Summary:
Fazenda is within the Rio Itapicuru Greenstone Belt (RIGB), a 100 km-long, 60 km-wide, north–southtrending volcano-sedimentary belt within the São Francisco Craton. The structural history of the area is complex, with at least three phases of ductile and ductile–brittle deformation followed by late brittle faulting.
Fazenda is an epigenetic, structurally controlled, and hydrothermally altered Precambrian quartzvein-hosted lode-gold deposit that has been subjected to greenschist facies metamorphism. The main gold mineralization is sulphide-bearing quartz veining associated with multiple deformation events. These vein systems vary in true width from 1.5 to 40 m, and horizontal s widths vary from 3 to 40 m. The regional strike of mineralization is north–south, while locally the veins are generally arcuate in an east–west trend, and south dipping at 40° to 70°, with a shallow-to-moderate east plunge. However, the plunge is quite variable, with some zones plunging westerly.
The bulk of gold mineralization is hosted by veins within the uppermost chlorite-magnetite schist (CLX1), and important mineralization is also found in the structurally lower CLX2 and Canto units. Economic mineralization occurs in horizontal to shallowly east- and west-plunging shoots, the locations of which are influenced by a combination of folding and shearing. Shoots range from tens of metres to hundreds of metres long, and tens of metres high.
Gold mineralization is related to multi-phase veining events, and grade increases with vein thickness and abundance. Vein thicknesses typically range from millimetre- to decimetre-scale. Veinlet networks are also common and occur in sets that vary in true width from 1.5 to 40 m, and in horizontal mining widths from 3 to 40 m. Vein composition is mainly quartz, albite, and interstitial calcite (95%) with the remaining 5% comprising iron sulphides and oxides, including pyrite, arsenopyrite, ilmenite, magnetite, pyrrhotite, and chalcopyrite. Sphalerite, pentlandite, galena, and gold are accessory minerals.
The major orebodies at the Fazenda mine are found within roughly E–W-trending and S-dipping brittle-ductile shear zones centred on the Fazenda Brasileiro unit. The bulk of gold mineralization is hosted by veins within the uppermost chlorite-magnetite schist (CLX1), and important mineralization is also found in the structurally lower CLX2 and Canto units. Economic mineralization occurs in horizontal to shallowly east- and west-plunging shoots, the locations of which are influenced by a combination of folding and shearing.
Shoots range from tens of metres to hundreds of metres long, and tens of metres high. Gold mineralization is related to multi-phase veining events, and grade increases with vein thickness and abundance.
Vein thicknesses typically range from millimetre- to decimetre-scale. Veinlet networks are also common and occur in sets that vary in true width from 1.5 to 40 m, and in horizontal mining widths from 3 to 40 m. Vein composition is mainly quartz, albite, and interstitial calcite (95%) with the remaining 5% comprising iron sulphides and oxides, including pyrite, arsenopyrite, ilmenite, magnetite, pyrrhotite, and chalcopyrite. Sphalerite, pentlandite, galena, and gold are accessory minerals.
Marcasite is formed by alteration of pyrrhotite, and covellite and digenite by alteration of chalcopyrite. Gold occurs mainly as inclusions within sulphide crystals, in veinlets, fractures, and intergranular spaces of sulphides and gangue minerals. Gold grains typically contain less than 5% Ag and are normally smaller than 20 µm.