Abstract

We have conducted a reflection seismic investigation over the apatite-iron deposit at Grängesberg in central Sweden. At the time of closure in 1989, the mine was operated using the sublevel caving method down to approximately a 650-m depth. This mining technique caused subsidence and generated a network of faults that propagated from excavated zones at depth up to the surface. The Grängesberg deposit is the largest iron oxide mineralization in central Sweden and is planned to be mined again in the coming years. It is therefore imperative to have a better understanding of the ore geometry and the fault network. A reconnaissance survey consisting of two seismic lines with a total length of 3.5 km was carried out to address these issues. The profiles intersect the Grängesberg deposit and open pit, as well as the major mining-induced fracture zone present in this area. A drop-hammer source mounted on a hydraulic truck was used to generate seismic signals; cabled and wireless receivers were used for the data recording. Preprocessing of the data first required the cable- and wireless-recorded data sets to be merged before stacking all data available at each shot point. Source gathers exhibit reflections from the near surface, probably generated at lithological boundaries hosting the iron mineralization and other geologic structures. Deeper reflections were also observed. The metavolcanic assemblage hosting the mineralization and the anthropogenic fault network were depicted in the stacked sections, bringing in new elements to refine the geologic model of the area. This study also illustrated the ability of reflection seismic methods to delineate mining-induced structures in hard-rock environments. Low-velocity anomalies from the open pit and adjacent structures were depicted in tomographic sections along the two lines, which showed good agreement with known geologic features and the reflection seismic results.

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