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Prestea |
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Ghana |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Presteagold mine is located approximately 85 km to the south-west of Obuasi, also on the north-western margin of the Ashanti volcanic belt of Ghana. It was owned by the State Gold Mining Corp. (Ghana) in late 1994, but may have subsequently been joint ventured to an outside party.
Production and reserve figures include:
5 to 19, av. 10.3 g/t Au = 280 t Au (Prod., 1885-1992, Dzigbodi-Adjimah, 1993).
6.08 mt @ 7.3 g/t Au = 44 t Au (Prov.+Prob.+Pos. Res., 1989, Suttill, 1989).
According to Suttill (1989), there is considerable potential for additional resources to be defined at Prestea.
The Prestea deposit occurs in the Upper Birimian near the contact with the Lower Birimian. The Upper Birimian is represented by a thick series of north-westerly dipping phyllites, carbonaceous phyllites, meta-tuffs, meta-greywackes and pelitic schists. These rocks have been isoclinally folded to form overthrust faults and shear planes dipping to the north-west. The fold axes trend in a north-easterly direction (Appiah, etal., 1991).
The mineralisation extends for about 10 km along strike. Free milling gold occurs in quartz veins, while fine disseminated gold is found in sulphide impregnated zones. The reefs dip at from 65š to nearly vertical and mineralisation is roughly parallel to the foliation. The quartz reefs are basically stratabound, following a highly distorted zone of carbonaceous phyllites sandwiched between harder meta-volcanic rocks to the east and massive sediments to the west. Mafic and felsic dykes parallel the ore. All are altered and some are mineralised. The Main Reef has two other splits (Appiah, etal., 1991).
The orebodies are in the form of discontinuous lenses that may be up to 1000 m long, pinching and swelling from 1 cm to 10 m in thickness. They consist of banded and brecciated quartz and wall rock with partings of carbonaceous phyllite. Sulphide mineralisation is associated with the carbonaceous partings, comprising arsenopyrite, chalcopyrite, sphalerite, galena, bournonite, pyrrhotite and some tetrahedrite. Gangue minerals include quartz, sericite, albite, chlorite and ankerite. Gold is commonly associated with galena and bournonite (Appiah, etal., 1991).
Disseminated mineralisation is found in the footwall in the form of fine grained gold in brecciated, silicified meta-sediments and meta-volcanic rocks with visible pyrite and needles of arsenopyrite. Minor amounts of fine grained chalcopyrite, sphalerite, pyrrhotite, bournonite and galena are present in areas flooded with quartz. This mineralisation averages 3 to 6 g/t Au and may be up to 60 m in width. Although the shape of these orebodies is not well known, they may be pod-like with lengths of several hundreds of metres. Frequent veinlets of quartz with minor sulphides form a stockwork. Gold occurs as 1 to 10 µm grains in grey quartz, in arsenopyrite associated with sphalerite and as micron sized particles in the groundmass (Appiah, etal., 1991).
The most recent source geological information used to prepare this decription 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.
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Appiah H, Norman D I, 1991 - The Geology of the Prestea and Ashanti Goldfields: A Comparative Study: in Ladeira E A (Ed.), Brazil Gold 91 Balkema, Rotterdam pp 247-255
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Mucke A, Dzigbodi-Adjimah K 1994 - Ore Textures and Paragenesis of the Prestea and Obuasi Gold Deposits in the Ashanti Belt of Ghana: An Ore Microscopic Study: in Geologisches Jahrbuch D100 pp 167-199
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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
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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
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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
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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