The Obuasi gold mine located in the Ashanti region of southern Ghana is being redeveloped into a modern and mechanised mine for longer operational life.
Owned and operated by Africa’s biggest gold miner AngloGold Ashanti, the Obuasi underground mine was placed under care and maintenance in 2016.
The Obuasi redevelopment project was officially launched in January 2019 after securing approval from the Government of Ghana in June 2018, while the first gold from the redevelopment project was poured in December 2019.
The Obuasi gold mine redevelopment project is being implemented in two phases with an estimated investment of £423m ($545m).
Phase one was completed in 2019, while phase two is scheduled for completion by the end of 2020.
Daily ore processing capacity under phase one is expected to be ramped up to 2,000 tonnes (t) in 2020, which is expected to be further increased to 4,000t per day from 2021 onwards following the completion of phase two redevelopment works.
The redevelopment project targets more than 30 million ounces (Moz) of undeveloped gold resources still present in the Obuasi gold deposit.
The average annual production from the redeveloped Obuasi mine is expected to be 400,000 ounces (oz) of gold at an average head grade of 8.1g/t during the initial ten years of operation which is expected to be increased up to 450,000oz during the next decade.
Location, geology, and mineralisation
The Obuasi mining operations are located on a 201km2 concession area in the Ashanti region of southern Ghana, approximately 60km south of Kumasi and 260km north-west of Accra.
The Obuasi gold deposit is situated within the north-central portion of the Ashanti Volcanic Belt which is predominantly made up of Birimian Supergroup sedimentary and volcanic rocks, Tarkwaian Group sedimentary rocks and other granitoid intrusions.
The ore body contains quartz vein as well as sulphide ore. Gold mineralisation is found hosted within graphite-chlorite-sericite fault zones associated with silica, carbonate and suphide hydrothermal alteration.
Obuasi gold reserves
The proven and probable gold reserves at Obuasi are estimated to be 5.8Moz, while the measured and indicated gold resources are estimated to be 34.05Moz.
Obuasi gold mine redevelopment background
The operational history of the Obuasi mine dates back to 1897. AngloGold became the owner and operator of the mine after acquiring Ghana's Ashanti Goldfields in a £600m ($1bn) deal in 2004.
The gold mine was placed on limited operation for the processing and clean-up of tailings, as well as maintenance of surface and underground infrastructure towards the end of 2014. The mine produced 243,000oz and 53,000oz of gold in 2014 and 2015, respectively.
In the meantime, AngloGold Ashanti initiated an optimised feasibility study (OFS) for Obuasi which was completed in June 2016.
A large number of illegal miners intruded into the mine complex in February 2016 and the illegal miners were forced to vacate the site with the support of the government in October 2016.
AngloGold Ashanti put the mine under care and maintenance in the same year and started negotiations with the government for the mine development in 2017.
The redevelopment plan, as well as a tax concession agreement, was ratified by the Ghana Parliament in June 2018, while key environmental permits for the project were secured in the same year.
Obuasi mine redevelopment details
Phase one of the mine redevelopment project involved mine rehabilitation as well as the demolition and refurbishment of parts of the processing plant.
Phase two involves refurbishment of the underground materials handling system, shafts and ventilation system as well as the upgrade of the processing plant to double its processing capacity.
The processing plant expansion includes a new primary crusher, a semi-autogenous grinding (SAG) and a ball mill circuit, a carbon regeneration facility, a new tailings storage facility and a new gold room.
Mining and processing methods
Obuasi is an underground mining operation involving stope mining (both longitudinal and transverse) as well as sub-levels caving methods.
The operating depth for the mine extends up to 1,500m-deep beneath the surface. The underground workings are reached through two main vertical shafts as well as one decline access. An additional shaft is used for the mine dewatering purpose.
The sulphide ore from the mine undergoes bacterial oxidation for mineral extraction at the South treatment plant at the mine site, while tailings are deposited at the South tailings storage facility located approximately 4km northwest of the ore treatment plant.
The Obuasi mine can be accessed through paved roads from Accra, with a travel time taken up to five hours.
The electricity for the project is supplied through three 33MVA lines by Volta River Authority (VRA), the main generator and supplier of electricity in Ghana.
The water for the processing plant operations is sourced from the Jimi River that runs 7.5km away from the project site.
Underground Mining Alliance (UMA), a joint venture between African Underground Mining Services (AUMS, 70%) and Ghanaian mining contractor Rocksure International (30%), was awarded a £292m ($375m) contract for providing full suite of underground mining services at Obuasi for a period of five years in October 2018.
AUMS was formerly a 50:50 joint venture between Ausdrill and Barminco, a subsidiary of Australia-based diversified global mining services group of Perenti.
AUMS became an operating business unit of Barminco after it acquired Ausdrill in November 2018.