The mixed carbonate-siliciclastic platforms are still poorly studied, although they represent important regional sedimentary successions in the Brasília Fold Belt. The aim of the present paper was to determine the deposition conditions of the Psammo-pelitic-carbonate Unit (top of the Paranoá Group) and the Sete Lagoas Formation (base of the Bambuí Group) in the Brasília Fold Belt, central Brazil. The sedimentation of the two sequences occurred in a mixed platform controlled by the bottom paleogeography where carbonate facies were deposited in shallow environment and detrital facies in deeper water. In the Paranoá Group, small limestone and dolomite lenses are found in the highest areas, whereas pelites were deposited in depressions and psammites inplatform channels. In the Sete Lagoas Formation, the major carbonate lenses were deposited over the highs derived from the post-glacial erosion surface that marks the unconformity between the Paranoá and Bambuí groups. In both units, micritic mud predominates, and reworking by waves occurred sporadically and associated with progradationretrogradation. The aggradation of terrigenous material caused deposition of pelitic facies. The dimensions, thickness, and distribution of carbonate lenses show that the paleorelief was more pronounced during the deposition of the Sete Lagoas Formation, when compared with the Psammo-Pelitic-Carbonate Unit. On the other hand, the sedimentation rate of Psammo-Pelitic-Carbonate Unit was greater than the one of the Sete Lagoas Formation.Keywords: Carbonate-siliciclastic platform; Bambuí Group; Paranoá Group.
ResumoSedimentologia da Unidade Psamo-pelito-carbonatada, Grupo Paranoá e da Formação Sete Lagoas, Grupo Bambuí: exemplos de sedimentação mista carbonática-siliciclástica no Proterozoico da Faixa Brasília. Apesar de muito importantes e de ocorrerem em várias sucessões litoestratigráficas na Faixa Brasília, as plataformas mistas de sedimentação silicilástica-carbonática ainda são pouco estudadas. O presente trabalho objetivou a determinação das condições deposicionais da unidade de topo do Grupo Paranoá (Unidade Psamo-Pelito-Carbonatada) e da Formação Sete Lagoas (base do Grupo Bambuí) na Faixa Brasília. A sedimentação das duas sucessões se deu em uma plataforma mista controlada pela paleogeografia de fundo em que as fácies carbonáticas foram depositadas em águas mais rasas e apenas ocasionalmente mais agitadas e as fácies terrígenas ocupavam as águas mais profundas. No Grupo Paranoá, as pequenas lentes de calcário e dolomito ocupavam as áreas mais elevadas, enquanto os pelitos se depositavam nas depressões e os psamitos correspondem aos canais na plataforma. Na Formação Sete Lagoas, a deposição das maiores lentes de rochas carbonáticas se dava sobre as elevações herdadas da superfície erosiva pós-glacial que marca a discordância entre os grupos Paranoá e Bambuí. Nas duas unidades, o processo de precipitação de lamas micríticas foi predominante nas fácies carbonáticas, enquanto o retrabalhamento por base de ondas ocorria de forma...
The Goiás Archean Block (GAB) in central Brazil is an important gold district that hosts several world-class orogenic gold deposits. A better comprehension of the crustal, tectono-magmatic, and metallogenic settings of the GAB is essential to accurately define its geological evolution, evaluate Archean crustal growth models, and target gold deposits. We present an overview of gold systems, regional whole-rock Sm-Nd analyses that have been used to constrain the geological evolution of the GAB, and augment this with new in situ zircon U-Pb and Hf-O isotope data. The orogenic gold deposits show variable host rocks, structural settings, hydrothermal alteration, and ore mineralogy, but they represent epigenetic deposits formed during the same regional hydrothermal event. The overprinting of metamorphic assemblages by ore mineralogy suggests the hydrothermal event is post-peak metamorphism. The metamorphic grade of the host rocks is predominantly greenschist, locally reaching amphibolite facies. Isotope-time trends support a Mesoarchean origin of the GAB, with ocean opening at 3000–2900 Ma, and reworking at 2800–2700 Ma. Crustal growth was dominated by subduction processes via in situ magmatic additions along lithospheric discontinuities and craton margins. This promoted a crustal architecture composed of young, juvenile intra-cratonic terranes and old, long-lived reworked crustal margins. This framework provided pathways for magmatism and fluids that drove the gold endowment of the GAB.
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