The gold operation at Obuasi, 200km north west of Accra in Ghana, was the major asset of Ashanti Goldfields Company (AGC), established in London in 1897. AGC started underground mining in 1907; Lonrho purchased AGC in 1969; and the Ghanaian Government reduced its holding in 1994, leaving Lonrho (now Lonmin) with a 33% stake and corporate plus private investors holding the remainder.
In the mid-1980s, the company launched a capital-intensive mechanisation programme that was completed during 1998/9. However, these efforts did not achieve the targeted gold output of 550,000oz/y to 650,000oz/y at a cash cost below $200/oz.
AGC reduced staffing at Obuasi from approximately 10,000 in 1996 to less than 6,600 in 2003 and invested in underground development and plant – an investment of US$37.6m during 2003, for instance. Even so, the management team formed after the April 2004 merger with AngloGold to form AngloGold Ashanti regarded the operation as being under-capitalised.
GEOLOGY AND RESERVES
The Obuasi deposits occur along a zone of intense shearing and faulting within precambrian greenstones. Mineralisation comprises two main types: quartz veins containing high-grade free gold and the main sulphide ore in which narrow veins contain gold trapped within arsenopyrite.
At end-2003, proven and probable reserves totalled 56.8Mt at 6.19g/t gold, while measured and indicated resources totalled 117.1Mt grading 8.71g/t gold. The amount of developed material was inadequate and the reserves were also restated – in light of intensive drilling and without remnant blocks and some surface reserves – at the end of 2004. Proved ore totalled 14.2Mt grading 2.95g/t, probable reserves were 36.3Mt at 7.05g/t, and measured plus indicated resources stood at 108.6Mt at 8.38g/t, giving 130.4t of contained gold in all.
In mid-2006 Anglogold Ashanti cited proved ore reserves at 10.7Mt and measured resources at 60.5Mt. The feasibility of developing the Obuasi Deeps deposits is the subject of intensive studies as exploiting this ore could extend the mine’s life by 35 years.
The mine has worked surface and underground mineralisation along an 8km north-south strike length but mining is now mainly underground, extending to depths of 1,600m. The mainly flat-back, cut-and-fill stopes used to exploit several ore blocks are being converted to mechanised open stopes. There are 15 shafts but the main hoisting shafts are modern – George Cappendell, Kwesi Mensah, Kwesi Renner (Stonewall) and Sansu. Obuasi has made extensive use of raise boring in developing the underground mine and has one of the world’s larger fleets of raise boring machines. During 2002–03, AGC upgraded the Brown sub-vertical shaft and bored a new ventilation shaft. Total hoisting capacity will remain at 6.2Mt/y between the Kwesi Renner, Kwesi Mensah and Sansu shafts.
The shafts are linked by an electric rail haulage system serving six dump stations on the 41 level. Designed by Nordic Mining Technology, this system comprises two eight-car trains of 14m³ side-hinged cars, hauled by 15t locomotives supplied by British manufacturer Clayton. The dump stations are fed by conveyors from ore passes to the haulage level. Late in 2004, Obuasi took delivery of new LHDs and haul trucks from Atlas Copco.
During the 1990s, AGC boosted output by surface mining and surface tonnage overtook underground in the middle of the decade. Having ceased surface mining during 2000 to contain costs, Ashanti opened the Homase pit in 2002, completed mining it during 2003 and started two more surface operations at the Kunka and Adubrem deposits. During 2003, underground production totalled over 2.3Mt, surface mining added 476,000t, and tailings recovery almost 2Mt.
Up to 7Mt/y of Obuasi ores have been processed in five treatment plants, but in 2000 throughput was reduced to 5.3Mt, with Pompora (built 1947) and the Oxide Treatment Plant (OTP) put on care-and-maintenance, while the Heap Leach plant did not operate at all. The Sulphide Treatment Plant (STP, commissioned 1994), which incorporates a large bio-oxidation facility and modern gravity separation circuits, treated 2.33Mt at 7.0g/t in 2003 with a recovery of 83.4%. With the restart of surface mining in 2002, the oxide plant was brought back into action and the Tailings Treatment Plant (TTP) increased throughput. However, by 2004 the oxide plant was only being used occasionally, and will be shut in 2006.
With renewed surface mining and increased tailings throughput, the three active process plants treated a total of 5.22Mt of ore in 2003, yielding 513,163oz at an average cash cost of $217/oz. However, for much of 2004 output was hampered by insufficient trackless-mining equipment and unplanned mill shutdowns.
During the period from May 2004, when AngloGold Ashanti started work, to the year-end, gold production totalled 255,000oz from 2.6Mt of ore, at total cash costs of US$305/oz. The company reported an increase in gold production to 391,000oz for the whole of 2005, at total cash costs of US$345/oz and total production costs of US$481/oz. The target for 2006 was 407,000–423,000 oz at a total cash cost of US$319–332/oz.