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Bergslagen District Base Metals - Falun, Garpenberg, Sala, Stollberg

Sweden

Main commodities: Cu Zn Pb Ag Au Fe
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The Bergslagen District in southern Sweden extends over an arcuate area of ~250 x 100 km some 180 km west of Stockholm and hosts a range of ore deposits that have been exploited over a long historical period. These include iron as well as base (Cu, Pb, Zn) and precious (Au, Ag) metals. This record concentrates on the base metal deposits, while a separate record covers the iron mineralisation.

In 1861 there were 511 operating iron mines in the district producing 425 000 tonnes of ore for the year, as well as 23 base metal mines with an output of 130 000 tonnes of ore.   By 1911 there were 242 iron mines for 2.15 Mt of ore and 26 base metal mines for 278 000 tonnes of ore.   In 1973 22 iron mines contributed 8.80 Mt of ore and 9 base metal mines yielded 1.22 Mt of sulphide ore.

The Bergslagen District is interpreted to have formed in an intracontinental extensional or continental margin back-arc region on continental crust (Weihed et al., 2005; Stephens et al., 2009). Regional metamorphism associated with the Svecokarelian orogeny (1.86 ±14 Ga; Stephens et al., 2009) reached the amphibolite facies over most of the district with greenschist facies rocks being preserved in western Bergslagen, and granulite facies rocks occurring to the SE and SW (Andersson et al., 1992; Wilkström and Larsson, 1993; Stephen et al., 2007, 2009). Stephens and Weihed (2013) divided the Bergslagen District into four metamorphic domains: northern migmatitic, central low-grade, central medium-grade upper greenschist-amphibolite facies, and southern migmatitic amphibolite-granulite facies.

The country rocks of the district comprise a Palaeoproterozoic metamorphosed supracrustal volcano-sedimentary succession composed mainly of submarine rhyolitic volcanic, sub-volcanic and volcaniclastics with subordinate mafic volcanics, chemical, epiclastic and organogenic (carbonate) sediments spread through the pile. These are accompanied by pre- to post-tectonic intrusive rocks (e.g., Allen et al., 1996). The magmatic rocks in the district range from 1.91 to 1.75 Ga, with pre-tectonic intrusive rocks consisting mainly of 1.90 to 1.87 Ga granite, tonalite, diorite and gabbro, known as the 'granitoid-dioritoid-gabbroid series' (e.g., Lundström, 1987; Stephens et al., 2009). Other intrusive rocks include the 1.88 to 1.76 Ga granite-syentoid-dioritoid-gabbroid rocks of Allen et al. (2008), metadolerite, alkaline intrusive rocks, and a 1.85 to 1.75 Ga granite-pegmatite suite.

The volcanics are described by a local term leptite for coarse metamotphosed acid volcanics.   In the Bergslagen district they form a 2000 m thick succession which varies from potash rich to extreme soda rich types (quartz-keratophyres) with subrodinate intermediate volcanics.   Calcareous intercalations are found within the volcanics, varying from magnesia poor to magnesia rich). Although these volcanic rocks include rhyolitic, dacitic, andesitic and basaltic compositions, the largely bimodal rhyolite-basalt suite is dominated by rhyolitic volcanic and pyroclastic material (Lagerblad 1988, Allen et al., 1996). These felsic rocks are interpreted to have been sourced from numerous caldera volcanoes in a shallow marine to subaerial basin environment. The meta-sedimentary rocks comprise metagreywacke, meta-argillites, feldspathic metasandstone, carbonates, and quartzite. Carbonate units, formed from stromatolite growth during hiatuses between eruptive events, extend intermittently for tens of kilometres and have been variably metamorphosed to marble and various skarn assemblages (Allen et al., 2003).

Over 6000 mineral deposits and occurrences are known in the Bergslagen region. Most are hosted by marble units within the metavolcanic succession, with the majority of the ores interpreted as being due to skarn alteration of the carbonates which are interlayered with (leptites) volcanogenic ash-siltstones, but apparently related to later sub-volcanic intrusives.   These hosts were subsequently overlain by argilites, greywackes, quartzites and conglomerates and intruded by early-orogenic ultramafics to granitoids, late-orogenic granites and post-orogenic plutons and even later dolerites.   The principal orogenesis is associated with the 1800 to 1750 Ma Svecikarelian orogenic event.

The great majority of the numerous occurrences and deposits represent iron mineralisation that has been sub-divided into the following groups:

• Banded quartz-hematite (±magnetite) ores - occurring as thin (mm to cm scale) alternating layers of hematite and quartz paralleling bedding. Hematite is locally reduced to magnetite.
• Skarn limestone magnetite ore - occurring as beds or massive to disseminated magnetite replacement of skarn altered carbonates, locally accompanied by manganese and or base metals.
• Massive apatite-rich magnetite (±hematite) ore - occurring as massive replacements in extrusive volcanic hosts and in part by sub-volcanic intrusives emplaced in the latter, and are interpretted to have a 'porphyry type' affiliation.
• Disseminated apatite-bearing magnetite ore - the apatite-rich magnetite ores commonly grade laterally and stratigraphically into disseminated mineralisation.

In addition to these iron ores (see the Bergslagen District Iron record for more detail) and occurrences however, base and precious metal deposits are also found in the district.

Base metals are found both as volcanic hosted massive sulphides and as massive or disseminated sulphides which may be closely associated with the iron ores.   The base metals are believed to be broadly coeval with volcanism and the emplacement of iron ores.   The majority of the significant base metals are restricted to a 120 km long and 30 km wide zone oriented NW-SE, normal to the main structural trend of the host leptites, but paprallel to a major fracture trend that may have controlled the emplacement of ore and possibly volcanics.

The tungsten deposits are also usually hosted by limestone or skarn altered rocks, locally overprinting the iron mineralisation.   Tungsten ores are believed to have developed around 100 Ma after than the iron ores, at a late stage of Svecokarelian metamorphism.

Gold mineralisation is also found overprinting the iron mineralisation, but is believed to be a younger replacement of favourable iron oxide rich units.


The most significant base and precious metal ore deposits of the Bergslagen District are as follows:

Falun Cu-Zn-Pb

  An east west 35 km long and 8 km wide belt of leptites bounded to the north and south by older Svecofennian granites (which are locally gneissic) passes through the town of Falun.   This belt of leptites hosts the richest sulphide ores in Sweden and the largest in the region.   It is believed to have historically yielded 35 Mt of ore and to be one of the largest copper producers in Europe over a period of several centuries with exploitation commencing in the mid 11th century.   The cumulative past production has also been estimated as: 28.1 Mt @ 2 to 4% Cu, 4% Zn, 1.55 Pb, 13 to 25 g/t Ag, 2 to 4 g/t Au (Allen et al., 1996; Kampmann et al., 2017).
  The main sulphide ores are all confined to a 15 km strike interval of the leptite belt where the volcanics have been metasomatically altered, transposed and foliated to varying degrees to mica schist and quartzite.
  The bulk of the ore at the main Falun mine is confined to the hinge axis of a tight drag fold plunging 70° S.   The largest orebody persisted to a depth of 332 m. The main zone covered a plan area of 40 000 square metres, with a main bulk occupying and area of some 120x280m with two attached thinner limbs which lensed out away from the main mass
  Three main types of ore are found in the Falun district, namely:
  i). Complex ores - mainly massive pyrite with considerable Cu, Pb & Pb, calculated to have averaged 0.7% Cu, 5% Zn, 1.7% Pb, 35% S.   Cu is richest towards the margins.   Small concentrations of magnetite have been recorded within the main complex orebody.   Quartz is the principal gangue with variable amount so skarn minerals, while relict carbonates (limestone and dolomite) are also found within the ore;
  ii). Disseminated ores are found on both to the east and west of the main body of complex ore and comprise impregnations and veinlets of chalcopyrite in the ore quartzites, accompanied by pyrite, pyrrhotite and sphalerite with magnetite and gahnite.   In the eastern part gold is also present together with weibullite, a Pb-Bi-Se mineral.   Veins of white quartz with chalcopyrite, galena and sphalerite are also found in the disseminated ore zone; and
  iii). Sköl (or gouge) ores - disseminations and veinlets of chalcopyrite or pyrite, pyrrhotite, sphalerite and galena found as 1 to 2 m thick bands immediately surrounding the margins of the orebodies.   They are characterised by biotite, chlorite, talc, amphibole and cordierite, andalusite or almandine.   The disseminated and gouge ore is calculated to have averaged 2.5% Cu, 0.5% Zn, 0.1% Pb, 10% S.
    Other smaller Cu and Zn mines such as Skytt and Näverberg are forund within a few kilometres in the same belt.   A similar 60 km long parallel belt of leptites are found a few kilometres to the north of Falun separated by granite.   This belt contains sulphide deposits similar to Falun, but generally of a small size as well as a large number of insignificant iron occurrences.


Garpenberg Cu-Zn-Pb-Ag-Au-Fe

  The Garpenberg Fe-oxide and polymetallic sulphide deposits lie within a 5 km wide and 25 km long enclave of 1.91 to 1.89 Ga Palaeoproterozoic, Svecofennian volcanic and volcaniclastic rocks with minor limestone, enclosed within a much more extensive, younger 1.90 to 1.87 Ga Svecokarelian meta-granitoid mass.
  Historic documents reveal systematic mining has been undertaken in Garpenberg since the 13th century, whilst studies from lake sediments provide evidence of early ore mining from the Middle Ages to the pre-Roman Iron Age as far back as ~400 BC. The main Garpenberg Mining operation was acquired b Boliden AB in 1957 from Zinkgruvor AB. Since then, a total of 51.5 Mt of ore has been processed. The ore treated has varied from 0.26 Mt @ 1.2 g/t Au, 69 g/t Ag, 2.84% Zn, 2.34% Pb in 1957, to 0.976 Mt @ 0.5 g/t Au, 141 g/t Ag, 3.9% Zn, 1.9% Pb in 2000, to 2.861 Mt @ 0.26 g/t Au, 118 g/t Ag, 4.1% Zn, 1.5% Pb in 2019 (Boliden Annual Reserve and Resource Report, 2019).
  The Garpenberg mine exploits the largest cluster of sulphide lenses in the region, and comprises more than 10 individual ore bodies distributed over a strike length of 4 km along an altered limestone unit. These steeply plunging composite ore shoots/orebodies are from SW to NE, the Strand-Lilla Strand-Kanalmalmen; Finnhyttan-Kyrkan-Tyskgården; Dammajön; Kvarnberget-Lappberget; Huvudmalmen; and Gransj¨n.
  The sequence in the district comprises, from the base to the core of the synform, a thick 'lower footwall' rhyolitic ash-siltstone with minor coarse volcaniclastic units. This is overlain by the 'upper footwall to lower hanging wall' rhyolitic-dacitic ash-siltstone, crystal sandstone and pumice±lithic breccias with intercalated limestone bands and lenses that largely host mineralisation. The next main unit is a feldspar-phyric rhyolitic-dacitic pumice±lithic volcaniclastic suite. These three units are intruded by and intercalated with with quartz-feldspar phyric rhyolitic pumice±lithic volcaniclastic units; mafic intrusions and volcanics rocks, and dacite to rhyolite sub-volcanic intrusions. The sequence is folded into a 2.5 km wide, NE plunging synform which broadens to the NE, and is intruded to the NW by a large 1.90 to 1.87 Ga meta-granitoid mass along a semi-concordant contact. To the SE, the synform is truncated by a NE-SW trending, SE vergent thrust separating the sequence above from a succession of undifferentiated felsic volcanic rocks, which are also intruded further to the SE by the same meta-granitoid. The Palaeproterozoic volcanosedimentary sequence of the syncline forms a large roof pendant or infolded enclave within the much more extensive 1.90 to 1.87 Ga Svecokarelian metamorphosed 'Early Granite' meta-granitoid.
  The principal host rock to ore is calcitic marble (limestone) that has been altered to dolomite and Mg ±Mn-rich skarns. The footwall to this mineralised limestone is composed of felsic volcaniclastic rocks that have been strongly altered to an assemblage of phlogopite-biotite-cordierite-sericite-quartz. In contrast the hanging wall comprises volcaniclastic and sedimentary rocks and dacitic intrusions that are relatively unaltered.
  The stratigraphic succession is interpreted to represent a volcanic cycle associated with a felsic caldera complex, and includes rhyolitic to dacitic, juvenile pumiceous, graded mass-flow breccia deposits and rhyolitic to dacitic ash-siltstone and sandstone in the footwall, whilst the hanging wall is composed of polymict conglomerates and juvenile, rhyolitic, pumiceous breccias. The pumiceous breccias of the hanging-wall are interpreted to represent a major eruption that formed a >500 m deep and >9 km diameter caldera in the Garpenberg area. The host limestone is interpreted to be a shallow marine stromatolitic carbonate reef formed at water depths of up to 50 m during hiatuses in volcanic activity (Allen et al., 2003).
  The geometry of the host limestone is complex, affected by large scale folding, shearing and faulting. The ore has been locally remobilised by folding and late faulting into fault- and fracture-hosted high grade, pyrite-poor, sphalerite-galena veins, some of which have been thick enough to be economic. This deformation has produced complex synforms and antiforms that have influenced the position, geometry and metal grades of the ore bodies, e.g., the Lappberget orebody is interpreted to be a >1.5 km long, subvertical anticlinal tube fold with the top of the antiform at just below 200 m depth and strike length of <200 m. The original main stage mineralisation and alteration at all the known Garpenberg ore bodies is interpreted to essentially be syn-volcanic, pre-dating regional metamorphism and deformation. The ore lenses occur along the contact zone between the limestone and underlying siltstones which is heavily skarn altered, while the more distal limestone has been converted to dolomite. Mineralisation is predominantly replacement style and is interpreted to have been the result of metal-bearing fluids penetrating along syn-volcanic, extensional faults and coming into contact with reactive limestone to form large, massive sulphide bodies.
  The main synformal structure described above is strongly isoclinally folded and the structure divided into blocks by consistently steeply dipping cross-folding and faulting. The different orebodies/shoots are strongly structurally controlled with the largest linked to antiforms e.g., Lappberget.
  The mineralisation occurs as massive, semi-massive and pod-like sulphide accumulations with sphalerite and galena being the dominant economic minerals, while chalcopyrite is locally present. Pyrite and pyrrhotite are subordinate to the base metal sulphides.
  Mineralisation associated alteration consists of Mg-rich tremolite-diopside skarns, accompanied by Mg-rich biotite-garnet-cordierite-quartz schist alteration, with silicification and K ±Mg alteration represented by muscovite-phlogopite-quartz schists (Vivallo 1985; Allen et al., 2008, 2013). The extent to which these mineral associations reflect primary alteration assemblages as distinct from metamorphic equivalents is uncertain (e.g. Allen et al.> 2008). There is locally a spatial association between sulphide mineralisation and Mn-rich magnetite mineralisation (e.g., Lappberget; Jansson 2011), although magnetite is generally subordinate at Garpenberg. Mineralisation in the Garpenberg district is characterised by an increase in Mg with proximity to sulphides (Vivallo 1985) reflected by the high Mn (up to 21.45% MnO) in skarn.
  Mineral Resources and Ore Reserves at the end of 2019 were (Boliden Annual Reserve and Resource Report, 2020):
  Proved + Probable Ore Reserves - 74.8 Mt @ 0.31 g/t Au, 96 g/t Ag, 0.05% Cu, 3.1% Zn. 1.4% Pb.
  Measured + Indicated Mineral Resources - 44.3 Mt @ 0.35 g/t Au, 90 g/t Ag, 0.05% Cu, 2.8% Zn. 1.4% Pb.
  Inferred Mineral Resources - 24.1 Mt @ 0.43 g/t Au, 59 g/t Ag, 0.07% Cu, 2.6% Zn. 1.5% Pb.
  Although not stated, it would seem the Mineral Resources listed are the balance of the total resource that has not been converted to Ore Reserves.

  Other historic and recent producing deposits within the Garpenberg district include the Ryllshyttan mine, which is believed to have produced 1 Mt of ore, 150 000 tonnes of which was iron ore.   Other mines include Garpenberg Odal, Garpenberg Nora and Smäaltarmossen.   In 1973 production included:   Garpenberg Odal -191 000 tonnes at 0.4% Cu, 4.0% Zn, 2.8% Pb, 86 g/t Ag, 1.0 g/t Au;   Garpenberg Nora -134 000 tonnes at 0.1% Cu, 2.1% Zn, 0.9% Pb, 156 g/t Ag, 0.2 g/t Au.   From 1935 to 1939, the last 5 years of it's operation, Ryllshyttan produced 164 000 tonnes @ 14.0% Zn, 6% Pb, 54 g/t Ag.


Sala Ag-Pb-Zn

  Sala is a silver rich deposit hosted by leptites, 40 km to the SE of Garpenberg and separated from the latter by granites and greenstones.   The leptites have intercalated limestones and dolomites which are thickest at Sala where ore is hosted by dolomite and serpentine rich carbonates.   The ore complex cuts diagonally across the carbonates with a vertical dip and plunge of 30 to 45° NW.   The ore zone is 80 to 100 m wide and follows a well developed gouge zone.   The ore minerals are argentiferous galena and sphalerite with minor pyrite, pyrrhotite and chalcopyrite, as well as locally significant mercury.   An estimated 5 Mt of ore has been mined.   The most recent mine in the field, Bronäs extracted 171 000 tonnes of ore averaging 350 g/t Ag, 2% Zn, 4% Pb from 1951 to 1962.


Stollberg Fe-Ag-Pb-Zn

  Stollberg comprises a cluster of deposits in a field located ~50 km WSW of Garpenberg that has been mined since the 14th century. Modern mining was restarted in the 1940s and ended in 1982 when the last mine closed after producing ~6.65 Mt of ore. The cluster includes several mines and ore fields, exploiting both iron-manganese and zinc, extending from the Lustigkulla mine in the north via the Svartberg and Dammberg fields to Brusmalmen in the south, a distance of ~4.5 km, which together constitute the Stollberg-Svartberg ore trend (Frank 2015). The mines were connected underground from Brusgruvan in the south to the Svartberg field in the north. This cluster is part of a more extensive trend that extends intermittently for almost 15 km that incorporates more than a dozen 'deposits'.
  The Stollberg-Svartberg ore trend occupies section of the eastern limb of the major north-south, upright to steeply east dipping and steeply south plunging 'Stollberg Syncline', although similar mineralisation is also known at Gränsgruvan on the northern section of the western limb.
  The 1.91 to 1.89 Ga Svecofennian metavolcanic rock sequence that form this syncline is largely surrounded by 1.90 to 1.87 Ga Svecokarelian meta-granitoids (the granitoid-dioritoid-gabbroid suite) and 1.87 to 1.67 Ga late- and post-Svecokarelian Orogeny intrusive rocks (the granite-syenitoid-dioritoid-gabbroid Suite).
  Westward, and stratigraphically upward from the granitoid contact, the sequence on the eastern limb of the syncline commences with a >1 km thick suite of poorly exposed felsic metavolcanic rocks which host several Mn-poor magnetite skarn deposits (Geijer and Magnusson, 1944).
  These are overlain by the Staren Marble,which is a white-orange coloured, coarse-grained calcite marble. It is one of the largest marbles of northwest Bergslagen, with a thickness of ~150 m and strike length of >7 km (Hjelmqvist, 1966). Local skarn alteration to is observed within the unit, as are interbeds of rhyolitic volcaniclastic silt-sandstone.
  This carbonate unit is succeeded by a sequence that includes rhyolitic siltstone, sandstone and breccia; feldspar-quartz-phyric rhyolitic pumice breccia-sandstone to rhyolitic sandstone; and massive polymict breccia-conglomerate which locally contains limestone clasts. It also includes other thin limestone/dolomite beds and lenses, as well as lensoid developments of amphibolite after mafic intrusion or volcanic rocks and coherent bodies of intrusive feldspar-porphyritic rhyolite.
  These rhyolitic rocks are overprinted by what is interpreted to be a metamorphosed alteration zone in the stratigraphically footwall of the mineralisation. This zone expands upward to the west, with a strike length that grows from <1 km to >4 km below the mineralised unit. The lower and lateral boundaries from the poorly altered footwall rhyolitic suite to sodic (gedrite-rich) altered rocks has been interpreted as an alteration front. The latter zone comprises gedrite-porphyroblastic albite + quartz ±garnet rocks (Ripa, 1988) which grades westward into quartzose biotite schists with local porphyroblasts of almandine, staurolite, cordierite, sillimanite and andalusite. These, in turn, grade westward into strongly porphyroblastic garnet-amphibole-biotite ±gahnite ±cordierite ±andalusite ±sillimanite rocks over the lowermost 5 to 50 m below the ore host unit, grading into marble, skarn and mineralisation. The schists and porphyroblastic rocks are interpreted to represent a metamorphosed zoned hydrothermal alteration envelope that is preferentially developed stratigraphically below the Stollberg string of deposits (Selinus, 1982; Ripa, 1988, 1994). The lower gedrite-porphyroblastic quartz + plagioclase ±garnet rocks were interpreted as metamorphosed chlorite + albite-rich zones whereas the overlying porphyroblastic garnet-amphibole-biotite rocks with gahnite and cordierite were interpreted to represent metamorphosed chlorite + sericite-rich alteration zones (Ripa, 1988). This footwall gedrite-albite-quartz alteration zone underlies the deposits in the centre of the Stollberg-Svartberg trend (i.e., Dammberget, Stollberg, Baklängan and Kogruvan-Myggruvan), but does not extend beneath deposits at the northern (Lustigkulla and Marnäs) and southern (Brusgruvan) ends of the trend (Frank et al., 2109).
  Stollberg Mn-rich dolomite or limestone, the host to the main ore deposits, which is intercalated with skarn and hydrothermally altered and metamorphosed rhyolitic volcaniclastic siltstone and has variable proportions of interbedded carbonate, iron oxides and fine-grained rhyolitic ash. At Gränsgruvan, the equivalent is an up to 30 m thick marble-skarn unit, which locally grades into a stratigraphically underlying massive diopside-hedenbergite skarn.
  The ore deposits are overlain by ~800 m of seemingly unaltered (in terms of alkali contents) massive to banded rhyolitic metavolcanic rocks (Ripa, 1988). The eastern and lowermost 1 to 20 m of this unit are replaced by strongly porphyroblastic garnet-amphibole-biotite ±gahnite ±cordierite ±andalusite ±sillimanite rocks similarly altered to those in the immediate footwall of the host unit to the east, with a gradational boundary with the marble, skarn and mineralisation. Above this basal alteration fringe, patchy, nodule-like aggregates are found, composed of microcline + quartz + zoisite + calcite + actinolite + grossular-rich garnet, with local zones of strong cordierite + muscovite + quartz alteration. A second, ~5 km long north-south lens of amphibolite, similar to that above the Staren Marble, occurs to the west of the Stollberg ore zone, and is interpreted to be extrusive in origin (Ripa 1988).
  The stratigraphically uppermost preserved unit in the syncline is composed of andalusite- and cordierite porphyroblastic metapelite and arenite (-Ripa, 1994; Stephens et al., 2009).
  The area is cut by NNW trending dolerite dykes, likely members of the anorogenic~1.48 to 1.47 Ga Tuna dolerite suite; and to the north by NE trending porphyritic dolerites with plagioclase and augite phenocrysts of the 0.98 to 0.95 Ga Sveconorwegian dolerite suite of Stephens et al., 2009. A few small pegmatites of unknown age cut all of the tectonic foliations.
  Arvanitidis and Rickard (1981) and Ripa (1988) used mineralogical zoning of the altered rocks and preserved stratigraphic younging indicators to infer a general westward younging of the succession to the synclinal core. A reversal in stratigraphic younging directions between the Gränsgruvan and Svartberg deposits, confirm both occupy the same stratigraphic level on opposite limbs of the Stollberg Syncline. Whilst Grip (1983) and Ripa (1988) interpreted the Staren Marble to be an equivalent of the Stollberg ore host marble across the core of an anticline,more recent study shows they are different stratigraphic units, and hence that a continuous, generally westward-younging stratigraphic sequence exists from belwo the Staren Marble to the centre of the Stollberg syncline (Jansson et al., 2103).
  The deposits are hosted by metavolcanic rocks, marble and skarn of the Stollberg host limestone in the semi-regional, north-south trending, steeply dipping, F2 'Stollberg Syncline'. Whilst F1 folds are not evident in the Stollberg area, the presence of a prominent S1 schistosity sub-parallel to S0 indicates that F1 folds are tight or isoclinal. Garnet overgrowths on S1, deformed bands of garnet alternating with quartz-biotite layers, and the localisation of fibrolitic sillimanite in the pressure shadows of garnet are interpreted to suggest that D1 and peak metamorphism outlasted S1 (Jansson et al., 2011, 2013). Peak metamorphism is suggested to have been to amphibolite facies conditions of 560 to 600°C and 2 to 3.5 kbar (Beetsma 1992), consistent with conditions indicated by the presence of both andalusite and sillimanite (Björklund, 2011; Jansson et al., 2013). Gedrite-garnet and andalusite-gahnite-cordierite-staurolite-clinopyroxene-K feldspar assemblages that are spatially associated with mineralisation, also formed after the development of S1. S2, which resulted from F2 folding along steep south-plunging axes is also locally cross-grown by gedrite [{Mg2}{Mg3Al2}(Al2Si6O22)(OH)2] (Ripa, 1996, 2012; Jansson et al., 2013). D3 is characterised by brittle NE- and minor NW-trending faults and shears.
  Three types of mineralisation have been recognised on the eastern limb of the Stollberg Syncline (Selinus 1983):  i). Iron oxide, mostly comprising 20 to 50% Fe magnetite, with variable amounts of galena, sphalerite and pyrrhotite (e.g., Stollmalmen);  ii). Sulphides in marble comprising disseminated to semi-massive sphalerite and galena with pyrrhotite, chalcopyrite, pyrite, magnetite and arsenopyrite (e.g., Dammberget); and  iii). Supergene-enriched Mn-bearing limonite-magnetite (e.g., between Stollmalmen and Brusgruvan; Frank 2015).
  The main Stollberg deposits comprise several semi-massive to massive base metal sulphide (sphalerite and galena, with subordinate pyrite, pyrrhotite, chalcopyrite, arsenopyrite and magnetite) and magnetite bodies spatially related to skarn altered carbonate rocks (Frank et al., 2019).
  The volcano-sedimentary country rocks are dominantly amphibolite facies metamorphosed rhyolitic pumice breccia and rhyolitic ash-siltstone with minor mafic sills.
  The exploited sulphide mineralisation, which is predominantly on the eastern limb of the Stollberg Syncline, occurs as what is interpreted to be stratabound pre-metamorphic replacement of volcaniclastic rocks and limestone that grades into an 'iron formation' (Frank et al., 2109). Ripa (1994) proposed that the Fe-Mn deposits formed formed first, with the polymetallic sulphides forming later, in conjunction with Mg alteration. Frank et al. (2109) suggest the skarn assemblages result from low-temperature replacement of limestone and volcaniclastic rocks, rather than by high-temperature metasomatism or synmetamorphic or late hydrothermal replacement of marble. As described above in the stratigraphy succession, the hydrothermally altered footwall rocks that have been metamorphosed on the eastern limb of the syncline are now predominantly composed of assemblages of garnet-biotite and gedrite-albite, with silica-altered rocks generally being subordinate in the ore field (Frank et al., 2019).
  The polymetallic sulphide deposits on the eastern side of the Stollberg-Svartberg ore trend contain three main skarn and ore assemblages: i). biotite-garnet ±gahnite ±sillimanite associated with pyrrhotite ±sphalerite; ii). amphibole-garnet ±andalusite ±staurolite ±cordierite ±biotite ±gahnite, associated with pyrrhotite ±sphalerite ±galena ±magnetite; and iii). olivine ±pyroxene ±garnet ±carbonate, associated with sphalerite-galena. These assemblages have been attributed to the composition of the pre-metamorphism altered protolith, with sphalerite and galena occurring in meta-limestone, while pyrrhotite, which may be accompanied by chalcopyrite, occurs in meta-rhyolite (Ripa 2012). Sulphides in these deposits are commonly zoned, e.g., skarn-hosted sphalerite and galena mineralisation at Dammberget occurs stratigraphically above garnet-biotite rock that is enriched in chalcopyrite and pyrrhotite (Ripa, 2012). At Dammberget also, the stratigraphically uppermost sulphide bearing zone in skarn is composed primarily of calcite, grunerite, actinolite, ferropargasite/hastingsite, clinopyroxene and garnet. In general, the mineralised zone of the Stollberg-Svartberg ore trend is a composite of these assemblages occurring as a layered sequence of green and red Mn skarn with skarn-altered metarhyolite. In addition to the garnet, calcite and amphiboles, the mineralised band includes gedrite, hornblende, diopside, epidote, gahnite, cordierite and staurolite, with rare fluorite. Serpentine occurs as an alteration product of olivine (Frank et al., 2019).
  However, at Gränsgruvan, on the western limb of the syncline, sulphides are located in a silicified zone, accompanied by metamorphosed altered rocks dominated by sericite and the assemblage quartz-garnet-pyroxene (diopside/hedenbergite) that is at least 120 m wide. The associated metallic minerals are mostly of sphalerite with galena and pyrite. An ~30 m thick skarn occurs in the hanging wall and footwall of sulphide mineralisation, but does not contain sulphides. This skarn comprises calcite, quartz and microcline with corroded poikilitic garnet, but locally consists of calcite with diopside/hedenbergite, tremolite, actinolite and magnesio-hornblende which is regarded as a likely correlative with that of similar composition hosting sulphides of the Stollberg-Svartberg trend on the eastern limb. This silicified zone appears to be an inner zone fringed by skarn that is absent from most the other deposits of the field where skarn is the innermost zone. The difference between the quartz-garnet-pyroxene and skarn is the significant volume of quartz, which is dominant in the former (Frank et al., 2019). A similar zone is known at Tvistbo, which is in a separate structure ~1.5 km north of Gränsgruvan, and contains more quartz and calcite and less garnet, tremolite, and diopside/hedenbergite than at the latter. The metallic minerals at Tvistbo are galena, sphalerite and magnetite, with minor pyrrhotite, pyrite and chalcopyrite (Frank et al., 2019).
  Garnet-biotite alteration occurs in both the stratigraphic hanging wall and footwall to sulphides along the Stollberg-Svartberg ore trend, outboard of the skarn zone, but is more intense in the footwall. It also appears in the same relative position at Gränsgruvan where it is >20m thick in the hanging wall and ~10 m in the footwall. Semi-massive sulphides occur within the garnet-biotite zone stratigraphically below the carbonate horizon and comprise galena, sphalerite and pyrrhotite, with lesser pyrite and chalcopyrite. The entire footwall garnet-biotite down to its contact with the underlying gedrite-albite rock, is K altered, but contains only minor or no Na, and is also enriched in Fe and Mg with a very variable mineralogy (Frank et al., 2019).
  As outlined in the stratigraphic sequence description above, the garnet-biotite zone below the central deposits of the Stollberg-Svartberg ore trend is stratigraphically underlain by a mass of gedrite-albite rock which extends over a strike length of ~4 km. A similar zone is also found locally in the hanging wall outboard of the garnet biotite interval also. However, it is apparently absent on the western limb of the Stollberg syncline. It is composed of fine-grained quartz, acicular to radiating ferrogedrite, albite, and rare cordierite and epidote (Frank et al., 2019).
  A cordierite-biotite zone is found over an interval of ~200 m into the hanging wall of the Gränsgruvan deposit on the western limb of the Stollberg Syncline, but elsewhere is only seen locally at the Stollmalmen deposit near the southern extremity of the Stollberg-Svartberg ore trend on the eastern limb. It comprises poikiloblastic cordierite with quartz inclusions, fine grained, equigranular and subhedral quartz and Mg-rich biotite cross-cut by muscovite plus local fibrolitic sillimanite.
  On the northern end of the mineral field, just west of the main synform closure, the Tvistbo and Norrgruvan prospects occur within the northern extension of the host carbonate. Whilst these prospects are small, they exhibit geologic characteristics that are transitional between mineralisation on the western and eastern limbs of the syncline. Ore at Tvistbo is hosted by skarn and is spatially associated with quartz-garnet-pyroxene rocks, whilst sulphides at Norrgruvan are hosted by quartz-fluorite rocks that are similar to those hosting the Brusgruvan deposit on the eastern limb of the syncline (Frank 2018; Frank et al., 2019). Brusgruvan, which is on the southern end of the Stollberg-Svartberg trend differs from other deposits on the eastern limb in that sulphide mineralisation is composed of a vein network of galena + fluorite ±pyrite with minor sphalerite and galena veinlets within the host rock. It was silicified and has a high pyrite to pyrrhotite ratio relative to the other deposits (Jansson et al., 2013). While skarn is present at Brusgruvan, it is not spatially associated with sulphides.
  The Gränsgruvan deposit is estimated to have produced 0.26 Mt @ 7.7% Zn, 2.6% Pb, 60 g/t Ag between 1972 and 1982 (Raat et al., 2013). Remaining individual lenses/prospects in the field include the Tvistbo prospect, near the northern end of the Stollberg syncline, which has a strike extent of ~200 m, is up to 55 m wide, and has a depth extent of at least 180 m with a dip of ~80°W. The indicated mineral resource, is ~0.575 Mt @ 3.3% Zn, 2.6% Pb, 22 g/t Ag, plus an inferred resources of 0.28 Mt @ 3% Zn, 2.5%, Pb, 20 g/t Ag.  Norrgruvan ~2 km south of Tvistbo, is hosted in a prominent marble horizon and contains sulphides over a 200 m strike length, ~5 m width and to a depth of ~100 m.
  Historic grades at other deposits/prospects in the Stollberg mineral field include:  Brusgruvan - 3.2% Zn, 15.6%, Pb, 320 g/t Ag, <1.23% Fe, <0.1% Mn, 13 ppm As;  Stollmalmen - 1 to 4% Zn, 0.5 to 1%, Pb, 10 g/t Ag, 20 to 35% Fe, 7% Mn, 2786 ppm As;  Dammberget - 3 to 5% Zn, 2 to 5%, Pb, 20 to 60 g/t Ag, 20 to 10% Fe;  Baklängan - 2.6% Zn, 4.8%, Pb, 43 g/t Ag, <26% Fe, <3.5% Mn;  Korrgruvan-Myggruvan - 0.5% Zn, 0.5%, Pb, 5 g/t Ag, 35% Fe, 11% Mn; Lustigkulla-Marnäs - 1% Zn, 0.5% Pb, 30 to 40% Fe, 10 to 15% Mn (Frank 2015).

The most recent source geological information used to prepare this summary was dated: 2019.     Record last updated: 22/6/2020
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
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Frank, K.S.,  2015 - Geological, mineralogical, and geochemical studies of the Paleoproterozoic base metal Stollberg ore field, Bergslagen, Sweden: in    Iowa State University, Ames, Iowa   Graduate Theses and Dissertations. 15911 166p.
Frank, K.S., Spry, P.G., Raat, H., Allen, R.L., Jansson, N.F. and Ripa, M.,  2019 - Variability in the Geologic, Mineralogical, and Geochemical Characteristics of Base Metal Sulfide Deposits in the Stollberg Ore Field, Bergslagen District, Sweden: in    Econ. Geol.   v.114, pp. 473-512.
Grip, E.,  1978 - Sweden - Extract on the Bergslagen District of Central Sweden: in Bowie, S.H.U., Kvalheim, A. and Haslam, H.W., 1978 Mineral Deposits of Europe, The IMM, London,   v.1, pp. 97-138.
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Jansson, N.F., Erismann, F., Lundstam, E. and Allen, R.L.,  2013 - Evolution of the Paleoproterozoic Volcanic-Limestone-Hydrothermal Sediment Succession and Zn-Pb-Ag and Iron Oxide Deposits at Stollberg, Bergslagen Region, Sweden : in    Econ. Geol.   v.108, pp. 309-335.
Jansson, N.F., Sadbom, S., Allen, R.L., Billstrom K. and Spry, P.G.,  2018 - The Lovisa Stratiform Zn-Pb Deposit, Bergslagen, Sweden: Structure, Stratigraphy, and Ore Genesis: in    Econ. Geol.   v.113, pp. 699-739.
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Kampmann, T.C., Jansson, N.F., Stephens, M.B., Majka, J. and Lasskogen, J.,  2017 - Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen Ore District, Fennoscandian Shield, Sweden: in    Econ. Geol.   v.112, pp. 1111-1152
Kampmann, T.C., Stephens, M.B. and Weihed, P.,  2016 - 3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden: in    Mineralium Deposita   v.51 pp. 665-680.
Raat, H., Jansson, N.F. and Lundstam, E.,  2013 - The Gransgruvan Zn-Pb-Ag deposit, an outsider in the Stollberg ore field, Bergslagen, Sweden: in Jonsson, E., et al. (Eds), 2013. 2013 Mineral deposit research for a high-tech world 12th SGA Biennial Meeting, 12-15 August 2013, Uppsala, Sweden.   Proceedings, v.4, pp. 1551-1554.
Tragardh, J.,  1991 - Metamorphism of magnesium-altered felsic volcanics rocks from Bergslagen, central Sweden. A transition from Mg-chlorite- to cordierite-rich rocks: in    Ore Geology Reviews   v.6, pp. 485-497.
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Wagner T, Jonsson E and Boyce A J  2005 - Metamorphic ore remobilization in the Hallefors district, Bergslagen, Sweden: constraints from mineralogical and small-scale sulphur isotope studies: in    Mineralium Deposita   v40 pp 100-114
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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