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

Rocks of the Rio das Velhas Supergroup comprise one of the most significant Archean greenstone-belt successions in Brazil, in both their appreciable m ineral productivity and extensive mineral potential. A large part of this g reenstone belt is contained within the Quadrilatero Ferrifero (Iron Quadran gle) region, Minas Gerais state, southeastern Brazil, which occupies the so uthernmost 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) e lastic-chemical, (4) volcaniclastic, and (5) resedimented rocks. Rocks of t he resedimented, volcanic-chemical, and mafic-ultramafic volcanic associati ons mainly host the most important gold deposits. An early compressional de formation occurs in the rocks of the Rio das Velhas greenstone belt and bas ement gneisses, with tangential thrusting from the north to the south or so uthwest. Structures generated during a second, compressional deformation, e ncompass 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 nort hwest 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 tha t hosts important gold deposits. The convergence of these two trends in the Nova Lima region is accommodated by roughly E-W-striking transcurrent faul ts, which are the most favored sites for large gold concentrations. Intracr atonic extension in Late Archean to early Paleoproterozoic times and NW-ver gent, Trans-Amazonian compressional deformation post-date gold deposition. Late extension during the Paleoproterozoic led to basin formation and the p rominent dome-and-keel architecture of the Quadrilatero Ferrifero region. D eformation related to the west-vergent thrust-and-fold belts of the Brasili ano orogeny is recognized at the eastern margin of the Quadrilatero Ferrife ro region. Defining structures as Archean, Trans-Amazonian, and Brasiliano in age is still difficult, although it is accepted that the gold-related Ar chean structures are best preserved in the central acid western parts of th e Quadrilatero Ferrifero. The principal source for the Rio das Velhas sedim entary rocks was probably the trondhjemite-tonalite-granodiorite crust that formed in multiple episodes after similar to3,500 Ma, and was widely metam orphosed and intruded at 2,880-2,850 Ma. Felsic volcanism at 2,772+/-6 Ma f ormed the Rio das Velhas greenstone belt. The volcanic succession was a sou rce for some overlying sedimentary rock units, as indicated by the presence of detrital zircons dated at 2,777-2,771 Ma. Strongly foliated granitic pl utons range between 2,712 + 5/-4 and 2,698+/-18 Ma. The age of gold mineral ization is inferred between 2,698 and 2,670 Ma. A metamorphic overprint dur ing the Trans-Amazonian orogeny is estimated at similar to2,050 Ma. There i s evidence of local isotopic disturbances because of post-Trans-Amazonian t ectonic events, at similar to1.8-1.7 and 0.6 Ga.