Gold mineralisation is associated with, and occurs within, graphite-chlorite-sericite fault zones. These shear zones are commonly associated with pervasive silica, carbonate and sulphide hydrothermal alteration and occur in tightly-folded Lower Birimian schists, phyllites meta-greywackes, and tuffs, along the eastern limb of the Kumasi anticlinorium.

Two main ore types are mined, namely quartz vein and sulphide ore. The quartz vein type consists mainly of quartz with free gold in association with lesser amounts of various metal sulphides containing iron, zinc, lead and copper. This ore type is generally nonrefractory. Sulphide ore is characterised by the inclusion of gold in the crystal structure of arsenopyrite minerals. Higher gold grades tend to be associated with finer grain arsenopyrite crystals. Sulphide ore is generally refractory.

The mine is located within the Obuasi concession area in south-western Ghana along the north easterly-striking Ashanti volcanic belt. The deposit is one of the most significant Proterozoic gold belts discovered to date. The Ashanti belt predominantly comprises sedimentary and mafic volcanic rocks, and is the most prominent of the five Birimian Supergroup gold belts found in Ghana. The belt is a 300km wrench fault system that propagated from Dixcove in the south-west to beyond Konongo in the north-east.

The Birimian was deformed, metamorphosed and intruded by syn- and post-tectonic granitoids during the Eburnean tectonothermal event around two billion years ago. Folding trends are dominantly north-northeast to north-east. Elongate syn-Birimian basins developed between the ridges of the Birimian system and these were filled with the Tarkwaian molasse sediments made up primarily of conglomerates, quartzose and arkosic sandstones and minor shale units. Major faulting has taken place along the same trends.

Obuasi Gold Mine is located in the municipality of Obuasi, in the Ashanti region of Ghana, about 240km northwest of the capital Accra and 60km south of Kumasi.

Obuasi is an underground operation with the main accesses into the mine consisting of shafts and a single access decline with interlevel development of between 15 and 30m. The Obuasi Deeps Decline, which is situated at the southern end of the mine, is designed to extend to a depth of about 1500m. The Obuasi Underground operation employs mostly Long Hole Open Stoping (LHOS) mining method for ore extraction. LHOS is a highly selective and productive method of mining that can be employed for orebodies of varying thickness and dips. The three main distinct variations of the LHOS used at Obuasi are Longitudinal Retreat Stoping (LRS), Longitudinal Open Stoping (LOS) and Transverse Open Stoping (TOS). The Blind Upper Stoping is a form of LRS or TOS used for partial sill pillar recovery. Underhand drift and fill (UHDF) which is used at Côte d’Or with extensive historical cut and fill mining is planned to be converted into LHOS.

At Obuasi, underground operations were suspended in May 2021 following a geotechnical event and fatality. A detailed review into the incident and its causes was followed by a thorough external review of future mining fronts covering the mine design, schedule and ground management plan. Underground mining activities remained suspended until mid-October 2021 when stoping activities restarted.

Since then, the restart plan, and in particular tonnage delivered to the mill, have tracked to schedule with the processing plant achieving 2,000 tonnes per day in January. The safe ramp-up to the full mining rate of 4,000 tonnes per day is expected by the end of June 2022.

A comprehensive series of protocols has been introduced to supplement existing operating procedures at Obuasi and they are expected to add about $10 to $20 per tonne to the mine’s operating costs, or about $50/oz. External consultants will continue their review of future mining areas. Areas of assessment completed include Sansu, Block 8 lower and the decline.

In terms of infrastructure, the work needed to support the ramp up to 4,000tpd is now complete (Phase 2). Phase 3 – which relates principally to extended capital expenditure to refurbish existing infrastructure around the KMS shaft and runs to end 2023 – is also proceeding according to schedule. This includes upgrading the KMS shaft and materials handling system, a new ventilation shaft, underground pump stations and refurbishment of the BSVS sub-shaft.

The milling circuit consists of two semi-autogenous (SAG) mills, SAG 1 and SAG 2, in open circuit and a Ball Mill in closed circuit with a cluster of cyclones. The SAG and Ball mills discharge is pumped to a set of primary cyclones whose under flow reports to the Knelson concentrators for free gold recovery while the overflow is screened, conditioned and subjected to conventional flotation.

Significant progress has been made since the first gold pour with production currently at 2,000tpd and planned for 4,000tpd in 2021. [2020 MR, 44]

The Sulphide Treatment Plant (STP) has a design capacity of 274 tons per hour (tph) of refractory transitional and primary sulphide sulphur ores yielding 30tph of sulphide sulphur concentrate for processing in its BIOX® facility. The treatment process flow includes primary crushing, semi-autogenous/ ball milling, flash flotation and conventional flotation, bio-oxidation and carbon in leach (CIL) gold recovery. Gravity Recoverable Gold (GRG) is recovered at the Mill and BIOX® Circuits via Knelson concentrators.

Flash flotation cells are fed with the Knelson concentrator tailings, to recover freshly liberated coarse sulphides which are further milled in a regrind mill and classified to obtain a finer material as product. Tailings of the flash cells gravitate into the ball mill whose discharge joins the SAG discharge.