| 1. |
- Bjärnborg, Karolina, et al.
(författare)
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Geochronology and geochemical evidence for a magmatic arc setting for the Ni-Cu mineralised 1.79Ga Kleva gabbro-diorite intrusive complex, southeast Sweden
- 2015
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Ingår i: GFF. - : Informa UK Limited. - 2000-0863 .- 1103-5897. ; 137:2, s. 83-101
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Tidskriftsartikel (refereegranskat)abstract
- Magmatic Ni-Cu sulphide deposits are commonly associated with mafic rocks within the marginal areas of Archean or Proterozoic cratons. The Kleva Ni-Cu sulphide deposit in southeast Sweden is hosted by gabbro-diorite rocks and is sandwiched between two Palaeoproterozoic magmatic arcs: the Transscandinavian Igneous Belt (TIB) and the Oskarshamn-Jonkoping Belt (OJB). Major and trace element data corroborate an arc origin for the Kleva intrusive complex. The variety of rocks and geochemical signatures suggest that it formed from several magmatic pulses. Despite the occurrence of supracrustal xenoliths, there is a lack of trace element evidence for significant assimilation. Mafic intrusions are abundant in southeast Sweden and are generally inferred to be synchronous with the TIB. Of these, Kleva is the only known Ni-Cu deposit, and it is unclear if it is associated with the OJB or TIB. Here, we present zircon and baddeleyite U-Pb dates of 1788 +/- 4Ma, 1788 +/- 5Ma and 1792 +/- 3Ma. We interpret a 1790Ma date to be the best estimate of the crystallisation age of the Kleva intrusive complex, indicating that the emplacement of mafic magma is coeval with the voluminous arc-related TIB magmatism in the area.
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| 2. |
- Bjärnborg, Karolina, et al.
(författare)
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Large spinel grains in a CM chondrite (Acfer 331): Implications for reconstructions of ancient meteorite fluxes
- 2013
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 48:2, s. 180-194
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Tidskriftsartikel (refereegranskat)abstract
- By dissolving 30400kg of marine limestone in HCl and HF acid, our group has previously recovered common relict chromite grains (approximately 63250m) from ordinary chondritic micrometeorites that fell on ancient sea floors, up to 500Myr old. Here, we evaluate if CM group carbonaceous chondritic material, which makes up an important fraction of the micrometeorite flux today, contains analogous grains that can be searched for in acid residues. We dissolved 8g of CM2 meteorite Acfer 331 in HF, which yielded a characteristic assemblage of both transparent Mg-Al- and opaque Cr-spinels >28m. We find on average 4.6 and 130 Mg-Al-spinel grains per gram in the 63250 and 2863m size fractions, respectively. These grains are mostly pink or colorless, and often characterized by heterogeneous Cr-content. Black, opaque Cr-spinel grains are absent from the >63m fraction, but in the 2863m fraction we find approximately 65 such grains per gram meteorite. The individual grains have a characteristic composition, with heterogeneous major element compositions (e.g., 44.461.7wt% Cr2O3), but narrow ranges for maximum TiO2 (0.61.6wt%) and V2O3 (0.51.0wt%) concentrations. The content of spinel grains in the 2863m fraction of CM meteorites appears comparable at the order of magnitude level with the content of >63m sized chromite grains in fossil L-chondrites from Ordovician limestone. Our approach of recovering meteoritic spinel from sediment may thus be extended to include CM meteorites, but the smaller size fraction of the acid residues should be searched.
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| 3. |
- Bjärnborg, Karolina
(författare)
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Origin of the Kleva Ni-Cu sulphide mineralisation in Småland, southeast Sweden
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Kleva Ni-Cu sulphide deposit is situated within a gabbro-diorite intrusive complex in southeast Sweden. The basement north of the intrusive complex is dominated by 1.81–1.77 Ga granites of the Palaeoproterozoic Transscandinavian Igneous Belt (TIB). Slightly older (1.83–1.82 Ga) rocks of the Oskarshamn Jönköping Belt, which hosts numerous syngenetic and epigenetic base metal mineralisations, occur just south of the Kleva intrusive complex. The aim of this PhD-thesis is to deduce the origin of the Kleva deposit, the mineralisation itself as well as its host rocks through geochemical, geochronological and petrological studies. U-Pb age determination of zircon dates igneous crystallisation to 1.79 Ga, which is the age of the Kleva intrusive complex and confirms its temporal association with the voluminous TIB magmatism. Major- and trace element systematics are in accordance with a basaltic magma that formed through partial melting of a metasomatically refertilised mantle wedge underneath an Andean-type continental magmatic arc. Lu-Hf signatures of zircon, together with other rocks of Palaeoproterozoic Fennoscandia indicate alternating stages of extension and compression across the subduction zone, facilitating ascent of the mafic magma. Evidence for contamination of the magma through crustal assimilation during its ascent are inconclusive. Low IPGE/Ni together with high S/Se, indicate sulphide melt saturation prior to final emplacement, possibly induced by crustal contamination. Nb/La vs La/Sm indicate contamination with mid-crustal rocks, and radiogenic Os of magmatic pyrite suggests <10% contamination with Archean crust. OJB aged rocks are thus unlikely contaminants, despite the numerous rock inclusions of similar geochemical composition within the intrusive complex. δ34S of Kleva mineralised rocks and the country rocks corresponds with the mantle range, and local or mantle origin of S can neither be proven nor rejected. Sulphide melt segregated from an evolved magma and partially accumulated into massive lenses, which is in accordance with a magmatic conduit setting. The mineralisation contains massive, net-textured and disseminated sulphides of typical magmatic association and is interpreted to be contemporaneous with silicate melt crystallisation, consistent with a Re-Os 1.71 ±0.2 Ga isochron for massive pyrite with magmatic texture. Re-Os isochrons of secondary pyrite indicate metamorphic disturbance of the mineralisation at least twice; at c. 1.61 Ga and 1.39 Ga, which can be linked to orogenic events further to the south and west. The mineralisation was heterogeneously affected by tectonic disturbance, resulting in remobilisation of chalcopyrite into veins, plastic deformation of sulphides and host rock, micro-faulting and brittle deformation of oxides and sulphides and recrystallisation of pyrite in fractures. To summarise, the deposit is an example of a subduction related magmatic Ni-Cu mineralisation affected by multi-stage deformation and alteration.
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| 4. |
- Bjärnborg, Karolina, et al.
(författare)
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The age of the Kleva intrusion, southeast Sweden
- 2013
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Ingår i: Mineral deposit research for a high-tech world : Proceedings of the 12th Biennial SGA Meeting, 12-15 August 2013, Uppsala, Sweden. - 9789174032079 ; , s. 1647-1649
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Konferensbidrag (refereegranskat)abstract
- The Kleva nickel-copper mineralization is situated within a gabbro-diorite intrusion on the border between the late Svecofennian rocks of the Oskarshamn-Jonkoping belt and rocks of the slightly younger Transscandinavian Igneous Belt (TIB). The sulphides present in the mafic intrusion are pyrrhotite, chalcopyrite, pentlandite and pyrite, occurring as massive mineralization, disseminations and thin, predominantly chalcopyrite-bearing veins. Age determinations of the gabbro-diorite were conducted to determine the age of the mineralization and its regional context. U-Pb analyses of baddeleyite (TIMS) and zircon (SIMS) from the gabbro-diorite and related rocks indicate an age of c. 1.79 Ga for the Kleva intrusion, broadly coeval with the TIB-rocks in the area, but younger than the late Svecofennian rocks. Further studies will help to constrain whether the sulphide mineralization was modified during later alteration, regional tectonism and metamorphism.
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| 5. |
- Petersson, Andreas, 1980-
(författare)
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Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes
- 2017
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 284–285, s. 122-131
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Tidskriftsartikel (refereegranskat)abstract
- Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.
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