Geology of the Dannemora field
The bedrock in the Dannemora field is dominated by 1.9 billion year old supracrustal rocks (formed on the surface) which include volcanoclastics (fragmented rocks of volcanic origin), carbonate rocks and metamorphosed variants of them. Although the rock has undergone considerable subsequent alteration, the primary structures are unusually well preserved.
The supracrustal rocks are part of an isoclinal fold structure comprising two synclines separated by an intermediate anticline. The Dannemora field is in the eastern syncline (“The Dannemora Syncline”), which consists of two steeply dipping limbs. The bottom of the syncline is known in the southern part of the field. In the northern part it has not been demonstrable despite diamond drilling to a depth of approx. 1,150 metres.
The volcanoclastic rock, which belongs to the leptite formation, has been classified as “pyroclastic flow deposit” or “pyroclastic fall deposit”, which in simple terms means they were formed during intense volcanic activity from gas-rich and fragment-rich ash masses flowing down the volcanoes respectively from material raining down from the clouds of ash ejected during eruptions. The field also contains layers of rock, mainly of the above types but which have been broken down by erosion and re-deposited on the earth’s surface or under the surface of the sea (“reworked pyroclastic deposit”).
The carbonate rocks in the field are classified as calcitic limestone or dolomitic limestone. Calcitic limestone, which is normally white or light grey, contains only small quantities of magnesium, iron and manganese, while dolomitic limestone is usually dark grey and has a considerably high content of these elements.
Two skarn types can be distinguished, depending on the manganese content. Manganese-poor skarn consists mainly of minerals such as diopside, actinolite and garnet, together with varying quantities of magnetite, while manganese-rich skarn, which can have a manganese content of up to 25 percent, predominantly contains minerals such as knebelite, dannemorite and serpentine, and often magnetite too. The latter is normally found in close proximity to the manganese-rich iron ore mineralisations.
Geological key map of the Dannemora area. I. Lager, 2001: The geology of the Palaeoproterozoic limestone-hosted Dannemora iron deposit,Sweden. SGU [Geological Survey of Sweden] Reports and Bulletins no.107.
The field is characterised by a large number of magmatic dikes and granodioritic intrusive rocks are common particularly in the southern part of the field.
Apart from the isoclinal fold, the tectonics in the field are characterised by a large number of faults. Some of these are nearly horizontal and in some cases have caused considerable displacement.