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Boke - Sangaredi, Bidikoum, Silidara, Ndanga, Boundou Waade, Paravi, Fria, Friguia, Kindia, Debete, Dian Dian |
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Guinea |
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Al
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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There are four main bauxite operations/projects in Guinea, west Africa, namely Sangarédi near Boke, Fria, some 80 km to the south-east of Boke, Kindia (Débété Mine) a further 80 km to the south-east of Fria and Dian Dian, which is ~250 km north Conakry. Sangarédi, Fria/Friguia and Kindia produced approximately 16 Mt of raw bauxite in 2003 and had capacities of 14 Mt at Sangarédi, 2.3 Mt at Fria, and 2.5 Mt at Kindia. The deposits are between 50 and 100 km of the Atlantic coast.
The bauxite deposits of Guinea are developed as a lateritic profile derived from the underlying Palaeozoic cover sequence of the West African Craton, which was uplifted along the Guinean Axis during the Tertiary. The Sangarédi deposits are developed from a stacked pile of argillite and siltstones with minor sandstone intercalations of the Lower and Upper Members of the Devonian Faro Suite. Other deposits are derived from this and members of the underlying succession that consists of horizontal to subhorizontal mudstones and siltstones of the Neoproterozoic Mali Suite, sandstones of the Cambrian Boundou and Ordovcian Pita suites, and mudstones, siltstones and aluminous schists of the Silurian Télimélé Suite. The sequence is widely intruded by laccoliths, sills and rare dykes of late Triassic to early Jurassic dolerite dated at 201 to 196 Ma.
Proven reserves at the Sangarédi deposit and the associated Bidikoum and Silidara pits amounted to 2.3 Gt in 2003, while resources at these deposits and at the N'danga, Boundou Waade and Paravi deposits was estimated at 18.6 Gt @ 40 to 50% Al2O3. The average thickness of the bauxite zone in these deposits is ~8 to 10 m with little soil cover and an iron-rich cap of about 1 to 2 m. Historically, grades at the main deposits of this group have been 56.58% Al2O3 at Sangarédi, 50% Al2O3 at Bidikoum, and 52% Al2O3 at Silidara. These deposits are operated by Cie des Bauxites de Guinée (CBG) which has produced over 220 Mt of bauxite since commencing operations in 1973.
The Fria or Friguia deposit bauxite was mainly developed over flat lying Silurian schists which are sandwiched between thick Ordovician and Devonian sandstones. These schists are exposed in a SE-NW trending belt which is 25 km wide and strongly fractured by a north-east trending Hercynian strike-slip fault pattern. The lower part of the schist sequence is composed of finely bedded blue to blue-grey aluminous schists overlain by shaley sandstone. The upper part comprises alternating sandstones and sericite schists. Up to 10 m thick dolerite sills and less common dykes intrude the whole schist sequence. Friguia is operated by Friguia S.A., which is controlled by UC Rusal of Russia who quote total Measured + Indicated + Inferred Mineral Resources of 322.4 Mt of bauxite as at 31 December 2020 (Rusal Annual Report 2020)
Only 30 Mt of reserves remained at Kindia in 2003, although as at 31 December 2020, the remaining Measured + Indicated + Inferred Mineral Resources was 85.6 Mt of bauxite (Rusal Annual Report 2020). Kindia is operated by Compagnie des Bauxites de Kindia.
The nearby Dian Dian deposit is controlled by Compagnie de Bauxites de Dian-Dian (Cobad), a subsidiary of UC Rusal of Russia who quote total Measured + Indicated + Inferred Mineral Resources of 744.4 Mt of bauxite as at 31 December 2020 (Rusal Annual Report 2020).
The terrane that contains this cluster of deposits has undergone lateritic weathering and been dissected to form a series of NW-SE elongated mesas separated by structural saddles or lateritic crusts. These mesas are at an elevation of 200 to 400 m above SL and have practically negligible soil and overburden. The development of bauxite is influenced by the bedrock lithology, geomorphology, vegetation and intensity of fracturing/faulting. In general, the bauxite is underlain by a stacked pile of argillite, siltstones and minor sandstone intercalations of the Lower and Upper Members of the Faro Suite, widely intruded by sills and dykes of Mesozoic dolerites.
The lateritic profile at Fria varies and may be as much as 25 to 30 m in thickness, divided into three layers, as follows:
Friable sandy-clay surficial layer - yellowish-brown to black in colour, influenced by vegetation cover. For herbaceous cover, it is only 0 to 0.2 m thick and contains blocks and rounded nodules of the underlying bauxitic crust which are covered by a black patina. In the forested sections, the layer may exceed 1 m, with an upper humic rich horizon, becoming lighter with depth, but still rich in nodules of bauxitic crust which are angular and have no patina. The matrix of this sandy clay layer is composed of kaolinite with small quartz grains.
Bauxitic crust - which varies from 1 to 6 m in thickness and is ubiquitous throughout the district. It comprises a dense and massive hardpan with physical characterisitic that vary with the topography. On hills it is fragmented to form large slabs or boulders up to several metres across separated by fractures, ofter occupied by tree roots. This type of bauxite is present in three facies, namely i). platy, ii). porous and vesicular and iii). composite. On structural saddles, the bauxitic crust is brown to deep reddish-brown, dense, massive and very ferruginous, and is either comp-act, breaking on clean fractures or porous and vesicular. On pediments or shallow slopes the crust is fragmented and heterogenous, composed of irregular hardpan fragments and nodules in a matrix of argillo-ferruginous material. With increasing slope angle, mechanical reqorking becomes more apparent, producing conglomeratic crusts. Nodules and boulders from the upper crust are mixed with pebbles from an alluvial terrace.
Weathered bedrock - separated from the bauxitic crust by a progressive, irregular transition zone that is usually 2 to 3 m thick. Included boulders and nodules decrease in frequency and size with depth and are embedded in a red clayey and friable matrix which becomes increasingly more dominant with depth. This layer has been subdivided into and upper "alloterite" in which the protolith texture has been totally destroyed to a lower "isalterite" in which the textures have been preserved. The dominant bedrocck is schistose sandstones and rarely dolerite.
The main bauxites contain 56 to 60% Al2O3, 1 to 8% Fe2O3 and 1 to 12% SiO2. They are platy, regularly bedded, exhibit the original schistose texture and are strongly jointed perpendicular to bedding. They are characterised by alternating lighter bands of well crystallised gibbsite and 30 to 50 cm thick darker bands of gibbsite, goehtite and minor hematite. Late stage gibbsite veins crosscut both bands. These bauxite have only ~2% of the alumina in the form of boehmite, particularly in the Boke region, and this is mainly concentrated on the top layer of the profile.
Pyrophyllite and andalusite rich bauxites are also evident particularly near the coastal belt, with a reduced available alumina content. These have a granular texture with platy or tubular jointing and composed of alternating dense grey and porous light red beds with a thickness varying from 0.5 mm to several cms.
Ferruginous bauxites occur in a range of facies, including most commonly porous and vesicular, with lesser platy or fragmental and nodular and friable, containing 38 to 42% Al2O3, 27 to 35% Fe2O3; <5% SiO2 and up to 3% TiO2.
Argillo-ferruginous bauxites are found at the base of the profile, underlying the main and ferruginous bauxites with a progressive, but irregular transition zone. This layer grades downward in turn into the lower clay as the density of bauxitic nodules decreases and the clay matrix becomed dominant. The underlying weathered schists are composed of alteranting millimetric grey micaceous and ferruginous beds and white gibbsite-kaolinite bands with associated muscovite.
One of the key features of these Guinea bauxites is the natural low silica content (~2.5%) and of that, only ~50% is reactive, which makes this ore most favourable for alumina production by low-temperature digestion.
The most recent source geological information used to prepare this decription was dated: 2020.
Record last updated: 25/2/2022
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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Boulange B, Bouzat G and Pouliquen M, 1996 - Mineralogical and geochemical characteristics of two bauxitic profiles, Fria, Guineau Republic: in Mineralium Deposita v31 pp 432-438
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