Structural Evolution of the Rio Das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits
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minerals Article Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits Orivaldo Ferreira Baltazar 1,* and Lydia Maria Lobato 2,* 1 Independent Consultant, Rua Ramalhete, 35/104, Anchieta, Belo Horizonte 30310-310, MG, Brazil 2 Instituto de Geociências, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Campus Pampulha, Belo Horizonte 31270-910, MG, Brazil * Correspondence: [email protected] (O.F.B.); [email protected] (L.M.L.) Received: 24 August 2020; Accepted: 26 October 2020; Published: 4 November 2020 Abstract: The Quadrilátero Ferrífero region is located in the extreme southeast of the Brasiliano São Francisco craton, Minas Gerais state, Brazil. It is composed of (i) Archean TTG granite-gneaissic terranes; (ii) the Archean Rio das Velhas greenstone belt; (iii) the Proterozoic metasedimentary and metavolcano-sedimentary covers. The Rio das Velhas rocks were deposited in the synformal NW–SE-directed Nova Lima basin. The Archean deformation converted the Nova Lima basin into an ample synclinorium with an eastern inverted flank. Archean orogenic gold mineralization within the Rio das Velhas greenstone belt rocks is controlled by NNW–SSE-directed, Archean regional shear zones subparallel to the strata of the Nova Lima synclinorium borders. Transamazonian and Brasiliano orogenies are superposed onto the Archean structures that control gold mineralization. In the eastern domain, Brasiliano fold-and-fault belts prevail, whereas in the western domain Archean and Transamazonian structures abound. The present study focus mainly is the western domain where the Cuiabá, Morro Velho, Raposos, Lamego and Faria deposits are located. Gold orebodies plunge to the E–NE and are tectonically controlled by the Archean D1–D2 deformation. The D3 Transamazonian compression—Which had a SE–NW vector sub-parallel to the regional mineralized Archean foliation/bedding—Buckled these structures, resulting in commonly open, synformal and antiformal regional folds. These are well documented near the gold deposits, with NE–SW axial traces and fold axes plunging to E–NE. Such folds are normal to inverted, NW-verging, with an axial planar foliation dipping moderately to the SE. The Transamazonian compression has only been responsible for the reorientation of the mineralized Archean gold ores, due to coaxial refolding characterized by an opposite tectonic transport. It has therefore not caused any other significant changes. Thrust shear zones, sub-parallel to the strong Transamazonian foliation, have given rise to localized metric segmentation and to the dislocation of gold orebodies. Throughout the region, along the towns of Nova Lima to Sabará, structures pertaining to the Brasiliano Araçuaí orogeny are represented only by gentle folding and by a discrete, non-pervasive crenulation cleavage. Thrust-shear zones and small-scale normal faults have caused, at most, metric dislocations along N–S-oriented planes. Keywords: Rio das Velhas greenstone belt; archean orogeny; paleoproterozoic orogeny; Quadrilátero Ferrífero; São Francisco Craton; gold deposits; tectonic-structural analysis 1. Introduction The Quadrilátero Ferrífero (QF) metallogenetic province is located in the southern border of the São Francisco Craton (CSF) [1]. The craton is part of the South American Platform, consolidated and Minerals 2020, 10, 983; doi:10.3390/min10110983 www.mdpi.com/journal/minerals Minerals 2020, 10, 983 2 of 38 stabilized as a result of successive geodynamic cycles. These cycles were, in turn, responsible for the breakdown and re-aggregation of large continental masses over geological time. The QF occupies an area of approximately 7000 km2 in the central and southern areas of Minas Gerais state. It constitutes the aggregation of Proterozoic and Archean terranes, consolidated at the end of the Minas orogeny, part of the Trasamazonian orogenic cycle. The region is bordered along its eastern and southeastern limits by the Araçuaí orogenic belt (Brasiliano orogenic cycle) (Figure1). Figure 1. Regional geological map of the southern São Francisco craton (modified from [2]). The Quadrilátero Ferrífero region is indicated. Insert refers to Figures 2, 4 and 5. Historically, the QF is one of the most important mineral provinces in Brazil [2,3], given its significant contribution to the country’s iron and gold production. These mineral deposits are hosted, respectively, within the banded iron formations of the Itabira Group—Paleoproterozoic (the metasedimentary chemical sequence of the Minas Supergroup)—And in the varied lithotypes of the Nova Lima Group—Archean (metasedimentary basal sequence of Rio das Velhas Supergroup). The orogenic gold mineralization [4] of the Rio das Velhas greenstone sequence is structurally controlled; the orebodies, contained within different lithotypes, are regionally distributed throughout Archean shear zones. The main gold hosts are banded iron formations and/or hydrothermal rocks known as lapa seca [5]. The so-called lapa seca is constituted by a set of quartz-carbonate hydrothermal rocks with sulfide minerals, white mica, and albite. The BIF and the lapa seca are responsible for around 49% and 47% of the existing gold, respectively, with the remaining 4% accounted for by mafic and ultramafic metavolcanics, as well as metasedimentary volcaniclastic and epiclastic rocks [2,3]. An Archean age has long been suggested for gold mineralization in the Nova Lima Group rocks [3,6,7], since the mineralized, regional shear zones predate the deposition of the Minas Supergroup. Robust U-Pb (SHRIMP) dating has been obtained in hydrothermal monazite for the Morro Velho and Minerals 2020, 10, 983 3 of 38 Cuiabá deposits [8], as well as for Lamego [9], setting ages of 2672 +/ 14 Ma and 2730 +/ 42 Ma, − − respectively, for gold mineralization in these deposits. The goal of the present article is to investigate the Archean deformation of the QF and to establish the nature of its relation to the associated gold mineralization in the Rio das Velhas greenstone belt. This is accomplished through a tectono-structural, regional evolutionary model that focuses on the western structural domain of the QF, that is, on the Nova Lima and Caeté gold districts—Where the referred deposits (Cuiabá, Lamego, Raposos, Morro Velho and Faria) are located- as well as their surroundings (Figure2, Figures 4 and 5) [9–18]. Figure 2. Geological map of the Quadrilátero Ferrífero and surroundings (modified from [19]). Batholiths and plutons: Pe—Pequi; Fl—Florestal; SN—Souza Noschese; Sa—Samambaia; Ma—Mamona; Ca—Caeté. Shear zones: PSZ—Pitangui; VSZ—São Vicente; RSZ—Raposos; CSZ—Congonhas. Megafolds: P—Pitangui; M—Mateus Leme; S—Souzas; M—Moeda; B—Dom Bosco; G—Gandarela; C—Conceição. Gold mines: M—Morro Velho; F—Faria; R—Raposos; L—Lamego; C— Cuiabá. Cities and localities: DV—Divinópolis; BH—Belo Horizonte; CA—Caeté; SB—Santa Bárbara; NL—Nova Lima; CL—Cláudio; BF—Bonfim; IT—Itabirito; OP—Ouro Preto; CG—Congonhas; ER—Entre Rios de Minas; CL—Conselheiro Lafaiete; PI—Pitangui. One of the fundamental questions of interest pertains to the influence of the Paleoproterozoic Minas orogeny on the Archean mineralization process. For this purpose, we present the following throughout the paper: (i) a revision of the cartography and stratigraphy of the QF; (ii) an integration of the data referring to the structural regional outline; and (iii) our reinterpretation of the data by other relevant authors. To complement this analysis, we also put forth the field data retrieved by this article’s authors, as well as other petrographic, geochemical, geophysical, and geochronological data (of recent publications) and summarized revisions of older works. Finally, for greater clarity, we have chosen to employ the original, pre-metamorphic terminology when referring to igneous and sedimentary rocks, thus avoiding stratigraphic nomenclature. 2. Geology and Structure of the Quadrilátero Ferrífero—A Revision A summary of the tectonic and geological evolution of the QF region can be found in the works of various authors [20–23]. The lithostratigraphic outline of the QF is formed by (i) Archean TTG granite-gneissic terranes; (ii) Paleo- to Neoarchean-metavolcano-sedimentary sequences, including the Rio das Velhas Supergroup; and (iii) Paleoproterozoic to Neoproterozoic, metavolcano-sedimentary Minerals 2020, 10, 983 4 of 38 and metasedimentary sequences, represented by the Minas and Espinhaço supergroups and by the Sabará and Itacolomi groups (Figures2 and3). Figure 3. Stratigraphic column of the supracrustal sequences in the Quadrilátero Ferrífero region (based on [19,23–25]). Synformal mega folds define the geometry of the QF and these are truncated by N–S-directed thrust faults in its eastern portion. They also define the QF western and southern borders, embodied by the Moeda and Dom Bosco synclines, respectively (Figures2,4 and5). To the north, the Serra do Curral homocline represents the inverted flank of a large synclinal fold [24,26–28]; to the east, the Gandarela, Ouro Fino, Conta História and Santa Rita synclines are disposed in an ample, N–S-directed arch, affected by Neoproterozoic shear zones of the Araçuaí orogeny, e.g., [6,22,23,29,30]. All these synformal Minerals 2020, 10, 983 5 of 38 troughs are filled by metasedimentary rocks that originate from the Minas Supergroup and the Sabará and Itacolomi groups. Figure 4. Structural map of the Quadrilátero Ferrífero showing the main structures