The Vargfors basin in the central part of the Skellefte mining district is an inverted sedimentary basin within a Palaeoproterozoic (1. 89 Ga) marine volcanic arc. The fault-segmented basin formed from upper-crustal extension and subsequent compression, following a period of intense sub-marine volcanism and VMS ore formation. New detailed mapping reveals variations in stratigraphy attributed to syn-extensional sedimentation, as well as provenance of conglomerate clasts associated with tectonic activity at the transition from extension to compression. The onset of fan delta to alluvial fan sedimentation associated with basin subsidence indicates that significant dip-slip displacement accommodating rapid uplift of the intrusive complex and/or subsidence of the adjacent volcano-sedimentary domain took place along a major fault zone at the southern margin of the intrusive complex. Subsidence of the Jörn intrusive complex and/or its burial by sedimentary units caused a break in erosion of the intrusion and favoured the deposition of a tonalite clast-barren conglomerate. Clast compositions of conglomerates show that the syn-extensional deposits become younger in the south-eastern parts of the basin, indicating that opening of the basin progressed from north-west to south-east. Subsequent basin inversion, associated with the accretion to the Karelian margin, involved reverse activation of the normal faults and development of related upright synclines. Progressive crustal shortening caused the formation of break-back faults accompanied by mafic volcanic activity that particularly affected the southern contact of the Jörn intrusive complex and the northern contact of the Vargfors basin

The Vargfors basin in the central Skellefte district, Sweden, is an inverted sedimentary sub-basin within a Palaeoproterozoic (1.89 Ga) marine volcanic arc. The sub-basin formed from upper-crustal extension and subsequent compression, following a period of intense marine volcanism and VMS ore formation. Detailed mapping and structural analysis reveals a pattern of SE–NW-striking normal faults and interlinked NE–SW-striking transfer faults, which define distinct fault-bound compartments, each with an individual structural geometry and stratigraphy. Constraints on the deformation style and mechanisms achieved by 2D forward modelling are in agreement with the previously inferred inversion of the early normal faults during a regional crustal shortening event. A rheologically weak carbonate-rich layer at the base of the sedimentary sequence favoured the fault inversion over more distributed shortening as the controlling deformation mechanism. Transposition of sedimentary strata into the approximately SE–NW faults led to formation of asymmetric synclines that were tightened during progressive shortening. Structural analysis infers a progressive opening of the basin towards SE and NW with time. Furthermore, it is inferred that a displacement gradient was developed along the main structural grain, with decreasing dip-slip displacements towards SE and NW, both during the extension and the structural inversion.VMS deposits in the vicinity of the contact between the volcanic and the overlying sedimentary rocks were formed along early normal faults, which reacted as fluid conduits. Subsequently, the deposits were transposed into the inverted faults during crustal shortening. Consequently, the inverted faults provide a useful tool for mineral exploration in the district.

The Palaeoproterozoic Skellefte Mining District is host to abundant ore deposits. Geological 3Dmodelling was performed using the gOcad software platform. Geological methods such as field mapping, structural analysis and facies analysis combined with geophysical techniques such as reflection seismic investigations, resistivity, magnetic, electromagnetic and gravimetric studies and analysis of potential field data provide a framework for the reconstruction of the crustal geometry and geological history of the district as a tool for modern ore exploration. Results will be furthermore utilized for kinematic 4-dimensional modelling

The Skellefte district in northern Sweden is host to abundant volcanogenic massive sulphide (VMS) deposits comprising pyritic, massive, semi-massive and disseminated Zn–Cu–Au ± Pb ores surrounded by disseminated pyrite and with or without stockwork mineralisation. The VMS deposits are associated with Palaeoproterozoic upper crustal extension (D1) that resulted in the development of normal faults and related transfer faults. The VMS ores formed as sub-seafloor replacement in both felsic volcaniclastic and sedimentary rocks and partly as exhalative deposits within the uppermost part of the volcanic stratigraphy. Subsequently, the district was subjected to deformation (D2) during crustal shortening. Comparing the distribution of VMS deposits with the regional fault pattern reveals a close spatial relationship of VMS deposits to the faults that formed during crustal extension (D1) utilising the syn-extensional faults as fluid conduits. Analysing the shape and orientation of VMS ore bodies shows how their deformation pattern mimics those of the hosting structures and results from the overprinting D2 deformation. Furthermore, regional structural transitions are imitated in the deformation patterns of the ore bodies. Plotting the aspect ratios of VMS ore bodies and the comparison with undeformed equivalents in the Hokuroko district, Japan allow an estimation of apparent strain and show correlation with the D2 deformation intensity of the certain structural domains. A comparison of the size of VMS deposits with their location shows that the smallest deposits are not related to known high-strain zones and the largest deposits are associated with regional-scale high-strain zones. The comparison of distribution and size with the pattern of high-strain zones provides an important tool for regional-scale mineral exploration in the Skellefte district, whereas the analysis of ore body shape and orientation can aid near-mine exploration activities.

Four-dimensional geological modelling has been conducted in the Palaeoproterozoic Skellefte mining district. 3D-modelling of volcanic-hosted massive sulphide deposits and associated host-rocks has been carried out in multiple scales from deposit to regional scale and is based on a combination of geological and geophysical investigations. A conceptual model founded on unravelling the structural control on sedimentation, volcanism and mineralization and the subsequent deformation patterns, acts as a base for geological modelling. The final 3D-model provides a structural framework in which the mineralizations can be studied by improved understanding of the structural evolution in the mine areas, and by comparing the regional structural patterns versus the form and attitude of ore deposits. Additionally, uncertainty and prospectivity models were constructed showing the distribution of data and the potential of discovering new ore deposits. Subsequent 4D-modelling adds the time aspect to the 3D-models and aims at visualizing and understanding the geological history in the district and as a support for ore targeting. Moreover, adding geological time to the modelling helps gaining confidence about both the conceptual models and the 3D-models. The final 3D- and 4D-models provide a regional three-dimensional context for both industrial and academic activities in the Skellefte district, and aid the understanding of large-scale tectonic processes.

The central part of the Palaeoproterozoic Skellefte District in northern Sweden is host to several VMS deposits. This area is dominated by upright folds with axial surfaces trending WNW - ESE. Northeast - SW trending faults crosscut WNW - ESE trending faults and impart a distinct fault pattern. Subvertical stretching as expressed by subvertical mineral lineations as well as gently W-plunging mineral lineations parallel to the F2 fold axes indicate not only significant vertical movement, but also pronounced lateral movement. The faults formed in an extensional stage and were reactivated during a compressional stage oblique to the earlier phase. This crustal shortening caused folding and development of the main foliation. Overturned, tight to isoclinal folds within the Vargfors meta-sediments coincide with 1st and 2nd order faults and are considered to be related to reactivation of the early normal and transfer faults. A three dimensional model taking into account the structures was constructed using the GoCAD 3D-modelling software.

The Kristineberg mining area is located in the western part of the Palaeoproterozoic Skellefte Ore District, one of the most important mining districts in Europe. As a part of a 3D geologic modeling project, two new reflection seismic profiles were acquired with a total length of about 20 km. One profile (HR), parallel to previous seismic profiles, was acquired using a 10 m receiver and source interval and crosses the steeply dipping structures of the Kristineberg mine. The other profile (Profile 2) runs perpendicular to all existing profiles in the area. Although the structural geology is complex, the processed seismic data reveal a series of steeply dipping to sub-horizontal reflections, some of which reach the surface and allow correlation with surface geology. Our general interpretation of the seismic images is that the Kristineberg mine and associated mineral horizon are located in the northern part of a series of steeply south dipping structures. Overall, main structures plunge to the west at about 30° - 40°. Cross-dip analysis and reflection modeling were carried out to obtain the 3D orientation of the main reflections and to provide insight into the possible contribution of out-of-the-plane reflections. This helped, for example, to obtain the 3D geometry of a deep reflection that was previously interpreted as structural basement to volcanic rocks. The new reflection seismic profiles have improved our understanding of shallow geological structures in the area and in conjunction with recently acquired potential field data, magnetotelluric data and geological observations will help to refine previous 3D geologic modeling interpretations that were aimed at larger scale structures.

The Kristineberg mining area in the western part of the Skellefte ore district is the largest base metal producer in northern Sweden and currently the subject of extensive geophysical and geologic studies aimed at constructing 3D geologic models. Seismic reflection data form the backbone of the geologic modeling in the study area. A geologic cross section close to the Kristineberg mine was used to generate synthetic seismic data using acoustic and elastic finite-difference algorithms to provide further insight about the nature of reflections and processing challenges when attempting to image the steeply dipping structures within the study area. Synthetic data suggest processing artifacts manifested themselves in the final 2D images as steeply dipping events that could be confused with reflections. Fewer artifacts are observed when the data are processed using prestack time migration. Prestack time migration also was performed on high-resolution seismic data recently collected near the Kristineberg mine and helped to image a high-amplitude, gently dipping reflection occurring stratigraphically above the extension of the deepest Kristineberg deposit. Swath 3D processing was applied to two crossing seismic lines, west of the Kristineberg mine, to provide information on the 3D geometry of an apparently flat-lying reflection observed in both of the profiles. The processing indicated that the reflection dips about 30° to the southwest and is generated at the contact between metasedimentary and metavolcanic rocks, the upper part of the latter unit being the most typical stratigraphic level for the massive sulfide deposits in the Skellefte district.

This paper exemplifies how the dynamic crustal evolution within geologically complex areas, such as the Palaeoproterozoic VMS-hosting Skellefte district in northern Sweden, may be illustrated by 4D-animations. Furthermore, the paper gives a brief description on how the animation was constructed. The modeled steps through the geological evolution include crustal extension accommodated by normal faults, synchronous volcanism and VMS deposition, block rotations and erosion post-dating the volcanism, deposition of post-volcanic sediments and, finally, inversion of the early normal faults and the related development of upright folds. Specific emphasis is put on illustrating how the originally stratabound VMS sheets were transposed into contrasting geometries depending on erosional features and variations in the tectonic overprint. Summing up, 4D-animations provide a powerful tool for sharing the scientific results and for use in educational purposes.

These intensely folded granitic veins cutting Palaeoproterozoic quartz-feldspathic gneisses mark both the peak in late-Svecofennian crustal melting at 1835-1825 Ma, expressed by emplacement of voluminous S-type granites, and synchronous penetrative flattening-style deformation of the crust. The flattening is considered to reflect mid-crustal extension that triggered melting of the crust that was previously thickened during the earlier stages of the Svecofennian orogeny (Skyttä and Mänttäri, 2008 P. Skyttä and I. Mänttäri, Structural setting of late Svecofennian granites and pegmatites in Uusimaa Belt, SW Finland: Age constraints and implications for crustal evolution, Precambrian Research 164 (2008), pp. 86-109. Article | PDF (6138 K) | View Record in Scopus | Cited By in Scopus (3)Skyttä and Mänttäri, 2008). Transpressional deformation and continued crustal melting characterize the subsequent evolution of the crust in the area.Photograph by Pietari Skyttä. View towards NNE. Hammer length 65 cm. Location: Inkoo, SW Finland; N666 3243, E33 0661 (EUREF-FIN).

Structural analysis of the Palaeoproterozoic volcanogenic massive sulfide (VMS) hosting Kristineberg area, Sweden, constrained by existing magnetotelluric (MT) and seismic reflection data, reveals that the complex geometry characterized by non-cylindrical antiformal structures is due to transpression along the termination of a major high-strain zone. Similar orientations of the host rock deformation fabrics and the VMS ore lenses indicate that the present-day geometry of the complex VMS deposits in the Kristineberg area may be attributed to tectonic transposition. The tectonic transposition was dominantly controlled by reverse shearing and related upright to overturned folding, with increasing contribution of strike-slip shearing and sub-horizontal flow towards greater crustal depths. Furthermore, the northerly dip of the previously recognized subsurface crustal reflector within the Kristineberg area is attributed to formation of crustal compartments with opposite polarities within the scale of the whole Skellefte district. The resulting structural framework of the main geological units is visualized in a 3-D model which is available as a 3-D PDF document through the publication website.

The complex structural evolution within the VMS-hosting Skellefte district, Sweden, has been investigated to provide a solid structural framework for the known mineral deposits in the area. The area occurs in a transition zone between dominantly N-S to NNE-WSW striking structures in the north and approximately WNW-ESE oriented structural trends in the south. The presence of high-strain zones with both the above orientations in the Skellefte district allows constraining their mutual relationship, as well as their significance for the build-up of the Svecokarelian orogen at around 1.89 Ga and for the following tectonic overprint between 1.87-1.80 Ga. The methods used in this study include structural analysis complemented by potential field modelling and SIMS U-Pb geochronology on zircon. Based on the results of this study, the earliest deformation (D1) is constrained at 1.89–1.88 (1.87) Ga and tentatively attributed to crustal extension occurring synchronously with volcanism. Deposition of the Skellefte Group metavolcanic rocks is inferred to have occurred in a pull-apart basin developed due to dextral strike-slip shearing along approximately N-S striking regional-scale shear zones. Variations in the development of deformation fabric across the district indicate that the crust was divided into an upper, un-metamorphosed domain and a lower, strongly metamorphosed domain during D1. We further infer that the transition from the upper to lower crust was locally coupled with development of low-angle crustal-scale detachment zones during D1. The heterogenous crust was subsequently overprinted by transpressional deformation which may be explained by two alternative models. According to the first model, one single SSE-NNW transpressional event with distinct strain partitioning between the coaxially deformed upper crust and the non-coaxially deformed lower crust is largely responsible for the present-day structural geometry. A post-folding rhyolite dyke, here dated at 1871 ± 4 Ma, constrains the minimum age of this event (D2). The alternative model includes two separate transpressional events: a SW-NE one at (1.88-) 1.87 Ga, followed by SSE-NNW transpression at 1.86 Ga. Recognition of the early-orogenic detachment zones allow us to suggest that many of the major crustal-scale shear zones in the central Fennoscandian Shield have originated as 1.89-1.87 Ga crustal detachment zones, i.e. earlier than typically considered.

The Palaeoproterozoic Kristineberg area in northern Sweden comprises five sulphide mineralizations occurring within a regional-scale, fault-controlled antiformal structure. The ~E-W faults show reverse shear senses with the largest dip-slip displacements in the east. However, based on both the steep orientations of the faults, as well as their close relationship with intense folding on the above metavolcanic rocks, the faults are considered to have their origin in a phase of crustal extension pre-dating compressional deformation. The SW-NE crustal shortening was regionally partitioned into both ~E-W and ~N-S trending high-strain structural domains, whereas the Kristineberg mine vicinity shows a pattern of sub-horizontal stretching with top-to-the-E kinematics and S-block-up reverse faulting. The above features together formed the Kristineberg deposit into its present shape with steep planar bodies close to the surface and elongate, gently W-plunging bodies indicating intense stretching at depth.

New U-Th-Pb zircon data (SIMS) from three intrusive phases of the Palaeoproterozoic Viterliden intrusion in the western Skellefte District, central Fennoscandian Shield, dates igneous emplacement in a narrow time interval at about 1.89 Ga. A locally occurring quartz-plagioclase porphyritic tonalite, here dated at 1889 ± 3 Ma, is considered the youngest of the intrusive units, based on the new age data and field evidence. This supports an existing interpretation of its fault-controlled emplacement after intrusion of the dominating hornblende-tonalite units, in this study dated at 1892 ± 3 Ma. The Viterliden magmatism was synchronous with the oldest units of the Jörn type early-orogenic intrusions in the eastern part of the district (1.89–1.88 Ga; cf. Gonzàles Roldán, 2010). A U-Pb zircon age for a felsic metavolcanic rock from the hanging-wall to the Kristineberg VMS deposit, immediately south of the Viterliden intrusion, is constrained at 1883 ± 6 Ma in this study. It provides a minimum age for the Kristineberg ore deposit and suggests contemporaneous igneous/volcanic activity throughout the Skellefte District. Furthermore, it supports the view that the Skellefte Group defines a laterally continuous belt throughout this "ore district". Tentative correlation of the 1889 ± 3 Ma quartz-plagioclase porphyritic tonalite with the Kristineberg "mine porphyry" suggests that these units are coeval at about 1.89 Ga. Based on the new age determinations, the Viterliden intrusion may equally well have intruded into or locally acted as a basement for the ore-hosting Skellefte Group volcanic rocks.

Outcrop-scale correlations of deformation fabrics with low-field anisotropy of magnetic susceptibility (AMS) measurements revealed a two-stage structural evolution of the pre-tectonic, Palaeoproterozoic Viterliden intrusion in the Skellefte District, Sweden. The first deformation event reflected ˜N-S compression during basin inversion, and comprised reverse dip-slip shearing along major ˜E-W faults, whereas the low-strain lenses in between experienced penetrative deformation with a component of NE-SW elongation along the main foliation. This event is largely responsible for the present structural geometry regionally and locally, and also for the magnetic fabric of the rocks. In particular, the sub-vertical maximum principal susceptibility axes (Kmax) within the high-strain zones are related to early dip-slip deformation, and were virtually unaffected by subsequent dextral strike-slip reactivation, which is recorded by sub-horizontal rock lineations. The strike-slip deformation reflects ˜E-W bulk shortening and may regionally be correlated with reverse faulting along a ˜N-S trending major shear zone east of the study area.

Complex late Svecofennian deformation patterns and voluminous granitic magmatism at ˜1.85-1.80 Ga characterise the Palaeoproterozoic Uusimaa Belt in SW Finland. Detailed structural analysis and ion microprobe U-Pb zircon geochronology were used to (i) evaluate the evidence for crustal extension that was assumed to pre-date the predominant contractional deformation, and (ii) integrate tectonic evolution with granitic magmatism.The early Svecofennian structures (D1 and D2) are only locally preserved in specific parts of the Uusimaa Belt, while the late Svecofennian structures occur throughout the area. Coaxially deformed granite dykes and weak S3 foliations in granite most likely indicate that crustal extension (D3) took place concurrent with the initial stages of crustal melting during ˜1835-1825 Ma. Subsequent evolution involved continued granitic magmatism and a shift to contraction at ˜1820 Ma. Contraction started under horizontal ˜N-S bulk compression and resulted in intense upright as well as N-NW vergent asymmetric folds (D4). Continued deformation with bulk compression shifting towards E-W at <1820 Ma caused strain localisation into sub-vertical transpressive shear zones with east-side-up shear sense (D5). Another expression of sub-vertical stretching during D5 is the generation of sheath and doubly plunging F5 folds.The volume of granite magmatism decreased from D3 to D5. Also the mode of granite emplacement shifted from large flat-lying sheets during D3 towards thinner migmatising sheets and veins during D4, while D5 involved intrusion of cross-cutting pegmatite dykes along sub-vertical crustal weakness zones.

Located in northern Sweden, the Skellefte mining district has been subject to several geological and geophysical investigations, as it is hosting abundant volcanic-hosted massive sulfide deposits. The importance of mineral exploration at greater depths in the Skellefte District has been increased since most of mineralization at shallow depths are already discovered and exploited. Therefore, geophysical methods become particularly important as they can improve our knowledge about spatial relationship between geological features at the depth. In the first part (local-scale) of this study, we used resistivity/IP data to map the subsurface geometry down to 430m. Furthermore, the results of the resistivity/IP studies were constrained with potential field data down to 1.5 km depth. In the second part (Regional-scale), potential field data were used to constrain the interpretation of the reflection-seismic data down to 5 km depth. The result from the first part indicated a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. In Part II, the gravity and magnetic data were investigated to better understand the contact between the Skellefte Group,volcanic rocks and the Bothnian Basin sedimentary rocks. Furthermore the method was used to constrain the geometry of late-orogenic gabbro-diorite and granite intrusions which occur along inferred shear zones that are only poorly indicated, or not visible at all on the reflection-seismic profiles. As the main outcome, the proposed integrated 3D model of the central Skellefte District (CSD) revealed crucial information about the spatial relationship between key lithologies which will be further used to understand the evolution of CSD in the 4th dimension, time.