Geophysical interpretation and rock-magnetic 
properties of ilmenite ores and host rocks

Dr. Suzanne McEnroe, NGU, Norway

Aeromagnetic surveys have been widely used in mineral exploration for many decades. In interpreting the magnetic response of mineral deposits, a fundamental understanding of magnetic rock properties is essential (Clark, 1997, 1999). A detailed high-resolution aeromagnetic map in the Sokndal region (Rønning, 1995) over the ilmenite norites and surrounding rocks shows induced and remanent anomalies. To aid interpretation, a research program on the rock-magnetic properties of anorthosite-norite-mangerite samples is underway (McEnroe, 1997; McEnroe et al., 1996, 1998; in press). We found that the magnetic response of the ores cannot be directly correlated with oxide abundance. In hemo-ilmenite ores submicron scale magnetic interactions may be important. Research to improve our understanding of ilmenite ores includes low temperature Mossbauer, TEM, and rock-magnetic measurements. We plan to sample the Bjerkreim-Sokndal intrusion, where a dramatic change in aeromagnetic signature occurs, due to abundance and exsolution state of the oxides. We will investigate the very different aeromagnetic signatures of the three anorthosite massifs that host the oxide intrusions. Distinct aeromagnetic signatures from the northern anorthosite may indicate unknown ore deposits or large blocks of economically valuable leuconorite. The strong negative magnetic anomaly over the Åna Sira anorthosite may be due to fine inclusions of hemo-ilmenite and ultra-fine needles of magnetite in the plagioclase. This work is complemented by research on another part of the Proterozoic Grenville plate, in the Adirondack Mountains, USA, which hosts classic Proterozoic magnetite-ilmenite deposits. The results of this work (McEnroe and Brown,2000) are directly applicable to the Rogaland province.