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- Billström, Kjell, et al.
(author)
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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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In: Precambrian Research. - 0301-9268 .- 1872-7433. ; 81:1-2, s. 83-99
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Journal article (peer-reviewed)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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- Frietsch, Rudyard, et al.
(author)
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Early Proterozoic Cu-(Au) and Fe ore deposits associated with regional Na-Cl metasomatism in northern Fennoscandia
- 1997
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In: Ore Geology Reviews. - 0169-1368 .- 1872-7360. ; 12:1, s. 1-34
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Journal article (peer-reviewed)abstract
- Scapolite is widely distributed in 1.9-2.5 Ga volcano-sedimentary rocks and 1.77-2.2 Ga igneous rocks over several hundred square kilometres in northern Fennoscandia, comprising northern Sweden, northern Finland and adjacent parts of Norway and Russia. This region is one of the largest scapolite-bearing Precambrian terranes in the world. Albitization, and to a lesser extent carbonatization, phyllic and tourmaline alteration, are spatially associated with scapolite. A number of epigenetic Cu-(Au) sulphide and Fe oxide deposits in northern Fennoscandia show a spatial and genetic relationship to this type of alteration, mainly scapolitization and albitization. The main metal occurrences are in 2.0-2.5 Ga mafic volcanics and sediments of the Lapponian Greenstone group and in 1.9 Ga intermediate-composition volcanic and volcaniclastic rocks of the Svecofennian Porphyry group. The scapolite is mainly a dipyre-mizzonite with Cl and CO3 and small amounts of SO4 and F, indicating high Na and Cl activity at the time of crystallization. Fluid inclusion data of the Lapponian Pahtohavare and similar Cu-Au deposits indicate formation temperatures of about 300°C and ore deposition from highly saline aqueous solutions. The deposition of copper and gold was in places regulated by a redox barrier; graphite in associated schists controlled the reduction reactions of the ore fluids and metals were precipitated. The Lapponian and Svecofennian sulphide deposits contain tourmaline of the schorl-dravite series. Aitik-Nautanen Cu-(Au) style deposits and in particular some deposits with vein-style iron ore, contain dravite-schorl deficient in Al and enriched in Fe3+, which is due to Fe-Al substitution in an oxidizing, relatively iron-rich environment. Scapolite and, probably also tourmaline, formed by a complex, multistage process. The source of the components in scapolite may have been evaporitic sequences or high salinity brines in Lapponian rift basins that contain 2.0-2.5 Ga mafic volcanics. During low to medium-grade (low P) regional metamorphism, the components that formed scapolite and tourmaline were mobilized and transported to their present positions in several metasomatic phases. Fault zones with fractures and breccias channeled the fluids, resulting in locally developed intense alteration. Gold and copper was transported by saline, high fO2, high temperature solutions as metal-chloride complexes. The ultimate source of fluids and heat sources is uncertain, but deep-seated crustal magmatic processes seem prerequisite. The alteration occurred mainly around 1.9 Ga at the peak of the main regional metamorphism and the intrusion of granitoids through to around 1.8 Ga. Cu-(Au) sulphide and Fe oxide ore deposits associated with large-scale scapolite-forming metasomatic processes are found elsewhere in the world (e.g., Australia, Kazakhstan, Russia) and show similarities with the Cu-(Au) deposits in northern Fennoscandia. However, the close genetic connection between scapolite-albite and ore formation of Fennoscandian deposits is not a common feature in other belts.
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- Frietsch, Rudyard, et al.
(author)
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Rare earth elements in apatite and magnetite in Kiruna-type iron ores and some other iron ore types
- 1995
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In: Ore Geology Reviews. - 0169-1368 .- 1872-7360. ; 9:6, s. 489-510
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Journal article (peer-reviewed)abstract
- An investigation has been conducted to determine the content and distribution of REE in apatite and magnetite in the iron ores of Kiruna type and some other iron ores. The purpose of this article is to discuss the results obtained from the investigation. In particular, it will be shown that REE in apatite and magnetite in different ore types exhibit characteristic patterns related to different modes of formation of the ores
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| 9. |
- Frietsch, Rudyard, et al.
(author)
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Sulphur isotopes in Lower Proterozoic iron and sulphide ores in northern Sweden
- 1995
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In: Mineralium Deposita. - 0026-4598 .- 1432-1866. ; 30:3-4, s. 275-284
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Journal article (peer-reviewed)abstract
- The present investigation deals with sulphur isotope distribution in Lower Proterozoic iron and sulphide mineralizations in northern Sweden. The contrasting sulphur isotope patterns are indicative of different genesis. Some 267 sulphur isotope analyses of pyrite, pyrrhotite, chalcopyrite, sphalerite, galena and bornite from 23 occurrences have been performed. Some deposits exhibit uniform compositions, although the mean δ34S values are clearly different, while other mineralizations have widely fluctuating values. The δ34S values in syngenetic, exhalative sedimentary skarn iron ores, quartz-banded iron ores and sulphide mineralizations of the 2.0-2.5 Ga old (Lapponian) Greenstone group show a large spread, supporting the existence of bacteriogenic sulphate reduction processes. The spread of the sulphur isotope values (δ34S = -8 to +25‰), and the non-equilibrium conditions, point to a biogenic rather than to an inorganic reduction of seawater sulphate. The isotopic composition of the sulphides in the epigenetic Lannavaara iron ores which were formed by a hydrothermal scapolite-tourmalme-related process, indicates a sulphur source similar to that of the Greenstone group. The δ34S values of Cu-(Au) sulphide mineralizations in the Malmberget region (e.g. Aitik), which were formed by a similar process and hosted by the volcanics-volcanoclastics of the 1.9 Ga old Porphyry group, are slightly below zero ‰, indicating a magmatic origin. The existence of different sulphur compositions for these mineralization types formed by a similar hydrothermal process, probably reflects the influence of the host rock, the solutions leaching pre-existing sulphides. In southern Norrbotten, epigenetic, Cu-Zn-Pb veintype mineralizations in metavolcanics and metasediments have δ34S values close to zero ‰ indicating a magmatic origin. The sulphur isotope data of the volcanogenic, massive sulphide ores of the Skellefte district, in particular the ores of the Adak dome, are close to zero ‰. The lead and sulphur isotopic features of the sulphides in northern Sweden show that the ore-forming processes were of a different nature on both sides of the Archean-Proterozoic border, implying differences in the crustal development. Lead isotopes show that lead was mobilized from specific sources on each side of the border. The sulphur of the sulphides in the Greenstone group in NE Sweden and Finland was introduced by sedimentary processes, whereas the sulphur of the sulphide occurrences towards the SW, mainly in the Porphyry group, is dominated by a magmatic sulphur component
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