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).

The Cuiabá Mine uses two mining methods: sub-level stoping and cut and fill. Sub-level stoping is the main mining method and applies in all of the thicker and steeper dipping parts of mineralisation. The cut and fill mining method was reintroduced to increase ore recovery. It is applied in the narrow veins below level 14.1 (Balancão, Galinheiro and Canta Galo orebodies) where the dip is lower. In the Galinheiro Footwall the mining method remains sub-level stoping as the orebody shows a reasonable steep dip and thickness.

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 ........