Having completed a feasibility study in December 2015 on the Prestea underground project, which confirmed the findings of the positive preliminary economic assessment undertaken in October 2014, Golden Star believes that with patience and responsible development, the underground mine can be a value-add to all stakeholders including the local community, the government of Ghana and the company’s shareholders.
Prestea mine overview
The Prestea mine consists of an established underground mine and adjacent surface deposits and is located 16 km south of the Bogoso refractory processing plant in Western Ghana. It is accessible by road from Accra as well as via the port city of Takoradi 150 km away. The underground mine at Prestea has been in production on an intermittent basis over the past 100 years and has historically produced in excess of 9 Moz. The mine was acquired by Golden Star in 2002 and placed on care and maintenance while evaluation and exploration activities continued to reassess its potential.
Prestea underground mine has a known reserve of 470 000 oz at an average grade of 14 g/t gold and indicated resources of 675 000 oz at an average grade of 18.4 g/t gold plus an additional 1.2 Moz of inferred resources. It has large areas unexplored and unexploited both above and below the deepest part of the mine, which is 1 350 m deep. Meanwhile, the surface deposits, which lie to south-east of the town of Prestea, have historically been exploited by artisanal miners, while the mining of the surface pits by Golden Star began in mid-2015.
The Prestea surface deposits currently have proven and probable mineral reserves of 122 000 oz of gold at an average grade of 2.24 g/t gold and measured and indicated mineral resources of 454 000 oz at 2.57 g/t gold inclusive of mineral reserves.
Feasibility study findings
Based on a gold price of US$1 150, the feasibility study shows positive economics for the mining of the West Reef at Prestea underground using shrinkage stoping, says Coetzer.
Based on this mine development plan, the feasibility study indicates an after tax internal Ghana-focused gold mining company Golden Star Resources, which completed its feasibility study in late 2015 for the Prestea underground mine, is optimistic for first ore production in early 2017, president and CEO Sam Coetzer tells Chantelle Kotze.
The 2.7 Mtpa carbon-in-leach Wassa processing plant MINING REVIEW AFRICA | FEBRUARY 2016 19 CANADIAN MINERS IN AFRICA • rate of return of 42% and after tax net present value of $124 million. The initial capital expenditure is estimated at $63 million and a payback period of 2.9 years with life of mine capital totaling $77 million net of revenue, processing costs and working capital during the development phase.
Capitalised operating costs relating to manpower, power, consumables and refurbishment costs during the pre-development and production ramp-up phase are included in this capital expenditure estimate.
The feasibility study proposes shrinkage stoping, which was the mining method historically used at Prestea, with the application of rock bolts and timber props to support the stope walls to maintain stope stability and control waste dilution, explains Coetzer.
The main haulage level will be established on the existing 24 level to move mineralised and waste rock to theCentral Shaft for hoisting to surface. An incline/decline system will be developed in the footwall of the mineralisation to access sub-levels at a vertical spacing of approximately 35 m to 40 m between existing levels 17 and 24 and 140 m below level 24. Shrinkage stopes will be developed between open raises spaced 60 m on strike.
Drawcones will be developed out of the sub-levels into the stopes and will be equipped with chutes for controlled shrinkage mucking into rail cars in the sub-levels. The stopes will be advanced up-dip with only the swell material (30% of total blasted) removed from the stopes during the mining phase.
When the stope is mined up to thesill pillar below the upper sub-level, the remaining mineralised material in the stope will be drawn as required. Despite this concise mine plan, Golden Star Resources will continue to look for new technologies to introduce to its mining methodology to enhance its safety and strength for the future, Coetzer notes. Prestea underground, in addition to the Wassa underground project, also currently in development, will ensure that Golden Star Resources adds value for its shareholders by enabling it to be a lower cost gold producer in the face of the current low gold price environment.
The feasibility study considers a new standalone plant; however, the company is reviewing the option of modifying its currently installed processing infrastructure to reduce capital spending yet maintain high levels of productivity and effi ciency. The metallurgical test work results indicate that the Prestea West Reef material is free milling with approximately 96% metallurgical recovery using gravity followed by carbon-in-leach (CIL) processing.
The proportion of gravity recoverable gold identifi ed in the test work is high at between 50% and 90%. The processing facility needed to do this will comprise a 500 tpd standalone plant using a standard comminution circuit, followed by gravity and CIL sections. The recovered gravity concentrate will be treated in the existing Acacia circuit at the Bogoso plant. Gold recovered from the CIL circuit will be further processed in the existing elution circuit, while the dore will be smelted in the existing gold room.
In addition to the processing synergies, much of the administrative work for Prestea will be done at the Bogoso complex, while the Prestea and Wassa mines share a common senior management team as well as finance and accounting systems. “Having used the synergies that existed it was possible to reduce the environmental impact and also swiftly introduce mining on surface that had a benefit to local contractors and employment, notes Coetzer.
2016 work plan
Since acquiring Prestea mine in 2002, Golden Star has invested in keeping the mine in good stead so when it decided to move it forward it had a good asset to work with, says Coetzer. Continuation of the refurbishment and upgrading of Prestea surface and underground infrastructure and stope development within the West Reef area has been underway since 2015 and continues to progress.
The infrastructure rehabilitation includes shaft and hoist upgrades, electrical infrastructure, process plant, ventilation, compressed air and pumping. This forms the bulk of the initial capital expenditure. Most recently, and in the run up to commercial production in early 2017, Golden Star is progressing as scheduled with rehabilitation work at the Central Shaft and refurbishment work on 17 and 24 levels.
All the bearer sets in Central Shaft were replaced during 2015, a critical step forward to full hoisting operations. The existing extensive infrastructure at Prestea, both on surface and underground, will be used in future production but will need considerable refurbishment and upgrading, for which capital investment will be used.
The benefit of the extensive underground infrastructure in place from previous mining will assist in the mining and haulage of ore from the West Reef area of Prestea underground.