Petrophysical Properties of Rocks in Central Kiruna Mining District, Sweden

Abstract: High economic and metallogenic significance of northern Norrbotten ore district in Sweden which hosts Kiruna mining district makes it a very important ore district in the world. Northern Norrbotten ore district contributes a combined 90 percent of iron production in Europe. Present day Kiruna mining district is host to Iron-oxide-Apatite (IOA), Viscaria, Iron-Oxide-Copper-Gold and Pahtohavare deposits. However, it is famous for its IOA Kirunavaara mineralization which is a giant ore body of high-grade magnetite. Results from earlier mineral prospecting activities indicates volume of orebody diminishes at depth especially towards the south. In this study, petrophysical measurements and data analysis of rock samples from outcrops in Kiruna mining district are performed. Physical rock properties govern geophysical survey techniques hence are used as petrophysical constraint in geophysical modelling to develop a common earth model of the mining district. Common earth modelling involves integrating all relevant geological, geophysical and petrophysical data in the model. This is to improve the understanding and knowledge of the orebody and aid long term mine planning activities in the mining district. Rock physical properties investigated include density, magnetic (anisotropy of magnetic susceptibility, natural remanent magnetisation) and electrical (complex electrical resistivity) properties. Block samples were collected from 34 outcrops in the Kiruna mining district and cut into cubes. Density of rocks samples ranged from 2388 to 3093 kgm-3. Rock types of most samples are belonging to the porphyry group based on previous gravity and density modelling of Kirunavaara mineralization. Variation in degree of induced (anisotropic) magnetic susceptibility (AMS), dominance of prolate ellipsoid of AMS parameters follow trend of geological structures are noted with reference to rock types. Mechanism of induced polarization (IP) in rock samples is of grain polarization type due to presence of metallic minerals disseminated in the rock samples. Complex electrical resistivity in wet rock samples increases with increasing frequency. Magnetisation in outcrops 01 and 21 is largely influenced by remanent magnetisation whereas magnetisation of outcrops 03 and 02 is influenced by induced magnetisation. Nature of magnetisation is determined based on computed magnitude of Königsberger ratio calculated from two types of measuring setups, here referred as gradiometer and oersted-meter approaches. Thus, the natural remanent magnetisation (NRM) in rocks is estimated in addition to induced magnetisation.