Controle estrutural da mineralização aurifera na mina de Cuiaba, setor noroeste do Greenstone Belt, Rio das Velhas, Quadrilatero Ferrifero, MG / Structural control of the gold mineralization at the Cuiaba mine, northwestern sector of the Rio das Velhas Greenstone Belt, Quadrilatero Ferrifero, MG

AUTOR(ES)
DATA DE PUBLICAÇÃO

1997

RESUMO

The Cuiabá Mine is located in the northem portion ofthe Quadrilátero Ferrífero, within a sequence of Archean rocks forrning the base of the Rio das Velhas greenstone belt. This research focused on geological mapping at a scale of l:100 and detailed studies of the tectonic structures observed in the Cuiabá Mine. Its main objective is to present the results of a qualitative structural analysis and its relationship to the ore bodies. The lithological sequence exposed at the mine is characterized by two distinct groups: (i) a basal group comprising sub-aquatic basaltic flows with carbonaceous phyllites and one intercalated layer of banded iron formation and; (ii) an essentially sedimentary group of rocks at the top, characterized by the altemation of carbonaceous pelites and graywackes, composing a typical turbiditic sequence. The deformation has a heterogeneous, non-coaxial and progressive character and occurred at different crustal levels, during three phases of successive deformation. The structures belonging to the DI and Dz phases have developed in a ductile to ductile-brittle crustal regime, under compressive stress oriented in the SE- NW direction. Both phases show a coaxial and progressive evolution with tectonic transport from SE to the NW. D3 structures were formed in a predominantly brittle-ductile regime and reflect the action of compressive stresses oriented in the E- W direction. The structural framework of the deposit is controlled by a large anticline (Fz), with an overtumed northem limb, outlined by the folding of the primary bedding. This fold presents a tubular geometry with the apex closure pointing to the northwest direction and axis inclined at about 30° to the southeast. The gold mineralization is hosted by the sulphidic zones in the banded iron forrnation layer. The gold is included in the pyrite crystals, which constitutes the most abundant sulphide in the ore bodies. These bodies are concordants to the primary bedding and their geometry is controlled by thrust and strike-slip faults developed during late stages of the Dz phase. The orientation of the sulphidic bodies is parallel to the stretching lineation Lez and to the axis of the tubular fold which control the structural framework of the deposit. Observation of the ore textures and structures revealed that the gold mineralization is mainly related to epigenetic processes, which include sulphidation of the banded iron formation around fractures and/or quartz-carbonate veins. In this context, two generation of sulphides could be established: (i) the first generation, pre- to early-DJ, is related to hydraulic fracturing systems, which created permeability to the access to the fluid, reaction with the banded iron formation, and deposition of gold and sulphides; (ii) the second generation, sin-Dz, is related to local remobilization of the mineralization, promoting sulphide concentrations in the hinge zones of mesoscopic folds (Fz)

ASSUNTO(S)

ouro quadrilatero ferrifero (mg) geologia estrutural

ACESSO AO ARTIGO

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