The area in which Cuiabá is located is known as the Iron Quadrangle and is host to a number of historic and current gold mining operations, as well as a number of open pit limestone and iron ore operations. The geology of the Iron Quadrangle is composed of Proterozoic and Archaean volcano-sedimentary sequences and pre-Cambrian granitic complexes.

Cuiabá is a “gold-only” Archaean banded iron formation (BIF)-hosted gold deposit. The deposit consists of an intermediate meta-mafic sequence of the Archaean Greenstone Belt type. It is characterised by hydrothermal alteration of the rocks, with the mineralisation occurring mainly in BIF layers, and subordinately in quartz veins or in the host schists. The host to the gold mineralisation is the volcano-sedimentary Nova Lima Group that occurs at the base of the Rio das Velhas Super Group. The upper sequence of the Rio das Velhas Super Group is the meta-sedimentary Maquiné Group. The gold mineralisation at Cuiabá has features and characteristics that are in agreement with the epigenetic orogenic gold deposit model presented for Archaean gold-lode deposits.

Cuiabá mine has gold mineralisation associated with sulphides and quartz veins in BIF and volcanic sequences. Structural control and fluid flow ascension are the most important factors for gold mineralisation with a common association between large-scale shear zones and their associated structures. Where BIF is mineralised the ore appears strongly stratiform due to the selective sulphidation of the iron rich layers. Steeply plunging shear zones tend to control the ore shoots, which commonly plunge parallel to intersections between the shears and other structures.

Apparent intersections of thrust faults with tight isoclinal folds, in a ductile environment, tend to control the mineralisation structures. The host rocks are primarily BIF and secondarily mafic volcanics (mainly basaltic). Mineralisation is believed to be due to the interaction of low salinity, carbon dioxide-rich gold-bearing fluids with the high-iron BIF, basalts and carbonaceous graphitic schists.

Sulphide mineralisation consists of pyrite and pyrrhotite with subordinate arsenopyrite and chalcopyrite; the latter tends to occur as a late-stage fracture fill and is not associated with gold mineralisation. Wallrock alteration is typically carbonate, potassic and silicic, showing clear zonation in the underground environment. The ore is mainly concentrated in the silicic and sulphidation zones, inside the BIF or in potassic (and sericitic) zones near the basalts. The main orebodies at Cuiabá are as follows:
• normal limb: Fonte Grande Sul and Serrotinho • overturned limb: Balancão, Galinheiro and Canta Galo.

Secondary orebodies occur in hydrothermaly altered schists at the footwall of Galinheiro (Galinheiro footwall orebody) and hydrothermaly altered schists/quartz veins near the footwall of Fonte Grande Sul – Serrotinho (quartz vein orebody).

Cuiabá mine has two mining methods: cut-and-fill and longhole stoping. To improve the safety and productivity of the operation, in 2011, the mining method was changed from cut-and-fill to longhole stoping (sub-level stoping and variations). Cut-and-fill is still applied in stopes with lower inclination, requiring the use of in-stope pillars in large-width stopes.

Cuiabá and Lamego Mines feed the Cuiabá Gold (flotation) and Queiroz (roaster, carbon circuit and refinery) plants, currently at 1.7Mtpa for a metallurgical recovery of 93.3%. At 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 it is in charge of pyrometallurgy and hydrometallurgy. The concentrate is roasted, 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.