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Stratigraphy and Geochemistry of the Palaeoproterozoic Dannemora inlier, north-eastern Bergslagen region, central Sweden.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Palaeoproterozoic Dannemora inlier is situated in the north-eastern Bergslagen region. The inlier consists of primary and reworked volcanic deposits, stromatolitic limestone and skarn that have been subjected to upper greenschist facies metamorphism. Thicknesses of the different volcanic deposits indicate deposition within a caldera, where syn-volcanic alkali alteration was strong. The deposition was submarine and below wave base in the eastern part of the inlier, but above wave base in the central part where erosion channels together with cross-bedding occurs frequently.

The Dannemora Formation is the volcanosedimentary succession of the inlier. Two borehole profiles, a northern and a southern, cover the whole Formation and show different alteration patterns. A strong depletion of Na2O and enrichment of K2O dominate in the southern profile, whereas this pattern is not as evident in the northern profile. The uppermost section of the totally eight constituting the Formation, is intercalated with ore-bearing dolomitic limestone and skarn, and has experienced at least two episodes of alteration. An anticline has been established lithogeochemically from immobile element ratios and the reoccurrence of an accretionary lapilli bed.

Numerous altered sub-alkaline, calc-alkaline and basaltic dykes have been recorded in the Dannemora inlier. They are the result of mixing and fractionation of at least three magmatic sources and carry a mixed signature of subduction zone and within-plate volcanic tectonic setting.

A seismic profile across the Dannemora inlier images a strong reflector package that dips c. 50° E to the east of the inlier. This package coincides with the polyphase, E-up reverse, brittle-ductile Österbybruk deformation zone (ÖDZ). Yet another steep reflector in the Dannemora ore-field extends to a depth of more than two kilometres. This reflector might represent either a deep-seated iron deposit or a fluid-bearing fault zone.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 50 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1209
Keyword [en]
Dannemora, Bergslagen region, volcaniclastic rocks, sedimentology, metasomatic alteration, lithogeochemistry, reflection seismic
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-236872ISBN: 978-91-554-9115-4 (print)OAI: oai:DiVA.org:uu-236872DiVA: diva2:765755
Public defence
2015-01-16, Axel Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2014-12-22 Created: 2014-11-24 Last updated: 2015-02-03
List of papers
1. Palaeoproterozoic metavolcanic and metasedimentary succession hostingthe Dannemora iron ore deposits, Bergslagen region, Sweden
Open this publication in new window or tab >>Palaeoproterozoic metavolcanic and metasedimentary succession hostingthe Dannemora iron ore deposits, Bergslagen region, Sweden
2012 (English)In: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 134, no 2, 71-85 p.Article in journal (Refereed) Published
Abstract [en]

The Dannemora supracrustal inlier is located in the north-eastern part of the Bergslagen regionin south-central Sweden and hosts the second largest iron ore deposit in the region. The metasupracrustalsuccession of the inlier consists of c. 1.9 Ga Palaeoproterozoic rocks that are mainly sub-alkaline, rhyoliticto dacitic, pyroclastic deposits, reworked pyroclastic deposits and metalimestone. It is c. 700–800-m thickand termed the Dannemora Formation. The formation is divided into lower and upper members and theformer is in turn subdivided into subunits 1 and 2. The great thickness of individual pyroclastic depositsindicates deposition within a caldera. The rocks show characteristics of a pyroclastic origin by containingabundant pumice, cuspate and Y-shaped former glass shards, and fragmented crystals of quartz andsubordinate feldspars. Scattered spherulites and lack of welding-compacted fiamme suggest that the lowermember was slightly welded, where as the upper member contains sericite-replaced glass shards withpreserved primary shapes indicating no welding. Undisturbed layers of ash-siltstone with normal gradingand fluid–escape structures are attributed to subaqueous deposition below storm wave base in the easternpart of the inlier, where as erosion channels and cross-bedding in some of the volcaniclastic deposits implydeposition and reworking above wave base in the central part of the inlier. Epidote spots, previouslyinterpreted as altered limestone fragments and an indicator for subaquatic deposition, are here reinterpretedas the result of selective alteration related to the intrusion of mafic dykes and to Ca release duringdolomitisation of limestone.

Place, publisher, year, edition, pages
Taylor & Francis, 2012
Keyword
Bergslagen; Dannemora; Palaeoproterozoic; pyroclastic rocks; volcanic textures; ignimbrites
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-182809 (URN)10.1080/11035897.2012.674551 (DOI)
Available from: 2012-10-15 Created: 2012-10-15 Last updated: 2017-12-07Bibliographically approved
2. Source character, mixing, fractionation and alkali metasomatismin in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden
Open this publication in new window or tab >>Source character, mixing, fractionation and alkali metasomatismin in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden
2013 (English)In: Geological Magazine, ISSN 0016-7568, E-ISSN 1469-5081, Vol. 151, no 04, 573-590 p.Article in journal (Refereed) Published
Abstract [en]

The geochemical and isotopic characteristics of metamorphosed Svecofennian maficdykes from the Dannemora area in the NE part of the Bergslagen region in central Sweden wereinvestigated and compared to mafic intrusive rocks in their vicinity. The dykes, with an inferred ageof c. 1860–1870 Ma, are calc-alkaline, sub-alkaline and basaltic in composition and have a mixedsubduction and within-plate geochemical affinity. They are the result of mixing of at least three mantlesource components with similar basaltic major element composition, but different concentrations ofincompatible trace elements. Magma M1 is strongly enriched both in Rare Earth Elements (REE)and High-Field-Strength Elements (HFSE); magma M2 is highly enriched in Large-Ion LithophileElements (LILE, except Sr) with only moderate enrichment in HFSE and REE (particularly low inHeavy Rare Earth Elements); and magma M3 is enriched in Sr and has a flat REE profile. MagmaM3 also has a somewhat more positive (depleted) initial εNd value of +1.8, compared to +0.4 to +0.5 for magmas M1 and M2. The magma evolution was controlled by a mixture of fractionation (mainlyaffecting the compatible elements) and mixing, best seen in the incompatible element concentrationsand the Nd isotope data. The basaltic overall composition indicates little or no wholesale contaminationby upper continental crust, but the dykes have undergone later metasomatic changes mainly affectingthe alkali elements.

Place, publisher, year, edition, pages
Cambridge Academic, 2013
Keyword
Svecofennian, Bergslagen region, Dannemora, greenstone dykes, alkali metasomatism
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-214689 (URN)10.1017/S0016756813000551 (DOI)000337751000001 ()
Available from: 2014-01-09 Created: 2014-01-09 Last updated: 2017-12-06Bibliographically approved
3. Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphasedeformation zones and magnetite‐skarn deposits
Open this publication in new window or tab >>Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphasedeformation zones and magnetite‐skarn deposits
Show others...
2011 (English)In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 116, B11307- p.Article in journal (Refereed) Published
Abstract [en]

The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora mining area. The interpretations are constrained by seismic wave velocity measurements on a series of rock samples, cross-dip analysis, prestack time migration, and swath 3-D imaging, as well as by other available geophysical and geological observations. A series of major fault zones is imaged by the seismic data, as is a large mafic intrusion. However, the most prominent feature is a package of east-dipping reflectors found east of the Dannemora area that extend down to at least 3 km depth. This package is associated with a polyphase, ductile-brittle deformation zone with the latest ductile movement showing east-side-up or reverse kinematics. Its total vertical displacement is estimated to be in the order of 2.5 km. Also clearly imaged in the seismic data is a steeply dipping reflector near the Dannemora mine that extends down to a depth of at least 2.2 km. The geological nature of this reflector is not known, but it could represent either a fluid-bearing fault zone or a deep-seated iron deposit, making it an important target for further detailed geophysical and geological investigations.

National Category
Geology
Identifiers
urn:nbn:se:uu:diva-166692 (URN)10.1029/2011JB008643 (DOI)000297675200001 ()
Available from: 2012-01-12 Created: 2012-01-12 Last updated: 2017-12-08Bibliographically approved
4. Lithogeochemistry and alteration of a Palaeoproterozoic metavolcanic succession in the Dannemora area, Bergslagen region, central Sweden
Open this publication in new window or tab >>Lithogeochemistry and alteration of a Palaeoproterozoic metavolcanic succession in the Dannemora area, Bergslagen region, central Sweden
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The rocks in the Dannemora area constitute the Dannemora Formation, which can be divided into eight different sections, A-H, on the basis of stratigraphy and geochemistry. The great thickness and alteration pattern of the deposit indicate deposition in a caldera environment, related to both intensive and waning volcanic stages and hydrothermal processes. The partly strong alteration is shown by the sericite dominated matrix and the completely sericite replaced plagioclase and volcaniclastic clasts. The northern part of the Dannemora area in general shows a lower degree of alteration than the central part in comparable stratigraphic levels, which implies that these parts were located in different positions to hydrothermal cells.

A lapilli-bearing ash layer is present between the massive pyroclastic beds in section A and C that have distinctly different alteration patterns suggesting that the strong alteration in section A occurred before deposition of section C. During the waning volcanic stage, the calmer environment allowed stromatolitic limestone interlayered with thin horizons of ash-siltstone to deposit. During subsequent dolomitisation of the limestone the interlayered ash-siltstone was affected by great enrichment of CaO and depletion of MgO. Despite this strong alteration this section is classified as least altered by alteration index, which most likely reflect a two stage alteration with initial alkali enrichment, followed by CaO enrichment that “reset” the alteration index. The great enrichment of Ba in the upper member is supported by the occurrence of hyalophane. On the basis of lithogeochemical signature of immobile element ratios together with the reoccurrence of an accretionary lapilli-bed the position of a previously inferred anticline has been established.

National Category
Geology
Identifiers
urn:nbn:se:uu:diva-236878 (URN)
Available from: 2014-11-25 Created: 2014-11-25 Last updated: 2015-02-03Bibliographically approved

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