Petrografia tridimensional de partículas de ouro orogênico do depósito Cuiabá, greenstone belt Rio das Velhas, Quadrilátero Ferrífero, Brasil : aplicação de tomografia computadorizada de raios-x de alta resolução para caracterização mineralógica

• Ano de defesa: 2023
• Autor(a) principal: Marília Ferraz da Costa
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; Brasil; IGC - DEPARTAMENTO DE GEOLOGIA; Programa de Pós-Graduação em Geologia; UFMG
• Palavras-chave em Português: Quadrilátero Ferrífero; Mina Cuiabá; Ouro Orogênico; Tomografia Computadorizada de Raios-X; Caracterização Mineralógica; Petrologia – Minas Gerais; Ouro – Minas e Mineração; Tomografia computadorizada por raios x; Quadrilátero Ferrífero (MG)
• Link de acesso: http://hdl.handle.net/1843/64095

Ore studies using conventional petrographic methods have an intrinsic limitation since spatial (three-dimensional, 3-D) distribution, orientation, shape and mineral associations have to be inferred by 2-D observations and measurements. Further, developing statistically significant data sets is particularly difficult for gold ores, given the fine grain size and low gold abundance even in high-grade ores. This study uses high-resolution X-ray computed tomography (HRXCT) and improved processing methods to provide 3-D data of ore textures and in-situ gold particles to discern mineralization styles and improve ore recovery at the Archean, worldclass Cuiabá orogenic gold deposit in the Quadrilátero Ferrífero province, the largest underground mine in Brazil (Anglogold Ashanti Ltda.). Cuiabá is hosted by rocks of the Rio das Velhas greenstone belt, southern portion of the São Francisco Craton, Minas Gerais state, Brazil. Selected samples represent distinct mineralization styles at Cuiabá. Ore hosted in banded iron formation (BIF) from the Fonte Grande Sul (FGS) orebody, and gold-bearing quartzcarbonate veins hosted in a metamafic unit from the Veio de Quartzo (VQZ) orebody. This study characterizes shape, size, orientation and mineral associations of over 16,000 gold particles. Segmentation and quantitative measurements of gold particles are particularly challenging due to their small grain size. This issue is addressed using the recently formalized partial volume and blurring (PVB) method. Results show that gold mineralization styles differ in 3-D textural and quantitative aspects including size and spatial distribution. Analysis of the VQZ sample shows that gold particles are mainly free in the quartz-carbonate matrix, with many particles in direct contact with a diverse sulfide mineral assemblage. Gold particles in the BIF-hosted FGS ore form bands in association with high pyrrhotite zones within iron carbonaterich bands. The PVB analysis allows us to show that the long axis of measured particles in both ore types ranges from ~25 μm to 1.4 mm, with most from 50 to 150 μm. Larger particles are dominantly sub-equant to prolate spheroids, with smaller particles appearing to be dominantly sub-equant to equant grains. The HRXCT data reveal that the kinematic processes affected gold shape and orientation and indicate the influence of deformation at the grain scale