Seismic surveys at the Per Geijer iron oxide-apatite mineralization in Kiruna, Sweden

Abstract. The mining company LKAB currently runs an intense exploration program focused on the Per Geijer iron oxide-apatite mineralization close to the active Kiruna mine, in northern Sweden. The iron mineralization is associated with high levels of phosphorus and REEs. As a major part of the mineralization occurs at depths >500 m, various deep-probing geophysical surveys have been used to guide the exploration throughout the years, recently also including seismic measurements. An initial pilot reflection profile in 2021 showed promising results, and three additional profiles were acquired shortly afterwards. These were followed by downhole seismics (VSP) in two drill holes. During 2024 an approximately 13 km2 large 3D survey was also acquired in the area. All these surveys utilized a Vibroseis as the source of the seismic signals. Clearly distinguishable P-wave reflections appear in the shot records from the surface seismics as well as in the downhole seismics, and the latter also show interpreted P-S reflections. The iron mineralization is dipping at an angle of 50–60°, making the data processing challenging. Some of the 2D profiles cross the mineralization at an oblique angle, and out-of-the-plane reflections were confirmed via cross-dip analysis. The resulting migrated sections and volumes show prominent reflections correlating with the Per Geijer mineralization and indicating extensions beyond the drill hole coverage. Reflections are also observed from the northern end of the nearby Kiirunavaara iron ore. Sonic and density logs indicate that the main cause of the reflections from the iron mineralization is the density contrast, as the velocity contrasts are rather small. A close comparison of the downhole seismic survey with the geological model further supports the interpretation that the iron mineralization is generating seismic reflections. Also notable in both 2D and 3D surface data is a high-amplitude reflection originating from a steeply dipping structure outside the survey area, which if projected to the surface correlates with older greenstones on the footwall (western) side of the Per Geijer mineralization and the Kiirunavaara orebody.