JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, B11307, doi: /2011jb008643, 2011

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2011jb008643, 2011 Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetite skarn deposits Alireza Malehmir, 1 Peter Dahlin, 1 Emil Lundberg, 1 Christopher Juhlin, 1 Håkan Sjöström, 1 and Karin Högdahl 1 Received 30 June 2011; revised 22 August 2011; accepted 26 August 2011; published 30 November [1] The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora mining area. The interpretations are constrained by seismic wave velocity measurements on a series of rock samples, cross dip analysis, prestack time migration, and swath 3 D imaging, as well as by other available geophysical and geological observations. A series of major fault zones is imaged by the seismic data, as is a large mafic intrusion. However, the most prominent feature is a package of east dipping reflectors found east of the Dannemora area that extend down to at least 3 km depth. This package is associated with a polyphase, ductile brittle deformation zone with the latest ductile movement showing east side up or reverse kinematics. Its total vertical displacement is estimated to be in the order of 2.5 km. Also clearly imaged in the seismic data is a steeply dipping reflector near the Dannemora mine that extends down to a depth of at least 2.2 km. The geological nature of this reflector is not known, but it could represent either a fluid bearing fault zone or a deep seated iron deposit, making it an important target for further detailed geophysical and geological investigations. Citation: Malehmir, A., P. Dahlin, E. Lundberg, C. Juhlin, H. Sjöström, and K. Högdahl (2011), Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetiteskarn deposits, J. Geophys. Res., 116,, doi: /2011jb Introduction [2] Integration of reflection seismic data with other available geophysical and geological information has been proven to be instrumental in providing key information on subsurface structures in the Scandinavian Precambrian crystalline crust [e.g., Malehmir et al., 2006, 2007; Juhlin and Stephens, 2006]. Such an approach to exploration has led to delineation of major lithological structural boundaries and structural discontinuities [Tryggvason et al., 2006; Malehmir et al., 2009a; Dehghannejad et al., 2010; Malehmir and Bellefleur, 2010]. Analysis of regional scale reflection seismic profiles in a geologically similar Bergslagen region (Figure 1) is likely to result in better understanding of the geometry of major crustal structures as well as of the mineralization potential at depth. Although 3 D seismic data would be ideal for these purposes, they are infrequently acquired due to the high cost and limited access. 3 D surveys are favored because, in a few places where have been conducted over crystalline bedrock, they have provided a basis for detailed exploration and mine planning [e.g., Milkereit et al., 1996, 2000; Adam et al., 2003; Malehmir 1 Department of Earth Sciences, Uppsala University, Uppsala, Sweden. Copyright 2011 by the American Geophysical Union /11/2011JB and Bellefleur, 2009]. However, 2 D seismic surveys confined to existing roads or forest tracks are often conducted prior to 3 D acquisition. [3] The shortcomings of 2 D seismic surveys, unless a dense network of surveys that cross each other exists, are mostly reflected in both inability to provide a complete picture of subsurface structures and ambiguity about the true geometry of the structures that were imaged. These shortcomings of 2 D data are less problematic if surveying is done on crooked lines, where source receiver midpoints are scattered around the seismic profile. In this situation, additional geometrical information about the reflections can be extracted through specialized analysis and processing of the data. For example, information about the out of the plane nature or, occasionally, about the true geometry of reflections can be obtained [e.g., Larner et al., 1979; DuBois et al., 1990; Bellefleur et al., 1995; Nedimović and West, 2002, 2003a]. Only a few published accounts make use of crookedline 2 D surveys to obtain information about the true geometry of reflections [e.g., Nedimović and West, 2003b; Malehmir et al., 2009b]. This is mainly due to the very narrow aperture of the 2 D crooked line data that normally does not allow for successful semi 3 D imaging. [4] In this study, the northeastern part of the Bergslagen region (Figure 1) has been investigated. This part was mapped at a 1:50,000 scale by the Geological Survey of 1of21

2 Figure 1. (top) Distribution of metallic mineral deposits and their close spatial relationship to Svecofennian, metavolcanic, subvolcanic, and metaintrusive rocks ( Ga). Inset in the top left corner shows the location of the general area of interest relative to Sweden. Dashed box shows the location of the study area. (bottom) Conceptual tectonic model of the Bergslagen region. Modified from Stephens et al. [2009]. 2 of 21

3 Figure 2. (a) Geological and (b) total field aeromagnetic maps of the study area see Stephens et al. [2009]. In Figures 2a and 2b, black and red lines, respectively, show the location of seismic reflection profiles 1 and 2 and those acquired at the Forsmark site [Juhlin and Stephens, 2006]. CMP lines are shown using blue color. Dashed line in Figure 2a shows the inferred location of the Österbybruk Deformation Zone (ÖDZ) near the Dannemora mine. Reflections observed from profiles 1 and 2 are projected to the surface, annotated, and shown in Figure 2b. Locations 1, 2, and 3 (see Figure 2a, white circles outlined by black) are discussed in section 3. Data provided by the Geological Survey of Sweden. Sweden [Stålhös, 1991], and regional compilations provide additional and valuable information on the structural characteristics [Bergman et al., 1996; Antal et al., 1998; Stephens et al., 2009]. [5] Hitherto, no long (>20 km) land reflection seismic profile has been acquired in the Bergslagen region. Such seismic profiles have proven to be a powerful tool to screen the crust, and have been acquired routinely in major mining areas of the world to better understand the crustal architecture of mineralized belts [e.g., Pretorius et al., 1989, 2003; Stevenson et al., 2003; Malehmir and Bellefleur, 2009; Cheraghi et al., 2011]. However, shorter seismic profiles have been acquired in the region that focuses on the Swedish nuclear waste repository site at Forsmark (Figure 1), located in an about Ga old, high strain belt [e.g., Juhlin and Stephens, 2006]. At greater distances from the Forsmark site, the geological structures are poorly understood at depth and limited to a wide angle reflection seismic profile [Law and Snyder, 1997; Korja and Heikkinen, 1995] recorded offshore by the BABEL project (Figure 1, BABEL line 7). BABEL line 7, spatially the closest regional reflection seismic profile to the Bergslagen region, shows a major crustal scale reflection package that dips toward the east and extends down to depths greater than 35 km, possibly down to the Moho discontinuity. Law and Snyder [1997] interpreted the package to originate from a zone of ductile brittle mylonites, related to the Singö Shear Zone (Figure 1). The detailed seismic reflection study by Juhlin and Stephens [2006] did not image such a zone at the Forsmark site, possibly due to the steep northeasterly dip of the Singö Shear Zone near the surface. Drilling results and recent work indicate that lithological contacts in this zone are subvertical (M. Stephens, personal communication, 2011). [6] In this study we focus on the top 3 km of upper crust with the main objectives of (1) imaging major geological features such as lithological boundaries and main deformation zones using two nearly perpendicular, about 25 km long reflection seismic profiles (Figure 2, profiles 1 and 2) and 3 of 21

4 with integration of available geological and geophysical information, and (2) constraining the 3 D geometry of the Dannemora iron ore body and immediate structures affecting the ore body and the host rock (Figure 2). We show how an additional component of about 50 source points in the downdip direction (a priori) has allowed treating the 2 D data in three dimensions and imaging, at depth, of what appears to be the Dannemora iron ore body and a smaller and partly mined mineralization (Diamant 2). Seismic data analysis has also resulted in images of a series of major deformation zones that are important for understanding the tectonic evolution of the study area. 2. Geological Setting 2.1. The Bergslagen Region [7] The Bergslagen region in south central Sweden is one of three major ore producing areas in Sweden (Figure 1). With more than 1000 years of continuous mining and more than 6000 known mineral occurrences, the region is historically considered the most prosperous ore district in the country [Allen et al., 1996; Stephens et al., 2009]. The Bergslagen region contains a diverse range of mineral deposits including banded iron formation (BIF), manganiferous skarn and carbonate hosted iron ore, rare earth elements (REE) deposits, tungsten (W) skarn deposits, apatite bearing iron ore, and volcanic hosted massive sulphide (VHMS) deposits [Allen et al., 1996; Stephens et al., 2009]. Currently, three polymetallic base metal sulphide deposits are being mined: Garpenberg, Zinkgruvan, and Lovisa (Figure 1). Abundant geological, geophysical, geochemical, and geochronological data are available from investigations at or near the ground surface [e.g., Stephens et al., 2009]. In addition, numerous petrophysical measurements such as density and magnetic susceptibility are available for different rock types at the Geological Survey of Sweden [e.g., Stephens et al., 2009]. [8] The Bergslagen region (Figure 1) was formed inboard of an active continental margin in a back arc tectonic setting characterized by extension and magmatism that has been overprinted by ductile deformation and metamorphism related to crustal shortening and tectonic switching (Figure 1) [Allen et al., 1996; Hermansson et al., 2008a]. The structural evolution is complex and may include structural inheritance and fault/shear zone inversion. An early orogenic Ga intrusive suite ranging from granite to gabbro dominates the lithology and typically contains narrow inliers of frequently mineralized Ga Svecofennian metavolcanic rocks [Stephens et al., 2009]. Clastic metasedimentary rocks are common in the southeastern part of the region where the rocks are of higher metamorphic grade. A younger, subordinate Ga intrusive suite of granite and pegmatite [Stephens et al., 2009] mainly exists in the central part of the region. Profile 2 (Figure 2) crosses the boundary of one of the granites [Stephens et al., 2007] The Dannemora Area [9] In the Dannemora area (Figures 1 and 2) about 25 mineralizations of both manganese rich and manganesepoor marble and skarn iron ores have been mined, producing about 37 Mt of ore grading 30% 50% iron [Lager, 2001]. The iron ore is mainly magnetite contaminated with calcite, dolomite, manganese, diopside, and actinolite. A small amount of sulphide mineralization from the Dannemora has also been reported, including sphalerite, galena, pyrite, chalcopyrite, and pyrrhotite (P. Svensson, personal communication, 2010). The Dannemora iron ore has been mined since at least the 14th century and mining was in operation until 1992 [Lager, 2001]. Dannemora Mineral AB, the present owner of the mine, estimates that at least 28 Mt of ore remains and plans to reopen the mine in 2011 (P. Svensson, personal communication, 2010). [10] Important contributions for the structural evolution of the northeastern part of the Bergslagen region are given by Talbot and Sokoutis [1995], Sjöström and Bergman [1998], Persson and Sjöström [2003], Hermansson et al. [2007, 2008a, 2008b], Sandström et al. [2009], and Högdahl et al. [2009]. Petrological/stratigraphical studies have been carried out by Allen et al. [1996], Lager [2001], and Stephens et al. [2003, 2007], and geochronological work constraining the timing of shear activities along major ductile deformation zones was carried out by Hermansson et al. [2007] and Högdahl et al. [2009]. [11] The regional and local geology of the Dannemora area has been described by Stålhös [1991] and Lager [2001], with particular emphasis on stratigraphy. The bedrock in Dannemora consists of Paleoproterozoic metavolcanic rocks and marble, the latter hosting the Dannemora iron ore, and is surrounded by slightly younger early orogenic granitoids (Figure 2). The metavolcanic rocks are well preserved and often show primary structures and textures. U Pb (zircon) geochronology yielded an age of 1894 ± 4 Ma [Stephens et al., 2009]. All rocks, except for some lateorogenic granitoids are deformed to various degrees [Stålhös, 1991; Persson and Sjöström, 2003]. Structurally, the Dannemora iron ore deposits are interpreted to occur within a syncline with a fold axis plunging about 5 10 to the northeast [Lager, 2001]. [12] Ongoing structural studies provide evidence of a major ductile shear zone, the Österbybruk Deformation Zone (ÖDZ), in the eastern limb of the Dannemora syncline [Dahlin and Sjöström, 2010] and previous mapping outlines a major fault 1.5 km to the east of the Dannemora mine [Stålhös, 1991; Bergman et al., 1996]. 3. Major Deformation Zones in the Dannemora Forsmark Area [13] The Singö Shear Zone (SSZ) [e.g., Eriksson and Henkel, 1988; Talbot and Sokoutis, 1995] and the Österbybruk Skyttorp Shear Zone (ÖSZ) [Persson and Sjöström, 2003] have been described and characterized, but the nature and significance of these zones are still being discussed [e.g., Talbot and Sokoutis, 1995; Law and Snyder, 1997; Persson and Sjöström, 2003]. Dahlin and Sjöström [2010] suggest that the ÖDZ is the northern continuation of the ÖSZ (Figure 2). Detailed studies of the latter show evidence of both ductile and brittle deformation [Bergman et al., 1996; Engström, 2001; Engström and Skelton, 2002; Persson and Sjöström, 2003]. Both mylonites and cohesive breccias are commonly observed along the ÖSZ and the Skyttorp Vattholma Fault Zone (SVFZ; Figure 2), indicating a com- 4of21

5 Figure 3. (a) Elevation (from and (b) Bouguer gravity anomaly [see Stephens et al., 2009] maps of the study area. The break in the topographic relief marked by the arrow represents the location of the Skyttorp Vattholma Fault Zone, which occurs along the ÖDZ. A sharp change in the gravity anomaly is also observed on the eastern and western sides of the deformation zone. Data provided by the Geological Survey of Sweden. plex evolution. Large areas of brecciated rocks exist locally [Stålhös, 1991]. The breccia fragments partly consist of ductile mylonites, showing that ductile deformation is re activated under brittle conditions. The rocks affected by faulting show complex hydrothermal alteration and a transition from brittle to semibrittle behavior during deformation [Engström and Skelton, 2002]. [14] The zone affected by ductile deformation at the surface along the ÖDZ in the Dannemora area is at least 500 m wide [Dahlin and Sjöström, 2010]. Temperature estimates based on microstructures and mineralogy of the ÖDZ shows that the temperature during the ductile deformation was 500 C [Dahlin and Sjöström, 2010], which is in accordance with previous estimates for the ÖSZ [Persson and Sjöström, 2003], while the temperature for the brittle/ semibrittle overprint along the SVFZ (Figure 2) was approximately 200 C 300 C [Engström, 2001]. The timing of deformation of the shear zones is not constrained, but ductile, pervasive deformation along the nearby SSZ occurred between 1.87 and 1.86 Ga [Hermansson et al., 2008a; Stephens et al., 2009], and more discrete ductile zones were active after 1.85 Ga [Hermansson et al., 2008b]. After Ga, the bedrock deformed in a brittle manner [Söderlund et al., 2009]. [15] The SVFZ is associated with a topographic break, which is part of a regional fault and fracture system that can be traced southward (Figure 3) for another 100 km [Lidmar Bergström, 1994]. East of the Dannemora area, the SVFZ is parallel and almost along the ÖDZ (Figures 2 and 3), indicating that the SVFZ is a reactivation of the ÖDZ. To the north, the ductile pattern rotates anticlockwise (Figure 2). Our kinematic mesoscale and microscale observations from strongly sheared granitoids and amphibolites (Figure 4; see Figure 2 for locations) suggest east side up movement along the ÖDZ near the Dannemora area, which is consistent with previous results along the same zone but further south [Engström, 2001; Persson and Sjöström, 2003]. [16] Further to the northeast, geological observations suggest that deformation along SSZ has played a major role for the structural evolution in the Dannemora area. Persson and Sjöström [2003] emphasized that rocks and structures to the south show a clockwise rotation into the SSZ, 5of21

6 Figure 4. (a) Photograph of a strongly sheared granitoid (Figure 2a, location 1). Short s type foliation, oriented N S, truncated by long c type shear bands, oriented NNW SSE, indicate a sinistral sense of shear and east side up kinematics. (b) Microphoto of a thin section of mylonitic metabasalt (Figure 2a, location 2). Short s type foliation (example shown by white solid line), oriented N S, truncated by long c type shear bands (example shown by black dashed line), oriented NNW SSE, indicates a sinistral sense of shear and east side up kinematics. implying that pre existing folds and associated lineations are affected and accentuated by the shear zone. Recently, the SSZ has been included within a separate tectonic domain [e.g., Hermansson et al., 2007] that forms the northern tectonic boundary of the Bergslagen region [Högdahl et al., 2009]. The geometry of the SSZ at depth has only been interpreted based on a wide angle reflection seismic data set, BABEL line 7, indicating a deep, listric, east dipping structure [Law and Snyder, 1997]. Recent drilling and underground tunneling shows that the SSZ is subvertical, 6of21

7 Table 1. Acquisition Parameters for the Reflection Seismic Survey, June 2010 Parameter Unit Survey Parameters Recording system SERCEL 408UL Profile 1 and 2 Spread geometry Asymmetric split spread (100 stations arm) No. of live channels 360 Maximum offset 7200 m Survey length Profile 1, 24 km; profile 2, 21 km Source VIBSIST Nominal CMP fold 75 Receiver spacing Source interval Recording length Sampling rate Spread Parameters 20 m (reduced to 10 m around Dannemora) 40 m (reduced to 10 m around Dannemora) 21 s (3 s after decoding) 1 ms Receiver and Source Parameters Geophone frequency 28 Hz Geophone per set Single Sweeps 3 5 Shots 1350 down to a depth of about 1.0 km (M. Stephens, personal communication, 2011). 4. Data Acquisition [17] Based on experience from reflection seismic surveys that aimed at imaging the upper few km of crust in the northeastern Bergslagen region [Juhlin and Stephens, 2006] and on present knowledge of geological structures, we used a receiver spacing of 20 m and a shot spacing of 40 m for the acquisition of the reflection seismic data. The reflection seismic recording system of Uppsala University (SERCEL 408UL), with a capability of recording up to 400 channels, was used. This capability allowed us to design an asymmetric split spread geometry using 360 active channels (with a 100 channel arm in the expected updip direction). Two nearly perpendicular seismic reflection profiles, each about 25 km long, were acquired (Figure 2, profiles 1 and 2). A mechanical hammer (VIBSIST) was used to generate the seismic signal [Juhlin et al., 2010; Dehghannejad et al., 2010]. Profile 1 starts 5 km south of the Forsmark site; it crosses a series of complex deformation zones, visible on the magnetic map (Figure 2), and continues to the south nearly 9 km southeast of the Dannemora iron mine. Profile 2 starts nearly 10 km east of the Dannemora mine and continues westward an additional 15 km to a large granitic intrusion (Figure 2). The profile crosses the ÖDZ east of Dannemora. [18] The receiver spacing across the Dannemora mine was reduced to 10 m to potentially provide a higher resolution image of the iron mineralization at this location. Shot spacing was also reduced to 10 m over the mine and partly to the west of the mine. To potentially provide additional information about the 3 D geometry of structures, a series of additional shot points west of the mine (in the downdip direction) were activated perpendicular to the actual profile (Figure 3). The additional shot points (about 50 shots) have been used in constraining the 3 D geometry of the Dannemora geological structures and the iron ore. In total, nearly 1350 shots were fired along both profiles. Gaps in shot points occurred mainly in the western part of the profile 2 where it crosses a main public road and a large forest. Table 1 summarizes the main acquisition parameters used for the seismic survey. 5. Petrophysical Measurements [19] To be effective, reflection seismic imaging requires significant rock property contrasts between the major geological entities. In fact, a contrast in acoustic impedance (the product of density and P wave velocity) between geological entities is a basic prerequisite for reflections to be generated. In order to better understand potential sources of reflectivity in the area, a series of rock samples from outcrops along the seismic profiles and from a few boreholes in the Dannemora mine were collected. Density and compressional seismic wave velocities were measured at Curtin University, Australia using an ultrasonic (0.5 MHz) P wave transducer at both room and elevated pressures (up to 65 MPa). Previous seismic wave velocity measurements [e.g., Law and Snyder, 1997; Juhlin and Stephens, 2006] and density data from the Geological Survey of Sweden database were also available. A compilation of all measurements is shown in Table 2. Geological features that will likely produce reflections include mafic bodies, and major ductile and brittle deformation zones. The very high density and high seismic wave velocity of iron mineralization (>4250 kg/m 3 and about 6500 m/s for Fe > 30%, mainly magnetite) should also be conducive to generation of reflections, if bodies of sufficient size and suitable geometry (e.g., not vertical) are present and if the background noise level is acceptable [Salisbury et al., 2000; Malehmir and Bellefleur, 2010]. 6. Conventional Data Processing [20] The main processing and imaging strategy in this study involved a traditional poststack migration algorithm. Similar processing flows have been effective for reflection seismic studies in the crystalline environment [e.g., Zaleski et al., 1997; Milkereit et al., 2000; Roberts et al., 2003; Table 2. Compilation of Physical Rock Property Measurements in the Northeastern Part of the Bergslagen Region Rock Type Density (kg/m 3 ) V P (m/s) Method Granitoid gneiss a Lab Metagranite b,c Sonic logs + Lab Mafic dyke a Lab Granitoid a,c Lab Amphibolite a,b,c Sonic logs + Lab Metagabrro a Lab Mylonite a Lab Fracture zone b Sonic logs ÖDZ c Lab Iron mineralization c Lab Felsic volcanics c Lab Skarn c Lab Marble c Lab a Law and Snyder [1997]. b Juhlin and Stephens [2006]. c This study (ultrasonic, room, and elevated pressure up to 65 MPa). 7of21

8 Table 3. Principal Processing Steps, 2011 Step Parameters 1 Read 21 s uncorrelated seismic data 2 Data correlation and reduction to 3 s 3 Build geometry data 4 Trace editing 5 Pick first breaks: full offset range, automatic neural network algorithm but manually inspected and corrected 6 Refraction static and elevation static corrections: datum 40 m, replacement velocity 5800 m/s, v m/s 7 Geometric spreading compensation: v 2 t 8 Band pass filtering: Hz 9 Surface consistent deconvolution: filter 140 ms, gap 15 ms, white noise 0.1% 10 Band pass filtering: Hz 11 Top mute: 30 ms after first breaks 12 Direct shear wave attenuation (near offset) 13 Air blast attenuation 14 Trace balance using data window 15 Velocity analysis (iterative) 16 Residual static corrections (iterative) 17 Normal moveout corrections (NMO): 60% stretch mute 18 Stack 19 f x deconvolution (poststack coherency filter) 20 Trace balance: ms 21 Phase shift migration: using constant velocity of 6000 m/s 22 Time to depth conversion: constant 6000 m/s Tryggvason et al., 2006; Schmelzbach et al., 2007; Malehmir and Bellefleur, 2009, 2010; Cheraghi et al., 2011]. Table 3 shows the principal processing steps used for the processing of the seismic data Prestack Data Enhancement [21] Figure 5 shows the increase in signal to noise ratio for a sample shot gather fired in profile 2 after various processing steps. A pronounced steeply dipping reflection package is clearly visible after a few steps of the processing. Band pass frequency and surface consistent deconvolution filters were designed to preserve the highest frequency content of the data with useful signal. The surface consistent deconvolution consisted of a 15 ms gap and a 140 ms operator and helped to improve resolution and to compensate for the effects of variable coupling conditions due to source and receivers being placed on exposed bedrock or overburden. [22] Obtaining a good refraction static solution is critical to effectively image structures with low reflection coefficients and that are steeply dipping in the crystalline environment [Juhlin, 1995; Roberts et al., 2003]. To calculate refraction static corrections, we picked first arrivals using an automatic neural network algorithm on the entire data set and corrected the picks, where needed, during manual inspection. The first arrivals were used to estimate receiver and shot static corrections. Near surface traveltime distortions were largely removed after the application of refraction static and elevation static corrections, and the coherency of reflections was markedly improved (Figure 5b). The first arrivals were further used to design a gentle trace top mute function to remove the direct P wave and refracted energy and to preserve, as much as possible, wide angle reflections/ diffractions and near surface reflections that could potentially be linked to the near surface geology. Part of the direct shear wave energy was reduced in the shot gathers after the implementation of the band pass filter and surface consistent deconvolution. Remaining direct shear wave energy, in a few shot gathers (e.g., Figure 5b), was subsequently attenuated at near offsets (<2000 m) using a median filter Poststack Migration [23] Geological observations [Stålhös, 1991; Law and Snyder, 1997; Persson and Sjöström, 2003; Juhlin and Stephens, 2006; Dahlin and Sjöström, 2010] and analysis of the processed shot gathers (e.g., Figure 5b) indicate that the geological structures are mainly steeply dipping. Anomalously high stacking velocities, up to 10,000 m/s, were necessary to allow dipping reflections to stack coherently. In this circumstance, reflections with one dip are enhanced at the expense of other reflections with different dips having different stacking velocities [Hale, 1991]. Application of dipmoveout (DMO) corrections can potentially help in imaging reflections with different dips [Deregowski, 1986]. However, DMO did not significantly improve the image quality. This is most likely due to the limitations of the DMO process when interface dips are large, cross dip moveout (CDMO) is present, and offset distribution is irregular. Figures 6a and 7a show the stacked sections using only normal moveout (NMO) corrections with several steeply dipping reflections clearly imaged, many of which almost reaching the surface. Several poststack migration algorithms were tested including Stolt, phase shift, and finite difference. Best results were obtained using a phase shift algorithm, which handles reflections with steep dips, a common characteristic of the data set (Figures 6b and 7b). A constant velocity of 6000 m/s was used for the time to depth conversion based on an average velocity of our first arrivals at far offset and sonic log measurements at the Forsmark site [Juhlin and Stephens, 2006]. 7. Advanced Data Processing and Analysis [24] In greenfield areas and where mining infrastructures exist, crooked line seismic surveys are common and require special processing strategies [Wu et al., 1995] to extract additional information about the subsurface structures. This is also the case in this survey, where seismic reflection data acquisition was restricted to the existing roads and forest tracks, and where additional shots were fired along a track perpendicular to profile 2. Prestack 2 D time migration, cross dip analysis, and 3 D binning and poststack migration were used where appropriate to extract additional information about the structures of special interest Prestack Migration [25] The common midpoint (CMP) fold and offset distribution is low and irregular in the western part of profile 2 and DMO corrections did not result in an improved seismic image between CMPs 400 and 900. Reflections in this area are not clear and seem to have varying dips (Figure 7a) but mostly in the inline direction. Therefore, 2 D prestack time migration, which does not take into account CDMO but does handle NMO and DMO simultaneously, was performed in an attempt to improve the image. [26] A prestack Kirchhoff time migration algorithm operating in the common offset domain produced migrated and stacked common reflection point gathers. Since no sonic log information is available, it was not possible to build a true 8of21

9 Figure 5. A shot gather (from profile 2) (a) before and (b) after processing steps that included refraction static corrections, surface consistent deconvolution, and band pass filters. Note the improvement in the quality of the steeply dipping reflections (S1) shown by arrows in Figure 5b. velocity model in this area. However, large, long wavelength velocity changes are not expected in this crystalline environment in comparison with sedimentary environments [Adam et al., 2003; Juhlin and Stephens, 2006]. Thus, our approach in this work was to use constant migration velocity stacks and inspect visually the quality of the obtained migrated stacked sections. We ran a series of prestack time migration tests using velocities ranging from 5500 m/s to 6500 m/s with 50 m/s increments. The best results were obtained using a velocity of 6000 m/s. Figure 8a shows the poststack migrated section of a portion of the seismic data along profile 2 between CMPs 400 and 900 (Figure 7a). Figure 8b shows the prestack time migrated section from the same portion using a migration velocity of 6000 m/s. A comparison between the two sections suggests that the main features are similar. However, reflections imaged by the prestack time migration algorithm show fewer artifacts, and are stronger and longer. Prestack time migration results from other portions of both profiles did not result in improved image quality, likely because of the significant component of reflector cross dip and, therefore, are not discussed or shown here. 9of21

10 Figure 6. (a) Stacked and (b) migrated seismic sections along profile 1, showing a series of northdipping reflections (I1, I2, F1, F2, F3) and a reflective zone (P1) in the southwestern and central parts of the sections, respectively. A south dipping reflection (P2) is also observed at the northernmost part of the section Cross Dip Analysis [27] Analysis and modeling of reflection traveltimes [e.g., Zaleski et al., 1997;Ayarza et al., 2000; Malehmir et al., 2006] have shown that planar geological features, which are out of the plane of the CMP line, but face this line, produce reflections that are imaged on a seismic profile. To study out of the plane reflections, an explicit assessment can be achieved through a simplified cross dip analysis [e.g., Larner et al., 1979; Bellefleur et al., 1995; Nedimović and West, 2003a; Rodriguez Tablante et al., 2007; Malehmir et al., 2009b]. Cross dip is the component of reflector dip in the vertical plane perpendicular to the seismic profile that produces reflection point scattering and associated time delays [O Dowd et al., 2004]. The impact of cross dip on seismic processing has been thoroughly summarized by Nedimović and West [2003a, 2003b]. We performed crossdip analysis in the easternmost portion of the stacked section along profile 2 (Figure 7a), in areas where a relatively large component of reflector cross dip is expected. No such area was indentified on profile 1 (Figure 2). [28] Since CDMO is a function of the stacking velocity [e.g., Larner et al., 1979; Nedimović and West, 2003a], we first performed a series of constant velocity stacks to obtain the optimum stacking velocity. The cross dip component was then estimated by visual inspection of stacks obtained with different cross dip corrections and a given constant stacking velocity. The most coherent and strongest reflection dictated the choice of the CDMO correction (Figure 9). [29] Figure 9a shows the original stacked section along the easternmost portion of the seismic profile. Figure 9b is the cross dip corrected stacked section from the same portion using a cross dip correction of 30 to the south and a stacking velocity of 9000 m/s. The cross dip corrected section especially at later arrival times shows a stronger and longer reflection than the original section (Figure 7, S1). We later interpret the reflection S1 to originate from the ÖDZ, which on the geological map (Figure 2) is oblique to the seismic profile and can have a southerly cross dip component. A 5 cross dip correction to the south between CMPs 1500 and 1700 in a time interval of s (Figure 9c) revealed a series of diffractions associated with the main reflection package. We associate these diffractions with small size inclusions, possibly mafic intrusions, within the ÖDZ. [30] Figure 9d has been cross dip corrected using 30 to the north and a stacking velocity of 7000 m/s. The result is interesting. A reflection with different dip direction, to the west, becomes visible in the section, but the S1 reflection disappears. Given the absence of geological observations at this depth, we can only speculate on the origin of the reflection. The structure may be another fault system that dips into the plane of the seismic profile from the south with a 30 cross dip component Swath 3 D Imaging [31] Geological structures in the crystalline environment can be complex and 3 D migration of high fold 2 D crooked 10 of 21

11 Figure 7. (a) Stacked section along profile 2, showing a strong and long series of east dipping reflections (S1), several east dipping and westward dipping reflections (R1, R2, R3, G1), and a potential diffraction (D1). (b) Migrated section showing that the steeply dipping reflections S1 and R1 may be correlated with the locations of the ÖDZ and the Dannemora main iron ore body, respectively. Dashed lines in both Figures 7a and 7b outline parts of the sections detailed in Figures 8 and 9. line data seems like the most desirable imaging process as it handles NMO, DMO, and CDMO simultaneously. However, swath 3 D data have a very small cross profile aperture, an uneven distribution of source and receiver midpoints, and a small range of source receiver azimuths, which might make the 3 D migration incomplete and therefore fruitless [Nedimović and West, 2003b]. In the case of the Dannemora data, extra shots were activated in a short profile (about 800 m long) nearly perpendicular to profile 2 at about m west of the mine. These data are collected to provide additional information on the Dannemora iron ore and structures hosting it (Figure 10). [32] Inspections of the shot gathers around the mine indicated that very shallow reflections were generated close to the abandoned open pit. These shallow reflections (Figure 11) are not observed on the 2 D seismic section presented in Figure 7, perhaps because of the crookedness of the line at this location and low fold coverage. To extract 3 D information about the geometry of the reflections near the Dannemora site and to preserve the shallow reflections for correlation with surface geology, swath 3 D processing and imaging of this portion of the data was carried out (Figure 12). [33] Tests were performed with 3 D prestack migration of the data, but the results were not satisfactory, perhaps because of the lack of an accurate velocity model. However, 3 D poststack migration provided promising results. The main processing steps and parameters were similar to the 2 D work on the shot gathers, except that shot statics for the extra shots were taken to be the same as the one at the nearest receiver location. The data were then binned in 3 D using 80 m (inline) by 20 m (crossline) CMP bins. This resulted in 17 inlines and 317 crosslines (Figure 12). The seismic cube was designed so that the inlines intersect the main geological features, visible on the geological map, at near right angles (Figure 10). After sorting the data to the CMP domain and constructing an initial 3 D velocity function, a preliminary stacked volume was produced. The preliminary stacked volume was improved after a series of residual static corrections linked to an iterative velocity analysis. 3 D migration of the stacked volume using a phaseshift algorithm produced useful results (Figure 13). [34] The swath 3 D processing and imaging strategy allowed us to successfully image a series of steeply dipping reflectors in the very shallow parts of the seismic volume. For example, a very steeply dipping reflection observed in inline 1006 becomes more gently dipping toward higher inline numbers (Figure 13). This reflection occurs in the vicinity of the Dannemora mine (also Figure 7). In comparison with the 2 D results, the 3 D processing strategy did not produce satisfactory results at depths greater than m, perhaps because low fold coverage of the 3 D bin- 11 of 21

12 Figure 8. (a) Poststack migrated section along a portion of profile 2 (Figure 7b) and (b) 2 D prestack migrated section along the same portion showing an improved image with west dipping and east dipping reflections possibly constituting a synformal structure, or two sets of fault systems. ning has a greater impact on the imaging process at depth compared to the 2 D processing. [35] 3 D visualization of the migrated seismic volume allowed the estimation of the strike of the main reflections (Figure 14). All reflections observed in the 3 D volume show a strike of about N25 E, which is consistent with the strikes observed on the geological map in this portion of the volume. 8. Results and Interpretations 8.1. Dannemora Iron Ore Body [36] Figure 15 shows a 3 D visualization of a portion of the migrated 2 D seismic data along profile 2 with surfaces constructed from drilling results showing the extent of the known iron mineralization in the Dannemora area. The main iron ore at Dannemora, as evident on the magnetic map shown in Figure 10, also produces a strong northeastsouthwest oriented magnetic signature. Petrophysical data indicate that the iron ore body should produce a strong reflection (Table 2). The visualization helps to correlate reflections observed on the data with the known iron mineralization in the area. The steepest reflection on the migrated seismic section (Figure 15, R1) occurs beneath the abandoned open pit of the Dannemora mine. We associate this reflection with the main ore body. The shallower part of the R1 reflector is imaged using the swath 3 D processing shown in Figure 14. If it corresponds to the main ore body, then the reflector dips about to the west with a strike of about N25 E (Figure 14) and may extend down to at least 2.2 km depth. An alternative scenario, albeit less likely, would be that R1 only represents the marble unit or a fluid bearing fault. [37] Reflection R2 (Figure 15) may be associated with another known iron mineralization (Diamant 2 ore body) with a shallower dip than the main ore body [Lager, 2001]. This ore body is present nearly 500 m west of the main ore and at a 500 m depth or more [Lager, 2001]. The swath 3 D imaging results suggest the presence of a very short, flatlying, high amplitude reflection attached to the R2 reflection (Figures 13 and 14). It is possible that this short reflection is caused by an open gallery from old mining activities or it may be a potentially unknown mineralization that has a horizontal continuation extending out from the dipping one. A 3 D seismic survey over the mine and the area around it would confirm the depth extension of the major iron mineralizations and allow the geometry and continuation of the main iron ore body to be mapped in detail Österbybruk Deformation Zone [38] The strongest and longest reflection observed in the seismic data occurs along profile 2 between CMPs 1300 and 1870 (Figures 7 and 15, S1). It comes close to or reaches the surface where the ÖDZ has been observed at the surface [Bergman et al., 1996; Persson and Sjöström, 2003; Dahlin and Sjöström, 2010] and at the inferred location of the SVFZ [Stålhös, 1991]. A few rock samples collected from outcrops of the deformation zone northeast of the study area (Figure 10) indicate a slightly lower density and seismic wave velocity for the deformation zone compared to the surrounding rocks (Table 2). Velocity measurements by Law and Snyder [1997] also suggest that the deformation zones in the area generally have lower sonic wave velocities in contrast to the surrounding rocks, making them potential reflectors to be imaged by seismic methods. It has not been possible to identify the polarity of the S1 reflections neither in the migrated section nor in the shot gathers (Figure 5b), a potential diagnostic clue in interpreting the origin of the reflection. A reverse movement is confirmed by the observed east side up ductile kinematics (Figure 4) and the easterly dip of the reflector. The ÖDZ and the SVFZ occur within a low resistivity zone (P. Svensson, personal communication, 2010), which might be an indication that the zone is in part a brittle structure with fluid filled fractures. Similar reflections were also observed at the Forsmark site [Juhlin and Stephens, 12 of 21

13 Figure 9. (a) Stacked section along a portion of profile 2 (Figure 7a), (b) cross dip corrected section of the same portion as Figure 9a using a cross dip of 30 to the south, (c) using a cross dip of 5 to the south, and (d) using a cross dip of 30 to the north. Note that a reflection appears in Figure 9c which is not visible in Figure 9a. 2006], but not as steep, long, or strong as those observed in profile 2. The series of diffractions (Figure 9c) associated with this zone may be an indication of the occurrence of small lenses composed of rocks with different acoustic impedance within the ÖDZ. Juhlin and Stephens [2006] suggested that amphibolite rock units within fracture zones increased reflection seismic amplitudes in the Forsmark area, as these rocks are observed both at the surface and in drill cores. This maybe a valid scenario also for the ÖDZ. Alternatively, the diffractions may represent fluid bearing, open and isolated fractures. At the absence of borehole geophysical data, it is not clear at what scale fluid filled fracture zones would produce or enhance reflectivity. However, reflections from fracture zones, especially those that are hydraulically conductive or dry, but have large aperture, are observed in the KTB deep drilling site, Germany [Lüschen et al., 1996] and in the Bergslagen region [Juhlin and Stephens, 2006]. What would be even more interesting to know is the origin of the fluids; whether they are meteoritic, metamorphic, mantle derived, or a combination of these. This is, however, beyond the scope of the current paper Other Major Geological Structures [39] Figure 6b shows the migrated seismic section along profile 1 with the main reflections marked. The most noticeable reflections are observed in the southernmost portion of the profile (e.g., Figure 6b, I1 and I2). The majority of the reflections are north dipping except for a package at the 13 of 21

14 Figure 10. (a) Close up of the geological map [see Stephens et al., 2009] and (b) corresponding highresolution helicopter total field aeromagnetic map (courtesy of Dannemora Mineral AB) of the Dannemora mining area, showing that a strong northeast southwest directed magnetic signature is produced by the main iron ore body. The magnetic high about 500 m west of the Dannemora main iron ore body is associated with the Diamant 2 iron mineralization. Locations 1 3 (Figure 10a) show where detailed geological studies were carried out (see Figure 4). Surface extent of the swath 3 D imaging cube is also shown in Figure 10. Data provided by the Geological Survey of Sweden. northernmost end of the profile (Figure 6b, P2). The central part of the profile does not show any distinct individual reflections, but appears as a strongly reflective zone. [40] We interpret I1 to be the top of a large mafic intrusion that approaches the surface to the south of the profile. Magnetic and gravity highs observed in the south could be associated with this intrusion (Figures 2 and 3). F1, F2, and F3 may represent a series of fault zones either occurring within the metagranodiorite or at the contact with the metavolcanic rocks. It is not clear what generates reflections I2, despite their strength and despite their successful imaging starting at depths of only a couple of hundred meters. Neither the magnetic nor the geological map indicates any change within the metagranodiorite at their projected surface (Figure 2). P1 and P2 represent two reflective zones. P1 is associated with a zone of intense deformation, which can be traced on the magnetic map of the study area (Figure 2), but no clear reflectivity pattern corresponding to this zone can be identified in this area. However, the P2 reflective package shows a consistent dip direction toward the south. This dip direction is consistent with those observed on the seismic profiles at Forsmark [Juhlin and Stephens, 2006]. The P2 package is very likely generated by a major NW SE striking deformation zone observed in this portion of the profile (see Figure 2). [41] Figure 7b shows the migrated seismic section along profile 2 with the main reflections marked. Reflection R3 and associated west dipping reflections seem to indicate the contact between the metavolcanic rocks and metagranodiorites. The associated weak reflections may represent structural fabrics (fractures or fault systems) within the metavolcanic rocks (Figure 8b). Geological observations within the metavolcanic rocks close to the boundary to the metagranitoids show that a lineation parallel to a fold axis trends to the northwest and plunges 55, i.e., is projecting below the metagranitoids in accordance with reflection R3 (Figure 10, location 3). The reflectivity pattern immediately west of the metavolcanic rocks shifts from being west dipping to east dipping (Figure 7). Reflection R4 and associated east dipping reflections most likely occur within the metagranodioritic 14 of 21

15 Figure 11. A raw shot gather recorded near the Dannemora mine showing a steeply dipping reflection generated below the Dannemora mine open pit, which is most likely a signal from the iron ore body. This reflection is not observed on the 2 D images shown in Figure 7. Figure 12. (a) Midpoint coverage along the central portion of profile 2 and (b) the 3 D bin fold coverage obtained using 80 m by 20 m CMP bins. Note the lateral extent of the trace midpoints in the vicinity of the Dannemora mine. 15 of 21

16 Figure of 21

17 Figure 14. The 3 D view from the migrated seismic volume demonstrating the main strike, which is N25 E, of what is probably the Dannemora iron ore (R1) at shallow depth. rocks. The two contrasting orientations of the reflectivity in the western part of the seismic profile (i.e., R3 and R4) may suggest the occurrences of blind faults or shear zones. [42] A short set of reflections (Figure 7, G1) at the westernmost end of the seismic profile 2 is associated with a gravity low (Figure 3). This geometry suggests that the maximum depth extend of the younger granitic body at the end of the profile to be approximately 1.5 km. This depth is estimated based on a background density of 2750 kg/m 3 and a density of 2650 kg/m 3 for the exposed granite [Malehmir et al., 2007]. A diffraction signal (Figure 7, D1) observed in the eastern part of profile 2 is interpreted to be from a small and isolated body, possibly a mafic intrusion within the granodioritic rocks. 9. Discussion [43] Results from the two regional reflection seismic profiles, the first such profiles recorded in the Bergslagen region, show that major lithological and geological structures have been successfully imaged. This further suggests that regional scale reflection seismic surveys in this region can be utilized for locating and identifying important geological structures at depth. [44] In the central part of the study area, the majority of the reflections dip toward the east northeast, except for the ÖDZ which dips east southeast (Figure 7, S1). Although seismic wave velocity measurements from the deformation zone indicate a slight decrease in velocity (Table 2), it is also possible that the reflections from the ÖDZ are, in part, caused by anisotropy [Jones and Nur, 1984]. Deformed rocks, with strong foliation, normally show an increased seismic velocity along the foliation fabric and a decreased seismic velocity perpendicular to the foliation. In order to produce strong reflections from mylonite zones, Jones and Nur [1984] suggested the presence of phyllosilicate and lamination (tectonic fabric) within the rocks, which would then enhance the reflected seismic amplitude. The reverse kinematics of the ÖDZ (Figure 4) and a sharp break in the gravity anomaly map (Figure 3b) along the zone, combined with the interpretation of a large mafic intrusion in the southernmost part of profile 1 (Figure 6), provide new insights. The gravity map suggests an east west trending, high density anomaly south of both profiles (Figure 3b). It is likely that the gravity high in the east and south of profile 1 is produced by a mafic intrusion that was brought to shallower depths by the ÖDZ during its reactivation episodes or even prior to that. The magnetic anomaly map also suggests a sharp change in the signature in the western side of the ÖDZ at this location (Figure 2b). The difference in gravity anomaly field is about 10 mgal, with more positive values toward the east. Although speculative, a rough 2 D estimation of the extra mass of a mafic intrusion located at a depth of about 1.5 km in the eastern side (estimated from the seismic data; Figure 6) would imply about 2.5 km of vertical displacement along the ÖDZ. This displacement is calculated from [Kearey and Brooks, 1991] d Dg 2GD ; where d denotes the approximate thickness crudely estimated from the maximum gravity anomaly by making use of the slab formula, G is the gravitational constant, and we Figure 13. A series of inlines ( ) extracted from the migrated seismic volume indicating a steeply dipping reflection (R1) at the Dannemora mine that becomes more gently dipping toward the higher inline numbers, a high amplitude reflection (R2) west of it, and another steeply dipping reflection (S1) in the easternmost part of the seismic volume. 17 of 21

18 Figure 15. The 3 D visualization of a portion of the 2 D migrated seismic data along profile 2 with the known iron mineralizations (red surfaces) obtained from drill cores. assume a density difference, Dr, of 250 kg/m 3. This estimation is based on an assumption that the mafic intrusion in the south extends laterally toward the west and the ÖDZ intersects the mafic intrusion. Given that, most mafic intrusions found in the study area are coeval with the metagranodiorite [Stålhös, 1991; Stephens et al., 2009], this assumption is likely valid. [45] Geological evidence suggests that the ÖDZ was reactivated at least twice and that there was a cyclic interaction between fluid flow and brittle deformation. This interaction resulted in softening, enhancing subsequent semibrittle deformation [Engström and Skelton, 2002]. Such a complex evolution, including volume change by mineral reactions and strain partitioning along the fault, may have played a profound role in constraining the rheology and wave velocities of the rocks. The polyphase activity includes ductile and brittle deformation along the structures that are spatially related in the reflection seismic profiles. This hampers the possibility to assign a specific fault rock to the main reflection (Figure 7, S1). However, the thickness of the reflection package is in best accordance with the ductile ÖDZ. It is well documented in other studies that low temperature deformation results in narrower, more localized, high strain zones than high temperature deformation [e.g., Stephens et al., 2009]. In addition, the detailed studies of the SSZ in the Forsmark area show that both ductile and more localized retrogressive strain made use of the previously developed planar anisotropy formed during pervasive ductile deformation, and that these anisotropies also controlled the orientation of deformation zones during reactivation in the brittle regime [Stephens et al., 2009]. These observations from a partly progressive evolution strongly suggest that there is an, at least, indirect link between the ductile shear on the ÖDZ and the main reflection S1. In addition, it is likely that the reflector geometry in this part of the profile is the result of ductile deformation. Based on results from Forsmark [Hermansson et al., 2007, 2008a, 2008b], the timing of this deformation is probably around Ga. [46] Figure 16 shows a sketch describing polyphase evolution of major deformation zones and folding phases in the Dannemoraarea[alsoAllen et al., 2003; Stephens et al., 2009]. The deformation zones, showing mainly reverse kinematics, were first generated due to a compressional tectonic setting along discrete zones and were later reactivated, changed to normal faulting, due to the change in the tectonic setting to extension. Much of the fault movements indicated by the seismic data, i.e., about 2.5 km reverse movement estimated in this paper, must then be related to the time prior to 1.80 Ga (also Figure 4). Mineral deposits formed during both the early igneous activity and metamorphism after that. [47] The mineral potential in the Bergslagen region, the tectonic setting, and the upper crustal architecture are of great interest to both geologists and geophysicists. Although direct delineation and targeting of new mineral deposits were not the main aim of this study, the seismic results suggest that the Dannemora main iron ore body possibly extends down to a depth of about 2.2 km. The very steep character of the R1 reflector, interpreted to be the Dannemora main ore, at least at shallow depths, is an indication that the seismic profile is almost perpendicular to its strike (Figure 14). The swath 3 D processing suggests a strike of N25 E and true dip of about 70 for the R1 reflector. The additional 50 shots and the swath 3 D processing allowed us to successfully image the very shallow parts of the reflectors for a better link with the observed geology. This was not possible with only the standard 2 D processing work. The swath 3 D processing turned out to be promising, mainly due to the additional shots that were placed at optimum locations, the downdip direction of the main geological features. Although swath 3 D processing with additional shots fired perpendicular to 2 D seismic profiles is helpful in extracting information about the 3 D geometry of struc- 18 of 21