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A three-stage structural evolution of the Quadrilátero Ferriífero: consequences for the Neoproterozoic age and the formation of gold concentrations of the Ouro Preto area, Minas Gerais, Brazil

1994; Elsevier BV; Volume: 68; Issue: 1-2 Linguagem: Inglês

10.1016/0301-9268(94)90069-8

1872-7433

Alain Chauvet, Michel Faure, Ivo Dossin, Jacques Charvet,

Geochemistry and Geologic Mapping

The Quadrilátero Ferrífero occurs in the southern part of the São Francisco Craton (Brazil), where three major Proterozoic supracrustal units (the “Rio das Velhas”, “Minas” and “Itacolomi” Supergroups) are exposed together with Archaean to Palaeoproterozoic granito-gneissic basement. Microtectonic and kinematic analyses have been undertaken in the area close to Ouro Preto in order to precise the tectonic evolution and to distinguish the tectonic features related to the Transamazonian and Brasiliano orogenies (respectively ∼ 2000 and ∼600 Ma). Three main tectonic events have been recognized. The earlier event, De, is clearly expressed within basement rocks of the Bação Complex and surrounding supracrustal units. A stretching lineation (Le lineation), roughly oriented N—S, can be observed. Numerous shear criteria such as drag folds and asymmetric pressure shadows indicate a top-to-the-south sense of movement along the southern border of the Bação Complex. This event was developed within amphibolite facies conditions, fact confirmed by quartz 〈c〉 axis fabrics. This earlier deformation is related to plutonic activity and reactivation of the Archaean basement expressed by the occurrence of scarce leucocratic granitoid rocks intruded within the Bação gneisses. It is here attributed to the Transamazonian orogeny in agreement with recent UPb dating (∼ 2000 Ma). The large-scale folds (i.e. Mariana anticline, Dom Bosco syncline) which define the dominant structural geometry of the Quadrilátero Ferrífero were formed during this event. The second tectonic event, Dm, has been defined as a major thrusting towards the WNW related to the Brasiliano cycle. A well-expressed stretching and mineral lineation, oriented E—W to NW—SE (Lm lineation) and associated NW-verging shear criteria characterize this event. Occurrence of muscovite, biotite and more scarcely sillimanite indicate that deformation developed in upper greenschist/lower amphibolite facies. Quartz 〈c〉 axis fabrics confirm the sense of shearing but the large scattering of several fabrics traduces a weak intensity of strain. All supracrustal and gneiss formations were affected by the thrusting which is developed on a folded surface previously formed during the De event. The third event, Dm′, is characterized by hangingwall down movement of the nappe pile in response to relaxation of the Brasiliano compressional Dm forces. This event, developed in greenschist facies, is marked by the occurrence of metre-scale drag-folds, sometimes associated to the development of a new lineation Lm′. Gold mineralization, which occurs along the Main Mineralized Contact (located between the “Minas” and “Rio das Velhas” Supergroups), is found within late veins which comprise large recrystallized quartz, carbonate, sulphide, chlorite and tourmaline. Microscopic analysis and veins geometry allow us to conclude that the veins were emplaced mostly after the ductile Dm deformation. It is proposed that the veins were formed by filling of open cavities created during Dm′, tectonics. This study highlights that the northwest-verging thrusting, which is the main tectonic event of the Quadrilátero Ferrífero, is only relevant of the Brasiliano orogeny. A model of structural evolution which involves plutonic activity at ∼ 2000 Ma within the Archaean Bação Complex, followed by Neoproterozoic thrust tectonics (Brasiliano) is proposed and discussed. The age of gold concentration, assumed to be achieved during the relaxation that occurred in response to the thrusting event, is re-interpreted and related to the late stage of the Brasiliano tectonics.