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Western Cluster - Mano River, Bea Mountain, Bomi Hills

Liberia

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The Western Cluster iron ore deposits of Mano River, Bea Mountain and Bomi Hills are distributed along the old railway from Mano River in the north to Monrovia on the coast, in western Liberia. The Mano River deposit is located immediately SE of the river of the same name, which forms the border between Liberia and Sierra Leone and is ~120 km NNW of Monrovia (#Location: 6° 49' 05"N, 10° 07' 34"W), while Bea Mountain is 20 km to the SSE. Bomi Hills is a further 30 km to the SSE (#Location: 6° 53' 53"N, 10° 49' 27"W), and 70 km north of Monrovia.

The overall structural grain of Liberia is dominated by a series of NE-SW trending Palaeo- to Mesoarchaean granite-greenstone terranes of the Archaean Man Shield, part of the West African craton. These terranes are truncated to the SW by the regional, NW-SE striking, composite, Todi Shear Zone, which separates them from a NW-SE trending, 20 to 40 km wide strip of high-temperature Pan African metamorphic rocks that occupy, and parallel the immediate NW-SE Atlantic coastline.

The granite-greenstone terranes are divided into two deformational provinces, with a diffuse boundary centred on the major NE-SW trending Cestos Shear Zone that stretches right across Liberia near its centre. The "Liberian-age province" to the NW, is characterised by both metamorphism and intrusion of plutonic rocks between 3.20 and 2.70 Ga. The rocks to the SE belong to the "Eburnean age province", metamorphosed at ~2.15 Ga. Rocks of the "Pan-African province" along the coastline, were metamorphosed and intruded by granitoids at ~500 Ma, although most of these reworked basement rocks are believed to have originally been part of the Liberian province. The Pan-African province rocks were uplifted relative to the Liberian province following the 500 Ma thermal event. During the Jurassic, a swarm of NW-SE trending dolerite dykes were intruded throughout all three provinces. By 120 Ma it is suggested that the Pan-African and Liberian provinces, on either side of the Todi shear zone, were in juxtaposition. The Todi shear zone is defined by a series of faults and wide zones of mylonite, steeply dipping to the SW below the Pan-African belt.

The granite-greenstone terranes are composed of felsic, mafic and composite gneisses, amphibolites, granites and ultramafic rocks, with zones of migmatite and pegmatite, and a series of NE-SW, steeply dipping shear zones which have major vertical and possibly lateral displacement. These shears are generally marked by broad zones of mylonite and differing rock types. Large diorite masses are found adjacent to the Cestos Shear Zone. Prominent, 10 to 100 km long ridges of iron formation parallel the grain of the granite-greenstone terranes and occur in all three provinces, and in mafic, felsic and composite gneiss belts. However, the iron formation is more prevalent in the Liberian-age province. The Western Cluster deposits are in the northwestern half of the "Eburnean age province".

The Mano River deposit is developed on the seven high hills adjacent to the Mano River, which are form a string of peaks along the ridge of underlying, resistive banded magnetite iron formation. The iron formation has been deformed into a tight, axial-plane faulted, 'S-shaped', synform-antiform pair. The mineralised iron formation has a width of up to 1 km, and occupies ~16 km of the ~30 km in the district. The mapped iron formation is compressed within an area of ~5 x 12 km and strikes ~NE-SW, with tight closures. The enclosing country rocks are predominantly felsic gneisses. The orebodies are composed of magnetite and goethite and contain up to 56% Fe, derived from the desilicification and lateritisation of the magnetite iron formation (Abdulla, UN, 1970). This was the former National Iron Ore Company (NIOC) mine that was worked until 1990.

The Bea River deposit corresponds to a pronounced ridge of banded magnetite iron formation that extends over an east-west trending interval of ~22 km, within granite gneiss country rock, several km north of the contact with a broad band of mafic gneisses. This resource had not been mined in 2012.

The Bomi Hills deposit consist of a chain of magnetite banded iron formation ridges, with a composite strike length of ~18 km. The Mineralisation comprises massive, coarse-grained magnetite containing ~20% hematite, and averaging 62 to 66% FeTotal. The amount of hematite decreases with depth and the gain-size increases. This was the former Liberia Mining Company (LMC) mine that was worked until 1990.

Reserves and resources are quoted as follows:
  Mano River
      80 Mt of ore at unspecified grade (Vedanta Annual Report, 2012)
      46 Mt @ 56% Fe (Abdulla, UN, 1970)
  Bea Mountain
      923 Mt of ore at unspecified grade (Vedanta Annual Report, 2012)
  Bomi Hills
      50 Mt of ore at unspecified grade (Vedanta Annual Report, 2012)
      50 Mt @ 62 to 66% Fe (Abdulla, UN, 1970).

Sesa Goa estimates that the remaining resource of just over 1 Gt of ore will yield ~330 Mt of saleable product (concentrate/pellets; Vedanta-Sesa Goa website, 2013). The same report indicates that drilling has increased the total resource to ~3 Gt at an unspecified grade.

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.


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