The Prestea-Bogoso area occurs at the southern termination of the Ashanti Belt, where the gold deposits, mined or under exploration, are localised principally along two steep to subvertical major crustal structures referred to as the Ashanti trend and the Akropong trend. The principal structures are graphitic shear zones and mineralised fault filled quartz veins. The Bogoso Prestea section of the Ashanti Trend shows a range of mineralisation styles associated with graphitic shear zone, which represents the principal displacement zone of a regional-scale shear zone that defines the mineral belt. These styles include laminated quartz vein deposits containing free gold, highly deformed graphitic shear zones containing disseminations of arsenopyrite as the principal gold bearing phase (e.g. Buesichem, Chujah-Dumasi and Bogoso North) and disseminations of sulphides in mafic/intermediate volcanic rocks generally found in the footwall of the main shear zone.

The Bogoso Prestea deposits can be classified as a lode gold deposits or orogenic mesothermal gold deposits, which are the most common gold systems found within Archean and Paleoproterozoic terrains. In the West African shield, orogenic gold deposits are typically underlain by geology considered to be of Eburnean age and are generally hosted by volcanosedimentary sequences. The Ashanti belt is considered prospective for orogenic mesothermal gold deposits and hosts numerous other lode gold deposits such at the Obuasi mine.

The Prestea concession lies within the southern portion of the Ashanti Greenstone Belt along the western margin of the belt. Rock assemblages from the southern area of the Ashanti Belt were formed between a period spanning from 2,080 to 2,240 Ma as illustrated in Figure 6, 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 concession is divided into three 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. From the eastern footwall to the western hanging wall, these packages are represented by the Tarkwaian litho-structural assemblage, the tectonic breccia assemblage, composed of sheared graphitic sediments and volcanic flows, and the last assemblage is composed of undeformed sedimentary units of the Kumasi Basin, which is located to the west of the Ashanti fault zone.

The various authors interpret the Main and West Reef structures to represent reactivated Eoeburnean reverse faults. However, there is some debate about when quartz veining, i.e., emplacement of the lode deposits, took place. Undoubtedly, there are several generations of quartz veining spawning over an extended period of time, most likely from the Eoeburnean orogeny to the end of the Eburnean reactivation. Gold within the West and Main Reefs is associated with smoky grey quartz veins which re-fracture the milky quartz veins that comprise the majority of the reef structures that are evidences of multiple fluid pulses along the major faults and most likely of Eburnean timing. To date, there has been no geochronological dating on the Prestea mineralization, but gold emplacement has been interpreted to be of syn-D3 to syn-D4 Eburnean timing.

Davis and Allibone (2004) show the Main Reef to be deformed in a variety of styles, including being affected by folding and boudinage associated with sinistral deformation and a third subordinate folding event.

The margins of the West Reef mineralized quartz vein are strongly sheared and comprise a brittleductile zone of deformation in the graphitic schists a few centimetres to up to 2 m in width on both sides of the vein at any locality. The deformation along the margins of the vein is interpreted to be due to post-mineralization deformation nucleating on the margins of the vein, which represents a strong competency contrast with the graphitic wall rocks. The kinematics of this deformation appears to be dextral. Over the length of the vein exposed on 17 L, several subsidiary shears were observed to cut through the vein which either caused the vein to be duplicated, causing local thickening of the mineralized vein over approximately 0 to 10 m, or caused extensional offsets of the vein. One 10-metre gap in the continuity of the resource on 17 L could be attributed to one of these shears. Overall however, these are relatively minor disturbances which just cause local irregularities in the vein and there should be no overall material loss.

Two distinct styles of mineralization are found on the Prestea mine site. The more extensive of the two mineralization styles are laminated fault fill quartz veins (Reef style mineralization), bound to the Main Reef fault or to the second order faults found in the Kumasi sedimentary hanging wall. The second mineralization style, which has never been mined in the past, consists of arsenopyrite rich, brecciated, and altered volcanic lenses.

Gold mineralization between Marlu and Bogoso North is restricted to a narrow graphitic fault zone located in the sedimentary hanging wall of the deposits with the footwall composed of volcanic lenses and sheared sedimentary rocks. The Bogoso North deposit consists of two splays of the main shear zones, a quartz vein dominated hanging wall splay, and a highly graphitic footwall structure. The two splays of the main shear zone at Bogoso North extend for approximately 500 m along strike and range in true width from 5 to 15 m. Gold mineralization at Bogoso North dips moderately to the northwest at 40 to 50º. The mineralisation is modelled over a 2 km strike at Bogoso North and an additional 2.7 km northwards to a depth of some 300 m. Bogoso North gold mineralization is associated with either quartz veins or graphitic cataclasites.

Quartz vein hosted gold mineralization generally is coincident with relatively high gold tenors, typically between 5 to 15 g/t Au. Vein gold is associated with abundant disseminated arsenopyrite in and around slices of graphitic mylonitic rock within the quartz veins. Although visible gold is occasionally observed in the veins at Bogoso North there are not abundant amounts of free gold as at Prestea and Ashanti. The quartz veins mark dilatant fractures, which focussed gold-bearing fluids, although gold deposition appears to have been promoted by interaction with adjacent wall rock fragments.

Mining activity includes both underground using mechanized shrinkage mining and longhole open-stoping, and a recent significant investment in open pit operations feeding both refractory and non-refractory processing plants.

The planned mining method is mechanized shrinkage mining. This method uses mechanical raise climber technology (Alimak) together with longholes drilled from the raise climber. The method is an advance in terms of safety and productivity from conventional hand-held shrinkage mining. Safety is greatly enhanced both in raise development and stoping operations by having miners protected from rockfalls and removed from working on the broken muck pile. The productivity (per miner) is enhanced by introducing short longholes for stope production.

A typical cycle for Alimak stoping is:
- Raise development in the centre of a stoping block;
- Raise hangingwall rock support;
- Drilling of longhole blast rings;
- Relocation of the raise climber infrastructure to the top of the stoping block, blasting of longhole rings and swell mucking; then,
- Final draw-down mucking and possible waste backfilling.

Small diesel and electric 2 m3 LHD’s will be used for rock haulage from the base of the stoping panel. Mine development will be by hand held drills with LHD mucking. Tramming of ore and waste along the two main levels (17 and 24) will be via an upgraded rail haulage system to Central Shaft.

The rib pillars between stopes are planned to be 3 m wide along the strike of the orebody. Stopes will be 20 m wide separated by these pillars. The pillars will run the entire length of each of the Alimak stopes and are not planned to be recovered.

Mine Ventilation Service’s (MVS) engineers completed a detailed ventilation system design for the mechanized shrinkage mining. The emphasis of the study was to determine ventilation infrastructure and fan duties required for the West Reef development, using phased ventilation modeling.

In areas where diesel equipment is used a factor of 0.08 m3 /s per kW of motor power is used. This is not only for dilution of diesel particulate matter but also for dilution of heat and dust.

The Prestea Underground (PUG) workings extend over a distance of 6 km along strike and down to a maximum depth of about 1,450 m below surface. The two primary shafts of the Prestea mine are the Central and Bondaye shafts.

The Central shaft is the primary access to the underground mining levels and it extends to a depth of 1,238 m below surface to 30 Level (L). Numerous levels were developed off the shaft to provide access to the Main Reef stoping areas. Traditional narrow vein mining methods were employed, primarily shrinkage stoping and captive cut and fill. Run of mine (RoM) material and waste were trammed to the Central shaft to loading pockets located below 20 L, 25 L, and 30 L, which served to load the RoM into skips for conveyance to the surface bins. The total capacity of the system at its peak may have been around 1,300 to 1,600 t/day.

The Bondaye shaft extends to a depth of 1,103 m, but unlike the Central shaft there is no dedicated rock handling system at Bondaye, and cars were loaded into the cages and raised to surface.

In addition to the Central and Bondaye shafts, there are several internal shafts. The No. 4 and No. 6 shafts are located to the south of the Central shaft. No. 4 shaft extends from 23 L to 35 L and was used as the primary access to 35 L, the lowest developed level in the mine. No. 6 shaft extends from 24 L to 31 L.

Crushing
A CAT988 front end loader (FEL) will reclaim ore from the stockpile on the ROM pad and deliver to the ROM bin. A 150mm aperture vibrating grizzly (01FE01) fitted with variable speed drive will allow material of -150mm to bypass the crusher and report to a belt conveyor (CV01). The +150mm oversize will gravitate into the KEMCO C160 Jaw crusher producing product of P80 - 150mm which also falls onto CV01 and the combine product collected at the stockpile. CV01 belt will be fitted with conveyor canopy to prevent washing of the ore during transport and rain.

Given the over capacity of the crushing unit, crushing will be done in batches. The high security risk associated with the PUG ore requires that large stocks will not be kept at the ROM pad.

The crushed material will be stockpiled on a 6000t capacity stockpile fitted with overhead canopy to keep out rainwater and also for dust suppression.

Milling and Classification
The crushed ore is removed from beneath the stockpile by a slot feeder (02-CH06) to a variable speed conveyor (02-CV-02) driven by hydraulic motor, which delivers the ore to the Mill feed conveyor (02-CV-03). The hydraulic drive unit of 02-CV-02 and the adjustment bars in the slot feeder allow the feed to the mill feed conveyor to be controlled.

The Mill feed conveyor is equipped with a weightometer that allows accurate control of Mill feed and associated controls. The mill feed conveyor will deliver the ore at nominal rate of 120tph into a 1.5MW 5.2x4.8 Morgardshammer SAG mill operating in open circuit. For maximum flexibility in adapting to ore competency changes, the Mill has a hydraulic coupling that allows the mill speed to be varied from 60% to 80% of critical speed. SAG mill discharge slurry of product size of P80- 700 micron enters the Mill discharge distribution box, which then directs the slurry into the Combined Mill discharge pumps hopper.

The Mill discharge pumps, delivers slurry with adjusted density of 55% solids to a bank of 8 GMAX 15-3117 Krebs cyclones. Cyclones overflow of P80 -75 micron gravitates to the vibrating trash screen before feeding into the 14M pre-leach thickener. The cyclone underflow is fed to the gravity circuit consisting of two(2) Operational 30” Knelson concentrators and one on standby, via a distribution box and scalping screen. The Knelson tails and scalping screen oversize (+4mm) are pumped to a cluster of 2 GMAX26-3332 Krebs dewatering cyclones.

The dewatering cyclones underflow feeds the 1.5MW 4.2x5.4 Morgardshammer Ball Mill. The overflow however gravitates to the Mill discharge hopper and serves as additional dilution water. The Ball mill is operates at a fixed speed of 75% critical speed through an air clutch system.

Ore from the PUG West Reef is trucked to GSR’s Bogoso operation, a distance of approximately 15 km, where the ore will be processed through the existing process facility producing 2,480 kg/a or 80,000 oz/a of gold at an overall recovery of 94%.

The processing facility at Bogoso consists two plants to separately process refractory and nonrefractory (oxide) ores. As the PUG ore is non- refractory ore it will be processed through a modified Oxide circuit that utilizes conventional CIL for gold recovery. The circuit includes the following unit processes:

- Run-of-mine (RoM) receiving;
- Crushing;
- Milling and classification circuit (cyclones);
- Gravity concentration;
- Thickening;
- Feed storage tanks;
- Carbon-in-leach (CIL) circuit;
- Upgrade/refurbishment of high intensity cyanide leach reactor (Acacia);
- Gold room and recovery;
- A new CIL tailings disposal line;
- Services (compressed air, instrument air, o ........