| 1. |
- Weihed, Jeanette Bergman, et al.
(författare)
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Geology, tectonic setting, and origin of the Paleoproterozoic Boliden Au-Cu-As deposit, Skellefte District, northern Sweden
- 1996
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 91:6, s. 1073-1097
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district in northern Sweden comprises more than 85 pyritic volcanic-hosted massive sulfide deposits which mainly occur within, and at the top of, a felsic-dominated volcanic unit overlain by a sedimentary sequence. The Boliden Au-Cu-As deposit was one of the first discovered in the district, and it has attracted a continuous interest since then due to its significant size and high gold grade (avg 15 ppm). The Boliden ore can be divided into massive ore, with arsenopyrite- and pyrite-dominated lenses, and vein ore which comprises a quartz-chalcopyrite-sulfosalt-dominated assemblage, occurring in brecciated parts of the arsenopyrite bodies, and quartz-tourmaline veins mainly in host rocks below the massive ore. As a rule, the gold is found in deformational structures in vein ore. Most gold is present as an Au-Ag-Hg alloy with variable compositions, from Au (sub 0.17) Ag (sub 0.68) Hg (sub 0.16) to Au (sub 0.93) Ag (sub 0.07) (in atomic proportions).For the last two decades, the approximately 1.88 Ga massive sulfide ores in the Skellefte district have collectively been interpreted as volcanic exhalative formations resembling the Miocene kuroko ores of Japan. However, this view has recently been challenged and a subsurface replacement origin has been proposed for some of the ores in the district.The Boliden ore is not bound to one particular host rock but occurs in feldspar porphyritic dacite, quartz porphyry, and basalt-andesite. Textural observations suggest that these rocks represent intrusions or lavas. Geochemically, they are typical calc-alkaline volcanic rocks, enriched in large ion lithophile elements, depleted in heavy rare earth elements, and with troughs for Th, Nb, Hf, and Ti. The ore zone, in its present setting, is in a more or less vertical position and oblique to lithological contacts. Ore-related hydrothermal and regional metamorphic processes (lower amphibolite facies) have created a complex alteration system around the ore. This forms a symmetric pattern with an inner sericite-rich zone, locally containing abundant andalusite, and an outer chlorite-dominated zone. The nature of the alteration is consistent with leaching of elements and a silica-alumina-rich residue--features which are often found in epithermal environments.Structural observations suggest that three ductile foliation-forming events have affected the rocks near the ore. These include a regional S (sub 1) foliation, formed during isoclinal folding, which was subsequently sheared causing formation of a strong cleavage S (sub s) and extensive deformation of the ore itself. A late S (sub 2) cleavage crenulated earlier fabrics.The available data and observations are not consistent with a volcanic exhalative model for the ore and the following scenario is favored. Shallow intrusions of dacite and andesite into unlithified sediments occurred around 1.87 Ga. At this time, the earlier marine environment had been lifted up to a shallow-marine or possibly subaerial position. Shortly thereafter, fluids which generated the massive ore at Boliden were focused along a fault, and arsenopyrite and pyrite lenses were precipitated in more than one host rock discordantly to lithological contacts. Regional deformation with folding and shearing, possibly at around 1.85 Ga, led to brecciation of previously formed ores and stretching of orebodies. In relation to this shearing event, Au was introduced and/or remobilized and concentrated in brecciated portions of the ore zone. Thereafter, ores and host rocks recrystallized during peak metamorphism at around 1.82 Ga, and a second deformation at around 1.80 Ga caused crenulation of early fabrics.The crosscutting nature of the ore with respect to the host rocks, the hydrothermal alteration pattern with strongly leached host rocks, and the ore association with early massive sulfides followed by gold, chalcopyrite, and sulfosalts in brittle structures all indicate that a modern analogue for ore formation may be a high-sulfidation epithermal environment. The epigenetic nature of the Boliden deposit has significant implications for exploration of gold deposits elsewhere in the region.
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| 2. |
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| 3. |
- Weihed, Pär, 1959-, et al.
(författare)
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Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden
- 2002
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 124:4, s. 201-210
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Tidskriftsartikel (refereegranskat)abstract
- The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphidequartz vein system with high gold grades was mined in the second half of the 1990’s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.
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| 4. |
- Weihed, Pär, 1959-, et al.
(författare)
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Relationship between 1.90–1.85 Ga accretionary processes and 1.82–1.80 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield
- 2002
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 124:3, s. 163-180
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Tidskriftsartikel (refereegranskat)abstract
- The three main intrusive suites: early calc-alkaline, late I/Atype, and late S-type intrusive rocks in relation to the Svecokarelian orogeny (1.9–1.8 Ga) have been dated at the Archaean craton margin in the Palaeoproterozoic Skellefte district and surrounding areas in northern Sweden. In addition, new SIMS data have been obtained on a calcalkaline intrusion for which unusually young TIMS ages existed, compared to similar calc-alkaline intrusions elsewhere in the region. Titanite and zircon from a subvolcanic intrusion affected by a major N–S trending shear zone have also been dated to constrain the last ductile deformation in the area. The 1895+14–12 Ma zircon age for a calc-alkaline intrusion is interpreted as the crystallisation age and is significantly older than the existing 1825 Ma age on titanite from a pyroxene skarn in a marble horizon close to the contact. The latter age is instead interpreted as the age of peak metamorphism in this area. The 1798±4 Ma age for the S-type granite confirms that the S-type magmatism is largely coeval with the I/A type magmatism previously dated at 1803±6 Ma. At a larger scale, a zoned belt over 2000 km long with A/I-type magmatism in the west and S-type magmatism in the east can be inferred. Either mafic underplating or Cordilleran type settings can explain the magmatic belt, which trends oblique to the roughly NE-directed subduction that led to the accretion of volcanic arcs onto the older craton between 1.95 and 1.87 Ga. An intimate temporal relationship between the extensive 1.80 Ga magmatism and regional N–S-trending shear zones in the area is confirmed by the titanite age of c. 1.80 Ga from one such shear zone. Kinematics on this shear zone suggest E–W shortening. SIMS data from a calc-alkaline intrusion at Sikträsk indicate that the previously obtained conventional zircon ages of 1.85–1.86 Ga are actually mixed ages of 1.88 Ga magmatic zircons, and c. 1.80 to 1.82 Ga metamorphic overgrowths. This shows that the 1.80 Ga event was not only constrained to shear zones. It is argued that both the 1.80 to 1.82 Ga deformation and metamorphism discussed here is related to E-W shortening and the voluminous magmatism at 1.82–1.80 Ga. This is in contrast to the older c. 1.88 Ga deformation identified to the north and east within the Karelian craton that was related to Svecokarelian accretionary processes.
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| 5. |
- Weihed, Pär, 1959-, et al.
(författare)
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Structural Evolution of the Björkdal Gold Deposit, Skellefte District, Northern Sweden: Implications for Early Proterozoic Mesothermal Gold in the Late Stage of the Svecokarelian Orogen
- 2003
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 98:7, s. 1291-1309
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Tidskriftsartikel (refereegranskat)abstract
- The Björkdal gold deposit is situated in the eastern part of the Paleoproterozoic Skellefte district in northern Sweden. The Skellefte district constitutes a 1.89 to 1.88 Ga volcanic arc with numerous volcanic massive sulfide deposits and lode gold deposits of which the Björkdal deposit is the largest, at ca. 20 Mt with 2.5 g/t Au. The gold at Björkdal is associated with centimeter- to meter-wide, subvertical quartz veins at the northwestern contact between a quartz-monzodioritic to tonalitic intrusion and the surrounding supracrustal rocks. The main quartz veins strike north-northeast, and a minor set of veins strike east-northeast. The quartz veins terminate against a major thrust duplex at the contact between the intrusion and the structurally overlying supracrustal rocks. The mylonitic thrust zone has a 20° to 40° dip toward north and trends approximately east-west. A few kinematic observations indicate reverse to obliquely reverse slip on the thrust. Deformed quartz veins exist in lithons between thrusts within the duplex. In the mine, the quartz veins in the footwall to the thrust are spatially and temporally associated with moderately to steeply west dipping reverse shear zones with a northeast strike. It is suggested here that the quartz veins and the steep reverse shear zones are related to the thrust duplex and formed more or less simultaneously. Fluid inclusion and isotopic results from previous studies indicate that juvenile magmatic fluids were responsible for the precipitation of quartz and sulfides at moderate temperatures and pressures. Furthermore, titanites from the quartz veins give ages of ca. 1.78 to 1.79 Ga, whereas the host pluton is dated at ca. 1.90 Ga, indicating a time gap of over 100 m.y. between the emplacement of the host rock and titanite growth in quartz veins. The regional deformation and metamorphism are poorly constrained in the area to some time between 1.87 and 1.80 Ga. As the quartz veins are virtually undeformed and do not exhibit metamorphic fluid inclusions or other evidence of premetamorphic origin, we interpret the titanite ages in the quartz veins as the age of emplacement of the veins. The ca. 1.78 to 1.79 Ga age is also constrained for the crustal-scale, north-south–striking shear zones in the area, and it is suggested here that the thrust duplex and steep reverse shear zones in the mine are third-order structures related to east-west shortening at ca. 1.80 Ga. Gravity data from the Björkdal area indicate the presence of a less dense body at depth beneath the Björkdal pluton. The geophysical signature is best explained by the presence of a 1.80 Ga Skellefte-type intrusion at depth. Magmatic fluids from this S-type granite may have interacted with the host pluton and precipitated gold in the more competent pluton during the east-west shortening. The common occurrence of scheelite in the quartz veins is further evidence for magmatic fluids derived from a younger pluton at depth.
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| 6. |
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| 7. |
- Juhlin, C., et al.
(författare)
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Crustal reflectivity near the Archean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area
- 2002
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 150:1, s. 180-197
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Tidskriftsartikel (refereegranskat)abstract
- Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south-southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high-resolution shallow component (1 kg shots) and a lower-resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans-Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high-amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west-dipping contact zone between meta-sediments and felsic volcanic rocks, however, the seismic data indicate a near-vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi-vergent geometry than previous interpretations. Comparison of the re-interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi-vergent crustal structure observed today.
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| 8. |
- Dehghannejad, Mahdieh, et al.
(författare)
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Reflection seismic imaging of the upper crust in the Kristineberg mining area, northern Sweden
- 2010
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 71:4, s. 125-136
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Tidskriftsartikel (refereegranskat)abstract
- 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 degrees-40 degrees. 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.
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| 9. |
- Malehmir, Alireza, et al.
(författare)
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Regional structural profiles in the western part of the Palaeoproterozoic Skellefte Ore District, northern Sweden
- 2007
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 159:1-2, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- The Kristineberg mining area is situated in the western part of the Palaeoproterozoic Skellefte Ore District, northern Sweden, and is well known for its VHMS base-metal and gold deposits. This paper presents five upper crustal geological cross sections that have been constructed and mainly constrained by seismic reflection data, potential field modeling as well as geological field observations. These profiles are visualized in 3D to highlight the three dimensionality and internal consistency of structures across the region. The resulting structural model for the Skellefte volcanics and overlying metasediments comprises two thrust-sheets that expose the Skellefte volcanics in the cores of hanging-wall anticlinal structures. A shear-zone is imaged as a band of seismic reflectivity terminated by the southern Revsund granite unit. Another shear-zone, possibly a continuation of the Skellefte Shear Zone (SSZ) runs through the centre of the region and accounts for some of the structural complexity and shearing observed between the two anticlinal exposures of the volcanics. Additional smaller scale shear-zones have been identified from geological and geophysical mapping within the main structural blocks of the Skellefte volcanics. The Mala volcanic rocks in the north are separated from the Skellefte volcanics by a fault that cuts discordant to the strike of the Mala volcanics. A structural basement has been proposed to the Skellefte volcanics, constrained by seismic reflection data. Exposures of Bothnian Basin rocks south of the Revsund granite outcrops, suggest that the domain beneath the north dipping reflectivity is associated with Bothnian Basin stratigraphy. The preferred interpretation for the contact between the Skellefte volcanics and the Bothnian Basin rocks is a thrust fault that brings the felsic volcanic rocks over the metasedimentary domain. The Revsund granites are divided into two major groups based on their present day thickness and shapes. Although parts of the Viterliden intrusion are almost undeformed, it is cut by a series of shear-zones, causing the magnetic lineations seen within these rocks. The structural profiles presented demonstrate that the Kristineberg ore is situated in the northern limb of a local synformal structure. The new crustal-scale structural model demonstrates the potential of integrating geophysical and geological data when modelling structures hosting mineralization in a complex region like the Skellefte District. The structural profiles presented in this paper, have greatly improved our understanding of the 3D tectonostratigraphy and architecture of the poly-deformed ca. 1.9 Ga the Skellefte Ore District.
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| 10. |
- Malehmir, Alireza, et al.
(författare)
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Seismic imaging and potential field modeling to delineate structures hosting VHMS deposits in the Skellefte Ore District, northern Sweden
- 2006
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 426:3-4, s. 319-334
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte District in northern Sweden is a roughly 150 by 50 km2 large early Proterozoic massive sulphide belt. Based on high-resolution reflection seismic data along two parallel seismic profiles, potential field modeling has been carried out and two geologic cross sections have been constructed that are consistent with the available geophysical data as well as surface geologic observations. The combined modelling suggests that the Kristineberg deposit occurs on the northern limb of a regional E–W striking syncline. The interpretations help to identify new prospective areas, both down-plunge from known ores, and on the ore-bearing horizon on the southern limb of the syncline. The new results suggest that the post-orogenic Revsund granites can be divided into two major types of intrusives, those which are intruded as domes/stokes with a maximum present day thickness of about 3–3.5 km and those which are intruded as thin sheets, with a maximum thickness of a few hundred meters. The margins of the intrusions are generally inclined inwards, suggesting that the current erosion level is near the middle, or toward the base, of the granites. The contact between the Skellefte volcanic rocks and the Bothnian Basin has been interpreted as a thrust fault. We also suggest that crustal thickening predates the Skellefte volcanism and that the interpreted Bothnian Basin rocks are either a structural basement or a separate terrane to the Skellefte volcanism. Diffraction patterns in the reflection seismic data can be interpreted as originating from either a mafic–ultramafic intrusion or a mineralization zone, similar to observations elsewhere in the world. The results obtained in this study have greatly improved our understanding of the tectonostratigraphic framework and architecture of the poly-deformed c. 1.9 Ga Skellefte VHMS belt and is a key step towards building a 3D geological model in the area.
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