as Mg-altered Fe
oxide skarn deposits.
Based on the occurrence of a distinct layering in many of the deposits and the overall
stratabound appearance, several workers have suggested that the Fe oxide deposits associated
with skarn or crystalline carbonate rock generally originated as volcanogenic, exhalativesedimentary deposits. The transitions from banded iron formation to Fe oxide skarn deposits
and from Fe oxide skarn deposits to Fe oxide deposits hosted by crystalline carbonate rock
provides some support to this hypothesis. The present mineral assemblages were most likely
formed in connection with low- to medium-grade regional metamorphism during the
Svecokarelian orogeny. This type of skarn has been referred to as ”reaction skarn”, in contrast
to ”primary skarns”, which are contact metasomatic deposits formed in association with the
intrusion of igneous rocks.
Fe oxide deposits in Mn-rich skarn or crystalline carbonate rock
The regional distribution of Fe oxide deposits associated with Mn-rich skarn or crystalline
carbonate rock mainly corresponds to that observed for the Mn-poor deposits described
above. However, the Mn-rich depoits appear to occur more frequently at higher stratigraphic
levels and are, to a larger extent, hosted by crystalline carbonate (marble) horizons. They are
generally stratbound to stratiform and lens-shaped.
Fe oxide deposits associated with Mn-rich skarn or crystalline carbonate rock include, for
example, the Dannemora deposit, the Tuna-Hästberg deposits, and the Bastkärn ore field and
Stollberg deposits. Stollberg is an example of a Fe oxide Mn-rich skarn deposit in which
disseminated sulphides grade into semi-massive to massive sulphide. All these deposits have
19
a tonnage >5 Mt and the largest is Dannemora with an estimated tonnage of 24 Mt.
Otherwise, most deposits of this type were minor operations with an estimated tonnage that is
<0.1 Mt. Mn and Fe contents in these deposits are generally c. 2-10 % and 30-50 %,
respectively.
The Fe oxide deposits associated with Mn-rich skarn or crystalline carbonate rock commonly
contain magnetite and manganiferous calc-silicates, including dannemorite, knebelite and
spessartine-rich garnet. These deposits contain >1 % MnO. The Mn content in magnetite is
generally low or very low, and Mn is accommodated in the calc-silicate and carbonate
minerals; marbles locally have up to 10 % MnCO3. The proportion of Mn-rich carbonate and
the Mn-bearing silicates is variable. The skarn ore parageneses generally contain minor
contents of graphite (0.5-1.5 % C) and impregnations of galena, sphalerite and arsenopyrite.
Examples of sulphide-rich deposits include the Dannemora deposit, the Stollberg ore field and
the silver-rich deposits at Hällefors.
Locally, Fe-Mn skarn mineralisations with little or no magnetite occur. These so-called
“eulysites” occur high in the stratigraphy, close to the overlying metasedimentary rocks.
Quartz-rich Fe oxide deposits, including banded iron formations (BIF)
Quartz-rich Fe oxide deposits occur mainly in the western part of Bergslagen. These
mineralisations are distinctly stratiform and locally developed as banded iron formations
(BIF). Several hundred minor deposits of this type occur over a distance of 100 km in the area
between Nora and Norberg. The most prominent deposits of this type that show a tonnage >5
Mt are also present in this part of Bergslagen (e.g. the Striberg, Stripa and Stråssa deposits).
Quartz-rich Fe oxide deposits also occur in the area north-west of Ludvika (e.g. the Laxsjö,
Ikorrbotten and Håksbergs ore fields). Single minor occurrences are present in the eastern
parts of Bergslagen, for example the Utö deposit in the Stockholm archipelago. Some
mineralisations of this type have been mined since the Middle Ages and some were
productive until the second half of the 20th century.
The dominant type of quartz-rich Fe oxide deposit is banded in a regular pattern with
hematite-rich and quartz-rich layers, and is hosted by the 1.91-1.89 Ga felsic metavolcanic
rocks. Individual hematite-rich and quartz-rich layers are generally 1-10 mm thick but in
places they are thicker. Locally, magnetite is relatively abundant and formed as an alteration
product after hematite. The iron content is generally between 30 and 55 %, but some deposits
that lack a distinct banding contain up to 60 % iron, for example, individual deposits in the
Bispberg ore field. Silica contents vary between 18 and 28 %, while P and Mn contents are
generally low, <0.03 % and <0.2 %, respectively. Impregnation of sulphides is common in
some occurrences, and the sulphur content lies in the range 0.001-0.1 %.
The presence of dark red layers of jasper with a fine dissemination of hematite in quartz is
typical for the quartz-rich Fe oxide deposits. Layers of skarn, with the calc-silicate minerals
actinolite, pyroxene and epidote, occur in some banded iron formations. As indicated above,
transitional varieties between skarn and quartz-rich Fe oxide deposits are developed.
Examples include the Grönvåld och Högban ore fields. The deposits in these ore fields are
thin, fine-grained and relatively rich hematite ores, with a local diffuse banding. They
alternate with layers of amphibole-pyroxene skarn with magnetite or hematite, weak hematitequartz mineralisations, felsic metavolcanic rock and marble.
20
Apatite-bearing Fe oxide deposits
Apatite-bearing Fe oxide deposits in Bergslagen are geographically restricted to the Ludvika
area in the western part of the region. Major deposits include the Grängesberg, Blötberget,
Fredmundsberg, Lekomberg and Idkerberget ore fields. This type of Fe oxide mineralisation
has produced more than 40 % of all the iron in Bergslagen, despite a limited number of
deposits. The largest and most famous ore field is Grängesberg, which has an estimated
tonnage of 150 Mt at 40-63 % Fe. The P content is 0.5-1.3 %.
The apatite-bearing Fe oxide deposits in Bergslagen are hosted by felsic to intermediate
metavolcanic rocks and generally form lens-shaped bodies. Their stratigraphic position is not
clear, but they appear to occur at a lower stratigraphic level than the skarn Fe oxide and BIF
deposits. Mineralogically, the apatite-bearing iron ores consist of magnetite and hematite
accompanied by apatite and small amounts of quartz and calc-silicate minerals. The latter
include actinolite and less abundant epidote, chlorite and garnet. Mg-rich silicates are also
present in the Blötberget deposit.
The apatite-bearing Fe oxide deposits have been interpreted as intrusive into, but more or less
coeval with, their felsic metavolcanic host rocks. Other theories on their origin suggest they
are volcano-sedimentary and sedimentary combined with later remobilisation into the
metavolcanic rocks. Recently, however, the spatial relationship between this type of
mineralisation and porphyritic, dacitic subvolcanic intrusions and dykes at the Grängesberg
deposit has been emphasised, and that these field relationships favour a magmatic
hydrothermal origin.
Sulphide dissemination in Fe oxide deposits
Several iron oxide deposits in Bergslagen contain, to variable extent, Fe and base metal
sulphides and some of these deposits have had a mining history for both Fe oxides and base
metals. Conspicuous examples include Dannemora and Stollberg. The Dannemora deposit
was mined for silver during the Middle Ages and for zinc and lead from 1890 to 1920. The
Stollberg ore field was also mined for silver during the Middle Ages and has later been a
major producer of zinc, lead and silver. In the SGU mineral and bedrock resource database
(mdep), these deposits are classified both as Fe oxide and sulphide deposits.
Mn oxide deposits
Mn oxide deposits in Bergslagen can be divided into two types:
1. Stratiform Mn oxide deposits associated with Mn-poor Fe oxide deposits;
2. Epigenetic deposits hosted by breccia.
Examples of the former are mainly situated in the Filipstad area, for example the Långban,
Jakobsbergsgruvan, Pajsberg-Harstigen and Sjögruvan deposits. The most famous of these
deposits is at Långban, and this type of mineralisation is generally referred to as the “Långban
type”.
The Mn oxide ores of Långban type are hosted by dolomitic marble and skarn. The ore
minerals are hausmannite and braunite, which are irregularily distributed but, locally,
arranged in layers giving the deposit a banded appearance. Associated skarn minerals are Mnrich and occur along the contacts between marble and the Mn oxide mineralisation. These
include manganiferous diopside, rhodonite, bustamite, manganiferous olivine and spessartine
garnet, as well as manganiferous varieties of phlogopite and richterite. In addition, the
Långban deposit is famous for its abundance of rare Pb, Sb and Ba minerals. These occur as
fissure and fracture fillings, which most likely formed in connection with hydrothermal
21
processes during later brittle deformation. The association that consists of native lead, barite
and calcite in veins is typical for the Långban deposit. The genesis of the Långban deposit has
attracted considerable attention, and sedimentary, metasomatic reaction and exhalativesedimentary origins have all been suggested.
Occurrences of epigenetic, Mn oxide deposits hosted by breccia include the Bölet deposit,
which consists of several minor mineralisations along the western shore of lake Vättern, and a
minor deposit north-east of lake Vättern.
W oxide deposits
W deposits in Bergslagen can be divided into two major groups:
1. Contact metasomatic W skarn deposits with scheelite and
2. W deposits in quartz veins with wolframite and scheelite.
The W skarn mineralisations are mainly located in the western part of Bergslagen and
comprise about 20 deposits in the Kopparberg-Ludvika area. The majority of these deposits
are minor occurrences or prospects and only a few have been mined, for example Yxsjöberg,
Sandudden, Wigström (Högfors) and Elgfall. The most prominent deposit is Yxsjöberg, with
an estimated tonnage of 5 Mt at 0.3-0.4 % W. It was mined until 1989 and was then the
largest W deposit in Scandinavia. The Elgfall and Wigström deposits (both <0.2 Mt) were
mined during two short periods (1970-1971 and 1978-1981, respectively) as satellite deposits
to Yxsjöberg.
All known W skarn deposits are hosted by the 1.91-1.89 Ga felsic metavolcanic rocks and
marbles. The mineralisations are mostly concordant or subparallel to bedding. Marble is
generally found as remnants in the skarn but is, in places, completely replaced by skarn.
Scheelite, generally with a significant component of powellite, is the only economically
important W mineral. The skarn assemblages consist of grossularite-andradite garnet,
hedenbergite-diopside pyroxene and hornblende. Scapolite, vesuvianite and wollastonite are
important constituents in some deposits, for example at Wigström and Elgfall. Fluorite is
common in many deposits and a strong positive correlation between F and W contents is
noted in the Yxsjöberg deposit, where fluorite was extracted as a by-product.
Molybdenite is a common feature in the W skarn deposits and, in many places, for example in
the Hörken deposit, scheelite and molybdenite occur in approximately equal proportions. In
general, other sulphide minerals are only present in subordinate amounts. An exception is
Yxsjöberg, where significant amounts of pyrrhotite and chalcopyrite are present. Indeed, this
mine opened its production as a copper mine. In the Wigström deposit, sphalerite is common
together with pyrrhotite and molybdenite. Minor amounts of gold have been noted in some W
skarn deposits.
Several authors have suggested that the W skarn deposits formed from hydrothermal fluids
generated in connection with the intrusion of igneous rocks, implying a contact metasomatic
origin. In general, the GP granites have been proposed as responsible for the formation of
these deposits (e.g. Yxsjöberg), although a genetic link to the GDG intrusive rocks has also
been proposed. Even an exhalative–sedimentary origin for the W-Mo mineralisations has
been suggested. However, well-constrained age data on titanite from the Yxsjöberg skarn and
Re-Os age determinations on molybdenite from the Wigström mineralisation strongly indicate
that at least these deposits are related to the granites in the GP suite.
22
W deposits hosted by quartz veins are rare in Bergslagen and most are minor occurrences, for
example, at Tyfors. The only occurrence that has been mined occurs at Baggetorp in southern
Bergslagen. This deposit was discovered in 1940 and mined between 1944 and 1958. It is
hosted by quartz veins that cross-cut a veined gneiss. The main ore minerals are wolframite
and scheelite, and wolframite is replaced and locally rimmed by scheelite. Molybdenite as
well as pyrite, chalcopyrite and bismuth minerals are also present. It has been suggested that
the Baggetorp deposit is genetically related to shear zones that formed prior to the intrusion of
the 1.81-1.76 Ga suite of GSDG granites.
Base metal, iron and other sulphide deposits
A current compilation by the SGU comprises more than 1000 mineral deposits characterised
as sulphide deposits. They are, with some exceptions, Fe and base metal sulphide deposits,
hosted by the 1.91-1.87 Ga Svecofennian mainly felsic metavolcanic rocks and intercalated
marbles.
Zn-Pb-Ag-(Fe-Cu-Co-Au) sulphide deposits
The base metal sulphide deposits in Bergslagen have traditionally been divided into two types
on the basis of their character of occurrence, metal content and host rock:
1. Stratiform, sheet-like, Zn-Pb-Ag-rich and Fe-Cu-poor deposits. The deposits in type 1
include, for example, the operating mine at Zinkgruvan. They have traditionally been referred
to as the “Åmmeberg type” and more recently as “stratiform ash-siltstone-hosted Zn-Pb-Ag
sulphide deposits (SAS type)”. In general, they are hosted by rhyolitic ash-siltstone
metavolcanic rocks. Crystalline carbonate rock (marble), calc-silicate rock and siliceous
chemical sediment beds are also present. The footwall lithologies generally show conspicuous
K, Si and subordinate Mg alteration.
2. Irregular, multi-lens and podiform, stratabound, massive and disseminated Zn-Pb-Ag-Cu
deposits. The deposits in type 2 include, for example, Sala and the operating deposits at
Garpenberg and Lovisa, as well as more massive, pyritic Cu-Zn-Pb-Ag-Au ores, for example
Falun. These deposits are traditionally referred to as the “Falun type” and more recently as
“stratabound, volcanic-associated, limestone-skarn Zn-Pb-Ag-Cu-Au sulphide deposits
(SVALS type)”. They are generally associated with felsic metavolcanic rocks interbedded
with crystalline carbonate rock (marble) and are closely associated with Mg-rich, calc-silicate
rock (skarn) and intense footwall Mg-rich±K alteration.
The tonnages of most of the base metal sulphide deposits in Bergslagen are relatively low,
generally <1 Mt. Exceptions are the Falun, Zinkgruvan, Garpenberg, Garpenberg Norra, Sala
and Saxberget deposits. A typical feature of the base metal sulphide deposits in Bergslagen is
a relatively high content of Zn. The base metal contents are on average 4.5% Zn, 2.5% Pb and
0.5% Cu. Ore minerals are pyrite, pyrrhotite, sphalerite, variably argentiferous galena and
chalcopyrite.
Ag has varied in significance in most of the base metal deposits in Bergslagen, from a major
commodity to an important by-product. Ag contents generally vary between 30 ppm and c.
1500 ppm. Historically, the most important silver producers in Bergslagen have been the
Falun, Sala, Hällefors (c. 1500 ppm), Guldsmedshyttan, Kaveltorp and Lövåsen deposits.
Au has never been an important commodity in any of the base metal deposits in Bergslagen.
However, at Falun, Garpenberg and Saxberget, it has formed a significant by-product. Au
23
contents are generally <1 ppm, which is comparable to the contents of this metal in base metal
sulphide deposits around the world. However, at Falun, higher grades of gold (2-3 ppm) occur
in quartz veins that are spatially associated with the base metal sulphide ore. Otherwise, goldbearing quartz veins have only been reported from a few localities in Bergslagen and all are
minor prospects.
Co is locally present and has been extracted as an economic by-product in some Cu-rich base
metal sulphide deposits in Bergslagen.
The base metal sulphide deposits are considered to be either synvolcanic, seafloor exhalative
or synvolcanic, sub-seafloor replacement mineralisations. Both mechanisms of ore formation
may be relevant for base metal sulphide deposits in the region and they are applicable to the
deposits identified as SAS type and SVALS type, respectively.
Mo sulphide deposits
Molybdenum sulphide deposits in Bergslagen are all minor (<0.1 Mt) and have never had any
economic significance. Nevertheless, most of these deposits were mined during the 2nd world
war. No mining has been carried out after this period. The mineralisations are generally
hosted by the granites and pegmatites that belong to the GP suite of intrusive rocks and
include, for example the Bispbergsklack, Pingstaberg and Uddgruvan deposits. The total
production from Swedish deposits is c. 0.2 Mt of MoS2, of which more than 50% originates
from Uddgruvan, which is a quartz-rich pegmatite deposit. Locally, Mo occurs as a minor
constituent in base metal, Fe oxide and W deposits.
Ni-Cu sulphide deposits and PGE deposits
Nickel is locally present as an economically significant component in some Cu-rich, base
metal sulphide deposits that are hosted by gabbro. The largest deposits in Sweden are the
Slättberg and Kuså deposits in northern Bergslagen. Other deposits of this type are the
Frustuna deposit in southern Bergslagen and the Ekedal and Gaddbo deposits in the central of
the region.
Platinum group elements (PGE) have never been mined in Bergslagen, and only one prospect,
the Flinten mineralisation in the Svärdsjö area in the northern part of the region is present.
This deposit is hosted by a layered gabbro, within which sulphide-bearing parts with
anomalous amounts of PGE and Au occur. Analysed samples contain up to 0.2 ppm Pd, 0.13
Pt and 1.8 ppm Au.
Greisen Sn (±Zn-Pb-Cu-Mo-W-Ag-Au-Be) deposits with oxides and sulphides
Greisen-type vein deposits hosted by the 1.70-1.67 Ga suite of GSDG intrusive rocks occur in
the north-westernmost part of the Bergslagen region. They are economically insignificant and
have never been mined. However, two occurrences have been in focus for exploration
activity, the Van and Stora Flaten deposits. The greisen-type vein deposits are cm to m thick,
dark alteration zones that are rich in quartz, mica (including Li-rich mica), chlorite and
contain variable amounts of metal-bearing oxides and sulphides, including cassiterite, galena,
sphalerite, chalcopyrite, arsenopyrite and pyrite. In general, fluorite and topaz are also present
in small amounts as well as molybdenite, wolframite and scheelite.
24
Excursion Route and Road Log
The field excursion will take an almost circular, anti-clockwise route through some of the
main mining areas in the Bergslagen region (Fig. 1). We will start from Arlanda/Stockholm
airport and head north to the Dannemora iron mine. From Dannemora the excursion will
travel back to Uppsala, an old historic capital city of Sweden, and then to the major historic
mining areas of Sala, Garpenberg, Stollberg, Grängesberg, Ställdalen, Kopparberg, Hällefors,
Nora and finally Zinkgruvan near Askersund. From Zinkgruvan there will be a 3.5 – 4 hours
drive back to Arlanda/Stockholm airport.
Excursion Stops
Day 1: Thursday 14 August
Introduction
On this first day we will drive from Arlanda/Stockholm airport to the Dannemora iron mine
(Fig. 1) and our accommodation at Gammel Tammen hotel, which is an old manor house in
the village of Österbybruk. This journey will take about 1.5 hours. There will be no geological
stops on this day. However, we will present an overview of the geology of Bergslagen during
the evening, or in the morning on Day 2.
25
Day 2: Friday 15 August, Dannemora Mine area
By Lena Landersjö (Dannemora Mineral) and Peter Dahlin (Uppsala University)
Introduction
The Dannemora iron ore deposit (Fig.11) is located at 17° 52' N and 60° 15' E, about 120
kilometres north of Stockholm and 45 kilometres north-northeast of Uppsala. It represents the
most significant iron ore deposit in the eastern part of Bergslagen and has been mined for
several hundred years until the mine was closed in 1992. The total production of iron ore from
the Dannemora mine up to 1992 is estimated at 36.8 million tonnes with an iron content
between 30 and 50 %. (Lager, 2001; Wik et al., 2006). Some polymetallic sulphide
mineralizations also occur within the deposit and a few have been mined. In fact, the earliest
documentation of mineralization in Dannemora, which dates back to the late 15th century,
refers to “the silver rock”.
Figure 11. Geological map of the Dannemora area (from Stålhös, 1988). Rock units: yellow –
rhyolite; blue – carbonate rocks (minor occurrences marked with K); green – gabbro, diorite; brown –
granodiorite, granite; red – granite. Excursion stops 1-4 indicated.
26
Significant amounts of iron ore remain in the Dannemora ore field, and today the high world
demand for iron has revived the interest of exploring for this commodity, and a re-opening of
the mine is planned for late 2009.
During the field excursion we will present the geological setting and outline of the
Dannemora ore field, demonstrate the mine model, and briefly mention the present activities
in the area (Stop 1). We will visit an outcrop in the mine area that gives insight into the type
of volcanic environment that prevailed during the ore-forming period (Stop 2), take a short
walk around parts of the old workings (Stop 3) and, finally, study a geologic profile west of
Dannemora (Stop 4). At the end of the day we will drive from Dannemora to Sätra Brunn near
Sala, where we will stay overnight at Brunnslogi guest-house.
Figure 12. Map of Dannemora with open pits and ponds marked. Excursion stops 1-3 indicated.
Stop 1: Dannemora iron ore deposit.
Location
Mine Office, Dannemora (N 6678050, E 1614000)
Description of the Dannemora deposit and the geological setting
The Dannemora deposit consists of about 25 bodies of magnetite ore, forming a 3 kilometres
long and 400 to 800 metres wide ore field, striking N30°E (Figs. 11 and 12). The ore is
27
characterized as a limestone and skarn iron ore, which contains between 0.2 and 6 %
manganese. The most recent and comprehensive information on the Dannemora deposit is
found in Lager (2001), where also relevant earlier references are listed. The iron ores have
been interpreted as stratabound and stratiform syn-volcanic hydrothermal deposits formed in
stromatolitic limestone (Lager, 2001).
The ore-host limestone and skarns are part of the uppermost unit in a 1.9 Ga old felsic
volcanic and sedimentary succession. During the ca 1.8 Ga Svecokarelian orogeny, the rocks
were tightly folded into a steep upright syncline with about parallel limbs, plunging a few
degrees to the northeast, and metamorphosed to greenschist facies. Compared to most other
parts of Bergslagen, primary rock textures are well preserved in the Dannemora district. In the
southern part of the deposit, the strata have fault-bounded contacts with a c. 1.89 Ga old, early
orogenic granodiorite, which has not been encountered in the northern part of the deposit.
The stratigraphic sequence in the Dannemora syncline (Fig. 13) has informally been divided
into a lower and an upper formation (Lager, 2001). The upper formation, in turn, is divided
into a basal unit and an upper unit. The lower formation is composed exclusively of
volcaniclastic rocks, as is the basal unit of the upper formation. However, the latter contains
clasts of iron ore. The upper unit of the upper formation consists of beds of volcaniclastic
rocks as well as carbonate rocks and beds of iron ore. All volcanic rocks have a rhyolitic
composition.
In the lower formation, the pyroclastic rocks typically contain numerous quartz phenocrysts
and/or pumice fragments, whereas in the upper formation, laminated ash-siltstone with
intercalations of carbonate predominates.
The carbonate rocks consist of dolomitic limestone and subordinate calcitic limestone.
Stromatolite-like structures are common and the carbonates are interpreted to be of biogenic
origin (Lager, 2001; Allen et al., 2003). The iron ore preferably occurs in dolomitic
sequences.
Stop 2: Outcrop with accretionary lapilli.
Location
Northwestern part of the Dannemora mining field (N 6678825, E 1614290).
Introduction
Here we can see ash-siltstone beds in the basal unit of the upper formation. This formation
forms the footwall to the limestone-hosted iron ore lenses, which occur some 50 metres to the
east. A rock sample from this locality yielded a UPb age of 1894 ± 4 Ma (Stephens et al., in
preparation). The intention was to date the very top of the lower formation, with volcanic
mass flows representing an active volcanic stage, followed by the laminated ash-siltstone in
the upper formation, representing a waning stage of the volcanism.
Description
In the brown and light-coloured laminated ash-siltstone at this locality, some beds with well
preserved accretionary lapilli are found (Fig. 14). Individual lapilli are up to 1 centimetre in
size and concentric layering may be seen. Normal grading is seen in some ash-siltstone beds,
indicating stratigraphic younging to the east.
28
Figure 13. Geological map of the Dannemora syncline at the 300-350 m level, and cross-sections
(inset). From Lager (2001). Surface projection of excursion stops 1-3 indicated.
29
Figure 14. Photograph of rhyolitic ash-siltstone containing accretionary lapilli. Outcrop in the basal
unit of the upper formation.
Stop 3: Svavelgruvan - Sulphide mineralization within the Dannemora deposit
Location
Southwest-central part of the Dannemora mining field (N 6677760, E 1613680)
Introduction
Subordinate Zn-Pb-FeS mineralization is found preferably in the upper formation, spatially
associated with iron ore mainly in the southern part of the mining field. In the early 1900s,
some of these sulphide bodies were extracted. Although the total tonnage of mined sulphide
ore amounted only to about 28,000 tonnes, the average grade recorded between 1911 and
1920 was 30-50% Zn.
Figure 15. Geological map at the 40 metre level of the massive sulphide deposit, Svavelgruvan mine
(from Lager, 2001).
30
Historically, the sulphide mineralization is described as bound to the iron ores, although often
remobilized and enriched due to deformation and faulting. Today, the deeper parts of this
mining area are inaccessible.
Description
The only massive sulphide vein deposit known in the Dannemora syncline occurs in the
Svavelgruvan mine. A view of the open pits demontrates the cross-cutting appearance of the
sulphide ore compared to the general strike of the iron ores (Fig. 15). Conformable sulphide
mineralization is, however, also known at depth. Rock samples from the waste dump contain
sphalerite, galena, arsenopyrite and pyrite.
Stop 4: The Bennbo profile
Location
Bennbo (N 6679030, E 1612115) is situated about 2 km west of Dannemora mine.
Introduction
The sequence is a c. 60 metre long profile oriented E-W and it consists of redeposited
pyroclastic deposits (Lager 2001).
Description
The sequence consists of well-preserved volcaniclastic sediments that were probably
redeposited in a beach-shoreface to shallow marine environment (Lager 2001), and reworked
by waves, currents and wind. This is evident from good sorting, ripple marks, graded bedding
and erosion channels (Fig. 16). Banded Iron Formations (BIFs) occur in several parts of
Bergslagen and were interpreted as a post-eruptive facies by Allen et al. (1996). At Bennbo
two texturally different variants of BIF can be distinguished. Variant one is dominated by
magnetite-rich layers (0.5-5 cm thick) interbedded with skarn-rich lamina (<1 cm). Variant
two consists of a rhyolitic fine-sand fraction with subordinate thin (<0.5 cm) magnetite-rich
laminae and skarn. In variant two, no laminae are thicker than 1 centimetre. The BIF has been
subjected to soft sediment deformation evident in water escape structures and convolute
bedding (Fig. 17). Plane-parallel bedding and a lack of traction current structures are evidence
that the BIF was formed in a quiet, below wave base environment.
Figure 16. Erosion channel.
Figure 17. Convolute bedding in BIF.
31
A c. 70 centimetre wide symmetrically zoned skarn occurrence, possibly of reaction skarn
type (Meinert 1992), is present in the sequence. The bedding (striking about N-S) is also cut
at low angle by a couple of decimetre to metre wide dykes, probably of intermediate
composition.
Stop 5: Early orogenic meta-granite
Location
Four km north of Vänge (N 6642800, E 1591550).
Introduction
In the Uppsala region, the older meta-intrusive rocks form two spatially (and genetically?)
distinct groups. One group consists of grey, enclave-bearing tonalites to granodiorites, the
other of red granites. The latter is spatially associated with mafic meta-intrusive rocks. On the
regional Bergslagen scale, there is an overall volumetric dominance of granites in the west
and tonalites-granodiorites in the east.
Description
Red to pale, unequigranular, undeformed to locally foliated and altered, early-orogenic
Svecokarelian metagranite. Deformation and alteration seem to be associated with
topographic lineaments. The alteration caused both sericitisation-saussuritisation and
chloritisation of feldspars. Chemically the alteration resulted in an increase in Mg while K,
Na, Ca and Mn were depleted and Al, Si and Ti remained immobile. Meta-granites of this
type have been dated at 1907±4 and 1891±6 Ma (not at this locality).
32
Day 3: Saturday 16 August, Sala Silver Mines
By Nils Jansson (Luleå University of Technology), Magnus Ripa (Geological Survey
of Sweden), Rodney Allen (Boliden Mineral and Luleå University of Technology) and
Stuart Bull (University of Tasmania)
Introduction
Sala township grew up in Mediaeval times around the most important silver mines in Sweden.
The ore deposits comprise silver-bearing Zn-Pb veins and stratabound polymetallic sulphideskarn ores within folded dolomitic marble (Allen et al., 1996, 2003; Jansson, 2007). The
marble formation is at least a few hundred metres in stratigraphic thickness and is interpreted
to overlie a thick volcanic succession that is exposed to the west and north (Fig. 18). The east
side of the marble is an intrusive contact against the Sala Granite, which has been dated by
zircon U-Pb to 1891 Ma. An accretionary lapilli-rich, volcanic air-fall bed within the Sala
dolomite, just west of the Sala Mines, has been dated by zircon U-Pb to 1894 ±4 Ma
(Stephens et al. 2001 and in prep).
We will visit excellent exposures of the dolomite, where stromatolite structures are evident,
the volcanic rocks that underlie and are interbedded with the dolomite, and we will have an
underground mine tour in the Mediaeval workings where the stratigraphy, structure and some
of the ores are still well exposed.
The dolomitic marble that hosts the Sala Zn-Pb-Ag mineralization
Careful examination of the marble units in the Sala area indicates that delicate sub-mm scale
microbial lamination associated with obvious stromatolite forms is locally preserved.
However, even in the areas of strong hydrothermal alteration, skarn alteration and higher
degrees of metamorphic recrystallisation and deformation, structures characteristic of a
stromatolitic origin survive (Allen et al. 2003). On a fine scale, the first order microbial
layering can be preserved as 0.5 to 3 cm spaced, often stylolitic, mm thick laminae defined by
traces of green skarn minerals. On a larger scale, stromatolite heads are obvious at the base of
the volcaniclastic interbeds, where they are defined by upward projecting lobate forms
separated by downward projecting triangular to flame-like patches of felsic material variably
altered to chlorite, phlogopite and skarn compositions. These structures survive all but the
most intense deformation and metamorphism and consequently provide the best evidence in
most areas that the marbles were stromatolite reefs. Marine 13C isotopic values are also
retained in the more recrystallized and deformed carbonates, but the 18O data is spread
towards lighter values.
The stromatolitic marble units are interbedded with thinner volcaniclastic units that represent
the reworked products of felsic pyroclastic volcanism deposited in sub- to locally above wave
base environments (Allen et al. 2003). Microbial activity that builds stromatolites is
dependent on photosynthesis and thus maximum water depths for stromatolite accumulation
depend on water clarity, which in Bergslagen would depend on the amount of suspended
vitric ash in the water column. Maximum water depths for stromatolite growth were probably
no more than 100-200 metres, and the stromatolite forms present at Sala suggest fluctuations
from sub-wave base to shallow inter-tidal conditions (c.f. Grotzinger, 1989).
33
Figure 18. Simplified geological map of the Sala area, modified after Jansson (2007), Ripa et al.
(2002), Holmgren (2001), Zenzen (1918-1919). Field excursion locations 1 to 6 are shown in the white
circles.
Brief description and history of the Sala silver mines
Sala Mine is approximately 700 m long from the Torg Shaft in the SE to the Carl XI Shaft in
the NW. The shafts are aligned S-SE / N-NW and the width of the mine area is approximately
80 m. The mine becomes progressively deeper from SE towards NW where a maximum depth
of 318 m is attained. The rake of the mined deposit is approximately 35o toward 325o which
corresponds to the axis of a major F1 syncline that hosts the ore deposit Figs. 18, 19).
34
Mining for silver began before the start of the16th century in the southernmost, now collapsed
parts of the mine. Early reckless mining caused several of these collapses and the mine was
almost abandoned already in the late 16th century even though more ore remained at depth.
Figure 19. Cross section of Sala Mine in a SSE-NNW orientation, parallel to the strike of the mine. The section
shows the major shafts as well as the major ore zones. The collapsed section of the mine in the south consists of
the Sandrymningen, Kungsrymningen and Herr Sten’s levels. No vertical exaggeration. From Jansson (2007).
Figure 20. 3D perspective view of Sala Mine seen towards the east, almost perpendicular to the strike. The
picture is extracted from the computer in Sala Silvergruva AB’s tourist reception (from Jansson 2007).
35
Figure 21. Geological plan of the Ulrika Eleonora Level (155 m level) in the Sala mine. From Jansson (2007).
The central shaft is the Queen Christina Shaft which was sunk in 1650-1660 in order to
improve access to ore buried by the collapses. The sinking of the shaft was a very risky, yet
36
highly successful project that allowed mining to continue uninterrupted until 1908. Since
then, only short-term mining for zinc has occurred 1950-1952. The mine is now a historical
heritage site with guided tours. The tourism venues are expanding fast and have greatly
increased the mine’s accessibility.
Sala Mine is known for its silver-richness, well-preserved 19th century pit-head buildings (of
which one is Sweden’s oldest), oval-shaped stopes made by fire-setting, shafts and
underground workings dating back to the 17th century. Unique for Bergslagen, the mine was
never filled and is not completely submerged under groundwater.
It total, approximately 450 tons of silver and 35 000 tons of lead was produced from 3-5
million tons of mined material. Though small figures in modern measures, it should be noted
that 200 tons of silver was produced during the mine’s heyday in the 16th century.
Geological interpretation of the Sala ore deposit
The Sala Zn-Pb-Ag deposit is interpreted by Jansson (2007) to have formed by sub-sea floor
infiltration of metalliferous fluids into a buried stromatolite reef. The reef contained sparsely
interbedded felsic volcanic rocks. During hydrothermal alteration and mineralization, these
interbeds have been Mg-altered and the stromatolitic limestone dolomitized. The ore mainly
consisted of argentiferous galena and sphalerite. Morover, historic reports suggest that highgrade sulphosalt- and amalgam-rich ore occurred in the now collapsed part of the mine.
The host rock has experienced several forms of deformation involving thrusting, ductile
extension, two-phase folding, reverse brittle-ductile shearing, as well as late brittle strike-slip
movement. The mine is mainly confined to two large tectonic structures: the NW-trending
Storgruvan Shear Zone (reverse kinematics) whose strike parallels the strike of the mine, and
the F1 Sala Syncline whose fold hinge parallels the plunge of the mineralization. The Sala
Syncline is moreover refolded leading to local reversals in the plunge of the ore bodies.
The carbonate has been metamorphosed under lower greenschist to lower amphibolite
conditions to dolomitic marble, yet stromatolitic textures are locally well-preserved. The
siliceous interbeds are metamorphosed to Chl–Phl-Trem +/- Ser +/- Diop +/- Ser +/- Qz +/Carbonate lithologies. A phase of retrograde alteration has also affected the rocks.
Sphalerite vein networks occur in proximity to well-preserved stromatolitic textures and
indicate that ore formation was not entirely texturally destructive. Stromatolitic way-up
determinations suggest that galena ore appears stratigraphically higher than the sphalerite ore.
The galena ore was generally richer in silver than the sphalerite ore (1 500-10 000 ppm vs.
150-200 (13) ppm) with silver mainly occurring as microscopic grains of antimonides,
sulphosalts and amalgam.
The relationships between the shapes of the workings and measured bedding attitudes suggest
that there was a stratigraphic control on ore formation. The ore was initially emplaced as
stratabound ore bodies but is now concentrated in the hinges of folds and near the boundary to
the Storgruvan Shear Zone.
37
Figure 22. Simplified geological map of the area directly west of Sala Silver Mines, modified after
Allen et al. (2003). Field excursion locations are shown in the white circles. North is up-page.
Stop 1: Bedding and structure in dolomite sequence at Tistbrottet
Location
Tistbrottet is an active dolomite quarry 200 m west of the Sala Silver Mines (N 1542580, E
6643440).
Introduction
The Sala dolomite comprises a regular, almost rhythmic alternation of stromatolitic dolomite
beds and thinner rhyolitic ash-siltstone/sandstone beds. The Tistbrottet and Finntorpet
dolomite quarries occur in parts of the Sala dolomite succession that are characterised by
thick sequences of 2-5 m thick, tabular dolomite beds with regular thin (0.01 – 0.5 m )
interbeds of felsic ash-siltstone/sandstone (Allen et al. 2003). Whole rock geochemical
analysis of this felsic material indicates juvenile rhyolitic compositions, and these rocks were
38
termed rhyolitic ash-siltstone (RAS) facies by Allen et al. (1996). This facies clearly
represents a quiet sub-wave base environment, and was interpreted by Allen et al (1996) to
represent mainly post-eruptive re-deposition of volcanic ash via a combination of turbiditic
and suspension processes. The rest of the Sala dolomite succession is similar but also contains
some thicker and coarser grained rhyolitic ash-siltstone/sandstone beds up to 3 m thick. The
thicker and coarser grained felsic interbeds 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 Bergslagen basin(s).
The Tistbrottet and Finntorpet quarries and nearby outcrops provide excellent evidence for the
microbial stromatolitic origin of the Bergslagen limestones.
Description
Tistbrottet quarry exposes a thick sequence of white, relatively pure dolomite of generally
massive appearance with lesser thin interbeds of pale brown rhyolitic ash-siltone and
sandstone. The bases of the dolomite beds (against the ash-siltstone beds) are sharp and
planar, whereas the tops of the dolomite beds typically have a crenulate wavy form. This
wavy form was interpreted by Allen et al. (2003) as the tops of domal stromatolites, which
have been covered by ash-siltstone/sandstone. The ash-siltsone/sandstone fills the interstitial
space between the individual stromatolite domes resulting in the crenulated wavy pattern that
can be readily seen even from this view point about one hundred metres away.
The beds strike E to NE, and dip and young to the north (Fig. 22). The main cleavage (S2)
strikes E to NE and has an associated steep stretching lineation. Deformation and
recrystallisation associated with a folded mylonitic S1 shear zone at the western end of the
quarry have obliterated primary structures in this area. The dolomite marble in the remainder
of Tistbrottet is generally massive, white and recrystallised with local stringers/patches of
green skarn minerals (mainly amphibole) and with the exception of bedding, primary
sedimentary textures are not generally preserved. Tectonic lineation, and bedding-parallel slip
focussed on the fine-grained siliceous interbeds during regional deformation have resulted in
variable amounts of deformation of the stromatolite heads. Locally, stromatolite columns are
also faintly outlined within the body of the carbonate beds by bedding-normal trains of calc
silicate minerals. These presumably formed by skarn reactions between the microbial
carbonate and trapped inter-column felsic clastic material.
Stop 2: Stromatolites in Finntorp Quarry, Sala
Location
One km SW of the Sala Silver Mines (N 1542540, E 6642580). The entrance to the quarry is
from the southwest.
Introduction
The following description of the dolomite succession at Finntorpsbrottet is taken from Allen
et al. (2003). The area around Finntorpsbrottet contains the best exposures of the structure of
the carbonate beds in the Sala region. At this locality, thick to very thick, tabular dolomite
beds strike mainly NW to NNW and dip steeply to the NE or to the SW (Fig. 22).
Stratigraphic younging indicators (stromatolite domes) from the NE side of the quarry and the
adjacent outcrops indicate a coherent northeast younging succession >200 m in thickness. The
39
main cleavage (S2) is moderate in intensity, strikes NNE to NE and contains a strong
stretching lineation that plunges 30-50 degrees to the SW. Small folds related to the main
cleavage (F2) plunge moderately to the NE. Unusually for Bergslagen, S2 is at a relatively
high angle to bedding and crenulates a weak earlier mica foliation (S1) that is sub-parallel to
bedding. A series of small NNW-plunging folds associated with a younging reversal at the
northeastern edge of the outcrop zone 200 m north of the quarry (Fig. 22) indicate an F1 fold
hinge in this area.
Figure 23. Sketch map and stratigraphic column through the dolomite succession at location 2 on the
NE wall of Fintorpsbrottet (Finntorps Quarry), southwest of Sala Silver Mines. Stromatolite forms
range from plane bedded to composite mounds. Black lenses are dark green-blue chlorite after altered
rhyolitic siltstone, which fills in the original depressions between stromatolite mounds. After Allen et
al. (2003).
Description
In spite of the deformation, sedimentary structures, including stromatolites are well preserved
in the NE half of Finntorpsbrottet, and irregularities in the quarry walls locally allow detailed
examination of structures in three dimensions. Individual stromatolites are predominantly
domal in form (Fig. 23), may be linked or non-linked and have diameters up to 65 cm and
synoptic relief of up to 50 cm. The typical structure is of 1 to 3 m thick tabular beds
comprising domal stromatolites that increase in diameter and decrease in synoptic relief up
40
section and grade to prone microbial mat at the bed top (Fig. 23). In similar vertical profiles,
domal stromatolites can also coalesce to form domal bioherms (~ patch reefs; e.g. Fig. 23 up
to 8 m). These structures have diameters and thicknesses of at least 3 m and a synoptic relief
of >1 m. Tabular 1-3 m thick beds may also be comprised of columnar domal stromatolites
with diameters of up to 20 cm, in which case there is no substantial relief on the upper
surface. In at least one such unit in Finntorpsbrottet (and in Sala silver mines), columns are
narrow and terminate at an acute angle giving the appearance of the distinctive "conophyton"
form.
Locally, the stromatolites have a well preserved internal layering, which consists of first order
1 mm thick laminae, spaced at 0.5 to 3 cm and defined by traces of green and grey skarn
minerals, with intervening second order, millimetre-spaced, wavy, sub-mm lamination.
However, in the bulk of the exposure only the first order laminae are preserved and these are
often stylolitic, and are crenulated when at a significant angle to S2. In thin section, the mmscale laminae are defined by trains of <1 mm crystals of Mg-rich chlorite and lesser tremolite.
They are interpreted as original fine wavy microbial laminae that were preserved where
draped by traces of fine-grained volcaniclastic material now replaced by chlorite and
tremolite.
Stop 3: Rhyolitic sandstone-siltstone interbed in dolomite, Sala
Location
In the forest about 100 m north of Finntorp quarry, Sala (N 6642797, E 1542500).
Introduction
The almost rhythmic alternation of thick stromatolitic dolomite beds and thinner rhyolitic ashsiltstone/sandstone beds that is so distinctive in Tistbrottet, occurs in all or nearly all of the
major calcitic and dolomitic limestone units in Bergslagen. Outcrop at stop 3, north of
Finntorpsbrottet, exposes an excellent example of one of these rhyolitic sandstone interbeds.
It is difficult to recognize stromatolite forms and internal structures within the dolomite beds
in the area of stop 3. However, the dolomite is regularly interbedded with felsic volcaniclastic
beds, and the stratigraphic tops of many of the dolomite beds (and bases of the volcaniclastic
beds) have an undulating, bulbous-lobate geometry with relief of up to >1 m, whereas the
bases of the dolomite beds (and tops of the volcaniclastic beds) are planar. The lobate shapes
at the top of the dolomite beds are similar to the domal stromatolite structures in
Finntorpsbrottet where good internal microbial layering can be observed. Consequently, these
beds are likewise interpreted to comprise coalesced domal stromatolite bioherms.
Examination of the siliceous interbeds within the Sala marble succession is complicated by
the development of coarse-grained tremolite skarn at the contacts with the enclosing dolomite.
These contact skarns are ubiquitous and commonly destroy primary textures throughout the
thinner silicic interbeds. As with the marbles themselves, primary textures in these
intervening siliceous units are best preserved here in the area of Finntorpsbrottet.
41
Figure 24. Stratigraphic column through the dolomite succession in the area of continuous outcrop
west of Sala Silver Mines. Field excursion locations 3 and 4 are shown in the white circles. Modified
after Allen et al. (2003).
Many marble beds in the Sala succession and elsewhere in Bergslagen are underlain by felsic
ash-siltstone deposited in a sub-wave base environment, but are immediately overlain by
intervals of storm wave reworked crystal sandstone. This indicates that growth of the
microbial stromatolites commenced in a sub-wave base environment and that penetration
above wave base resulted in interruption of carbonate accumulation. Partially reworked
accretionary lapilli layers suggest that some of the storm wave reworked volcaniclastic
deposits were emplaced synchronously with phreatomagmatic felsic volcanism, and could
represent air-falls reworked by eruption-generated tsunamis. The thicker carbonate reefs show
a cyclic interbedding of thick (average 5 m) limestone and thinner (average 10-100 cm) felsic
ash-siltstone/sandstone beds. The cycles are attributed to growth of the reef up to wave base,
followed by dumping of felsic detritus onto the reef via ash fall eruptions and wave action
(?including eruption generated tsunamis), resulting in smothering of the microbial
community, continued basin subsidence back to deeper water (sub-wave base) conditions, recolonisation by the microbial community, growth to sea level again, and so on.
42
Consequently, in the Bergslagen basin, viable microbial communities were largely restricted
to sub-wave base environments. The current affected above wave base environments were
rendered inhospitable by the large volume of mobile unconsolidated felsic pyroclastic ejecta
that was circulating in these environments (Allen et al., 2003).
Description
Stop 3 contains a 1-2 m thick, white-weathering, stratified rhyolitic sandstone-siltstone bed
with thicker brown-weathering dolomite beds above and below. The top of the lower dolomite
bed shows a lobate geometry attributed to the heads of stromatolite domes, whereas the base
of the overlying dolomite bed is relatively sharp and planar. The base of the ash bed is coarse
grained and rich in volcanic quartz and feldspar crystal grains. The lower to middle part of the
bed has diffuse low angle cross stratification. Units of this type have been referred to as
rhyolitic stratified crystal-vitric±lithic sandstone (SCS) facies and generally interpreted to
represent shallow water or subaerial tractional sedimentation (Allen et al., 1996). The lower
part of the rhyolitic bed has abundant, close packed to scattered accretionary lapilli. The
lapilli have a pale rim and a dark core and are mostly intact, although broken lapilli are also
present. Most are ellipsoidal shaped due to stretching in the plane of the main cleavage and
have long axes of up to 1 cm. They are interpreted as the partly reworked deposit of a
rhyolitic phreatomagmatic airfall eruption.
The bed here at stop 3 is typical of the thicker (>1 m) felsic volcaniclastic interbeds in the
Sala area and elsewhere in Bergslagen where primary rock textures are preserved. These beds
tend to have a distinctive cyclic structure, comprising a basal interval of SCS facies and an
overlying interval of RAS facies. The base of the sandstone has in-filled topographic
irregularities of up to 1 m relief created by stromatolite heads at the top of the underlying
marble. The basal part of the sandstone is massive and grades upwards into diffuse to
moderately well developed, planar to low-amplitude wavy bedforms. These have wavelengths
of 1 to 2 m, and consist of packets of planar to wedging lamination that truncate each other at
low angles. They are interpreted as low-angle cross stratification that records storm wave
reworking of the substrate. The “hummocky” bedforms include a layer of partly reworked
accretionary lapilli. The overlying RAS facies interval varies from massive to tabular bedded,
fine-grained sandstone to siltstone. Beds may be internally massive or normally graded and
are interbedded with massive to planar-laminated ash-siltstone. The RAS facies indicate subwave base sedimentation via a combination of turbiditic and suspension processes.
Stop 4: Rhyolitic sandstone interbed with reworked accretionary lapilli, Sala
Location
In the forest about 250 m north of Finntorp quarry, Sala (N 6642835, E 1542607).
Introduction
Several rhyolitic sandtone/siltstone interbeds in the Sala dolomite sequence have well
preserved accretionary lapilli. These beds, including the accretionary lapilli, show varying
degrees of reworking. In this outcrop, the accretionary lappili show evidence of reworking by
strong traction currents. The common association in Bergslagen between the rhyolitic
sandstone/siltstone interbeds in limestone/dolomite, accretionary lapilli and low-angle to
normal cross bedding, led Allen and Bull (2003) to suggest that many of the rhyolitic
interbeds represent pyroclastic fall deposits and that these beds may have been reworked by
storms and tsunamis generated by the same volcanic eruptions that created the fall deposits.
43
Description
A 2-3 m thick, white-weathering, stratified rhyolitic sandstone-siltstone bed occurs here in the
dolomite succession. The middle part of the rhyolitic bed contains a 30-50 cm thick interval
of cross bedded accretionary lapilli. Many of the lapilli are broken and the sandy matrix
between the larger lapilli fragments contains white-weathering curved shell-like fragments,
which are the outer rinds of broken lapilli. This indicates that the lapilli-rich pyroclastic fall
horizon was reworked by strong currents during or soon after the airfall eruption.
Stop 5: Sala Mine tour
Location
Directly south of Sala town (N 1543080, E 6643330: Drottning Kristinas shaft).
Introduction
Many features of the host dolomite with rhyolitic siltstone-sandstone interbeds will be seen
underground in the ancient drifts. Rich Zn mineralization can be seen in the roof of a crosscut on the Ulrika Eleonora Level (155 m level).
Figure 25. Stromatolitic dolomite in Södra Wallbergs drift, old Sala Silver Mines.
Stop 6: Early-orogenic meta-granite, Stora Ensta
Location
Northern outskirts of Sala, on the road to Saladamm (N 6646670, E 1545406).
Introduction
This is an alternative to stop 5 on day 2. See stop 5 day 2 for more information on the older
granitoids in Bergslagen.
Description
Red, massive, early-orogenic metagranite. The cores of plagioclase crystals are moderately
altered to green epidote-sericite. The metamorphic overprinting is not always macroscopically
44
obvious and some older granites may be mistaken for syn- to late orogenic Svecokarelian
(anatectic) granites. However, a sample of this granite taken at a locality c. 2 km east of here
was dated at 1891±6 Ma.
Stop 7: Subvolcanic intrusion, north of Sala
Location
North of the “Terrasit” factory, 3 km north of Sala (N 6647982, E 1544488).
Introduction
In the Sala area, there is a spatial relation between subvolcanic intrusions and Fe sulphidechalcopyrite mineralisation. ”Terrasit-” is the commercial name of a lime product used on
walls on buildings and indicates that the nearby carbonate rocks are less dolomitised than at
the Tist- and Finntorps-quarries.
Description
Strongly porphyritic rhyolite with about 50% 3-4 mm phenocrysts of feldspar, quartz and
possibly biotite after hornblende, in a grey-brown fine grained groundmass. This rhyolite
shows weak epidote-biotite alteration. The rock is interpreted to be a subvolcanic intrusion
and has been dated at 1892+5/-4 Ma by the TIMS U-Pb zircon method.
Stop 8: Felsic meta-volcanic clastic facies, north of Sala
Location
Sandtorpsbergen, 4 km north of Sala (N 6649202, E 1544026).
Introduction
The parts of the supracrustal successions in Bergslagen, stratigraphically well below thick
meta-limestone units (as at Sala) are typically dominated by juvenile, massive to diffusely
stratified, felsic volcanic rocks. These rocks are interpreted to represent the intense volcanic
stage of Allen et al. (1996). In the less metamorphosed, deformed and hydrothermally altered
areas, many of these rocks contain abundant recognisable pumice clasts and/or glass shard
grains. In the more metamorphosed, deformed and altered areas, the coarse grained rocks may
display a diffuse mottled matrix texture that can be attributed to tightly packed pumice clasts,
whereas the silty to sandy grain size rocks are commonly massive and homogeneous with a
sugary granoblastic texture devoid of primary volcanic textures.
Description
There are three prominent outcrops on this low ridge. The southern-most outcrop contains
massive monomict rhyolitic ash-sandstone with scattered small lens-shaped clasts that may be
flattened pumice clasts (fiamme), and some more blocky possible rhyolite lithic clasts. The
next outcrop to the north (middle outcrop) comprises diffuse to well planar stratified rhyolitic
ash-sandstone. The third and northern-most outcrop comprises diffuse to well planar
stratified, moderately sorted, fine to medium grained volcanic breccia with abundant dense
blocky rhyolite clasts.
45
Day 4: Sunday 17 August, Garpenberg mines and region
By Rodney Allen (Boliden Mineral and Luleå University of Technology), Nils Jansson
(Luleå University of Technology), Erik Lundstam and Rolf Jonsson (Boliden Mineral),
and Stuart Bull (University of Tasmania)
Introduction
The Garpenberg mining operations exploit one of the most important styles of mineralization
in Bergslagen, termed Falun-type by Geijer (1917) and Koark (1962), or “stratabound,
volcanic-associated, limestone-skarn-hosted Zn-Pb-Ag-Cu-Au sulfide deposits” (SVALStype) by Allen et al. (1996). Mining of copper veins started well before the 13th century and
mining of complex sulphide ore started in the 13th century and continues to date. The ore is
rich in silver and zinc, but contains also copper, lead and gold in payable amounts. The
mineralised system extends along strike for 7 km and down dip for over 1.5 km, and is
focussed at the margins of a folded, 10-80 m thick, calcite marble horizon within volcanic
rocks. This marble horizon also hosts the major Smältarmossan magnetite deposit, just 50-100
m north of the Garpenberg (South) deposit. Other thinner marble horizons several hundred
metres lower in the stratigraphy host a number of other magnetite-skarn iron ore deposits,
including the Ryllshyttan deposit from which 1 MT of high grade Zn ore was produced from
within a magnetite-skarn deposit. At the Garpenberg mines, the total production, plus reserves
and known mineralization amount to about 80 MT. Proven reserves are 17 MT at 5.6% Zn,
2.3% Pb, 104 g/t Ag, 0.3 g/t Au (Boliden Mineral data). The Zn-Pb-Ag 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 ores and alteration assemblages
are partly similar to both VMS and intrusion-related skarn deposits. The ore deposits have
been modified by subsequent deformation and metamorphism, and the location of economic
ore bodies within the mineralized system is strongly influenced by these later events.
Figure 26. The mine chapel, from the early 1600’s, in front of the modern Garpenberg and
Garpenberg Norra (near the horizon) headframes.
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We will have an underground mine tour to the Lappberget ore body at Garpenberg Norra,
study drill cores, and visit outcrops of the volcanic host sequence, limestone and skarn. We
will also visit outcrops at the Ryllshyttan magnetite-zinc mine where bedded skarn deposits
with stratiform magnetite layers and diverse skarn compositions are particularly well exposed.
Geology of the Garpenberg region
The Garpenberg region comprises an inlier of volcanic rocks with subordinate marble
horizons, enclosed by early orogenic granites (Fig. 27). The supracrustal rocks occupy a
northeast trending, mainly steeply northeast to southwest plunging, tight to isoclinal syncline.
In detail the syncline has a complex geometry due to many parasitic folds and faults. Axial
surfaces to the folds are inclined steeply to the SE, and the SE limb of the main syncline is
truncated by a major NE trending shear zone that also dips SE and has shear sense indicators
that record reverse dip-slip displacement. The supracrustal succession NW of the fault has a
stratigraphic thickness of about 2 km, is of lower amphibolite facies metamorphic grade and
hosts the Garpenberg ore deposits. Rocks on the SE side of the fault are undifferentiated
metavolcanic rocks that appear to be of higher metamorphic grade, but still amphibolite
facies.
The main folds are D2 structures related to a strong NE-trending penetrative cleavage and a
strong, mainly steep, stretching lineation. Shears and faults are common, both parallel and
oblique to cleavage (Allen et al., 2003). F2 parasitic folds have undulating fold axes and grade
into cone- or sheath-shapes due to a steep, strong but inhomogeneous stretching strain
associated with the folding. Basically, in the areas of strongest strain, the F2 anticlines have
been stretched into cone shapes and the synclines into funnel shapes. The large shear zone
trending northeast along the southeast limb of the regional syncline is interpreted as a major
D2 structure with several km of displacement. Most other faults and shears recognised in the
region have displacements of less than 1 km, overprint S2 foliation, and are designated as D3
structures. They could be related to D3 reactivation of the D2 shear zone. Stephens et al.
(2001) suggest that F2 folding in much of Bergslagen is a direct result of regional scale D2
shearing. The structure of the Garpenberg region supports this interpretation.
First generation tectonic structures are commonly masked by the strong overprint of the
second generation structures. However, S1 foliation is locally well developed and is mainly
sub-parallel to bedding. F1 folds appear to be uncommon or are of such large scale that they
cannot be mapped in outcrop.
From top to bottom, the stratigraphic succession at the Garpenberg mines comprises (Fig. 30):
1. Upper hanging-wall rhyolitic pumice breccia
2. Lower hanging-wall limestone-volcanic breccia-conglomerate sequence
2a. Matrix-supported, polymict limestone-volcanic breccia with dacitic tuffaceous
matrix
2b. Lenses of rhyolitic and dacitic pumice breccia and ash-siltstone
2c. Basaltic to andesitic volcanic breccia, sandstone and siltstone
2d. Clast-supported, polymict limestone-volcanic breccia, crystal-rich sandstone,
siltstone
2e. Clast-supported, limestone breccia-conglomerate
3. Limestone (including calcitic marble, dolomite, skarn)
4. Upper footwall felsic volcaniclastic succession
4a. Rhyolitic ash-siltstone
4b. Rhyolitic pumice breccia-sandstone
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4c. Dacitic pumice breccia-sandstone
4d. Rhyolitic pumice breccia-sandstone, ash-siltstone
5. Basalt lava and volcaniclastic rocks
6. Lower footwall rhyolitic ash-siltstone and minor coarse volcaniclastic units
In addition there are several shallow intrusions:
7. Weakly feldspar-porphyritic dacite
8. Strongly feldspar-porphyritic dacite
9. Weakly feldspar-porphyritic basalt and andesite (commonly actinolite-biotite-epidote rock)
Figure 27. Geological map of the Garpenberg region, modified after Allen et al. (2003). Field
excursion stops are shown by the numbers 1-10.
In the Garpenberg area, the carbonate and equivalent skarn units are a minor component of a
felsic volcaniclastic-dominated stratigraphy, and stromatolitic or other microbial structures
are poorly preserved. However, the setting and internal structure of these carbonate units is
the same as for the obviously microbial carbonates at Sala and Älvlången. The thin marble
and skarn beds in the Garpenberg area are also similar to the individual carbonate beds that
are amalgamated to form the thick regionally extensive marbles in the Sala and Älvlången
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areas. Consequently, the Garpenberg marble units are also interpreted to represent reefal
structures, although in this area the microbial communities were unable to establish
themselves on a large scale, with the exception of the main mineralized marble horizon that
hosts the sulphide mines. In comparison with the situation at Älvlången, where the microbial
communities became dominant once volcanism waned, the occurrence of thick, juvenile
volcaniclastic units scattered throughout the Garpenberg succession may indicate that the
environment was too volcanically active to allow substantial reef formation (Allen et al.,
2003).
Bedforms and facies associations of the stratigraphic succession both proximal and distal to
the Garpenberg ore deposits indicate that the thick footwall succession accumulated mainly
below wave base in a broad, relatively shallow basin. The main limestone unit records a
volcanic hiatus with relatively stable, relatively shallow sub-wave base marine conditions that
favoured building of an extensive stromatolite reef. The lower hanging-wall stratigraphic
package directly above the main limestone records uplift, exposure, erosion and shallow water
environments, followed by subsidence to deep water conditions (Allen et al., 2003).
Comparison of the stratigraphies at Garpenberg Norra and the distal areas indicates that in the
area of the ore deposits, the footwall succession contains much thicker and more abundant
juvenile felsic pumice-rich mass flow deposits than the distal area. This suggests that the
footwall succession records development of a marine rhyolite-dacite volcano, with the vent
area at or close to the area of the ore deposits. The upper hanging-wall rhyolitic pumice
breccia unit is over 300 m thick and over 10 km in strike extent (Fig. 27). This unit was
interpreted by Allen et al. (2003) as an intra-caldera pyroclastic flow deposit, and the extent
of the unit indicates that the whole Garpenberg area lies within or directly beneath this
caldera.
Consequently, the Garpenberg succession records a large marine felsic volcanic system, with
two major active volcanic periods, separated by a volcanic hiatus during which a stromatolite
reef formed. The second period of intense volcanic activity (hanging-wall pumice unit)
comprised a climactic caldera-forming eruption accompanied by major caldera subsidence
over most or all of the Garpenberg region. The Garpenberg polymetallic sulphide deposits
formed at the synvolcanic stage, within the proximal (near vent) part of this large rhyolitedacite volcano.
The Garpenberg deposits
The Garpenberg deposits are irregular, stratabound, multi-lens and pod-like concentrations of
sphalerite, galena and lesser pyrite, pyrrhotite, chalcopyrite, tetrahedrite, argentite and
alabandite. The ores range from massive to semi-massive, to strong disseminations and vein
networks. A few small copper-dominant ore bodies comprising networks of quartzchalcopyrite-pyrite-pyrrhotite-fluorite veins were mined from the upper footwall of the
Garpenberg (South) deposit in early years of mining. The main Zn-Pb-Ag ore bodies are
hosted in the main 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) (Christofferson et al.,
1986; Vivallo, 1985a and 1985b).
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Figure 28. Surface projection of the main ore bodies and drifts in the Garpenberg area (unpublished
Boliden Mineral data). Mine grid system is in metres.
Garpenberg
Dammsjön
Lappberget
Garpenberg Norra
Figure 29. Longitudinal projection of the main ore bodies and drifts in the Garpenberg region
(unpublished Boliden Mineral data). Mine grid system is in metres.
The Garpenberg deposits lie mainly on the NW limb of a regional F2 syncline and display a
complex pattern of parasitic F2 folds and post-F2 shears and faults (Allen et al., 2003). The
parasitic folds form a series of doubly plunging arches and cones, separated by irregular
synforms. Consequently, the distribution of ore bodies is controlled by a combination of
stratigraphy and structure. Five scenarios are identified for the development of economic
orebodies within the Garpenberg ore deposits:
(1) Tectonic remobilised vein ores in faults and shear zones in the footwall felsic rocks.
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(2) Skarn ore zones in the limestone, especially around the crests of parasitic anticlines, and
especially where shear zones intersect these parasitic folds.
(3) Tectonic remobilised vein ores in faults along the hanging-wall contact of the limestone
and in the hanging-wall rocks in the crests of parasitic anticlines.
(4) Impregnation ore in quartz±skarn rocks at the base of the limestone unit.
(5) Impregnation ore in strongly silicified rocks at the top of the felsic footwall sequence.
The limestone ore host unit shows a consistent alteration zonation from unaltered calcitic
marble, through dolomite with network tremolite±galena-sphalerite veins, to mineralized
skarn, as the skarn orebodies are approached. This zonation occurs in all directions: along
strike, down-dip and up through the limestone host unit. Most of the major limestone-skarn
hosted ore bodies occur at or adjacent to the basal contact of the limestone. It is inferred that
the alteration zonation also reflects the chemical evolution and growth of the alteration system
with time. Consequently, this zonation indicates first the hydrothermal addition of Mg and
some Mn to the limestone (to form dolomite), then silica and some base metals (tremolitesulphide veins cutting the dolomite), and then silica, Fe, more Mn and ore metals (the skarn
ores) (Allen et al., 2003).
The basal 0.2-3 m of skarn directly adjacent to the top contact of the felsic footwall rocks is
generally a distinct tremolite ± talc ± spessartine skarn or in some cases actinolite-spessartine
skarn. The skarns stratigraphically above this “contact skarn”, in the lower to middle part of
the limestone, comprise an irregular mosaic of diopside-spessartine and tremolite>spessartine
skarns with lesser actinolite-spessartine and diopside-actinolite-spessartine skarn. At the top
of the limestone, the skarns tend to be more Fe-rich with hedenbergite-andradite ± spessartine
and actinolite-andradite ± spessartine skarns (Allen et al., 2003). All of these skarn types can
host economic mineralization. Mineralization in the upper footwall and lower to middle parts
of the limestone is dominated by sphalerite, galena and pyrite, whereas like the skarns, the
mineralization in the upper part of the limestone and base of the hanging-wall is more Ferich, typically with pyrrhotite-sphalerite-galena (e.g. F ore zones, Figs. 30, 31). The hangingwall alteration above these upper mineralized zones also tends to have higher Fe/Mg contents
than the footwall alteration zones.
The footwall alteration comprises a mosaic of interfingering phlogopite ±cordierite ±garnet,
quartz-muscovite, and silicification zones, within felsic volcanic rocks. The uppermost 20-70
m of the footwall contains strong silicification that is overprinted locally by cross-cutting
phlogopite-rich zones, which connect spatially with major skarn zones in the adjacent
limestone unit. Alteration commonly decreases markedly just a few metres stratigraphically
above the main limestone. The hanging-wall alteration mainly comprises weak to moderate
epidote-actinolite-quartz alteration, with local zones of phlogopite-cordierite and silicification
above the ore bodies. This indicates a strong asymmetry in the Garpenberg alteration system,
with extensive strong alteration in the stratigraphic footwall and weak to moderate alteration
in the hanging-wall. The epidote-actinolite-quartz alteration could represent a regional scale
alteration envelope around the Garpenberg alteration system (Allen et al., 2003).
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Figure 30. Geological cross section through the middle of the Garpenberg Norra deposit (from Allen
et al., 2003). Volcanic facies, structure, ore bodies and skarn are shown.
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Figure 31. Geological cross section through the middle of the Garpenberg Norra deposit (from Allen
et al., 2003). Same geological section as Fig. 30 but showing alteration facies.
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Chemically, the phlogopite-(±biotite) ±cordierite ±garnet (spessartine>almandine-grossular)
zones contain high Mg ±Fe ±K, and very low Na ±Ca compared to normal felsic volcanic
compositions (Vivallo, 1985a and 1985b; Allen et al., 2003). They also range from high to
low Al2O3 compared to normal felsic volcanic compositions, which indicates that some of the
rocks have been strongly leached of silica (mass loss), whereas some others have undergone
mass gain through addition of silica, Mg, K and Fe. These zones are interpreted as metachlorite alteration. The quartz-muscovite zones show high K ±SiO2 and very low Na ±Ca, and
are interpreted as meta-quartz-sericite alteration. The silicification zones show high SiO2 ±K,
moderate to low Mg and Fe, and very low Na ±Ca. These zones are interpreted as metasilicified rocks. Three different types of silicification are distinguished. The most common
form is strong silicification of the felsic rocks in which the rocks take on a white to pale grey
quartzite-like appearance, but can still be recognised as felsic volcaniclastic rocks. The
deepest part of the mine also contains zones of extremely intense silicification in which the
rocks are grey to almost transparent cherty quartz rocks with either a minor component of
aluminous phyllosilicate minerals (phlogopite, muscovite) and aluminous skarn (aluminous
garnet), or a minor component of non-aluminous skarn. The aluminous quartz rocks have
immobile element ratios similar to nearby less altered felsic rocks and are therefore
interpreted as extremely silicified felsic rocks. The non-aluminous quartz rocks are interpreted
as extremely silicified limestone. The alumina content of the aluminous quartz rocks is
commonly very low (commonly less than 5% Al2O3) and the silica content is commonly 8090%, which indicates that these silicified felsic rocks have suffered extreme mass gain
through addition of silica (Allen et al., 2003).
Garpenberg-type mineralization is characterised by relatively high Mn, and especially Mn/Fe,
compared to the regional skarns and iron ore skarns. Regional depletion in 18O occurs in
carbonates in the intensely mineralised Garpenberg area and 13C depletion occurs in the
dolomite close to ore (Allen et al., 2003). Thus C and O isotopic data may also provide a
potential method for discriminating regional dolomitisation from ore-related dolomites.
Timing and genesis of Garpenberg mineralization
At the ore deposit scale there is a clear spatial association between mineralization and strong
alteration in the footwall rocks and the limestone. Consequently it can be inferred that the
alteration in both the footwall volcanic rocks and the limestone are directly related to ore
formation. The mica-rich alteration zones preserve S1, S2 and S3 foliations, which indicates
that the alteration system, and therefore the mineralization, was formed before S1 or early
during development of S1. The dacite intrusions in the footwall and lower hanging-wall are
mineralogically and chemically similar to some of the juvenile dacitic volcaniclastic rocks in
the footwall succession and consequently can be regarded as broadly co-magmatic with the
volcanic succession. These intrusions are relatively unaltered, appear to cut strong phlogopiterich alteration in the footwall on section 4540 Y (Fig. 31) and also appear to cut the northern
end of the Lappberget orebody on section 4000 Y. These relationships suggest that these synvolcanic intrusions are post-ore in timing, and are compelling evidence that the ore deposits
are synvolcanic in timing (Allen et al., 2003).
The skarns are intrepreted to represent Mg-rich hydrothermal alteration zones in limestone
and volcaniclastic-limestone mixtures that were subsequently changed to coarse grained
skarns during regional metamorphism (Allen et al., 1996, 2003). Alternatively, the skarns
could be primary intrusion-related skarns that have been modified during subsequent regional
deformation and metamorphism. In this second scenario the intrusions are likely to have been
essentially synvolcanic intrusions. Allen et al. (1996 and 2003) suggested that hot metal
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bearing hydrothermal solutions penetrated up through the volcaniclastic succession in a
number of places within the Garpenberg volcanic centre and spread laterally through the
upper footwall rocks to form extensive zones of sub-conformable alteration. An extensive
stromatolitic limestone reef, represented by the main limestone unit, formed both a barrier and
a chemically reactive trap that focused precipitation of metals along the base of the limestone.
The climactic explosive eruption of the Garpenberg volcano and the associated main stage of
caldera subsidence occurred shortly after deposition of the regionally extensive limestone
reef. The Zn-Pb-Ag mineralization may have formed just after this caldera-forming eruption.
Allen et al. (1996, 2003) showed that although the ores are synvolcanic, there is no evidence
for any truly exhalitive mineralization and that most if not all the mineralization formed in the
subsurface as various types of veins and replacements. The ores are attributed to either to a
sub-sea floor, marine, volcanogenic hydrothermal system (VMS system), or alternatively to
the upper levels of an intrusion-related hydrothermal system associated with the early
synvolcanic granitoids.
Evidence consistent with the former volcanogenic alternative:
(1) The Garpenberg ore deposits coincide in space and time with development of a marine
felsic volcanic centre and the ore deposits occur within the caldera vent of this volcano.
(2) The alteration system has an extreme asymmetry, with extensive strong alteration in the
stratigraphic footwall and only weak to moderate alteration in the hanging-wall. This
indicates a strong stratigraphic control on mineralization.
(3) With the exception of the extremely silicified “quartz rocks”, the alteration types and
alteration pattern are similar to the alteration developed around VMS deposits in
volcaniclastic-dominated successions.
(4) No specific plutons or stocks can be directly related to mineralization.
Evidence consistent with a synvolcanic, intrusion-related origin for the ores includes:
(1) The ore deposits occur in the proximal/central part of a major magmatic system that is
dominated by marine volcaniclastic rocks but also includes shallow intrusions. The dacite
intrusions could be a high-level phase of an intrusion-skarn system.
(2) The abundance of skarns that are texturally and mineralogically similar to intrusionrelated contact metamorphic skarns. Simple coarse grained skarns are probably more
consistent with direct replacement of dolomite by skarn, rather than isochemical
metamorphism of volcanogenic alteration.
(3) At Garpenberg Norra, the intense “quartz rock” alteration in the lower part of section
4540 Y, and the apparent possible zoning up-dip along the limestone unit from quartzrock, to skarn, to dolomite, to unaltered calcitic limestone, could be interpreted as related
to silica (+Mg, Fe, base metals) influx and skarn formation related to an unknown
intrusion somewhere below 1400 m level.
(4) ”Quartz rock” alteration is unusual in VMS, but common in intrusion-related skarn
systems.
(5) Bergslagen’s early granitoids are synvolcanic, so a distinction between volcanic- and
intrusion-related ores may be ambiguous !
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The carbonate and calc-silicate hosted Ryllshyttan Zn-Pb-Ag-(Cu) and Fe-oxide
deposit
Ryllshyttan is located 2.5-3 km SW of the Garpenberg mine (Fig. 27). During its 450 years of
mining (16th century – 1944), Ryllshyttan underwent shifts from the mining of silver to iron
and eventually zinc. These shifts reflect the unique geology of the deposit, which displays
characteristics of VMS, calcic-magnesian iron skarn and Banded Iron Formation (BIF)
deposits. In total, 1 Mt of massive to semi-massive sphalerite>galena>chalcopyrite ore and 3
Mt of semi-massive magnetite ore were mined.
The sulphide ore is unevenly distributed within a heterogeneously skarn-altered limestone. In
the north, the ore zone is underlain by a massive pumiceous rhyolitic sandstone-breccia
interpreted as a mass flow deposit of juvenile pyroclastic debris. The stratigraphic hangingwall consists of epidote-garnet-quartz +/- magnetite +/- actinolite calc-silicate layers interstratified with rhyolitic ash-siltstones. The hanging-wall rocks are interpreted as
metamorphosed interbedded exhalitic/chemical and volcaniclastic sediments. Directly W and
NW of the Ryllshyttan mine, the stratigraphic succession has been intruded by an early
orogenic granodiorite (Fig. 32).
Prior to formation of the sulphide mineralization, the supracrustal rocks were intruded by a
rhyolite porphyry which extends into the hanging-wall. Several mafic intrusions also occur in
the mine area and intrude both the sulphides and the calc-silicates. All footwall and hangingwall lithologies except for the granodiorite have undergone various degrees of silicification,
phl-qz, chl-qz and bio-qz alteration; alteration being most intense near the largest sphalerite
ore body (Kompanigruvan). Field relationships and lithogeochemistry suggest that this
alteration was synchronous with massive epigenetic influx of silica, iron, magnesium and
sulphides into the limestone ore horizon. In contrast, the presence of magnetite BIF and
abundant Fe-rich calc-silicates in the hanging-wall suggest that Fe-oxides were an intrinsic
part of the sedimentary environment during accumulation of the succession. Consequently,
Ryllshyttan may represent a limestone and stratiform iron formation overprinted by later
magmatic intrusions, hydrothermal alteration and sulphide mineralization.
The succession has experienced multiple deformation involving folding and shearing along
NE-SW (D2) to ENE-WSW (late D2-D3) trending axial planes as well as late brittle faulting
(D4). Folding has established strong S-symmetries in the Ryllshyttan area, though the pattern
is disrupted by numerous later shears and faults. Observations on all scales suggest that the
sulphide ore has experienced syn-kinematic and syn-metamorphic remobilization whereas the
magnetite skarn has behaved more competantly during deformation.
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Figure 32. Geological plan of the 126 m level, Ryllshyttan mine, Garpenberg area.
Figure 33. Geological cross section through the Ryllshyttan mine, Garpenberg area. The cross section is
oriented N 25 W, approximately perpendicular to the strike of the main folds.
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Stop 1: Underground mine tour to the Lappberget ore body, Garpenberg Norra
Location
The Lappberget ore body is accessed via the Garpenberg Södra or Garpenberg Norra shafts
(N 6690495, E 1523340: Garpenberg Norra shaft).
Introduction and description
The Garpenberg ore deposits and their geological setting are summarized above. The
Lappberget ore body is the most important of several new discoveries by Boliden Mineral in
the Garpenberg area. The ore body is currently being developed and mining has recently
commenced. We will receive a brief introduction to the mine by the Mine Geology staff and
then go underground. The underground workings are constantly changing due to mining and
new development. It is likely that we will be able to see exposures in the upper footwall
rocks, the main limestone-skarn and the ore.
Stop 2: Drill core shed, Garpenberg
Location
South of the Garpenberg Södra headframe and mill complex, adjacent to the old Ulrika’s shaft
which is now an air vent shaft, and close to the old mine chapel (N 6687895, E 1521185).
Introduction and description
Drill cores from the Lappberget and Ryllshyttan deposits will be examined. The geology and
mineralization of these deposits are summarized above.
Stop 3: Ryllshyttan mine area
Location
South of Garpenberg, adjacent to the road to Fors and Avesta (N 6687400, E 1519600).
Introduction
The geology and a preliminary interpretation of the genesis of the Ryllshyttan ore deposit are
summarized above. The area around the old opencuts and shafts is very dangerous and is
fenced off. We will not go into the fenced off area. Instead, we will examine a drill core (stop
2) that intersects the magnetite and Zn-Pb mineralization in the Kompanigruvan part of the
mine, and a superb large outcrop directly north of the Ryllshyttan deposit. This outcrop
exposes the immediate hanging-wall rocks to the limestone ore-host unit, and includes
magnetite banded iron formation, felsic ash-siltstone facies, an enigmatic brecciaconglomerate, several different skarn facies, and several felsic and mafic intrusions.
Description
This outcrop contains 0.2 – 1 m thick beds of skarn separated by ~ 1 m thick beds of banded
rhyolitic ash-siltstone. The ash-siltstone has a relatively juvenile rhyolitic composition and by
comparison with other similar but texturally better preserved ash-siltstones in west
Bergslagen, Sala and Dannemora, is interpreted to be composed of resedimented, glass shardrich, rhyolitic pyroclastic ash (see Allen et al., 1996 for more detail). The rhyolitic ashsiltstone beds in this outcrop have a sodium-rich composition, probably due to alkali
exchange during hydrothermal alteration. The skarn is dominated by garnet and epidote in
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varying proportions. Garnet-dominated beds are generally massive whereas epidote
dominated beds contain planar stratification. Skarn beds, where garnet and epidote occur
together, generally have garnet-rich cores and epidote-rich margins. Epidote-dominated beds
locally contain up to 20 % planar laminae of magnetite. These magnetite laminae appear to be
primary bedding and suggest that the magnetite layers are a type of stratiform, syn-genetic,
banded iron formation (BIF).
Garnet compositions differ from bed to bed, ranging from brown-yellow spessartinedominated to brown-red “grandite”-dominated garnets. An interesting feature of one skarnbed is a lateral transition from Ep > Mag skarn to massive spessartine skarn over a distance of
less than one meter, suggesting significant post-depositional hydrothermal modification.
These extraordinary rocks have historically been referred to as ‘skarn-banded leptites’ (cf.
Magnusson, 1970). The details of their origin are still to some extent unknown. Allen et al.,
(2003) suggested that they may represent metamorphosed calcareous sediment beds, which
contain a detrital rhyolitic ash component, a detrital carbonate component, and a hydrothermal
sediment component. Different compositions represent different proportions of these three
components. An alternative origin could be heterogeneous metasomatism along beds of
varying permeability, composition and reactivity.
The outcrop also contains several intrusions, the oldest being a quartz-porphyritic rhyolite
with a peperitic margin. Roughly 50 m to the west is the intrusive contact with the orogenic
granitoid that borders the Garpenberg supracrustal enclave to the NW. This intrusion has a
dacitic/granodioritic composition and is lithogeochemically similar to syn-volcanic intrusions
near the Garpenberg ore deposits.
ENE-trending amphibolite dikes with sharp margins can also be observed. The margins of the
dykes have undergone intense phlogopite alteration whereas the cores are weakly altered.
Similar amphibolites crosscut the dacite/granodiorite, suggesting they are younger, yet the
phlogopite margins suggest they pre-date the last Mg-alteration in the Ryllshyttan area.
Tectonic fabrics and metamorphic mineralogy suggest that all the intrusions pre-date peak
deformation and regional metamorphism.
Figure 34. Photograph of a part of the “Sjöhällen” outcrop at stop 3, Ryllshyttan, showing interstratified
rhyolitic ash-siltstone (cream to pink colours) and skarns (green, brown). The tape measure is 1.5 m long.
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Stop 4: Skarn and volcanic sandstone-siltstone, south of Ryllshyttan
Location
Southern end of the Garpenberg supracrustal enclave, south of Ryllshyttan mine (N 6686115,
E 1519570).
Introduction
One of the most common facies associations in Bergslagen consists of rhyolitic ash-siltstone
with lesser interbeds of limestone and/or skarn. This facies assocation can be regarded as an
end-member in a spectrum that ranges to the thick limestone successions with lesser interbeds
of ash siltstone-sandstone (e.g. Sala) and also to the interbedded ash-siltstone and skarn
association seen at stop 3 near Ryllshyttan. The ash-siltstone facies generally represents
volcanic ash that was sedimented into the sub-wave base environment by dilute turbidity
currents and hemipelagic suspension settling (Allen et al., 1996). This facies represents the
dominant ambient sedimentation in the Bergslagen basin. The limestone beds represent
stromatolite reefs that developed during time intervals of low detrital (ash-siltstone)
sedimentation (Allen et al., 2003). As shown at Sala, the stromatolite reefs in the shallow
Bergslagen basin, either became covered with more ash-siltstone in the sub-wave base
environment, or they were covered by coarse sand facies deposited by storm waves. The latter
environment may have been between storm wave base and normal wave base, or above
normal wave base. Consequently, these facies associations provide an outstanding record of
the fluctuations in depositional environment during evolution of the Bergslagen basin.
Description
There are two interesting outcrops at this location. The larger southern outcrop exposes a
garnet-rich skarn that is conformably overlain to the NE by crystal-rich rhyolitic sandstone.
The skarn is interpreted to have replaced a 2-3 m thick limestone bed. The crystal-rich
sandstone is about 1 m thick, is massive to diffusely stratified and grades upwards into fine
sandstone and ash-siltstone. This cycle of limestone (here skarn) overlain by crystal-rich
sandstone that grades upwards to ash-siltstone is reminiscent of the succession at Sala, and a
similar environmental interpretation is envisaged. The northern outcrop comprises ashsiltstone with lesser thin interbeds of epidote-amphibole±garnet skarn. The rocks have been
folded into an elongate dome and basin pattern. A faint S1 foliation can be seen parallel to
bedding, whereas the regional S2 foliation is steeply dipping and strikes NE parallel to the
long axes of the dome and basin structures. No subsequent foliation or folding is evident,
which suggests that the dome and basin pattern may have been produced by inhomogeneous
strain during the F2-S2 deformation rather than the overprinting of two (F2 and F3) folding
events.
Stop 5: Basalt pillow lava, Garpenberg village
Location
Outcrop on the NW side of the main road in Garpenberg village (N 6688785, E 1521640).
Introduction
Mafic rocks are prominent 300-500 m stratigraphically below the main Garpenberg limestone
unit, through much of the Garpenberg syncline (Fig. 27). All of these mafic rocks are now
amphibolites. However, relict primary textures indicate that some of them are meta-dolerite
60
sills, whereas others are volcanic and volcaniclastic sedimentary facies (Fig. 27). The
volcanic facies include pillow lava, hyaloclastite and fire fountain deposits. The sedimentary
facies are stratified basaltic breccias and sandstones with or without lesser felsic clasts, and
presumably result from resedimentation or reworking of the primary volcanic facies. Based
on volcanic textures and bedforms, all of these volcanic and sedimentary facies are interpreted
to have been deposited in a subaqueous environment. These mafic rocks are typical of the
mafic component of the generally bimodal Bergslagen volcanic successions. The abundance
of mafic volcanic and sedimentary rocks at this stratigraphic horizon provides an important
stratigraphic marker within the otherwise felsic volcanic succession in the Garpenberg region.
Description
This outcrop has superb examples of elongate basaltic pillow lobes enclosed within blocky
hyaloclastite breccia. The pillows have calcite-filled amygdales, which grade from large and
widely scattered in the cores of the pillows to small and close-packed at the pillow margins.
The hyaloclastite clasts are angular and locally show the classic curvi-planar or broad curved
clast margins that are characteristic of hyaloclastite. These clasts also show remnants of the
amygdaloidal textures described above, indicating that they are quenched and broken pieces
of the pillow basalt.
Stop 6: Limestone quarry, Dammsjön
Location
Large quarry on the E side of the road between Garpenberg and Garpenberg Norra (N
6689180, E 1522090).
Introduction
Limestone was mined from this quarry as early as the 1400’s for smelter flux in the
Garpenberg smelters. A renovated old lime kiln from 1860 is standing in the centre of the
quarry. Although this quarry lies on the main Garpenberg limestone horizon and occurs close
to the Dammsjö ore bodies, the limestone here is relatively unaltered and primary textures are
relatively well preserved. The exposures in the quarry also display well the style of
deformation that is common in this main limestone unit.
Description
The “limestone” comprises grey-weathering, folded, thinly banded, white calcitic marble with
lesser 5 cm to 1 m thick interbeds of brown felsic ash-silstone. The banding in the limestone
is overprinted by the earliest detectable tectonic foliation and is very similar to the microbial
stromatolitic layering in the well preserved limestones at Sala. Consequently, this banding is
regarded as relict primary microbial layering. The limestone behaved plastically during
deformation whereas the felsic interbeds behaved more rigidly and have been folded and
boudinaged. These rocks are regarded as a strongly deformed equivalent of the well preserved
interbedded stromatolitic limestone and felsic sandstone-siltstone facies association at Sala.
Stop 7: Hanging-wall pumice deposit and dacite intrusion, paste plant area
Location
Close to the main road to the Garpenberg Norra mine, south of the main shaft (N 6689950, E
1523335).
61
Introduction
The main two facies in the hanging-wall succession of the Garpenberg ore deposits are
rhyolitic pumice breccia and dacite intrusions. Both are exposed at this locality.
Description
The rhyolitic pumice breccia is a massive, cream-weathering, weakly feldspar-quartz
porphyritic rhyolitic rock with a diffuse mottled texture of white weathering ragged pumice
clasts close-packed within a slightly darker, cream-yellow weathering pumiceous matrix.
Scattered 2-50 cm long, dark brown-green biotite-rich lenses are scattered randomly through
the rock. These darker lenses are aligned parallel to each other, and are folded by the main F2S2 deformation. These lenses were interpreted by Allen et al. (1996) to be diagenetically
compacted, large pumice clasts, which are one type of “fiamme”. The alignment of the clasts
defines a folded, diagenetic compaction foliation or fiamme foliation that can be used as a
bedding marker in these otherwise massive unbedded rocks. The smaller white pumice clasts
are relatively uncompacted, and are lineated in the regional L2 tectonic lineation. This pumice
breccia texture is remarkably well preserved here and in several other areas in Bergslagen,
and is similar to the textures of other well preserved pumice-rich subaqueous mass flow
deposits in younger volcanic terranes (Allen et al., 1996). The dacite intrusion is massive and
weakly feldspar porphyritc and shows a sharp and irregular contact against the pumice
breccia. This intrusion is the same one that occurs in the stratigraphic hanging-wall of the
Lappberget ore body directly below this outcrop.
Stop 8: Hanging-wall pumice deposit, AK-road east of Garpenberg Norra
Location
On the edge of a gravel forest road E of Garpenberg Norra mine. Access to this forest road is
from the main Garpenberg – Horndal road (N 6690140, E 1524130).
Introduction and description
This small outcrop is a superb example of the hanging-wall rhyolite pumice breccia in the
middle of the Garpenberg syncline. At this locality the diagenetic compaction foliation is
approximately perpendicular to the regional S2 foliation, indicating that the outcrop lies close
to the F2 fold axis. Both the fiamme textures and the uncompacted pumice clasts in the matrix
are well preserved.
Stop 9: Dacite intrusion in hanging-wall
Location
Högtjärnklack hill, NE of Garpenberg Norra (N 6691720, E 1525940).
Introduction and description
The top of this large hill contains several outcrops of massive, weakly feldspar-porphyritic
dacite. This dacite forms a large thick sill within the hanging-wall pumice breccia in the
northern part of the Garpenberg syncline and is similar to dacite sills that occur in the
hanging-wall close to the Garpenberg Norra ore bodies.
62
Stop 10: Mafic and felsic volcaniclastic rocks in upper footwall, pipeline road
Location
Outcrops along the SE side of a new gravel road and pipeline, directly south of Lake
Dammsjön (N 6688295, E 1522920).
Introduction
This outcrop section lies on the SE limb of the Garpenberg syncline, about 200-400 m
stratigraphically below the hanging-wall rhyolitic pumice breccia (Fig. 27). The outcrops are
thought to also lie stratigraphically below the main Garpenberg limestone horizon. The
section contains a ? 50 m thick mafic volcanic interval, which is interpreted to be
stratigraphically equivalent to the mafic rocks at stop 5.
Description
This outcrop section contains a variety of rhyolitic, ?dacitic and mafic volcaniclastic rocks.
The rhyolitic rocks comprise siliceous, tuffaceous, volcanic sandstone and siltstone; the
sandstones contain quartz and feldspar crystal grains. The interpreted dacitic rocks comprise
volcanic sandstone, siltstone and matrix-supported breccia of dark feldspar-porphyritic
?pumice clasts set in a silty-sandy matrix. Compared to the rhyolitic rocks the dacitic rocks
have a less siliceous, more feldspathic groundmass, a brown-grey groundmass colour, and are
feldspar-porphyritic with no quartz crystal grains. The mafic clastic rocks comprise diffuse to
well stratified tuffaceous siltstone, sandstone and matrix-supported ?pumice/scoria breccia.
These mafic rocks are characterized by dark green-grey, biotite-amphibole-feldspar-rich
compostion. The sandstones and breccias are distinctly feldspar-porphyritic. The outcrop
section also contains a couple of enigmatic breccias of pale siliceous blocks in a darker
matrix. Locally the breccias display a jigsaw puzzle “in situ” breccia texture. Possible origins
include hydrothermal alteration and brecciation of a volcanic rock, or intrusion of an igneous
rock into clastic volcaniclastic sedimentary rocks (peperite).
63
Day 5: Monday 18 August, Stollberg Mines
By Magnus Ripa (Geological Survey of Sweden), Rodney Allen (Boliden Mineral and
Luleå University of Technology) and Erik Lundstam (Boliden Mineral)
Introduction
On this day we will drive from Garpenberg to the Stollberg mines, which are located 5 km NE
of the town of Ludvika. The mines 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 (Figs. 35-37). 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 and volcaniclastic sedimentary rocks (Ripa,
1988, 1996). The Brusmalmen ore body comprised a disseminated to massive galenasphalerite replacement of limestone and is associated with intensely silicified limestone. Ripa
(1994, 1996) envisaged that the ores were 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.
Figure 35. Geology, mineral deposits and mineral occurrences of the Stollberg region (after Ripa, 1996; and
Boliden Mineral unpublished data). Locations of field trip stops 1-3 are shown.
64
Figure 36. Geology of the Stollberg mines area (from Ripa, 1996). The letters A to M denote individual ore
bodies and these are also shown on the cross section figure 37. K = Stollmalmen, J = Dammberget, M =
Brusmalmen. The insets show schematic representations of the ores and associated skarn minerals. Oamp =
orthoamphibole, px = pyroxene, oli = olivine, serp = serpentine, chl = chlorite, grt = garnet, amp = amphibole,
tlc = talc, bio = biotite, chp = chalcopyrite, po = pyrrhotite, sph = sphalerite, gal = galena, diss = dissemination.
Field relations, mineralogy and geochemistry suggest that the felsic metavolcanic rocks were
originally rhyolites. The footwall and host-rock successions were intensely altered during the
ore-forming event (Fig. 38). Subsequently(?), the rocks were folded and metamorphosed in
65
the amphibolite facies (510-560 C at c. 3 kbar). In the altered footwall lithologies,
metamorphic gedrite, biotite, and muscovite formed from the original alteration minerals. The
metamorphic alteration assemblages of the ore host rocks consist of biotite-garnet, amphibolegarnet ±andalusite ±staurolite ±cordierite ±biotite and olivine ±pyroxene ±garnet parageneses.
The latter three assemblages are associated with pyrrhotite ±sphalerite, pyrrhotite ±sphalerite
±galena ±magnetite and sphalerite-galena mineralisations, respectively, and thus define a
metal and mineral zonation. The variations in alteration assemblage mineralogy are related to
the varying proportions of carbonate rock to volcanic debris in the premetamorphic
assemblage, and to the character of hydrothermal alteration (Ripa 1996 and unpublished data).
Subsequently, the Stollberg rocks were affected by retrograde metamorphism and
deformation at 400-500 C and less than 3.5 kbar. Gedrite was altered to chlorite and lizardite,
biotite to chlorite and plagioclase to epidote. Mineral chemistry and high-resolution
transmission electron microscopy observations suggest that the retrograde reactions were
metasomatic. The last event to affect the rocks was brittle faulting, which caused partial
mechanical remobilisation of the ores, especially into fractures. These remobilised ores
display poorly sorted breccia textures and contain angular to rounded clasts of wall rock
lithologies (“ball ores”).
Apparently biotite, gedrite-garnet(I) and andalusite-gahnite-corderite-stauroliteclinopyroxene-K feldspar-ore(I) alteration were synchronous with or post-dated the formation
of penetrative and plastic S1 foliation. These parageneses were overprinted by quartz-biotitegarnet(II)-ore(II) alteration, which in turn was overprinted by penetrative and plastic S2
foliation and retrogression. Textures suggest that sulphide formation post-dated magnetite
formation.
In a regional context, the relations at Stollberg suggest that the retrograde event corresponds
to regional metamorphism at c. 1800 Ma and that ore formation and prograde metamorphism
were synvolcanic to synplutonic at c. 1900 Ma.
Figure 37. Longitudinal section of the Stollberg mines area (from Ripa, 1996). The letters A to M denote
individual ore bodies and these are also shown on the map figure 36. The inset shows grades for the individual
ore bodies or group of ore bodies.
66
Figure 38. Schematic stratigraphic section through the host rocks and mineralization at Stollberg (from Ripa,
1996).
Stop 1: The Stollmalmen deposit, Stollberg
Location
The tower on the crest of the hill overlooks the Stollmalmen ore bodies (N 6674432, E
1470805).
Introduction
At this location we will provide a brief introduction to the geology of the Stollberg area. The
tower provides a good overview of the old workings and the surrounding country-side. In
clear weather the outskirts of Ludvika can be seen to the SSW. The Stollmalmen ore bodies
comprised magnetite skarn with variable amounts of disseminated sulphide mineralization.
Description
Dark silver-grey manganiferous magnetite skarn ore is well exposed in the sides of the old
workings below the viewing tower. These ores are interpreted to have replaced a several
67
metres thick limestone unit. At this location, the original carbonate rocks have been almost
completely replaced by skarn and ores. Variably skarn-altered to skarn-banded and
mineralised metavolcanic siltstones that form part of the host rocks are well exposed directly
west of the workings. These rocks are interpreted to form the immediate hanging-wall to the
mineralization. The strata trend N-S and dip steep to vertical; younging is (probably) to the
west. Biotite, clino- and orthoamphiboles and garnet ±andalusite are meta-alteration minerals.
Locally, arsenopyrite may be found.
Stop 2: The Dammberget sphalerite-galena deposit, Stollberg
Location
The Dammberget deposit lies about 1 km north of the Stollmalmen deposit, along the same
original limestone bed (N 6675391, E 1470805).
Introduction and description
This stop provides a short overview of the old workings; then we will walk a short traverse
roughly from west to east, from stop 2 to stop 3. At stop 2 beside the old workings there is an
outcrop of variably biotite- and biotite-garnet-altered, rhyolitic ash-siltstone, which is
probably the volcanic facies that hosts the mineralized limestone unit (not exposed). The
traverse passes from these host rock lithologies into the underlying plagioclase-phyric
metavolcanic rock. Note the change in alteration style from biotite-bearing to gedrite-bearing
without any obvious textural change.
Stop 3: Footwall gedrite alteration, Stollberg
Location
On the eastern side of the Stollberg ridge (N 6675050, E 1471019).
Introduction
This outcrop lies at the end of the traverse into the footwall rocks below the Dammberget
deposit.
Description
This outcrop shows the subtle but relatively abrupt contact between biotite-altered and
gedrite-altered rock. The gedrite is readily visible due to the distinct fan-radiating sheath-like
morphology. The original rock type appears to be a massive homogeneous rhyolitic rock and
could have been a rhyolite lava or a juvenile pyroclastic deposit.
68
Ställdalen
By Rodney Allen (Boliden Mineral and Luleå University of Technology), Magnus Ripa
and Ingmar Lundström (Geological Survey of Sweden)
Introduction
Allen et al. (1996) distinguished two main volcano types in Bergslagen: shallow marinesubaerial, rhyolitic pyroclastic caldera volcanoes, and shallow marine-subaerial, daciterhyolite complexes. Proximal (vent) areas were identified as the areas of very thick, relatively
coarse-grained, syn-eruptive volcaniclastic deposits and abundant porphyritic intrusions.
Figure 5 shows a regional facies model for these volcano types and associated mineralization
in Bergslagen. Rhyolitic pyroclastic rocks and reworked pyroclastic rocks are dominant
throughout most of the Bergslagen region and consequently rhyolitic pyroclastic caldera
volcanoes are inferred to have been the dominant volcano type. Several rhyolitic vent
complexes are inferred, but Godegård in the Zinkgruvan area, and Sången-Hällefors, are best
known.
The lower part of the stratigraphy in the Ställdalen - Grängesberg area contains abundant
dacitic intrusions and thick dacitic pyroclastic units (Fig. 3) and was consequently interpreted
by Allen et al. (1996) to represent the caldera vent(s) of one or more nested dacite-rhyolite
caldera volcanoes. The thickness and inferred large volume of pyroclastic units at Ställdalen
suggest that caldera collapse events probably occurred, and the common occurrence of
shallow water depositional environments, suggests that the vents were near sea level. On a
regional scale, the proximal dacitic rocks at Ställdalen are overlain by distal rhyolitic facies
from an adjacent rhyolitic pyroclastic caldera volcano (Fig. 3). Major skarn iron ore deposits
and minor polymetallic sulphide deposits are associated with limestone beds in the distal
rhyolitic facies that overlie the Ställdalen caldera succession.
This excursion will visit the pyroclastic flow and fallout succession of a major dacitic vent at
Himmelriksbacken, SW of Ställdalen.
Stop 4: Dacitic breccia, Himmelriksbacken, Ställdalen
Location
Himmelriksbacken, west of the main road between Ställdalen and Kopparberg (N 6642810, E
1449707). The outcrops are on the side of a hill, directly west of the gravel forest road.
Introduction
This hillside has several good outcrops of coarse grained, dacitic volcanic rocks. The dacitic
composition is reflected in the feldspar-porphyritic textures, absence of quartz phenocrysts,
and the relatively biotite-amphibole rich composition compared to rhyolitic rocks in
Bergslagen. The metamorphic grade is amphibolite facies and the rocks have a medium
grained granoblastic metamorphic grain size and a moderate to strong, steep, stretching
lineation. However, despite this metamorphism and deformation, primary volcanic rock
textures are relatively well preserved at outcrop scale. Regional and local stratigraphic
younging directions are consistently to the east and these outcrops lie stratigraphically near
the top of a several hundred metre thick interval with massive, coarsely feldspar-porphyritic
dacitic rocks. Some of these rocks may be porphyritic dacitic intrusions. However, locally
69
preserved clastic textures indicate that some of the rocks are also massive pyroclastic rocks.
These relict clastic textures and diffuse stratification become distinct towards the top of the
thick dacitic succession and are well exposed in the outcrops at stop 4.
Description
Two main types of dacitic breccia have been noted in these outcrops. The first is matrix to
clast-supported and contains dark feldspar-porphytitic dacitic blocks with broadly curved
cuspate margins, which suggest that the fragments were glassy and have been quench
fragmented (Fig. 39A). The matrix is pale coloured, slightly silicified, sandy in grainsize and
probably of ?tuffaceous dacitic composition. The dacite clasts could represent quenched
pumice blocks, or quenched blocks of poorly vesicular lava. The deposit could be a mass flow
breccia of pyroclastic blocks, or a hyaloclastite breccia, or an intrusive hyaloclastite breccia,
which is called “peperite”. More information about contact relationships and bedforms is
required to be certain of the origin.
The second type of breccia is also matrix- to clast-supported and comprises irregular, angular,
feldspar-porphyritic dacite blocks and lenses with a preferred orientation (Fig. 39B). The
matrix is similar to the first breccia type. The dacite blocks appear to have been flattened
parallel to bedding or S1 and then foliated and folded in the regional S2-F2. It is likely that the
blocks are pumice clasts that were flattened parallel to bedding during diagenesis and were
then folded during regional deformation. The rock is interpreted to be a subaqueous
suspension deposit of pumiceous dacitic pyroclastic debris (water-settled pumice deposit),
and is probably a proximal (near vent) subaqueous pyroclastic fall deposit or the upper part of
a subaqueous mass flow deposit of pumiceous debris.
Figure 39. Two breccia facies from Himmelriksbacken, Ställdalen. A. Dacite breccia with blocky clasts with
cuspate margins. B. Dacite breccia with irregular elongate clasts.
Stop 5: Dacitic proximal pyroclastic fall deposits, Himmelriksbacken, Ställdalen
Location
Himmelriksbacken, west of the main road between Ställdalen and Kopparberg. The outcrops
are about 300 m south of stop 4 and lie about 50 m E of the gravel forest road (N 6642551, E
1449752).
70
Introduction
A series of outcrops exposes the thick, crudely normal-graded top of the dacitic breccias from
stop 4, and an overlying succession of thin to medium-bedded, dacitc tuffaceous sandstone
and siltstone. All of these rocks are interpreted to be juvenile pyroclastic deposits and their
slightly reworked equivalents. The upper, bedded succession is at least 30 m thick, however it
could be considerably thicker as the top is not exposed.
Description
The lower part of the bedded succession is well exposed at stop 5. It comprises planar thinbedded dacitic sandstone, with several discrete horizons of tight basin-like fold structures.
The folds range up to 1.5 m in amplitude and several decimetres wide. A single angular dacite
block can be seen at the nose of at least two of the folds. This part of the succession is
interpreted to be shallow subaqueous, pyroclastic fallout deposits with discrete layers of
abundant bomb sag structures. The abundance of bomb sags and the thickness of the whole
succession, indicate that this location is within a couple of kilometres from the pyroclastic
vent to this eruption. Such well preserved proximal fall deposits and bomb sags are rare in
ancient volcanic successions and this is probably one of the very best exposed in
metamorphosed Precambrian rocks!
Stop 6: Reworked dacitic pyroclastic fall deposits, Himmelriksbacken,
Ställdalen
Location
Same general area as stop 5, but about 50 m to the NE (N 6642595, E 1449780).
Introduction and description
The pyroclastic fall deposits of stop 5 are overlain by at least 10 m of similar bedded, dacitic
sandstones, but which display soft-sediment slump structures, erosion surfaces, bimodal wave
ripples and large amplitude cross-bed dunes. These deposits are interpreted to be pyroclastic
fall deposits that have been reworked in situ by waves and tidal currents in a shallow
subaqueous environment.
71
Day 6: Tuesday 19 August, Hällefors and Viker-Älvlången areas
By Rodney Allen (Boliden Mineral and Luleå University of Technology), Magnus Ripa
and Ingmar Lundström (Geological Survey of Sweden)
Introduction
The Hällefors-Grythyttan area is regarded as a type area for Bergslagen geology and mineral
deposits. Sundius (1923) wrote a classic report on this area, which was decades ahead of it’s
time scientifically, and is still the best description of the area. This area has a relatively low
metamorphic grade (greenschist to lower amphibolite facies) and moderate deformation.
Consequently, primary rock textures are well preserved in outcrop. The volcanic succession in
this part of Bergslagen is over 7 km thick and grades upwards into an at least 2 km thick
succession of grey argillites and local conglomerate (Fig. 3; Allen et al., 1996). However,
neither the base of the volcanic rocks nor the top of the sedimentary rocks is preserved. The
volcanic succession is strongly bimodal, comprising almost entirely calc-alkaline rhyolitic
rocks and subordinate tholeiitic basaltic rocks. The basaltic rocks are dolerite sills and basaltic
dykes, except for a horizon of extrusive pillow basalt near the top of the volcanic succession
(4.9 km level in Grythyttan-Sången stratigraphic column in figure 3; Sundius, 1923). The
rhyolitic rocks are mainly non-welded to poorly welded pyroclastic flow and fallout units, and
their rapidly resedimented subaerial and subaqueous equivalents. Subvolcanic porphyritic
intrusions and cryptodomes are also abundant. The lower to middle part of the volcanic
succession is dominated by juvenile, thick, poorly stratified rhyolitic pyroclastic rocks,
whereas the upper part is dominated by stratified rhyolitic ash-siltstone and sandstones (Fig.
3). The ash-siltstone and sandstone are interpreted to be mainly rapidly reworked pyroclastic
ash that has been resedimented into below wave base subaqueous environments.
Several vent complexes are inferred, but Sången-Hällefors is the best known (Allen et al.,
1996). The proximal (vent) area is about 20 km diameter and contains a extremely thick
pyroclastic succession (> 3-7 km thick) of very thick (up to 500-1000 m), texturally distinct,
pyroclastic flow units. Each of these units reflects a large volume pyroclastic eruption, which
probably formed a collapse caldera or an increment of caldera collapse. Consequently, the
extremely thick proximal pyroclastic successions probably reflect ponding of pyroclastic
deposits in compound (nested) caldera subsidence structures. Intrusive rhyolites are abundant
and are concentrated to the top of the proximal pyroclastic deposits and occur in the overlying
thick ash-siltstone sequence (Fig. 3). These rhyolites are attributed to the passive
emplacement of degassed magma following major explosive volcanic periods. The rhyolitic
volcanic centres in the Grythyttan-Hällefors area and elsewhere in Bergslagen are similar to
well studied pyroclastic calderas in younger better exposed terranes (e.g. Lipman, 1984;
Howells et al., 1991).
Depositional environments of the proximal and medial facies associations were mainly
shallow water to subaerial, whereas the medial to distal ash-siltstone facies associations
formed in mainly shallow to deeper marine environments. From this and analogy with
modern rhyolite caldera volcanoes we envisage that each rhyolite volcano formed a broad,
low angle shield, with a (proximal) caldera subsidence area in the middle (Fig. 5). Flanks of
the shields (medial to distal areas) were constructed of pyroclastic outflow sheets alternating
with reworked pyroclastic debris and mainly comprised relatively shallow seas.
72
During this day of the excursion we will visit two different rhyolitic pyroclastic successions:
the first at Bergtjärnsåsen was interpreted by Allen et al. (1996) to be a proximal to medial,
subaerial pyroclastic succession, whereas the second at Älvlången is interpreted to be a
proximal to medial, marine pyroclastic succession that grades up into a limestone reef
succession.
Stop 1: Rhyolitic proximal-medial pyroclastic flow and fall deposits,
Bergtjärnsåsen, Hällefors
Location
Bergtjärnsåsen lies between Kopparberg and Hällefors (Fig. 1), several km north of the main
road (N 6641628, E 1436045). Access is via a gravel forest road that leaves the main
Kopparberg-Hällefors road directly east of Sikfors.
Introduction
This locality comprises a prominent ridge with abundant good outcrop. The area lies in the
middle of the thick volcanic domain between Hällefors and Kopparberg (Fig. 1) and is
inferred to occur low down in the regional stratigraphic succession. The local geological
context of the outcrops is not well known. However, a superb succession of juvenile
pyroclastic flow and fall deposits is exposed on the ridge. Due to the presence of erosion
surfaces, lack of graded bedding, an abundance of accretionary lapilli, and the bimodal sorting
of the lapilli beds, the succession was interpreted by Allen et al. (1996) to have accumulated
in a subaerial environment. The area is conspicuously devoid of both iron ore and sulphide
ore deposits. Allen et al. (1996) suggested that the sulfide deposits, skarn- and carbonatehosted iron deposits, and banded iron formations in Bergslagen, all preferentially occur in
below fairweather wave base facies associations of rhyolitic ash-siltstone, normal graded
vitric-crystal breccia-sandstone and limestone. They concluded that the Bergtjärnsåsen area is
devoid of mineralization probably as a result of the subaerial depositional environment.
Description
The NE end and top of the ridge exposes a >30 m thick, orange-red coloured, massive,
matrix-supported, quartz-feldspar-porphyritic, rhyolitic breccia-sandstone (Fig. 40A). Many
of the crystal grains have angular broken shapes. Abundant 0.5-10 cm, diffuse, irregular,
ragged, clasts can be detected on slightly weathered outcrop surfaces. These clasts are
interpreted to be non-welded pumice clasts and the matrix is interpreted to be vitric ash. The
deposit is interpreted to be a rhyolitic non-welded pyroclastic flow deposit.
The base of the pyroclastic flow deposit contains large irregular clasts (up to 1m) of
tuffaceous sandstone with abundant accretionary lapilli (Fig. 40B). These are inferred to be
rip-up clasts from a 10 cm – 2 m thick accretionary lapilli-bearing airfall deposit that directly
underlies the pyroclastic flow deposit. This airfall deposit lies in turn on a >50 m thick unit of
diffusely stratified, well sorted, coarse grained, feldspar-quartz crystal rich, volcanic
sandstone. The sandstone is interpreted to be reworked pyroclastic ash, deposited by traction
currents in a fluvial or shallow subaqueous environment. The contact between the sandstone
and the overlying airfall deposit is an abrupt erosion surface.
73
Figure 40. Bergtjärnsåsen, N of Sikfors A. Rhyolitic matrix-supported pumice breccia = pyroclastic flow
deposit. B. Part of a large clast of an accretionary lapilli fall deposit that occurs within the pyroclastic flow
deposit.
Stop 2: Banded Iron Formation and exhalative Zn-Mn horizon, Älvlången
Location
On the shore of lake Älvlången within the nature reserve just south of Viker (N 6591218, E
1442525). This is a nature reserve, so do not use geology hammers at this locality.
Introduction
The most extensive marble horizon in Bergslagen is a one to several km thick, northeasterly
trending unit that extends for 40 km between Viker and Stråssa (Fig. 1). This marble unit
conformably overlies a thick rhyolitic volcanic succession. However, similar to the limestones
at Sala and Dannemora, the Viker-Stråssa limestone occupies the core of a syncline and is
structurally truncated on the eastern side, such that the stratigraphic top of the unit is not
exposed and it is not known what stratigraphic unit originally lay above.
Around Lake Älvlången, Allen et al. (1996, 2003) mapped field and drill core traverses across
the transition from the rhyolitic volcaniclastic succession to the marble-dominated succession
(Figs. 41, 42). This transition contains the Älvlången-Viker Zn prospect, an example of
laterally extensive, stratiform Zn-Pb-Ag sulfide mineralisation, which is an important style of
base metal mineralisation in Bergslagen. The prospect comprises a 10 cm - 1 m thick, 7 km
long zone of stratiform Zn mineralisation associated with Fe-Mg-Mn-rich metasediments and
banded iron formation (BIF) (Figs. 41, 42; Hellingwerf et al., 1988). The mineralized horizon
is exposed on the western shore of lake Älvlången and has been intersected in several
exploration diamond drill profiles at the northern end of the lake (Fig. 41).
The ambient background facies in the volcaniclastic-dominated lower 500 m of the
statigraphic section in figure 42 is the sub-wave base rhyolitic ash-siltstone (RAS) facies,
within which there are three thick, massive, juvenile volcaniclastic units (Allen et al., 1996,
2003). The basal and upper (stop 4) units are ~ 140 and 25 to >40 m thick respectively (Fig.
42, 60-200m and 420-445m), and comprise pumice clasts dispersed in an ash matrix with ~
10% scattered 1-1.5 mm quartz and feldspar phenocrysts. They have been interpreted as
pyroclastic flow deposits emplaced into the subaqueous environment represented by the
enveloping RAS facies (Allen et al., 1996). These authors interpreted a thin interval of
stratified crystal sandstone at the top of the upper unit to represent storm wave or tidal
reworking of the top of the pyroclastic flow deposit, indicating that the unit was deposited in
74
water depths of no more than 50 to 100 m. The other juvenile volcaniclastic unit (Stop 3) is a
14 m thick accretionary lapilli deposit (Fig. 42, 290 m). Distinctive accretionary lapillibearing units of this type have been interpreted as water-settled phreatomagmatic airfall
deposits by Allen et al. (1996). This particular unit may be the thickest accretionary lapilli
airfall deposit described anywhere in the world! It records a very large phreatomagmatic
eruption in the Viker-Älvlången volcanic centre.
Figure 41. Simplified geological map of the Lake Älvlången area, Viker (modified from Allen et al. 2003,
Stephens et al., 1998). The location of field trip stops 2-4 and the Viker exhalative Zn mineralization are shown.
Although internal primary textures are rarely preserved in the carbonate rocks in the
Älvlången profile, the carbonate-dominated upper half of the profile is clearly similar in
structure to the Sala succession (Allen et al., 2003). The marble units are therefore also
considered to have accumulated as stromatolitic reefs, as evidenced particularly by the lobate
75
casts on the upper surfaces of many beds. The range and arrangement of facies in the
volcaniclastic interbeds is also similar, and in this case storm wave reworking associated with
the SCS facies at the base of the sandstone-siltstone cycles in the thicker interbeds is
confirmed by the presence of well preserved hummocky cross stratification. The significance
of the Älvlången profile as a whole, is that water depths fluctuated from below wave base to
near wave base throughout accumulation of the succession. However, the lack of substantial
marble units in the lower half of the succession demonstrates that the microbial communities
were unable to colonise the substrate and promote carbonate accumulation during periods of
intense volcanism and associated volcaniclastic sedimentation.
The discrete interval of thin bedded and laminated calcitic marble, siltstone and iron
formation, which hosts the Viker Zn mineralization, records a distinct but transient period of
different environmental conditions. The ubiquitous thin planar bedding and lamination,
absence of domal stromatolites, and absence of SCS sandstone facies, all suggest that this
interval records quiet, deeper water conditions (Allen et al., 2003). These conditions favoured
suspension sedimentation of distal fine-grained carbonate, volcanic siltstone, and Fe-Mn and
Zn-bearing chemical sediments.
Figure 42. Stratigraphic column through the transition from rhyolite volcanic-dominated to marble-dominated
facies on the western side of Lake Älvlången, Viker area (modified from Allen et al. 2003). The location of field
trip stops 2-4 and the Viker exhalative Zn mineralization are shown.
76
Description
This location occurs approximately at the stratigraphic position of a change from underlying
thick-bedded stromatolitic dolomitic limestones to overlying planar thin-bedded calcitic
limestone, BIF and stratiform Zn-Pb-Mn-Fe mineralization (Fig. 42). On the E side of the
narrow point (directly E of the picnic area) at about the water level, red and black banded iron
formation is exposed within thin-bedded siliceous and calcitic rocks. 15-20 m further E, also
at water level, is an outcrop of thin-bedded to laminated calcitic limestone with abundant
laminae of hematitic quarz (meta-jasper). About 100 m S along the shoreline of Lake
Älvlången (N 6591094, E 1442540), the Viker mineralized horizon is exposed in some small
outcrops at water level. The rocks at this location are moderately to strongly folded and
sheared. The mineralized horizon at this locality consists of fine to medium grained, dark
green, Fe-Mn-Mg amphibole-garnet skarn with bands and disseminations of
sphalerite>galena. The host rocks appear to be thin-bedded calcitic and siliceous sedimentary
rocks.
Stop 3: Accretionary lapilli fall deposit, Älvlången
Location
These outcrops lie south of Lake Älvlången, near the small lake Ormtjärn. Stop 3 and stop 4
occur on a ridge of relatively good outcrop mapped by Allen et al. (1996). Stop 3 is near the
start of this ridge traverse at N 6588118, E 1440765.
Introduction
Although the stratigraphy in the Lake Älvlången area mainly youngs to the SE, stops 3 and 4
lie near the axis of the regional syncline and younging is to the NW. Stratigraphic correlation
can confidently be made between the areas of SE and NW younging due to the presence in
both areas of a distinctive thick accretionary lapilli unit (Fig. 41; Fig. 42, 300 m) and a
distinctive thick, massive pyroclastic flow deposit (Fig. 42, 430 m) just below the marbledominated part of the succession. These accretionary lapilli and pyroclastic flow units can be
distinguished by rock texture, phenocryst mineralogy and chemistry. Individual very large
phreatomagmatic airfall eruptions, such as the one that caused this deposit, probably covered
a large part of Bergslagen. Consequently, these airfalls are one of the best potential marker
horizons that with further work might be correlated over a large area.
Description
The accretionary lapilli unit conformably overlies a thick succession of grey, massive to
diffuse stratified, rhyolitic ash-siltstone, which is interpreted to be reworked rhyolitic ash
deposited in a subaqueous, below wave base environment. The accretionary lapilli unit
comprises diffuse to well planar-stratified, clast- to matrix-supported, 10 to 15 cm thick
accretionary lapilli beds with a volcaniclastic, originally vitric, matrix of sand grainsize. A 1
m thick, cross-bedded, crystal-rich interval near the top of the unit indicates some tractional
reworking of the substrate, most likely in a shallow marine environment. Weak to moderate
epidote-quartz alteration spots have nucleated on the lapilli and subsequently the lapilli and
these alteration spots have been stretched in the regional L2 lineation.
77
Stop 4: Rhyolitic ash fall and pyroclastic flow deposit, Älvlången
Location
Stop 3 and stop 4 lie on a ridge of relatively good outcrop mapped by Allen et al. (1996). Stop
4 is near the end of this ridge traverse at N 6588170, E 1440736.
Introduction
The accretionary lapilli fall deposit is overlain by rhyolitic ash-siltstone, stromatolitic
limestone, and then about 50 m of juvenile rhyolitic deposits that record the next major
rhyolitic eruption in the area (Fig. 42). Stop 4 provides an outcrop section through the lower
part of this eruption sequence.
Description
The sequence begins with several metres of planar stratified, non-graded to normal-graded
rhyolitic ash deposits. Several 0.5 – 10 cm thick, continuous planar beds of sand-gravel
grainsize are interbedded with the diffuse stratified stratified ash siltstone-sandstone. These
coarser beds mainly consist of small, matrix-supported, flattened pumice clasts, scattered
volcanic quartz and feldspar crystals, and fine-grained ash matrix. These deposits are
interpreted to be water-settled airfall. The thickness and grainsize of the deposits indicate a
major phreatomagmatic eruption, probably within the Viker-Älvlången region.
The fall deposits are overlain conformably by a >40 m thick, massive rhyolitic pumice
breccia-sandstone unit, which except for the basal few metres, comprises moderately flattened
pumice clasts dispersed in an ash matrix with about 10% scattered 1-1.5 mm quartz and
feldspar phenocrysts. This unit is interpreted to be a pyroclastic flow deposit. The basal few
metres are more crystal rich and contain irregular pods and bands of quartz-feldspar crystalrich composition dispersed within the “normal” massive pyroclastic flow facies. This basal
facies is a more extreme version of the basal facies of the pyroclastic flow deposit we studied
at Bergtjärnsåsen. It is interpreted as the basal crystal-rich ground layer of the pyroclastic
flow that has been variably ripped up and incorporated into the overriding flow during
emplacement of the flow. Crystal- and or lithic-rich ground layers are well documented at the
base of many well-studied young pyroclastic flow deposits, but have rarely been documented
in Precambrian metavolcanic rocks. The base of the pyroclastic flow was emplaced in water.
However, other outcrops to the north expose a shallow marine erosion surface at the top of the
deposit. This suggests that the pyroclastic flow deposit initially filled the shallow basin at this
location and was then rapidly eroded down to wave base by shallow marine wave activity.
The thickness of the pyroclastic flow therefore indicates that the basin at this location and this
time was about 50 m deep.
78
Day 7: Wednesday 20 August, Zinkgruvan mine area
By Lars Malmström (Zinkgruvan Mining) and Rodney Allen (Boliden Mineral and
Luleå University of Technology)
Introduction
Zinkgruvan is the largest and most important stratiform Zn-Pb-Ag-Cu deposit in Bergslagen.
The Zinkgruvan mines have past production and reserves of over 50 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. The deposit is 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 are deformed, some primary textures can be observed in the less
altered host rocks and bedding is preserved in the sulphide ores. Magnusson (1948, 1960)
considered the Zinkgruvan (Åmmeberg-type) ores as metasomatic ores that formed at the
migmatisation front. However, more recently the Zinkgruvan ores have been interpreted as
syn-sedimentary exhalative ores formed on the seafloor 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).
Stop 5, Mine museum
Stop 4, Quartz – Microcline Rocks
Stop 1,
Mine tour
Stop 2,
Drill core
Stop 3,
Metasediments
Figure 43. Simplified geological map of the Zinkgruvan area showing locations of the field excursion stops.
79
Stop 1: Zinkgruvan mine tour
Location
Zinkgruvan mine (N 6521620, E 1459550).
Introduction
The visit under ground will focus on the meta-volcanic succession in which the stratiform
sphalerite-galena mineralization is found. The Nygruvan section of the mine, which has
provided the bulk of production until a few years ago, is situated to the east of the fault zone
and consists of a fairly regular tabular 1-15 m thick horizon. This horizon is striking NW-SE,
dipping 60-80 degrees and has a near vertical plunge. The Knalla section of the mine is
situated to the west of the fault zone striking NE-SW, consists of several bodies of variable
thickness (1m – 30 m) and dipping variably from near vertical to sub-horizontal to the NW.
The plunge is variable. In the central part of the deposit in the immediate stratigraphic foot
wall of the zinc-lead mineralization is a copper (chalcopyrite) zone hosted by a dolomitic
marble. It varies from 5 m to 40 m in thickness and no production has yet come from this
mineralization. Mining activities are going on between the 300 m and 1 100 m levels. The
mining methods are drift and benching, and panel stoping. The ore production for 2007 was
875 kt @ 8.3 % Zn, 4.4 % Pb and 85 g/t Ag.
Description
Mine tour showing the mine host package, i.e. ore zone and country rock.
Reserves
Resources
Cu-Resources
Mined out area
Figure 44. Simplified 3D plan of the Zingruvan ore bodies and mine workings (Lunding Mining data).
80
Stop 2: Drill core archive, Zinkgruvan
Location
Zinkgruvan mine (N 6521620, E 1459550).
Introduction and description
A drill core will be shown that has been drilled through the whole stratigraphic package, i.e
through the three delimited lithostratigraphic groups. From the stratigraphic footwall (quartzmicrocline rock) through the meta-volcanic succession with the mineralization (mine
package) and out into the stratigraphic hanging-wall (metasediments).
M ig m a tit e , S illim an it e - b io t ite - q u a rtz - f e ld s p a r
M e ta s e d im e n ts
P y rrh o t it e m in e r aliz a tio n
V o lc an ic la s tic s
M a rb le, w o lla s t o n ite - s k a rn - v e s u v ia n it e - g a rn et
Z n - P b O RE
V o lc an ic la s tic s
(c u)
M a rb le, f o rs te rit e - s e rp e n t in e - ( m a g n e tit e ) - c a lc ite
M in e P a c k a g e
V o lc an ic la s tic s
Z n - P b m in e ra liz at io n
V o lc an ic la s tic s
Q u a rt z - M i c ro c l in e
ro c k
Q u a rt z - M ic ro c lin e ro c k
Figure 44. Simplified stratigraphic column for the Zinkgruvan mine package (Zinkgruvan Mining data).
Stop 3: South of Zinkgruvan
Location
Outlet tube (N 65206300, E 14595300).
Introduction
Three well delimited lithostratigraphic groups can be distinguished close to the Zinkgruvan
ore deposit. The youngest, metasedimentary rocks, occur in the south. The metasediments
consist of veined gneisses, migmatites and granitoid rocks. They are rich in biotite and
sometimes also andalusite and sillimanite. Their protoliths were probably epiclasic sediments
with high clay content. In the veined gneisses (migmatites) there are some concordant beds
with disseminated pyrrhotite.
Description
Outcrops along the outlet tube, 700 m south of Nygruvan mine; veined gneiss, migmatites,
granites.
81
Stop 4: North of Zinkgruvan
Location
An outcrop in the footwall rocks (N 6522130, E 1460190).
Introduction
The oldest of the three lithographic groups, quartz-microcline rock, occurs to the north of
Zinkgruvan and covers an area of about 7 km2. It is totally dominated by a massive rhyolitic,
red fine-grained quartz-microcline rock, strongly enriched in K and depleted in Na. The
distance between the ore zone and the quartz-microcline rock various but in the eastern part of
the Knalla mine they are almost adjacent to each other.
Description
This stop is a road cutting, 600 m northeast of the Zinkgruvan mine. K-rich altered
metavolcanic rocks are exposed.
Stop 5: Mine museum, Zinkgruvan
Location
Mine museum (N 6521180, E 1458680)
Introduction
The mine museum is situated in the old buildings of the Knalla mine. It is known for its
model over the Zinkgruvan deposit, which is made at a scale of 1:800 and is kept updated.
Among other things, old tools and photos show the history of Zinkgruvan all the way back to
1857, when the mine was started by Vieille Montagne company from Belgium.
Description
Mine museum, including a model of the Zinkgruvan deposit.
82
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