Orebodies in the Dannemora field
The iron-ore-bearing part of the Dannemora synclinal has a length of approx. 3 km and a width ranging between 400 and 800 metres. As a result of the primary formation of iron-rich mineralisations and the later tectonisation, a large number of orebodies occur throughout the strike of the field and at various depths. A total of twenty five orebodies in the field have been mined.
The orebodies generally occur as more or less stratiform lenticular bodies in the carbonate rock. Close to the surface, the orebodies strike in approx. N30oE and dip between 90o and 80o towards the northwest, while a less steep dip is encountered deeper down. Thicknesses vary from a few metres to approx. 60 metres. The iron-ore-bearing formation has a considerable depth in the northern part of the field, where magnetite-rich mineralisations are intersected at a depth of approx. 1,000 metres with core drill holes.
Longitudinal section of the Dannemora field showing mined out ore and remaining ore down to the depth investigated by core drilling.
The predominant (and only economically interesting) iron mineral in the field is magnetite, a mineral composed of iron and oxygen (Fe3O4). Normally the magnetite is fine-grained (0.04-0.1 millimetres).
The orebodies in the Dannemora field have relatively high manganese grades. In one of the orebodies the manganese is bound to magnetite (up to 3 percent), but in the majority of the orebodies it is silicate-bound and is contained in skarn minerals such as knebelite and dannemorite which occur in (and close to) several of the iron orebodies.
Historically, the mined iron ores in the Dannemora field have been classified into three main types:
- Manganese-rich skarn iron ores consisting mainly of magnetite, knebelite, dannemorite and serpentine. The iron content in these orebodies varies between 30 and 50 percent, while the manganese content is between 1 and 6 percent.
- Manganese-poor skarn iron ores consisting of magnetite, diopside and actinolite. The iron content is between 30 and 50 percent and the manganese grade normally lies between 0.2 and 1.0 percent.
- Calcareous iron ores consisting of magnetite, calcite, dolomite and sometimes iron-bearing carbonate minerals.
All orebodies in the field have low phosphorous content (0.001-0.007 percent). The sulphur content originates mainley from pyrite and pyrrhotite and varies between the orebodies.
A number of small sulphide occurrences are known in the Dannemora field. Some of these have high zinc grades and have been mined during different periods. As production in the mine was focused entirely on iron ore during the previous operating period, the sulphide mineralisations have not been investigated to any appreciable, extent. Consequently, it has not yet been possible to calculate tonnages and grades.
The sulphide mineralisations can in principle be divided into two main categories based on their mineralogical composition.
- Mineralisations with sphalerite, galena, pyrite and arsenopyrite (also has silver grades up to 20 grams per tonne).
- Mineralisations with chalcopyrite, pyrite, pyrrhotite and arsenopyrite (contains occasionally gold up to 4 grams of gold per tonne).
Geological map of “Svavelgruvan” at a depth of 40 metres. I. Lager, SGU [Geological Survey of Sweden] Reports and Bulletins 107.