Chemical characterisation of ilmenite, magnetite and apatite in the Bjerkreim-Sokndal Layered Intrusion, Rogaland, South Norway

Abstract

During 2001 cooperation was established between NGU and Norsk Hydro Agri in order to locate combined resources of apatite, ilmenite and vanadium magnetite in the Bjerkreim-Sokndal layered Intrusion in the Rogaland Anorthosite Province of aouth Norway. It had previously been recognized that a potential existed for finding coexisting apatite, low-Mg= ilmenite and vanadium-rich magnetite in the northern part (the Bjerkreim-lobe) of the layered intrusion. It had also become apparent that none of the minerals are present in amounts that can support exploitation of one mineral alone. Fieldwork was undertaken during July 2001 with the purpose of locating and constraining the most promising areas in terms of volume and grade of the three target minerals in the Bjerkreim-lobe. Three apatite-bearing and oxide-rich zones were singled out, sampled and preliminarily constrained. These zones are located with one area in mega-cyclic unit (MCU) IBe (Åsen-Bjerkreim), containing up to 41% of the three minerals, two areas in MCU IIIe (Helland and Terland respectively), with up to 34% of the three minerals, and in MCU IVe (Teksevatnet-Vasshus), with 32% ilmenite, magnetite and apatite. The fieldwork and subsequent whole-rock chemical analyses (XRF) were described and discussed by Schiellerup et al. (2001). In the apatite. and oxide-rich part of MCU IBe the MgO-content of ilmenite vary between 1.36 and 2.26% with an average of 1.8%. Stratigraphically dependant variation has not been resolved and the highest MgO contents in ilmenite tend to occur in the most mafic layers. In the two MCU III prospects at Helleland and Terland the MgO-content in ilmenite averages 2.2 and 2.5% respectively, whereas the Teksevatnet profile yield an average of 1.4% MgO in ilmenite. In the Teksevatnet profile the modal contrast between mafic and felsic layers is exceptional and the MgO-content of ilmenite is consi