A model for the formation of the iron,manganese and sulphide ores of central Sweden

The non-apatitic iron ores, manganese ores and sulphide ores of central Sweden, which occur in a volcano-sedimentary complex of Early Proterozoic age, have a common volcanogenic origin. The close association between specific ore types and metavolcanics suggests a stratigraphic control of the ores on a regional scale. Deposited in basins, the ores formed with increasing water depth (increasing pH and decreasing Eh) in the following order: quartz-banded iron ores mostly lacking sulphides; non-man-ganiferous skarn iron ores with Cu-Fe sulphides; manganiferous skarn iron ores with Zn-Pb sulphides, and Fe²+-Mn²+ silicates (eulysites). The latter two ore types occur adjacent to argillites/greywackes indicating a reducing environment. The manganese oxide ores of the Långban type represent a formation intermediate between the quartzbanded iron ores and the manganiferous skarn iron ores.The main structural pattern of the ore-bearing district is made up of NE-NNW trending isoclinal folds, in which the non-manganiferous skarn iron ores occur in antiforms, in contrast to the quartz-banded iron ores, the manganiferous skarn iron ores and the eulysites which occur in synforms. The sulphide ores often occur close to the iron-bearing horizons. Associated magnesia-rich alterations, paracontemporaneous with the volcanism, follow the iron ore-bearing horizons within the synforms.

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Zusammenfassung Die zentralschwedischen Eisen-, Mangan- und Sulfidlagerstätten, die in einem vulkanisch-sedimentären Komplex Früh-Proterozoischen Alters liegen, haben einen gemeinsamen vulkanogenen Ursprung. Die nahe Assoziation zwischen bestimmten Erztypen und Meta-Vulkaniten spricht für eine stratigraphische Anlegung der Erze im regionalen Sinne. Abgesetz in Becken, erfolgte die Bildung der Erze mit zunehmender Wassertiefe (steigendes pH und abnehmendes Eh) in der folgenden Ordnung: quarzgebänderte Eisenerze häufig ohne Sulfide, manganfreie Skarneisenerze mit Cu-Fe-Sulfiden, manganführende Skarneisenerze mit Zn-Pb-Sulfiden sowie Fe²+-Mn²+-Silicate (Eulysite). Die letztgenannten Erztypen treten in Nähe von Meta-Argilliten und Meta-Grauwacken auf, was ein reduzierendes Milieu andeutet. Die Manganoxiderze vom Typus Långban stellen eine Bildung zwischen den quarzgebänderten Eisenerzen und den manganführenden Skarneisenerzen dar.Die hauptsächlichen Strukturen der erzführenden Provinz sind NO-NNW isoklinale Falten, in welchen sich die manganfreien Skarneisenerze in den Antiformen befinden und die quarzgebänderten Eisenerze, die manganführende Skarneisenerze und die Eulysite in den Synformen. Die Sulfiden finden sich oft nahe zu den Horizonten mit Eisenerzen. Assozierte magnesiumreiche Umwandlungen, die zu dem Vulkanismus gehören, folgen den eisenführenden Horizonten in den Synformen.

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Résumé Les minerais de fer, de manganèse et de sulfures, exempts d'apatite, de la Suède centrale, qui se présentent dans un complexe volcano-sédimentaire d'âge protérozoïque ancien, ont une origine volcanique commune. L'association étroite entre certains types de minerai et les méta-volcanites, indique un contrôle stratigraphique à l'échelle régionale. Déposés dans des bassins, les minerais se sont formés sous une profondeur d'eau croissante (pH augmentant et Eh diminuant) dans l'ordre suivant: minerais de fer siliceux-rubanés le plus souvent sans sulfures, minerais de fer skarnique non manganésifères avec sulfures de Cu et Fe, minerais de fer en skarn, manganésifères avec des sulfures de Zn et Pb, et finalement des silicates à base de Fe²⁺–Mn²⁺ (eulysites). Les deux derniers types sont à proximité des méta-argillites et méta-grauwackes, indiquant un milieu réducteur. Les minerais d'oxydes manganeux du type Långban représentent une formation intermédiaire entre les minerais de fer siliceux-rubanés et les minerais de fer en skarns manganésifères.Les structures principales de la région métallifère consistent en plis isoclinaux NE-NNW, les minerais de fer en skarns sont dans les antiformes, et les minerais de fer siliceux-rubanés, les minerais de fer en skarns manganésifères et les eulysites dans les synformes. Les sulfures se trouvent à proximité des horizons ferrifères. Les transformations associées riches en magnesium, qui sont liées à l'activité volcanique, se produisent suivant les horizons ferrifères dans les synformes.

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On the chemical composition of the ore breccia at Luossavaara,northern Sweden

An important argument for a magmatic affiliation of the Kiruna type of ore is the presence of ore breccia which accompany the main ore bodies. According to Parák (1975a and b) the main ore bodies are not of the same origin as the breccias. This is based on data from the Luossavaara deposit in northern Sweden: the main ore and the breccia have different chemical compositions. Microprobe analyses made in the present study, however, clearly demonstrate similarity in chemical composition, strengthening the hypothesis of a common origin for both formations.     

Rare earth elements in apatite and magnetite in Kiruna-type iron ores and some other iron ore types

An investigation of the content and distribution of REE in apatite and magnetite in the iron ores of Kiruna type and some other iron ores is presented. REE in apatite and magnetite in different ore types show characteristic patterns which are related to different modes of formation of the ores.The magnetite-apatite iron ores of the world can be divided into two types: (a) Kiruna iron ores proper which occur in volcanic rocks, and (b) iron ores connected with deuteric processes and/or related to intrusive rocks. Apatite of the Kiruna ores proper in Fennoscandia (e.g. Kiirunavaara, Malmberget and Grängesberg) shows a common pattern with 2000–7000 ppm REE, a weak to moderate LREE/HREE fractionation and negative Eu anomalies. In the Kiruna area, apatite of the main, P-poor ores and of the later, hydrothermal-exhalative P-rich ores, have the same REE distribution which indicates a common source. There is a similar REE distribution in magnetite-apatite trachytic-rhyodacitic host rock which confirms a close magmatic relationship. Apatite in phosphorites (such as the Paleoproterozoic Påläng deposit in northern Sweden) has a different composition (< 1000 ppm REE with Ce depletion) which excludes a sedimentary origin of the Kiruna apatite.Apatite in other volcanogenic magnetite-apatite ores outside Fennoscandia differ by a stronger LREE/HREE fractionation and by a medium to large Eu depletion, partly indicating a relationship to alkaline intrusions. The Avnik apatite, Turkey, shows a weak differentiation in combination with a pronounced negative Eu anomaly, indicating provenance from silicic magmatic sources.The REE pattern of apatite in the deuteric-hydrothermal apatite-bearing iron ores is in general similar to that of apatite in the Kiruna iron ores proper. The similarity indicates a common process of formation for both ore types.The apatite-iron ores of the Kiruna type proper were formed by a late-magmatic differentiation. The ores of the Kiruna area are, in similarity with some other magnetite-apatite ores, emplaced along regional fracture-fault lines and close to an older basement. In general the REE pattern of apatite in the different deposits shows an affinity to alkaline or sub-alkaline magmas, indicating a rifting environment. The alkaline, trachytic volcanics hosting the Kiruna ores in northern Sweden are clearly related to an extensional setting where rifting was important. A probable source for this large-scale ore-forming process was partial melting of deep-seated rocks. The ores evolved in an intracontinental setting with magma generation caused by underplating of older crust.The process giving rise to magnetite-apatite ores of the Kiruna type has occurred during the time span from Paleoproterozoic to Tertiary. The Proterozoic ores occur mainly in cratonized areas, whereas the younger ones occur in fold belts. The amount of ore formed in post-Proterozoic time is as large as that formed in Proterozoic time.     

Sulphur isotopes in Lower Proterozoic iron and sulphide ores in northern Sweden

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 ³⁴S values are clearly different, while other mineralizations have widely fluctuating values.The ³⁴S 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 ( ³⁴S = -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 ³⁴S 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 ³⁴S 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.