The Prestea-Bogoso mineralization occurs at the southern end of the Ashanti Greenstone Belt, where 11 gold deposits, mined or under exploration, are localized principally along three steep to subvertical major crustal structures. Rock assemblages from the southern area of the Ashanti Belt were formed during a period spanning from 2,080 to 2,240 million years (Ma) with the Sefwi Group being the oldest rock package and the Tarkwa sediments being the youngest. The Ashanti Belt is host to numerous gold occurrences, which are believed to be related to various stages of the Eoeburnean and Eburnean deformational events.

The geology of the Prestea mine site is divided into four main litho-structural assemblages, which are fault bounded and steeply dipping to the west. This suggests that the contacts are structurally controlled and that the litho structural assemblages are unconformable. These packages are, from the eastern footwall to the western hanging wall, the Tarkwaian litho structural assemblage, the tectonic breccia assemblage, the graphitic Birimian sedimentary assemblage, and the undeformed Birimian sedimentary assemblage.

At Prestea, the principal structure is the mineralized quartz vein, known as the Main Reef, which is relatively continuous and has been modelled and mined over a strike length of some 6 km and to a depth of approximately 1,450 m below surface (35 Level [L]). The subordinate West Reef and East Reef, in the immediate hanging wall and footwall, respectively, of the former structure, are discontinuous. West Reef occurs some 200 m into the hanging wall of the Main Reef structure and, at present, is known to occur over a strike length of 800 m and has currently been defined by underground drilling between 550 to 1,150 m below topography as far as the 30 L.

The existing infrastructure includes the Central and Bondaye shafts, the 17 L and 24 L lateral drifts, which have been developed to the West Reef deposit, the South Waste Shaft (exhaust ventilation), and the 11 L and 12 L lateral drifts, which will be the exhaust air routes above the West Reef area. The declines and incline are designed at a gradient of -/+28 degrees (°) with dimensions of 2.7 m width by 2.7 m height.

Traditional shrinkage mining with the application of rock bolts and timber props to support stope walls is the mining method planned. The mine plan includes 9% ore tonnes (7% gold ounces) from on vein sill development and 91% ore tonnes (93% gold ounces) from shrinkage stoping.

The mining methods used at the Bogoso main and satellite pits are conventional excavator and truck methods typical for this style of gold mineralisation. Drilling and blasting of ore and waste is conducted over bench heights of 6 m or 9 m and explosives are delivered to the hole by the manufacturer. Oxide or weathered ore is generally only required to be lightly blasted or in some areas can be excavated as ‘free dig’. Hydraulic excavators are used to achieve good selectivity in conjunction with good blasting practice and mine to a 2.5 or 3.0 m flitch height. Ore and waste is generally loaded to 95 t capacity off highway haul trucks to a central stockpile or to the waste dump.

GSR intends to process the Prestea West Reef ore through a low throughput stream of the CIL plant within the confines of the existing Bogoso processing operation, a distance of approximately 15 km from the mine. The plant consists of a two stage crushing circuit, ball mill, gravity circuit (incorporating the existing Acacia high intensity cyanidation reactor) and CIL circuit, with associated services. The existing non refractory plant elution circuit is used for gold recovery from the activated carbon, which is then recovered as metal by electrowinning in the existing Bogoso gold room. Reagents such as cyanide, lime, flocculent, and oxygen
are supplied from the existing Bogoso plant services.