The gold mineralisation at Lamego is characterised by orebodies associated with two horizons of chemical sedimentary rocks: BIF and metachert (MCH), and also with shear zones containing abundant quartz veinlets. The proportions of these lithotypes vary substantially from one deposit to another. In the BIF, sulphide mineralisation is associated with gold, while in the MCH and quartz veins, the gold occurs either as native gold or in sulphides. Lamego shows similar rock assemblage, but with higher structural complexity than Cuiabá. The BIF which contains the mineralisation is more structurally deformed and is sometimes described as a ‘metachert’.

The mineralisation is characterised by sulphidation in the form of disseminated sulphide bands or as fracture filling and, more rarely, as massive sulphide hosted in BIF/MCH. Sulphide bands are rare in MCH. The plunge of the mineralised zones coincides with both the fold axis of the first two structural events and the mineral stretching.

The Arco da Velha deposit is located on the eastern side of a large fold and extends for 250m along the strike. In the north-eastern portion, the mineralisation is concentrated in the MCH, while in the south-western portion it is concentrated in the BIF. Carbonaceous phyllite and chlorite-sericite schists occur in the hangingwall contact, while hydrothermally-altered meta-andesite occurs in the footwall. The Cabeça de Pedra deposit is located in the hinge region of the large Lamego structure. The area which has shown the best economic potential contains BIF and MCH (80% of the area consists of BIF and the remaining 20% is MCH). The presence of faulting makes the stratigraphy complex in some areas. The carbonaceous phillite and clorite/sericite schists normally occur in the hangingwall and meta-andesites in the footwall.

CdS is located in the eastern part of the lower to middle greenschist facies archaean Rio das Velhas greenstone belt. The CdS I, II and III gold deposits and associated targets are located in a gold trend that extends for about 14km in a north-easterly direction, from Grota Funda (CdS I areas) in the south to Jambeiro (CdS III areas) in the north. The main gold targets and deposits are distributed over three trends, namely the CdS trend, the Donana Trend and the Cristina Trend. CdS mineralisation occurs in a greenstone belt geological environment, associated with quartz and sulphides (mainly arsenopyrite) in a structurally controlled corridor approximately 16 – 20km in strike length and about 500m vertical extent.

The CdS deposits consist of narrow north east/south-west elongated lenses of mineralisation dipping 60° to 70° south-east and plunging 20‚ to 30° north-east. CdS is an orogenic type deposit and comprises many hydrothermal lodes with quartz veins and low sulphide content disseminated in the wall rocks. The deposits are narrow, elongated and folded. In general, the mineralisation consists of sericitic zones and quartz veinlets.

Cuiabá mine has two mining methods: cut-and-fill and longhole stoping. Cut-and-fill is adopted at stopes with low inclination, requiring the use of in-stope pillars in large-width stopes. In 2011, Cuiabá mine started changing the mining method from cut-and-fill to longhole stoping with the aim of improving safety, productivity and selectivity for the mining operation.

The mining methods at Lamego are cut-and-fill and longhole open stope. Cut-and-fill is typically used for the larger mining areas, when the orebodies exceed 20m spans. While this method allows for selectivity, it has constraints in terms of productivity. Longhole open stope on the other hand is less selective, but allows higher productivity.

The mining method for CdS I is sub-level stoping. Each panel consists of three levels, with secondary development of 300m from the cross cut in a north-east direction and 300m from cross cut in a south-west direction. The stopes are 15m in height. Some of the mining sequence is bottom/up, though all of CdS I is being converted to top/down sequences. According to geotechnical guidance, a sill pillar of 4m in height is designed between panels, and 4m rib pillars are used each 30m along the strike. The blast drilling for stopes is executed via fan drilling in ascending and descending directions. The loading and hauling operations are performed by with 8t front-end loaders and 30t articulated trucks, at an approximate rate as of 1,500tpd.

There are two metallurgical plants in CdS: the heap-leach plant for the oxide ore and the sulphide plant.

The sulphide process consists of crushing, grinding and gravity concentration, flotation, thickening, acidulation, pressure oxidation (POX autoclave), CCD (counter current decantation), CIL extraction, elution, neutralisation, electro-winning and tailings disposal. The plant and POX circuit have a capacity as of 600ktpy.

The heap-leaching process consist of crushing, agglomeration, stacking, leaching, adsorption, elution and electro-winning.

Cuiabá and Lamego mines feed the Cuiabá gold plant (flotation) and Queiroz (roaster, carbon circuit and refinery) plants, currently at 1.7Mtpa for a metallurgical recovery of 93.3%. At the Cuiabá gold plant, crushing and milling of the ore is followed by flotation and filtration in order to produce a concentrate, which is transported by aerial ropeway to Queiroz for further treatment. Approximately 25% to 30% of gold is recovered through a gravity circuit at the Cuiabá plant. The backfill plant is also located at Cuiabá. The Queiroz plant is located in Nova Lima, it was built with two different metallurgical routes to treat refractory ore and non-refractory ore. The concentrate is burned, and the calcine proceeds to a carbon circuit for further refining. The sulphide gas is captured for processing through the acid plant; approximately 230ktpa of sulphuric acid are produced as a by-product.

Gold production process starts at Cuiabá plant where the Mechanical Treatment is situated. After that the concentrate containing gold goes to Queiroz Plant where the Pyromettalurgy and Hydrometallurgy are responsible of the end of the process. A description of each process is given below:

Cuiabá Plant
Cuiabá plant contains the Mechanical Treatment that consist in reduce size and concentrate material that we have interest, and to do that, it uses these following process: Crushing, Milling, Flotation and Filtering to take off water from the pulp, so that it can be transported to Queiroz Plant using the Aerial Ropeway.

Crushing
There is a primary crushing inside the mine and after that the ore is sent to the surface crushing. Crushing is just a process to reduce size of ore stones so that it can feed the Mill with a small size.

Milling
Milling process is responsible to reduce ore size so that it assumes a powder size. After that the ore goes through gravimetric concentrators so the free gold can be taken of the gold. However, approximately 85% of the gold is included in sulfur molecule and is necessary additional processes to recovery it.

Flotation
All the ore coming from milling pass through flotation area that concentrates the sulfur by the reduction of mass. Since the gold molecule is included in sulfur it’s grade becomes higher with sulfur concentration. The concentrate produced must have a minimum of 28% of sulfur because that’s a very important parameter for the Pyromettalurgy process.

Filtering
So the concentrate can be sent to Queiroz plant through Aerial Ropeway, it is necessary that the amount of water in it suffer reduction. To get this the concentrate is filtered and the amount of water decreases to 11% of mass.

Queiroz Plant
At Queiroz plant the chemical processes are made to recover the gold from the concentrate. Queiroz plant process is parted in to distinct areas: Pyromettalurgy and Hydrometallurgy.

Pyromettalurgy
Once the concentrate receives water to become a pulp again, it is pump to Pyromettalurgy area so it can be roast. The concentrate must have a minimum of 28% of sulfur and a density of approximately 2.0 t/m³ so the exothermic reaction of oxidation can occur.

In the roaster the concentrate is oxidized by the addition of atmospheric air and a temperature of 700ºC inside the roaster. In these conditions the sulfur is oxidized generating sulfur dioxide and exposing the gold to cyanide attack in the Hydrometallurgy process.

The sulfur dioxide is converted into sulfur trioxide and then it’s used to make sulfuric acid.

The concentrate coming out of the roaster then is called calcine. The Calcined is quenches, thickened and finally pumped to Hydrometallurgy area.

Hydrometallurgy
The calcine received from Pyromettalurgy at this time can be leach by cyanide. The calcine goes to tanks with addition of air. The pH rate is controlled with addition of lime. If the pulp with a pH in a rate less than 10.5 receives, a solution of sodium cyanide there is formation of cyanuric acid in gas form, which is very dangerous.

Since the pH rate is controlled in a rate between 10.8 – 11.3 a solution with 10% of sodium cyanide is addicted to the pulp so that the gold can be solubilized.

The bigger part of gold is solubilize and after solid / liquid separations, the rich liquor (High level of gold) is send to the precipitation area, which precipitates the gold with addition of zinc and plumb nitrate. This precipitated material is sent to the refinery so that the gold bars can be made.

However not all of gold is taken off the pulp in the first stage of leaching. The pulp returns to leaching and then goes to CIP (carbon in Pulp) where the soluble gold is adsorbed in activated coal. Since the coal is loaded with gold, it passes through an elution process to recover the gold. The rich liquor is sent to precipitation area.

When all the possible gold is taken off (minimum of 94, 5%), the pulp and the barren solution (low level of gold) is discharged to the calcine dam. However before that all this pulp receives a solution of ferrous chloride to reduce cyanide level to less than 50 ppm.