Tonkolili - Kasafoni, Numbara, Marampon, Simbili

Sierra Leone

Main commodities: Fe
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The Tonkolili iron deposits are located within the north-east trending Sula Mountains of northern Sierra Leone, west Africa, within the Man Shield of the West African Craton.

The deposits lie within a district in which Archaean (Leonian) granitic basement is unconformably overlain by the Kambui Supergroup which has been regionally metamorphosed to greenschist facies. The Kambui Supergroup includes the Sula Group which is composed of a lower volcanic group, the Sonfon formation, which is overlain by the sedimentary Tonkolili formation. The Sonfon formation comprises basaltic pillow lavas and ultramafic rocks, while the Tonkolili formation consists of conglomerates, psammites, pelites and tuffs. These formations were metamorphosed during the Liberian orogenic episode (~2.7 Ga), to produce amphibolite/hornblende schists, quartz mica schists and BIFs.

The Tonkolili deposits are dominated by primary magnetite mineralisation in the form of a fresh metamorphosed BIF featuring a uniform mineral assemblage of quartz-magnetite ± ferro-silicate minerals, that constitutes a resource of 11.6 Gt averaging 30.1% Fe.

Two well-developed weathering zones exist within the deposits, and are associated with the progressive development of a tropical weathering profile overprinting the primary BIF. These zones feature a near surface hard indurated duricrust up to 60 m thick, and a soft progressively developed saprolite between duricrust and fresh BIF. These zones are amenable to producing iron ores including DSO lump and sinter fines from the duricrust, and concentrate from the saprolite.

Supergene enrichment of the BIFs has produced an indurated duricrust composed of cemented relict iron oxides and detrital material. Within the hardcap there is a depletion of primary silica by dissolution, and enrichment of iron oxides by chemical reprecipitation and physical accumulation. The dominant mineralogy is hematite and goethite, with accessory martite and limonite.

The duricrust is underlain by a saprolite zone, which is subdivided into upper and lower intervals, based on textural and chemical properties. The upper saprolite is chemically oxidised with iron present as Fe3+ and an absence of primary textures, while the lower saprolite is less weathered and retains primary textural fabrics and a similar mineral assemblage to the underlying fresh BIF. An oxidation front forms the division between the upper and lower saprolite, with the latter being characterised by the predominance of Fe2+. The mineral assemblages of the saprolitic ores are characterised as being goethite-hematite and maghemite-magnetite with accessory clay minerals and progressively weathered silica. The lower saprolite, hasa predominance of strongly magnetic iron minerals.

The primary BIF is characterised by finely alternating bands of subparallel bedded, crystalline magnetite and silica. The BIF may be subdivided into two units based on age, grade and mineralogy. The upper formation is noted as a quartz-magnetite BIF with a magnetite content of 30 to 40%, while the lower formation is a quartz-magnetite-silicate BIF with a lower of 20 to 30% magnetite content. The ferromagnesian minerals are discretely interbedded with quartz-magnetite layers, and are interpreted to be tuffs that have been metamorphosed to an amphibole-pyroxene-garnet assemblage.

The total Tonkolili resource is divided between the Kasafoni, Numbara, Marampon and Simbili deposits.

Resource (measured+indicated+inferred) estimates are as follows (Reston et al., 2011):
    Hematite cap >56.5% Fe - 126.5 Mt @ 58.10% Fe, 2.43% SiO2, 6.0% Al2O3, 0.09% P;
    Hematite cap and sparoilite, and magnetite saprolite 26.5 to 56.5% Fe - 1124.4 Mt @ 40.0% Fe, 21.4% SiO
2, 11.96% Al2O3, 0.09% P;
    Magnetite BIF >20.0% Fe - 1160 Mt @ 30.1% Fe, 45.0% SiO
2, 10.5% Al2O3, 0.06% P.

The most recent source geological information used to prepare this summary was dated: 2011.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Reston, M.S., Baker, H.T., Elvish, R.D., Reardon C.A. and Young, B.J.W.,  2011 - The Tonkolili Iron Ore Deposits, Sierra Leone: in   Proceedings, Iron Ore 2011 Conference, 11-13 July 2011, Perth, Western Australia, The Australasian Institute of Mining and Metallurgy, Melbourne,    pp. 133-145

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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