Luleå, Sweden
Luleå, Sweden

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Petersson J.,Vattenfall | Stephens M.B.,Geological Survey of Sweden | Stephens M.B.,Lund University | Mattsson H.,Lulea University of Technology | Moller C.,GeoVista AB
Lithos | Year: 2012

Hydrothermal alteration resulting in albitization and quartz dissolution has been identified in Paleoproterozoic metagranites down to - 1000. m elevation at Forsmark, Sweden. The alteration features were discovered during investigations to locate a site for the disposal of spent nuclear fuel in a deep geological repository. In general, albitization occurs extensively, but it is also observed locally adjacent to minor intrusive bodies of amphibolite. The altered rocks show a marked decrease in K-feldspar and an increase in quartz relative to the unaltered equivalents, resulting in an epitonalitic composition. Plagioclase is metamorphic in character and generally richer in albite than in the unaltered rocks. It is inferred that albitization was triggered by the input of basic or intermediate melts into the crust during igneous activity close to the peak of regional metamorphism at 1.87-1.86. Ga. The mineralogy of the epitonalites gives rise to an increased thermal conductivity and, thereby, a positive influence for the design and safety of a deep geological repository for spent nuclear fuel. However, the increased frequency of low conductive amphibolite in the albitized volumes, consistent with the proposed mechanism for alteration, gives a negative influence. In sharp contrast to the albitization, a majority of the occurrences of quartz dissolution, which resulted in the formation of episyenite, are located along fracture zones. Quartz dissolution took place between or after 1.8-1.7. Ga, when the bedrock was able to respond to deformation in a brittle manner. Most of the vugs left after the removal of quartz are, to a variable extent, refilled by hydrothermal assemblages, including quartz, albite, K-feldspar, hematite, chlorite and calcite. The geometry and spatial distribution of episyenite argue against an extreme fluid/rock ratio and it is inferred that the fluids had at least a moderate salinity with a temperature in excess of 300 °C. The dissolution process was promoted by the generation of secondary permeability localized in columnar or pipe-like volumes. The close spatial connection to fracture zones provides a basis to avoid bedrock affected by this type of alteration and, thereby, reduce the negative mechanical and hydrogeological aspects for a deep geological repository. © 2012 Elsevier B.V.

Mainali G.,Lulea University of Technology | Nordlund E.,Lulea University of Technology | Knutsson S.,Lulea University of Technology | Thunehed H.,GeoVista AB
Electronic Journal of Geotechnical Engineering | Year: 2015

Tailings dam failures have been occurring in recent years. Many of these failures have caused human casualties, destruction of property, and damage to environment and huge economic loss to the mining industry. The monitoring of the dam is essential to know the existing state of the dams and to ensure the safety of the dam over its life time. The present study has been conducted to test the applicability of electrical resistivity and self-potential (SP), for detecting anomalous seepage through mine tailings dams in Sweden and monitoring the physical condition of the dam. This study has demonstrated the potential of using geoelectrical methods for monitoring the conditions of the tailing dams related to seepage. © 2015 ejge.

Stephens M.B.,Geological Survey of Sweden | Stephens M.B.,Lulea University of Technology | Follin S.,SF GeoLogic AB | Petersson J.,Norconsult AB | And 3 more authors.
Tectonophysics | Year: 2015

This paper presents a review of the data sets and methodologies used to construct deterministic models for the spatial distribution of deformation zones and intervening fracture domains in 3-D space at Forsmark, Fennoscandian Shield, Sweden. These models formed part of the investigations to characterize this site, recently proposed as a repository for the storage of spent nuclear fuel in Sweden. The pronounced spatial variability in the distribution of bedrock structures, formed under ductile (lower amphibolite- or greenschist-facies) and subsequently brittle conditions, was controlled by two factors; firstly, the multiphase reactivation, around and after 1.8. Ga, of older ductile structures with a strong anisotropy formed under higher-temperature conditions at 1.87-1.86. Ga; and, secondly, by the release of rock stresses in connection with loading and unloading cycles, after 1.6. Ga. The spatial variability in bedrock structures is accompanied by a significant heterogeneity in the hydraulic flow properties, the most transmissive fractures being sub-horizontal or gently dipping. Although the bedrock structures at Forsmark are ancient features, the present-day aperture of fractures and their hydraulic tranmissivity are inferred to be influenced by the current stress state. It is apparent that the aperture of fractures can change throughout geological time as the stress field evolves. For this reason, the assessment of the long-term (up to 100,000. years) safety of a site for the storage of spent nuclear fuel in crystalline bedrock requires an evaluation of all fractures at the site, not only the currently open fractures that are connected and conductive to groundwater flow. This study also highlights the need for an integration of structural data from the ground surface and boreholes with magnetic field and seismic reflection data with high spatial resolution, during the characterization of structures at a possible site for the storage of spent nuclear fuel in crystalline bedrock. © 2015 Elsevier B.V.

Tavakoli S.,Lulea University of Technology | Bauer T.E.,Lulea University of Technology | Elming S.-A.,Lulea University of Technology | Thunehed H.,Geovista AB | Weihed P.,Lulea University of Technology
Journal of Applied Geophysics | Year: 2012

The Skellefte district in northern Sweden is one of the most important mining districts in Europe hosting approximately 80 volcanic massive sulfide (VMS) deposits. Due to its economical importance, geological and geophysical studies were carried out in order to create an image of the geometry of the upper crustal structure and integral geological elements and to evaluate their relationship to mineral deposits. Consequently, seismic reflection data along three sub-parallel profiles were acquired during 2009-2010 to map the spatial relationships between the geological structures down to a depth of ~. 4.5. km. Although these seismic studies helped researchers understand the regional relationship between geologic units in the central Skellefte district (CSD), the seismic reflection data did not succeed entirely in mapping the lithological contacts in the area.In this study, the model derived from the seismic reflection data was examined by using 2.5D modeling of potential field data (down to a 5. km depth) constrained by physical properties of the rocks and surface geology. Moreover, we modeled gravity and magnetic data along the non-reflective or poorly reflective parts of the seismic profiles to identify major lithological contacts and shear zones in the CSD, which could not be modeled on the basis of the seismic reflection data. Gravity and magnetic data helped reveal the spatial relationship between the Skellefte volcanic rocks, Vargfors group meta-sedimentary rocks and two meta-intrusive complexes. Results suggest a maximum depth extent of 2.1. km for the tectonic contact at the southern border of the Jörn granitoid. Furthermore, this north-dipping Skellefte-Jörn contact coincides closely with magnetic lows and gravity highs, which implies that the Jörn intrusive rocks have a greater thickness than the underlying basalt. Further to the NW, gravity and magnetic data suggest a depth extent of 2. km for the Gallejaur complex, which coincides with a set of gently dipping reflectors. In addition, this study supports previous concepts of fault geometries and fault patterns as a result of upper-crustal extension and subsequent inversion during crustal shortening. In the final model interpretations of the IP data were included, thus relating indications of mineralization to the geological structures. © 2012 Elsevier B.V.

Tavakoli S.,Lulea University of Technology | Elming S.-A.,Lulea University of Technology | Thunehed H.,Geovista AB
Journal of Applied Geophysics | Year: 2012

The central Skellefte district (CSD) is a part of a major ore-bearing district in northern Sweden. Studying the depth and patterns of the contact relationship between the two major stratigraphic units of the CSD, the Skellefte Group and the Vargfors Group, is a key issue to understand the geometry and structure of the area and to guide exploration of base metals. In this study, we interpret geoelectrical data collected along two profiles and magnetic and gravity data obtained from the database of the Swedish Geological Survey (SGU) and Boliden Mineral, to reveal contact relationship and depth extension of the major geological structures. Petrophysical analyses of the different lithologies were conducted on samples from the database of the SGU. Electric resistivity, induced polarisation (IP), magnetic susceptibility and density were determined on 154 core samples representing the different lithologies of the area. The resistivity/IP data were acquired to define structural relations down to a maximum depth of ~. 430. m. The major contact between sediments of the Vargfors basin and volcanic rocks of the Skellefte Group were outlined from the inversion of the resistivity/IP sections, suggesting a synform boundary between the Vargfors Group and Skellefte Group. The contact relationship between the felsic and mafic volcanic rocks of the Skellefte Group is also understood with the help of the resistivity/IP data. The resistivity models were tested using the magnetic data and magnetic susceptibility inferred on the resistivity bodies. The result suggests a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. The integration and interpretation of geological and geophysical data improved the basic understanding of the geometry of CSD. Based on previous geological investigations, the potential ore deposits are believed to be found along the volcano-sedimentary contact. The result from this study can thus be used for the base metal exploration, finding the locations of potential sulphide deposits and give a better understanding about spatial relationship between different geology structures in the CSD. © 2012 Elsevier B.V.

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