The dominating rocks of the super group are various types of mica shists, dolomitic marbles, limestones, amphibolites and quartzites. The iron ore contains an average of 33% iron in the forms of the oxide minerals hematite and magnetite. Hematite makes up 97.5% to 98% of the iron oxides. Magnetite corresponds to 2.5% to 2.0% in the Kvannevann mine. Other ore bodies may contain up to 15% Magnetite.
Due to tectonic overprint of the host rock and the ore, the rock formation is grossly folded and shows a distinct cleavage underlined by the occurrence of flaky hematite crystals (specularite). The magnetite crystals of the ore are on the contrary mainly isometric and partly show idiomorphic crystal faces. The main gangue minerals are silicates, quartz and carbonates.
The hematite is proven to contain minor but detectable amounts of titanium (Ti) and very low contents of heavy metals. The magnetite is slightly enriched in manganese oxide (MnO) while the total amount of heavy metals does not exceed 360 ppm. The chemical and mineralogical properties of the ore minerals are proven to be stable throughout the mining areas.
The host rock for the iron oxide minerals is made up by quartz, feldspar, Fe-Al-silicates and carbonates. The ore seams are from a few meters to hundred meters thick and strongly folded. In the Ørtfjell area they almost are vertical with unknown extension to depth.
The ore concentrations are spread over an area of approximately 45 km2, and the current mining is concentrated around Ørtfjell with five major deposits. The iron ore resources have been estimated to approximately 509 mill tonnes until 100 m above sea-level, expected to ensuring production for several decades.
Sub level caving
After more than 30 years of open pit mining, Rana Gruber AS started underground mining using the sublevel open stoping method at the Kvannevann Mine in 2000. The mining has been continuously improved and further developed to suit the difficult rock conditions in the area and to reduce operating costs. A combination of low-grade iron ore and significant rock mechanics challenges (large horizontal stresses) has made the mining in the Kvannevann Mine extraordinary.
Sublevel caving involves dividing the ore body in horizontal layers, starting with the first layer on top of the ore body and then work downwards. In sublevel caving the production is divided into four different stages of operation.
This method allows working in parallel on different sublevels in the mine, and continuous excavation works can be on-going at one sub level without interfering with another.
Iron ore mine
Iron oxide mining started in the Dunderland valley more than 100 years ago. Until 1999 iron ore was only mined by open pit methods. Today Rana Gruber operates the Kvannevann underground mine as well as satellite deposits which are mined by open-pit methods.
The ore in the underground mine is transported by trucks and crushed at the new underground crushing station. Conveyor belts transports the ore to the central underground storage silo. Ore from the open pit operations is taken the main crusher and blended with the underground ore in the silo. The crushed ore from the silo is filled in railway wagons. This filling takes place underground due to the special climate conditions of the area. The ore is then transported by rail to the dressing plant at Gullsmedvik.