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The Bong iron deposit is located ~75 km NE of Monrovia in northwestern Liberia (#Location: 6° 49' 05"N, 10° 19' 32"W).
Iron formations were first discovered in the Bong Range in 1934, although the first commercial mining was commenced in 1965 by Gewerkschaft Exploration of Düsseldorf, later to be known as Exploration und Bergbau GmbH, to supply a consortium of German and Italian steel mills. It closed in 1990 by a civil war. In January 2009, the Liberian government signed an investment agreement with China Union Investment Company (Wuhan Iron & Steel Group) to re-open and operate the mine.
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 Bong deposits are located within the "Eburnean age province".
The Bong deposit is developed towards the western end of an ~40 km long, by ~1 km wide prominent ridge of ironstone (the Bong Range), within a 20 to 60 km wide composite gneiss belt of felsic and mafic composition, that stretches across the country from the NE border with Guinea, to the Atlantic ocean. Several other similar sized ridges are mapped along strike for that at Bong. Most of the host sequence has been heavily lateritised (SRK, 2011).
The ironstone ridge at Bong is formed by the outcrop of the Bong formation, principally composed of banded iron formation (itabirite), with interbeds of hornblende and mica schist. The itabirite occurs as the remnants of a series of tight synclines and anticlines, the axes of which are flat, undulose and shallowly plunging.
The folded banded iron formation is a magnetite itabirite, that has been weathered to depths of ~40 to 50 m to form a soft 'hematite-martite-goethite' ore (with <:10% magnetite), passing into a transitional ore and then the underlying magnetite mineralisation at a depth of 80 to 100 m (Abdulla, UN, 1970; Bong Town website www.bong-town.de/, 2013).
The soft hematite and transitional ores average: ~38 to 40% FeTotal, 39 to 41% SiO2, 0.01% S, 0.02% P;
and were concentrated to: ~65 to 66% FeTotal, 4 to 5% SiO2, 0.005% S, 0.03% P, 6 to 7% LOI;
the magnetite itabarite ore averages: ~37 to 38% FeTotal, 41 to 42% SiO2, 0.1% S, 0.05% P;
and was concentrated to: ~65 to 66% FeTotal, 7 to 8% SiO2, 0.03% S, 0.03% P, 8 to 9% LOI (Abdulla, UN, 1970);
Quartz is the principal gangue mineral. The mean grain size of iron minerals is ~0.15 mm and quartz is ~0.12 mm (Bong Town website www.bong-town.de/, 2013).
The ore within the soft and transitional zones average 10% goethite, 35% hematite and 55% magnetite. This ore was beneficiated with spirals and the tails magnetically separated to produce fines concentrates and pellets (Abdulla, UN, 1970).
The deposit is estimated to contain a total resource of 4 Gt @ 36% Fe (McGraw Hill Financial - Platts website, 2013).
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.
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