Search

The Itareru tonalite is one of the Paleoproterozoic plutonic rock units that intruded the Serrinha block, an Archean-Paleoproterozoic crustal fragment of the São Francisco Craton (Bahia, Brazil). It is characterized by a ca. 80 km long, NW-SE elongated leaf-shaped body, mainly comprised of tonalite-diorite to granodiorite rocks with numerous dioritic enclaves. Magnetic studies were performed on 26 sites of the Itareru pluton (including country rocks). Anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) are coaxial, indicating that all minerals have the same orientation, partially controlled by regional strain. Rock-magnetism measurements reveal that paramagnetic–matrix minerals contributed significantly to the magnetic susceptibility and AMS while magnetite grains in the range of 2–5 μm are responsible for AARM. Magnetic fabrics in the central region of the pluton are probably primary (magmatic), which was acquired when the rocks were crystallized, reflecting magma flow linked to a magma feeder zone (magma chamber). The moderated magnetic lineation plunges in the northern part of the pluton suggest that this region was close to the feeder zone. In contrast, the southern region should be far from it, since the magnetic lineations are horizontal to sub-horizontal. The Itareru tonalite emplacement was also controlled by regional strains related to the collision of the Paleoproterozoic Rio Itapicuru greenstone Belt with the Archean basement. • AMS and AARM are coaxial indicating that all minerals have the same orientation. • Magnetic fabrics in the central region of the pluton are probably magmatic acquired when the rocks were crystallized. • Magnetic fabrics in the central region of the pluton are related to magma flow. • The emplacement of the pluton was controlled by regional strains. • Itareru emplacement is related to the collision of the RIGB with the Archean basement. [ABSTRACT FROM AUTHOR]

Hypozonal orogenic gold mineralization has been recently recognized in the Troia Massif, an Archean/Paleoproterozoic basement inlier within the Neoproterozoic Borborema Province, NE Brazil. The mineralization occurs in association with quartz veins hosted by the amphibolite facies Paleoproterozoic Serra das Pipocas greenstone belt. The main mineralized areas are located near parallel to lithological contacts, siting on shear zones established at the metavolcanic and metasedimentary boundary. Four deformation events are recognized: (1) Dn was responsible for the early Sn foliation, parallel to the bedding (So) of the greenstone units. (2) Dn+1 is characterized by a pervasive, southeasterly–dipping Sn+1 foliation that is axial–planar to several asymmetric, tight to isoclinal and recumbent folds. (3) Dn+2 represents a strike–slip NE–trending shear zones, and (4) the late Dn+3 event is characterized by ductile–brittle deformation. Evolution from Dn+1 to Dn+3 is interpreted as a progressive deformation, with the maximum compressive stress in the WNW–ESE direction. The main stage of gold mineralization is associated with the Dn event, and comprised by deformed quartz veins and associated high–temperature calc–silicate alteration (diopside, hornblende, titanite) and albitization of the host rocks. Lower temperature gold (±Te, Ag) occurs along late stage brittle structures (Dn+3). The U–Pb titanite age of 2029 ± 28 Ma for the calc–silicate alteration of the early gold mineralization is similar to the age of many Transamazonian/Eburnean gold mineralizations elsewhere. However, the strong Pb loss of titanite grains defines a 574 ± 7 Ma lower intercept age, indicating that early gold mineralization was affected (remobilized) by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan–African orogeny). This is in agreement with the U–Pb zircon age of 575 ± 3 Ma obtained for the syn–tectonic Guaribas dikes, which bracketed the age of Dn+3. Therefore, as Dn+1, Dn+2 and Dn+3 evolved by progressive deformation; they are all of Neoproterozoic age. This two–stage gold mineralization is here interpreted as product of crustal exhumation, with (1) early hypozonal orogenic gold mineralization occurring shortly after the high–grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile; and (2) late gold mineralization occurring at shallow levels (second exhumation process), associated with the Neoproterozoic Brasiliano/Pan–African orogeny. Image 1 • Four deformation events are recognized (Dn, Dn+1, Dn+2, Dn+3). • The main stage of gold mineralization comprises deformed quartz veins. • The U–Pb titanite age of 2029 ± 28 Ma was obtained for early gold mineralization. • Late (c. 580 Ma) gold mineralization is found in brittle–ductile structures. • A two–stage exhumation–driven gold mineralization is the proposed model. [ABSTRACT FROM AUTHOR]

Abstract The origin and significance of the metavolcanic rocks of the Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of the Guyana Shield in Paleoproterozoic times. This work presents new LA-MC-ICPMS U-Pb zircon data and geochemical analyses carried out on metavolcanic rocks of the VNGB. This information was supplemented by petrography, stratigraphic and structural data acquired through geologic mapping and boreholes descriptions. In the Vila Nova region, the VNGB units rest on the Archean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB consists of metabasalts and andesitic metabasalts, covered by an upper metasedimentary domain with subordinate metavolcanic and exhalative chemical rocks. The metavolcanic rocks include metabasalts, meta-andesites, amphibolites and amphibole schists, deformed and elongated according to the regional NW-SE structure. U-Pb zircon data yielded an age of 2154 ± 6 Ma for a meta-andesite of the lower portion of VNGB. One orogenic metamorphism followed by three deformational events were recognized. The D 1 and D 2 events formed the schistosity (S 1), preserved as intrafolial folds (F 2) and the crenulation cleavage (S 2) associated with the development of the oblique shear zones. Two distinct assemblages: 1) garnet, hornblende and biotite, and 2) hornblende, plagioclase and diopside define two distinct metamorphic peaks: M 1 and M 2 with temperatures of 450 and 650 °C, and lithostatic pressure between 4 and 6 kbar. The metabasites comprise geochemical compositions of Fe- and Mg-tholeiites with komatiitic affinity. The first group is enriched in LREE and HFSE. The second group shows MORB-like REE patterns in association with negative Nb, Ti and P anomalies. The observed features indicate that the metavolcanic rocks of the VNGB represent the evolution of back-arc basins and island arcs associated with the Trans-Amazonian Orogeny. These rocks characterize the Rhyacian evolution of the Maroni-Itacaiunas Province in the eastern portion of the Guyana Shield. Graphical abstract Image 1 Highlights • La-ICP-MS U-Pb zircon geochronology data on meta-andesite established a Rhyacian age (2,154 ± 6 Ma) for the magmatism. • The protoliths are tholeiitic basalts and andesites with geochemistry associated with an island arc-back-arc system. • E-MORB, N-MORB and plume type sources were involved in the formation of the volcanic rocks. • The volcanic rocks were affected by metamorphic events under greenschist-amphibolite and hb-px hornfels facies conditions. [ABSTRACT FROM AUTHOR]

The structural architecture of an orogenic deposit is essential for its metallogenetic understanding, as both subjects are interconnected in this type of environment. Cuiabá gold deposit, located in Archean rocks from Rio das Velhas Greenstone Belt, is the subject of numerous studies focused mainly on metallogenetic issues. From the point of view of structural characterization, two divergent models have been studied: the sheath-fold model and the refolding model. Such hypothesis is different in terms of defining an exploratory research plan and, therefore, the detailed structural characterization is still not well understood. As a way of filling this gap, the main objective of this work is to propose a structural characterization of the entire deposit in order to demonstrate evidence for a new model of tectonic evolution. For this, the methodologies applied for this work were: underground geological mapping in all available ore galleries and definition of geological-structural domains whose main criteria are related to down-plunge view of fold asymmetry and stratigraphic indicators. Structural geology is characterized by 3 deformation phases related to 2 tectonic events. The first event has two ductile-progressive deformation phases (D1 and D2), with NE-SW strike-slip direction of transport associated coaxial fold interference patterns with cylindrical geometry plunging to ESE, which represents the main structure at the mine. The second event is a brittle-ductile deformational phase (D3) with W-verging reverse fault systems, probably associated to Brasiliano Pan-African Orogeny. Gold mineralization is associated with silica and sulfide-rich hydrothermal fluid introduction through D1 and D2 axial-plane foliation. Phase D1 is pyrrhotite-associated mineralization, with coarse gold presence. D2 phase has pyrite-associated mineralization with fine gold. There is no gold in phase D3, although it has late-tectonic sulphides formation. Therefore, there is no field evidence that corroborate the existence of sheath folds for the deposit. Fold interference patterns associated to deposit morphology suggests SW and N prospective targets, with strong possibility of structural connection between other gold deposits, such as Lamego and Descoberto targets. • Geological mapping reveals a pre-Cambrian fold interference pattern with no field evidence for sheath folds. • Gold inside BIF is related to this framework at two stages: (1) pyrrhotite with coarse gold and (2) pyrite with fine gold. • A NE-SW ductile strike-slip transport at a convergent tectonic setting with inclined folds plunging to ESE is suggested. • Gold was carried out throughout axial-plane foliation and were placed where coaxial refolding was locally presented. • A post-mineralization tectonic event was responsible for orthogonal displacements at the entire deposit. [ABSTRACT FROM AUTHOR]

Potential-field edge detection filters are powerful fault detection tools in mineral exploration. The application of these filters to electromagnetic (EM) data is not as accepted as those associated with potential field data. But they may be advantageous when dealing with mineralizations associated with non-magnetic rocks. This paper introduces the multiscale edge detection filter to provide an enhanced image of fault zones when applied to apparent conductivity data. The Rio das Velhas Greenstone Belt, the studied area, hosts several gold mineralizations controlled by northwest–southeast shear zones subparallel to non-magnetic metasedimentary rocks that are typical candidates for the EM interpretation. Several northwest–southeast trending faults were highlighted in the studied area. We employed a data fusion interpretation strategy by correlating the detected EM faults with radiometric anomalies to classify high-priority spots for future exploration. We tagged four known gold deposits and new targets with similar characteristics following the proposed guidelines. • We show that the multiscale filter (worming) can be used for electromagnetic (EM) data. • The EM worming screened non-magnetic mineralized fault zones in the Rio das Velhas Greenstone Belt. • Combining EM-worming and radiometric data, we selected sites for follow-up surveys. [ABSTRACT FROM AUTHOR]

Spinifex-textured komatiites in the Selva greenstone belt are the first unequivocal examples of komatiites in the Transition Subdomain of the Carajás Mineral Province. Outcrops of spinifex-textured komatiites, located ∼1.5 km to the south of the Carajás ridge, were discovered during regional exploration for Ni–Cu–(PGE) sulfide deposits by VALE. They are associated with a 3.8 km long unit consisting of variable types of ultramafic rocks (talc schist, serpentinite and spinifex-textured komatiite). This ultramafic unit follows the steep dipping NW–SE trending Selva greenstone belt composed mainly by quartz-chlorite schists (interpreted as metasediments) and chlorite-actinolite schists (interpreted as metabasalts). Greenschist facies metamorphic parageneses characterize all rock types in the Selva greenstone belt. The komatiitic rocks in the Selva belt comprise a sequence of flows consisting of an upper spinifex-textured layer and a lower olivine cumulate layer. Although the spinifex and cumulus textures are well preserved in the field, the primary mineralogy of the komatiites has been completely replaced by greenschist facies metamorphic minerals. Platy olivine spinifex texture, consisting of an array of roughly parallel olivine plates, and random spinifex texture, consisting of randomly oriented olivine plates, are the most common primary volcanic textures in komatiites in the Selva greenstone belt. Platy and random spinifex texture is defined by former plates of olivine replaced by serpentine with minor actinolite, chlorite and magnetite, alternating with former matrix replaced by abundant actinolite and minor chlorite, talc, serpentine, and magnetite. The domains between olivine plates in both platy and random spinifex-textured rocks contain irregular arrays of fine-grained parallel crystals, representing primary fine-grained “quench” clinopyroxene crystals replaced by actinolite. Spinifex-textured komatiites have MgO contents bracket between 22.8 and 26.9 wt.%, and cumulate textured komatiites have MgO contents up to 40.6 wt.%. When plotted vs MgO contents, most major and minor elements fall on well-defined linear trends indicating control by olivine fractionation or accumulation. Komatiites from the Selva and Seringa (located in the Rio Maria Domain) belts are Al-undepleted with Al 2 O 3 /TiO 2 ratios close to 20. Results for CaO, Na 2 O, and REE suggest that these elements were mobile and their abundances have been modified during metasomatic alteration. REE contents in some samples are very high (up to 40 times primitive mantle values) and REE patterns vary from flat (La/Yb MN ∼ 1) to highly enriched in LREE (La/Yb MN up to ∼ 10). The REE mobility may be related to hydrothermal alteration associated to Cu–Au mineralization in the region. The identification of spinifex-textured komatiites close to the Carajás Basin suggests the continuation of 3.0–2.9 Ga greenstone belts of the Rio Maria Domain within the Transition Subdomain, and enlarges the area with potential to host komatiite-associated Ni–Cu–PGE deposits. [ABSTRACT FROM AUTHOR]

Archean orogenic gold deposits are hosted in the Rio das Velhas greenstone belt in the eastern sector of the Quadrilátero Ferrífero, Southern São Francisco Craton region, southeastern Brazil. These include the gold deposits along the NE-trending Córrego do Sítio lineament, stretching for some 15 km, and the Pilar deposit, object of the present study. These deposits occur in an area close to the boundary of the São Francisco craton, which is commonly interpreted to have preserved the Archean and Paleoproterozoic crust from the effects of Ediacaran-Cambrian Araçuaí orogenic front. The gold deposits were formerly interpreted as exclusively Archean (ca. 2.7 Ga). However, recent geochronological data suggest the imprint of the late Ediacaran-Cambrian Brasiliano Orogeny in host rocks of these deposits, as structural modification or hydrothermal alteration assemblages that postdates Archean mineralization. To elucidate those issues, contextual (thin section) in-situ U–Pb SHRIMP dating was conducted on hydrothermal monazite crystals from the Carvoaria and Cachorro Bravo deposits of the Córrego do Sítio lineament and from the Pilar gold deposit. Hydrothermal monazite in mineralized metapelites from Carvoaria yielded a U–Pb Discordia with intercepts at 2,514 ± 22 Ma and 555 ± 19 Ma. Three younger, age-equivalent crystals are concordant and yielded a U–Pb Concordia age of 539 ± 9 Ma, identical within uncertainties to the lower intercept age. Monazite from the Cachorro Bravo deposit yielded U–Pb Concordia ages of 551 ± 10 Ma and 510 ± 11 Ma. Monazite from the veined (silicified) sulfide-schist from the Pilar deposit is depleted in U, precluding the calculation of robust U–Pb ages, and disclosed a mean 208Pb/232Th average age of 508.2 ± 6.4 Ma. These results reinforce the proposal of a strong Ediacaran-Cambrian imprint related to the final stages of the Brasiliano orogenic event, affecting Archean gold deposits throughout the Quadrilátero Ferrífero. Consequently, the results point to the importance of mapping Brasiliano-related structures that control the spatial arrangement of the gold deposits, such as the Córrego do Sítio lineament, consisting of an important exploration target. This major NE-SW trending strike-slip shear zone hosts several gold deposits and might represent an Archean structure reactivated during the Brasiliano Orogeny that possibly led the large volume of Ediacaran-Cambrian post-collisional hydrothermal fluids among the east sector of the Quadrilátero Ferrífero. • U–Pb SHRIMP dating of gold-related hydrothermal monazite on thin section. • Cambrian ages indicate resetting of the Archean mineralization by fluid flow. • Influence of the Brasiliano Araçuaí Orogen over the São Francisco Craton border. [ABSTRACT FROM AUTHOR]

The Pitangui Greenstone Belt (PGB) is a Meso to Neoarchean metavolcano-sedimentary sequence, inserted in the Pitangui Synclinorium, located in the northwest portion of the Archean-Paleoproterozoic metallogenic province of the Quadrilátero Ferrífero, southern of the São Francisco craton (Brazil). The PGB holds important gold mines like the Turmalina Complex and recently discovered deposits, as São Sebastião. The PGB is mapped in detail scale, but there is still little information about its subsurface configuration. This paper presents gravity surveys, constrained by geological, structural, boreholes, aeromagnetic and seismic data, aiming to provide a better understanding of the 3D geometry of this granite-greenstone terrane. The treatment of the available data resulted in Bouguer anomaly maps, 2D gravity modelling and a 3D geological-geophysical integrated model. The gravity Bouguer anomalies of the region present high amplitudes associated with the PGB, alternating with low zones related to the surrounding granite-gneiss rocks. The profiles and the model suggest that the maximum depth of the greenstone belt is approximately 5 km in its central portion, being strangled by granitic intrusions in its southernmost region. The PGB is about 100 km long, but most of it is covered by Neoproterozoic sediments of the Bambuí Group and Quaternary deposits, which significantly increase its metallogenic potential, especially in regions with few mineral exploration works. The 3D model contributes to the understanding of the tectonic framework and evolution of the PGB, showing the spatial relation with the surrounding granite-gneiss basement and intrusions, and younger sedimentary covers, providing valuable insights about their depth, vertical and lateral geometry. Additionally, this new data can become a valuable tool for developing strategies to select deep mineral targets, expanding the exploratory potential for this mineral province. • Pitangui greenstone belt present high amplitude Bouguer anomaly. • The greenstone belt extends in a much larger area than it outcrops. • The greenstone belt has 5 km depth on the central portion. • 3D model present the geometry of this granite-greenstone terrane. [ABSTRACT FROM AUTHOR]

The 642 Ma-old Brauna Kimberlite Field is located on the northeastern sector of the São Francisco Craton (Serrinha block) and is one of the rare Neoproterozoic kimberlitic events in South America. Zircon xenocrysts from the volumetric most important kimberlite pipes Brauna 03, Brauna 07 and Brauna 04 were used as a tool to identify different components of the lithosphere beneath the northeast region of the São Francisco craton. A composite kimberlite sample of eight representative and different drill holes and three samples of the host rocks (Nordestina granodiorite) were sampled for SHRIMP geochronology. The results were compared with precise U–Pb age data for the regional rocks, i.e. the Archaean basement and the Palaeoproterozoic Rio Itapicuru greenstone belt. Samples from the Nordestina granodiorite gave three different ages: 2155 Ma in the western part of the batholith, 2139 Ma in its central part, and 2132 Ma in its eastern part. Zircon 207Pb/206Pb ages of the Brauna kimberlite zircon grains spread over the timespan 2107–2223 Ma and indicate four age groups at 2105 ± 3 Ma, 2138 ± 7 Ma, 2166 ± 5 Ma, and 2198 ± 4 Ma. Source rocks for the three former age groups can be found in the Rio Itapicuru greenstone belt, including zircon xenocrysts from the Nordestina granodiorite, whereas the latter age group has not yet been reported in the Serrinha block. The new zircon ages show that only rocks of the Palaeoproterozoic Rio Itapicuru greenstone belt and of a hidden 2.17–2.20 Ga crust were sampled by the kimberlite magma during its ascent through the lithosphere. It is proposed that there is none or a few Archaean crust beneath the kimberlite emplacement area, hence implying a thick Palaeoproterozoic lithosphere for this portion of the São Francisco craton. [ABSTRACT FROM AUTHOR]

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, TDM and εNd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean TDM, inherited Pb–Pb zircon ages and strongly negative εNd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision. [ABSTRACT FROM AUTHOR]

Abstract: A study of sediment provenance and tectonic setting was carried out for the first time on metasedimentary rocks of the Rio Itapicuru greenstone belt, Serrinha Block, São Francisco craton, Brazil, using a combination of LA-ICP-MS zircon U–Pb geochronology, whole-rock geochemistry and Sm–Nd isotope data. The protoliths of the studied phyllites, micaschists and biotite gneisses were classified mostly as mudstones and impure sandstones, i.e. shales and greywackes. The chemical index of alteration (CIA) ranges from 39 to 70 indicating moderate chemical weathering in source areas. Intermediate and felsic rocks (granite, granodiorite, rhyolite and andesite) are the likely main source rocks, and less often mafic rocks (basalt and gabbro). The depleted-mantle Nd model ages (T DM), most in the range 2.0–2.2Ga, and the positive ɛ Nd values for the proposed deposition time (ca. 2115Ma) suggest source rocks mainly in the greenstone belt. The U–Pb data on detrital zircon grains of four samples yielded 207Pb/206Pb age mostly in the timespan 2125–2237Ma, with age populations clustering at 2164 ± 4Ma and 2209 ± 4Ma, and minor outliers at ca. 2112Ma, 2270Ma, and 2415Ma. The results indicated that the metasedimentary rocks of the Rio Itapicuru greenstone belt have source areas mainly in Palaeoproteroic terranes such as the Rio Itapicuru and Rio Capim greenstone belts. The zircon populations between 2.17Ga and 2.24Ga are not found in the Serrinha block, thus requiring sources that are not known in the region. Diagrams of tectonic setting suggest that the sediments may have been deposited on continental island arc. Our data support a model in which basalts and arc granites of the Rio Itapicuru greenstone belt were accreted onto an unknown continental or arc margin and the entire pile collided with, and were thrust onto a microcontinent represented by the adjacent Archaean basement complex. [Copyright &y& Elsevier]

Abstract: Several granitic plutons have intruded the Palaeoproterozoic Rio Itapicuru greenstone belt, São Francisco craton, Brazil, in the time interval 2163-2080 Ma, but their tectonic significance is poorly understood. The Fazenda Gavião granodiorite (FGG) is one of a set of plutons emplaced along the western boundary of the greenstone belt with Archaean migmatite-gneiss basement. The pluton is mostly composed of hornblende granodiorite, occasionally crosscut by syn-plutonic mafic dykes. The FGG is metaluminous, medium- to high-K calc-alkaline with relatively constant silica abundances (SiO2 ∼ 63–66 wt%), high Sr (900–800 ppm) and high Ba (1000–1500 ppm). The associated mafic dykes are ultrapotassic, with high abundances of Ba, Sr, MgO, Ni, Cr, and light rare earth elements, suggesting derivation from partial melts of an enriched mantle source. The FGG originated probably by fractional crystallization from a primitive K-rich mafic magma that interacted with crustal melts. Its zircon U–Pb SHRIMP age of 2106 ± 6 Ma indicates that the FGG is younger than the early (2163-2127 Ma) tonalite-trondhjemite-granodiorite (TTG) and calc-alkaline arc plutons of the greenstone belt, and is closely related in time and space with potassic to ultrapotassic plutons (ca. 2110-2105 Ma). The negative εNd(t) of FGG and coeval K-rich plutons of the Rio Itapicuru greenstone belt contrasts markedly with the positive εNd(t) of the older arc plutons, indicating a major change of isotope signatures in granites of the Rio Itapicuru greenstone belt with time. This isotope shift may be related to magma contamination with older continental material and/or derivation of the parental potassic magma from enriched lithospheric mantle sources. We suggest that the K-rich plutons were emplaced during or shortly after Palaeoproterozoic arc-continent collision. [Copyright &y& Elsevier]

The Mamão gold deposit, located in the eastern portion of the Amazonian Craton, northern Brazil, is hosted by mafic rocks of the Mesoarchean Andorinhas greenstone belt. An underground mine was operated in this area by Troy Resources Ltd., which produced approximately 350 koz of gold between 2008 and 2015. The hydrothermal alteration halos are characterized by a distal zone (10- to 80 m wide) with a chlorite-albite-epidote-actinolite assemblage, an intermediate zone (5- to 20 m wide) marked by calcite-albite-chlorite-actinolite-quartz, a proximal zone (0.5–5 m wide) composed of biotite-quartz-calcite-pyrite-(gold) and a quartz vein-hosted orebody zone (0.5- to 6 m wide) composed by quartz-calcite-pyrite-(gold). Geochemical data indicate that the metabasalt, metandesite and metadacite host rocks display a tholeiitic N-MORB signature, suggesting that they were formed in ancient oceanic spreading centers. Hydrothermal chlorite of the Mamão deposit is classified as ripidolite and brunsvigite. Biotite is the main mica and increases in modal content from the distal alteration to the vein zone, displaying compositions within the annite domain. Muscovite occurs as very fine lamellae and is generally located in the intermediate zone of the altered region. Amphiboles are calcium-rich and display compositions mostly varying from hornblende (Mg-hornblende to Fe-hornblende) in the distal zone, to actinolite in the intermediate zone. Sulfidation occurs in all hydrothermal alteration zones, with both fine- and coarse-grained pyrite as the main sulfide phase, and lesser amounts of chalcopyrite, pyrrhotite, cobaltite and sphalerite. Fine-grained pyrite contains many gold inclusions and is typical of the orebody zone preferentially in close contact with the proximal hydrothermal zone, whereas coarse-grained pyrite (without recoverable gold) occurs in the veins. From the edges to the core of the hydrothermal alteration halos, the As, Sb, Ag and Au contents in the pyrite increase, whereas Pb, Co and Mo decrease. Gold occurs as free crystals (2- to 0.1 mm) disseminated within the gangue or as inclusions in the fine-grained pyrite. Temperature estimated via chlorite geothermometer ranges from 284- to 320 °C, with pressure between 1.12 and 1.34 kbar according to biotite geobarometer calculations for the hydrothermal zones. The C–O–S isotope analyses of the calcite and pyrite from the orebody zone yielded δ13C, δ18O and δ34S values ranging from −4.23 to −3.54‰, 8.60–9.77‰ and 3.37–3.52‰ respectively, which suggests that the hydrothermal fluid is predominantly of metamorphic origin. The εNd (3.0) calculated using isotopic analyses of the metavolcanic host rocks varies between +0.11 and + 4.25. A whole-rock Sm–Nd isochron age of 2952 ± 14 Ma was obtained from the host rocks, while a post-mineralized pyrite yielded a Sm–Nd isochron age of 2446 ± 35 Ma. The deposit exhibits mineralogical and isotopic characteristics consistent with those of orogenic type containing gold-bearing, sulfide-rich quartz veins. The Mamão deposit's host rocks are among the oldest rocks described so far in the Carajás Province. The age of the mineralizing event is limited between 2.95 and 2.45 Ga (probably ca. 2.7 Ga), which is the major metallogenic event of gold-deposit formation in several granite-greenstone terrains worldwide. [Display omitted] • Characterization of hydrothermal alteration halos. • Geochemistry and mineral chemistry of hydrothermal alteration minerals. • P-T conditions and source of hydrothermal fluid. • Sm–Nd and C–O–S isotopes of orogenic gold deposit. • Archean gold deposit in the Amazonian Craton. [ABSTRACT FROM AUTHOR]

Orogenic Au deposits represent one of the largest sources of gold in the world and in Brazil, particularly at the Quadrilátero Ferrífero, southeastern portion of the São Francisco Craton. In the last decades, advances in in-situ analytical techniques allowed a detailed investigation of the chemistry of sulfides. This can reveal information about the origin and composition of the mineralizing fluids, potential sources of metals and sulfur and the fluid-rock interaction during the ore-forming processes. In this paper we combine Electron Microprobe Analysis and LA-ICP-MS analyses to put constraints on the nature and evolution of the Pitangui gold deposit. The Pitangui deposit represents an important Orogenic Au deposit hosted in clastic metasedimentary rocks of the Pitangui greenstone belt, northwestern portion of the Quadrilátero Ferrífero. Detailed drill hole, petrographic observations and in-situ chemical analyses of the hydrothermal alteration zones show that pyrite is the dominant ore mineral in the deposit and two possible origins are suggest including (i) syngenetic pyrite development at the early stages of the greenstone belt evolution and (ii) epigenetic pyrite, which is associated with hydrothermal processes during the Orogenic Au event. Syngenetic pyrite (PyI) has high Co, Bi, Pb, Sb, Te, Se, Ag and Au concentrations. The PyI could represent an important source of metals, a set of trace elements and likely sulfur for the later Orogenic Au mineralization. Epigenetic pyrites (PyII, PyIII and PyIV), conversely, are mainly found in association with hydrothermal minerals and within the mineralized carbonate-quartz-vein. The variations at Co, Ni, Pb, Te, Sn, Se, As, Bi, Sb, Ag, Mo and W in the epigenetic pyrite indicate that the dehydration PGB sequences during orogenesis constitutes the main fluids sources. The PyV, in turn, only appears at metamafic dyke, but its origin remains unclear. • The Pitangui Orogenic Au deposit records a complex metallogenetic evolution. • In-situ EPMA and LA-ICP-MS trace elements geochemistry can reveal the ore-forming processes. • Syngenetic and epigenetic sulfides display different trace elements composition and suggest different origins. [ABSTRACT FROM AUTHOR]

The Crixa´s greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brası´lia belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe- and Al-rich metasedimentary rocks of the upper unit.Sm–Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00±0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show TDM model ages of mostly 2.33–2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite–garnet schist yields an age of 505±7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixa´s represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny. [Copyright &y& Elsevier]

The Archean nucleus of the Amazonian Craton, represented by the Carajás Mineral Province (northern Brazil), crops out as a broad exposition of igneous, sedimentary, and metamorphic rocks showing a complex polyphase geologic history. A succession of orthogneisses (metagranitoids), amphibolites, and metavolcano-sedimentary rocks that show greenstone belt characteristics occur in Tancredo Neves Village district, northeast of São Félix do Xingu city (state of Pará), is the focus of this study. Detailed field, petrographic, and stratigraphic data reveal that these metamorphic rocks underwent two different tectono-metamorphic events: (i) the regional metamorphism (>2.8 Ga), which occurred under greenschist-to mid-upper amphibolite-facies conditions; and (ii) the dynamic metamorphism (>2.6 Ga) which reached amphibolite-facies conditions and overprinted previous metamorphic textures. Associated regional strike-slip NW–SE to EW-trending ductile shear zones generated mylonitic, ultra-mylonitic, and migmatitic zones. Whole-rock geochemistry discriminates four groups of metagranitoid protoliths. G1 group is composed of peraluminous trondhjemitic rocks samples, calc-alkaline, weakly fractionated, and enriched in HREE. G2 group comprises metaluminous and alkaline monzogranite samples with high Zr, Y, Nd, HREE, and intermediate fractionation. U–Pb (LA-MC-ICP-MS) dating of G2 zircons yields a crystallization age of 2745.6 ± 5.0 Ma. Groups G3 and G4 encompass peraluminous, calc-alkaline to alkali-calcic, anorogenic, fractionated monzogranitic and granodioritic rocks. G3 and G4 groups are further distinguished by high and low La/Yb (N) ratios, respectively. The results obtained for Tancredo Neves Village district very much resemble the Canaã dos Carajás (Carajás Mineral Province) area characteristics, comprising intense magmatism (2.7–3.0 Ga) and reworking related to at least two distinct metamorphic events in the Archean. As Canaã dos Carajás, Tancredo Neves Village can be a promising area for mining exploration. • Comprises the Archean rocks from the Tancredo Neves Village region, northern Brazil. • Geological, petrographical, and geocronological data for mylonitic gneissic granitoids. • Definition of a regional large-scale metamorphic event transposed by a dynamic event. • Petrogenetic features and comparisons with the easternmost Carajás Mineral Province. [ABSTRACT FROM AUTHOR]

The Crixás Greenstone Belt, located in an Archean-Paleoproterozoic terrane within the Neoproterozoic Tocantins Province, hosts an important gold deposit in Brazil with an endowment of 7 Moz currently operated by AngloGold Ashanti. Its stratigraphy is composed of an Archean komatiite and basalt basal sequence followed by a Paleoproterozoic sedimentary sequence composed of carbonaceous phyllites, dolomite-rich marbles with minor siliciclastic lenses (Ribeirão das Antas Formation) followed by a uniform siliciclastic sequence (Córrego Geral Formation). At the mining area, the entire sequence is stratigraphically overturned and metamorphosed under greenschist to lower amphibolite facies conditions. The orebodies are structurally controlled by thin-skinned low-angle thrust faults, and are mainly hosted by the sedimentary sequence in dilation zones named locally, from the shallowest to the deepest, as Structure Palmeiras, Structure IV and Structure III. The gold mineralization occurs in association with disseminated sulfides, massive sulfide lenses or quartz veins. Structure III is the deepest mineralized structure with gold mostly occurring in quartz veins cutting carbonaceous phyllites. Siliciclastic lenses occurring within the Structure III interval show variable gold grades, while siliciclastic rocks from the uniform deeper sequence just below Structure III are barren. The chronology of the sedimentary deposition, metamorphism and mineralization is still poorly understood. Detailed drill core logging, petrography and U–Pb geochronology of detrital zircons from siliciclastic rocks were carried out in order to better understand the relationship between the two metasedimentary packages and gold distribution. Petrographically there are no relevant differences between Au-bearing siliciclastic rocks (Ribeirão das Antas Formation) and barren siliciclastic rocks (Córrego Geral Formation), except for the presence of sulfides in the former. The new dataset of detrital zircon U–Pb ages revealed a ubiquitous Paleoproterozoic peak in all samples, with nearly no Archean contribution. K-feldspar and lithic angular fragments, zircons with low to medium roundness, and the U–Pb results indicate a Rhyacian provenance from a proximal source. Neoproterozoic overgrowths are found on detrital zircons from the barren siliciclastic interval of the Córrego Geral Formation, providing unequivocal evidence of a Neoproterozoic event around 600 Ma that affected the rocks. The Neoproterozoic partial reset, possibly related to the overriding of the Mara Rosa Magmatic Arc onto the Crixás Greenstone Belt, is unlikely to be linked to the mineralization considering that zircons from the barren rocks are the most thermally affected by this event. The Au mineralization is more likely related to Paleoproterozoic events, a time period known as a major global episode of orogenic gold deposit formation. • New U–Pb DZ data from the Crixás Greenstone Belt gold deposit. • Comparison between the Au-bearing and barren siliciclastic rocks. • Ubiquitous unimodal Rhyacian provenance. • Neoproterozoic partial resetting stronger in DZ from barren rocks. • Mineralization possibly related to the Paleoproterozoic major global episode of orogenic gold [ABSTRACT FROM AUTHOR]

The Quadrilátero Ferrífero (QF) is an important, polydeformed mineral province, located in the southernmost portion of São Francisco Craton. The Rio das Velhas greenstone belt (RVGB) is an Archean meta-volcano-sedimentary succession from the QF, the Córrego do Sítio unit (CdS) being its most widely distributed sedimentary sequence. Detailed sedimentology study allied to x-ray diffraction-XRD and geochronological (LA-ICP-MS in detrital zircon) analyses provide evidence for a new interpretation of the CdS. The identified sedimentary structures of the CdS (e.g. flaser and lenticular bedding, sharp and erosional base, normal grading, mud drapes, quartz granules, syndepositional deformation, and ball & pillow) led the interpretation of these couplets as turbidity currents deposits corresponding to TA and TE facies of the classic Bouma Sequence. The CdS sequence was deposited on distal portions of an ancient submarine fan environment by turbidity current mechanisms, then it was partially reworked by tidal bottom currents. Geochronological data pointed out the main influence of the Rio das Velhas II magmatic event in the deposition of the eastern QF metasedimentary sequence, as evidenced by the maximum depositional age at 2818 ± 31 Ma. The surrounding metamorphic complexes terrains may have acted as source areas during the deposition of the sedimentary sequences, probably in a convergent to a collisional marginal tectonic setting (subduction stage). • Córrego do Sitio unit sedimentary sequences interpreted as turbiditity currents partially influenced by tidal bottom currents. • Eastern Rio das Velhas greenstone belt metagraywacke maximum depositional age at 2818 ± 31 Ma. • Sedimentary provenance from surrounding Archean metamorphic complexes. [ABSTRACT FROM AUTHOR]

The Pitangui greenstone belt (PGB) – located in the southern São Francisco Craton, (Brazil) – is a volcano-sedimentary assemblage that is tectonically imbricated over the orthogneiss and granitoid rocks collectively termed the Divinópolis Complex. One diorite gneiss sample from the northern Divinópolis Complex yields an igneous zircon age of 2878 ± 5.7 Ma, constraining the time of crystallization. The basal sequence of PGB predominantly comprise metabasalt with restricted bodies of metagabbro/metadiorite, metaultramafic volcanic rocks and BIF. The upper sequences of the PGB exhibit a maturation of the basin, grading to more felsic and sedimentary lithologies, such as clastic metasedimentary rocks, intermediate metavolcanic rocks, and metapyroclastic rocks. The first recognized deformation phase in the PGB (D n) is associated with greenschist facies conditions and the development of the main tectonic foliation. The second deformation phase (D n+1) is associated with amphibolite facies metamorphism, while a later (D n+2) phase may be related to younger Proterozoic deformation (ca. 2.0 Ga). PGB metabasalts yield a crystallization age of 2740.2 ± 12 Ma and are characterized by a tholeiitic affinity and flat LREE patterns. Based on mixed MORB- and arc-affinities, a back-arc basin setting is suggested for the formation of PGB basalt. This mafic magmatism (2740 Ma) is slightly younger than the ages reported for the Rio das Velhas greenstone belt, with which the PGB is generally correlated. The metasedimentary rocks of the PGB have age intervals spanning from 2690 to 3250 Ma. One mica-quartz phyllite has a maximum depositional age of 2769.8 ± 8.9 Ma while a chlorite phyllite records and age of 2693.2 ± 7.9 Ma. The metasedimentary rocks are derived from a source area of mafic to felsic composition that is similar to TTGs, cratonic Archean shales and E-MORB. The predominant sources were likely the sialic Archean crust of the granite-gneiss complexes (e.g. the Divinópolis Complex and potassic granitoids of the Mamona Event) and the mafic sequences of the arc (e.g. PGB back-arc basalt). One late potassic pluton (Casquilho monzogranite) crosscutting the fabrics of the volcano-sedimentary assemblage and yields an intrusion age of 2694.7 ± 9 Ma, constraining the minimum sedimentation age assembly of the package. Collectively, a Neoarchean primitive island arc or back-arc basin is proposed to have generated the PGB, which is here interpreted as distinct from the Rio das Velhas greenstone belt. We suggest that PGB developed during an accretion-collision event in the southern São Francisco Craton before cratonic stabilization of the crust by the end of the potassic 2760-2680 Ma Mamona Event. • Crystallization age of the Divinópolis Complex diorite gneiss is 2878 Ma. • Mafic magmatism of Pitangui greenstone belt at 2740 Ma. • Metasedimentary rocks originated from sources between 2690 and 3350 Ma of felsic and mafic composition. • The minimum depositional age of sediments is constrained by the intrusion of the Casquilho granite at 2694 Ma. • Pitangui greenstone belt is younger than Rio das Velhas greenstone belt. [ABSTRACT FROM AUTHOR]

Metamorphosed mafic-ultramafic rocks from the central portion of the Rio Grande do Norte Domain of the Borborema Province (BP), northeastern Brazil, have been studied here with the objective of gaining a better understanding of the Archean evolution of this portion of the BP. The study area is underlain by gneissic-migmatitic basement as well as biotite schists and paragneisses of the Seridó and Jucurutu formations of the Seridó Group. The mafic-ultramafic rocks (serpentinites, amphibole-schists and amphibolites) occur in lenses for 30 km along strike following the NNE regional foliation. The serpentinites occur as relatively small exposures of elongated bodies within paragneisses of the Jucurutu Formation and orthogneisses of the Caicó Complex. They consist of xenoblastic serpentine (75–99%) that has replaced the primary ferromagnesian minerals of the protolith. The amphibole schists and amphibolites are whitish gray to green, depending on the dominant mineral phase, tremolite, talc or actinolite. The mafic-ultramafic rocks present high MgO, Ni and Cr 2 O 3 contents and very low TiO 2 , Na 2 O and K 2 O contents, being compositionally similar to mafic-ultramafic rocks from other Archean greenstone belts worldwide. U-Pb (SHRIMP) dating of zircons returned concordia ages of 3526 ± 5 Ma (MSWD = 0.0084) for the serpentinites of the Serra Verde Mine and 3747 ± 12 Ma (MSWD = 9.8) for serpentinites from the Oiticica Mine. The petrographic and geochemical characteristics as well as the Eo to Paleoarchean ages of the studied rocks are significant as the basement of this portion of the BP is dominantly of Paleoproterozoic age. It is envisaged that these results will lead to further debate and a greater understanding of crustal evolution of the BP. We propose that these mafic-ultramafic rocks are part of a dismembered Archean greenstone belt, which present the oldest ages so far determined for the South American Platform. • The mafic-ultramafic rocks have geochemical signatures akin to Archean komatiites. • The mafic-ultramafic rocks may be a part of a dismembered greenstone belt. • Zircon U-Pb dating indicates Eo to Paleoarchean crystallization ages for the protoliths. • Archean crust in this part of Borborema Province may be more extensive than previously thought. [ABSTRACT FROM AUTHOR]

The Crixás Greenstone Belt is a supracrustal rock sequence of the allochthonous Archean-Paleoproterozoic Crixás-Goiás Terrane of the Neoproterozoic Tocantins Province of Central Brazil. The belt occurs between Mesoarchean (2.9-2.8 Ga) primitive orthogneiss complexes intruded by Siderian intracratonic mafic dikes and stocks. The rock succession of the greenstone belt consists of metakomatiites followed by metabasalts, both of greenschist to low amphibolite facies, underlying metasedimentary rocks of low greenschist facies. The greenstone belt is divided into the southern, central, and northwestern sectors, with different rock associations. The northwestern sector tectonically underlies a small exotic block known as the Mina Inglesa Sequence. The sequence consists of a low greenschist facies volcano-sedimentary package similar to a greenstone belt, but of unknown origin. The latest stratigraphic nomenclature for the belt labels its rock package as the Crixás Group. From base to top, the Group comprises the Córrego Alagadinho, Rio Vermelho, and Ribeirão das Antas Formations, made up of metakomatiites, metabasalts, and metasedimentary rock, respectively. The Mina Inglesa Sequence is an independent unit of the Crixás Group, in spite of the similar rock assemblage of both units. Protoliths of the Ribeirão das Antas Formation comprise shallow water black shales with dolomite lenses, laterally wedge-shaped with impure turbiditic sandstones (wakes). Published U/Pb and Sm/Nd whole-rock isotopic data indicate that the metakomatiites and metabasalts erupted during the Mesoarchean (3.0 Ga). Detrital zircon grains from the sedimentary protoliths suggest a mixed Mesoarchean and Rhyacian source-area (3.2 and 2.2 Ga). The U/Pb age from hydrothermal zircon grains of a gold-bearing quartz vein indicates that the sedimentation ended just before 2.1 Ga. Field, drilling, and the isotopic data led the authors to review the stratigraphic content of the Crixás Group by subdividing it into an Upper and Lower Subgroups. The Lower Subgroup comprises the Mesoarchean volcanic section subdivided, from base to top, into the currently adopted Córrego Alagadinho and Rio Vermelho Formations. The Upper Subgroup rests ubnconformable on the Lower Subgroup, and comprisess the Rhyacian metasedimentary section. The Upper Subgroup contains two rock units deposited in different environments. The currently used Ribeirão das Antas Formation is the lower unit, which is redefined as composed of carbonaceous phyllites with dolomite lenses. The upper unit comprises the new proposed Córrego Geral Formation, composed of metagraywackes. The wedge-shaped contact between both sedimentary units indicates that they were coevally deposited. The Paleoproterozoic sedimentary basin consisted of a small depression formed during the sagduction of the volcanic pile, triggered by the diapiric rise of the adjacent orthogneiss protoliths. The sediment deposition started with black shales in response to a slow rise of the seawater and paleoclimatic conditions. An increasing input of siliciclastic turbidites occurred in response to the uplift of a Rhyacian/Mesoarchean source-area that provided a mixed clastic load. Anomalous positive δ13C data from the dolomite lenses indicate that they deposited during the 2.2-2.1 Ga Lomagundi Event, which is compatible with the dominant Rhyacian zircon grains of the graywackes. The Crixás-Goiás Terrane lacks geologic evidence for the existence of an internal Rhyacian crustal block to justify the youngest detrital zircon grains of the graywackes. These observations render uncertainty about the geographic position of the rising crust. Paleoproterozoic crustal shortening of the belt resulted in stacked thin-skinned thrust-faults that host gold orebodies. An eastward age decrease of the crustal shortening from Crixás to an easterly belt leads the authors to propose that the clastic load source could occur to the West of the Terrane. The Mina Inglesa Sequence is an allochthonous block with lithologic and isotopic features that distinguish it from the greenstone belt. The authors also describe that there is no isotopic evidence for Meso- to Neoproterozoic source-areas, neither for the Córrego Geral Formation nor for the Mina Inglesa Sequence. These findings raise a suggestion that the granite-greenstone belt association of the Terrane may represent a craton border that remained stable before its amalgamation to the province during the Neoproterozoic. • Stratigraphic update of the Crixás Greenstone Belt, Central Brazil. • Subdivision of the Crixás Group into a Lower Sub-group of volcanic rocks, and an Upper Sub-group of metasedimentary rocks. • Subdivision of the metasedimentary section into two coeval formations. • U-Pb data from detrital zircon grains and discussion about source-areas. • Discussion about tectonic and depositional environments of the greenstone belt. [ABSTRACT FROM AUTHOR]

Field, petrographic, lithogeochemical, LA-ICP-MS in situ U-Pb zircon geochronology, and Nd and Sr isotopic data were used to determine the geologic evolution, timing of volcanism and tectonic setting of the Mundo Novo greenstone belt (MNGB) including nearby units, such as the basement and younger granitic rocks in the eastern São Francisco Craton (NE Brazil). The basement of the greenstone belt corresponds to the Gavião Block, which comprises tonalite-trondhjemite-granodiorite orthogneisses, migmatites and subordinate metarhyolites and metagranites. The MNGB is divided into three lithological sequences: (i) the lowermost unit is composed of metakomatiites; (ii) the middle unit is composed mainly of metabasalts, metadacites and metasedimentary chemical rocks; and (iii) the uppermost unit is composed of metasedimentary siliciclastic rocks. Additionally, two Rhyacian-Orosirian granitic plutons occur in the area, the Areia Branca and Jequitibá metagranites. U-Pb zircon data from the Miguel Calmon and Fazenda Coqueiro metagranites in the Gavião Block yield crystallization ages of 3355 ± 16 Ma and 3227 ± 23 Ma, respectively, and both metagranites were derived from the lower continental crust based on low 87Sr/86Sr i values (0.699 and 0.705, respectively) and negative values of Ɛ Nd(t) (−3.7 and −3.6, respectively). The 2595 ± 21 Ma U-Pb zircon crystallization age of the metadacites allowed the determination of the timing of volcanism in the middle sequence of the MNGB. The 2106 ± 71 Ma and 1975 ± 36 Ma U-Pb zircon crystallization ages for the Areia Branca and Jequitibá metagranites, respectively, which are both interpreted as derived from crustal protoliths based on high 87Sr/86Sr i values (0.744 and 0.730, respectively) and negative values of Ɛ Nd(t) (−7.7 and −6.5, respectively), record a Rhyacian-Orosirian tectonothermal event that affected the MNGB and the adjoining region. The whole-rock geochemical data in tectonic discrimination and multielement diagrams show that the volcanic rocks of the MNGB formed in island arc and back-arc basin settings related to the oceanic crust. Therefore, the following five stages are proposed for the tectonic evolution of the study region: stages (i) and (ii) were characterized by the establishment of the TTG basement by 3.35 Ga, when the Miguel Calmon granite was consolidated, followed by a rifting that formed the rhyolites, and subsequent granite formations occurred at 3.22 Ga (Fazenda Coqueiro granite formation), followed by the abort of the rift and a closure event. The volcanism during stage (iii) at 2.59 Ga in the MNGB occurred in island arc and back-arc basins in the oceanic crust between Archean cratonic blocks, and stages (iv) and (v) between 2.10 Ga and 1.97 Ga represent two stages of a progressive collision event in which the oceanic crust was compressed between the Gavião, Mairi, Jequié and Serrinha blocks and the Itabuna-Salvador-Curaçá Belt from the eastern part of the São Francisco Craton. The collisional event of stages (iv) and (v) was coeval with the Rhyacian-Orosirian formation of granites and basins such as that where the uppermost siliciclastic sequence of the MNGB was deposited. • The Paleoarchean metagranites were derived from the lower continental crust. • The Mundo Novo greenstone belt formed in island arc and back-arc basin settings. • The volcanism in the Mundo Novo greenstone belt occurred at 2595 ± 21 Ma. • The Rhyacian-Orosirian granitic bodies were derived from crustal protoliths. • The oceanic crust was compressed between cratonic blocks during the Paleoproterozoic. [ABSTRACT FROM AUTHOR]

The Serra das Pipocas Greenstone Belt is one of the rare Paleoproterozoic greenstone belts within the Borborema Province. It is on the west border of the Archean/Paleoproterozoic Nucleus (Cruzeta and Mombaça Complex) of the Ceará Central Domain, northern portion of the Borborema Province. The Serra das Pipocas Greenstone Belt has an elongated sigmoidal shape encompassing about 1000 km2 of the Ceará Central Domain. It consists of a metavolcanosedimentary sequence formed by metasedimentary psammitic-pelitic-marly rocks and interleaved metaultramafic and metamafic rocks, represented by chlorite-anthophyllite-actinolite/tremolite schists and amphibolites. Layers of metatuffs, metabasic, meta-acid sills (metadacites), metacherts, gondites and banded iron formations occur within the metamafic rocks. Actinolite, metagabbro, metadiorite bodies, metabasic dike, Neoproterozoic metagranodiorites and leucogranites crosscut the metavolcanosedimentary sequence. Close to the borders of metagranodioritic bodies (zircon U–Pb 2181 ± 4.4 Ma), the metavolcanosedimentary sequence is hydrothermally altered. The metaultramafic rock (chlorite-anthophyllite-actinolite/tremolite schist) contains high MgO (>18 wt%), low alkali (≤1 wt%) and TiO 2 (<0.8 wt%) and Al 2 O 3 /TiO 2 ratios between 9.94 and 27.26, with a mean value of 17.13. Values of (Gd/Yb) N vary between 1.00 and 1.88, which attest affiliation with komatiite protoliths, between the Munro and Barberton types. The amphibolites are usually fine-grained and depleted in ΣREE (33.14–45.02 ppm), with low (La/Yb) CH values (0.74–3.82). This reflects weak LREE enrichment in relation to HREE, and no Ce and Eu anomalies [(Eu/Eu*) n between 0.98 and 1.04]. Such patterns are similar to T-MORB (1.7–4.3). Additionally, some mafic-ultramafic metaplutonic rocks present chemical composition similar to their volcanic counterparts, and this may indicate cumulates or sills contemporary to this sequence. Zircon U–Pb dating using LA-ICP-MS of three distinct metadacite occurrences (western border: 2234 ± 13 Ma, eastern border: 2212 ± 13 Ma, and central portion: 2156 ± 45 Ma) and metagranodiorites (2181 ± 4.4 Ma) indicates that the sequence was formed during the Rhyacian/Orosirian and later affected by late Neoproterozoic. [ABSTRACT FROM AUTHOR]

Serpentinized peridotites and basaltic volcanic rocks from the Quatipuru, Serra do Tapa, and Morro do Agostinho complexes are part of dismembered ophiolite bodies hosted by metasedimentary units of the Neoproterozoic Araguaia Belt, Tocantins Province, central Brazil. Approximately 10 lenticular ultramafic bodies, which contain primitive (serpentinite) and more evolved (actinolite schist, actinolite-talc schist, actinolite-serpentinite schist, talc schist, talc-serpentine schist, actinolitite, cloritite) rocks, were identified in the Rio do Coco Group (RCG), a metavolcanic-sedimentary sequence in the southern portion of the belt. These bodies are intercalated with metasedimentary rocks. All lithologies are metamorphosed to upper greenschist and lower amphibolite facies. The sequence was initially characterized as a classic Archean greenstone belt, but the present study suggests that it should be considered an ophiolite suite. On the basis of the Ɛ Nd(t) values the RCG rocks can be subdivided into two groups: Group 1, with positive Ɛ Nd(t) values, neodymium model ages (T DM) of ca. 1.0 to 1.1 Ga, with ocean-island-basalt (OIB) affinity, and being less contaminated with metasedimentary rocks of the Baixo Araguaia Supergroup; and Group 2, with negative Ɛ Nd(t) values, T DM ranging from 1.2 to 1.7 Ga, with a variable ocean island basalt (OIB), plume or enriched mid-ocean-ridge basalt (P- and E-MORB) affinity, and being more contaminated with metasedimentary rocks. Two samples were analyzed by the U-Pb method on zircon (LA-MC-ICPMS). The weighted average age from a serpentinite (715 ± 26 Ma) and an actinolite schist (702 ± 24) indicates a maximum emplacement age for this sequence at 708 ± 35 Ma, which is similar to the previously published age for the Quatipuru Complex (Sm-Nd isochron age of 757 ± 49 Ma). This may suggest that the metavolcanic-sedimentary sequence of the Rio do Coco Group and the Quatipuru ophiolite system could be part of the same dismembered oceanic lithosphere. • U-Pb on zircon data from the Rio do Coco Group relate it to the evolution of the Neoproterozoic Araguaia Belt. • Geochemistry, Nd isotopes and U-Pb on zircon for the Rio do Coco Group suggest that these rocks are related with the Quatipuru Complex. • The tectonic setting for the Rio do Coco Group mafic-ultramafic rocks is discussed in terms of mantle sources. [ABSTRACT FROM AUTHOR]