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| 3. |
- Billström, Kjell, et al.
(författare)
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Age and provenance of host rocks and ores in the Paleoproterozoic 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. 1054-1072
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district in northern Sweden is a ca. 1.9 Ga, extensively mineralized, mainly felsic, submarine volcanic belt. Within the district, the volcanic rocks (Skellefte Group) are overlain by turbiditic sedimentary rocks and coarser clastic rocks, as well as younger, mainly mafic, volcanic rocks (Vargfors Group). To the north, subaerial volcanic rocks of the Arvidsjaur Group are probably coeval with the Vargfors Group. The sedimentation in the Bothnian basin, south of the Skellefte district, appears to have started at ca. 2.0 Ga and continued until ca. 1.86 Ga, as indicated by the presence of granitoids spanning this time interval. The first main magmatic episode in the Skellefte district was a felsic stage at around 1.89 Ga as confirmed by two new U-Pb zircon ages from volcanic rocks situated in the central and eastern part of the district (Bjurvattnet, 1884 + or - 6 Ma; Melestj rn, 1889 + or - 4 Ma). No basement is known to the felsic magmatism, but granitoids occurring to the south of the district, which have been dated at 2.0 to 1.9 Ga, could constitute remnants of a basement which was destroyed by 1.89 Ga arc volcanism within the Skellefte district. The Vargfors Group overlies the Skellefte Group with no major unconformity, and one new age from an ignimbrite in the Vargfors Group (1875 + or - 4 Ma) confirms the temporal relationship with the deposition of subaerial volcanic rocks of the Arvidsjaur Group.An evaluation of age data for the early, synvolcanic (ca. 1890 Ma) Joern-type granitoids suggests that these should be further subdivided. Three different generations of Joern-type granitoids may exist. The GI phase has an age of about 1.89 Ga, the GII and GIII phases within the major Joern batholith probably formed at around 1.87 Ga, and the Siktr sk intrusion in the southern part of the district, has a crystallization age of ca. 1.86 Ga.A number of distinctive isotopic characteristics have been observed, e.g., significant data scatter for Sr whole-rock data, reversely discordant zircon data, and unusually young lower intercept ages for zircon discordia. These features seem to relate preferentially to volcanic rocks, and it is suggested that this behavior is due to Phanerozoic hydrothermal processes that have mobilized elements at different scales. Upper intercepts for zircon discordia, however, are with one exception thought to represent true crystallization ages. The 1847 + or - 3 Ma age for a mass flow at Petiktr sk, as defined by a three-point discordia, is for geologic reasons too young, but a considerably higher (super 207) Pb/ (super 206) Pb age at 1890 Ma for one zircon fraction is more consistent with the field relationships.Volcanic-hosted massive sulfide ores occur in the upper part of the volcanic sequence of the Skellefte Group and, in some cases, also in the lower part of the Vargfors Group. A good approximation of the age of massive ore formation is provided by the age of the host rocks. It is suggested that two main depositional stages of massive ore occurred at ca. 1885 to 1880 Ma and at ca. 1875 Ma. Gold occurs in two principal settings, as a constituent in the volcanic-hosted massive sulfide ores, and related to quartz veins found both in intrusive and supracrustal rocks. In the massive ores, gold was probably emplaced in connection with the hydrothermal processes which concentrated the base metals. Gold in some major intrusive-related Au deposits (e.g., Bjoerkdal) is likely to have concentrated at a premetamorphic stage, tentatively at 1.87 Ga, and still other Au ores (e.g., Boliden) may be epithermal in origin and were possibly formed at a relatively late stage at ca. 1.85 Ga. Later, during peak metamorphic conditions, some mesothermal Au-As vein deposits (e.g., Grundfors) formed at ca. 1.84 to 1.82 Ga.
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| 4. |
- Billström, Kjell, et al.
(författare)
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Geology and Age Constraints on the Origin of the Intrusion-Related, Sheeted Vein-Type Åkerberg Gold Deposit, Skellefte District, Sweden
- 2012
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Ingår i: Minerals. - : MDPI AG. - 2075-163X. ; 2:4, s. 385-416
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Tidskriftsartikel (refereegranskat)abstract
- The Early Proterozoic (~1.9 Ga) Skellefte mining district in northern Sweden hosts abundant base metal deposits, but there are also gold-only deposits. The Åkerberg gold ore is unusual given the noted lack of alteration, a scarcity of sulfides and gold associated with thin (mm-cm wide) parallel quartz veins hosted in a gabbro. The gold content is positively correlated with the density of quartz veins, but gold often occurs between veins and also in parts of the gabbro where there is no veining. The gabbro is intruded by a granodiorite and associated pegmatite bodies, and U-Pb dating of zircon and baddeleyite suggest that these lithologies developed close in time at around 1.88 Ga ago. There are no primary inclusions in quartz veins, but different types of secondary aqueous inclusions occur. The Åkerberg ore is interpreted as a sheeted vein complex, with veins constrained to tensional cracks induced when a granodioritic magma intruded the competent, sheet-like gabbro intrusion. It is suggested that unmixing of the felsic magma also produced pegmatite bodies and a gel-like melt which invaded fractures in the gabbro and deposited silica. In a comparison, the Åkerberg ore shares many characteristics with the intrusion-related style of gold mineralizations.
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| 5. |
- Billström, Kjell, et al.
(författare)
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IOCG and related mineral deposits of the northern Fennoscandian Shield
- 2011
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Ingår i: Hydrothermal iron oxide copper-gold & related topics. - Adelaide : PGC Publishing. ; , s. 381-414
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The northernmost Fennoscandian shield comprises Archaean and Palaeoproterozoic rocks. Unlike most other shield areas, economic mineral deposits are largely restricted to its Palaeoproterozoic parts. The latter are characterised by intracratonic basin evolution between ca. 2.5 and 2.0 Ga, involving recurrent mantle hotspot activity with numerous layered intrusions, komatiite and picrite eruptions, but no signs of accretionary phases or formation of major new felsic crust. Accretion and continent-continent collision followed from ca. 1.9 to 1.8 Ga, during the Svecofennian orogeny. A range of mineralisation styles are hosted by extensive ca. 2.5 to 2.0 Ga greenstone belts and younger, subduction-related 1.9 to 1.8 Ga Svecofennian intrusive and extrusive settings. These mineralisation styles partially overlap, and individual deposits may not readily be placed into genetic classification schemes. A provisional grouping of observed mineralisation styles comprises (1) stratiform-stratabound sulphide, (2) apatite-iron, (3) skarn-related iron and BIF, and (4) epigenetic(±syngenetic?) Au and Cu-Au deposits. The descriptive section of this paper also highlights features that may relate to orogenic-gold, IOCG and 'atypical metal association' categories of mineralisation. The assumption made is that the deposition of a diverse range of ore deposits was made possible by a long and complex geological evolution. This involved an initial (sowing) stage where iron, and to some extent copper and gold, were concentrated during 2.3 to 2.1 Ga (Karelian) rock-forming processes. Following this, ore elements were mobilised during two younger (Svecofennian) stages at 1.92 to 1.87 and 1.85 to 1.79 Ga, respectively. The latter were triggered by metamorphic and magmatic episodes, and fluids liberated during these stages precipitated IOCG and related deposits when fluids met structural and chemical traps in suitable host rocks. Ore fluids are generally saline, and their development probably involved incorporation of evaporates and, at least locally, also felsic magmatism may have played a role. Skarn-related mineralisation, hosted by ca. 2.1 Ga greenstones, occurs both as a BIF type in Sweden (formed at around 2.1 Ga), and as a gold-copper enriched variety (the result of Svecofennian epigenetic processes) in the Kolari region of Finland. The huge Kiirunavaara deposit is the type example of apatite iron ores, and is here considered to have formed from a magma at ca. 1.88 Ga, although it also has features best explained by a magmatic-hydrothermal overprint. A younger, less prominent, stage of apatite iron ore formation took place at approximately 1.78 Ga. Epigenetic gold and copper-gold deposits are particularly hard to classify as these show mixed ore characteristics, and to some extent this is likely to be due to multiple mineralisation stages (cf. the huge, low grade Aitik deposit in Sweden which is interpreted to be a hybrid porphyry-IOCG-type of ore). Structurally controlled, orogenic-gold mineralisation is common in the Central Lapland greenstone belt, although there are also gold deposits with enhanced contents of e.g., copper, cobalt and uranium (e.g., at Saatopoora). The latter, sometimes referred to as being of an 'atypical metal association' type, could potentially also include syngenetic mineralisation (e.g., at Juomasou). The range of epigenetic (±syngenetic) gold and copper-gold deposits could possibly be related to a vague east-west trend defined by gold-rich deposits in the east (Finland), followed by IOCG (copper±gold) and more iron-dominant ore types near the Finnish-Swedish border and further west into Sweden.
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| 8. |
- Billström, Kjell, 1954-
(författare)
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Isotopic studies of two early Proterozoic sulphide ores in the Bergslagen district, south-central Sweden
- 1985
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two sulphide ores in Beraslaqen (Svärdsio and Ammeberq), south-central Sweden, have been examined for their isotope characteristics in order to elucidate their qenetic oriqin. The obtained isotopic data point to a common volcanic-exhalative mode of formation in aqreement with recent qeochemical data. Ore elements were mainly leached from the volcanics, closely after their crystallization. The banded impreqnation ore in Ammeberq is interpreted as a distal on-surface deposit emplaced in a basin where reducinq conditions prevailed, while the Svärdsjö massive, complex ore corresponds to a proximal subsurface deposit. Despite their common origin several dissimilarities exist with respect to sulphur isotopes between the two deposits, while ore lead data and carbon and oxyqen isotope ratios show no major variations. The reason for this is probably that ore lead was derived principally from one source (the volcanic basement), while sulphur probably oriqinated from more than one source (various proportions of leached maqmatic sulphides and H2S derived by reduction of sea water sulphate). It is believed that the measured 6 3'4S values in principal reflect the conditions at the primary ore formation and thus mirror the specific processes takinq place when the brine(s) ascended towards the sea floor, thouqh secondary processes have affected the sulphur isotope distribution and particularly the 6 13C and 6 ls0 values of the ore-associated carbonates. Differences in the type of hydrothermal convection system in Svärdsjö and Ammeberq may also have been a decisive factor for the type of ore formed, the former deposit suqqested to be formed from a hiqh-power discharqe system and the latter from a low-power steady-state system. The ore lead isotopic composition is almost identical in Svards 10 and Ammeberq (and are in fact very similar all over the ore province of Berqslaqen), a feature explained by the fact that the volcanic basement at the time of ore formation was isotopically homoqeneous. Ore lead model ages are too low in comparison with the accepted time of ore formation, which is probably due to a U-Th enrichment of the volcanite-forminq maqmas.
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| 11. |
- Billström, Kjell, et al.
(författare)
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Regional variations in the Pb isotopic compositions of ore galena across the Archaean-Proterozoic border in northern Sweden
- 1997
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 81:1-2, s. 83-99
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Tidskriftsartikel (refereegranskat)abstract
- Galena is a rare component in the Palaeoproterozic mineralizations in northernmost Sweden. In the present work Pb isotope data from galena from nineteen deposits tend to vary as a function of geographical location. Basically, the zone dividing radiogenic and non-radiogenic lead isotopic compositions is consistent with the position of the inferred Proterozoic-Archaean border in northern Sweden. Radiogenic lead is generally found in galena of the 2.0-2.7 Ga old Greenstone group in the northeast along the Finnish border, and in some Cu occurrences in the ∼ 1.9 Ga old Porphyry group near Malmberget. Non-radiogenic lead is found in the Huornaisenvuoma Zn-Pb ore in the Greenstone group, in epigenetic Cu-Zn-Pb ores in the Porphyry group in southern Norrbotten, and in similar deposits near the Caledonides. The lead isotopic compositions of galena in the Cu-Zn-Pb ores of southern Norrbotten are fairly homogeneous with 206Pb/204Pb = 15.34 to 15.50, 207Pb/204Pb = 15.16 to 15.30, similar to those of the sulphide deposits associated with the Central Finnish batholith and to data from the North-central Skellefte district. The coincidence of data for the areas of southern Norrbotten and the North-central Skellefte district is interpreted such that these terrains formed one crustal unit at the time of ore deposition. Tentatively, this event took place at ∼1.87 Ga or slightly thereafter, and was related to the suturing of the Skellefte arc to the northerly inferred continent. Although Pb model ages should be treated with caution when applied in Proterozoic terrains in Sweden, the present data suggest an age difference between the deposits (and between source rocks) at Tjåmotis and Huornaisenvouma (∼2.0 Ga old) on one hand, and the remaining Cu-Zn-Pb occurrences (∼1.9 Ga old) on the other hand. Possibly, the indicated depositional event at ∼2.0 Ga is an expression of simultaneous, large-scale crust-mantle interactions, in accordance with the hypothesis of magma-forming processes at that time.In the Pb-Pb diagrams, data for galena in northern Sweden indicate linear relationships. The radiogenic lead is the result of leaching processes in Phanerozoic (Caledonian) times, which released rock lead evolved in the time interval ∼2.0-1.9 Ga to 0.4 Ga. The data furthermore suggest that radiogenic galena was formed by mobilizing lead from U-rich rocks (238U/204Pb ratios ca. 25-32) of the Greenstone group which are likely to have a similar age, possibly close to 2.0 Ga. Since the deposits with radiogenic lead are distributed over a very large area, and still define linear trends, it is likely that the ∼2.0 Ga age is typical for the bulk of Greenstone rocks in Norrbotten. From the slopes of straight-line relations for the Pb isotope data, it is inferred that only Proterozoic rock lead was mobilized at ∼0.4 Ga and that no significant Archaean lead was involved in such processes. Accordingly, lead mobilization was quite local and only involved the upper portions of the crust. As the radiogenic lead is found at localities far away from the Caledonian front, the indicated Pb mobilization processes were most probably connected with reactivation of old Proterozoic fracture zones.
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| 12. |
- Billström, Kjell, et al.
(författare)
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Sandstone-hosted Pb-Zn deposits along the margin of the Scandinavian Caledonides and their possible relationship with nearby Pb-Zn vein mineralisation
- 2020
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 127
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Forskningsöversikt (refereegranskat)abstract
- Numerous sandstone-hosted Pb-Zn deposits occur along the present-day erosional front of the eastern Scandinavian Caledonides. The largest deposit is Laisvall (64.3 Mt at 4.0% Pb, 0.6% Zn and 9.0 g/t Ag) and since mineralisations generally share similar characteristics (reminding of both SEDEX and MVT-style) the term Laisvall-type has often been used. Typically, mineralised zones occur along sedimentary bedding and consist of disseminated galena and sphalerite and lesser amounts of calcite, fluorite, baryte, pyrite and sericite forming a cement that fill interstitial pores in Neoproterozoic/Eocambrian (e.g. Laisvall) to Cambrian (e.g. Vassbo) sandstones. Deposits occur both in autochtonous and allochtonous sedimentary rocks, and a broad consensus exists about their epigenetic nature, their spatial relationships to syn-sedimentary faults and that ore fluids have scavenged metals from the crystalline basement. However, the detailed ore depositional history and the timing of ore deposition have remained more controversial. New analyses aimed to complement earlier Rb-Sr data (crush-leach technique using sphalerite) fail to support a published three-point isochron age of 467 +/- 5 Ma. This is probably due to syn-ore mixing between fluids carrying isotopically variable strontium and inherited problems to analyse sphalerite grains that strictly were deposited from a single ore pulse. Tentatively, strontium in the ores originate from a mix of components derived from the basement, seawater and the local sedimentary host sequences. The lead component has highly radiogenic compositions, and data define sub-parallel linear arrays interpreted to essentially represent mixing of isotopically different types of lead released from regional basement rocks. There are obvious similarities when comparing features of deposits representing two Pb-Zn ore styles, the sandstone-hosted dissemination and the fracture-controlled mineralisation in the granite-dominated basement occurring further east of the Caledonian margin. These include low temperature brines responsible for mineral deposition, the mineralogy and the nature of Rb-Sr and Pb isotope data. We suggest that these types of mineralisation have a common origin and time of emplacement, but it is elusive to propose a well-constrained age. Nonetheless, field observations and other evidence suggest that ore formation is due to large-scale fluid flow triggered by the transition from an extensional to compressional tectonic setting at about 500 Ma. Connected to this mid-Cambrian stage was the development of syn-sedimentary faults and fractures in the basement and in overlying consolidated sandstones. The opening of such zones of weakness enabled a movement of ore-forming fluids infilling pore space in sandstones (disseminated ore) and fractures in the basement (vein ore).
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| 13. |
- Billström, Kjell, et al.
(författare)
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Solstad, a Co-Se-bearing copper ore in the Västervik quartzites, Sweden
- 2023
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 144:3-4, s. 156-169
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Tidskriftsartikel (refereegranskat)abstract
- The Solstad copper deposit, located in SE Sweden, is hosted by a quartz-rich rock sliver surrounded by a granite belonging to the 1.8 Ga Transscandinavian Igneous Belt. Ore petrographic studies have revealed a number of previously unrecognized opaque phases, including several Co phases, selenides and tellurides. Based on an in situ U-Pb investigation of zircons from a mineralized sample, it is suggested that zircons have a detrital origin and that the quartz-rich host rock is a xenolith belonging to the c. 1.88–1.86 Ga Västervik quartzite formation. A low-radiogenic galena sample implies that the source for the metals in the ore has a primitive origin, probably the basaltic lavas (now amphibolites) that are intercalated in the Västervik quartzite. Fluid inclusion studies in quartz distinguish four distinct ore fluids: (1) a hypersaline halite-bearing aqueous fluid related to an early (1.85–1.86 Ga) chalcopyrite depositional stage, (2) a subsequent CO2-rich fluid, that deposited native gold, tellurides, selenides and bismuthinite, developed (at ≥1.8 Ga) as a result of a phase separation, (3), a moderate- to high-salinity aqueous fluid did also develop at this event and led to the deposition of bornite and (4) a concluding, low-salinity aqueous fluid stage (at ≤1.8 Ga) caused oxidation to covelline and digenite of previously formed phases. It is proposed that the Solstad deposit and other Cu ± Co-rich sulphide (± magnetite) occurrences in the Västervik region along the southernmost margin of the 1.9–1.8 Ga Svecofennian Domain, represent a distinct ore type associated with quartzites and amphibolites.
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| 14. |
- Billström, Kjell, et al.
(författare)
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U-Pb zircon age of the Rostberget W-occurrence, northern Sweden
- 1989
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 111:2, s. 121-126
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Tidskriftsartikel (refereegranskat)abstract
- Anomalously U-rich granites of the so-called Revsund type, associated with W- and Sn-enrichments, occur in the Gunnarn area close to Storuman in northern Sweden. At Rostberget, a swarm of thin undeformed wolframite-bearing greisen veins are situated in the interior of the Joran granite intrusion. We have dated zircons from a sample of the greisen veins with the U-Pb method. The veins are less than 1 cm wide and it is likely that the sample also contains zircons from the immediately surrounding altered granite. The mineralizing event and connected metasomatic processes probably released Zr from the granite which became available for zircon growth processes in the greisen veins. One notable feature, probably due to the greisen-forming processes, is the very high common Pb concentrations characterizing the zircons. By varying the assumed common Pb composition in the calculations, the effect on resulting intercept ages has been evaluated. The obtained age interval (1775-1782 Ma) is typical of formation ages for Revsund granites, which indicates that the mineralization event at Rostberget followed closely upon the granite emplacement. We conclude that the Revsund-type granites, which cover large areas, were generated in a narrow time interval 1790-1775 Ma ago.
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| 15. |
- Billström, Kjell, et al.
(författare)
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Zn-Pb Ores of Mississippi Valley Type in the Lycksele-Storuman District, Northern Sweden : A Possible Rift-Related Cambrian Mineralisation Event
- 2012
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Ingår i: Minerals. - : MDPI AG. - 2075-163X. ; 2:3, s. 169-207
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Tidskriftsartikel (refereegranskat)abstract
- The epigenetic Zn-Pb deposits in the Lycksele-Storuman ore district, northern Sweden, are hosted by Paleoproterozoic basement near the margin of the Caledonian mountains. A paleogeographic reconstruction suggests that platform sediments, including Cambrian shales, overlaid the mineralised basement. The mineralisation type, containing sphalerite, galena, calcite and fluorite, is confined to veins and breccias and interpreted to be of Mississippi Valley Type (MVT) style. There is no appreciable wall rock alteration. Fluid inclusion work reveals coexisting aqueous and hydrocarbon fluids. Ore deposition is interpreted to have occurred during mixing of two fluids; a cool (<70 degrees C) brine with a salinity of 30 eq. mass% CaCl2 and a hot (similar to 200 degrees C) brine with about 18 eq. mass% CaCl2. The mixing led to complex Sr isotope systematics in the analysed minerals. A tentative sphalerite Rb-Sr isochron age of a 534 +/- 13 Ma probably dates mineralisation. The isotope systematics of Pb and Nd are less complex and both elements were essentially leached from basement rocks and transported by the hot fluid. Ore formation is considered to have taken place during rifting, related to the opening of the Iapetus Ocean.
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| 21. |
- Dekov, Vesselin, et al.
(författare)
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Ferrihydrite precipitation in goundwater-fed river systems (Nete and Demer river basins, Belgium): Insights from a combined Fe-Zn-Sr-Nd-Pb-isotope study.
- 2014
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Ingår i: Chemical Geology. - : Elsevier. - 0009-2541 .- 1872-6836. ; 386, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- Two groundwater-fed river systems (Nete and Demer, Belgium) carry red suspended material that settles on the river bed forming red sediments. The local aquifer that feeds these river systems is a glauconite-rich sand, which provides most of the dissolved Fe to the rivers. The solid component of these systems, i.e., the red suspended material and sediments, has a simple mineralogy (predominantly ferrihydrite), but shows a complex geochemistry pointing out the different processes contributing to the river chemistry: (1) the red sediments have higher transition metal (excluding Cu) and detrital element (e.g., Si, Al, K, Rb, etc.) concentrations than the red suspended matter because of their longer residence time in the river and higher contribution of the background (aquifer) component, respectively; (2) the red suspended material and sediments have inherited their rare earth element (REE) patterns from the aquifer; (3) the origin of Sr present in the red suspended matter and red sediments is predominantly marine (i.e., Quaternary calcareous rocks), but a small amount is geogenic (i.e., from detrital rocks); (4) Pb in both solids originates mostly from anthropogenic and geogenic sources; (5) all of the anthropogenic Pb in the red suspended material and sediments is hosted by the ferrihydrite; (6) Nd budget of the red riverine samples is controlled by the geogenic source and shows little anthropogenic component; (7) the signi- ficant Fe- and Zn-isotope fractionations are in line with the previous studies. Their fractionation patterns do not correlate, suggesting that the processes controlling the isotope geochemistry of Fe and Zn are different: oxidation/reduction most likely governs the Fe-isotope fractionation, whereas adsorption/desorption or admixing of anthropogenic sources controls the isotope fractionation of Zn.
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| 25. |
- Guitreau, Martin, et al.
(författare)
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Hafnium isotope evidence for early-Proterozoic volcanic arc reworking in the Skellefte district (northern Sweden) and implications for the Svecofennian orogeny
- 2014
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Ingår i: Precambrian Research. - : Elsevier. - 0301-9268 .- 1872-7433. ; 252, s. 39-52
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district is a seemingly juvenile and heavily mineralized crustal domain in northern Sweden that formed between 1.90 and 1.87 Ga. It is commonly interpreted as a volcanic arc deposited on a basement (known variously as the Bothnian or the Knaften-Barsele group) that could be represented by older rocks (1.96-1.94 Ga) found in the vicinity. In order to understand the potential genetic relationship between the arc and the basement, Hf and Pb isotopes in magmatic zircons from key lithologies were measured by solution multi-collector inductively-coupled plasma mass spectrometry. It is shown that both geological groups display similar Hf isotope compositions, which translate into decreasing εHf with time. Overall, the data are compatible with reworking of the Knaften-Barsele arc to produce the Skellefte rocks over a short time interval from 1.90 to 1.87 Ga in a context of crustal extension with ongoing subduction. When the data presented here are integrated with general models of tectonic evolution of the Svecofennian orogen, they fit a scenario in which the juvenile Knaften-Barsele arc formed between 1.96 and 1.94 Ga and became accreted onto the Karelian continent located further north at about 1.92-1.91 Ga. Systematic north to south variations in Pb, Nd, and Hf isotope compositions throughout the Svecofennides, interpreted as resulting from an increase in Archean crust involvement towards the south, indicate a genetic link between the Proterozoic crustal domains of Sweden and Finland.
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