Magma chamber processes in the Tellnes ilmenite deposit (Rogaland Anorthosite Province, SW Norway) and the formation of Fe–Ti ores in massif-type anorthosites
Introduction
Economic igneous Fe–Ti oxide occurrences are commonly associated with massif-type anorthosites and two hard-rock deposits are currently being mined: the Tellnes ilmenite deposit (Rogaland Anorthosite Province, SW Norway) and the Lac Tio deposit (Allard Lake anorthosite, Quebec, Canada). The scarcity of Fe–Ti deposits has hampered research on the formation of such ore bodies and particularly on mechanisms responsible for ilmenite enrichment. There is no consensus as to whether accumulation processes, immiscibility, magma mixing, fractional crystallization, solid-state remobilisation, cotectic crystallization, or a combination of these processes or others, control the formation of Fe–Ti oxide ores. Nevertheless, the genesis of massif-type anorthosites and related rocks has been thoroughly debated (e.g. Ashwal, 1993, Longhi et al., 1999, Vander Auwera et al., 2000, Bédard, 2001, Longhi, 2005) and the Rogaland Anorthosite Province in particular has been the subject of numerous investigations (see Duchesne, 2001 for a review). In this province, the Bjerkreim-Sokndal layered intrusion, which contains all rock types belonging to the AMC series (Anorthosite–Mangerite–Charnockite), has been extensively studied (see Wilson et al., 1996 for a review). The petrogenesis of the Bjerkreim-Sokndal Intrusion has greatly improved our understanding of the origin of massif-type anorthosites and related rocks. An important feature of the AMC series in Rogaland is the inferred jotunitic (hypersthene monzodiorite) parental magma. These liquids, also called ferrodiorites (Ashwal, 1993, Emslie et al., 1994), are Fe-, Ti- and P-rich and, contrary to basaltic magmas, crystallize ilmenite as an early liquidus mineral. The liquid line of descent of jotunitic magma is well known due to the occurrence of fine-grained jotunitic dykes that represent liquid compositions (Duchesne et al., 1989) and as a result of experimental studies (Vander Auwera et al., 1998) performed on such compositions.
In this well-documented geological context, a comprehensive genetic model for the Tellnes ilmenite deposit remains to be established. This deposit, which has been mined since 1960 by TITANIA A/S, was previously described as a rather homogeneous ore body with plagioclase, ilmenite, and orthopyroxene as the major minerals (Gierth and Krause, 1973). However, an extensive database of whole-rock compositions built up by TITANIA A/S over the last twenty years indicates that compositional variations are larger than previously believed. A detailed petrographical and geochemical investigation of more than 100 samples from the deposit has thus been undertaken in order to constrain a genetic model for the ore body and Fe–Ti ore-forming processes in massif-type anorthosites in general.
Section snippets
The Rogaland Anorthosite Province
The Tellnes ilmenite deposit belongs to the late-Proterozoic Rogaland Anorthosite Province (see Duchesne, 2001 for a review), emplaced in the south-western part of the Sveconorwegian orogenic belt (SW Norway) (Fig. 1). The Province comprises different units of the AMC series: four large massif-type anorthosite plutons (Egersund-Ogna, Håland, Helleren and Åna-Sira), the Bjerkreim-Sokndal layered intrusion (Wilson et al., 1996), two smaller leuconoritic bodies (Hidra and Garsaknatt) (Demaiffe,
Previous studies
Detailed petrographical descriptions of the Tellnes ilmenite deposit have been presented by Gierth (1970), Gierth and Krause, 1973, Gierth and Krause, 1974, Gierth et al. (1982) and Gierth (1983). Although these authors describe the mineralogical and chemical composition of the ore body as very uniform, they noticed some variability. Most important are the lower ilmenite and the higher plagioclase and apatite contents at the margins of the ore body. The upper part contains increasing amounts of
Samples and analytical methods
More than 100 samples were selected from various drill cores in three SSW–NNE sections (800, 1200 and 1600), more or less perpendicular to the elongation of the ore body (Fig. 4). Each sample corresponds to 40 cm to 1 m of a split drill core. Altered portions of rocks have been avoided during sampling. A few rocks were also collected in the open-pit. Petrographic studies were carried out on polished thin sections for all rocks.
Forty seven samples were selected from sections 800, 1200 and 1600
Mineralogy and textural relationships
The Tellnes ore is an equigranular, massive, medium-grained (∼ 0.5–2 mm) ilmenite-rich norite. The norite rarely displays modal layering and appears extremely homogeneous in outcrop. Plagioclase, ilmenite and orthopyroxene are the main minerals. Olivine, magnetite, clinopyroxene, biotite, apatite and hornblende are subordinate. Traces of Fe–Ni–Co–Cu sulphides are ubiquitous. Other accessory phases include aluminous spinel, baddeleyite and zircon.
Plagioclase is the most abundant mineral in the
Plagioclase compositions
Plagioclase compositions are widely used to characterize stratigraphic variations (cryptic layering) in cumulates in layered intrusions in order to identify magma chamber processes such as fractional crystallization, magma recharge, assimilation or trapped liquid crystallization. Even though plagioclase is known to retain its primary composition as a result of slow NaSi–CaAl exchange (Grove et al., 1981), reequilibration and diffusion could nevertheless be significant in deep-seated plutonic
Sequence of crystallization: comparison with the Bjerkreim-Sokndal layered intrusion
The Bjerkreim-Sokndal layered intrusion crops out in the central part of the Rogaland Anorthosite Province (Michot, 1965, Duchesne, 1987, Wilson et al., 1996). The lower part of this intrusion consists of a 7000-m thick series of cumulates (the “Layered Series”) and the upper part comprises acidic rocks, many of which are interpreted to represent liquid compositions (Duchesne and Wilmart, 1997). All units of the AMC series are developed: anorthosite, troctolite, leuconorite, norite,
Conclusions
The Tellnes ilmenite deposit is not an homogeneous ore body. Study of three N–S cross-sections reveals systematic variations in the cumulus assemblage. In the lower part (LCZ), plagioclase and ilmenite are the only cumulus minerals, followed by the appearance of orthopyroxene and olivine in the upper part (UCZ). Marginal Zones (UMZ and LMZ) are characterized by high plagioclase and apatite contents and lower proportions of ilmenite. Phase diagrams for magmas related to massif-type anorthosites
Acknowledgements
This work was funded by the Belgian Fund for Joint Research and the Fund for Research in Industry and Agriculture. Fieldwork was partly supported by the Fourmarier Foundation. TITANIA A/S is gratefully acknowledged for providing access to chemical data on the Tellnes deposit and for supporting the cost of new analyses. Kari Berge and Ragnar Hagen are thanked for their continuous interest and encouragement. We also acknowledge Kåre Kullerud for sharing unpublished microprobe data as well as
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