Mineralização aurífera associada aos veios quartzo-carbonáticos hospedados na unidade máfica basal da jazida Cuiabá, greenstone belt Rio das Velhas, Quadrilátero Ferrífero, Minas Gerais, Brasil

Abstract

The Cuiabá deposit, located in the Quadrilátero Ferríferro district (QF) in the southeastern part of the state Minas Gerais, is the largest underground mine in Brazil (5.78 Moz resources of 10.26 g/t). This world-class orogenic gold deposit is part of the Rio das Velhas greenstone belt, a typical association of mafic volcanic rocks, banded iron formation (BIF), carbonaceous phyllite and micaceous phyllite, metamorphosed to the greenschist facies conditions. The depositional structure and orebodies are controlled by an anticline fold, indicating an inverted north flank and a normal sequence in the south. The fold axis dips to 116° azimuth direction and plunges between 35° and 12°. The predominant sulfide mineralization is associated with the Cuiabá-BIF, and related to hydrothermal alteration zones. The main orebodies are Fonte Grande Sul, Serrotinho, Balanção and Galinheiro, with Galinheiro Extensão, Surucucu, Canta Galo and Dom Domingos of secondary importance. Other two-mineralization styles are shear-related quartz veins and disseminated sulfides minerals in Fe-rich, upper mafic volcanic unit, such as the locally mined Viana, Galinheiro Quartzo and Galinheiro Footwall orebodies, associated with the upper mafic unit. Recently three new gold mineralized zones were discovered and named VQZ, Serrotinho Footwall and Fonte Grande Sul Footwall, where the mineralization is a shear-related quartz-carbonate veins type hosted in the lower mafic volcanic unit, associated to distal (chlorite), intermediate (carbonate) and proximal (sericite) hydrothermal alteration zones. Only one structure generation (G1) is related to the lower metamafic unit hosting gold-bearing quartz-carbonate veins. The event is characterized by a SSE-NNW directed, transpressive stress under ductile to brittle conditions, with structural elements including the compositional banding S0, flexural foliation S1, and a mylonitic foliation S2 accompanied by the development of mineral lineation Lmin1, intersection lineation Lint1, isocline and intrafolial folds F1 and quartz veins. Four quartz-carbonate vein types are identified in the lower metamafic unit: V1, V2, V3 and V4. The V1 system is a S2-controled fault-fill type, and the main source of gold. The V2 veins are also mineralized, controlled by the S1 foliation and subdivided in three sub-types, according to internal crystals orientation: V2a is classified as fault-fill, while V2b and V2c are extensional veins. The V3 and V4 systems are also extensional veins. All vein systems are related to D1 shearing and the veins structural analyses indicates a shorting vector rotation during the deformational event and hydrothermal fluid input. A proposed evolutionary model for the vein systems suggest that the ore-forming fluid responsible for gold mineralization channeled along D1 shear zones and dispersed along the S0-1 structures. The internal vein features and the trapping temperatures indicates a progressively tectonic activity decrease. Ore characterization studies, conducted by the MLA (Mineral Liberation Analyser) technique, demonstrate a quartz-pyrite-ankerite composition with 98 % of liberated gold. Gold grains range in size between 1 and 250 m and occur as Au-Ag electrum. Although the majority is present as liberated gold, high quantities of small gold particles may be only extracted after ultra-fine milling followed by cyanide leaching.