Brazil's premier gold province. Part I: The tectonic, magmatic, and structural setting of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero

Abstract

Rocks of the Rio das Velhas Supergroup comprise one of the most significant Archean greenstone-belt successions in Brazil, in both their appreciable mineral productivity and extensive mineral potential. A large part of this greenstone belt is contained within the Quadrilatero Ferrifero (Iron Quadrangle) region, Minas Gerais state, southeastern Brazil, which occupies the southernmost portion of the Sao Francisco craton. The Nova Lima Group rocks, at the base of the Rio das Velhas greenstone belt, host important orogenic gold deposits. The group contains lithological associations from bottom to top as follows: (1) mafic-ultramafic volcanic, (2) volcanic-chemical, (3) clastic-chemical, (4) volcaniclastic, and (5) resedimented rocks. Rocks of the resedimented, volcanic-chemical, and mafic-ultramafic volcanic associations mainly host the most important gold deposits. An early compressional deformation occurs in the rocks of the Rio das Velhas greenstone belt and basement gneisses, with tangential thrusting from the north to the south or southwest. Structures generated during a second, compressional deformation, encompass NW-striking thrust faults and SW-vergent, tight to isoclinal folds, inferring a general southwest transport direction. In the central portion of the Quadrilatero Ferrifero, the Paciencia lineament, which strikes northwest and dips to the northeast in the south, or strikes northeast and dips to the southeast in the north, is a thrust-related, oblique ramp fault that hosts important gold deposits. The convergence of these two trends in the Nova Lima region is accommodated by roughly E–W-striking transcurrent faults, which are the most favored sites for large gold concentrations. Intracratonic extension in Late Archean to early Paleoproterozoic times and NW-vergent, Trans-Amazonian compressional deformation post-date gold deposition. Late extension during the Paleoproterozoic led to basin formation and the prominent dome-and-keel architecture of the Quadrilatero Ferrifero region. Deformation related to the west-vergent thrust-and-fold belts of the Brasiliano orogeny is recognized at the eastern margin of the Quadrilatero Ferrifero region. Defining structures as Archean, Trans-Amazonian, and Brasiliano in age is still difficult, although it is accepted that the gold-related Archean structures are best preserved in the central and western parts of the Quadrilatero Ferrifero. The principal source for the Rio das Velhas sedimentary rocks was probably the trondhjemite–tonalite–granodiorite crust that formed in multiple episodes after ~3,500 Ma, and was widely metamorphosed and intruded at 2,880–2,850 Ma. Felsic volcanism at 2,772±6 Ma formed the Rio das Velhas greenstone belt. The volcanic succession was a source for some overlying sedimentary rock units, as indicated by the presence of detrital zircons dated at 2,777–2,771 Ma. Strongly foliated granitic plutons range between 2,712+5/–4 and 2,698±18 Ma. The age of gold mineralization is inferred between 2,698 and 2,670 Ma. A metamorphic overprint during the Trans-Amazonian orogeny is estimated at ~2,050 Ma. There is evidence of local isotopic disturbances because of post-Trans-Amazonian tectonic events, at ~1.8–1.7 and 0.6 Ga.