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Boke - Sangaredi, Fria, Kindia

Guinea

Main commodities: Al
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There are three main bauxite operations in Guinea, west Africa, namely Sangarédi near Boke, Fria, some 80 km to the south-east of Boke and Kindia a further 80 km to the south-east of Fria. These three operations produced approximately 16 Mt of raw bauxite in 2003 and have 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.

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. 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 deposit was formed on the Palaeozoic cover sequence of the West African Craton which was uplifted along the Guinean Axis during the Tertiary. Bauxite is 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.

Only 30 Mt of reserves remained at Kindia in 2003, although at the nearby Dian Dian deposit there is a resources estimated at 1000 Mt @ 50% Al
2O3.

The terrane has undergone lateritic weathering and been dissected to form a series of NW-SE elongated mesas separated by structural saddles or lateritic crusts. The development of bauxite is influenced by the bedrock lithology, geomorphology, vegetation and intensity of fracturing/faulting.

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% Al
2O3, 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.
Pyrophyllite rich bauxites are also evident. 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% Al
2O3, 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.

The most recent source geological information used to prepare this summary was dated: 2003.    
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 & Additional Information
   Selected References:
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


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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