A rock magnetic study of the Åkerberg gold deposit, northern Sweden

Abstract: A rock magnetic study has been preformed of the Åkerberg gold deposit, located 30 km NE of Boliden in northern Sweden. The gold mineralization is located to a zone of subparallel quartz veins in a layered gabbro. Ground surveys show a lowering of the magnetic properties at the ore zone. This thesis addresses the factors causing the anomaly by investigating the magnetic properties of different parts of the gabbro. The thesis is also concerned with displacement of tectonic blocks, which is indicated by tilted magmatic layers in the immediate surroundings of the ore zone. Thermal and alternating field demagnetization and susceptibility and thermomagnetic measurements have been conducted on the samples collected at the ore zone and in distant parts relative to the ore zone. ChRM-directions have been analyzed with principal component analyses and one Virtual Geomagnetic Pole (VGP) was calculated. Mineral composition was examined with a Scanning Electron Microscope and conventional microscopy in both transmitted and reflected light. This study shows that the sampled outcrop distant to the ore zone is magnetite poor and that the gabbro lacks magnetite in the ore zone and in immediate surrounding areas. The NRM (Natural Remanent Magnetization) in the ore zone is carried by monoclinic pyrrhotite. The magnetic anomaly over the ore zone is caused by a lower amount of pyrrhotite compared to the surrounding areas. This study also indicates that the NRM carried by monoclinic pyrrhotite in the ore zone and in the distant parts of the gabbro was acquired during the same event. The timing of this event, however, is unclear since only one VGP could be calculated.