The Tellnes ilmenite deposit (Rogaland, South Norway): magnetic and petrofabric evidence for emplacement of a Ti-enriched noritic crystal mush in a fracture zone Herve ´ Diot a, * , Olivier Bolle b,1 , Jean-Marc Lambert b , Patrick Launeau c , Jean-Clair Duchesne b a Universite ´ de La Rochelle, Av. M. Cre ´peau, F-17042 La Rochelle Cedex 01, France b Universite ´ de Lie `ge, Bd. du Rectorat, Ba ˆt. B20, B-4000 Sart Tilman, Belgium c Universite ´ de Nantes, 2 rue de la Houssinie `re, BP 92208, F-44322 Nantes, France Received 7 August 2001; accepted 21 December 2001 Abstract The Tellnes ilmenite deposit, a world class titanium deposit, occurs in the A ˚ na – Sira anorthosite (Rogaland anorthosite province, South Norway). It is mainly made up of an ilmenite-rich norite that has been previously interpreted as injected in a crystal mush state, in a weakness zone of the enclosing anorthosite. This emplacement mechanism has produced a faint orientation in the ore due to the flow of the mush. The internal flow structure of the orebody is studied here using the low-field anisotropy of magnetic susceptibility (AMS) method. Partial anhysteretic remanent magnetization (pAARM) indicates that coarse magnetite is the main mineral responsible for the magnetic fabric. Parallelism of the magnetic fabric with the shape-preferred orientation of the ore-forming minerals is checked using image analysis (IA) from oriented sections (intercept method). Interpretation of the AMS data verified by pAARM and IA, provides information on the magmatic foliation and lineation. Emplacement flow of the ilmenite norite crystal mush occurred in the direction of the orebody, parallel to its walls, and with an average SE 188 plunge. The feeder zone was likely situated below a network of veins on the SE end of the orebody. The sickle- shaped outcrop of the deposit suggests a transcurrent, dextral opening of a WNW – ESE-striking weakness zone across the anorthosite pluton. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Titanium deposit; Petrofabric; Anisotropy of magnetic susceptibility (AMS); Rogaland, Norway 1. Introduction The Tellnes ilmenite deposit (TID) (Sokndal district, Rogaland, South Norway) is the largest Ti–Fe orebody in Europe, and the second most important ilmenite orebody in production in the world today, just after Lake Tio deposit (Allard Lake district, Quebec). In 1999, its yearly production was 550,000 tons of ilmenite concentrate, averaging 44.5% TiO 2 , and represented 5% of the world’s TiO 2 production (Titania A/S, pers. comm. 1999). The TID is a hard-rock orebody of igneous origin, which consists of an ilmenite norite lens emplaced in the Rogaland anortho- site province (e.g. Duchesne et al., 1985), an AMC (anorthosite – mangerite – charnockite) 2 igneous complex of Late-Proterozoic age, characterized by huge anorthosite bodies (Krause and Pedall, 1980; Krause et al., 1985; Duchesne, 1999). Ilmenite and magnetite magmatic depos- its of subeconomic to economic grade are widespread in this area but Tellnes is today the largest and richest deposit (Force, 1991). The Rogaland anorthosite province is part of the Sveconorwegian orogenic province, correlated with the Greenville province in North America. In sharp contrast to many North American occurrences, however, the Rogaland anorthosites and associated Ti–Fe deposits were emplaced during a post-collisional magmatic activity period (Scha ¨rer et al., 1996; Duchesne et al., 1999). The Rogaland Ti–Fe deposits thus escaped regional tectonic reworking and metamorphism, even if most of them crystallized or recrystallized during syn-plutonic deformation of the diapirically emplaced anorthosites (Duchesne et al., 1985; 0191-8141/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(02)00050-0 Journal of Structural Geology 25 (2003) 481–501 www.elsevier.com/locate/jstrugeo 1 Post-doctoral Researcher from the Belgian F.N.R.S. (National Fund for Scientific Research, Belgium). 2 Nomenclature of the AMC rocks is (Streckeisen, 1974): anorthosite ¼ rock with plagioclase content . 90%; (leuco)(mela) norite ¼ hypersthene (leuco)(mela)gabbro; jotunite ¼ hypersthene monzogabbro; (quartz) mangerite ¼ hypersthene (quartz) monzonite; charnockite ¼ hypersthene granite. * Corresponding author. Tel.: þ 33-5-46-45-86-18; fax: þ33-5-46-45-86- 18. E-mail address: hdiot@univ-lr.fr (H. Diot).