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.

Mineralization occurs in the hydrothermal/shear zone and gold grades are closely related to the sulfide content, especially arsenopyrite. Gold occurs as microscopic native-gold grains - typically, finer than 400 mesh (37 microns) - at contacts between recrystallized quartz grains, muscovite grains and as inclusions in arsenopyrite, and less commonly in pyrrhotite, quartzveinlets, 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 quite high with an average of approximately 3,935 ppm As for samples greater than 1.0 g/t Au. Silver content is very low with the average Ag/Au ratio equal to 0.5 for samples with a gold concentration of greater than 1.0 g/t Au. Antimony, copper, lead, and zinc are mostly non-anomalous.

The gold mineralization is closely related to the structural fabric. The main mineralizing fluid upward flow was possibly along the thrust movement direction with lateral escape along interconnecting sub-horizontal structural fabric. Brittle deformation at the Riacho dos Machados gold deposit is limited to poorly defined cross-faults which may have anomalous geochemistry but do not host gold mineralization.

Conventional open pit mining methods are employed at the MRDM open pit including drilling, blasting, loading, and hauling.

The bench height is 12 m, however, mining occurs on six meter lifts (flitches) in waste, and three meter lifts in ore. Road widths are 21 m. Berm widths normally are six meters in the oxide and transition ore, eight meters in the hanging wall fresh rock, and 12 m in the footwall fresh rock.

The processing plant was designed to process 7,000 tpd, with the potential to expand to 9,000 tpd (3.28 Mtpa) with some modifications, using three-stage crushing, ball mill grinding, CIL, sulfur dioxide-air cyanide detoxification, and a gold adsorption, desorption, and recovery (ADR) plant.

The crushing circuit includes a three-stage crushing plant with an availability based on 80% for the crushing circuit. ROM ore is dumped into a feed hopper. A variable speed apron feeder reclaims ore from the feed hopper onto a vibrating grizzly with openings set at 152 mm (6 in). Oversize from the grizzly discharges into the primary Metso model 125 jaw crusher. Undersize from the grizzly combines with the discharge from the jaw crusher onto a conveyor belt. The conveyor belt feeds the primary double deck vibrating screen. Undersize from the screen is conveyed to a fine ore bin via a conveyor belt. Oversize from the screen is fed to the secondary Metso HP-500 cone crusher. Product from the secondary cone crusher is transferred to the vibrating single deck secondary screen. Undersize from the screen is conveyed to the fine ore bin. Oversize from the screen feeds the tertiary crusher. Discharge from the tertiary crusher is returned to the feed of the secondary screen, thus operating the tertiary crusher in closed circuit.

Ore was originally reclaimed from the fine ore bin by four apron feeders and designed to be fed to the ball mill at the rate of 310 tph. Since the fine ore bin did not have sufficient capacity to operate the plant efficiently, modifications were made to use one of the apron feeders to feed a parallel fine ore storage system. This system consists of a transfer conveyor, radial stacker, and a reclaim feeder. The reclaim feeder is fed by a rubber tired front end loader to place the crushed ore on mill feed conveyor.

Ore is transferred from the fine ore storage to the ball mill feed conveyor. Water mixes with the ore in the feed to the ball mill. The mill is 6.7 m diameter by 11.1 m long with twin 4,500 kW motors. Slurry discharges from the ball mill into the cyclone feed box where the slurry is pumped to the cyclones. Underflow from the cyclones returns to the ball mill feed. The cyclone overflow is the product from the comminution circuit. It flows by gravity to a linear trash screen. The underflow from the trash screen flows by gravity into a thickener feed box. The slurry is pumped to slurry holding tanks that feed the CIL circuit. The thickener overflow solution is returned to the process water tank.

The CIL circuit includes two leach tanks followed by six CIL tanks. The slurry flows by gravity from the first tank to the last. Cyanide and milk of lime are added to the circuit to obtain the correct pH and to extract the gold from the ore. Activated carbon is advanced counter currently through the circuit from the last tank to the first. From the first CIL tank, the activated carbon is pumped to a loaded carbon tank. From the tank it is transferred to the ADR plant.

A sulfur dioxide-air cyanide destruction circuit is provided to reduce the cyanide concentration in the tailings to less than 1.0 ppm of total cyanide in order to meet discharge criteria in the International Cyanide Management Code.

Gold is recovered from the activated carbon in the ADR plant which is designed to handle carbon in three tonne batches. Loaded carbon is transferred from the loaded carbon tank to a hydrochloric acid wash tank. After acid ashing, the carbon is rinsed, neutralized with caustic soda to increase the pH, and transferred to a vertical pressure strip vessel. In the vessel, cyanide-caustic solution is used to strip the gold from the activated carbon. The pregnant solution is stored in the pregnant solution tank. From there it is pumped to the electrowinning cells where the gold is plated onto cathodes. The sludge from the cathodes is removed by high pressure washing and filtered to remove excess water. The filter sludge is dried and smelted in an electric induction furnace to produce gold doré that is shipped off site for further refining and sale.

After the stripping circuit, the activated carbon is fed to the carbon regeneration kiln for removal of any organic contaminants in order to maintain the carbon activity required to adsorb gold from the CIL solution. After regeneration, the carbon is pumped to the last CIL tank.

The slurry discharging from the cyanide destruction circuit are the final tailings from the plant. They flow to a tailings pump box. From the pump box they are pumped to the TSF. The tailings dam is designed to be raised on an annual basis. An area of the TSF is available for water storage. A small dike isolates the water storage area from the rest of the TSF.