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The Sulitjelma Area includes more than 20 deposits with a total tonnage in excess of 35 Mt (Cook et al. 1990). The average grades of the 25 Mt mined from 11 deposits between 1887 and 1991 were 1.84 % Cu and 0.86 % Zn. Contents of precious metals were 10 g/t Ag and 0.25 g/t Au (Cook et al. 1990). The deposits are concentrated in an area of 25 km2, and are located at varying stratigraphic-structural levels in the Otervann Volcanic Formation (Cook et al., 1990), also known as the Sulitjelma amphibolites, in the Upper Allochthon Køli Nappe Complex.
The palaeotectonic setting of the volcanic rocks is disputed. According to Boyle (1989), the volcanic rocks form the extrusive part of the Sulitjelma ophiolite complex, underlain by sheeted dikes of the Mietjerpakte Intrusive Complex and below that the Sulitjelma Gabbro Complex. Inclusions of Precambrian gneiss in the Gabbro have been interpreted to show that the ophiolite complex represents a fragment of an ensialic marginal basin (Cook et al. 1990). The gabbro has been dated to 437 ± 2 Ma (Pedersen et al. 1991), and because of the cogenetic relationships between the young gabbro and the volcanic rocks, it has been argued that the term ophiolite complex is inappropriate (Grenne et al. 1999). Structurally below the volcanic rocks, there is a thick sedimentary sequence of schists, the Furulund Group, which contains fossils of Upper Ordovician to Lower Silurian age (Spjeldnæs 1985). The whole rock package has been folded isoclinally, leading to a large-scale repetition of sedimentary and volcanic units. This also means that the ore units are repeated (Cook et al. 1990).
The volcanic pile is divided into several units, and the sulphide deposits are found at different levels within these units. The deposits are further divided into a Southern and a Northern Ore Field. The Jakobsbakken, Sagmo and Anna deposits are in the southern part of the ore field. The Jakobsbakken deposit is the most zinc-rich of the deposits, containing 4.5 Mt of ore with 1.55 % Cu and 2.42 % Zn. It also is at a structurally lower position than the other two main deposits in the Southern Ore Field, closer to the underlying Furulund Schist.
Most of the deposits are in the Northern Ore Field. The Ny-Sulitjelma and Bursi deposits are the structurally the lowest in this ore field, and are situated between overlying metabasalts of the so-called Giken Amphibolite Unit and the Furulund Schist (Cook et al. 1990). Structurally higher up in the Giken Amphibolite Unit is the Hankabakken–Palmberg ore zone. The largest of the deposits in the Sulitjelma field, Giken, is situated structurally above the Giken Amphibolite Unit and below a schist unit (the Leirelva Schist) equivalent to the Furulund Schist. The Giken deposit contained 10.5 Mt of ore, of which 5.8 Mt was mined, containing 2.25 % Cu and 0.7 % Zn. Further up structurally is the Charlotta deposit, which is located at the contact between the Leirelva Schist and the Lower Amphibolite Unit. It is possible that this is the same level as the Sagmo deposit in the Southern Ore Field. The Mons Petter deposit is the structurally highest ore zone in the Northern Ore Field, and is situated between the Lower and Middle Amphibolite Unit. The westernmost, small Furuhaugen deposit (0.52 Mt) has an uncertain structural position, but is possibly the highest deposit in the stratigraphy.
Polyphase deformation with a high degree of folding and amphibolite-grade metamorphism has obliterated most primary textures and mineralogical zonation patterns in the Sulitjelma Ore Field. However, several of the deposits are underlain by alteration zones, and display zonation in base metals in accordance with classical VMS type deposits (Cook et al. 1990).
References
Boyle, A.P. 1989: The geochemistry of the Sulitjelma ophiolite and associated basic volcanics: Tectonic implications. in Gayer, R.A. (ed.): The Caledonide geology of Scandinavia, London, Graham and Trottman, p. 153–163.
Cook, N.J., Halls, C. & Kaspersen, P. 1990: The geology of the Sulitjelma ore field, Northern Norway – some new interpretations. Economic Geology, vol. 85, 1720–1737.
Grenne, T., Ihlen, P. M. & Vokes, F. M. 1999: Scandinavian Caledonide metallogeny in a plate tectonic perspective. Mineralium Deposita, v. 34, p. 422–471.
Pedersen, R.B., Furnes, H. & Dunning, G. 1991: A U/Pb age for the Sulitjelma Gabbro, north Norway: Further evidence for the development of a Caledonian marginal basin in Ashgill–Llandovery time. Geological Magazine, vol. 128, p. 141–153.
Spjeldnæs, N. 1985: Biostratigraphy of the Scandinavian Caledonides. In Gee, D.G. and Sturt, B.A. (eds.): The Caledonide orogen – Scandinavia and related areas. Wiley, Chichester, p.317–329. |