Ti deposits in anorthosite : geochemical constraints on the economic value J.C. Duchesne & J. Vander Auwera Laboratoires associés de Géologie, Pétrologie et Géochimie, University of Liège, Sart Tilman, Belgium. Recent experimental data (Fram & Longhi 1992; Longhi et al. 1993; Vander Auwera & Longhi 1994; Vander Auwera et al. 1998; Longhi et al. 1999) indicate that parental magmas of the anorthosite- mangerite-charnockite (AMC) suite probably encompass a large continuum of compositions ranging from high-Al basalts (HLCA, Table 1) to more ferroan and potassic compositions, represented by the primitive jotunites (hypersthene-bearing monzodiorites)(TJ, Table 1). Table 1 Endmember composition of the norite series TJ HLCA Vander Auwera & Longhi, CMP 1994 Fram & Longhi, Am Min 1992 SiO2 49.39 50.02 TiO2 3.67 1.85 Al2O3 15.81 17.51 FeO 13.11 10.97 MgO 4.54 6.67 MnO 0.13 0.15 CaO 6.87 8.78 K2O 0.96 0.44 Na2O 3.50 2.93 P2O5 0.71 0.16 Experimental phase equilibria show that both endmember magmas can account for the norite series which fractionates at 3-5 kb to silica-enriched liquids (Longhi et al. 1999). In Rogaland (Fig. 1), comparison between phases experimentally obtained on TJ primitive jotunite (i.e. plag An47 and orthopyroxene En66, Vander Auwera & Longhi 1994) and natural phases from the anorthosite massifs (i.e. An49 and En74 in the Åna- Sira anorthosite massif and An57 and En75 for the Egersund-Ogna anorthosite massif) and from the Bjerkreim-Sokndal layered intrusion (An52 and En77) suggests that a liquid generally similar to this primitive jotunite was parental to both types of intrusions. Interestingly, Fe-Ti-V-P deposits have been recognized in massive anorthosites and in the Bjerkreim-Sogndal layered intrusion (Duchesne 1999). In massive anorthosites, the ore-bodies occur as (deformed) dykes or pods ranging in composition from pure hemo-ilmenite (Jerneld) to ilmenite norite (Tellnes, Storgangen). Polybaric fractional crystallization and synemplacement deformation in rising anorthosite diapirs lead to relatively Mg- and Cr-rich ilmenite (± V-magnetite) deposits. The high contents in Cr and Mg are deleterious for the new chlorination process that tends to substitute to the classical sulfatation process used in the TiO2 pigment industry. On the other hand fractional crystallization of jotunite magmas in layered magma chambers, such as the Bjerkreim-Sokndal inytrusion, gives rise to voluminous “ disseminated ” mineralizations, containing low Mg and Cr ilmenite + Ti-magnetite ± REE-rich apatite, more adequate to the chlorination process but still of sub-economic value. Immiscibility is not the controlling mechanism, except maybe in some rare nelsonites (Hesnes). Subsolidus re-equilibration leads to a thorough change in the oxide mineral composition towards an