Nimba Range - Mount Nimba, Chateau, Grands Rochers, Pierre-Richaud, Sempere
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The Nimba Range iron ore deposit are distributed over a NE-SW aligned area of 40 x 10 km, straddling the border between central north of Liberia and far south-western Guinea in West Africa, close to the border junction between Liberia, Guinea and Cote d'Ivoire. It is 360 km ENE of Monrovia and 270 km NE of the Atlantic port of Buchanan (#Location: 7° 32' 24"N, 8° 29' 53"W).
NOTE: This same description is shared by the separates records for the Nimba Range deposits in Liberia and in Guinea.
The Nimba Mountains form a fractured chain that rise from the surrounding lowlands that are at about 400 masl in Liberia in the southwest. The main ridge reaches its highest point of 1752 m in the centre, shared between Guinea and Cote d’Ivoire. Much of the central and northern portions of the ridgeline exceed 1500 m altitude, rising 1000 to 1250 m above the surrounding plains that have risen to ~500m asl in Guinea. The mountains form a prominent topographical feature with steep and varied terrain. Because of their wide biodiversity and their representation of on-going ecological and biological processes in the evolution of ecosystems and species, the Guinean and Ivoirian Nimba Mountains Strict Nature Reserves (SNRs) were included into the Mount Nimba Natural World Heritage Site. In Guinea, the Guinean SNR is also a core area of the Nimba Mountains Biosphere Reserve. The Nimba mining concession is adjacent to and has been excised from the SNR as a 15.2 km2 Mining Enclave (The Société des Mines de Fer de Guinée [SMFG] website, viewed 2022).
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 Meso to Neoarchaean (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:
• 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;
• Nimba Itabirite - a 250 to 400 m thickness of predominantly magnetite-quartz banded iron formation (BIF) which hosts the main orebody;
• 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 Range has been folded, in places isoclinally, and metamorphosed to epidote-amphibolite facies at around 2700 Ma.
In the main Nimba orebody in Liberia 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 to a depth of up to 670 m below the pre-mining surface. NOTE: 'Banded iron formation' (BIF) and 'Itabirite' are used interchangeably in this summary.
The altered itabirite has been grouped into:
• Soft siliceous ore with 47 to 66% Fe;
• Hard to medium hard siliceous ore with 52 to 66% Fe;
• Soft high grade ore with >66% Fe; and
• Hard to medium hard high grade ore with >66% Fe.
Medium hard ores consist of compact, but un-cemented anhedral hematite, while 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 Nimba deposit also includes detrital iron mineralisation known as 'canga', 'deposited in palaeochannels adjacent to the Mount Nimba mountain range. The range is composed of itabirite, quartzite and other schists emplaced onto a terrane of tonalitic granite-gneiss, migmatite and sedimentary gneisses. It stretches over three plateaus with a total combined aerial extent of ~35 km2 (mining-technology.com, 2014).
The original Mount Nimba deposit in Liberia was quoted as having contained >250 Mt @ 50 to >66% Fe, or >150 Mt @ 66 to 68% Fe, much of which has been mined (Berg et al., 1977). Prior to mining the deposit was 250 to 300 m thick, 800 m long and had been traced to a depth of 670 m. Following the discovery of the deposit in Liberia in 1955, the Liberian-American-Swedish Mining Company (LAMCO) was formed to exploit the resource and development commenced, including ~360 km standard gauge railway to the Atlantic port of Buchanan. Production commenced in 1963 and peaked at 12 Mtpa in 1974, subsequently declining due to market conditions. Production started at Mount Tokadeh in 1985 to extend the life of the Nimba mine but operations ceased in 1992 with the onset of civil war.
The ArcelorMittal iron ore deposit at Yekepa, also known as the Nimba No. 1 Mine and referred to above as Mount Tokadeh, is also located in Liberia, <5 km SW of the Guinea-Liberia-Core d'Voire border intersection and 5 km from the original Nimba mine in Liberia. Following the civil war and subsequent changes in control, ArcelorMittal commenced mining of the Mount Tokadeh deposit at Yekepa, in 2011. Mineralisation occurs as a siliceous goethite/hematite/magnetite ore, containing ~60% Fe, 10% SiO2, 2% Al2O3 and 0.1% P. It is derived from a banded iron formation on Mount Gangra in the Yekepa Range. The ore is transported to Mount Tokadeh, ~20 km to the SW, where it is lightly refined by crushing and screening into direct shipping iron ore (DSO) for transport 243 km by rail to the port of Buchanan on the Liberian Atlantic coast for export. This deposit had remaining reserves of 501 Mt @ 48.3% Fe in 2014 (ArcelorMittal, 2015). Remaining reserves and resources at the ArcelorMittal Mt. Tokadeh, Mt. Gangra and Mt. Yuelliton deposits as at 31 December, 2020 were: Proved + Probable Reserves - 475 Mt @ 47.7% Fe; Measured + Indicated Resources - 45 Mt @ 43.6% Fe; Inferred Resources - 2211 Mt @ 38.8% Fe (ArcelorMittal, Annual Report, 2020; where Resources are exclusive of Reserves).
The Société des Mines de Fer de Guinée (SMFG)-HPX-Ivanhoe Mines Nimba Range Project is located towards the northern end of the Nimba Ranges in Guinea ~15 km NE of the Guinea-Liberia-Core d'Voire border intersection. The first exploration in this part of the Nimba Ranges in Guinea was commenced in 1957. This was followed up in two phases of drilling and trenching. The first stage, assisted by the UNDP in 1969-72, was followed by the establishment of a mining company, MIFERGUI, to exploit the deposit which was divided into into four mining areas, namely: Château, Grands Rochers, Pierre-Richaud and Sempere. MIFERGUI estimated a resource of 246 @ 60% Fe, based on this first stage (SMFG, 2020). The second stage of testing was conducted by the American company Kaiser who performed a feasibility study on the project.
Exploration of the Nimba deposit in Guinea concentrated on the Pierre Richaud mining area which had an estimated resource of 463 Mt @ 65% Fe. This appears to be the main deposit as known in 1990 and presumably includes the earlier MIFERGUI resource. It lies within the core of a syncline with lower grade 50% Fe banded iron formation (BIF) limbs. The deposit, as known in 1990, had an areal extent of 2700 x 500 m and in its central portion was ~400 m thick. To the north it also grades into BIF, while to the south it passes into the separate Grand Rochers deposit. Numerous bands of soft to medium hard waste BIF are intercalated within the deposit, particularly near the hanging wall, although waste is minimal towards the footwall. The original protore is a fine and grained, recrystallised, thinly banded BIF with alternating beds of quartz and iron oxides, including magnetite, hematite and martite. The total, unenriched iron content in this protore contains 30 to 38% Fe. Alumina and phosphorus are negligible. The two main ore types as recognised at Pierre Richaud, are described as follows:
• Blue Ore - which is primarily anhedral hematite with minor martite and magnetite. Where high grade, this ore contains in excess of 68% Fe, with total granular quartz and alumina of <3%. The phosphorus content is comparable to the BIF protore at ~ 0.033% P.
• Brown Ore - is anhedral hematite with varying amounts of goethite. The quartz content is <2% at surface, but increases steadily towards the BIF contact. Loss of ignition is >2% because of the goethite content and the presence of intercalated iron silicate laminae. The phosphorus content is higher than that of the blue ore and it may occasionally be >0.1% P.
The transition from high grade mineralisation to BIF is locally marked by a narrow interval of siliceous rock with a content of between 50 and 60% Fe. The mineralisation is capped by a hard crust, typically to depth of 1 to 2 m, characterised by higher alumina, phosphorus and loss of ignition.
This Pierre Richaud description is drawn from Morgon, Izon and Sow, 1992 - The Mineral Economy of Guinea, US Bureau of Mines. 25p.
No signifiant mining had been undertaken on the Guinea Nimba deposit to 2022. However, in 2022 it was undergoing a pre-feasibility study by SMFG-HPX/Ivanhoe Mines for development, with a short railway line connecting the project to Mount Tokadeh across the border in Liberia for transport on the same line ArcelorMittal utilises to Buchanan. Mineral Resources have note been sited as of early 2022, but the project is said to have a potential mine life of at least 20 years, with an initial production from ~2026 of 10 to 15 Mt per annum, quickly ramping up to 30 Mt pa. Nimba north is quoted as containing an additional 1.5 Gt of lower grade ore suitable for blending or upgrading using amortised infrastructure (Ivanhoe Liberia press release, November 2021). However, an article in Mining.com (ArcelorMittal buys Mount Nimba from BHP, August 1, 2014) states the deposit has high quality resources of ore totalling 935 Mt of direct shipping ore @ an average grade of 63.5% Fe. In 2021, SMFG was 95% majority-owned by High Power Exploration through its indirectly owned subsidiary Euronimba Limited, and by the Government of Guinea. Euronimba is an affiliate of Ivanhoe Mines.
The most recent source geological information used to prepare this summary was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
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