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Location: 49 km NW from Takoradi, Ghana
Western RegionPO Box 283TarkwaGhana
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GeologyIduapriem mine is located within the Tarkwaian Group which forms part of the West African Craton that is covered to a large extent by metavolcanics and metasediments of the Birimian Supergroup. In Ghana, the Birimian terrane consists of northeastsouthwest trending volcanic belts separated by basins, and the Tarkwaian Group was deposited in these basins as shallow water deltaic sediments. The Tarkwaian lithologies are considered to represent the erosion products that accumulated following uplift and deformation of the underlying Birimian rocks during the Eburnean orogeny. The basins (grabens) are believed to have formed as a result of rifting, preferentially in the central parts of the Birimian volcanic belts. The Tarkwaian Group consists of a thick sequence of clastic metasedimentary rocks that have undergone low-grade regional metamorphism.Deposit typeAt Tarkwa, the entire Tarkwaian Group has been folded into a broad syncline and is locally referred to as the Tarkwa Syncline. The Banket Series Formation comprises a sequence of individual quartz pebble conglomerates (Banket beds), breccia conglomerates, and metasandstones (also called quartzites and grits). All known gold mineralisation within the Banket Formation is associated with the conglomerates and is found within the matrix that binds the pebbles together. Gold content is a function of the size and amount (packing) of quartz pebbles present within a conglomeratic unit – the bigger and/or more pebbles present, the higher the gold grade. The upper stratigraphic limit of the Banket Series Formation is marked by the hangingwall quartzite which exhibits well-developed and characteristic trough- and crossbedded haematitic black sand banding. The hangingwall quartzite also contains thin discontinuous grit interbeds. Dykes and sills of doleritic composition intrude the sedimentary sequence and frequently occur adjacent to complex structural zones.Mineralisation styleThere are four recognised conglomerate reefs namely A, B, C and D which are equivalent to the Tarkwaian Sub-basal, Basal (or Main), Middle (or West), and Breccia Reefs respectively. The B and C Reefs are oligomictic and consist of well-sorted conglomerates and have been mined underground in some areas more than a century ago. The A and D Reefs have a lower gold tenor and are polymictic containing both well-rounded and angular fragments. Gold is found within the matrix that binds the pebbles together. The gold content is a function of the size and amount (packing) of quartz pebbles present within a conglomeratic unit, the more pebbles present suggests more gold. The gold is fine-grained, particulate and free-milling. Mineralogical studies indicate that the grain size of native gold particles ranges between 2 and 500 microns (0.002 to 0.5mm) and averages 130 microns (0.13mm). The thickness of the main mineralised B and C Reefs are approximately 15m and 6m respectively. The mineralised sequence dips at angles varying from steep and sub-vertical, at Blocks 1 and 2, to steep (70° to 80° north-dipping) towards Block 3 East. The dip continues to be shallower at Block 3 West (50°), through Blocks 4 and 5 (45°) to become 35° at Block 7 South and 30° at Block 8. At Ajopa, the average dip is 50° to 60°. At Blocks 7 and 8, the western limb of the syncline extends over 4km on the property, with the eastern limb reaching the surface just beyond the eastern boundary of the concession. The western and the eastern limbs outcrop about 4km apart with the mineralised horizons buried some 400m below the surface at the centre of the syncline.Mineralisation characteristicsThe gold is fine-grained, free milling and not associated with sulphides.
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