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Bogosu

Ghana

Main commodities: Au
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The Bogosu (or Bogoso) Project is spread over a strike length of 18 km along the main Ghanaian Shear Zone that runs along the north-eastern margin of the Ashanti volcanic belt. This shear also localises the Prestea deposit approximately 15 km to the south-west, and Ashanti/Obuasi which is 70 km to the north-east. The area of the current concession was worked from 1933 to 1955 by the Marlu Gold Mining Areas Ltd, with a break between 1943 and 1946. The ore exploited was mainly oxides taken from a series of open pits situated along the extent of a range of low lying hills. During this period approximately 30 t of Au was extracted. Marlu also sank a 250 m shaft and extracted a limited amount of high grade ore. Operations ceased due to poor recoveries. Very little surface oxide ore remains (Suttill, 1989).

Work carried out by the State Gold Mining Corporation and the UNDP during the 1970's indicated potential for further ore. After the application for title by Denison Mines Ltd in 1980, recommendations by a number of parties, including Watts, Griffiths & McOuat Ltd, and the formation of a joint venture partnership with Sikaman Gold Resources, a concession was granted in early 1986 to Canadian Bogosu Resources Ltd. Extensive surface trenching, sampling of Marlu's underground workings and a series of adits, and 17 000 m of drilling in 90 holes, was undertaken, followed by a feasibility study by Minproc Engineers in 1988. Further exploration has been carried out since then to expand the reserves. In 1987 the International Finance Corp. acquired an interest, followed in 1988 by Billiton International Metals. RTZ Consultants undertook a technical and financial audit of the project in 1989 (Suttill, 1989; Griffis & Flach, 1990).

Published production and reserve figures include:

    7.5 mt @ 4.1 g/t Au = 30 t Au (Prod., 1936-1955, Griffis & Flach, 1990).
    14 mt @ 3.8 g/t Au = 53 t Au (Total Res., 1989, Suttill, 1989).
    9.3 mt @ 3.8 g/t Au (Mineable Res., @ 1.4 g/t c'off, 1989, Suttill, 1989).
    8.0 mt @ 4.1 g/t Au (Mineable Res., @ 2.0 g/t c'off, 1989, Suttill, 1989).     8 mt @ 3.45 g/t Au (Sulphide Res., 1994, Ord Minnett, 1994).
    1.6 mt @ 3.2 g/t Au (Mixed Oxide/Sulph. Res., 1994, Ord Minnett, 1994).
    1 mt @ 2.4 g/t Au (Oxide Res., 1994, Ord Minnett, 1994).
    10.4 mt @ 3.7 g/t Au (Res., 1993, AME, 1994).

The reserve is contained within five individual deposits, namely Nankafa, Dumassic, Chujah, Marlu and Bogosu North. The overall planned waste:ore ratio was 6.5:1, with expected recoveries of 83% from sulphide ore and 78% from oxides (Suttill, 1989).

All of the ore lies within the crush zone between the Birimian and Tarkwaian series, which in most places is about 30 m wide (Suttill, 1989). The western and central sections of the mining area are dominated by meta-sediments which dip steeply to the north-west. These sediments are typically thin bedded wackes and shales exhibiting graded bedding and thick alternating sequences of the component clastics. In many areas, in particular in the proximity of mineralisation, the phyllites are carbonaceous and commonly contain considerable pyrite. It is also fairly common to find bands and lenses of carbonate, of dolomitic or ankeritic composition, associated with graphitic units and gold mineralisation. In the central and eastern part of the mining area, volcanics become increasingly common (Griffis & Flach, 1990).

The volcanics are mainly pyroclastics, interbedded with sediments, and occasional mafic lavas. The very eastern section of the area is occupied by Tarkwaian sediments, including thickly bedded quartzite, phyllite, sandstone and occasional narrow conglomerates. The Tarkwaian is steeply dipping adjacent to the contact, but is more gently dipping further east. The contact between the Tarkwaian and the Birimian is interpreted to be a fault. Mafic dykes and sills cut both the Birimian and Tarkwaian sediments (Griffis & Flach, 1990).

The most striking feature of the area is the faulting which is concentrated in a zone some 1 to 1.5 km wide which is dominated by interbedded metasediments and metavolcanics. Exposures of highly tectonised units are common. Since this zone contains significant amounts of quartz veins and stockworks, it generally forms a topographic high. These zones, which are referred to as 'crush zones', appear to have undergone a complex history with numerous periods of reactivation. The most important mineralisation is associated with the 'crush zone'. Folding within the Birimian is generally isoclinal and more complex than that of the Tarkwaian (Griffis & Flach, 1990).

Most of the important gold deposits at Bogosu are found within the crush zone, in the interval where sediments and volcanics are intercalated. Virtually the whole of the crush zone is anomalous in gold. Individual ore deposits occur as lozenge shaped bodies within the crush zone and vary considerably in grade and size. Many are relatively small with strike lengths of 100 to 200 m and widths of 10 to 15 m, others are larger. The orebodies are defined by cut-off grades rather than by geological characteristics. In one interval of 3 km the ore blocks are continuous, although several prospects are defined where grade increases and the ore thickens. Within this interval mineralisation of >2 g/t Au varies from 10 to 50 m in width. Similarly, individual open pits may vary from a few hundred thousand to several million tonnes (Griffis & Flach, 1990).

Veining is developed in a number of directions and has a variation in thickness, ranging from thin veinlets to thick (30 cm) late stage veins which parallel the trend of the ore zone. Gold occurs both within the vein stockwork and the intervening country rock. This applies both to the oxide ore where gold has been redistributed and to hypogene ore. Graphitic or carbonaceous material is common in most of the mine area, although widespread carbonaceous material does not necessarily correlate with higher grades. While much of the mineralisation is found in the main crush zone, other lesser, parallel structures are also mineralised with veins that may be 50 to 200 cm thick and traceable for a hundred metres or more (Griffis & Flach, 1990).

At Bogosu the ore is refractory with micro-fine gold being intimately associated with pyrite and arsenopyrite. No free gold has ever been seen (Suttill, 1989). The pyrite to arsenopyrite ratio is of the order of 4:1. Within the pyrite, gold occurs along grain boundaries and as discrete particles totally enclosed within the grains, and varies from 0.5 to 12 Ám in diameter. In arsenopyrite the gold is rarely visible and is considered to be in solid solution within the arsenopyrite lattice. The total amount of gold within the two sulphides varies, but appears to be linked to the presence of arsenopyrite, with a good correlation between the Au and As grades. The ore must be roasted to liberate the gold. In oxide ore the gold is freed by the oxidation of sulphides to hydrated iron oxides and the free gold can be treated by cyanidation. In the oxide zone there has been leaching and concentration of supergene mineralisation (Griffis & Flach, 1990).

The most recent source geological information used to prepare this summary was dated: 1996.    
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:
Allibone A, Teasdal J, Cameron G, Etheridge M, Uttley P, Soboh A, Appiah-Kubi J, Adanu A, Arthur R, Mamphey J, Odoom B, Zuta J, Tsikata A, Pataye F, Famiyeh, Lamb E  2002 - Timing and structural controls on Gold mineralization at the Bogoso Gold mine, Ghana, West Africa: in    Econ. Geol.   v97 pp 949-969
Mumin A H, Fleet M E, Longstaffe F J  1996 - Evolution of hydrothermal fluids in the Ashanti Gold Belt, Ghana: stable isotope geochemistry of Carbonates, Graphite, and Quartz: in    Econ. Geol.   v91 pp 135-148
Oberthur T, Schmidt Mumm A, Vetter U, Simon K, Amanor J A  1996 - Gold mineralization in the Ashanti Belt of Ghana: genetic constraints of the stable isotope geochemistry: in    Econ. Geol.   v 91 pp 289-301
Oberthur T, Weiser T, Amanor J, Chryssoulis S L  1997 - Mineralogical Siting and Distribution of Gold in Quartz Veins and Sulfide Ores of the Ashanti Mine and Other Deposits in the Ashanti Belt of Ghana: Genetic Implications: in    Mineralium Deposita   32 (1997) pp 2-15


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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