The gold mineralization at Jacobina is hosted almost entirely within quartz pebble conglomerates of the Serra do Córrego Formation, the lowermost sequence of the Proterozoic age Jacobina Group. This formation is typically 500 m thick but locally achieves thicknesses of up to one kilometre. The gold-bearing reefs range in size from 1.5 m to 25 m in width and can be followed along strike for hundreds of metres, and in some cases for kilometres. Although they are quite homogeneous along their strike and dip extensions, the mineralized conglomerates differ from one another in stratigraphic position and mineralization patterns. The differences are likely due to changes in the depositional environment, and possibly also in the source areas. Some contacts between the reefs and crosscutting mafic and ultramafic intrusives are enriched in gold. Not all conglomerates of the Serra do Córrego Formation are economically mineralized and many of them are completely barren of gold. Intra-reef quartzites typically contain low gold grades (<0.70 g/t Au).

The vast majority of significant gold mineralization occurs within the matrix of the conglomerates. Gold occurs as very fine grains of native gold typically 20 µm to 50 µm in size. Gold mineralization rarely occurs in the pebbles, but when it does, it is along fractures. Interbedded quartzites host gold mineralization almost exclusively along fractures especially near late mafic dikes. Fracture-controlled mineralization in the quartzites is rare in the absence of late mafic dikes in the Jacobina Mine area, however, structurally controlled gold mineralization in pyrite-tourmaline-bearing quartz veins is abundant in the Pindobaçu area, 50 km north of Jacobina.

Teixeira et al. (1999 and 2001), classified the gold deposit/occurrences of the Jacobina range into four groups, mainly based on the nature of the host rock. These groups are:

1) Conglomerate-hosted gold deposits – encompassing the pyritic, gold-bearing, quartz- pebble conglomerates and quartzites of the Serra do Córrego Formation, which host the gold deposits of the Jacobina gold district;

2) Mafic-hosted gold deposits represented by small gold workings where gold is associated with disseminated hydrothermal pyrite near tension gashes and quartz-pyrite veins and veinlets, hosted by late-tectonic gabbroic and dioritic dikes, which cut the Serra do Córrego and the Rio do Ouro Formations;

3) Ultramafic-hosted gold deposits represented by narrow quartz veins with pyrite and arsenopyrite developed along the sheared footwall contact between metamorphosed peridotite and pyroxenite dikes and quartzites of the Serra do Córrego or Rio do Ouro Formations (e.g., Mina Velha and Jacinto); and

4) Quartzite-hosted gold deposits represented by shear zone-related quartz veins hosted by quartzite of the Rio do Ouro and “Cruz das Almas” Formations (e.g., Maravilha and Goela da Ema).

The Jacobina Mine Complex uses both selective and non-selective mining methods in the form of Cut and Fill (CAF) mining for the shallow dipping zones and Sublevel Longhole Stoping (SLS) for the zones with dips that allow for bulk type mining. Access to the mineralized zones is by ramp, hence allowing for a high degree of flexibility. The various cut-off grades for each mining area typically range from 1.29 g/t Au to 2.1 g/t Au. The cut-off grades were estimated for both the CAF and SLS methods which also considered the cases of “development realized” (dr) and “development not realized” (dnr) for the two methods and with or without fill.

The process involves the following activities: three-stage crushing of the ore, grinding of the crushed material into a pulp, gravity concentration of coarse gold, leaching of the pulp in a conventional cyanide leaching process, gold extraction of the enriched solution in CIL and CIP tanks, and gold recovery in a electrolytic circuit.

The run-of-mine (ROM) material is trucked to the crushing facilities located adjacent to the processing plant. The broken ore is passed through a grizzly (80% <180 mm) and fed to the jaw crusher with a capacity of 800 tonnes per hour. The coarsely crushed material is then passed through secondary and tertiary cone crushers with a capacity of approximately 600 tonnes per hour. The secondary crusher reduces the size of the feed to 80% <100 mm and the tertiary crusher further reduces the feed to 80% <25 mm.

The product of the crushers is fed to two storage silos and then into two ball mills where the material is ground to 80% <13 mm. The No. 1 mill has dimensions of 3,658 mm by 6,706 mm and is powered by a 1,342 kW motor and the No. 2 mill has dimensions of 4,572 mm by 9,144 mm is powered by a 2,500 kW motor.

Part of the material from the grinding circuit is pumped to the thickener followed by the leach tanks. The leaching circuit consists of seven CIL tanks with a total capacity of 5,350 m3.

The pulp is next sent to the CIP circuit which is made up of six 180 m3 mechanically agitated CIP tanks. Part of the pulp stream containing the coarse gold is processed through Knelson Concentrators and an Acacia Reactor. It is estimated that 50% of the gold in the plant is handled by the concentrator/reactor combination.

The enriched carbon from the CIP circuit is removed and the gold is placed into solution. The pregnant solution from the CIP circuit and from the Acacia Reactor is circulated through electrolytic cells and a doré bar is produced at the induction furnace. The doré bars have a nominal composition of 97% gold and 3% silver. The doré bars are transported by air to Umicore in Sao Paulo for refining.