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Reflection seismic investigations in the western part of the paleoproterozoic VHMS-bearing Skellefte district, northern Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
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2006 (English)In: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 101, no 5, 1039-1054 p.Article in journal (Refereed) Published
Description
Abstract [en]

The Skellefte district forms part of the Svecofennian ca. 1.90 to 1.80 Ga, snpracrustal sequence and associated intrusive rocks in the northern part of Sweden. The western part of the Skellefte district, which is the most important metallogenic province in northern Sweden today, hosts major volcanic-hosted massive Sulfide (VHMS) deposits (e.g., the 23 million metric tons (Mt) Kristineberg Cu-Zn-Pb-Ag-Au deposit). In order to obtain a better understanding of the VHMS ore potential at depth, new seismic reflection data were acquired along two parallel and 25-km-long profiles in the Kristineberg area in 2003. The data were collected with the purpose of obtaining high-resolution images of the top 10 kin of the crust and are presented here for the first time. Although the structural setting is very complex, the stacked sections reveal numerous reflections that can be correlated with surface geology. Visible on both profiles is a pronounced north-dipping hand of reflections marking a boundary between relatively transparent crust above and significantly more reflective crust beneath it. We interpret this reflective crust to represent a structural basement to the ore-bearing Skellefte Group, possibly constituting Bothnian basin metasedimentary rocks bordering the Skellefte district to the south. This new interpretation is important for the understanding of the tectonic evolution of the Skellefte district and for defining exploration strategies in the area. The seismic results suggest that the Kristineberg and Ravliden deposits occur on the northern limb of a kilometer-scale local second-order syncline within the hinge zone of a major antiform. Results from a profile located approximately 8 kin to the west of the Kristineberg mine indicate that the Revsund granitoid has a thickness of about 3 to 3.5 km. Ultramafic rocks are also imaged clearly Diffraction patterns and bright-spot reflectivity is interpreted as originating from either mafic to ultramafic intrusions ora mineralization zone at 3- to 5-km depth. These results help to identify new prospective areas and mineral potential, both downplunge from known ores and on the same stratigraphic horizon on the southern limb of the ore-bearing syncline. The seismic reflection profiling has been effective in imaging the major structures around the Kristineberg orebody, demonstrating that this technique can be used for delineating complex structures significant for mineral exploration.

Place, publisher, year, edition, pages
2006. Vol. 101, no 5, 1039-1054 p.
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-94065DOI: 10.2113/gsecongeo.101.5.1039ISI: 000242694300007OAI: oai:DiVA.org:uu-94065DiVA: diva2:167785
Available from: 2006-03-02 Created: 2006-03-02 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Extracting 3D Information from 2D Crooked Line Seismic Data on Hardrock Environments
Open this publication in new window or tab >>Extracting 3D Information from 2D Crooked Line Seismic Data on Hardrock Environments
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seismic methods have been used in sedimentary environment for almost 80 years. During that time, exploration geophysicists have developed a number of techniques to handle specific aspects of working in sedimentary areas. This is not the case for studies in the hardrock environment, where significantly less time and money have been invested on seismic investigations. Therefore, there is still a need to develop the right techniques appropriate for working in hardrock environments. The research presented here, covers aspects of acquisition, processing and interpretation in hardrock environments. A cost-effective alternative for two-dimensional data acquisition is presented. Acquisition parameters are also discussed and recommendations for future work are given. The main effort of this thesis, however, was to find appropriate processing methods to address some of the different problems present in datasets acquired in the hardrock environment. Comparison of two computer programs for first arrival seismic tomography was performed in order to find the most suitable one for processing crooked line geometries. Three-dimensional pre-stack depth migration was also tested to find a detailed near-surface image. A processing method geared to enhance the signal-to-noise ratio was applied to the dataset with the lowest signal amplitudes to improve the quality of the stack. Finally, cross-dip analysis and corrections were performed on two of the three datasets included in this thesis. Cross-dip analysis was also applied as an interpretation tool to provide the information needed for estimation of the true dip of some of the reflectors related to geological structures. The results presented in this thesis indicate that cross-dip analysis and corrections are one of the most powerful tools for processing and interpretation in the presence of complex geology. Therefore, it is recommended to include this method as a standard step in the processing and interpretation sequence of data acquired in hardrock environments.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 69 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 150
Keyword
Earth sciences, Hardrock Environment, Crooked line, Reflection Seismics, signal-to-noise, cross-dip, cross-profile, Geovetenskap
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-6510 (URN)91-554-6476-9 (ISBN)
Public defence
2006-03-24, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00
Opponent
Supervisors
Available from: 2006-03-02 Created: 2006-03-02 Last updated: 2016-05-13Bibliographically approved
2. 3D Geophysical and Geological Modeling in the Skellefte District: Implications for Targeting Ore Deposits
Open this publication in new window or tab >>3D Geophysical and Geological Modeling in the Skellefte District: Implications for Targeting Ore Deposits
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the advancements in acquisition and processing of seismic reflection data recorded over crystalline rocks, building three-dimensional geologic models becomes increasingly favorable. Because of little available petrophysical data, interpretations of seismic reflection data in hardrock terrains are often speculative. Potential field data modeling are sometimes performed in order to reduce the ambiguity of seismic reflection interpretations. The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District was chosen to construct a pilot three-dimensional geologic model in an attempt to understand the crustal architecture in the region and how the major mineral systems operated in this architecture. To contribute to this aim, two parallel seismic reflection profiles were acquired in 2003 and processed to 20 sec with special attention to the top 4 sec of data. Several reflections were imaged and interpreted by the aid of reflector modeling, borehole data, 2.5D and 3D potential field modeling, and geological observations. Interpretations are informative at the crustal scale and help to construct a three-dimensional geologic model of the Kristineberg mining area. The three-dimensional geologic model covers an area of 30×30 km2 down to a depth of 12 km. The integrations help to interpret a structural basement to the Skellefte volcanic rocks, possibly with Bothnian Basin metasedimentary affinity. The contact is a shear-zone that separates the two units, generating large fold structures, which can be observed in the region. The interpretations help to divide the Revsund granitic rocks into two major groups based on their present shape and thickness. A large gravity low in the south is best represented by the intrusion of thick dome of Revsund granite. In the north, the low-gravity corresponds to the intrusion of sheet-like Revsund granites. In general, the structure associated with the Skellefte volcanics and the overlying metasedimentary rocks are two thrusts exposing the Skellefte volcanic rocks in the cores of hanging wall anticlinal structures. Lack of coherent reflectivity in the seismic reflection data may be due to complex faulting and folding systems observed in the Skellefte volcanics. Ultramafic sills within the metasedimentary rocks are interpreted to extend down to depths of about 5-6 km. The interpretations are helpful for targeting new VHMS deposits and areas with gold potential. For VHMS deposits, these are situated in the southern limb of a local synformal structure south of the Kristineberg mine, on the contact between the Revsund granite and the Skellefte volcanic rocks. A combination of metasedimentary and mafic-ultramafic rocks are highly gold prospective in the west, similar to observations elsewhere in the region. There are still questions that remain unanswered and need more work. New data in the study area will help to answer questions related to e.g., an enigmatic diffraction seismic signal in Profile 5 and the structural relationship between the Skellefte volcanic rocks and the Malå volcanics. Although the derived 3D geologic model is preliminary and constructed at the crustal scale, it provides useful information to better understand the tectonic evolution of the Kristineberg mining area.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 84 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 337
Keyword
Geophysics, Seismic reflection, Hardrock, VHMS deposits, Potential field, 3D geologic model, Skellefte District, Geofysik
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-8188 (URN)978-91-554-6957-3 (ISBN)
Public defence
2007-09-27, Axel Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00
Opponent
Supervisors
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2016-05-13Bibliographically approved

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