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Geological setting
Bergslagen region
Zinkgruvan is a massive Zn-Pb-Ag-(Cu) deposit situated in the southern part of the Bergslagen region, in south-central Sweden (Figure 2.1) (Hedtröm et al., 1989; Jansson et al., 2017). The region is one the most prosperous mineral districts in Sweden with more than 1000 years of continuous mining hosting over 6000 occurrences in the country (Stephens et al., 2009; Malehmir et al., 2011). The Bergslagen region is believed to have been developed on an active continental boundary in a back-arc tectonic setting featured by extension and magmatism influenced by ductile deformation and metamorphism associated with crustal shortening/subduction and tectonic alteration (Allen et al., 1996; Hermansson et al., 2008). The region is part of the southern volcanic belt of the Svecofennian domain, which is part of the Baltic Shield that mostly contains supracrustal Precambrian rocks with an age of approximately 1.90 to 1.86 Ga (Gorbatschev and Bogdanova 1993; Gaál and Gorbatschev 1987). The supracrustal rocks in Bergslagen are mostly felsic metavolcanic rocks that are estimated to be as thick as 10 km (Kumpulainen et al., 1996).
Currently, in the Bergslagen region, three polymetallic sulphide mines are being extracted: Garpenberg, Lovisa, and Zinkgruvan; the latter is the focus of this study. The predominant mineral deposits in the region are iron-oxide deposits, while polymetallic (primarily massive sulphides) are subordinates (Allen et al., 2003).
Zinkgruvan mining area
The Zinkgruvan deposit is the southernmost underground mine in Sweden and supplies Zn, Pb and Cu (Figure 2.1). The deposit is mined by Zinkgruvan Mining AB Company, which is a subsidiary of Lundin Mining Corporation. Allen et al. (1996) referred the deposit as “stratiform ash-siltstone-hosted Zn-Pb-Ag sulphide deposit (SAS-type)”, which was previously accredited as “Åmmeberg-type” (Geijer, 1917). Hydrothermal alteration is evident in the mine sequence and majority of the metallic deposits in the district are spatially associated with hydrothermally altered felsic volcanic rocks (rhyolitic to dacitic), marble, skarn and metasedimentary rocks.
A detailed description of the stratigraphic setting of the Zinkgruvan deposit has been provided by Hedström et al. (1989), Allen et al. (1996) and Kumpulainen et al. (1996). The deposit features distinctive stratification and spreads for more than 5-km-along strike and to the depths of 1,600 m. The Zinkgruvan formation was characterized as the region squeezed between the ’Mariedamm volcanic unit’ and the ’Vintergölen formation’, which are informally known as ‘Emme group’ (Hedström et al., 1989; Kumpulainen et al., 1996). The Zinkgruvan formation is a succession of grey, mainly fine-grained, biotite-bearing quartz-feldspathic rocks (the metatuffites unite), calc-silicate units and marble. Jansson et al. (2017) suggested that the Zinkgruvan formation is hosting the stratiform Zn-Pb-Ag mineralization.
Zinkgruvan is a complex stratiform deposit consisting of several thin layers, which is highly tectonized, folded and deformed by various major secondary faults in the area. There are two major fault systems that have been influencing the position and continuation of the deposit: Knalla and Nygruvan faults. The western part of the Knalla fault contains several ore lenses, which have been influenced by various folding and deformation, while on the other side of the fault (the eastern side), which is the Nygruvan ore body comprises an individual ore lens. The Nygruvan produced the majority of the historical extractions from the mine (Mining data solution. Various tectonic events and alterations influence the primary structures and lithologies making identification of the original structure uncertain (Bengtsson, 2000).
The deposit is a complex hardrock setting folded in different directions and the objective of this study is to image the massive sulphide mineralization in the formation and the structures hosting it. Figure 2.1 Geological map of the Zinkgruvan mining area showing the seismic profiles (blue, orange and grey lines) and major lithological units (modified from Stephens et al., 2009; SGU map). P1, P3 and P4 profiles are the focus of this study. Adapted based on Gil et al. (2020)
Seismic data acquisitions
The data presented in this study were acquired as part of a large scientific-industrial research project called Seismic Imaging Techniques for Mineral Exploration (SIT4ME). It involves several European institutions and co-funded by the EIT Raw Materials. As part of the project, a dense multi-method seismic dataset was acquired in the Zinkgruvan mining area of the Bergslagen mineral district of Sweden. This thesis, only focuses on a portion of the dataset that includes three 2D crooked seismic reflection profiles (P1, P3 and P4) extracted from the rather sparse 3D dataset. They are processed in a combined manner in order to explore combined 2D potential of the dataset (Figure 2.1).
Figure 3.1 An illustration of the combined CDP lines and CDP midpoint coverage of the data used in this project work.
Black dots represent the stations and red dots the shot points.
In November 2018, a multi-method seismic dataset was acquired in the Zinkgruvan mining area, including a combination of dense 2D profiles and sparse 3D grid in an area of 6 km2 for the project (Figure 3.1) (Gil et al., 2020). The dataset was acquired using a 32t seismic vibrator (10-150 Hz) of TU Bergakademie Freiberg that was activated at every 10 m along the profiles (Figure 3.2). The acquisition spread involved 425, 1C-28 Hz cabled along with 192-3C (10 Hz) sensors simultaneously recorded 368 shots from the three different profiles (Figure 3.2). One northeast-southwest oriented seismic line (P1) with a total length of 4.25 km was acquired using 10 m receiver and source intervals, while the other two lines (P3 and P4) with a total length of 2.46 km and 1.38 km, respectively, were acquired using 20 m receiver and 10 m source spacing. The data were recorded using SERCEL 428 recording system and wireless recorders (Table 3.1). This setup provided a comprehensive dataset, which has been used for a multitude of processing and imaging approaches. Table 3.1 summarizes the main acquisition parameters of the acquired seismic lines (P1, P3 and P4).
Figure 3.2 Example field photos during the seismic data acquisition showing (a) 32t seismic vibrator (10-150 Hz sweeps used) of TU Bergakademie Freiberg and (b) 1C cabled geophone, (c) 3C geophone, and (d) seismic line setup with conventional geophones (orange cables). Photos by A. Malehmir (November 2018).
The profiles processed (A, B, and C), in this thesis work are a combination of the three P1, P3 and P4 profiles. Profile A and B as shown in Figure 3.1 are different combinations of P1 and P3. Profile A provides an approximately east-west line with a total length of approximately 4 km with a CDP spacing of 5 m. Along P1 receiver and shot spacing was set 10 m. Profile B which is a combination of northern part of P1 and P3, has a length of approximately 2.7 km with the same CDP spacing of 5 m. Profile C is a combination of P4 and part of P3 with a length of approximately 2.2 km and with CDP spacing of 10 m given that a receiver spacing of 20 m was used along P3 and P4 (Figure 3.1).
In this study only vertical component data are used. For geodetic surveying, a Differential Global Positioning System (DGPS) was used and where relevant LiDAR (Laser imaging, detection, and ranging) data were used to correct for poor quality elevation data.
Table of contents :
1 Introduction
1.1 Motivation and main objectives
2 Geological setting
2.1 Bergslagen region
2.2 Zinkgruvan mining area
3 Seismic data acquisition
4 Methodology
4.1 Seismic waves
4.2 Reflection seismic data processing
4.2.1 Geometry application and first arrival picking
4.2.2 Refraction static corrections
4.2.3 CDP sorting
4.2.4 Amplitude recovery and filtering
4.2.5 Velocity analysis and NMO corrections
4.2.6 Unmigrated stack
4.2.7 Post-stack migration
5 Results and interpretations
5.1 General observations
5.2 3D visualization and correlation between profiles
6 Discussion
7 Conclusions
8 Acknowledgements
9 References
