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  • 1.
    Bender, H.
    et al.
    Department of Geological Sciences, Stockholm University, Stockholm, Sweden.
    Ring, U.
    Department of Earth Sciences, Geophysics, Uppsala University, Uppsala, Sweden.
    Almqvist, B.S.G.
    Department of Earth Sciences, Geophysics, Uppsala University, Uppsala, Sweden.
    Grasemann, B.
    Department of Geodynamics and Sedimentology, University of Vienna, Vienna, Austria.
    Stephens, Michael B.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones: Sequential Accretion of the Caledonian Internides, Central Sweden2018In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 37, no 10, p. 3545-3576Article in journal (Refereed)
    Abstract [en]

    Exhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.

  • 2.
    Brojerdi, Fatemeh Sharifi
    et al.
    Department of Earth Sciences, Uppsala University.
    Juhlin, Christopher
    Department of Earth Sciences, Uppsala University.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Malehmir, Alireza
    Department of Earth Sciences, Uppsala University.
    Reflection seismic imaging of the deeper structures at the Forsmark spent nuclear fuel repository site, central Sweden2013In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 89, p. 21-34Article in journal (Refereed)
    Abstract [en]

    The Forsmark area belongs to the Paleoproterozoic Svecokarelian orogen (c. 1.9-1.8 Ga), the principal geological entity inside the Fennoscandian Shield, and is the site where Sweden has proposed to store its spent nuclear fuel. Three major sub-vertical (at the surface), composite ductile and brittle deformation zones that strike in a WNW or NW direction are present in the area. In between these zones the bedrock is less deformed and considered suitable for a repository. We present reprocessed reflection seismic data from seven profiles in which we have focused on improving the images in the depth range 1–5 km by passing lower frequencies through the processing flow at the cost of poorer resolution in the near-surface realm. The new images indicate that sub-horizontal to moderately dipping structures are possibly more extensive at depth than previously thought. Three main deeper reflective zones have been identified, one that is sub-horizontal and two that dip moderately to the southwest. The sub-horizontal reflective zone may represent a 1.27-1.26 Ga dolerite sill at about 3 km depth. One of the moderately dipping reflective zones may originate either from another dolerite sill or from a brittle fault system. The other moderately dipping structure may be present throughout most of the area and could cut all three sub-vertical deformation zones at depth. The new images and corresponding interpretation do not require a re-evaluation of the Forsmark site for storage of spent fuel, but they do influence how to interpret the deeper structures and, as a consequence, the tectonic evolution of the area.

  • 3.
    Johansson, Åke
    et al.
    Swedish Museum of Natural History, Department of Geosciences.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Timing of magmatism and migmatization in the 2.0–1.8 Ga accretionary Svecokarelian orogen, south-central Sweden2017In: International journal of earth sciences, ISSN 1437-3254, E-ISSN 1437-3262, Vol. 106, no 3, p. 783-810Article in journal (Refereed)
    Abstract [en]

    The Palaeoproterozoic (2.0–1.8 Ga) Svecokarelian orogen in central Sweden consists of a low-pressure, predominantly medium-grade metamorphic domain (central part of Bergslagen lithotectonic unit), enclosed to the north and south by low-pressure migmatite belts. Two periods of metamorphism (1.87–1.85 and 1.83–1.79 Ga) are known in the migmatite belts. In this study, new U–Th–Pb ion microprobe data on zircon and monazite from twelve samples of locally migmatized gneisses and felsic intrusive bodies determine both protolith and metamorphic ages in four sample areas north of Stockholm, inside or immediately adjacent to the medium-grade metamorphic domain. Two orthogneiss samples from the Rimbo area yield unusually old protolith ages of 1909 ± 4 and 1908 ± 4 Ma, while three orthogneisses from the Skutskär and Forsmark areas yield more typical protolith ages between 1901 ± 3 and 1888 ± 3 Ma. Migmatized paragneiss samples from this and two earlier studies contain a significant detrital component sourced from this 1.9 Ga magmatic suite. They are interpreted to be deposited contemporaneously with or shortly after this magmatism. Migmatization of the paragneiss at Rimbo was followed by intrusion of leucogranite at 1846 ± 3 Ma. Even in the other sample areas to the north (Hedesunda-Tierp, Skutskär and Forsmark), metamorphism including migmatization is constrained to the 1.87–1.85 Ga interval and penetrative ductile deformation is limited by earlier studies in the Forsmark area to 1.87–1.86 Ga. However, apart from a metamorphic monazite age of 1863 ± 1 Ma, precise ages were not possible to obtain due to the presence of only partially reset recrystallized domains in zircon, or highly discordant U-rich metamict and altered metamorphic rims. Migmatization was contemporaneous with magmatic activity at 1.87–1.84 Ga in the Bergslagen lithotectonic unit involving a mantle-derived component, and there is a spatial connection between migmatization and this magmatic phase in the Hedesunda-Tierp sample area. The close spatial and temporal interplay between ductile deformation, magmatism and migmatization, the P–T metamorphic conditions, and the continuation of similar magmatic activity around and after 1.8 Ga support solely accretionary rather than combined accretionary and collisional orogenic processes as an explanation for the metamorphism. The generally lower metamorphic grade and restricted influence of the younger metamorphic episode, at least at the ground surface level, distinguishes the central part of the Bergslagen lithotectonic unit from the migmatite belts further north and south.

  • 4.
    Kampmann, Tobias Christoph
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Jansson, Nils F.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael B.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Olin, Paul H.
    CODES ARC Centre of Excellence and TMVC ARC Research Hub, University of Tasmania.
    Gilbert, Sarah
    CODES ARC Centre of Excellence and TMVC ARC Research Hub, University of Tasmania.
    Wanhainen, Christina
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Syn-tectonic sulphide remobilization and trace element redistribution at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden2018In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360, Vol. 96, p. 48-71Article in journal (Refereed)
    Abstract [en]

    Mineralization types at the Palaeoproterozoic Falun base metal sulphide deposit are predominantly pyritic Zn-Pb-Cu-rich massive sulphide mineralization, disseminated to semi-massive Cu-Au mineralization, auriferous quartz veins, and mineralized shear zones of talc-chlorite-dominated schist. The massive and disseminated to semi-massive sulphide mineralization types were subject to polyphase ductile deformation (D1 and D2) and metamorphism under low-P, lower-amphibolite facies conditions, which led to the development of ore textures and paragenetic relationships indicating both mechanical and chemical remobilization of sulphides. In the massive sulphide mineralization, rare inclusion-rich pyrite occurs as relic cores inside inclusion-poor metamorphosed pyrite. Imaging and spot analysis using multielement laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) reveal that inclusion-poor pyrite was depleted in trace elements, which were originally present as non-stoichiometric lattice substitutions or in mineral inclusions. The inclusion-rich pyrite was shielded from depletion and, at least partly, retained its initially higher trace element concentrations, including Au.

    Gold is also associated with chalcopyrite in the disseminated to semi-massive Cu-Au mineralization and in the system of auriferous quartz veins hosted therein, the latter being also affected by the D2 ductile strain. It is inferred that emplacement of the vein system took place after the peak of metamorphism, which occurred between D1 and D2, but prior to and possibly even shortly after completion of the D2 deformational event. Similarities in trace element signatures in chalcopyrite are compatible with the interpretation that the quartz veins formed by local chemical remobilization of components from the Cu-Au mineralization. Transport of liberated Au from pyrite during grain growth in the massive sulphide mineralization may have upgraded the Au endowment in the quartz veins, leading to the additional formation of native gold in the veins. A strong correspondence between elements liberated from pyrite (e.g. Pb, Bi, Se and Au) and those forming discrete and characteristic mineral phases in the quartz veins (Pb-Bi sulphosalts, native gold) supports this hypothesis.

    Trace element signatures for the main sulphide minerals pyrite, chalcopyrite, sphalerite and galena are similar to previously published data from other metamorphosed massive sulphide deposits. The association of the Falun mineralization with elevated Bi is reflected by its occurrence in sulphide minerals (e.g. galena) and in abundant mineral inclusions of Pb-Bi sulphosalts (e.g. weibullite), especially in the disseminated to semi-massive Cu-Au mineralization. Elevated Sn concentrations in the lattice and/or as cassiterite inclusions in chalcopyrite, sphalerite and galena are compatible with a hot, acidic and reducing fluid during formation of the syn-volcanic, base metal sulphide mineralization and associated host-rock alteration.

  • 5.
    Kampmann, Tobias Christoph
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Jansson, Nils
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Majka, Jarosław
    Uppsala universitet, AGH University of Science and Technology, Mickiewicza.
    Lasskogen, Jonas
    Boliden Mineral AB.
    Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen ore district, Fennoscandian Shield, Sweden2017In: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 112, no 5, p. 1111-1152Article in journal (Refereed)
    Abstract [en]

    The Paleoproterozoic Falun Zn-Pb-Cu-(Au-Ag) pyritic sulfide deposit in the Bergslagen ore district, Sweden, is enveloped by hydrothermally altered rocks metamorphosed to the lower amphibolite facies. Immobile-element ratios suggest that the alteration precursors were volcanic rocks of mainly rhyolitic to dacitic composition. Least altered examples of these rocks plot along magmatic fractionation trends outlined by late- to post-ore feldspar-phyric metadacite dikes and post-ore granitoid plutons, consistent with a comagmatic relationship between these calc-alkaline, coeval (<10-m.y.) suites. Dolomite or calcite marble, as well as diopside-hedenbergite or tremolite skarn, form subordinate but important lithologic components in the hydrothermally altered zone. Marble occurs as fragments in the massive pyritic sulfide mineralization, suggesting that at least some mineralization formed by carbonate replacement.

    Mass-change calculations suggest that the hydrothermally altered volcanic rocks gained Mg and Fe and generally lost Ca, K, and Na. Proximal, quartz-anthophyllite-rich altered rocks additionally gained Si, whereas several types of biotite-rich altered rocks lost this element. These mass changes along with mineral chemical data for anthophyllite, biotite, cordierite, and garnet, and the common occurrence of quartz indicate that chloritization, sericitization, and silicification were the dominant premetamorphic alteration styles. A zonation from distal sericitized and silicified volcanic rocks to intermediate sericitized rocks, partly overprinted by chloritization (Mg-rich chlorite), and proximal siliceous and intensely chloritized (Fe-rich chlorite) rocks has been identified. Furthermore, mass changes in more peripheral parts of the altered zone toward the southeast of the deposit suggest that the alteration weakens gradationally toward the volcanic and subvolcanic rocks surrounding the deposit. These patterns represent vectors toward mineralization.

    Intensely chloritized rocks, largely represented by a single, rhyolitic precursor, envelop the central pyritic massive sulfide bodies to the east, south, and west, supporting a structural model in which the massive sulfide mineralization formed the stratigraphically highest preserved unit in the center, surrounded in a tubular manner by stratigraphic footwall rocks. The northern side represents a portion of the footwall, which was separated by a major shear zone. These spatial relationships also have implications for near-mine exploration, since quartz-rich footwall rocks locally host disseminated to semimassive stockwork Cu-Au mineralization.

    Cooling of a hot (300°–400°C), acidic (pH ≤4) and reducing fluid carrying metals and sulfur is suggested for formation of stockwork Cu-Au vein mineralization and hydrothermal alteration in the stratigraphic footwall. The Zn-Pb-Cu-rich massive sulfide mineralization is inferred to have formed by fluid neutralization upon interaction with carbonates and mixing with cooler seawater upon fluid entry into porous pumice breccia in a subseafloor setting. Dissolution processes, primary porosity in the pumice breccia, and secondary porosity produced during synvolcanic faulting are all suggested to have contributed to the creation of space necessary for the formation of the massive sulfide mineralization. Falun differs from other deposits of the same type in Bergslagen mainly in the high pyrite content of the massive sulfide mineralization, the absence of related Fe oxide deposits, as well as the dominant replacement of volcaniclastic sediments compared to carbonates. The types of host rocks, the inferred premetamorphic feldspar-destructive alteration types, and the style of mineralization and alteration zonation at the deposit are reminiscent of pyritic volcanogenic massive sulfide (VMS) deposits. However, the importance of chemical trapping by fluid-limestone interaction, as well as the spatial association with subordinate skarn alteration constitute important differences to a classic VMS model.

  • 6.
    Kampmann, Tobias Christoph
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Structural geology and spatial patterns of hydrothermal alteration at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen region, south-central Sweden2014Conference paper (Refereed)
  • 7.
    Kampmann, Tobias Christoph
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    3D modelling and sheath folding at the Falun Zn-Pb-Cu-(Au-Ag) massive sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden2016In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 51, no 5, p. 665-680Article in journal (Refereed)
    Abstract [en]

    The Falun pyritic Zn-Pb-Cu-(Au-Ag) deposit, situated in the Palaeoproterozoic (1.9–1.8 Ga) Bergslagen lithotectonic unit in the south-western part of the Fennoscandian Shield, is one of the major base metal sulphide deposits in Sweden. Altered rocks and ore types at Falun have been metamorphosed and deformed in a heterogeneous ductile manner, strongly modifying mineral assemblages in the original hydrothermal alteration system and the geometry of the deposit. Using a combined methodological approach, including surface mapping of lithologies and structures, drill core logging and microstructural investigation, the polyphase character (D1 and D2) of the ductile deformation is demonstrated and a 3D model for the deposit created. F2 sheath folding along axes that plunge steeply to the south-south-east, parallel to a mineral stretching lineation and the dip direction of the S2 foliation, is suggested as a key deformation mechanism forming steeply plunging, rod-shaped ore bodies. This is in contrast to previous structural models involving fold interference and, in turn, has implications for near-mine exploration, the occurrence of hanging-wall components to the ore body being questioned. Typical rock-forming minerals in the Falun alteration aureole include quartz, biotite/phlogopite, cordierite, anthophyllite and minor almandine, andalusite and chlorite, as well as dolomite, tremolite and actinolite. Where observable, the silicate minerals in the alteration rocks show growth patterns during different phases of the tectonothermal evolution, considerable static grain growth occurring between D1 and D2 and even after D2. A major high-strain zone, characterized by the mineral assemblage talc-chlorite-(quartz-biotite/phlogopite) defines a boundary between northern and southern structural domains at the deposit, and is closely spatially associated with the polymetallic massive sulphide ores. A possible role as a metal-bearing fluid conduit during ore genesis is discussed.

  • 8.
    Kampmann, Tobias Christoph
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Hydrothermal alteration, 3D modeling and sheath folding at the volcanic-hosted Falun Zn-Pb-Cu-(Au-Ag) deposit – implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, central Sweden2014Conference paper (Refereed)
    Abstract [en]

    The Paleoproterozoic (1.9 Ga), volcanic-hosted Zn-Pb-Cu-(Au-Ag) sulfide deposit at Falun is located in the Bergslagen lithotectonic unit, central Sweden, which includes one of the major ore districts in the Fennoscandian Shield, northern Europe. The Falun deposit is known mainly as one of the world’s leading copper suppliers over many centuries. During the 20th century, the mine was a major base (Zn, Pb, Cu) and precious (Ag, Au) metal producer until it closed during 1992.This study has the following four aims: (i) Identify the style and spatial distribution of hydrothermal alteration; (ii) determine the geometry of the different types of ore bodies; (iii) provide a mechanism for the structure of the deposit; and (iv) address broader implications for the Bergslagen ore district. Petrographic and structural data were collected during surface mapping and microscope work; modeling of the different ore bodies in 3D space was completed using available mine level maps and data collected during new logging of available drill cores. The Falun deposit is affected by polyphase ductile deformation and metamorphism under amphibolite facies conditions. The metamorphosed alteration rocks are dominated by distal quartz-mica-cordierite-(anthophyllite) and proximal quartz-anthophyllite assemblages, interpreted to represent Si-, Fe-, Mg-metasomatism of felsic volcanic rocks. Dolomite and calc-silicate (tremolite, actinolite, diopside)-skarn assemblages are interpreted as the equivalent alteration of carbonate rocks. Surface mapping in the open pit indicates that the ore bodies are completely enveloped by these altered rocks. Structural data suggest the presence of a reclined F2 fold that plunges steeply to the southeast, with a stretching component defined by a linear grain-shape fabric sub-parallel to the fold axis. Modeling in 3D space reveals the presence of several rod-shaped ore bodies that also plunge steeply to the southeast and thicken and merge upwards into a single ore body that is up to 270 m in diameter at the ground surface. The ore body close to this surface is zoned in a concentric pattern, from a massive, pyritic Zn-Pb-Cu-sulfide core in the inner part to a more Cu-rich sulfide zone and then a semi-massive to disseminated Cu-Au mineralization in the outer part. The cone-shaped and zoned ore bodies are interpreted as steeply-plunging megascopic sheath folds, formed in a ductile, high-strain tectonic regime. The viscosity contrast between competent, strongly silicified and metamorphosed felsic volcanic host rock and softer massive sulfide ore is suggested to have enhanced the development of these sheath folds. Steeply plunging, rod-shape geometries have commonly been reported for several volcanic-hosted sulfide and Fe oxide ore deposits in the Bergslagen lithotectonic unit and megascopic sheath folds have been identified in a high-strain belt in the northern part of this unit. If sheath fold formation can be confirmed as a key deformation mechanism for ore bodies in this mineral district, in contrast to the classical model of dome and basin fold interference structure, this will influence near-mine exploration strategies. Previous structural concepts and models for footwall/hanging wall relationships will need radical revision and areas previously considered as barren hanging wall lithologies may have a higher exploration potential.

  • 9.
    Kampmann, Tobias Christoph
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael B.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Structural investigation and 3D modelling of the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen region, south-central Sweden2013Conference paper (Refereed)
    Abstract [en]

    One of the prominent ore deposits of the Bergslagen region in south-central Sweden is located in Falun in the northern part of this mining district. A belt of 1.91-1.89 Ga metavolcanic rocks hosts both the ores and an alteration aureole of several hundreds to thousands of metres in extent at the ground surface. Analysis of the structures in the area reveals a polyphase ductile deformational history and a major, steeply plunging F2 reclined fold. 3D modelling of the boundary surface to the pyritic Zn-Pb-Cu sulphide ore has been used to visualize the geometry of this ore body at depth, constraining its steeply plunging rod-like and NW-SE elongate shape and a geometric control by the F2 fold structure.

  • 10.
    Kampmann, Tobias
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Jansson, Nils
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Majka, Jarosław
    Uppsala universitet.
    Systematics of hydrothermal alteration at the volcanic-hosted Falun Zn-Pb-Cu-(Au-Ag) deposit: implications for ore genesis, structure and exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden2016In: Geophysical Research Abstracts, ISSN 1029-7006, E-ISSN 1607-7962, Vol. 18Article in journal (Refereed)
  • 11.
    Kampmann, Tobias
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Sveriges Geologiska Undersökning.
    Ripa, Magnus
    Sveriges Geologiska Undersökning.
    Hellström, Fredrik
    Sveriges Geologiska Undersökning.
    Timing of magmatism and mineralisation at Falun, a major base metal sulphide deposit in the Fennoscandian Shield, Sweden2015Conference paper (Refereed)
  • 12.
    Kampmann, Tobias
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Sveriges Geologiska Undersökning.
    Ripa, Magnus
    Sveriges Geologiska Undersökning.
    Hellström, Fredrik
    Sveriges Geologiska Undersökning.
    Majka, Jarosław
    Uppsala universitet.
    Time constraints on magmatism, mineralisation and metamorphism at the Falun base metal sulphide deposit, Sweden, using U-Pb geochronology on zircon and monazite2016In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 278, p. 52-68Article in journal (Refereed)
    Abstract [en]

    U–Th–Pb (zircon and monazite) ion probe data have provided constraints on the timing of emplacement and metamorphism of magmatic rocks close to the Palaeoproterozoic, Falun base metal sulphide deposit in the Bergslagen lithotectonic unit, Fennoscandian Shield, Sweden, and, thereby the timing of mineralisation. Hydrothermal alteration and mineralisation at Falun are constrained to a short interval of several million years between a 207Pb/206Pb weighted average age of 1894 ± 3 Ma for a rhyolitic sub-volcanic rock in the felsic volcanic to sub-volcanic host rock suite, and a 207Pb/206Pb weighted average age of 1891 ± 3 Ma for a post-sulphide, porphyritic dacite dyke. Magmatism also included the emplacement of granite plutons with igneous crystallization ages of 1894 ± 3, 1894 ± 2 Ma and 1893 ± 3 Ma. The felsicsub-volcanic to volcanic activity and the emplacement of dacite dykes and granite plutons overlap in age within their respective analytical uncertainties, indicating hydrothermal alteration and sulphide mineralisation inside a narrow time span of intense magmatic activity, and burial of the supracrustal rocks.Two distinct patchy and homogeneous metamorphic monazite types in a felsic volcanic rock around and hydrothermally altered rocks at the Falun deposit yield 207Pb/206Pb weighted average ages of 1831 ± 8 Ma and 1822 ± 5 Ma, respectively. These ages fall well within the temporal range of a younger1.84–1.81 Ga (M2) metamorphic episode during the 2.0–1.8 Ga Svecokarelian orogeny, with the older episode (M1) inside the Bergslagen lithotectonic unit at around 1.86 Ga. This shows the major influence of the M2 event in the north-western part of this unit, leading to a complete resetting of the U–Th–Pb isotopesystem in monazite.

  • 13.
    Möller, Charlotte
    et al.
    Department of Geology, University of Lund.
    Bingen, Bernard
    Geological Survey of Norway, Trondheim.
    Andersson, Jenny
    Geological Survey of Sweden.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Viola, Giulio
    Geological Survey of Norway, Trondheim.
    Scherstén, Anders
    Department of Geology, University of Lund.
    A non-collisional, accretionary Sveconorwegian orogen: comment2013In: Terra Nova, ISSN 0954-4879, E-ISSN 1365-3121, Vol. 25, no 2, p. 165-168Article in journal (Refereed)
  • 14.
    Petersson, Jesper
    et al.
    Vattenfall Research & Development.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mattsson, Håkan
    Geovista AB.
    Möller, Charlotte
    University of Lund.
    Albitization and quartz dissolution in Paleoproterozoic metagranite, central Sweden: implications for the disposal of spent nuclear fuel in a deep geological repository2012In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 148, p. 10-26Article in journal (Refereed)
    Abstract [en]

    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.

  • 15.
    Saintilan, Nicholas J.
    et al.
    Section of Earth and Environmental Sciences, University of Geneva.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Spikings, Richard A.
    Section of Earth and Environmental Sciences, University of Geneva.
    Schneider, Jens C.
    Department of Mineralogy, Technische Universität Bergakademie Freiberg.
    Chiaradia, Massimo
    Section of Earth and Environmental Sciences, University of Geneva.
    Spangenberg, Jorge E.
    Institute of Earth Surface Dynamics, University of Lausanne.
    Ulianov, Alexey
    Institute of Earth Sciences, University of Lausanne.
    Fontboté, Lluis
    Section of Earth and Environmental Sciences, University of Geneva.
    Polyphase vein mineralization in the Fennoscandian Shield at Åkerlandet, Järvsand, and Laisvall along the erosional front of the Caledonian orogen, Sweden2017In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 52, no 6, p. 823-844Article in journal (Refereed)
    Abstract [en]

    The Åkerlandet, Järvsand, and Laisvall deposits in Sweden are calcite-fluorite-sulfide vein deposits and occurrences located close to the current erosional front of the Caledonian orogen and hosted by crystalline basement rocks in the Fennoscandian Shield. At Laisvall, basement-hosted veinlets occur beneath Ediacaran to Cambrian sandstones that host a strata-bound Pb-Zn deposit. The mineralized fractures at Åkerlandet and Järvsand occur along fault systems oriented N–S to NNW–SSE. Veins or veinlets strike NNW–SSE and NW–SE at Åkerlandet, NNE–SSW at Järvsand, and NNW–SSE and NNE–SSW to NE–SW at Laisvall. At Åkerlandet and Järvsand, fractures acted as conduits for hydrothermal fluids of variable composition and formed during separate tectonic events. At Åkerlandet, the fault zone with NNW–SSE strike shows kinematic indicators consistent with ~NE–SW bulk horizontal extension. At Järvsand, the calcite-fluorite-galena veins formed along R-Riedel shears related to the host N–S to NNW–SSE fault system. The kinematic indicators are consistent with ~NW–SE bulk horizontal extension, similar to the extensional deformation during the later part of the Caledonian orogeny (Silurian to Devonian). At Åkerlandet, adularia-quartz deposition was followed by sphalerite ± galena and finally by precipitation of fluorite and calcite. 40Ar-39Ar thermochronology of a single adularia sample did not yield a well-defined plateau age but the gas released at higher temperatures suggests an early Tonian (980 to 950 Ma) crystallization age, i.e., during the later part of the Sveconorwegian orogeny, although the data do not exclude other less likely interpretations. Previous fluid inclusion microthermometry and geochronological studies and new petrographic and geochemical results suggest that sphalerite ± galena mineralization formed from saline, relatively oxidizing, moderate-temperature, and slightly acidic hydrothermal fluids, either during the Ediacaran or the Middle Ordovician. Metals and H2S were derived from local basement rocks. Based on petrographic evidence, rare earth element composition, and S, C, and O isotope data, fluorite and calcite precipitated under near neutral and relatively reducing conditions. Occurrence of solid bitumen in veins at Åkerlandet and C and O isotope data of calcite at Åkerlandet and in the Laisvall basement veinlets suggest that the precipitation of calcite and fluorite was triggered by interaction of hot and evolved hydrothermal fluids (87Sr/86Sr = 0.718–0.732) with organic matter. Structural, petrographic, and geochemical data at Laisvall suggest that the basement structures hosting calcite-fluorite ± pyrite veinlets were utilized in the Middle Ordovician as the plumbing system for the oxidizing, slightly acidic, metal-bearing brines that caused the economic Pb-Zn mineralization in the overlying sandstones

  • 16.
    Saintilan, Nicolas J. D
    et al.
    University of Geneva, Department of Earth and Environmental Sciences, Section of Earth and Environmental Sciences, University of Geneva.
    Schneider, Jens C.
    Department of Mineralogy, Technische Universität Bergakademie Freiberg.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Chiaradia, Massimo
    Section of Earth and Environmental Sciences, University of Geneva.
    Kouzmanov, Kalin
    Section of Earth and Environmental Sciences, University of Geneva.
    Wälle, Marküs
    Institute of Geochemistry and Petrology, ETH Zürich.
    Fontboté, Lluís
    Section of Earth and Environmental Sciences, University of Geneva.
    A middle ordovician age for the laisvall sandstone-hosted Pb-Zn deposit, Sweden: A response to early caledonian orogenic activity2015In: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 110, no 7, p. 1779-1801Article in journal (Refereed)
    Abstract [en]

    Ten sphalerite separates isolated from mineralized samples in proximal and distal positions relative to the proposed main feeder fault systems at the Laisvall deposit were used to obtain an absolute age determination of this world-class Pb-Zn deposit hosted by autochthonous Ediacaran to Lower Cambrian sandstone and located currently along the erosional front of the Scandinavian Caledonides in northern Sweden. Residue and leachate fractions of each separate were obtained using the crush-leaching technique. All samples correspond to sphalerite formed using reduced sulfur derived from thermochemical sulfate reduction, three of them from disseminated ore in the Lower Sandstone, two from the disseminated ore in the Upper Sandstone, and five from steeply dipping galena-sphalerite-calcite veinlets interpreted in previous works as remobilization of disseminated ores. The isotope dilution-thermal ionization mass spectrometry (ID-TIMS) data yield an overall complex Rb-Sr isotope pattern with two distinct trends in the 87Sr/86Sr vs. 87Rb/86Sr isochron diagram. The three sphalerite residues of disseminated mineralization from the Lower Sandstone orebody show Rb-Sr isotope systematics indicative of undisturbed primary precipitates, and yield an isochron model age of 467 ± 5 Ma (mean square weighted deviation, MSWD, 1.4). Since the isochron is based on three points, the obtained age is to be considered as preliminary. Yet, the obtained age is fully consistent with geologic evidence reported by previous authors and pointing to Middle Ordovician timing of ore formation. The ID-TIMS data were complemented by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses on the same sphalerite samples. The data support the hypothesis that the measured ID-TIMS Rb and Sr contents in these sphalerite residues are held in the sphalerite structure itself and are not related to micro-inclusions. The most viable hypothesis, in agreement with published work, is that during rapid growth, sphalerite may incorporate Rb and Sr ions from the hydrothermal fluids in its structure, most probably in octahedral voids. By contrast, the second trend in the 87Sr/86Sr vs. 87Rb/86Sr space defined by most other sphalerite residues and corresponding inclusion fluid leachates from the Upper Sandstone orebody and the veinlet samples is too steep to account for a realistic isochron age determination. This steep linear trend is interpreted to represent a postmineralization disturbance involving fluids rich in Sr. This disturbance of the Rb-Sr isotope system is consistent with the presence of the steeply dipping galena-sphalerite-calcite veinlets and the fact that the Upper Sandstone is, in places, tectonically disrupted because of its proximity to the basal Caledonian décollement. The attempt to date the Granberget deposit, located in tectonically disrupted allochthonous units inside the Caledonian orogen, failed because the Rb-Sr isotope systematics of the three analyzed sphalerite samples are also disturbed. The obtained Middle Ordovician (467 ± 5 Ma) mineralization age at Laisvall can be interpreted as a far-field foreland response to an early Caledonian arc-continent collision and the subsequent development of a foreland basin. Basinal brines formed in the foredeep of the orogen could be conveyed cratonward, interact with permeable Baltica crystalline basement rocks, and resurge as metal-bearing fluids in sandstone at Laisvall along reactivated Paleoproterozoic crystalline basement faults. Mixing of metal-bearing brines with hydrocarbon and H2S-rich fluids in Ediacaran to Lower Cambrian sandstone may explain the initial Sr isotope signature (87Sr/86Sr = 0.715900 ± 60) of the isochron intersect

  • 17.
    Saintilan, Nicolas J. D
    et al.
    University of Geneva, Department of Earth and Environmental Sciences.
    Spangenberg, Jorge E.
    Institute of Earth Surface Dynamics, University of Lausanne.
    Samankassou, Elias
    University of Geneva, Department of Earth and Environmental Sciences.
    Kouzmanov, Kalin
    Section of Earth and Environmental Sciences, University of Geneva, University of Geneva, Department of Earth and Environmental Sciences.
    Chiaradia, Massimo
    Section of Earth and Environmental Sciences, University of Geneva, University of Geneva, Department of Earth and Environmental Sciences.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Fontboté, Lluís
    University of Geneva, Department of Earth and Environmental Sciences, Section of Earth and Environmental Sciences, University of Geneva.
    A refined genetic model for the Laisvall and Vassbo Mississippi Valley-type sandstone-hosted deposits, Sweden: constraints from paragenetic studies, organic geochemistry, and S, C, N, and Sr isotope data2016In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 51, no 5, p. 639-664Article in journal (Refereed)
    Abstract [en]

    The current study has aimed to refine the previously proposed two-fluid mixing model for the Laisvall (sphalerite Rb-Sr age of 467 ± 5 Ma) and Vassbo Mississippi Valley-type deposits hosted in Ediacaran to Cambrian sandstone, Sweden. Premineralization cements include authigenic monazite, fluorapatite, and anatase in the Upper Sandstone at Laisvall, reflecting anoxic conditions during sandstone burial influenced by the euxinic character of the overlying carbonaceous middle Cambrian to Lower Ordovician Alum Shale Formation (δ 13Corg = −33.0 to −29.5 ‰, δ 15Norg = 1.5 to 3.3 ‰, 0.33 to 3.03 wt% C, 0.02 to 0.08 wt% N). The available porosity for epigenetic mineralization, including that produced by subsequent partial dissolution of pre-Pb-Zn sulfide calcite and barite cements, was much higher in calcite- and barite-cemented sandstone paleoaquifers (29 % by QEMSCAN mapping) than in those mainly cemented by quartz (8 %). A major change in the Laisvall plumbing system is recognized by the transition from barite cementation to Pb-Zn sulfide precipitation in sandstone. Ba-bearing, reduced, and neutral fluids had a long premineralization residence time (highly radiogenic 87S/86Sr ratios of 0.718 to 0.723) in basement structures. As a result of an early Caledonian arc-continent collision and the development of a foreland basin, fluids migrated toward the craton and expelled Ba-bearing fluids from their host structures into overlying sandstone where they deposited barite upon mixing with a sulfate pool (δ 34Sbarite = 14 to 33 ‰). Subsequently, slightly acidic brines initially residing in pre-Ediacaran rift sediments in the foredeep of the early Caledonian foreland basin migrated through the same plumbing system and acquired metals on the way. The bulk of Pb-Zn mineralization formed at temperatures between 120 and 180 °C by mixing of these brines with a pool of H2S (δ 34S = 24 to 29 ‰) produced via thermochemical sulfate reduction (TSR) with oxidation of hydrocarbons in sandstone. Other minor H2S sources are identified. Upward migration and fluctuation of the hydrocarbon-water interface in sandstone below shale aquicludes and the formation of H2S along this interface explain the shape of the orebodies that splay out like smoke from a chimney and the conspicuous alternating layers of galena and sphalerite. Intimate intergrowth of bitumen with sphalerite suggests that subordinate amounts of H2S might have been produced by TSR during Pb-Zn mineralization. Gas chromatograms of the saturated hydrocarbon fraction from organic-rich shale and from both mineralized and barren sandstone samples indicate that hydrocarbons migrated from source rocks in the overlying Alum Shale Formation buried in the foredeep into sandstone, where they accumulated in favorable traps in the forebulge setting.

  • 18.
    Saintilan, Nicolas J. D
    et al.
    University of Geneva, Department of Earth and Environmental Sciences.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Lundstam, Erik
    Boliden AB.
    Fontboté, Lluís
    University of Geneva, Department of Earth and Environmental Sciences.
    Control of Reactivated Proterozoic Basement Structures on Sandstone-Hosted Pb-Zn Deposits along the Caledonian Front, Sweden: Evidence from Airborne Magnetic Data, Structural Analysis, and Ore-Grade Modeling2015In: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 110, no 1, p. 91-117Article in journal (Refereed)
    Abstract [en]

    Strata-bound, nonstratiform, epigenetic galena-sphalerite-cement mineralization in Ediacaran-Cambrian sandstone, including the previously mined deposits at Laisvall and Vassbo, occurs along the eastern erosional front of the Caledonian orogen in Sweden. The sandstone is part of an autochthonous siliciclastic sedimentary sequence that rests unconformably on top of Proterozoic crystalline basement beneath the Caledonian thrust nappes. Linear anomalies have been identified in high-resolution airborne magnetic data that correspond to geologic features in the Proterozoic basement. Furthermore, the Laisvall and Vassbo strata-bound Pb-Zn deposits are both spatially associated with areas of change in the trend of the magnetic lineaments. Magnetic anomalies, trending either N-S to NE-SW and WNW-ESE to NW-SE in the Laisvall area, and NNE-SSW to NNW-SSE and NW-SE to W-E in the Vassbo area, were identified. In the Laisvall area, some magnetic minima and edges along magnetic gradients can be correlated with faults in the Proterozoic basement. The reactivation of these basement structures is expressed in the Ediacaran-Cambrian sedimentary cover rocks as newly formed faults with Phanerozoic displacement. Along individual faults belonging to two sets (NE-SW to N-S and WNW-ESE to NW-SE), synsedimentary block movement has been recognized. The highest Pb and Zn grades in Laisvall delineate orebodies and orebody trends that follow these faults. Areas where the faults change strike contain some of the largest and richest orebodies. In the Vassbo area, the orebody footprint reflects a folded dolerite dike in the underlying Proterozoic basement. The dike, modeled on the basis of borehole data, is recognized by a magnetic maximum and an edge along a magnetic gradient. No faults have been mapped at the ground surface as being related to the location of dolerite dikes in the basement. However, it is considered that the basement dikes illustrate a structural control, emplacement either producing a local fracture network or being driven by preexisting basement structures. The main orebodies in both deposits display funnel-shape geometry, fault-rooted in Laisvall and located close to the hinges of the folded dolerite dike in the basement at Vassbo. Metal distribution patterns are similar in both deposits and are characterized by Pb-rich cores proximal to the basement-steered structures while Zn-rich shells are distal from these structures. The funnel-shaped ore geometry is interpreted to reflect a fault-rooted migration path and the metal precipitation mechanism. In both deposits, the highest Pb and Zn grades occur at the top of sandstone paleoaquifers. Similar mineralization footprints, variation in grades, and paleoaquifer settings were recognized in several carbonate-hosted Mississippi Valley-type (MVT) Zn-Pb deposits (e.g., San Vicente deposit, Peru; Topla-Mežica deposits, Slovenia). This geometry is suggestive of a sour gas trap that accumulated by density at the top of paleoaquifers. This gas could have provided H2S by thermogenic sulfate reduction to the metal-bearing fluids and triggered precipitation of Pb-Zn sulfides. The combined evidence from the airborne magnetic data, the structural analysis and the geometry of the orebodies, and metal distribution suggests that the basement faults reactivated during the Ediacaran-Cambrian sedimentation, acted at a later time as feeders for the metal-bearing fluids to fertile horizons for mineralization, and localized deformation during postsedimentary and postmineralization tectonics.

  • 19.
    Saintilan, Nicolas J.
    et al.
    Department of Earth Sciences, University of Geneva, Geneva, Switzerland.
    Spangenberg, Jorge E.
    Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland.
    Chiaradia, Massimo
    Department of Earth Sciences, University of Geneva , Geneva, Switzerland.
    Chelle-Michou, Cyril
    Department of Earth Sciences, University of Geneva, Geneva, Switzerland.
    Stephens, Michael B.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Formerly Geological Survey of Sweden (SGU), Uppsala, Sweden.
    Fontboté, Lluís
    Department of Earth Sciences, University of Geneva, Geneva, Switzerland.
    Petroleum as source and carrier of metals in epigenetic sediment-hosted mineralization2019In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 8283Article in journal (Refereed)
    Abstract [en]

    Sediment-hosted ore deposits contribute a significant amount (up to 65%) of the global resources of lead and zinc. Among them, the Mississippi-Valley type deposits and related oil fields often comprise large-scale hydrothermal systems where regional host rocks are stained with disseminated liquid petroleum (crude oil) and other organic compounds. Current models for the formation of those epigenetic Pb-Zn sulphide deposits consider that metals are mostly leached from basement rocks and their detrital erosional products, and transported by oxidized basinal hydrothermal fluids as chloride complexes. Sulphide precipitation mainly occurs when these basinal brines interact with fluids rich in reduced sulphur species produced mostly by thermochemical sulphate reduction (TSR) mediated by hydrocarbons. Here, using organic geochemistry and Pb isotopes, we provide evidence that petroleum and associated water were key for the formation of sulphide mineralization in the world-class sandstone-hosted ore deposit at Laisvall, not only by supplying reduced sulphur but also by contributing metals in significant amounts. The lead originally found in bitumen of the Alum Shale Formation was transported —during an arc-continent collisional event— by liquid petroleum and associated water to the site of sulphide mineralization. The alteration of petroleum by TSR made lead available for precipitation as sulphide. The petroleum-associated lead represents 40 to 60% of the metal budget in the deposit, the remainder being sourced by leaching of basement rocks.

  • 20.
    Stephens, Michael
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Andersson, Jenny
    Sveriges Geologiska Undersökning, Geological Survey of Sweden.
    Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0−1.8 Ga accretionary orogen, Sweden2015In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 264, p. 235-257Article in journal (Refereed)
    Abstract [en]

    Absolute ages of migmatization and protolith formation, and constraints on the timing of ductile deformation in two major lithotectonic units in the south-western part of a 2.0–1.8 Ga orogenic belt in the Fennoscandian Shield, south-eastern Sweden, have been determined using U-Pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite. Detrital and xenocrystic zircon in paragneiss and garnet-bearing leucogranite, respectively, in the Bergslagen lithotectonic unit show ages of 2.1–2.0 Ga and 1.9 Ga. Deposition of the sedimentary material occurred during or after a subduction-related magmatic event at 1.91–1.87 Ga. Two orthogneiss protoliths formed during this magmatic event around 1.88 Ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1.85 Ga igneous protolith. A polyphase tectonothermal evolution with anatexis under low-P metamorphic conditions around 1.86 Ga (M1) and 1.84–1.81 Ga (M2) is inferred for the migmatitic gneisses in the Bergslagen unit; garnet-bearing leucogranite crystallized around 1.84–1.83 Ga, close in time to major folding of the M1 gneissic fabric. A previously unrecognised 1.86–1.85 Ga ductile deformational event under medium-grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south (Småland lithotectonic unit), close in time to the M1 event in the Bergslagen unit to the north.By constraining the timing of anatexis and comparing with information bearing on crustal thickness, excess mass at depth and the character and age of magmatic activity, it is inferred that anatexis in the Bergslagen lithotectonic unit is related to pulses of mafic underplating, during the early stages of two separate, subduction-related magmatic episodes after the 1.91–1.87 Ga magmatic event. It is suggested that each pulse was related to intra- or back-arc spreading above a subduction boundary, which had entered a retreating mode with separate, long periods (20–50 Ma) of extension or transtension. This study challenges the need to invoke crustal thickening related to plate collision at 1.9–1.8 Ga as a mechanism to explain high-grade metamorphism in the southern part of the 2.0–1.8 Ga orogen. Instead, a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred.

  • 21.
    Stephens, Michael
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Follin, Sven
    SF GeoLogic AB, Domherrevägen 9, SE-183 51 Täby.
    Petersson, Jesper
    Norconsult AB, Box 8774, SE-402 76 Göteborg.
    Isaksson, Hans
    Geovista AB.
    Juhlin, Christopher
    Department of Earth Sciences, Uppsala University, Uppsala universitet.
    Simenonov, Assen
    Swedish Nuclear Fuel and Waste Management Company.
    Review of the deterministic modelling of deformation zones and fracture domains at the site proposed for a spent nuclear fuel repository, Sweden, and consequences of structural anisotropy2015In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 653, p. 68-94Article in journal (Refereed)
    Abstract [en]

    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.

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