Mapping Remanent and Induced Magnetic Sources from the Km to Micron-scale

Significant resources of titanium (hemo-ilmenite) and magnetite are associated with the 7000m- thick, 930 Ma, Bjerkreim-Sokndal Layered (BKS) Intrusion. Recently NGU acquired high-resolution aeromagnetic surveys and we have complemented these by ground-magnetic surveys, and with thin section magnetic maps. The BKS, with an area of 250 km², lies in a tight, doubly plunging syncline produced during high- temperature solid-state deformation. Each unit began with intrusion of new magma that mixed with more evolved magma already in the chamber, temporarily producing plagioclase- and hemo-ilmenite-rich cumulates. Later the magma returned to normal fractional crystallization, leading to evolved compositions, and precipitation of Fe- richer silicates, Ti- richer ilmenite, and magnetite. Generally the dominant oxides in the layers result in negative remanent magnetic anomalies over hemo- ilmenite-rich cumulates and positive induced anomalies over cumulates dominated by magnetite and Ti- rich ilmenite. Magnetic contrasts are striking over the top MCU IV, where a negative remanent anomaly traces for >15 km in the northern Bjerkreim Lobe. Here magnetic intensity in the trough of the ground anomaly varies, depending on position in the south plunging syncline. At the northern hinge, where layering and foliation dip south, the trough is 3000 nT below background where as on the east limb at Heskestad, 15km away, where the distal edge of Unit IV of the layered series abuts against the basement below the intrusion, layering and foliation are vertical, parallel to the reversed Neoproterozoic magnetizing field. Here the anomaly is 12 000 nT below background in helicopter survey, and the ground anomaly locally is 30 000 nT below background. Strong lattice-preferred orientations hemo-ilmenite (001) planes and orthopyroxene c- axes lie quasi-parallel to the Neoproterozoic magnetizing field and could lead to enhanced remanence related to hematite exsolution in ilmenite and to exsolved rods and blades of hemo- ilmenite oriented parallel to c- axes in orthopyroxene. Electron probe element mapping and magnetic scanning microscope map individual grains for NRM and induced magnetization to locate the sources of the remanent magnetization.