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- 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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| 3. |
- Frietsch, Rudyard, et al.
(författare)
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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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Ingår i: Ore Geology Reviews. - 0169-1368 .- 1872-7360. ; 12:1, s. 1-34
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Frietsch, Rudyard
(författare)
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Formation of Mg-bearing magnetite and serpentine in skarn iron ores in northern Sweden
- 1984
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 106:3, s. 219-230
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Tidskriftsartikel (refereegranskat)abstract
- This paper deals with the formation of magnetite in connection with sulphurization and serpentinization of skarn iron ores in Early Proterozoic (1.9 Ga) metasediments and mafic metavolcanics in northern Sweden. The ores contain Ca-Mg and Mg silicates which are characterized by a high Mg/(Mg + Fe) ratio, being mainly higher than 0.8. This feature is possibly due to an original high content of sulphur in the chemically precipitated ore material. During regional metamorphism involving heat transfer and recrystallization there occurred internal reactions. Sulphur reacting with primary, more Fe-rich silicates, gave rise to more Mg-rich silicates and a simultaneous formation of iron sulphides and magnetite. Calculations made on the Stora Sahavaara ore show, however, that the sulphide-silicate reactions could only have attributed to relatively restricted amounts of magnetite. The magnetite of the ores often contains small amounts of magnesium, at maximum 6 weight% Mg. The magnesium-bearing magnetite is mostly associated with serpentine which is an alteration product of olivine, tremolite and diopside. The occurrence of the magnesium-bearing magnetite is attributed to serpentinization which released iron from the silicates, and their high magnesium/iron ratio could in part be related to this process. However, the formation of magnetite in connection with serpentinization is only known in some deposits, and it is therefore plausible that serpentinization, in similarity with the sulphide-silicate reactions, has given rise to restricted amounts of magnetite in comparison to the amount of magnetite (or its precursor) which was deposited as a primary chemical precipitate.
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| 7. |
- Frietsch, Rudyard
(författare)
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On the magmatic origin of iron ores of the Kiruna type : reply
- 1984
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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. ; 79:8, s. 1949-1951
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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