The Nimba iron ore deposit is located in the far north-west of Liberian in West Africa, close to the border junction with Guinea and Cote d'Ivoire. It is 360 km ENE of Monrovia and 270 km NE of the Atlantic port of Buchanan. The ore deposits is located within the north-south trending Nimba Range which extends over the border into Guinea where additional resources have been delineated (see below).
The Nimba deposit is located in the southern part of the West African Craton which is exposed in two regions, i). the Man Shield to the south along the south coast of West Africa in Sierra Leone, Liberia and south-east Guinea, and ii). the Reguibat Shield to the north in Mauritania and neighbouring Morrocco and Algeria.
These two shield areas are composed of: i). Cores of Archaean (3100 to 2500 Ma) crystalline rocks. The Archaean of the Man Shield is a granite-greenstone terrane, which comprises 85% granitoid basement of overall granodiorite composition, but ranging from diorite through tonalite to granite, occurring as granitic gneisses and migmatites, mostly quartz-feldspar-biotite and amphibole bearing, metamorphosed to amphibolite and granulite facies. The greenstones occur as linear infolded, north to NE trending remnants of ultramafic lavas and sills, now serpentinites and chloritic schists, with intercalated amphibolites and altered metasediments; and ii). Eastern blocks of Paleoproterozoic (2000 ±200 Ma) 'Eburnean/Birimian terranes', predominantly composed of intrusive granites in the west and volcanic formations in the east, particularly in Ghana, Cote d'Ivoire and Mali on the Man Shield.
The Man and Reguibat shields are separated by the broad, shallow 'down-sag', Taoudeni Basin infilled by Neoproterozoic to Devonian continental to shallow marine sedimentary strata unconformably overlying the crystalline basement represented by the shields to the north and south. The Taoudeni Basin sequence does not exceed 5000 m in thickness, and though it varies locally, is a very homogenous lithological sequence.
The craton, including the Man and Reguibat shields and the Taoudeni Basin are bounded to the north, east and west by Pan-African (late Neoproterozoic) mobile belts, and to the west by a the superimposed Hercynian mobile belt of the Mauritanides.
The iron deposits of the Man Shield are mainly within the greenstone belts of the Archaean basement in Liberia, Guinea and Cote d'Ivoire.
The Nimba deposits are hosted by the 1400 m thick Nimba Supergroup exposed as ridges bounded by lowlands of the older Yekepa Supergroup of quartzo-feldspathic gneisses and orthoamphibolites. The lower half of the Nimba Supergroup is comprises metamorphosed mafic volcanics of the Seka Valley Schists. The upper half of the supergroup is composed of metasediments subdivided into the:
i). Gbahm Ridge Formation - 250 m of quartz-chlorite-graphite phyllite with accessory muscovite, garnet, biotite, carbonate and tourmaline. The upper 50 m contains unward increasing interbeds of metachert;
ii). Nimba Itabirite - a 250 to 400 m thickness of predominantly magnetite-quartz banded iron formation (BIF) which hosts the main orebody;
iii). Mt Alpha Formation - >100m thickness of phyllite mainly composed of quartz and biotite with subordinate chlorite, muscovite, garnet, amphibole and pyrite.
The sequence in the Nimba area has been folded, in places isoclinally, and metamorphosed to epidote-amphibolite facies at around 2700 Ma.
In the main orebody area, the normally 350 to 400 m of Nimba Itabirite has been reduced to 250 to 350 m. The orebody is composed of blue altered itabirite composed of anhedral hematite and martite with quartz as the only significant impurity. The blue ore is formed by alteration of the protolith grey itabirite of magnetite and quartz BIF. The ore zone is approximately 1500 m long and has been explored toa depth of up to 670 m below the pre-mining surface.
The altered itabirite has been grouped into:
i). Soft siliceous ore with 47 to 66% Fe;
ii). Hard to medium hard siliceous ore with 52 to 66% Fe;
iii). Soft high grade ore with >66% Fe; and
iv). Hard to medium hard high grade ore with >66% Fe.
Medium hard ores consist of compact, but un-cemented anhedral hematite, while and hard ores are composed of cemented anhedral hematite of syn-orogenic origin. The proportion of soft ores increase with depth. Siliceous ores in general have <66% Fe and >2.5% SiO2.
The 25 Km Nimba Range, which straddles the border between Liberia and Guinea, has been divided into four mining areas in Guinea, namely: Château, Grands Rochers, Pierre-Richaud and Sempere.
The main Mount Nimba deposit in Liberia is quoted as having contained >250 Mt @ 50 to >66% Fe, much of which was mined.
Current resources (2005) in the Liberian segment of the ranges are quoted at 800 Mt @ 67% Fe.
Resources on the Guinean segment of the Nimba Ranges include:
Mifergui - 246 @ 60% Fe;
Pierre Richaud - 463 Mt @ 65% Fe.
The most recent source geological information used to prepare this summary was dated 1977.
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 to this deposit in the PGC Literature Service Collection:||
Berge J W, Johansson K and Jack J, 1977 - Geology and origin of the hematite ores of the Nimba Range, Liberia: in Econ. Geol. v72 pp 582-607|
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