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

Abstract. LKAB currently runs an intense exploration program focused on the Per Geijer (PG) iron oxide-apatite mineralization, close to the active Kiruna mine, in northern Sweden. The mineralization and the hosting volcanic units dip at an angle of 50–60 degrees and are overlain by quartzite. Greenstones are also present in the area. 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 geophysical surveys have been used to assist 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, all with Vibroseis as a source. These were followed by downhole seismics (VSP) in two drill holes. During 2024 a 3D survey covering the Per Geijer mineralization was also acquired. Clear P-wave reflections are visible 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 steeply dipping structures in the area complicate the data processing, and out-of-the-plane reflections are present in some of the 2D profiles, the latter confirmed via cross-dip analysis. The resulting migrated sections and volumes show prominent reflections, some of which correlate with the Per Geijer mineralization or units close to it and suggest some extensions of the known mineralization. Reflections are also observed from the northern end of the nearby Kiirunavaara ore. Sonic and density logs are available to support the interpretation. These indicate that the main cause of the reflections from the iron mineralization is the density contrast, rather than the velocity contrasts. The downhole seismics also allow a closer link to the drilling information. Further notable in both our 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 western side of the Kiirunavaara orebody.