The Obuasi gold mine, located 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 1998-1999. 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. The mine is 100% owned by AngloGold Ashanti, which was formed after AGC merged with AngloGold in April 2004.
The Obuasi gold mine restarted production in December 2019 after a five-year suspension to undertake its redevelopment. As a result of the redevelopment, the mine is expected to operate for around 21 years with average gold production of 400,000oz a year.
Geology and reserves of the Obuasi deposits
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.
As of 31 December 2017, the Obuasi gold mine is estimated to contain proved and probable reserves of 20.28Mt graded 9g/t of gold (Au).
Mining at Obuasi
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.
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. In late 2004, Obuasi took delivery of new LHDs and haul trucks from Atlas Copco.
The mine produced 243,000oz in 2014, before being placed on limited operations to amend its programme of mining operations. An optimised feasibility study (OFS) was completed in 2016 and the mine was placed on care and maintenance in 2016 pending the start of the redevelopment project.
Obuasi project redevelopment
The Obuasi redevelopment project is being carried out in two phases and the first blast was conducted in February 2019.
The first phase includes project establishment, mine rehabilitation and development. It also involves upgrades to plant and infrastructure to facilitate a production rate of 2,000tpd during the first year of operation.
AngloGold Ashanti will undertake the refurbishment of the underground materials handling system, shafts and ventilation under phase two. Other works in the phase will include the construction of carbon regeneration, primary crusher, the semi-autogenous grinding (SAG)/ball circuit and a new gold room and tailings storage facility (TSF).
Ore processing at Obuasi gold mine
Up to 7Mtpa 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.
The existing process plants at the site are capable of treating both oxide and fresh material. The main plant can undertake flotation and BIOX treatment needed for underground refractory sulphide ore type.
The project will mine 2,000tpd during 2020 and increase to 4,000tpd by the end of the year.