Structure and stratigraphy of the Dannemora inlier, eastern Bergslagen Region
Primary volcanic textures, geochemistry and deformation
Peter Dahlin and Håkan Sjöström
Department of Earth Sciences, Uppsala University,
Villavägen 16, 75236 Uppsala
Index
Introduction ... 4
Geological setting ... 5
Volcanic textures and interpretations ... 6
Volcanic textures ... 6
Phenocrysts ... 6
Glass fragments and devitrification ... 7
Lithophysae ... 8
Fiamme ... 8
Accretionary lapilli ... 9
Pumice block deposit ... 10
Discussion and summary of volcanic textures ... 11
Stratigraphy ... 13
Summary of stratigraphic results ... 16
Geochemistry and metamorphism ... 17
Greenstone dikes ... 17
Metamorphic conditions ... 19
Summary of geochemistry and metamorphism ... 19
Tectonic structures ... 20
Metavolcanic rocks ... 20
Metagranitoids ... 23
Shear zones ... 24
Shear zone displacement ... 26
Summary of tectonic structures ... 26
Dannemora inlier in the regional tectonic framework ... 27
Discussion and conclusions ... 28
Acknowledgement ... 29
References ... 30
Appendix 1 ... 34
Appendix 2 ... 35
Introduction
The aim of this project was to build a 3‐4 D structural model for the Dannemora iron deposit. This model will enable interpretation of the geometry, stratigraphy, deformation history and genesis of the Dannemora ore deposit. The low metamorphic grade and exceptionally well preserved primary rock textures at Dannemora should allow the construction of a high resolution model that can be applied to other more highly metamorphosed ore deposits in Bergslagen.
The project was initiated by Lennart Falk, Dannemora Mineral AB, and further developed by in particular Rodney Allen, New Boliden, with support from Håkan Sjöström, Uppsala University, Magnus Ripa, SGU, and Pär Weihed, LTU, among others. The original application by Rodney Allen and Håkan Sjöström included two projects: 1) The origin of iron ores in Bergslagen and their relationships with polymetallic sulphide ores (SGU‐FoU project 60‐1451/2006), and 2) Structure and stratigraphy of the Dannemora iron deposit. This was a collaboration project between LTU, UU, New Boliden and Dannemora Mineral AB, and Rodney Allen. For practical reasons, this project was separated into the two parts, but collaboration continued. Structure and stratigraphy of the Dannemora iron deposit co‐financed by Dannemora Mineral AB (main part) and SGU (SGU‐
FoU project 60‐1453/2006). 2009 the project was entirely financed by SGU.
Two master theses sprang from the Structure and stratigraphy of the Dannemora iron deposit project and they are attached as appendices. One thesis focussed on kinematics of a major shear zone and the deformation mechanisms in different minerals, a method to deduce the temperature that prevailed during the tectonic deformation. A third task was to find possible minerals for radiometric dating, both the magmatic and deformation ages. The second thesis deals with geothermobarometry of sulfides: Fe‐content in sphalerite is pressure dependent and the As‐content in arsenopyrite is temperature dependent.
Four stops prepared during the research project and by Dannemora Mineral AB, were exhibited in the Dannemora area during the excursion that covered the Bergslagen region as part of the 33rd International Geological Congress 2008.
Geological setting
The Bergslagen region is situated in the south‐central part of the Fennoscandian Shield (Fig. 1), which is dominated by metamorphic rock of the GDG‐suite, i.e. granitoid‐dioritoid‐gabbroid, with an age of c. 1.90‐1.87 Ga (Stephens et al., 2007). Subordinate c. 1.91‐1.89 Ga are metavolcanic and metasedimentary rocks and marble, also exist as well as less common younger intrusive rocks, with the age span 1.87‐1.75 Ga (Stephens et al., 2007). The Bergslagen region has been interpreted to represent a continental back‐arc magmatic region (Allen et al., 1996).
The metasupracrustal sequences in Bergslagen province were deposited mainly as pyroclastic flows (Lundström et al., 1998), in an extensively thinned continental back arc magmatic region (Allen et al., 1996). The Taupo Volcanic Zone in New Zealand, a flooded continental margin dominated by caldera related ignimbrites is the best recent analogy today to Bergslagen (Allen et al., 1996).
In the Dannemora inlier, located in the NE part of the Bergslagen region (Fig. 1), consists of metavolcanic rocks and marble, surrounded by metagranitoids. The metavolcanic rocks show textural evidence for being emplaced mostly as pyroclastic currents and subordinate air‐fall deposits (Lager, 2001). The Dannemora ore field is well known for its iron ore, which generally is hosted by marble that locally contain stromatolitic structures (Lager, 2001). Dating of a pyroclastic flow deposit in the Dannemora inlier gave the TIMS U‐Pb zircon age of 1894±4 Ma (Stephens et al., 2009).
A B
Fig. 1. A) Fennoscandian Shield. Bergslagen Region is encircled area. (Modified after Papunen and Gorbunov (1985) and Lundqvist (2000)). B) Geological map of the Bergslagen Region. Yellow = metavolcanic rocks, light blue =
metasedimentary rocks, GDG = metagranitoids, red, orange and pink = GSDG (granitoids‐syenitoids‐dioritoids‐gabbroids (From Stephens et al., 2007)
Dannemora inlier
Volcanic textures and interpretations
The metavolcanic rocks in the Dannemora inlier are among the best preserved in Bergslagen (Stephens, pers. comm. 2009), although the rocks have been affected by greenschist facies metamorphism and at least two episodes of deformation (e.g. Stålhös, 1991, Stephens et al., 2009).
The composition of the metavolcanic rocks varies from dacitic to rhyolitic and they have been interpreted mainly as ignimbrites (Lager, 2001), emplaced by pyroclastic density currents (PDCs).
PDC is the general term for a ground hugging current of gas and clasts, i.e. juvenile, lithic and crystal fragments (Branney and Kokelaar, 2002). Ignimbrites are deposits of PDCs “rich in pumices fragments, commonly dominated by poorly sorted beds, typically made up of various massive and stratified pumiceous beds and some subordinate pumice‐poor beds” (Branney and Kokelaar, 2002).
Volcanic textures
This chapter describes volcanic textures and also a texture that is lacking in the Dannemora inlier.
The well preserved microtextures give a hint of the origin of the subsequently metamorphosed and deformed supracrustal rocks. The textures were formed before eruption (e.g. phenocrysts) and during deposition (e.g. accretionary lapilli) or after deposition (e.g. spherulites). Certain textures provide evidence of the eruption mechanism, emplacement temperature, distance to source region, depositional environment and stratigraphic younging direction etc.
Phenocrysts
The abundance of phenocrysts can distinguish between different emplacement processes such as pyroclastic falls and density currents, and also the distance to the source. Distal pyroclastic fall deposits rarely show high abundance of phenocrysts however they might be part of proximal ballistic deposit. PDCs may carry crystals for long distances and they can be enriched by the processes within currents due to the abrasion of pumice fragments containing crystals (Walker, 1972). Embayments in phenocrysts form when the growth is inhibited by mineral grains or bubbles in the magma (Kozlowski, 1981). The roundness of phenocrysts is due to magmatic resorption (Donaldson and Hendersson, 1988).
Phenocrysts are very common in the metavolcanic rocks in the Dannemora inlier. Quartz is the dominating phenocryst, usually as 0.5‐5 mm euhedral to subhedral crystals. However roundness and embayment (Fig. 2A) in the quartz phenocrysts, are common and sometimes show polygonization within them (Fig. 2B). Polygonization is caused by sub‐grain formation during tectonic deformation (Passchier and Trouw, 2005). Subordinate phenocrysts are plagioclase, mainly albite, but calcic varieties exist locally, which is apparent by subsequent epidotisation (Fig. 3A).
A B
Fig. 2. A) Microphotograph of quartz phenocryst with embayment. Scale bar is 0.5mm and crossed polarised light. B) Field photograph of crystal rich ignimbrite containing quartz phenocryst with encircled polygonization. Pencil point for scale.
A B
Fig. 3. A) Epidotised plagioclase. Scale bar 1 mm and crossed polarised light. B) Field photograph of pink devitrified, tectonically flattened and rod‐like glass fragments from the lower member. Width of picture 12 cm.
Glass fragments and devitrification
According to the laws of thermodynamics, glass is unstable (Marshall, 1961) and hence prone to devitrification i.e. crystallisation (de Latin for changed from and, vitrum means glass) and alteration (McPhie et al., 1993). High temperature devitrification of silicic glass produce different styles of crystal aggregates of cristobalite and/or feldspars collectively named spherulites (e.g. Lofgren, 1971) and these are characteristic for hot pyroclastic deposits and lavas (Ross and Smith, 1961), but occur in colder deposits too.
During upwards transport in the crust the magma is subjected to progressive decompression and dissolved gases start to exsolve, resulting in the onset of bubble formation and eventually to fragmentation in the conduit (Cashman et al., 2001) and/or at the surface (Legros and Kelfoun, 2000). These fragments consist of glass shards and pumice (i.e. juvenile fragments) and their presence help distinguishing pyroclastic rocks from coherent rocks, as the latter do not usually contain juvenile fragments.
In pyroclastic deposits hotter than the glass transition temperature (Tg) welding can take place which is ductile deformation of juvenile fragments. The degree of welding within pyroclastic deposits depends on eruption temperature and the retained heat, volatile content, and compression by the overburden (Ross and Smith, 1961).
Spherulites have been found in Haglösa, N‐NV of Gruvsjön (Fig. 19) in massive pyroclastic rocks and in layered, reworked epiclastic rocks, stratigraphically located in the lower member (see Stratigraphy). Such epiclastic deposits formed due to weathering of rocks to produce new particles different in size and shape from those formed and distributed by an eruption (White and Houghton, 2006). In addition, layered rocks with traction current bedforms set the depositional environment to shallow subaqueous (McPhie et al., 1993). The spherulites have a diameter of < 1 mm and consist of densely packed radiating mineral fibres, and they are present both in the matrix (Fig. 4A) and in former large juvenile fragments (Fig. 4B) in the pyroclastic rocks, but only in the fragments in the epiclastic rocks. Juvenile fragments contain small spherulites and show welding compaction, clearly around phenocrysts (Fig. 5A). Later the devitrification accentuated the compaction foliation. From a drill core (with uncertain stratigraphic position, possibly lower member) devitrification has produced pectinate spherulites (Fig. 5B), radiating from the surface into the former juvenile fragments.
Lithophysae
Spherulites with a central cavity are called lithophysae (Ross and Smith, 1961) and they start to form at an early stage of the cooling of lavas and densely welded pyroclastic deposits (McPhie et al., 1993). Lithophysae have been described from numerous ignimbrite occurrences in Sweden e.g. Lake Rakkur, Norrbotten, (Lilljequist and Svensson, 1974) and Hällefors, Bergslagen (Lundström, 1995).
This texture is has not been recorded in the Dannemora inlier, indicating that the rocks were neither lavas nor densely welded pyroclastic deposits.
Fiamme
The flattening of pumice fragments and formation of fiamme (Italian for flames) can be ascribed to welding compaction, diagenetic compaction and/or tectonic deformation in pyroclastic deposits/rocks (Gifkins et al., 2005). Fiamme formed by welding compaction, has feathery tips and are ragged compared to diagenetically and tectonically flattened pumices which have pinched tips (Bull and McPhie, 2007). In low temperature pyroclastic deposits, the glassy fragments are replaced during diagenesis by phyllosilicates such as clay minerals (McPhie et al., 1993). The mechanically weak pumice fragments are easily eroded during the eruption and subsequent flow. Abrasion produces fine ash that contributes to the vast amount of ash in the so‐called co‐ignimbrite air fall (Sparks and Walker, 1977).
In the Dannemora inlier parts of the lower member contain slightly prolate‐shaped red to pinkish devitrified pumice fragments (Fig. 3B). However, welding produces oblate‐shaped fiamme and that would have been folded by subsequent tectonic deformation and not welding. Therefore the prolate‐shape is most truly due to tectonic deformation. Former glassy fragments in the upper member are now sericite aggregates but still show the primary cuspate‐ and Y‐shape, and thread‐
like delimitations (Fig. 6). The sericite replacement probably occurred during low temperature metamorphism. Due to the lack of spherulites in the upper member it is concluded that the deposition was distal, emplaced at low temperature and/or too thin to retain the heat during welding.
A B
Fig. 4. Microphotographs with crossed polarised light. A) Spherulites growing in the matrix of the pyroclastic deposit.
Scale bar is 0.5mm. B) Spherulites growing in the glass fragment of the pyroclastic deposit. Scale bar is 1 mm.
A B
Fig. 5. A) High temperature quartz and feldspar replacing pumice fragment with more coarse crystals compared to the matrix. Notice the compaction foliation in the pumice (left side of the black phenocryst), accentuated by grains coarser than the matrix. The coarse grains are due to devitrification. If the pumice is scrutinized numerous spherical spherulites can be discerned. Scale bar 1 mm. B) Micro photo of pectinate spherulites. Crossed polarised light, scale bar is 0.1 mm.
A B
Fig. 6. Sericite‐replaced glass fragments with preserved cuspate‐shaped delimitations, from the upper member. Crossed polarised light. A) Scale bar 0.5 mm. B) Scale bar 1 mm
Accretionary lapilli
Huge air suspended ash clouds are produced via fragmentation in and close to the conduit (Cashman et al., 2001, Legros and Kelfoun, 2000) and from elutriation of ash from PDC’s (Sparks and Walker, 1977). Water drops falling through the ash cloud start to accrete the ash to form aggregates, accretionary lapilli. Schumacher and Schmincke (1991) distinguished two types of accretionary lapilli;
The record of accretionary lapilli is the only firm evidence for pyroclastic fall deposits in the Dannemora inlier (Fig. 7). They have been found at one outcrop in the Dannemora syncline, located in the easternmost part of Gruvsjön. With a maximum diameter of 10 mm for the accretionary lapilli, the estimated ash cloud thickness was > 4 km (Gilbert and Lane, Fig. 15, 1994) and this requires subaerial eruption (Schumacher and Schmincke, 1991). Consequently the accretionary lapilli are in favour of subaerial eruption although the depositional environment was subaqueous, evident from layers with normal grading and water‐escape structures.
A
B
Fig. 7. A) Rim‐type (dark rim and light coloured core) accretionary lapilli which are the manifestation of pyroclastic fall deposit. Pencil for scale. B) Microphotograph of rim‐type accretionary lapilli. Scale is 1 mm and parallel polarized light.
sinks. The resulting deposit consists of pumice embedded in ash that originates from both air‐fall and attrition of the pumice (Fisher and Schmincke, 1984). As floating pumice is easily affected by wind and waves, the formation of a deposit with ash and pumice from the same eruption requires settling of pumice shortly after the eruption or in a constricted area such as a caldera or a lake that hinder the dispersion of the pumice. From subaqueous eruptions in oceans, floating pumice rafts has been reported to travel a great distance from the eruption site (e.g. Shane et al., 1998). Cold pumice absorbs water slower than hot pumice (Whitham and Sparks, 1986), because vacuum is created when the hot gas in the vesicles cools and consequently the surrounding sea water is sucked in to fill the empty space in the pumices and they get water‐logged and sink (McPhie et al., 1993).
North of Bennbo (Fig. 19) is a bed of phenocryst‐rich pumice blocks intercalated with ash‐siltstone (Fig. 8). This kind of deposit is an excellent marker bed in metamorphosed and folded metavolcanic rocks. The marker bed also gives a reliable younging direction, in this case grading from non‐layered ignimbrite to the mixture between pumice and ash‐siltstone. Such deposits are quite common in the Skellefte district in northern Sweden (Rodney Allen pers. com 2008).
Fig. 8. Phenocryst‐rich pumice blocks (grey) intercalated with ash‐siltstone (cream coloured), north of Bennbo. The layering is steep to vertical. Stratigraphic younging direction is towards east i.e. down in the figure. Hammer for scale.
Discussion and summary of volcanic textures
High abundance of phenocrysts indicates deposition from PDC in the lower member.
McArthur et al. (1998) concluded that in the margin or distally in the Garth Tuff, Wales, the spherulites were compact, spherical and smaller compared to the ones within the proximal parts.
The pectinate‐shaped spherulites occurred in non‐welded to slightly welded parts (Fig. 9). The spherical, small and compact spherulites in the Dannemora inlier consequently indicate a marginal
or distal deposit. Pectinate‐shaped spherulites are located in the uppermost part of the deposit (Fig.
9), hence in the E fold limb of the anticline, and relative above the spherical type, indicates a younging direction towards E. The lower member contain non‐flattened pumice fragments replaced by feldspar i.e. the deposits never got welded.
Within the upper member, the juvenile fragments still got their original cuspate shape and are replaced with sericite. In the highest level of the stratigraphy consists of marble intercalated with ash‐siltstone that lacks large phenocrysts, indicating air‐fall deposition of volcanic ash during the formation of the carbonate.
Fiamme in the Dannemora inlier got their shape from diagenetic and/or tectonic compaction. No preserved evidence for welding compacted fiamme has been recorded.
Accretionary lapilli, present in the W limb of the Dannemora syncline, formed during a subaerial eruption but were deposited subaqueously.
The pumice blocks bed was deposited subaqueous in an isolated sedimentary basin.
Fig. 9. Idealised profile of the Garth Tuff, Wales with the defined textural zones. The boundaries between zones are gradational. In Dannemora inlier spherical spherulites occur in the sparsely spherulitic zone, e.g. Fig. 4, and pectinate shaped spherulites in the uppermost part of the ignimbrite, e.g. Fig. 5B. Notice that lithophysae missing in the Dannemora inlier grow in the central part of the ignimbrite. (From McArthur et al., 1998)
Stratigraphy
Two synclines (Dannemora and Bennbo synclines) and an intervening anticline were interpreted by Lager (2001) (Fig. 19). The ore bearing Dannemora syncline in the E and the Bennbo syncline located c. 2 km westwards, both coincide with strong positive magnetic anomalies (Stålhös, 1991). The anticline west of Gruvsjön (Fig. 19) exposes the deepest part of the stratigraphy.
A correlation between the two drill cores, 276 and 286, used by Lager (2001) and the drill core, 494, that was drilled approximately 1 km southward and analysed in this study, would have been ideal.
However most of drill core 286 is missing, which makes it impossible to correlate the two borehole profiles.
A valid stratigraphy is vital to determine the position of the ore bodies in and the 3D structure of the Dannemora inlier. Lager (2001) divided the supracrustal succession into upper and lower formation, but the boundaries were not defined. The basis for the division of the stratigraphy into a large number of pyroclastic flow deposits is not specified (Lager, 2001), but colour variation is the most probable explanation. The numerous beds or layers of different deposits have not been possible to define during drill core logging and field work. The revised interpretation is based on textural changes, which are more reliable to separate different deposits than variation in colour. Earlier work by Lager (2001) was very detailed and he described and interpreted the stratigraphy as very variable with hundreds of “pyroclastic flow deposits or ignimbrites” (the quotation marks was used in Lager, 2001). Based on both field observation and logging of a c. 1240 m long drill core 494, that interpretation has been revised in this study.
The supracrustal succession here referred to as the Dannemora Formation (DFm), is divided into an upper and a lower member (Fig. 10). The members can be correlated with the two volcanic stages of the evolution of the Bergslagen region described by Allen et al., (1996): the lower member was deposited during the intense stage and upper member consequently during the waning stage. The lower boundary of DFm is not exposed and the highest level of DFm is exposed in the cores of the Dannemora and the Bennbo synclines. The lower member is subdivided into subunit 1 (lower) and subunit 2 (upper), each consisting of a flow deposit (bottom) and air‐fall deposit with accretionary lapilli (top) (Fig. 10). The partly missing accretionary lapilli deposit in subunit 2 is discussed below.
The thickness of the DFm is not stated in Lager (2001), but a rough estimate is 700‐800 m (cf. Plate 2 in Lager, 2001). Based on logging of a drill core 494 which penetrates the whole succession, the thickness of the DFm is here estimated to 600‐700 m. The starting point of the borehole is in the western limb of the anticline i.e. in the lower member (Fig. 10). The two recorded occurrences of accretionary lapilli are interpreted as the same bed, divided by the axial plane of the anticline (Fig. 10). If the occurrences instead represent two separate accretionary lapilli beds, the estimated thickness is at least 300 m larger and the lower member would have an additional subunit. Another complication is that subunit 2 of the lower member is > 400 m thick, but as the borehole goes through a parasitic z‐fold (cf. Fig. 4, Lager 2001), the true thickness is consequently < 400 m.
One key unit has not been recorded during logging of drill core 494; the accretionary lapilli beds that are interpreted as the top of subunit 2 of the lower member (Lager, 2001 and Allen et al., 1996). The borehole goes through the stratigraphic level with the accretionary lapilli bed about 450 m
horisontal distance SW of the outcrop with the same bed. A missing bed can be due to faulting, reworking or erosion. No fault breccias or shear zones were recorded during the logging of the drill core. And even if such structures would have been encountered, the true orientation of them cannot be achieved because there is no information about the borehole orientation available. Reworking and erosion of a deposit might be selective and also cause lateral differences along the beds.
Fig. 10. Stratigraphic column of Dannemora Formation based on drill core Bh 494 (left). The formation is divided into an upper and a lower member. Subdivision of the lower member is subunit 1 (lower) and subunit 2 (upper). Repetition of the lower member is due to folding and as the borehole starts in the western limb (right) it crosses the inferred axial plane.
Based on profiles drawn from mine plans and the assumption that the ore bearing layers are located in the uppermost part of the succession, the axial trace of the Dannemora syncline has been modified to be located 2‐300 m westward compared to the interpretation by Stålhös (1991).
In the W limb of the Bennbo syncline, there is an E‐W almost continuously exposed profile showing important details concerning stratigraphy, depositional environment and tectonic structures. The steeply dipping layers contain numerous sedimentary structures such as cross bedding, water‐
escape structures, soft‐sediment folding and erosion channels (Fig. 11). These structures all show an unambiguous stratigraphic younging direction towards E. Water depth must have varied during deposition as the grain size ranges from silt to coarse sand. The main source of sediments is primary
Based on the amount of barium, here c. 8 wt‐percent, the hyalophane formed during diagenesis (Essene et al., 2005). It is not known if the occurrence of hyalophane linked to the formation of the BIF. However numerous occurrences of coexisting Fe‐Mn‐mineralisations and hyalophane have been described e.g. Lillsjön, Bergslagen, Sweden (Lundström and Wadsten, 1979), Jakobsberg, Bergslagen, Sweden (Igelström, 1867) and Cuyuna Iron Range, Minnesota, USA (McSwiggen et al., 1994).
A B
Fig. 11. A) BIF with soft‐sediment deformed vertical dipping beds, both within the bed and at the upper boundary. N is to the left and younging direction upwards in the picture. Horisontal surface, younging direction is up in the figure and width of view c. 1 m. B) Erosion channel (beige, upper part) cutting down into a reworked layered ignimbrite, with white devitrified pumice fragments. Vertical dipping beds and younging direction is up in the figure. Pencil for scale.
The occurrences of epidote fragments (Fig. 12) were interpreted to indicate marine deposition and also change of depositional sequence (Lager, 2001). They were considered to represent pieces of carbonate lithoclasts replaced by epidote in the pyroclastic flow (Lager, 2001). We suggest that they represent pumice fragments that acted as traps for fluids during alteration and metamorphism. This interpretation is based on the fact that the fragments have the same texture as the matrix including quartz phenocrysts. Epidotisation occur almost exclusively in the vicinity of greenstone dikes, i.e. the alteration might be related to the intrusion event or the regional metamorphism of the dikes and the metavolcanic rocks.
A B
Fig. 12. Epidote fragment from the lower member A) Drill core. Notice the quartz phenocrysts within the fragment.
Scale = top to bottom 3 cm. B) Outcrop. Most obvious epidote fragment is lower left. Black pencil for scale.
Summary of stratigraphic results
Dannemora Formation is divided into a lower and the upper member. The lower member is subdivided into two subunits. Total thickness is approximately 600‐700 m. However with respect to folding, this approximation is a maximum value.
The division into hundreds of beds recorded by Lager (2001) was probably based on colour variations. This division is not supported by this study, as textural changes are a more reliable feature when dividing a succession into units and beds. However, the depositional sequences (Lager, 2001) can to certain degree be correlated to member 1 and 2.
Epidote fragments are not former carbonate lithoclasts that indicate marine deposition. Texturally these fragments are pumices. The pumices acted as traps for hydrothermal fluids during dike intrusion and/or during regional metamorphism.
Geochemistry and metamorphism
Greenstone dikes
Numerous greenstone dikes have intruded the rocks in the Dannemora inlier and surrounding metagranitoids. The greenstone dikes are mainly greenish grey to greyish green, very fine grained and dominated by epidote and chlorite with minor amounts of amphiboles and sphene. The opaque phase consists of euhedral, scattered < 1 mm pyrite grains.
Seventeen greenstone dikes have been analysed for major and trace elements. In a TAS diagram, (Fig. 13) they plot as basalt to picrite. In the Nb/Y vs. Zr/TiO2 discrimination diagram they plot as sub‐
alkaline basalts (Winchester and Floyd, 1977).
A B
Fig. 13. A) TAS‐diagram, the greenstone dikes (n=17) plot as picrite to basalt (boundaries after Le Maitre, 1989).
B) Discrimination diagram Nb/Y vs. Zr/TiO2 of Winchester and Floyd (1977). All analyses, except one, plot as sub‐alkaline basalt.
The Spider diagram (Fig. 14) of rock/primodial mantle shows that LILE (large lithophile elements), i.e.
Ba to K, with one exception, are enriched relative to HFSE (high field strength elements), and that Nb and Ta are depleted. REE normalized to chondrite are enriched in the LREE compared to HREE that levels out below 10x the chondrite values, and a small, mainly positive peak in europium with Eu/Eu* = 0.91‐1.29. The latter indicates that plagioclase was not part of the crystallisation.
A B
Fig. 14. A) Spider diagram of rock normalised to primordial mantle. A general enrichment of LILE compared to HFSE, depletion of Nb and Ta. Primordial mantle values after McDonough and Sun (1995). B) REE in rock analyses normalised rock to chondrite, with enrichment in LREE compared to HREE. Eu has a slight positive peak. Chondrite values after McDonough and Sun (1995).
In a Y‐La‐Nb discrimination diagram (Fig. 15) the analyses plot in the calc‐alkali basalt field (1A) and continental basalt field (2A). Analyses that plot away from the Nb apex are typical for subduction related environment or continental crust contamination (Wilson, 1989). The cluster in the calc‐
alkaline field is in favour of subduction environment. However in Ti‐Zr‐Y discrimination diagram (Pearce and Cann, 1973) the analyses scatter mainly as WPB (n=8) and in the mixed field B (n=5).
Stephens et al., (2009) concluded that mafic metavolcanic rocks from Bergslagen region, plot in fields B, C and D in a Zr‐Ti‐Y diagram (cf. Fig. 13), i.e. some of the analyses are classified as calc‐
alkaline formed in an active continental margin environment.
A B
Fig. 15. A) Y‐La‐Nb discrimination diagram of Cabanis and Lecolle (1989), mainly discriminate the analyses as calc‐alkali basalts (1A) and continental basalts (2A). 1C is volcanic‐arc tholeiites and 1B transition between 1A and 1C, 2B is back‐
arc basin and field 3 is oceanic basalts. B) Ti‐Zr‐Y discrimination diagram of Pearce and Cann (1973), the analyses scatter within all fields except A. A = Calc‐alkali basalts, B = Ocean‐floor basalts, with low‐K tholeiites and calc‐alkaline basalts, C = Low‐K tholeiites, D = WPB.
Based on the relation between Ta/Yb and Th/Yb (Fig. 16), the tectonic environment for the analysed greenstone dikes scatter within the field defined for continental arc basalt (CAB), within active continental margin, and within‐plate volcanic zone (WPVZ), i.e. a back‐arc basin environment (Gorton and Schandl, 2000).
Yttrium can be highly mobile during alteration and weathering resulting in depletion (Hill et al., 2000), consequently the sampling here was focussed on fresh and unaltered parts of the greenstone dikes. The mobility and depletion of Y due to alteration should be detected in a Zr‐Ti‐Y‐diagram (Fig.
15) and if altered the analyses plot away from the Y apex and outside the defined field. This is possible to observe even for weakly altered samples (cf. Hill et al., 2000). Mobility of both Zr and TiO2 should be revealed in the same way. The conclusion is that Y, Zr and TiO2 is reliable in tectonic setting diagrams. It should be mentioned that fractionation can result in plots indicating false tectonic environments. The low Mg# ranging from 25 to 47 is an indication of fractionation.
Fig. 16. Discrimination diagram based on Ta/Yb vs. Th/Yb. The analyses scatter within active continental margin and within‐plate volcanic zone. MORB = Middle ocean ridge basalt, SHO = Shoshonite, CAB = Continental Arc Basalt, IAT = Island Arc Tholeiite. (Combined diagram of Gorton and Schandl (2000) and Pearce et al., (1981))
The Bergslagen region has been suggested to be an extensional continental back‐arc region (Allen et al., 1996). The scatter of the analysed greenstone dikes from Dannemora from CAB to WPVZ could be interpreted as a similar change of tectonic setting, as described by Pearce et al., (1981), i.e. that dikes intruded both during a back‐arc spreading episode and an arc episode, evident in changes in geochemical affinity. In addition, the ratio between subduction zone component and MORB component can change over time (Pearce and Peate, 1995). The influence of subduction zone component decreases when the back‐arc region “matures” into back‐arc spreading (Pearce et al., 1981). The depletion of Nb and Ta could be ascribed to subduction related processes (Kelemen et al., 2003).
Metamorphic conditions
The greenstone dikes consist of actinolite + chlorite + epidote ± hornblende ± albite i.e. typical greenschist facies mineralogy for a basaltic composition. In the rhyolites, prograde muscovite and chlorite coexist along the tectonic fabric, which indicate that the metamorphic grade never reached amphibolite facies (Winkler, 1979) in the Dannemora inlier. Deformation mechanisms in the ÖSZ (see later text) give temperature ≤ 500°C (Passchier and Trouw, 2005). These findings show that the metamorphism never exceeded greenschist facies in the Dannemora inlier, but also that the previous estimate of lower greenschist facies conditions (Lager, 2001) was too low.
Summary of geochemistry and metamorphism
The greenstone dikes from Dannemora inlier and surroundings are geochemically classified by minor elements as subalkaline basalts. Tectonic environment diagrams classify these subalkaline greenstone dikes as arc‐basalts with continental affinity to within‐plate volcanic zone. The trough in Nb/Ta also indicates a subduction related tectonic environment, i.e. an active continental margin.
On the basis of both the mineralogy in the greenstone dikes and the deformation mechanisms in deformed rocks, the metamorphic temperature was ≤ 500°C i.e. uppermost greenschist facies conditions.
Tectonic structures
The intrusion of the early orogenic granitoids has previously been suggested to result in the isoclinal folding and steeping of the supracrustal rocks in the area as well as in Bergslagen in general (e.g.
Stålhös, 1991). However many studies have shown that granitic magma generally crystallises as tabular rather than diapir‐shaped bodies (e.g. McCaffrey and Petford, 1997 and Brown, 1994). Also the lack of metamorphic aureoles around the granitoids contradicts large diapiric bodies. Already Mahon (1988) concluded that granitoid diapirism would suffer “thermal death” and lock up in the middle crust. Therefore the tight (F1‐) folding in the supracrustal inliers in the Dannemora area (and Bergslagen) must be due to subsolidus deformation affecting these rocks and the surrounding granitoids. The regional tectonic deformation affected both lithologies and occurred after the intrusion of the granitoids. Stephens et al., (2009) concluded the 1.87‐1.84 Ga GSDG suite intruded under transtensional regime of regional scale dextral strike‐slip structures.
It is vital in any study of a structural evolution to identify the number and character of foliations. In this study, three tectonic foliations (S1, S2 and S3) have been recorded in the metasupracrustal rocks in the Dannemora inlier. Tectonic foliation only localised to faults (Lager, 2001) is thus not supported. A direct correlation of structures with earlier studies in the area (Stålhös, 1991) is not possible as that work did not separate bedding from tectonic foliation in the supracrustal rocks. This excludes the use of the published data for bedding cleavage analysis to define large‐scale folds, but the differentiation of rock types, intense foliation etc on the map of Stålhös (1991) help outlining the general trends of the different rocks and also some large‐scale folds. In the ongoing work we integrate this data with bedding/cleavage analysis to define F1 and F2 folds. Another important task is to relate shape and localisation of ore/mineralisation to the structural elements. It is implicit that the iron ore is concordant to S0/S1 but also affected by subsequent folding and the development of the pronounced lineation. However, the sulphide mineralisation in Svavelgruvan is obviously discordant to S0/S1 (Fig. 51 in Lager, 2001) and therefore most likely structurally controlled.
The record of one major and some smaller ductile shear zones affecting both the plutonic and the supracrustal rocks is the most significant new structural element for the interpretation of the structural geometry of the Dannemora area. If displacement distances are large this may affect also the interpretation of the stratigraphy as previous interpretations are based on a continuous stratigraphy without tectonic breaks.
The absence of layering and the less variable character of the tectonic foliations within the metagranitoids make the definition of foliations less of a problem. HOwever the correlation of foliation(s) in the plutonic and supracrustal lithologies is less obvious and important for the structural interpretation.
Metavolcanic rocks
S1 is a continuous cleavage defined by sericite/chlorite and is in most cases parallel to sub‐parallel to
However, meso‐scale isoclinal F1‐folds with S1 defining the axial planar foliation and verified by refolding by F2 are only occasionally observed in the Dannemora inlier like e.g. in the Garpenberg area (Allen et al., 2003) and the entire Bergslagen region. The large‐scale isoclinal F1 folding of ore bearing stratigraphy envisaged by mining is thus not reflected by frequent meso‐scale structures.
This discrepancy is probably significant for the folding mechanism and the structural evolution.
In the Bennbo area (Fig. 19) there is a 100m scale tight F1 fold. The bedding cleavage relationship between S0 and S1 is reversed in the limbs and way up shifts from E in the western limb to W in the eastern limb. The constructed fold axes is sub‐parallel to the measured lineations in the area (Fig. 17)
S2 is a grain shape fabric, which is most distinct in rocks with quartz phenocrysts, but also present in massive metavolcanic rocks in which S2 results in kinking of S1. The orientation of S2 is mainly WNW‐
ESE and it has a distinct angle to S0 i.e. to the main strike direction of the Dannemora syncline and shear zones (Fig. 19). Within layered varieties of the metavolcanic rocks S2 is related to asymmetric meso‐scale F2‐folds. The pronounced lineation in the area is sub‐parallel to the axes of these folds hence the lineation is referred to as L2.
Parasitic F2‐folds on the geological map (Fig. 19) are derived from different sources. In the Dannemora syncline the z‐fold is traced from a folded accretionary lapilli bed from the mine maps (Lager, 2001). The shape of the z‐fold in the anticline is based on bedding measurements and that fold closure is > 400 m wide.
A B
Fig. 17. A) Stereonet displaying poles to S0 (filled circles, n=24), S1(open squares, n=13) and S2(open triangles, n=10). B) All measured lineations in Dannemora inlier (diamonds, n=30) with mean vector (blue circle displaying the scattering interval) and S1 (open squares, n=13) and the cylindrical best fit (red great circle) for S1.
The relationship between F2‐folds and thin (< 1 dm wide), ductile shear zones (Fig. 18) indicates that the latter formed during or after D2. The strong dextral shear of WNW‐ESE, steeply dipping, greenstone dikes was developed during D2 (Fig. 18). A moderately to steeply plunging lineation (L2), resulting from strong mineral stretching and/or intersection between S1/S2 is parallel to sub‐parallel to the axes of F2. The stretching related to L2 has most probably affected the distribution and shape
Approximately N‐S striking greenstone dikes truncating the bedding show L2 but are also boudinaged. If L2 and the boudinage developed simultaneously, this implies transpressive deformation during that event.
A B
Fig. 18. A) Subhorizontal surface showing reworked, graded bedding in steep dipping metavolcaniclastic rock. A possible F1‐fold (highlighted by hatched line) is refolded by an F2 fold with a grain‐shape fabric, axial planar S2. F2 is truncated by a thin dextral shear zone. All structures are cut by the local S3 cleavage (parallel to string of compass; mirror pointing N).
B) Greenstone dike with dextral S‐C' fabric (WNW towards left).
S3 has only been found locally. It is a moderately dipping, spaced cleavage truncating S1, S2 and F2 (Fig. 18), and the significance of this foliation is not yet established. S3 is striking 190‐230° which is roughly the strike of the main shear zone east of Dannemora syncline, but the foliation has a gentler dip. Table 1 shows a summary of structures in the Dannemora inlier.
Table 1: Summary of structures in the Dannemora inlier
Geological event Foliation Lineation Fold Phase
Deposition of the supracrustal rocks and intrusion of
granitoids
S0 Syn‐sedimention
folds
Extension
(?) Intrusion of dikes (?)
Extension?
D1 S1
continuous, defined by sericite/chlorite
and parallel with axial plane to F1
Possibly horisontal and parallel to fold
axis to F1
F1 Isoclinal, N‐S striking and with steeply dipping axial
plane
Compression
(?) Intrusion of dikes (?)
Extension?
D2 S2
GSF, spaced and most pronounced in
rocks with phenocrysts
L2 Stretching‐ and
intersection lineation between
S1/S2
F2 Asymmetric, z‐folds
Compression:
formation of folds and shear zones
D3 S3
local
Compression
Metagranitoids
The metagranitoids surrounding the Dannemora inlier share a more or less pronounced lineation with the supracrustal rocks (this study and Stålhös, 1991) suggesting that it corresponds to L2 in the latter. The steep foliation to the W of the magmatic contact to the supracrustal rocks is partly at a high angle to the general NNE‐SSW trend of the rocks of the inlier and parallel to the strike of S2 in the latter. These patterns indicate that the granitoids and the rocks of the inlier were affected by approximately N‐S shortening during D2 and that the foliation subsequently rotated into the shear
Fig. 19. Top ‐ Map of the Dannemora inlier. Inferred F1 synclines (X) and anticline (<>) with asymmetric F2‐fold.
Simplified map from 12I Af161 Östhammar NV (Stålhös, 1991). Bottom – A schematic NW‐SE profile across the
Dannemora inlier as follows from A to B: metagranitoid in the west with “baked” contact to the metavolcanic rock. Next is the Bennbo syncline with marble recorded only in the western fold limb and the anticline west of Gruvsjön (see top figure). The contact between the metagranitoid and the metavolcanic rock west of the Dannemora syncline is sheared with east‐up movement. In the deepest part of the syncline there is a granitoid intrusion with skarn‐envelope, known from drill core logging. The relationship between this intrusion and the one immediately to the west is unknown. The Dannemora syncline consists of intercalations of marble and metavolcanic ash‐siltstones. The regional scale shear zone (ÖSZ, see below) defines the contact to the metagranitoid in the east.
zone. Alternatively the shear zone is an earlier structure that has been rotated clockwise by folding, as suggested by Persson and Sjöström (2003).
Shear zones
The northwards extension of the Österbybruk‐Skyttorp zone, ÖSZ, (Persson and Sjöström, 2003) has been identified east of Lake Filmsjön. The ÖSZ belongs to a shear zone system splaying off from the Singö Shear Zone in the north (Persson and Sjöström 2003). The system is folded (rotated anticlockwise) and continues E‐W along the supracrustal rocks including the Ramhäll ores, ca. 12 km S of Dannemora. This folded shear zone system makes up the western and southern boundary of a major tectonic lens of metagranitoids. Its eastern boundary is outlined by the Gimo Shear Zone (GZ) Persson and Sjöström, 2003). The ÖSZ and the GZ appear distinct in regional formlines pattern (Fig. 20) (Bergman et al., 1996).
The steep ÖSZ truncating the inlier east of Lake Filmsjön is at least 500 m wide and affects granitoids, rhyolites and basalt (cf. Stålhös, 1991). The lineation in the mylonites is often steeper (more down‐dip) than in the surrounding, less deformed rocks. The foliation in the granitoids rotates clockwise approaching the shear zone truncating the eastern margin of the inlier. In that part it is parallel to the general trend of the rocks of the inliers. To the E of the shear zone, the foliation in the metagranitoids rotates anticlockwise from NNE‐SSW to E‐W in an at least c. 3 km wide zone (Fig. 20).
The latest and dominating ductile movement of the shear zone east of Lake Filmsjön is E‐up, which is in agreement with earlier work to the S (Engström 2001, Persson and Sjöström, 2003). Hence the granitoids to the E have been uplifted relative to the inlier. Ductile shear structures have also been recorded underground in the mine at 175 m level. Way up structures towards NW show that this shearing occurred in the eastern limb of the Dannemora syncline.
A revised excerpt from a master thesis by Linn Björbrand describes the microtextures of the ÖSZ (Appendix 1).
Fig. 20. Map of formlines and interpreted deformation zones in NO Uppland (Bergman et al., 1996).
The mine map of Konstängsfältet shows, that seven drill cores truncated “skölzoner”. As ductile shear zones are apt to be reactivated it is very likely that at least some of these semi‐brittle shear zones (“skölzoner”) formed by reactivation. Therefore, the ductile shear zone system in the eastern limb of the Dannemora syncline probably influenced the location and orientation of the subsequently developed semi‐brittle system. Such repeated activity has been recorded along the shear zone between Skyttorp and Vattholma (Engström, 2001).
Also the tabular granitoid W of the Dannemora syncline and S of Gruvsjön is mylonitised (along both boundaries) and shows again E‐up kinematics. In addition, the tabular granitoid at Film, possibly the same as at Dannemora, is strongly deformed at its western margin. This repeated structural pattern suggests that E‐up shear zones significantly affect the eastern part of the Dannemora inlier, including the Dannemora ore bearing syncline. Also in Garpenberg in northern Bergslagen (Allen et al., 2003) and Viker‐Älvlången in the southwest (Stephens et al., 2001), there are approximately NE‐SW striking shear zones truncating the eastern limb of the ore bearing synclines.
The regional consistency of shear zones probably affected the local distribution of ore bodies but may also have contributed to the unusual fold geometries and their relation to the pronounced, often steep lineations typical of Bergslagen.
Shear zone displacement
The lack of marker horizons on each side of the ÖSZ makes it impossible to get a direct measurement of the displacement during ductile shear. However, it is possible to get an estimate by comparing the mineralogy and in particular the mineral chemistry in similar rocks across the shear zone. The titanium content and Fe/Fe+Mg in biotite (Henry et al., 2005) in comparable granodiorites complemented by plagioclase‐amphibole thermometry in dikes on each side will be applied to give at least a crude estimate. A study of the dikes seems most promising in this context as they have greenschist facies mineralogy in the inlier (see next paragraph) and a more amphibolitic field appearance in the granitoids to the E of the ÖSZ.
An eastwards increase in metamorphic grade is indicated in the study by Persson and Sjöström (2003) showing that the shear zone related mineralogy in the GZ indicated higher metamorphic grade than that of the ÖSZ. In addition, to the SE, in the Norrtälje synform, there are migmatites and anatectic granitoids (Stephens et al., 2007). This pattern, and the thermobarometry result by Sjöström and Bergman (1998) indicate an increase in metamorphic grade both to the NE (the Singö shear zone) and SE. Combined with the suggestion of a west‐vergent fold front (Stålhös, 1991) or thrust (Sjöström et al. 2008) in the Norrtälje synform, structurally controlled metamorphic variations in NE Bergslagen may therefore exist although metamorphism outlasting deformation is a general feature (Allen et al., 1996). This contradiction may reflect that Bergslagen has been affected by two metamorphic episodes as suggested by Andersson et al., (2006).
Summary of tectonic structures
It is vital in any study of a structural evolution to identify the number and character of foliations. In this study, three tectonic foliations (S1, S2 and S3) have been recorded in the metasupracrustal rocks in the Dannemora inlier. Tectonic foliation only localised to faults (Lager, 2001) is thus not supported.
The at least 500 m wide continuation of the ÖSZ affecting both the plutonic and the supracrustal rocks along the E limb of the Dannemora syncline is the most significant new structural element.
S1 is a sericite/chlorite defined continuous cleavage, in most cases parallel to sub‐parallel to S0, i.e.
steep and strikes N‐S to NNE‐SSW.
S2 is a grain shape fabric, which is most distinct in rocks with quartz phenocrysts, but also present in massive metavolcanic rocks in which S2 results in kinking of S1.
S3 has only been found locally, is spaced and has a moderately dip.
A moderately to steeply plunging lineation (L2), resulting from strong mineral stretching and/or intersection between S1/S2 is parallel to sub‐parallel to the axes of F2. The stretching related to L2 has most probably affected the distribution and shape of the ore bodies.
Dannemora inlier in the regional tectonic framework
The regional tectonic framework surrounding the Dannemora inlier is fairly well established and defines boundary conditions for the evolution also within the inlier. The northern margin of Bergslagen region, although its location at the present erosion level is discussed, probably coincides with crustal scale tectonic boundaries. The Singö shear zone (SSZ) in the NE was pervasively deformed at 1.86‐1.85 Ga and shows evidence of localised deformation 1.83‐1.80 Ga (Hermansson et al., 2008a; Hermansson et al., 2008b). Högdahl et al. (2009) suggested that the SSZ and the Gävle‐
Rättvik zone (GRZ) (Tirén and Beckholmen 1990, Stephens et al., 1997) are both parts of a coherent arcuate structure defining the northern boundary of the Bergslagen region. In addition, it was suggested that this boundary is related to structures in the middle and lower crust and possibly represents a terrane boundary, which is in accordance with the interpretation that a terrane or plate boundary exist in this part of the Fennoscandian Shield (Lahtinen et al., 2005).
In a local study Persson and Sjöström (2003) identified the ductile shear zones in the in Österbybruk‐
Skyttorp and Gimo areas, the ÖSZ and GZ, respectively, and interpreted these as enveloping a large scale tectonic lens or lenses consisting of granitoids. Steeply plunging east‐verging folds (F3) refolded this patterns and the strong stretching lineation (L2). Several such east‐verging large‐scale folds exist to the south of the GRZ‐SSZ indicating a common evolution including dextral shear. Högdahl et al., (2009) suggested that that there was a progressive evolution from dextral shear at 1.87—1.86 Ga to pure shear along the GRZ‐SSZ and that this resulted in shift of activity northwards to the Hagsta Gneiss Zone (HGZ) at 1.81 Ga. Such a progressive evolution is consistent with c. N‐S shortening and (growing) regional transpression. Also the large‐scale fold structure of western Bergslagen, with a clockwise rotation of lithologies approaching the GRZ is consistent with such an evolution.
All these structures are mainly related to shortening of the crust, i.e. the evolution after the extension of the continental crust that was related to the back‐arc magmatism and ore formation. It is implicit that this extensional period generated large‐scale (listric?) faults that, despite voluminous intrusion of granitoids, represented crustal anisotropies localizing strain (e.g. by fault inversion) during subsequent crustal shortening. Inversion of extensional faults during significant shortening could be expected to result in thrusts rotating to steep reverse shear zones like the ÖSZ.
Discussion and conclusions
The supracrustal succession is here referred to as the Dannemora Formation and it is divided into a lower and an upper member, and the former is subdivided into subunit 1 and 2. The deposition environment is not as variable as suggested by Lager (2001). And the 1000’s of thin beds recorded by Lager (2001) was probably based on colour variation rather than textural variations. The occurrence of epidote lithoclasts, interpreted as former carbonate fragments, as an indication of marine environment (Lager, 2001) is not supported by this study, because these epidote fragments texturally resemble pumices. In addition the epidotisation seems to be related to the intrusion of dikes and/or regional metamorphism.
Depositional temperature and processes were different in the lower and the upper member. The lower member contains small, scattered, spherical and pectinate spherulites, indicating distal or marginal parts of a non‐welded to slightly welded pyroclastic flow deposit. The upper member never underwent any welding, which is evident by preserved bubble‐walled and cuspate former glass shards that were later replaced by sericite.
The recorded fiamme were flattened due to diagenetic and/or tectonic deformation.
The size of the accretionary lapilli in Dannemora inlier is in favour of subaerial eruption (Schumacher and Schmincke, 1991) although the depositional environment was subaqueous, evident from layers with normal grading and water‐escape structures.
Beds with pumice blocks indicate subaqueous deposition in an isolated basin. Floating pumice is easily dispersed and, for such a deposit to form, it has to be emplaced in an isolated basin or if the pumice blocks sank instantaneously, after eruption close to the eruption site.
Based on the geochemistry of the greenstone dikes, the tectonic setting was as the transition between ACM and WPVZ, i.e. a subduction zone and a back‐arc basin. The greenstone dikes are classified as subalkaline, generally with a continental affinity. The strong deformation of certain greenstone dikes indicates that they intruded before D2. Different cross cutting greenstone dikes have been affected by folding or formation of boudinage, i.e. their original orientations were different.
The interpretation that tectonic foliations are only related to faults (Lager, 2001) can be rejected, as three different foliations except the bedding, has been identified.
The boundary between the metagranitoids along the western margin of the Dannemora inlier and the metasupracrustal rocks is magmatic, ~500 m wide, and most of the recorded contacts elsewhere show evidence of tectonic deformation.
The eastern limb of the Dannemora ore‐bearing syncline is truncated by a major shear zone with E‐up kinematics during the latest ductile deformation. This shear zone represents the continuation
