A 3-STAGE STRUCTURAL EVOLUTION OF THE QUADRILATERO-FERRIFERO - CONSEQUENCES FOR THE NEOPROTEROZOIC AGE AND THE FORMATION OF GOLD CONCENTRATIONS OF THE OURO-PRETO AREA, MINAS-GERAIS, BRAZIL

The Quadrilatero Ferrifero occurs in the southern part of the Sao Fran cisco Craton (Brazil), where three major Proterozoic supracrustal unit s (the ''Rio das Velhas'', ''Minas'' and ''Itacolomi'' Supergroups) ar e exposed together with Archaean to Palaeoproterozoic granito-gneissic basement. Microtectonic and kinematic analyses have been undertaken i n the area close to Ouro Preto in order to precise the tectonic evolut ion and to distinguish the tectonic features related to the Transamazo nian and Brasiliano orogenies (respectively approximately 2000 and app roximately 600 Ma). Three main tectonic events have been recognized. T he earlier event, D(e), is clearly expressed within basement rocks of the Bacao Complex and surrounding supracrustal units. A stretching lin eation (L(e) lineation), roughly oriented N-S, can be observed. Numero us shear criteria such as drag folds and asymmetric pressure shadows i ndicate a top-to-the-south sense of movement along the southern border of the Bacao Complex. This event was developed within amphibolite fac ies conditions, fact confirmed by quartz <c> axis fabrics. This earlie r deformation is related to plutonic activity and reactivation of the Archaean basement expressed by the occurrence of scarce leucocratic gr anitoid rocks intruded within the Bacao gneisses. It is here attribute d to the Transamazonian orogeny in agreement with recent U-Pb dating ( approximately 2000 Ma). The large-scale folds (i.e. Mariana anticline, Dom Bosco syncline) which define the dominant structural geometry of the Quadrilatero Ferrifero were formed during this event. The second t ectonic event, D(m), 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 (L(m) lineation) and associat ed NW-verging shear criteria characterize this event. Occurrence of mu scovite, biotite and more scarcely sillimanite indicate that deformati on developed in upper greenschist/lower amphibolite facies. Quartz <c> axis fabrics confirm the sense of shearing but the large scattering o f several fabrics traduces a weak intensity of strain. All supracrusta l and gneiss formations were affected by the thrusting which is develo ped on a folded surface previously formed during the D(e) event. The t hird event, D(m'), is characterized by hangingwall down movement of th e nappe pile in response to relaxation of the Brasiliano compressional D(m) forces. This event, developed in greenschist facies, is marked b y the occurrence of metre-scale drag-folds, sometimes associated to th e development of a new lineation L(m'). Gold mineralization, which occ urs 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 an d tourmaline. Microscopic analysis and veins geometry allow us to conc lude that the veins were emplaced mostly after the ductile D(m) deform ation. It is proposed that the veins were formed by filling of open ca vities created during D(m') tectonics. This study highlights that the northwest-verging thrusting, which is the main tectonic event of the Q uadrilatero Ferrifero, is only relevant of the Brasiliano orogeny. A m odel of structural evolution which involves plutonic activity at appro ximately 2000 Ma within the Archaean Bacao Complex, followed by Neopro terozoic thrust tectonics (Brasiliano) is proposed and discussed. The age of gold concentration, assumed to be achieved during the relaxatio n that occurred in response to the thrusting event, is re-interpreted and related to the late stage of the Brasiliano tectonics.