The Kirunavaara hangingwall : a geophysical and petrophysical analysis

Abstract: Underground mining of the Kirunavaara iron mineralization is causing large-scale deformation of the hangingwall of the orebody. To understand and possibly even predict this deformation, a good understanding of the elastic and mechanical properties of the rocks in the hangingwall is necessary. This thesis presents results from the study of Anisotropy of magnetic susceptibility (AMS), magneto-mineralogy, fracture frequency (FF), and rock quality (RQD). If a link can be found between AMS and the rock mechanical parameters FF and RQD, AMS may serve as a cheap tool for a rough indicator of rock mechanical properties. Two parallel reflection seismic profiles were shot within the town of Kiruna in order to locate deformation zones and lithological boundaries in the hangingwall. This work was done in cooperation with Uppsala University. AMS, FF and RQD data was obtained from three drillcores in the hangingwall and 13 outcrops. The drillcores are located along the southernmost of the seismic profiles. The AMS study showed that the magnetic fabric varies very much between rock types, and even within what has previously been thought of as homogeneous porphyry. Based on the magnetic fabric, 5 sub-groups of porphyries (group I-V) were identified. Results of the magneto-mineralogy study further indicate that magnetite controls the magnetic susceptibility of not just the porphyries, but all the rock types in the Kirunavaara hangingwall. The only exception is the paramagnetic type IV porphyry. Data from outcrops show a decrease in the dip of the magnetic foliation plane (F) and the degree of magnetic anisotropy (Pj) towards the east, and this was confirmed by data from drillcores. The decrease in dip also corresponds to previous studies of the structural geology in the area. For the porphyries, a qualitative correlation was found between the FF and the magnetic shape parameter (T), but no solid correlation could be established between the RQD and Pj, F or T. The magnetic data suggests that the rocks in the uppermost part of the Kirunavaara hangingwall (down to 151-185m) have been subjected to shearing while deeper rocks (below 151-185m) show signs of folding. This sub-horizontal layering is also observed in the magneto-mineralogy and RQD-data. A possible explanation is that the porphyries of the Kirunavaara hangingwall originate from (at least) two different lava-flows, the resultant rocks having a different mineralogical composition and thus different magnetic and rock mechanical properties. Five seismic reflectors, corresponding to five lithological boundaries, were located, and their strike and dip calculated. What appears to be the upper part of the Kirunavaara mineralization was also mapped, as a bonus. Tomographic modelling of first arrivals indicates the position of low velocity (deformation) zones in the near-surface, down to 200m.