Palaeoproterozoic metavolcanic and metasedimentary succession hosting the Dannemora iron ore deposits, Bergslagen region, Sweden

Abstract

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

Keywords:

• Bergslagen
• Dannemora
• Palaeoproterozoic
• pyroclastic rocks
• volcanic textures
• ignimbrites

Acknowledgements

We thank Dr Lennart Falk, Dannemora Mineral AB, for suggesting this study and Dannemora Mineral AB and the Geological Survey of Sweden for financing the first years of the project. Our colleagues at Dannemora Mineral AB are thanked for access to drill cores and support in various practical matters. The manuscript has benefited greatly from the reviews by Dr Frank Beunk and Dr Markku Väisänen. The following persons are thanked for improving earlier versions of the manuscript: Prof. Hemin Koyi, Dr Örjan Amcoff, Dr Karin Högdahl and Dr Magnus Ripa.