The Paracatu deposit is a metamorphic gold system with finely disseminated gold mineralization hosted within metasedimentary rocks. Very fine and evenly distributed gold is disseminated throughout a thinly bedded, highly deformed phyllite of Upper Proterozoic age. The deposit is part of a northwest-southeast lineament of gold occurrences including Cabeça Seca and Luziânia.

Gold mineralization was introduced syn-tectonically as the result of metamorphic alteration during thrusting of the Morro do Ouro Sequence over the rocks of the younger Vazante Formation. Structural interpretation suggests that mineralization was precipitated within a high strain zone where silica and carbonate were depleted from host phyllites, resulting in an increase in graphite content that may have acted as a chemical trap, precipitating out gold and sulphide mineralization remobilized during the metamorphic alteration of the Morro do Ouro Sequence.

The deposit has extraordinary lateral and longitudinal continuity. The majority of exploration efforts have sought to better define the continuous longitudinal continuity of mineralized phyllites at depth west of Rico Creek and the lateral limits of the economic mineralization.

The Paracatu mineralization is subdivided into four horizons defined by the degree of oxidation, surface weathering and sulphide mineralization. These units are, from surface, the C, T, B1 and B2 horizons. Figure 7-6 presents the conceptual pre-mining weathering surface and establishes the relative relationship between the various zones. Mining to date has exhausted the C and T horizons. The remaining mineral reserves are hosted exclusively in the B1 and B2 horizons. For this geological model, the interpreted geologic profiles combines the C, T and B1 horizons into a single unit.

Type C mineralization occurs at surface and extends to a depth of 20 m to 30 m. Type C mineralization is completely altered with no remaining sulphides. It also features localized laterite development.

The T horizon is generally only several metres thick, and marks the transition from the C horizon to the B1 horizon.

The B1 horizon is dark in colour and carbonaceous, with less oxidation than the C horizon. Sulphides have been completely oxidized but some fresh sulphide material is visible in the quartz boudins.

B2 mineralization was originally described as unweathered or fresh mineralization with primary sulphides.

The contact between unmineralized host rock (Type A – Oxide and Fresh) and the various mineralized horizons is gradational, occurring over a 10 m thick interval that is characterized by arsenic values of 200 ppm to 500 ppm and gold values of up to 0.2 g/t.

The mineralization at Paracatu is indicative of metamorphic alteration of lower and sericite make up 80% of the rock mass. Carbon occurs in the form of a fine opaque dust disseminated within the individual sericite bands. Carbon content varies from 5% to 20%. Minor amounts of ilmenite, tourmaline, anatase, rutile and limonite are also commonly observed.

Petrography indicates that 60% to 90% of unoxidized phyllites were composed of variable amounts of quartz and sericite to produce a distinctive banding. Individual bands are typically less than two centimetres thick. The phyllites also contain up to 20% carbonate (calcite and ankerite). Fine grained carbon was also observed in the less weathered samples. Accessory minerals included muscovite, biotite, albite, tourmaline, ilmenite, chlorite, zircon and rutile.

The amount of sulphides present does not typically exceed 3% to 4%. The most common sulphides observed are arsenopyrite, pyrite and pyrrhotite. Galena is relatively common and may be accompanied by sphalerite. Chalcopyrite occurs locally in fractures within the main sulphide minerals listed above. The sulphides typically occur as individual crystals or coarse crystalline aggregates. Arsenopyrite is the most common sulphide and occurs as a fine grained (<1 mm) to coarse grained (>3 mm) aggregates. Crystals up to one centimetre in size are not uncommon. Arsenopyrite crystals increase in size to the southwest.

The mineralization at Paracatu exhibits distinct mineralogical zoning with the arsenopyrite content increasing towards the center and west and in the zones of intense deformation. Gold grade increases with increasing arsenopyrite content.

Pyrrhotite occurs in the western part of the deposit and gold grades are elevated where pyrrhotite increases. There is evidence for a pyrrhotite-rich deposit at depth, which has been intersected in a number of drill holes.

The deposit formation model proposed for Paracatu suggests that gold and arsenopyrite were introduced concurrently during the deformation event.

The quartz boudins typically observed in the higher grade portions of the Paracatu deposit represent original, attenuated quartz veins. The boudins crosscut bedding at a shallow angle. The boudin thickness likely represents the original thickness of the quartz veins, which have been stretched considerably, implying moderately high to very high strain in the system (Holcombe, 2005). The boudin formation has been interpreted as a two-stage process. First, quartz veins are emplaced early in the deformation event. As stress builds, these veins are folded, boudinaged and separated. Mineralized boudins are parallel to foliation. A final barren quartz stockwork phase cross cuts foliation in the low grade hanging wall.

Gold occurs either as free gold or electrum. Microscopic analysis indicates that 92% of the gold at Paracatu is free-milling with less than 8% encapsulated by sulphide grains or silica.

The Paracatu Mine is a traditional open pit truck/shovel operation.

The production fleet consists of ten Caterpillar 777 haul trucks and thirteen Caterpillar 793 haul trucks. In 2014, the Caterpillar 793 fleet will expand to fifteen haul trucks.

The loading fleet consists of three Caterpillar 992 loaders, three Caterpillar 994 loaders and two Bucyrus 495HD shovels.

The diesel-powered drill fleet is comprised of two Atlas Copco DM45 rigs, two Atlas Copco DM50 rigs, one Bucyrus 35R and one Atlas Copco ROC D7. Shovel and drill fleets will be replaced with comparable units upon retirement.

Paracatu has two mineral processing plants known as Plant I and Plant II with extraction of gold using gravity/ flotation/ carbon-in-leach (CIL) recovery processes.

Plant I treats the softer near-surface B1 ore at a design throughput of 20 Mt/a, while Plant II treats the harder B2 ore at a design throughput of 41 Mt/a.

In Plant I, ore is crushed through two stages and ground in ball mills prior to gold recovery by jigs and flotation. The concentrate is treated by gravimetric methods first and the coarser gold is recovered. The flotation and gravity concentrate is then leached with cyanide in a carbon-in-leach (CIL) circuit, followed by carbon elution and electrowinning to recover gold which is then smelted to form gold bars.

Currently, Plant II consists of an in-pit MMD crusher, a 1.8 km conveyor to a covered stockpile area, an 11.6 m diameter SAG mill, and four ball mills. The ore recovery process uses gravity flotation to produce concentrate which is leached with cyanide in a CIL circuit, followed by carbon elution, electrowinning and smelting into gold bars.

The primary crusher is located within the open pit and is scheduled to operate 18 hours per day. Run-of-mine (ROM) ore is delivered by 240 t rear-dump, haul trucks to the 480 t crusher dump hopper. An apron feeder withdraws ROM ore from the dump hopper and feeds it at a controlled rate of approximately 7,000 t/h to an MMD 1300 Series Twin Shaft Sizer. The MMD sizer crushes the rock from a maximum size of 1,300 mm to a nominal size of 350 mm and discharges material directly onto a sacrificial conveyor, which in turn discharges onto the overland conveyor.

The Plant II grinding circuit is designed to operate at 5,400 t/h with an availability of 92%. The grinding circuit consists of one 11.6 m diameter by 6.7 m long Effective Grinding Length (EGL) 20 MW gearless drive SAG mill, two 7.3 m diameter by 12.0 m long EGL 13 MW ball mills and two 8 m diameter by 12.8 m long EGL 15 MW ball mills. Ball mills are equipped with dual pinion gear drives. The SAG mill operates in closed circuit with a trommel screen and vibrating screen, and the ball mills operate in closed circuit with hydrocyclones and flash flotation cells. Plant capacity has been selected to give a nominal flotation feed grind of 80% passing 75 µm.

The flotation circuit consists of rougher flotation followed by desulphurization cells and cleaning cells. Cleaner tails are recirculated to the rougher flotation cells and the cleaner concentrate is ground using a vertical stirred regrind mill operating in closed circuit with hydrocyclones. Rougher tailings are discarded to the tailings mix tank.

Concentrate thickener underflow is pumped to a new trash screen located over the pre-aeration tank at the CIL plant. The cleaned slurry passes through the screen to the 750 m3 agitated pre-aeration tank where milk of lime slurry is added. This tank overflows to four 750 m3 CIL tanks. The flow from the fourth tank splits into two lines that flow into four 300 m3 CIL tanks. Carbon is retained in each tank by swept North Kalgoorlie Mines (NKM) type screens. Cyanide and lime from existing make-up systems are staged and added to each tank train. On an intermittent basis, loaded carbon is pumped countercurrent to the slurry flow in order to increase the gold loading. Loaded carbon is removed from the first CIL tank, drained and washed on two vibrating screens and transferred to the loaded carbon bin. A carbon safety screen is located over the CIL tails pump box.

The pregnant and barren eluate tanks are located in an area previously occupied by two small regeneration kilns. The new 600 kg/h kiln will be constructed and commissioned prior to demolishing the smaller existing kilns.

Loaded carbon is pumped to an acid wash vessel. After acid washing with 3% HCl is complete, the spent acid is neutralized with sodium hydroxide from the existing makeup system before discarding it to the tails pump box.

The elution cycle operates with a 0.2% sodium cyanide and 1% sodium hydroxide solution at a temperature of 145°C and a pressure of 450 kPa for approximately eight hours.

Stripped carbon is evacuated from the bottom of the elution vessel and this is processed in a new regeneration kiln to restore carbon activity.

Pregnant solution is pumped to four electrowinning cells located adjacent to a single cell on the upper floor of the existing refinery. Gold is electrowon on the stainless steel wool cathodes. At the end of each cycle, the cathodes are removed from the cells and the gold-bearing sludge is recovered in a filter press. The resulting filter cake is dried, mixed with fluxes, usually borax, soda ash and occasionally sodium nitrate and fed to an existing electric induction furnace. The doré metal and slag separate in the furnace, and the slag is poured off into slag pots. Then the doré metal is poured into bars for shipment.