Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil: a review of the setting of gold deposits

Abstract

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1)mafic– ultramafic volcanic; (2)volcano–chemical–sedimentary;(3)clastic–chemical–sedimentary,(4)volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite;(5)resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6)coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7)non-marine association with the lithofacies:conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures