Structural Evolution of the Rio das Velhas Greenstone Belt, Quadrilátero Ferrífero, Brazil: Influence of Proterozoic Orogenies on Its Western Archean Gold Deposits

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.

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 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) (Figure 1).

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 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 (Figure 2, Figures 4 and 5) [9,10,11,12,13,14,15,16,17,18].

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 [20,21,22,23]. 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 and metasedimentary sequences, represented by the Minas and Espinhaço supergroups and by the Sabará and Itacolomi groups (Figure 2 and Figure 3).

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 (Figure 2, Figure 4 and Figure 5). To the north, the Serra do Curral homocline represents the inverted flank of a large synclinal fold [24,26,27,28]; 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 troughs are filled by metasedimentary rocks that originate from the Minas Supergroup and the Sabará and Itacolomi groups.

Juxtaposed unto the eastern synclines are the Vargem do Lima syncline and the Mariana anticline—the former constituted by Rio das Velhas Supergroup lithotypes. Rocks from the Rio das Velhas and Minas supergroups compose the Mariana anticline, which is referred to as the Rio das Velhas uplift [24] in its northern section. These supracrustal sequences are molded around granite-gneissic complexes: the Bação, Bonfim, Belo Horizonte, Caeté, Santa Rita and Santa Bárbara domes (Figure 2, Figure 4 and Figure 5).

2.1. Geological Units

There are seven important lithostratigraphic units present in the QF: (i) the TTG granite-gneissic basement; (ii) the Rio das Velhas Supergroup; (iii) the Minas Supergroup; (iv) the Sabará Group; (v) the Itacolomi Group; (vi) the Post-Itacolomi mafic dikes; (vii) the Espinhaço Supergroup (Figure 3).

TTG Granite-Gneissic Basement—This includes the Belo Horizonte, Bonfim, Bação, Caeté, Santa Bárbara, Santa Rita and Campo Belo complexes (Figure 2), with Paleo- to Mesoarchean ~3.2 Ga to 2.8 Ga evolution [31,32,33,34,35,36,37,38,39,40]. Geochronological dating (U-Pb, SHRIMP and LA-ICP-MS in zircon [19,41,42] indicates four significant magmatic events: (i) Paleoarchean (3220–3200 Ma) Santa Bárbara, with an older TTG crust; (ii) Mesoarchean (2930–2850 Ma) Rio das Velhas I, with juvenile TTG magmatism and the accretion of greenstone mafic-ultramafic terranes to the pre-existing crust [41]; (iii) Neoarchean (2800–2760 Ma) Rio das Velhas II, with final production of TTG crust, plutonism, felsic volcanism, and Rio das Velhas greenstone belt sedimentation; (iv) Neoarchean Mamona, with the presence of potassic magmatism related to the Rio das Velhas greenstone belt evolution [42]. The Santa Bárbara and Rio das Velhas I events compose the TTG granite-gneissic crust that acted as a basement for the Rio das Velhas greenstone belt, whereas Rio das Velhas II and Mamona represent cycles involving oceanic crust subduction and collision of two continental blocks [19].

Rio das Velhas Supergroup—Defined by [43], it is subdivided from base to top into the Nova Lima and Maquiné groups [24]. The former consists of ultramafic, mafic, and felsic volcanic rocks, banded iron formations (BIF), sandstone and greywacke. The Maquiné Group that overlaps it—alternating along unconformable or gradational contacts—was divided into the Palmital [44] and Casa Forte [5] formations. The basal Palmital Formation is constituted by sandstones and quartz sandstones, and the Casa Forte by sandstones and conglomerates [24]. The first description of the Rio das Velhas sequence as a greenstone belt is due to [45,46]. Ladeira in [47] formalized the Rio das Velhas Supergroup designation; subsequently, the Archean peridotitic komatiites of the eastern QF were defined as Quebra Osso Group, a basal unit of Rio das Velhas greenstone belt [48,49]. Finally, a subdivision proposal was presented for the Nova Lima and Maquiné groups [6] based on the recognition of lithofacies associations [6,50].

Minas Supergroup—The Minas Supergroup plataformal clasto-chemical sedimentation [24,43,47] unconformably overlies the Rio das Velhas Supergroup. The basal units are sandstones and fluvial conglomerates, which transition into marine argillites (Tamanduá and Caraça groups). The transitionally overlapped Itabira Group has a basal Lake-Superior-type BIF, which transitions into the Gandarela Formation carbonate sequence. Shallow-water, deltaic to marine sediments of the Piracicaba Group unconformably rest over the Gandarela Formation. The Minas Supergroup must have been deposited around the 2580–2150 Ma interval [51]. Contributions to the current body of knowledge about its evolution may be found in [52,53,54,55]; these analyses mainly take into consideration the U-Pb dating of detrital zircons within the sedimentary rocks of this supergroup.

Sabará Group—The Sabará Group [24,43,51] is a volcano-sedimentary sequence. Flysh-type sedimentary rocks predominate, occurring with angular unconformity over the Piracicaba Group rocks. Its deposition took place in the 2.12–2.09 Ga interval [56,57,58,59,60]. The Sabará rocks were deposited in a compartmented foreland basin [61].

Itacolomi Group—Sandstones, conglomerates and pelites of the Itacolomi Group rest with angular unconformity over the Piracicaba and Sabará groups [24] and produce deposits in intermontane troughs [23,24,62]. The U-Pb zircon dating in sandstones of 2.05 Ga sets the limit for its deposition age [57,58].

Post-Itacolomi Mafic Dikes and Sills—These N- to NW-directed dikes crosscut the Paleoproterozoic sedimentary rocks and other basal units of the QF. The age of 1.70 Ga that has been obtained for one of these dikes near the town of Ibirité (north of the QF) suggests an intrusion related to the opening of the Espinhaço basin [63].

Espinhaço Supergroup—In the extreme northeastern portion of the QF, in the Cambotas and Tamanduá mountain ranges, sandstones and conglomerates of the Cambotas Formation outcrop. These are equivalent to the basal unit of the Espinhaço Supergroup [24,64,65,66].

The metamorphism in the granite-gneissic basement (Paleo- to Mesoarchean) of the QF reaches the granulite facies. Mineral assemblages of rocks from the Rio das Velhas greenstone belt attain amphibolite to greenschist metamorphic conditions. The sedimentary platformal Minas rocks metamorphism is of the greenschist facies [24,32,43,67,68,69]. According to [22], the Rio das Velhas and Minas supergroups were subject to two greenschist facies metamorphic events during the Paleo- and Neoproterozoic.

2.2. Structural Evolution

2.2.1. Summary of Existing Literature

Workers have debated the structural evolution of the QF since the middle of the last century. The following geological interpretations are important landmarks: (i) the first proposal for stratigraphic column and structural model [70]; (ii) identification of an angular unconformity at the base of the Itacolomi Group [71]; (iii) description of an angular and erosive unconformity at the base of the Minas Supergroup [72]; (iv) an unconformity between rocks of the Piracicaba and Sabará groups [24,26,73] (Figure 3).

Although all of the above-mentioned authors concur on a polycyclic and polyphasic evolution, most of their structural syntheses on the QF tend to focus on the deformation of the Minas Supergroup sedimentary rocks. They particularly recognize the roles of the Minas and Araçuaí orogenies, which correspond to the Transamazonian and Brasiliano orogenic cycles, respectively. This is evidenced by some well-known contributions, such as [22,23,30]. Also, as suggested by [74], the the QF could have experienced a single polyphasic and progressive, compressional, ductile deformation during the Brasiliano orogenic cycle. More recently, some researchers attribute the structural framework of the QF to nappe tectonics related to the Transamazonian event [75].

Dorr [24] does not recognize that a pre-Minas deformation event affected the TTG basement, which was considered as a set of intrusive granitic rocks. He does, however, acknowledge: (i) a pre-Minas deformation period, which solely affected the Rio das Velhas Supergroup; (ii) the deposition, arching, and uplift of the Minas Supergroup; and (iii) a younger major orogeny affecting the Itacolomi Group, responsible for the fold-and-fault belt in the eastern domain.

Years later, other authors acknowledged that the pre-Minas deformation also affected the basement [2,6,9,29,76,77,78,79].

2.2.2. Deformation Phases—D1 to D4 and DE

Five sets of structural, planar, and linear elements have been summarized based on the data collected from the existing literature on the structural evolution of the QF [6,22,23,24,29,30,72,76,79]. Grouped into four deformational, compressional phases (D1 to D4) and one extensional phase (DE), they are ordered and described from older to younger, following criteria such as overlay, different styles, orientation and cross-cutting relationships (Figure 4). Tectono-stratigraphic and structural relationships, in addition to the scarce geochronological data, make it possible to relate these phases to their respective orogenies or regional events.

A pre-Rio das Velhas greenstone belt deformation, exclusive to the TTG basement, is represented by a subvertical, N–S foliation, associated with transcurrent shear zones. The best outcrops/exposures are in the Alberto Flores Gneiss [32] of the Bonfim dome [30,78] (Figure 4).

D1 structures are associated with the evolution of the Rio das Velhas greenstone belt—Rio das Velhas orogeny [33] of the Jequié orogenic cycle (Neoarchean). They occur in the Nova Lima Group [6]. The main structures are the S-verging, E–W oriented Bem-Te-Vi thrust shear zone and the likely Andaime fold, located N of the Bação dome [6] (Figure 4). The axial-planar foliation associated with folding is a transposition-like mylonitic foliation, arising in highly deformed zones. Its general orientation is E–W, with a high dip angle and a parallel mineral lineation that plunges to the north. Kinematic markers of the Bem-Te-Vi shear zone—such as S/C foliations, mini-fold vergences, and mineral/stretch lineation—define the N to S tectonic transport, thus producing a fold-thrust belt due to tangential, compressional, ductile Neoarchean tectonics. The preservation of D1 structures is attributed to their specific orientation, which is almost orthogonal to the direction of the tectonic transport of the ensuing D2 deformation [6].

The evidence for D1 is also described by [24,72]. D1 is related to transcurrent Archean tectonics [79], following the model proposed by [78,80].

D2 structures crop up in the Maquiné and Nova Lima groups [6]. They are best featured in the São Vicente and Raposos shear zones (Figure 4), associated with SW-vergent, tight-to-isoclinal folds, and an axial planar to transposition/mylonitic foliation, with an average attitude of 060/35 [81]. Along the foliation planes, the mineral lineation varies between 070/25 [81] and 060/20 [79]. In the Andaimes range, to the north of the Bação dome, such NW–SE structures truncate and dislocate the D1 structures (Figure 4).

To the northeast of the town of Rio Acima, the Casa Forte Formation rests with an angular unconformity over the Nova Lima Group top turbidites; pebbles of deformed BIF are found in basal conglomerates of this group [6]. It is noteworthy that the Casa Forte Formation was deformed by D2. Both the SW vergence of D2 folds and the existing ENE–WSW-directed stretch lineation indicates mass movement towards WNW along NW-SE shear planes, with medium dips to NE. These are thrust shear zones with a subordinate sinistral, directional component [6].

The D2-related structures have been described by [3,76,79,81,82].

D3 structures are registered in the Sabará Group, as well as in the Minas and Rio das Velhas supergroups. They produced a NE–SW-oriented, NW-vergent fold-thrust belt in a ductile-brittle regime, with an SE to NW tectonic transport. The belt, in turn, was responsible for the aggregation of large-scale folded structures—such as the Serra do Curral homocline, the Serra da Piedade and Gandarela synclines, and the Conceição anticline [22,23] (Figure 4). Smaller-scale structures include: (i) the Serra do Curral anticline, located at the junction between the Serra do Curral homocline and the Moeda syncline [23,83,84]; (ii) S-shaped, drag mesofolds located on the eastern border of the Moeda syncline, as is the case in the Gama mountain range area [22,30]; and (iii) NE–SW oriented and NW-vergent asymmetrical tight folds and thrust faults in the western extremity of the QF [22]. Structures of the Minas Supergroup also attributed to D3 are: (i) directional displacements along the Mutuca and Gorduras shear zones [24,84], on the eastern flank of the Moeda syncline; (ii) mesofolds within phyllites of the Piracicaba Group, belonging to the core of the syncline, with axial traces transversal to the axis of the syncline [85].

In the Nova Lima Group, the D3 foliation is recognized by all researchers who have investigated the Nova Lima and Caeté gold districts [2,3,6,13,14,15,16,76,86,87,88,89,90,91,92,93,94]. The D3 structures are attributed to the compressional phase of the Paleoproterozoic Transamazonian orogenic cycle [22,23,75]. They correspond to the Minas accretionary orogeny in the extreme SE of the São Francisco Craton [95].

DE structures are of extensional character and associated with the collapse phase of the Trasamazonian orogeny. They are, therefore, related to the uplift of the basement complexes [22,23] especially evident around the Bação dome (Figure 4 and Figure 5). Not only did this generate a contact metamorphic aureole, but the Bação ascension also imprinted normal shear zones on its borders. These present a mylonitic, high-angle foliation, and a down-dip mineral lineation that plunges outwards the dome—printed on both the dome itself and on the rocks of the Nova Lima Group [6].

Also related to DE are the normal faults registered in the contact zones between the Bonfim dome and the Moeda syncline [96], in addition to the Belo Horizonte dome and the Serra do Curral homocline, on the northern and western borders of the Bonfim dome [23,30] (Figure 4 and Figure 5). Such faults are WNW–ESE-oriented in terms of their regional field vectors [30]. The Itacolomi Group is associated with the formation of intermontane basins during the DE phase [23]. DE developed a dome-and-keel structure in the QF region [22,23,97] around 2095 Ma (whole-rock Sm-Nd; [59]). Finally, the uplift of the basement blocks as metamorphic core complexes is proposed [6,30].

D4 structures are related to the E–W compressional Neoproterozoic Brasiliano orogeny. This molded the borders of the São Francisco Craton to what we know them to be today, greatly impacting the eastern half of the QF and imprinting a W-vergent, N–S-oriented fold-thrust belt [6,22,23,24,29,30,76]. The Araçuaí belt constitutes the outer domain of the Araçuaí-Western Congo orogeny, an Ediacaran-Cambrian orogenic system that ensued between the São Francisco and the Congo cratonic regions during the Western Gondwana amalgamation [98,99,100].

The D4 structures affect all Proterozoic as well as subjacent units [6,22,23,24,29,30,76]. In the QF region, the related shear zones have a mylonitic foliation plunging to E, an ESE stretching lineation, and kinematic markers indicating thrusting to W. The W-vergent, isoclinal-to-tight folds, with axial-planar foliation and lineation following the dip direction associate with zones of high deformation. They progressively open when furthering away from these zones.

In the eastern domain of the QF, D4 progressively overlaps younger thrust fronts to the east; its final pulse is an open fold, with associated N–S crenulation cleavage. In the north-central part of the QF, an E–W-directed mylonitic zone has a moderate to shallow dip to south. It constitutes an interference zone that results from the superposition of the Ribeirão da Prata and Fundão-Cambotas thrust fronts over the Caeté and Córrego do Garimpo thrust fronts, respectively to the south and north (Figure 4). Sub-horizontal mineral and stretching lineations as well as kynematic markers define this structure as a lateral ramp, associated with such Brasiliano-age thrust faults [6,76]. In addition to the N–S-directed fold-and-thrust belt, E–W and N–S folds and cleavages of the QF are also considered to belong, respectively, to the second and third deformation phases of the Brasiliano orogeny [30].

3. The Rio das Velhas Supergroup

3.1. Stratigraphy

The Rio das Velhas Supergroup is informally subdivided into seven lithofacies associations [6], thus preserving the lithostratigraphy of [24] for the Nova Lima and Maquiné Groups (respectively, the base and top of the sequence). These associations are characterized as: (i) mafic-ultramafic volcanic (peridotitic komatiite, magnesian basalt); (ii) chemical-volcano-sedimentary (tholeiitic basalt, BIF); (iii) clasto-chemical sedimentary (pelite, BIF); (iv) volcaniclastic (agglomerate, tuff, sandstone, turbidite); (v) resedimented (epiclastic turbidite); (vi) coastal (sandstone, rythmite); and (vii) non-marine (sandstone, conglomerate). The Quebra Osso Group’s peridotitic komatiites [49] and the Córrego dos Boiadeiros mafic-ultramafic sequence [101] are both included in the mafic-ultramafic basal association. The Palmital Formation is repositioned from the base of the Maquiné Group to the top of the Nova Lima Group [6].

For the basaltic volcanism, whole-rock Sm-Nd dating provided the age of ca. 2927 +/− 180 Ma [2], with a considerable error margin. Felsic volcanism provided ages around the 2776–2772 Ma interval (U-Pb in zircon) [102]); ages of ca. 2792 +/− 11, 2773 +/− 7 and 2751 +/− 9 Ma (SHRIMP and ID-TIMS, U-Pb in zircon) were obtained for a volcaniclastic greywacke, and a ca. 40 m. y. interval was proposed for the felsic volcanism [103]. The U-Pb dating of detrital zircons of sedimentary rocks have, in turn, determined the maximum deposition ages for the: (i) Non-marine Association (Maquiné Group), at ca. 2730 Ma [25]); (ii) Resedimented Association (Palmital Formation) at ca. 2744 Ma [25]; and (iii) Coastal Association at ca. 2750 Ma [52].

3.2. Deformation Phases (Dn1 to Dn4) and Metamorphism

In the past decades, most of the work focusing on the structural progress of the Rio das Velhas Supergroup, particularly the Nova Lima Group’s, results from the investigations held near to the Nova Lima and Caeté gold districts. Four deformation phases are indicated [88,90,104,105] and are hitherto named Dn1 (the oldest) to Dn4:

• The interpretation of Dn1 is based on a mylonitic foliation parallel to the bedding, with isoclinal folds as well as mineral and intersection lineations, and with dips to the east (Figure 6a). Dn1 is described in the Cuiabá, Lamego, Raposos, Faria and Morro Velho deposits [9,14,16,17,88,106,107,108,109]. In Raposos, Dn1 shear zones are attributed to dextral compressive movements and have ‘z’ folds associated with them [109]. Evidence of transcurrent movements in Dn1 shear zones is apparent both in Raposos and Morro Velho [12,89]. In Cuiabá, dextral movement is also registered in the Dn1 shear zones [105].
• Dn2 presents more pronounced foliation and lineation patterns, both related to a close-to-isoclinal folding of the bedding/foliation of Dn1. The Dn1 and Dn2 folds are coaxial (Figure 6a), with axes to E. Thrust shear zones developed at the end of Dn2 [9,14,15,16,89,105]. In Cuiabá and Lamego, the late-stage, Dn2, oblique thrust-shear zones intercept the larger Dn1 folds [9,14,15,16,86,105] (Figure 6b).
• Dn3 shows an E–W/55N crenulation cleavage and an axial plane of open, normal-to-inclined folds, with axes plunging at a low angle to the E, registered in the region of the Raposos deposit [104].
• Dn4 has a N–S/40–65E crenulation cleavage, an axial plane of gentle folds—the result of its previous foliations—And a S10W/30 to S10E/35 crenulation lineation [88,104].

The open folds of Dn3 and Dn4 and their respective cleavages are omnipresent, though not pervasive. They are overprinted on the planar and linear structures of Dn1 and Dn2.

The Nova Lima Group deformation is differently interpreted by some authors. Although a few of them also consider the deformation to be progressive [13,14,16,17,18], they divide it into three phases, from Dn1 to Dn3. In this classification, all the N–S and E–W cleavages are grouped into the same third phase (Dn3). Such criteria were established in the study of certain gold deposits:

• Raposos. According to [106], the N–NW orientation of the Rio das Velhas rocks (Dn1) in the Raposos deposit area precedes the deposition of the (then, so-called) Minas Series. The author also states that the direction and plunge of folds to E (Dn2) that crosscut the Dn1 structures were produced by the same N–NW movements that reversed rocks of the Minas Series [5]. On the other hand, the Dn2 foliation in Raposos is associated to a deformation event that would have followed the Minas Supergroup sedimentation [104]. Dn2 in Raposos is attributed to the Brasiliano orogeny by [107].
• Cuiabá. Three deformation phases are acknowledged [14,15]. They are attributed to a compressive and progressive event originated during the Brasiliano and/or Paleoproterozoic, given their common tectonic transport direction—from E–SE to W–NW.
• Lamego. Dn1 and Dn2 are regarded as progressive Archean phases, responsible for gold mineralization, whilst the N–S cleavage (Dn3) is ascribed to the Brasiliano compression [9].

Three phases of deformation are recognized in the Nova Lima Group, in the region between Raposos, Caeté and Sabará: (i) foliation parallel to the E–W bedding, of Archean age; (ii) penetrative NE–SW, Transamazonian foliation; and (iii) NS crenulation cleavage related to the Brasiliano orogeny [92].

The regional metamorphism of the Rio das Velhas greenstone belt rocks is of low greenschist facies inside the QF, except around the Bação dome, where it is of medium greenschist to medium amphibolite facies, with retro-metamorphism to greenschist facies conditions [6,112]. According to [32], the regional greenschist-to-amphibolite metamorphism occurred around 2700 Ma and was associated with the mineral paragenesis oriented along the Archean metamorphic-mylonitic foliation [6,112].

The contact metamorphism that affected metapelites surrounding granite-gneissic domes is assigned to their uplift due to the Transamazonian orogenic collapse. The process also produced some dome-and-keel structures [22,23]. Contrarily, dome-and-keel structures were formed beforehand, during the Archean, owing to an orogenic collapse around 2700 Ma [113]. The authors suggest the structures were reactivated during the Transamazonian extension and intercepted the Archean troughs, thus generating metamorphic aureoles.

Greenschist facies metamorphic conditions have also affected the Minas Supergroup and are related to the Transamazonian compression, which occurred around 2000 Ma [22,69]. Finally, a metamorphic event for the QF region could have ensued from the Brasiliano orogeny at the 400–600 Ma interval [22].

4. Structural Control of the Archean Gold Deposits Hosted in the Rio das Velhas Supergroup

4.1. Regional and Deposit Scales

The most important gold mineralizations of the QF occur in the Nova Lima Group. They have been historically explored by prospectors since the 18th century [114]. The hydrothermal, “gold-only”, orogenic-type mineralization [4,115,116,117,118] is controlled by shear zones. Chlorite, carbonate, and sericite zones are common throughout the host rocks, with silicification and sulfide enrichment in their innermost parts.

The importance of structural control for the distribution of gold deposits has been a known factor for quite some time [114]. Although they are hosted in any one of the Rio das Velhas greenstone belt lithotypes, the most productive deposits are in BIF ± ferruginous chert [2,118] and lapa seca [118,119].

Regional NW–SE shear zones control the distribution of the gold mineralization in the QF region and its surroundings. Mineralization is associated with a mylonitic foliation, parallel to subparallel to the folded and transposed bedding. The Raposos and São Vicente shear zones (Figure 2, Figure 4 and Figure 5)—Intercepted at their edges by the deposition of synformal throughs of the Minas Supergroup rocks—are examples of this [6]. To the NW of the QF, the Pará de Minas and Pequi shear zones are parallel to the strata and maintain an NW–SE direction [120]. As for the SE portion, mineralization gathers along the Congonhas shear zone (Figure 2) [121,122].

At the deposit scale, mesofolds and smaller shear zones control the orebodies. They may be directional or thrust, of second and third orders. Sulfidized gold zones are distributed throughout the whole folded layer, particularly BIF and lapa seca [118]; they are, however, preferentially concentrated within the hinges of subsidiary mini-folds, or in the boudins of their flanks. Such folds are intrafolial and molded onto the bedding, parallel and subparallel to a mylonitic foliation. In some deposits, sulfidized and mineralized quartz-carbonate veins are contained in mylonitic foliation of oblique, thrust shear zones. They are either subparallel to or crosscut the previously mineralized structures at high angles (Figure 6b). Finally, the plunge of orebodies is controlled by the axes of these Archean folds, and is parallel to a stretching lineation [9,14,15].

In the Faria deposit, orebodies are distributed throughout five BIF layers, folded onto reversed and conjugated anticlines and synclines (Figure 6c) [86]. In Cuiabá, gold orebodies associated with the Dn1 shear zone are distributed along the entire Cuiabá fold [105]. The oblique thrust shear zones, formed late in the Dn2 phase, intercept the Dn1 large folds and act as a conduit to the main gold zones of BIF (Figure 6b) [14,15,16,86,105]. In Raposos, one of Dn1 shear zones controls the BIF orebodies [13].

In the Morro Velho deposit, located in a segment of the São Vicente shear zone, lapa seca sets up a tight-to-isoclinal, large ‘z’ fold [123], with an axial E–W oriented trace [17]. Subsidiary folds presenting the same ‘z’ vorticity (attributed to Dn1) control the orebodies (Figure 7) [17,119]. The South and Main orebodies occupy anticlinal hinge-zones [119], that set forth ‘z’ shapes of the Dn1 first deformation phase (Figure 7) [17].

At the orebody scale, structures that control gold vary significantly. They include: (i) small-scale, ductile to ductile-brittle shear zones; (ii) axial planar cleavages; (iii) fractures and joints; (iv) mini-fold axes; (v) breccia zones; (vi) boudinaged bodies; and (vii) tear faults [2,6,9,11,14,15,87,88,105,124,125].

4.2. Gold Mineralization Styles, Dn1 and Dn2

Three main mineralization styles were introduced, at the time referred to as “types” [88]. The authors correlated these styles to their associated sulfide minerals and to the deformation phases Dn1 and Dn2. The study focused on deposits and occurrences located within the Nova Lima and Caeté gold districts, where the main host types are BIF and lapa seca. It described the following phenomena:

• A stratabound, replacement style, hosted in BIF and lapa seca, where the original bedding is commonly obliterated and anastomosed. The process is controlled by Dn1 shear zones and associated folds. Quartz boudins, tension gashes, and massive sulfide zones are typical. Pyrrhotite is dominant, with pyrite and arsenopyrite as subordinate sulfides—Type 1.
• Replacement style in BIF and lapa seca layers, migrating from Dn2 fractures and shear zones that cross-cut Dn1 host rock structures, in either a subparallel or high angle. Banded or disseminated sulfide minerals are typical. They are rich in pyrite, arsenical pyrite and arsenopyrite—Type 2.
• Quartz-sulfide vein style in sericite and carbonate hydrothermal alteration zones of phyllites and schists, occurring in a variety of host sedimentary and volcanic rocks. This is pyrite rich, typically disseminated, with scattered, subordinate pyrrhotite. Veins form along Dn1 and Dn2 shear zones—Type 3.

Type 1 is dominant in Raposos [88,90,126] and Morro Velho [89]. It is scarce in Cuiabá [89], where type 2 is abundant [88,89]. Type 2 is also dominant in Lamego, and subordinate in Raposos [88,90,126], Morro Velho and Faria [89]. Type 3 occurs subordinately in most deposits, such as in the Viana and Galinheiro Quartzo orebodies in Cuiabá [16,94] and the Ouro Preto orebody in Raposos [13]. This is also typical for deposits of the Nova Lima Group’s other gold districts.

5. Discussion

5.1. Geological Evolution of the Quadrilátero Ferrífero

5.1.1. Archean

The NW–SE-oriented synclinorial basins seem to have dominated the QF region throughout its evolution process. They are implanted on the Paleo- to Mesoarchean, TTG granite-gneissic basement and are filled by volcano-sedimentary sequences. The analysis of the geographic and geometric distribution of the strata that make up the Rio das Velhas greenstone belt inside the QF and its correlation with similar sequences, mapped in the NW and SE portions, evidence such a regional arrangement.

To the NW of the QF, the Pitangui syncline makes up a synformal trough, filled by the Archean volcano-sedimentary sequence of the Pitangui Group. The sequence has around 50 km of longitudinal extension with a NW–SE direction; its NE and SW borders are marked by left-lateral reverse shear zones, into the regional stratification [120]. This is the Pitangui–Pequi Syncline of [127], which continues towards SE in the form of the Mateus Leme and Souza synclines [23], (Figure 2). To the S–SE of the QF, the Congonhas-Itaverava volcano-sedimentary belt also configures a syncline, with an NW-SE axis and steeply dipping flanks, marked by the Congonhas shear zone [122,128]. The Congonhas shear zone, in turn, has a left-lateral reverse movement, is subparallel to the original bedding, and hosts gold mineralization [122] (Figure 2).

Despite the overlap of three orogenic events, i.e., Rio das Velhas, Minas, and Araçuaí, the original distribution of the Nova Lima Group strata is subject to reconstitution. The large tectono-stratigraphic units of the Rio das Velhas greenstone belt still preserve a general NW–SE orientation and show some symmetry and polarity through the repetition of units in their lateral distribution along the main NW–SE axis—which approximately accompanies the axis of the Vargem do Lima syncline (Figure 5 and Figure 8a). In both NE and SW positions, towards this axis, there are: (i) basal mafic-ultramafic volcanic rocks in contact with the TTG gneisses; (ii) tholeiitic basalts with BIF; and, in the innermost parts, (iii) volcaniclastic rocks and epiclastic turbidites. This ensemble constitutes what we hitherto define as the Nova Lima synclinal basin (Figure 8b).

The present contribution reinterprets the basal portion of the Rio das Velhas Supergroup stratigraphy. Such stratigraphical reassessment better envisages the regional distribution of these Archean units and of the proposed symmetry [6] that subdivides the Nova Lima and Maquiné groups into seven lithofacies associations. We suggest that the mafic-ultramafic rocks at the base of the Nova Lima Group, which are part of the Mafic-Ultramafic Association [6], should be correlated to the Quebra Osso Group [48,49] (Figure 5). This would include the rocks located: (i) west of Caeté [74]; (ii) in the Córrego dos Boiadeiros [101]; and (iii) by Piedade do Paraopeba [129], which komatiites and basalts present rhyodacite intercalations [129]. Acidic to intermediate volcanic rocks mapped west of Caeté (Figures 5 and 12a) [74] are at the interface with the Volcano-Chemical-Sedimentary Association. These volcanic rocks are here included at the top of the Quebra Osso Group, where it seems to represent the closure of the basal volcanic cycle.

According to this new interpretation, the Volcano-Chemical-Sedimentary Association constitutes the base of the Nova Lima Group. The subsequent Clasto-Chemical-Sedimentary Association is characterized by pelagic sedimentation that followed basaltic volcanism, from the final extension/opening phase of the Nova Lima basin (Figure 9a). On the other hand, the Volcaniclastic and Resedimented associations represent, respectively, felsic volcanic centers and orogenic sedimentation of the basin’s inversion (Figure 9b). The sandstones and rhythmites of the Coastal Association are, in turn, platform sedimentary rocks on a stable continental margin [6] (Figures 3 and 5). They manifest as a contemporary deposition onto the turbidites of the Palmital Formation, at the top of the Nova Lima Group.

Nova Lima basin—The Nova Lima basin comprises mafic-ultramafic volcanic rocks belonging to the Quebra Osso Group, set in a relatively external position at its NE and SW limits and found in contact with basement TTG gneisses. Tholeiitic basalts with BIF from the Volcano-Chemical- Sedimentary Association at the base of the Nova Lima Group occur internally and in two bands, that is, the regions of Caeté and Nova Lima-Raposos. The Volcaniclastic and Resedimented lithofacies associations are the basin’s innermost units, with the latter dominating its central corridor. Active margin granitoid rocks (the Caeté Granodiorite and the Samambaia Tonalite [102]) are intrusive near the basement/greenstone sequence contact, respectively E and W (Figure 2, Figure 5 and Figure 9b).

The evolution of the Rio das Velhas greenstone belt started with the extrusive komatiitic ultramafic-mafic magmatism of the Quebra Osso Group [131] onto a submerged TTG sialic crust, with the Caeté [74] and Piedade do Paraopeba [129] felsic volcanism closing the basal volcanic cycle.

The predominance of epiclastic and volcaniclastic sedimentary rocks within the Nova Lima Group (Figure 5 and Figure 8) and the pre-existence of a sialic basement (Figure 2 and Figure 5) [41] suggest that bimodal volcanism dominated during the evolution of the Rio das Velhas greenstone belt. Such volcanism varied from an early tholeiitic basaltic-andesitic magma to a later dacitic-rhyolitic one, with the simultaneous existence of mafic and felsic magmas [132]. Turbidites from the Resedimented Association comprise the final stages of the basin’s inversion.

The tectono-stratigraphic characteristics of the Quebra Osso-Nova Lima’s volcano-sedimentary sequence suggest a back-arc basin as the main deposition site. This interpretation is corroborated by the Nova Lima basin’s elongated synformal geometry, which preserves the same symmetry and polarity in the distribution of volcanic and sedimentary rocks, in addition to the marginal positioning of granitoid magmatism of the active margin (Figure 8 and Figure 9).

The stratigraphic sequence at the Cuiabá gold deposit has andesite at the base and overlapping basalt, separated by BIF (main gold host) and carbonaceous pelite, with rocks from the Volcaniclastic Association at the top [14,15,16,87,88,89,105]. This must indicate the occurrence of mafic flows onto volcaniclastic sedimentary rocks on the flanks of volcanic felsic centers, with an evolution similar to that proposed by [133] for part of the Abitibi greenstone belt, Canada (Figure 10).

Maquiné basin—At the end of the Nova Lima basin inversion—therefore, already in the syn-collisional stage of the Rio das Velhas orogeny—the implantation of the Maquiné basin began, with the deposition of continental sediments on the lower strata with angular unconformity (Figure 9b and Figure 11b). The characteristic sedimentation of the Casa Forte Formation, in the case of the Vargem do Lima syncline, with alluvial sedimentary rocks lying unconformable over the sediments belonging to the top of the Nova Lima Group, indicate a retro-arc foreland basin [6]. The Maquiné sedimentation took place in a converging margin basin, with the rapid exhumation of its sediment source and a clear aging of the source area throughout its evolution [25]. As for the Rio das Velhas orogeny, deformation phases D1–D2 (Table 2) converted the Nova Lima and Maquiné synformal basins into the inverted Nova Lima synclinal, an NNW–SSE-oriented, SSW-vergent fold belt (Figure 9b and Figure 11a,b).

5.1.2. Paleoproterozoic

During the Paleoproterozoic, the southern portion of the São Francisco Craton was affected by the Mineiro belt (2.47–2.10 Ga). The belt represents a segment of a complex system of long-lasting, continental and oceanic magmatic arches that preceded the amalgamation of the São Francisco-Congo paleocontinent [95].

The large iron ore deposits of the QF are located in the foreland zone of this Transamazonian orogen, where the interconnected Minas basins were implanted at the end of the Archean and beginning of the Paleoproterozoic [51]. The sedimentary rocks of the Minas Supergroup were originally deposited in a passive continental margin environment, over the TTG-gneiss basement and the previously deformed Archean greenstone sequences. The Sabará Group’s volcano-sedimentary sequence rests on an erosional surface cut on rocks of the Piracicaba Group and was dominated by turbiditic synorogenic sediments.

The Minas and Sabará basins were inverted from SE to NW during the D3 compressional phase (

Table 1. Synthesis of structural evolution of the Archean Rio das Velhas greenstone belt and Proterozoic cover sequences in the Quadrilátero Ferrífero region.

Tectonic Event	Phase	Regimen	Tectonic Transport	Main Structures
Rio das Velhas orogeny	D1 1	Compressive, simple shear	NNE to SSW	NNW-striking, dextral transcurrent shear zone; ENE plunging, tight to isoclinal, ‘z’ intrafolial folds. E-striking, S-verging transpressive shear zone; S-verging tight to isoclinal folds. Inversion of the Nova Lima basin.
	D2 1	Compressive, simple shear	ENE to WSW	NNW-striking thrust shear zones. Reactivation of D1 shear zones. NW-verging, ENE-plunging tight to isoclinal folds. ENE-plunging stretching/mineral lineation. Inversion of the Maquiné basin.
Minas orogeny	D3 2	Compressive, simple shear	SE to NW	NE-striking, NW-verging thrusts. NW-verging tight to open folds. Stretching and mineral lineations plunging towards SE. EW- striking crenulation cleavage. Inversion of the Minas and Sabará basins.
	DE 2	Extensional	WNW to ESE	Uplift of granite- gneissic basement as domes. Normal faults around the domes. Intermontana Itacolomi basins.
Araçuaí orogeny	D4 3	Compressive, simple shear	E to W	NS-striking, W-verging thrusts. W-verging tight to isoclinal folds and open, normal folds. Stretching and mineral lineations plunging towards ESE. NS-striking crenulation cleavage. Inversion of the Itacolomi basin.

¹ [6], and this work; ² [23]; ³ [30].

) of the Transamazonian orogeny. The first to affect the Minas Supergroup (and the Sabará Group), D3 created most of the Supergroup’s large structures, such as the Serra do Curral homocline, the Piedade syncline, and the Gandarela syncline [23]. D3 also generated the Serra do Curral anticline, at the junction of the Serra do Curral homocline/Moeda syncline [23]. This nucleation may have been controlled by a section from the top of the basal substrate, formed when the Minas interconnected basins were opened, due to the effect of boudinage in turn related to the regional crustal extension (see Figure 14.21, page 280 of [134]). The Serra do Curral anticline should, therefore, solely account for one single segment of the large Serra do Curral homocline. The large, open-fold inflection in the Moeda syncline, located close to its junction with the Serra do Curral homocline, is also attributed to D3 (Figure 4). Although the nucleation of the Dom Bosco syncline is attributed to the Transamazonian DE extension [30], it is likely that its beginning still occurred during D3, when its southern flank was inverted. Like the Serra do Curral homocline, the Dom Bosco syncline has an axis almost orthogonal to the D3 tectonic transport direction.

Within the QF, the Rio das Velhas greenstone belt was affected by a wide D3 folding, with the occurrence of local shear zones mainly close to the contacts with strata of the Minas Supergroup (Figure 4). The NE–SW-oriented foliation, plunging to SE, is pervasive throughout the Nova Lima and Caeté auriferous districts and is plane-axial of this D3 fold (Figure 11c).

The DE deformation seems to have been responsible for the constriction of the Moeda syncline at the junction with the Dom Bosco syncline. The eastern flank of the Moeda syncline is molded onto the shape of the Bação dome, with bedding parallel to its border foliation. To the north of the Bação dome, the syncline’s flank is more steeply inverted, and segmented by D4 shear zones. DE also strictly reoriented some of the Rio das Velhas greenstone belt’s strata around the Bação dome (Figure 4).

5.1.3. Neoproterozoic

Fault-folding systems of the Araçuaí Neoproterozoic belt reached the entire eastern half of the QF, in response to an E–W compressional stress field. Their main structural elements are thrust-fault and N–S-directed folding systems that verge to W; they affect the gneissic basement, the Archean volcano-sedimentary sequences and the Proterozoic sedimentary and volcano-sedimentary covers. They were the first deformation to affect the Paleoproterozoic Itacolomi Group and the Paleo- to Mesoproterozoic Espinhaço Supergroup (Figure 2, Figure 4 and Figure 5). In the western half of the QF, the range of compression D4 is greatly attenuated in the Rio das Velhas greenstone belt, with smooth, N–S-oriented folds and a crenulation/fracture cleavage printed onto the pre-existing foliations (Figure 11d). Small magnitude shear zones are restricted.

5.2. Structural Evolution of the Rio das Velhas Greenstone Belt and Implication for Its Gold Mineralization

The four deformation phases (Dn1 to Dn4) of the Rio das Velhas greenstone belt, when interpreted for the Nova Lima and Caeté gold districts, may be correlated to the more regional scale five deformation phases of the QF (D1 to D4 and DE; synthesis in Table 1 and

Table 2. Correlation of regional vs. local deformation of the Archean Rio das Velhas greenstone belt in the Quadrilátero Ferrífero region.

Regional	Nova Lima-Caeté District
D1 NNW-striking bedding parallel, dextral strike-slip shear zone. EW-striking transpressive zone. Isoclinal intrafolial megafolds. E-plunging, z-shaped folds.	Dn1 Mylonitic foliation parallel to bedding. E-plunging isoclinal folds and intersection/mineral lineation. Z-shaped folds, suggesting dextral slip1.
	Dn2 Striking NE-SW foliation and lineation. Tight folds coaxial with D1.
D2 NNW-striking thrust shear zones with sinistral, directional component. S-shaped folds. Reactivation of D1 shear zones.	Dn2 Late thrust shear zones2.
D3/Dn3: Crenulation cleavage: E-W/55N; E-plunging open folds3.
D4/Dn4: Crenulation cleavage: N-S/40-65E; NS-striking open folds; crenulation lineation: N-S/30-35S.

¹ [109]; ² [9,14,15,16,89,90]; ³ [88,90,104].

):

• Dn1 and Dn2, progressive and coaxial, with foliation parallel to the bedding and associated mineralization, correspond to the regional D1 phase.
• The thrust shear zones of the final Dn2 phase are correlated to the regional D2 phase.
• Dn2, in the Raposos deposit region (open-fold, plane-axial foliation of Dn1 Archean foliation/bedding) corresponds to the regional D3 phase [5,104,106].
• The Dn3 and Dn4 structures are of evident correlation with the regional D3 and D4 phases.

5.2.1. Gold-Related, Archean D1 and D2 Deformations

The initial phase of the Rio das Velhas D1 deformation generated the large Nova Lima syncline through the compression and inversion of the Nova Lima synformal basin. The Bem-Te-Vi shear zone is the main regional structure of this phase. It is preserved to the north of the Bação dome [6] and is interpreted as an E–W transpressive segment of an NW–SE-oriented regional shear zone. It is in an inflection zone, related to an emerged Archean magmatic arch—the Bação dome itself—which, in turn, is the source for the platform sediments of the Coastal and Resedimented lithofacies associations in its surroundings. The Samambaia Tonalite (2780 Ma; [102]), to the west of the Moeda syncline, and the high-K calcium-alkaline granitoids (2750–2730 Ma; [41,42]), intrusive in the TTG gneisses of the Bação dome, both present contemporary felsic volcanisms. Both the tonalite and high-K granitoids indicate an active continental margin environment and suggest prolonged basement exposure during the Archean (Figure 2 and Figure 5).

During D1, the N-to-S tectonic transport direction, preserved in the Bem-Te-Vi transpressive segment, makes a small angle with the general NNW direction of the regional stratification, at the edges of the Nova Lima basin. The shear component of the compressive D1 vector dominated in relation to the normal component. Thus, interstate landslides were more effective than the tipping caused by the normal component. Dextral directional shear zones, with a mylonitic foliation subparallel to bedding, were the dominant regional structures in D1 (Figure 11a). These inter-strata glides generated shear folds with a ‘z’ vorticity in the more competent strata. They are notable in BIF and lapa seca, with axes oriented towards ENE, in a position almost orthogonal to that of tectonic transport [135]. These ‘z’ folds—isoclinal and intrafolial, broken in areas of greater deformation—are omnipresent in the deposits of the Nova Lima and Caeté gold districts (Figure 6b,c and Figure 7).

In Raposos, the BIF layers have a general NW-SE direction and a dominant ‘z’ fold pattern [106]. Ref. [109] attributes Raposos’s ‘z’ folds—referent to the shear zone with a mylonitic foliation, parallel to the BIF bedding—to “a compressive dextral binary”. Folds with a ‘z’ vorticity are especially striking in the Cuiabá [93], Raposos [106,109], Faria [86], and Morro Velho deposits [13,17,108,119]. In Morro Velho, the South and Main orebodies are concentrated on the hinges of these small ‘z’ folds, which belong to the first deformation phase (D1) recorded in the deposit [17]. The smaller folds and respective orebodies are positioned on the flank of the large ‘z’ fold in lapa seca, mapped along the São Vicente shear zone [24,123]. This fold closure is to E; it has a WNW–ESE axial trace and a vergence to NNE (Figure 7).

The mylonitic foliation, parallel to bedding (D1), is recognized by the various authors who have researched the auriferous districts of Nova Lima and Caeté [9,13,14,15,17,88,92,106,107,108,109], as the first deformation phase to affect the Nova Lima schists. On a deposit scale, gold mineralization in the Morro Velho, Raposos and Faria deposits, located on the western flank of the Nova Lima syncline, is controlled by D1 deformation phase shear zones.

On the other hand, the occurrence of D2 structures is related to the compression and inversion of the Maquiné synformal basin, which seems to have been entirely implanted onto the Nova Lima volcano-sedimentary strata, previously deformed in D1. No pebbles or fragments of the TTG granite-gneiss basement have been located thus far in the Casa Forte conglomerates of the Maquiné Group. Its basal conglomerates have deformed BIF pebbles from the Nova Lima Group. Angular unconformity, between the sandstones and conglomerates of the Casa Forte Formation and the turbidites at the top of the Nova Lima Group, is apparent to the NE of the Rio Acima municipality [6].

The compressive D2 vector, oriented from E–NE to W–SW (in a direction almost orthogonal to the NNW–SSE direction of the Nova Lima fault-fold belt) generated and inverted the Maquiné synclinal basin, as well as reactivated the directional D1 shear zones, and generated new thrusts with a subordinate sinistral directional component. The sinistral directional component of the D2 thrusts generated restricted folds with a ‘s’ vorticity in the more competent strata, such as BIF and lapa seca (Figure 6b,c and Figure 11b).

The Vargem do Lima syncline constitutes a large D2 fold with an inverted NE flank that is molded onto the Casa Forte alluvial-fluvial sediments. Its NW–SE axial trace [24] is subparallel to the inverted Nova Lima syncline regional direction (Figure 9b and Figure 11b).

In the Cuiabá and Lamego deposits, D2 thrusts crosscut the host rocks of the gold mineralization in either subparallel or high angles, in this case along the hinge zones of folds generated by the D1 deformation phase [12,125,126] (Figure 6b,d). In Cuiabá, such thrusts are responsible for the type-2 BIF gold mineralization [14,15,16,89,93,94,105,107,136]. The D2 deformation was more intense in the Sabará-Caeté region than in the Nova Lima-Raposos region [105]. Thus, on the eastern flank of the Nova Lima syncline, the D2 thrust structures are more common and pervasive than on the western flank, where the directional dextral movements and ‘z’ folds of the D1 deformation dominate.

The Rio das Velhas Archean deformation was continuous and progressive [6,9]. It started with the nucleation of the Nova Lima syncline in D1, with a NNE to SSW tectonic transport. It was followed by its amplification and inversion during the implantation of the Maquiné retroarc basins in D2, and then with tectonic transport from ESE to WNW in an angular unconformity (Figure 11a,b).

The D1 and D2 structures hallmark the initial and final phases of the progressive deformation of the Rio das Velhas greenstone belt, that is, the Rio das Velhas orogeny during the Neoarchean (Table 1 and Table 2). The regional NW–SE shear zones initially presented dextral, strike-slip movements (D1); they were subsequently reactivated with a reverse-sinistral movement (D2), which occurred in response to the rotation of the compressive vector in a clockwise direction. The NE-to-SW compressive efforts of D1–D2 onto the strata of a general NW–SE orientation then resulted in the inversion and greater deformation of both the Nova Lima syncline and the Vargem do Lima syncline NE flanks.

By the end of the Rio das Velhas orogeny, the Rio das Velhas greenstone belt made up a broad syncline with an inverted eastern flank, which was converted into a fold-fault belt. It had a W–SW vergence, a general NNW–SSE direction, and an average-to-low dip to ENE (Figure 9b and Figure 11b). In this belt, the bedding-parallel mylonitic foliation defined important shear zones—such as the São Vicente and Raposos in the central portion of the QF, both gold bearing, with D2 reverse-sinistral movements [6] superposed to directional dextral D1 structures.

The peak of D1 is estimated around the ca. 2750–2735 Ma interval: (i) the first age corresponds to the last manifestation of felsic volcanism [103] and to the maximum age for the Palmital Formation deposition, top of the Nova Lima Group [25]; (ii) the second age is the maximum deposition age of the Maquiné Group [25]. Gold mineralization at the Lamego deposit, ca. 2730 Ma [9], occurred at the end of this interval.

The time span of D2 may be restricted to the 2735–2700 Ma interval, corresponding respectively to the maximum deposition age of the Maquiné Group [25] and to the Archean orogenic collapse [113]. The age of ca. 2670 Ma was obtained for the mineralization at the Morro Velho and Cuiabá deposits [8], which suggests that the process may have partly occurred during the final extensional phase of the Rio da Velhas orogeny, at the end of the Mamona event (2760–2680 Ma; [19,42]).

The evolution of the Rio das Velhas greenstone belt (Rio das Velhas orogeny) occurred in the span of 2800–2670 Ma [6]. The 2800–2780 Ma interval was the extensional phase, which marks the beginning of the opening and inversion of the Nova Lima basin. The period between 2780–2700 Ma represents the closing of the basin, from the beginning of subduction to the orogenic collapse. Finally, the final placement of granodioritic and granitic bodies occurred between 2750 and 2670 Ma [6]. This evolution process corresponds to the tectono-magmatic Rio das Velhas II (2800–2760 Ma; [19,41]) and Mamona (2760–2680 Ma; [19,42]) events.

The age of ca. 518.5 +/– 9 Ma (U-Pb, SHRIMP) was obtained for xenotime for the Lamego deposit and reflects the overall impact of the Brasiliano orogeny—represented by a high-dipping, N–S cleavage, but with no record of hydrothermal alteration [9]. In the Córrego do Sítio deposit, however, the hydrothermal monazite present in the orebodies yielded a ca. 534–555 Ma dating (U-Pb, SHRIMP). This indicates the possible influence of the hydrothermal activity associated to the collapse of the Araçuaí Orogen in the Cambrian [137].

Fluid circulation derived from the orogenic, collapse-related magmatism of the Araçuaí Orogeny [138], represented by the G4 (530–500 Ma) and G5 (520–480 Ma) granitoid supersuites [139,140,141], could have imprinted similar modifications (with U-Pb ages between 515 to ca. 495 Ma). This process would have occurred in the hydrothermal minerals associated with the mineralized systems of the São Francisco craton and Araçuaí belt (southern Espinhaço Range and QF).

5.2.2. The Proterozoic Orogenies and Their Impact on the Archean Western Gold Deposits

The Proterozoic Minas and Araçuaí orogenies—related, respectively, to the Transamazonian and Brasiliano orogenic cycles—have structurally impacted the QF region in different ways.

Paleoproterozoic—The Paleoproterozoic-Rhyacian Minas orogeny affected the QF region more gently, and mainly in its south-southeastern portion. Thrust fronts of the Mineiro belt [36,142], belonging to the Minas accretionary orogeny [95], reached the Minas Supergroup [22,23], the volcano-sedimentary sequence of the Sabará Group foreland basins [61] and the Rio das Velhas greenstone belt. The age of 2387 +/− 46 Ma (U-Pb, SHRIMP), obtained from hydrothermal monazite in the Lamego deposit, records the reach of the Minas accretionary orogeny in the QF region and its impression onto the Rio das Velhas greenstone belt [9].

This is the D3 deformation, with a compressive vector directed from SE to NW, subparallel to the direction of the D1–D2 Archean bedding-parallel mylonitic foliation. The Rio das Velhas fault-fold belt (NW–SE-oriented, with dips to the NE) was folded by buckling. These folds are regionally evident south of the Serra do Curral, throughout segments of the Volcano-Chemical-Sedimentary- and Volcaniclastic associations. Aeromagnetic and gamma-spectrometric maps clearly show the geometry of this folding in anomalous bands, correspondent to these two lithofacies associations (Figures 5 and 12) [6]. In the region between Caeté and Sabará, the presence of tufts and felsic volcanic breccias from the Volcaniclastic Association serves as an important structural guide that reveals, in a precise manner, the style of the D3 folding over the Nova Lima strata, with an excellent record in gamma-spectrometric aerogeophysical maps (Figure 12a). In the Nova Lima region, this D3 folding is evident in a magnetometric map of the Analytical Signal (Figure 12b). They are open folds, with NW–SE wrap surfaces, NE–SW axial traces, and a well-developed and pervasive plane-axial foliation, with high-to-medium, SE-oriented dips. These folds are tightened to isoclinal near the Serra do Curral, verging towards N–NW, with inverted NW flanks and axial lines tending towards the E-W direction.

The D3 compression also reoriented and even amplified mineralized Archean folds, thus reorienting Archean-evolved gold-mineralized orebodies [144]. The combined anticline and syncline folds of the Faria deposit, verging to the NW and with NE–SW fold axial traces, are D1–D2 Archean structures, which have been reoriented and inverted in D3, when they assumed an axial trace parallel to the D3 foliation general direction (Figure 6c). The large ‘z’ fold in lapa seca, originally mapped out by [123] in the Morro Velho deposit, is an Archean structure, rotated and inverted in D3 (Figure 7). Cuiabá and Lamego are isoclinal and intrafolial folds of the inverted NE flank, belonging to the Nova Lima Archean syncline. Both were reoriented during D3 and are located, in this phase, on the flanks of great folds, which then refolded the great syncline (Figure 6b,d and Figure 13).

On a deposit scale, the D3 fold is characteristic of the Raposos deposit. Here, the D1–D2 Archean foliation/bedding exhibits an open fold with an E-axis plunge. The plane-axial foliation of these folds has average N30–60E/45SE attitude on the surface, with a NW–SE general direction enveloping surface that still preserves the Archean D1–D2 trend. Still in Raposos, within the BIF segment that hosts the Espírito Santo and Espírito W orebodies, the general NW–SE direction of the Archean D1–D2 foliation/bedding [13] is clearly evident, and gently folded in D3 (Figure 14).

The field data obtained from geological maps at a scale of 1:50,000 [145], regarding the Nova Lima-Caeté range, when complemented with data recently collected by the first author, indicate: (i) Transamazonian D3 folding axes plunge with low angle to E, therefore subparallel to the axes of the Archean folds; (ii) an S3, plane-axial foliation in such Transamazonian folds, with the average attitudes of N45E/48SE and N62E/38SE, respectively, for the regions of Nova Lima-Raposos and Caeté-Sabará (Figure 15).

The local structural modifications generally attributed to the Brasiliano D4 compression are here reinterpreted as part of the final stages of D3. These alterations include:

i.

The subvertical E–W crenulation cleavage associated with the D3 phase smooth folding [88,104,105]. It shows a regional distribution pattern that has been attributed to the final stages of the Brasiliano D4 deformation by [30]. In the Morro do Sino region, on the inverted flank of the Faria anticline, folds with WNW–ESE axial traces are refolded by folds of a lesser amplitude, that have N–S axial traces as a result of the Brasiliano D4 compression. These EW folds are common in the Nova Lima Group. Although they are not pervasive, they are here attributed to the final stages of D3, instead of D4, given the clockwise rotation of the Transamazonian compression vector—from SE–NW to S–N (Figure 6c).

ii.

The Geriza structure (south of Caeté) is considered by [74] to be an antiformal fold originating from the Brasiliano D4 deformation. It is reinterpreted here as an anticlinal synformal fold, nucleated in D3 on the eastern flank of the Nova Lima syncline. This flank of the Nova Lima syncline was previously inverted and sheared in D1–D2, during the Archean (Figure 4 and Figure 5). The Geriza fold was amplified, assuming an isoclinal boomerang shape due to the control exercized by the Ribeirão da Prata and Fundão-Cambotas shear zones, in overlap with the Caeté and Córrego do Garimpo shear zones (Figure 4).

iii.

The Conceição syncline [146] (Figures 4 and 5) was nucleated during the Archean, according to [6], amplified and inverted during the Brasiliano event. Contrarily, it is here reinterpreted as an antiformal synclinal fold with an NE axial plunge that was generated in D3, later amplified, reoriented and segmented in D4. Despite being closed in between shear zones of the Brasiliano compression, it remains in structural discordance in relation to the adjacent Gandarela syncline (Figure 4 and Figure 5).

The Transamazonian D3 compression acted in approximately the same direction, but in the opposite direction to that of the Archean D1 compression. Therefore, the folds with a ‘z’ vorticity, mineralized in BIF and lapa seca in D1, were refolded, with subparallel axes in D3. During D3, an inversion of the Minas basins occurred, and they were nucleated over the Rio das Velhas greenstone belt and its granite-gneiss basement. Consequently, D3 refolded the great Archean synclinal structure—oriented towards NNW–SSE, with E–SE dips, and axes subparallel to those of the D1–D2 folds (Figure 8 and Figure 16). The subparallelism present in these axes of Archean and Transamazonian folds may be responsible, in part, for the twisted-to-helicoidal aspect that is registered in the orebodies that occur in the deposits of the Nova Lima and Caeté gold districts. In such districts, this twist is attributed to the Dn1–Dn2 coaxial Archean refolding [12,88,105], in turn related to the regional D1 deformation.

As for the Rio das Velhas greenstone belt, the DE dome-like ascension reoriented the planar and linear structures and, consequently, the potentially associated gold mineralizations. In the surroundings of the Bação dome, the D1–D2 Archean mylonitic foliation of the overlying greenstone cover was reoriented and created synformal folds, with N–S and NE–SW axes (on the east and west edges of the dome, respectively). These synforms were amplified during the Brasiliano D4 compressional event. In their subvertical flanks, the mineral lineations and the axes of intrafolial folds in the foliation/bedding of the Nova Lima rocks assume, respectively, sub-horizontal and subvertical dips [6,76].

Neoproterozoic—The Neoproterozoic Araçuaí orogeny evolved in the 625–500 Ma interval [98] and deeply affected the eastern sector of the QF. It is W-vergent, N–S-oriented fault-fold belt, which we refer to as the D4 deformation. In the western sector, among the auriferous Nova Lima and Caeté districts, D4 is only manifested in a smooth bending of the Rio das Velhas greenstone belt previous structures, which generated crenulation cleavages and a subvertical, non-pervasive N–S fracture. Thrust shear zones are significant only on the contact edges with the Caeté granodiorite, to the east, and with the quartzites at the edge of the Moeda Formation, to the west. Within these shear zones, however, sulfidized smoky quartz veins, commonly mineralized in gold, may be in the form of boudins and be reoriented to a general N–S direction, presenting sub-horizontal axes, as is the case in the Caeté shear zone. These are localized thrust zones, causing small displacements of the pre-existing structures and gold orebodies. Nevertheless, normal faults, also of a small magnitude and attributed to the extensional phase of this orogeny, are able to displace orebodies, as in the Raposos deposit [13]. To the south of Caeté, an EW mylonitic strip with a dip to the south—a lateral ramp segment of the Ribeirão da Prata shear zone, generated in D4—controls the orebodies that reorient subparall to the sub-horizontal mineral lineation (Figure 4 and Figure 5).

5.3. Mineralization Styles and Relation to Deformation Phases D1 and D2

The mineralization styles referred to as types 1 and 2 by [88] are typified, respectively, by the dominance of pyrrhotite (1) and pyrite (2) [12,118,125,126]. They may represent the hydrothermal conditions during the early (D1) and late (D2) Archean deformation.

As pointed out [147], the mineralogical, geochemical, and fluid inclusion constraints of the hydrothermal alteration are dependent of ƒ_O2, X_CO2, X_CH4 and a_ΣS, as well as the activities of K⁺/H⁺ and Na⁺/H⁺ [148]. Ref. [3,147] indicate that pyrrhotite is the dominant early-stage iron sulfide replacing Fe-bearing metamorphic minerals, such as magnetite and the siderite of BIF (São Bento and Raposos). Based on the logƒ_O2 vs a_ΣS diagram, at 350 °C and 2 kb of Figure 2.12 of [149] and Figure 5 of [148], the former authors suggest that ƒ_O2-buffered sulfidation reactions, associated with the incipient alteration stages of BIF, took place at fluid a_ΣS-equilibrium conditions favorable for pyrrhotite development. A type-1 scenario was thus associated with the D1 deformation. During this stage, the ratio of sulfur/H₂O + carbon was relatively low. However, as extensive, widespread carbonate-alteration developed, with CO₂ buffering and the decrease in the fluid X_CO2, slightly higher levels of a_ΣS fluid must have been attained. This would have allowed for pervasive pyrite formation replacing carbonates, consisting in the type 2 mineralization style associated with the D2 deformation.

These observations further corroborate the interpretation that gold mineralization took place in a continuum from D1 to D2, with gold being associated with both pyrrhotite and pyrite. An additional stage of hydrothermal gold mineralization is also the interpretation of [150], based on fluid inclusion studies in Cuiabá [118,150].

6. Conclusions

This contribution investigates the deformational evolution in the Quadrilátero Ferrífero (QF) region, where the Rio das Velhas greenstone belt is loci to Archean orogenic gold mineralization. A tectono-structural, regional evolutionary model focuses on the western structural regional domain, where a number of well-known deposits (Cuiabá, Lamego, Raposos, Morro Velho and Faria) are mainly distributed in the Nova Lima and Caeté gold districts (Figure 4 and Figure 5) [9,10,11,12,13,14,15,16,17,18].

One of our particular interest is to examine how later orogenic events of Proterozoic age impacted the Archean mineralization process. These are the main conclusions:

• In the QF, one can find records for three regional scale orogenic events: (i) the Neoarchean Rio das Velhas (corresponding to phases D1 and D2, that is, the deformation of the Rio das Velhas greenstone belt); (ii) the Paleoproterozoic-Rhyacian Minas event (phases D3 and DE, the deformation of the Minas Supergroup and Sabará Group); and (iii) the Neoproterozoic Araçuaí event (phase D4, the deformation of the Itacolomi Group and the Espinhaço Supergroup).
• The geographic distribution and geometric arrangement of the Quebra Osso and Nova Lima groups (base of Rio das Velhas Supergroup), which represents the metavolcanic and metavolcano-sedimentary stratas within the Rio das Velhas greenstone belt, suggest the occurrence of a deposition in a NW–SE-oriented syncline basin, implanted on a Paleo-Mesoarchean TTG granite-gneiss basement. The polarity and symmetry in the distribution of the Quebra Osso and Nova Lima stratas and their litho-structural and textural characteristics indicate a back-arc basin as the main deposition environment.
• The alluvial-fluvial sediments of the Maquiné Group (top of Rio das Velhas Supergroup) were also deposited in a NW–SE-oriented syncline basin, with a slight angular unconformity over the Nova Lima Group strata. The litho-stratigraphic, textural, and structural characteristics of the Maquiné strata suggest a retro-arc foreland basin as the main deposition site.
• The Rio das Velhas Neoarchean orogeny deformed the marine metavolcanic and metavolcano-sedimentary sequences of the Quebra Osso and Nova Lima groups (phase D1) and the continental meta-sedimentary sequences of the Maquiné Group (phase D2). It also converted the Nova Lima and Maquiné basins into a wide, inverted, NW–SE directed, and SW-vergent syncline.
• Gold mineralization, hosted within lithotypes of the Nova Lima Group, occurred in two stages, which are mineralization styles referred to as types 1 and 2. Type 1 (phyrrotite dominated) and type 2 (pyrite and arsenopyrite dominated) are related, respectively, to the D1 and D2 phases of the progressive Archean deformation.
• The D1 deformation (compressive N–S vector) inverted the Nova Lima basin, with a NW-SE regional, transcurrent dextral shear and ‘z’ vorticity folds, at all scales.
• The gold mineralization at the western flank of the Nova Lima syncline is mainly controlled by shear zones and related folds (with axes of orebodies plunging towards ENE) of the D1 phase; this consists in the type 1 mineralization at Morro Velho, Raposos, and Faria deposits.
• The D2 deformation (ENE–WSW compressive, progressing to D1) inverted the Maquiné basin that was implanted on the Nova Lima strata, thus reactivating previous structures with thrust movements.
• The D2 thrusts are more effective on the eastern flank of the Nova Lima syncline. They are responsible for the pyrite-dominated type 2 gold mineralization occurring in the Cuiabá and Lamego deposits; in Cuiabá, they intercept the BIF layer that had been previously mineralized in D1 (type 1 mineralization).
• The obtained age for gold mineralization in Lamego, ca. 2730 Ma, may be related to the final stages of D1, which correspond to the maximum deposition age of the Maquiné sediments. The 2670 Ma age at Morro Velho and Cuiabá, in turn, suggests that the mineralization there might have occurred in the final stages of the Mamona event (2760–2680 Ma), which was already under an extensional regime. Figure 17 attempts to integrate the Archean D1–D2 deformations with the evolutionary, magmatic Rio das Velhas II and Mamona events as well as gold mineralization ages as we know them until now.
• The Paleoproterozoic Minas orogeny has an SE–NW compressive vector (phase D3) and is represented, in the QF region, by the Mineiro belt. It inverted the Minas and Sabará Paleoproterozoic basins and affected their basements, that is, the Paleo- to Mesoarchean TTG gneisses and the Rio das Velhas greenstone belt. In the Lamego deposit, the obtained age of 2387 +/– 46 Ma, in hydrothermal monazite (U-Pb, SHRIMP), registers the reach of the accretionary Minas orogeny.
• The D3 compression (from SE to NW) refolded through buckling the gold-mineralized Archean structures. The folds are, in general, open with E–SE oriented axes, subparallel to those of the Archean folds. They present a pervasive, NE–SW oriented planar-axial foliation that plunges towards SE. D3 was responsible solely for the redirection of the orebodies generated during the D1 and D2 Archean deformations, thus causing their rotation and translation on the flanks of this phase’s larger folds—the case of the Morro Velho, Faria, Cuiabá and Lamego deposits.
• The Geriza and Conceição do Rio Acima folds, molded onto the Nova Lima strata belonging to the eastern flank of the Nova Lima syncline, are D3 structures, later rotated and amplified in D4.
• On a deposit scale, the D3 folds may be responsible, at least in part, for the twisting of the orebodies, generally attributed to the coaxial refolding phase of D1.
• Thrust shear zones, subparallel to the intense D3 foliation, caused the segmentation and metric displacements of orebodies in a very localized manner.
• There appears to be no record of interference from structures generated during the Transamazonian DE extension in relation to orebodies.
• The Araçuaí orogeny of the Neoproterozoic (E–W compression, phase D4) printed a fold-fault belt onto the eastern sector of the QF, where it deeply affected the Rio das Velhas greenstone belt. In the deposits of the Córrego do Sítio lineament, hydrothermal monazite yielded ages of ca. 534–555 Ma (U-Pb, SHRIMP).
• In the Nova Lima-Sabará range, the Araçuaí orogeny is represented solely by smooth folding and discreet and non-pervasive crenulation cleavage; smaller thrust shear zones, on the other hand, only cause metric displacements, located on N–S-oriented planes. In Lamego, ca. 518 Ma was obtained in xenotime (U-Pb, SHRIMP).
• D4 thrusts are important only at the edges of the Nova Lima basin, where sulfide-bearing smoky quartz veins, in the form of boudins and with sub-horizontalized axes in the Nova Lima pelites, are redirected to the general N–S direction of these thrusts.
• Other normal faults, also of small magnitude and attributed to the extensional phase of the Brasiliano orogeny, can also displace gold orebodies.