3D gravity and magnetic model of the Rogaland Igneous Complex in southwest Norway: a tank for ilmenite, apatite and magnetite resources

The Rogaland Igneous Complex (RIC), in southwest Norway, is well known for its iron-titanium ore deposits (i.e. Storgangen and Tellnes), and potential apatite and vanadium-rich magnetite deposits. A better understanding of the subsurface structure of the complex and surrounding anorthosites will help to locate new mineral deposits, and in estimating the extent of the known mineralized zones. The RIC consists of anorthosites, leuconorites, mangerites, and the Bjerkreim-Sokndal (BK) layered intrusion. These igneous rocks were intruded into granulite facies rocks at 0.93-0.92 Ga, during the late-stage of the Sveconorwegian orogeny.

There is a strong correlation between the geology of the RIC and the magnetic and gravity anomaly patterns, with contrasting signatures between the three large anorthosite bodies (Egersund-Ogna, Haland-Helleren, and Ana-Sira) and the extensive BK layered intrusion.

Particularly, the Bouguer gravity map shows gravity lows over the anorthosites and the granulites, while a positive gravity anomaly ranging from 10 to 30 mGal correlates with the norite and mangerite rocks. In the aeromagnetic anomaly map, the anorthosites correlate with moderate to strong negative magnetic anomalies (below background) while mangerites and granulites have positive anomalies. More complex is the magnetic pattern over the BK layered intrusion. The latter is made up by 6 mega-cyclic units subdivided into a sequence of zones, defined by the presence or absence of certain index minerals which control the magnetic properties of the rocks and the magnetic pattern. This is clearly visible in the striking negative anomaly observed on the east limb of the Bjerkrem Lobe at Heskestad, with amplitude of -13000 nT in a high-resolution helicopter survey, and below -30000 nT in ground magnetic survey.

This area has long been explored, and a large set of geophysical data have been collected during multiple campaigns including gravity, seismic, airborne magnetic and radiometric data. Recently acquired ground magnetic data over the BK layered intrusion complement these data. Here, we used the geophysical data, and an extensive petrophysical dataset of over 1000 samples to investigate the shallow and deep structure of the RIC. A 3D gravity and magnetic model of the study area, built across multiple cross-sections, is presented. The BK layered intrusion is modeled in a doubly-plunging syncline structure and has a preliminary depth extent of approximately 4 km which agrees with previous seismic interpretations indicating the base at 4 to 5 km.