This expansion will see the CIL plant increase its processing design capacity from the current 4.2 million tonnes a year and the mine’s further transition from heap leach processing will take place between 2009 and 2012.
Tarkwa was originally a small underground mine when it was purchased by Gold Fields in the mid-1990s. The company saw that the best route for mining Tarkwa was predominantly as an opencast operation and the underground operation was closed some years later, in 1999. In 2000 some of the assets of the adjacent former Teberebie opencast mine were purchased and merged with Tarkwa, and at the end of 2003 Tarkwa commissioned its CIL plant, having previously been exclusively a heap leach operation. With the expansion of the CIL plant its South heap leach plant, which was the former Teberebie plant, will be shut down, and Tarkwa’s North heap leach plant will taper off but continue to treat the material that is not passed through the CIL plant.
Tarkwa, as a surface mine that moves some 220,000 to 270,000 tonnes a day of ore and waste material and produces 700,000 ounces a year of gold, has been very successful for its owners (Gold Fields with 71.1%, Iamgold with 18.9% and the government of Ghana 10%). It currently mines from nine open pits that will eventually coalesce into four pits ranging between two and four kilometres in length. It does selective mining with backhoes to avoid dilution from an orebody that consists of strata bound reef conglomerates mined within a quartzite sequence. An advantage of Tarkwa’s very low 1.3 g/t grade is that artisanal miners do not find its orebody worthwhile, and thus a problem that other operations in West Africa have to deal with is avoided. The reef thicknesses at Tarkwa range from 1.5 to 9 metres, with an average around three metres. The reef dips range from 10O to 40O with an average of 25O.
With its selective mining method, all digging of ore at Tarkwa is controlled visually by a geologist or geotechnician to ensure minimal dilution. The dilution skins at Tarkwa are 30 cm on the hangingwall and 20 cm on the footwall and have an estimated grade of 0.15 g/t. Because of this approach the mine has a large geological staff. Tarkwa employs 29 geologists and 77 geotechnicians on grade control, and three additional expatriate geologists. The mine also employs 13 geologists and 32 geotechnicians to do grade control and resource drilling as well as resource modelling, and it has two geologists and a geotechnician focused on geotechnical engineering. In addition the mine employs 14 mining engineers and two mechanical engineers to manage the mining operation which employs some 486 people including 120 maintenance contractors. Overall Tarkwa employs 1,550 people as well as 1,700 contractors.
Gold Fields has increased the mine’s reserves extensively since 1998 when these amounted to 3 million ounces of Gold Fields’ volume driven opencast Tarkwa gold mine, which mines a low grade orebody, has completed its transition from contractor to owner mining and is planning what will be a tripling of the size of its carbon-in-leach (CIL) gold processing plant from its original design capacity as it prepares to ease itself away from heap leach processing. to complete migration from heap leach to CIL gold. In 2000 the reserve had increased to 6.4 million ounces and at the end of 2005 the reserve was 324 million tonnes at 1.29 g/t for 13.4 million ounces of gold at a gold price of US$375/oz. This will ensure at least a further 20 years of life at Tarkwa. In part the increase in reserves has been due to increases in mining efficiency with the conversion to owner mining that allows the operation to take its cut off grades for its heap leached material to 0.35 g/t and its milled material head grade cut-off down to 0.6 g/t. This, together with the introduction of the mill has allowed Tarkwa to process ores where this had previously not been possible.
As of 2005 Tarkwa’s resource was 410 million tonnes at a grade of 1.48 g/t for 19.5 million ounces of gold calculated using a gold price of US$450/oz. This includes an inferred underground resource of 18.6 million tonnes containing 2.4 million ounces of gold. The underground potential of Tarkwa will only be considered for mining after the depletion of the life of the opencast operations. Possibilities for underground mining include beneath the Akotansi, Kottraverchy and Teberebie pit areas.
The total Tarkwa’s exploration plan is done with the aim of maintaining its resource base at current levels. For its current financial year this includes a US$400 to US$450/oz open cut in the Akontansi West complex area, with 7,320 metres to be drilled in the Mantraim, Pepe West, Kottraverchy and Teberebie locations. In its next financial year the exploration will include the drilling of a US$450/oz pit in the Pepe North area followed by geotechnical and structural analysis boreholes, with an estimated 7,800 metres to be drilled.
With it being a very exploration focused operation, Tarkwa spends US$95.43 per metre for its diamond drilling of evaluation pit boreholes, of which it undertakes some 9,000 metres a year for an expenditure of US$1.2 million. It spends US$24.3/m on reverse circulation drilling using a small rig and US$36/m using a large rig for grade control drilling, averaging at US$30.9/m. Tarkwa undertakes some 110,000 metres of reverse circulation drilling a year for an expenditure of US$3.2 million.
However, the mine’s biggest cost is haulage, and it has spent US$80,000/km on developing its roads. Tarkwa has some 52 km of haul roads. Its equipment comprises two Liebherr 994B 295 tonne excavators with 16 m3 bucket size, four Liebherr 994 (200) 230 tonne excavators with 14 m3 bucket size and three Liebherr 984 excavators each with a 120 tonne capacity and a bucket size of 7 m3. For rehandling Tarkwa uses four Cat 992G FELs, with a bucket size 11.5 m3, and one Cat 966 FEL machine with bucket size of 3.6 m3. There are 27 Cat 785 haul trucks, two Cat 773 water trucks, four Cat D9 dozers and five D10 dozers, both of these types being track dozers. There are two Cat 844 wheel dozers, and four Cat 16H graders, as well as 13 Pantera 1500 drill rigs. Truck allocation is done by a GPS assist modular mining feel management system. The mine plans to increase the fleet with a further six Cat trucks and two Liebherr excavators.
The operators of the mobile equipment at Tarkwa are considered among the best in the world. The loading rate is 1,680 tonnes an hour, and the mine aims for 1,730 tonnes an hour. Explosives for the drill blast and haul operation are supplied by AEL in the form of a down-the-hole service with six metre benches blasted and loaded out on three metre flitches. Only some 6% of the ore is free dig, where no blasting is required. The mine uses over 2,000 tonnes of explosives a month. The life of mine stripping ratio is 3.3:1.
Ore is hauled to three different crushers, two at the North plant and one at the South plant. The North stockpile is divided into four piles, these being heap leach medium grade and high grade stockpiles and CIL medium and high grade stockpiles. The CIL process plant takes in a head grade of 1.8 g/t and has a gold recovery rate of 96.3%. The mine boasts the largest SAG mill in West Africa and while it does sacrifice some flexibility as a result it means less spares than would be the case had it gone for two smaller mills.
The North heap leach plant has a crushing capacity of 29,000 tonnes a day, in a three stage circuit that reduces 80% of the ore to less than 14 mm, while the South heap leach plant can crush 18,000 tonnes a day in a four stage crushing plant that reduces 80% of the ore to less than 10 mm.
The North heap leach area has 53 million tonnes under leach on 22 cells on four lifts, with each cell 75 metres wide and having an 11 metres lift height. Each subsequent lift is in pyramid form and ultimately there will be six lifts for a height of 70 metres. Ore is delivered to the pad stacker by a string of mobile conveyor belts known as Pregnant solution is collected from heap leach pads at Tarkwa. grasshoppers and the pad stacker continually stacks the ore without interruption to production as it moves. Tarkwa has some 8 km of conveyor belting, made up of some 27 long belts and 50 smaller grasshoppers. Ore is stacked on a 400 mm bed of coarse ore for drainage from each lift, and about 60% of the pregnant solution is collected in the interlift layer, the balance passing to lower lifts for extended leach time. The emitter tubing is designed for an application rate of 10 litres/hour/m2 of leach solution. Some 100,000 m3 of leach solution containing 3 g/t litre (3,000 ppm) of cyanide is pumped daily to the North leach pads and 40,000 m3 to the South leach pads. Cyanide strength is about 250 ppm in the intermediate solutions and 25 ppm in the barren solutions, from which the gold has been removed by pumping the pregnant solution through beds of activated carbon. Some 40,000 m3 of pregnant solution at a typical grade of 0.75 ppm gold is processed daily at the North area and 22,000 m3 of solution at the South area. Overall the North and South heap leach plants process 16.1 million tonnes a year at a grade of 1.2 g/t and a recovery rate of 67%. The average heap leach production cost is US$3.27/tonne compared with US$5.03/tonne of ore processed at the CIL plant.
Tarkwa is essentially a large excavation project and so has some 35% of its total costs made up of fuel costs. The mine has been optimised to be a model volume driven operation with its cost per tonne of ore processed being the cheapest in West Africa and fuel efficiency has improved by about 8% since Gold Fields opted for owner operator mining. The trucks use 93 litres of diesel an hour and the excavators use 154 litres an hour, so the mine’s daily fuel consumption comes to 110,000 litres a day.
The mine has managed to avoid its machines being stranded through a lack of tyres for its fleet, a problem that has pervaded the industry globally, and it has sufficient tyres stored up to see it through the next 18 months. With the maintenance and upgrade of the haul roads a tyre at Tarkwa, which costs US$15,000, has a forecast life of 5,800 hours. The average haul distance is 2 km, with the furthest pit being 7 km from the plant. The operational cost of mining and hauling the material to the plant is about US$0.96/tonne.
With the cost increases in commodities such as cement, steel, cyanide and diesel matching those of the gold price in 2005 the margins of Tarkwa remained constant until recently when the gold price began to accelerate rather dramatically. Tarkwa spends over US$5 million a month on commodities, of which about half is spent on diesel, with sodium cyanide the next biggest contributor, followed by cement, tyres, and grinding balls. The cost of diesel alone went up 14% in 2005 from US0.68/l to US0.84/l, and overall the increase of commodity prices has increased costs by US$4 million a month. Total cash costs at Tarkwa are US$280/oz of gold produced.
Though it now does its own mining Tarkwa makes use of a local contractor, which has a US$30 million contract over five years to do the soft loading, including rehabilitation and civil works loading. The mine spends about 14% of its total costs on other contractors in Ghana and results in the employment of an estimated 1,700 contractors. Tarkwa paid the government of Ghana more than US$23 million in direct tax in 2005, up from 9.7 million in 2000, another indication of the successful growth of the operation. Tarkwa, which originally had to compensate or re-accommodate 20,000 people to undertake the surface mining on its 208 km² mining lease area, also spends US$1/oz on the local communities in Ghana, which is equivalent to 0.5% of pre-tax profit, and has invested in programmes to create sustainable employment beyond mining, such as the farming of oil palms.MRA