The Riacho dos Machados gold deposit is considered to be a classic mesothermal orogenic gold deposit in a sheared and deformed Archean to Proterozoic age greenstone belt sequence. It is comprised of metamorphosed volcanic-sedimentary rock units intruded by slightly younger post-tectonic igneous bodies.

The principal host for the gold mineralization is the quartz-muscovite schist of the RMG. The mineralization occurs in a belt of hydrothermally altered rock developed along a district-scale shear zone that extends almost 30 km along a N20°E strike direction and dips 40° to 45° east. The mineralization has a typical amphibolite facies mineral association which is progressively altered to greenschist facies assemblage. The gold mineralization occurs as “stacked” tabular horizons that are mostly concordant with the principal rock foliation (shear zone). These tabular zones typically consist of a main zone which may be sided by a thinner footwall or hanging wall zones, separated by three meters to ten meters of unmineralized rock. Continuity along strike and at depth is good with gold mineralization occurring continuously over a 2,000 m strike length and up to 1,000 m down dip.

Gold grades in the mineralized zone are closely related to sulphide content, especially arsenopyrite. Gold occurs as microscopic native-gold grains typically finer than 400 mesh (37 µm). The gold grains occur interstitial to quartz grains, muscovite grains, and sulphide grains, and also as inclusions in arsenopyrite, and less commonly in pyrrhotite, quartz-veinlets, tourmaline, and pyrite.

The following minerals and features, listed from high to low importance, are noted as indicators of gold mineralization:

-Arsenopyrite (both anhedral and euhedral needles);
- Pyrrhotite;
- Abundant quartz veinlets (sheared into foliation plane);
- Pyrite;
- Crenulation folding;
- Tourmaline veins (fine-grained massive intergrown).

The arsenic content of the mineralization is relatively high, with an average of approximately 4,000 ppm for samples greater than 1.0 g/t Au. Silver content is very low, with the average Ag/Au ratio of 0.5:1.0 for samples with a gold concentration of greater than 1.0 g/t. Antimony, copper, lead, and zinc are not commonly anomalous.

The gold mineralization is closely related to the structural fabric. The mineralizing fluid was probably channelled upward in the thrust-related shear zones with minor or local lateral escape into intersecting shear zones. Brittle deformation at the RDM deposit is limited to poorly defined crossfaults that may have anomalous geochemistry but do not host gold mineralization.

The area presents the occurrence of graphite, which influences the metallurgical behaviour of the ore. Graphite occurs in shear zones filling intrafolial planes, ranging from 2 to 20 cm thick, and in the form of boulders, with lenticular geometry or symmetrical boudins associated with quartz veins, with thicknesses of up to 1.5 m. They have dark grey and black coloration and low hardness, and due to graphite's lubricating characteristics, act as zones of geomechanical weakness. Graphite is in nonmineralized areas, associated with domains of marginal or waste contents, or in mineralized areas with the presence of high sulphidation.

Conventional open pit mining methods are employed at the Mine, including drilling, blasting, loading, and hauling. Waste mining is performed by a contractor, while ore mining is conducted using an owner’s fleet.

The operation’s strip ratio is relatively high, at approximately 6.9:1 (waste:ore). The mining rate will be relatively constant throughout the life-of-mine (LOM); total daily waste material movement is estimated at 70,000 t/d and ore mining at 7,890 t/d (2.88 Mt/a).

Current pit bottom elevations of the north and south ends of the open pit are approximately 717 and 705 masl, respectively, and the crusher elevation is 865 masl. Surface rights cover a sufficient area to locate all the mining infrastructure required for the LOM, including waste management facilities (WMF), TSF, WSF, and processing plant.

Pit dewatering is carried out using in-pit sumps and perimeters wells.

The mine design bench height is 12 m; however, mining occurs on 4 m lifts in waste and 3 m lifts in ore.

The final pit design is approximately 1,750 m long and 750 m wide, with elevations that range from 600 to 900 masl. Final pit exits for the south and north ramps are at approximately 860 masl. The crusher dump pocket is less than 500 m horizontal distance from the pit exit. Adequate buffers exist around the open pit for possible future expansion, for example if the gold price increases. Some condemnation holes have been drilled in the infrastructure and waste dump areas to ensure economical mineralization is not sterilized.

The pit slopes used in the mine design for material on the west side (footwall) of the pit are up to 60°, and hanging wall bench face angles are up to 85°. Operating practices can have an impact on pit slopes, and careful blasting practices will be required near the final pit slopes. Final pit slopes have begun to be established on the west side (footwall) of the pit. There is not much potential for increasing the ultimate pit slopes, based on field investigation and operational challenges.

Bench-by-bench, monthly production schedules have been developed to identify ore types, waste removal, and stripping requirements. A lower stripping ratio will be realized in the later years. The granularity of the mine planning process is appropriate for this stage of production. The primary reason for this level of detail of the mine scheduling is the need to have accurate information for production and budgeting.

Crushing
Briefly, the process entails dumping ore into a feed hopper from which a variable-speed apron feeder reclaims the ore onto a vibrating grizzly with openings set at 152 mm. The oversize from the grizzly feeds a Metso C140 primary jaw crusher with a closed side setting of 130 mm. The undersize from the grizzly is combined with the crushed product from the jaw crusher and fed onto a conveyor belt. The belt feeds the primary double-deck vibrating screen. The first deck opening is set at 56 mm; the second is set to 19 mm. The undersize (<19 mm) is conveyed to a fine-ore bin via conveyor belt; the oversize feeds a secondary Metso HP500 cone crusher with a closed side setting of 24 mm. The product from the secondary cone crusher is transferred to a secondary double-deck vibrating screen with openings set at 34 and 19 mm. The undersize from the secondary screen is conveyed to the same fine-ore bin. The oversize feeds a tertiary Metso HP500 cone crusher with a closed side setting of 13 mm.

The product from the tertiary crusher returns to the feed of the secondary screen, thus operating the tertiary crusher in a closed circuit.

The nominal rate of the crushing circuit is designed to be 408 t/h. The plant can potentially be expanded to 475 t/h with the addition of another tertiary crusher and another tertiary screen.

The ore is reclaimed from the fine-ore bin by four feeders with a nominal rate capacity of 342 t/h.

The ore from the fines bin can be fed to an emergency stockpile with a capacity of 10 kt. The emergency stockpile provides feed to the ball mill during crushing circuit shutdowns.

Grinding
The crushed ore is transferred from the ore storage bin to the ball mill feed conveyor. The water mixes with the ore in the ball mill feed chute and the hydrocyclone underflow feeds the ball mill through the feed chute, closing the circuit.

The mill has a diameter of 6.7 m and is 11.1 m long, with twin 4500 kW GE motors. After the power line was constructed, the steel load was increased from 19% to 27% (the current operational ball charge). The maximum steel ball charge is 33%.

The ground slurry passes through the trommel screens to remove ball chips. The ball mill BWi of open pit ores averages 17.4 kWh/t, and the ball consumption is in the range of 1,300 g/t. The ball mill operates at a slurry solids content of approximately 72%. The ground slurry is pumped to a battery of ten Weir Cavex 500 hydrocyclones. Nine hydrocyclones are operational, with one on standby, for spigot and vortex finder changes and to do maintenance. The hydrocyclones operate with an opening set at 90 mm diameter spigot and 140 mm diameter vortex finder. The hydrocyclone feed slurry density is controlled at approximately 40% solids, resulting in an overflow with 30% solids and a particle size distribution of P80 74 µm (the objective is to achieve P80 53 µm). The underflow solids content is approximately 75%.

The processing plant was designed to process 7,000 tpd (2.55 Mtpa). The plant can potentially be expanded to 9,000 tpd (3.28 Mtpa) with completion of the power line, addition of another tertiary crusher, installation of a larger mill discharge pump, and installation of larger conveyor belts.

The overall process flowsheet consists of:
• Three-stage crushing circuit;
• Ball mill grinding, in closed circuit with hydrocyclones;
• Thickening to produce a leach feed of up to 39% solids;
• CIL circuit;
• Cyanide detoxification;
• Zadra pressure stripping of the loaded carbon;
• Electrowinning of the carbon eluent;
• Casting of gold bars in an induction furnace.

Carbon in Leaching
The CIL tanks are fed with a volume of about 700 m3/h. The circuit consists of 10 tanks in total; each holds 1,924 m3 and is 13.5 m in diameter and 14.5 m high. The first tank is used as a pre-lime and pre-aeration tank.

A process improveme ........