The TELLNES ilmenite deposit 
GEOLOGY

   See the overview geological map of the Rogaland anorthosite province and miscellaneous ore photographs from Tellnes.

  The Tellnes deposit has been extensively studied by two groups: Krause’s group from Clausthal and Duchesne's group from Liége. See Krause et al. (1985) and Duchesne (1999), respectively.

  The Tellnes deposit is an ilmenite-rich norite intrusion into the Åna-Sira anorthosite, as evidenced by sharp contacts, numerous apophyses cutting the surrounding anorthosite, intrusive breccias, and xenoliths of anorthosites.

  It has a sickle-shaped outcrop more than 2700 m long and is more than 400m wide in its central part (geological map). The north-western contact dips 50-45^o SW and the opposite contact varies from almost vertical to 45^o dip. Two faults dipping westward and two subvertical basalt dikes, striking E-W, intersect the outcrop (Krause et al. 1985). The vertical displacement of the orebody, caused by the faults, is in the order of 80-100m.

  The orebody has a complicated relationship to a jotunitic to mangeritic dyke system.

  U-Pb ages on zircon and baddeleyite from the ilmenite orebody are 920 ± 3 Ma, thus slightly younger than the jotunitic dyke (931 ± 5 Ma) (Schärer et al. 1996)

  Isotopically, the ilmenite norite can be derived from the crystallisation of a noritic liquid related to the crystallisation of the surrounding Åna-Sira anorthosite (Wilmart et al. 1989). Liquids of appropriate composition are, however, not known elsewhere in the massif, thus the exact nature of the parent magma of the Tellnes deposit is ill-defined (Duchesne 1999).

  The Tellnes deposit is homogeneous in a large scale; euhedral plagioclase (An45-42), only locally slightly bent and granulated, and euhedral bronzite (En77-75) with subordinate olivin (Fo80) are enclosed by an interstitial hemo-ilmenite (Hem13) and Fe-Ni-Cu-Co sulfides. 

  The average modal composition (vol. %) of the ilmenite norite is: plagioclase 53.2%, ilmenite (28.6%), orthopyroxene (10.2%), biotite (3.9%), magnetite (0.7-2.5%), accessories (3.4%) (Gierth 1970). The accessories comprise at least 24 minerals including apatite.

 Among the accessory opaque minerals. baddeleyite is ubiquitous (Gierth & Krause 1974). The sulfides include several subsolidus transformations of a primary Fe-Ni-(Co)-Cu association. Bravoite, chalcopyrite, covelite, marcasite, millerite, pentlandite, pyrite, pyrrhotite, siegenite and violaritehave been described (Gierth, Krause & Schott 1982).

 Two types of symplectitic intergrowths are present in the Tellnes ore. Symplectitic intergrowths of bronzite and magnetite (with some ilmenite) are considered to be due to the breakdown of primary olivine. Such textures are found almost exclusively along the margins of the orebody. Symplectitic intergrowths of oxides, sulfides and bronzite, replacing olivine, occur in the sulfide-rich upper part of the orebody (Gierth 1970, Gierth & Krause 1973).

 The concentration of Cr and V in the ilmenite are closely linked with the exsolved hematite; this was first pointed out by Gierth & Krause (1974).