Brasiliano Orogeny Research Articles

Geology and metamorphism of the neoproterozoic Mangabal Complex: An example of Ni[sbnd]Cu-PGE mineralized intrusion in the Goiás Magmatic Arc, central Brazil

Inserted in the context of the Goiás Magmatic Arc, the Mangabal Complex represents a partially metamorphosed mafic-ultramafic intrusion, which is part of a cluster of neoproterozoic layered bodies formed during the Brasiliano Orogeny in central Brazil. The Mangabal Complex is composed of two exposures, the northern and the southern members, which exhibit the similar igneous mineralogy, crystallization sequence and were affected by the same metamorphic conditions. The stratigraphy of the Mangabal Complex can be divided into three main zones: i. the Lower Mafic Zone (LMZ), composed of adcumulatic norite and, forming the basal portion of the southern member; it is composed of adcumulatic norite; ii. the Ultramafic Zone (UZ), characterized by dunite and harzburgite cumulates, and iii. the Upper Mafic Zone (UMZ), consisting predominantly of norite, with restricted interlayeres of feldspathic dunite-orthocumulate. Petrographic data reveal a crystallization sequence for the Mangabal Complex formed by Olivine + Cr-Spinel > Olivine + Orthopyroxene > Orthopyroxene + Plagioclase > Clinopyroxene. In the olivine cumulus, the forsterita (Fo) content (Fo 78–85) suggest a moderately primitive parental magma. The NiCu-PGE mineralization occurs as disseminated sulfides, located in the UMZ, UZ and LMZ cumulate rocks, or forming massive to net-textured bodies in the metamorphic rocks of the UMZ. The main sulphide assemblage is composed of pyrrhotite + pentlandite + chalcopyrite ± pyrite, with minor pyrite + millerite. Disseminated sulphides are present in igneous rocks as well as in the metamorphic rocks that still partially preserve their original primary texture. This fact, together with the occurrence of abundant olivine and chromium-bearing spinel cumulates, suggests a fertile magma for the Mangabal Complex. However, the highest S, Ni, Cu, and PGE contents correlate positively with and are related to amphibolite and calc-silicate rocks associated with shear zones, indicating that metamorphism and deformation processes were critical in promoting the remobilization of sulphides and their deposition along shear zones. The primary mineral assemblage was largely replaced due to high-pressure upper-amphibolite facies metamorphism. The paragenesis representing metamorphic peak conditions is composed of kyanite + orthoamphibole + hornblende, which reveals pressures of approximately 8.5 kbar and a temperature up to 750 °C. The metamorphic mineral assemblage, together with the proximity to the São Luis de Montes Belos shear zone, suggests that kyanite crystallization in the Mangabal Complex and similar layered intrusions worldwide might be linked to the development of major crustal-scale structures.

The Horto-Baratinha itabirite-hosted iron ore: A basal fragment of the Espinhaço basin in the eastern São Francisco Craton

The Horto-Baratinha (HBD) iron ore deposit is located at the eastern border of São Francisco Craton, comprising BIF-hosted high-grade bodies (>60 wt.% Fe) associated with polydeformed quartz-mica-schists, amphibole-schist of Statherian maximum deposition age, enclosed by Statherian granitoids of the Borrachudos Suite and Neoarchean gneiss. All the sequence is crosscut by undeformed dikes and sills of pegmatitic bodies probably formed during Late Ediacaran-Cambrian. The metasedimentary sequence is stratigraphically correlatable with the Orosirian-Statherian Serra da Serpentina and Serra de São José Groups that comprise the basal units of the Espinhaço Supergroup and was intensively segmented into distinct tectonic blocks. The sedimentary/diagenetic bedding of the metamorphosed BIF (itabirite) is generally transposed by an axial planar schistosity. The lamellar hematite from itabirite is the oldest iron oxide generation, which was formed during the syn-deformational stage, parallel-oriented to the rock foliation. The (keno)magnetite grains from itabirite, iron ore and pegmatite bodies developed as idioblasts that grew over the foliation formed during late and post-deformational stages. Magnetite oxidizes subsequently to martite and granular hematite. Coarse lamellar hematite crystals randomly oriented in the border of the pegmatitic bodies also formed during the post-deformational stage due to hydrothermal reaction with itabirite. The country rocks have undergone at least three stages of deformation developed during the syn-collisional and late-collisional (Ediacaran to early-Cambrian) phases of the Brasiliano Orogeny: stage 1 with the development of a pervasive foliation (S1), parallel to axial plane to tight folds and transposition of all sedimentary structures; stage 2 with folding of S1; stage 3 with refolding of S1. Both fold systems interfere with each other making up a dome and basin refolding shape. During the late-collisional (Ediacaran to early-Cambrian) and post-collisional/gravitational collapse (Cambrian) the sequence was intruded by anatectic pegmatitic bodies, which are part of the Eastern Brazilian Pegmatite Province, one of the most significant pegmatitic regions worldwide. The fluid related with these intrusions could be related with the Si leaching, crystallization of magnetite and granular hematite, and consequent formation of high-grade iron bodies.

Neoproterozoic magmatic and metamorphic events in the Cuchilla Dionisio Terrane, Uruguay, and possible correlations across the South Atlantic

The Cuchilla Dionisio (or Punta del Este) Terrane of the southern Dom Feliciano Belt, eastern Uruguay, experienced a prolonged Neoproterozoic to Cambrian tectonothermal evolution. Zircon and monazite age data from orthogneisses of the basement of the terrane (Cerro Olivo Complex) provide evidence of widespread magmatism at c. 780 Ma and high-grade, up to granulite-facies metamorphism between 655 and 632 Ma. Isotope data imply that the orthogneiss protolithic melts formed in a within-plate, rift-related setting by Neoproterozoic (re-) melting of lower crustal material that was extracted from the mantle during the Palaeoproterozoic. We show that the terrane extends further north than previously thought. The presence of inherited Meso- and Palaeoproterozoic zircon grains indicates the existence of older crustal components in the terrane. The ages of the detrital zircon grains and their Hf isotope signatures cannot be explained by a Río de la Plata Craton provenance but are consistent with a southern African Kalahari Craton origin. Ar-Ar ages of 628 and 625 Ma were obtained on magnesiohornblende in amphibolite from the northern Cuchilla Dionisio Terrane, whose protolith formed in an Ediacaran back-arc basin. These ages are interpreted as the time of amphibolite-facies metamorphism. Thus, crustal convergence between the Kalahari (or Congo?) and the Río de la Plata Craton and subduction of the intervening oceanic basin must have been underway between 655 and 625 Ma.The early Neoproterozoic basement of the Cuchilla Dionisio Terrane is part of the southern African Namaqua Province that was separated from the Kalahari Craton during Rodinia break-up and opening of the southern Adamastor Ocean in the Tonian. Closure of this ocean in the late Cryogenian to early Ediacaran was associated with arc magmatism and high-temperature metamorphism of the Cuchilla Dionisio Terrane basement. A back-arc basin formed and was closed between this arc and the western African margin. Relics of this basin were obducted onto the Cuchilla Dionisio Terrane at c. 625 Ma. Late Neoproterozoic intrusive igneous rocks from the Cuchilla Dionisio and the Coastal terranes were emplaced in a supra-subduction setting (at least in Uruguay) and have the same Hf isotope signature.

The carbonate-hosted MVT Morro Agudo Zn-Pb deposit, central Brazil

Morro Agudo is the largest known Mississippi-Valley Type deposit (MVT) in Brazil with original estimated reserves of 18.3 Mt @ 5.08% Zn and 1.75% Pb of which 3.92 Mt@ 4% Zn and 1.45% Pb remained in 2017. In spite of having been mined for more than 30 years, a comprehensive study of Morro Agudo metallogenic features was still lacking in the international literature. This deposit is hosted within the 250 km long Vazante Sequence, a N-S-trending Mesoproterozoic-Neoproterozoic carbonate-dominant sedimentary sequence thrusted over Neoproterozoic rocks during the Brasiliano Orogeny. Sulfide bodies in Morro Agudo are restricted to the Morro do Calcário Formation as a 1300 m long ore envelope averaging 9 m thick, comprising stacked up stratabound orebodies that thin out down dip and are limited to the NE by the Main Fault. Five orebodies are described: (a) Breccia-hosted orebody “GHI”, (b) Sphalerite-rich doloarenite orebody “JK”, (c) Galena-rich doloarenite orebody “L”; (d) Lower replaced stratiform orebody “M”; (e) Upper replaced stratiform orebody “N”. Comparison of these orebodies reveals mineral zonation patterns and variations in Zn-Pb grade consistent with sulfide textural relationships. These relationships indicate five mineralization styles that provide spatial and timing constraints for Morro Agudo; (a) Stratiform sulfides, (b) Massive Ore, (c) Laminated Ore, (d) Breccia-hosted Ore, (e) Late stage fracture-filling Ore. Carbon and oxygen stable isotope data for each mineralization style shows a pattern of mixing of signatures of mineralizing fluids and barren host rocks and supports our interpretation that these mineralization styles occurred in more than one stage of ore formation. The ore was later offset by a set of strike slip normal faults associated with compression related to the Brasiliano Orogeny, of which the Main Fault (Falha Principal) is the most important because it limits the deposit to the NE. Previous interpretation of upper orebodies as part of a SEDEX system is revisited in the present work. In our model, data used to support a syngenetic origin for Morro Agudo, such as synsedimentary structures, Pb isotopes in sulfides and fluid inclusion data indicate a younger, barren generation of sulfides deposited long before Zn-Pb mineralization, during sedimentation of the Morro do Calcário Formation in the Mesoproterozoic. These early sulfides were preserved in cherts and as massive pyrite beds that were later overprinted by sphalerite and galena. Overprinting thus mixed Meso and Neoproterozoic Pb isotopic sources and led to a long standing argument over whether the Morro Agudo ores were syngenetic or epigenetic. This interpretation is also in agreement with the current classification of the Vazante Sequence as containing imbricated Neoproterozoic and Mesoproterozoic sedimentary rocks. Based on our work of sulfide textural relationships, stable isotopes, whole-rock geochemistry and previously published data, we argue that Morro Agudo ores formed by at least two stages of percolation of mildly hot hydrothermal fluid in the Neoproterozoic. The primary reason for sulfide precipitation is interpreted to be mixing of mildly-hot, metal-bearing basinal brines and seawater/connate waters.

Connecting the Araçuaí and Ribeira belts (SE – Brazil): Progressive transition from contractional to transpressive strain regime during the Brasiliano orogeny

Whether the Araçuaí and the Ribeira Neoproterozoic belts in southeast Brazil represent a continuous or two distinct orogenic belts is still a debated question. We compile existing geologic and geophysical data and argue that the two belts, in spite of differences in tectonic style and kinematics, should be considered as part of an orogenic continuity that formed during the mostly Late Proterozoic Brasiliano orogenic evolution. Structural mapping supported by Anisotropy of Magnetic Susceptibility data shows that the transition between the two belts is gradual, with a progressive change from a NE-trending subvertical foliation and subhorizontal stretching lineation in the Ribeira belt to gently dipping and less pronounced fabrics in the Araçuaí belt. The lineation progressively changes northward from NE to E-W, suggesting a transition from overall transcurrent to thrust kinematics, and the solid-state finite strain seems to get progressively higher into the Ribeira belt. Differences in tectonic style are explained by the southward termination of the rigid São Francisco craton, which caused oblique collision and lateral escape, as supported by numerical modeling. Shear-wave splitting measurements suggest that the transcurrent deformation in the Ribeira belt affected the entire lithosphere. In the transitional zone, the seismic anisotropy pattern is more complex and the delay time between the fast and slow shear-waves is smaller. These observations, together with a similar record of magmatism and timing of orogenic events and P-T conditions during peak metamorphism strongly support continuity between the Araçuaí and the Ribeira belts. This model is a "quasi-facsimile" of the Paleoproterozoic deformation that occurred in the Great Slave Lake area in Canada.

Structural framework from gravity and magnetic data in the paleo/mesoproterozoic Araí rift-sag Basin, Central Brazil

The Araí Basin comprises a paleo/mesoproterozoic rift-sag basin inverted in a thick-skinned regime during the Brasiliano Orogeny. Its depth to basement, geometry, and lateral extension remain unknown. We have aimed to determine this information based on geophysical/geologic data to clarify its main structures and gain an evolutional understanding. Gravity and magnetic data allowed us to delineate the rift system — its major faults and adjacent postrift sedimentary structures — over its entire length. The geophysical investigation included radial power spectrum analysis and Euler deconvolution alongside the application of edge structure enhancement filters on magnetic anomaly and matched filter grids. A 2.5D forward gravity model was generated using all results, with geophysical sources at depths of 35–42, 20.9–22.6, 11–15.6, 9.68, 8–2.3, and 0.37–0.54 km, respectively, interpreted as the Mohorovičić Discontinuity, Curie Surface, Conrad Discontinuity, granitic intrusions, basement depths, and superficial faults. The main interpreted tectonic framework of the entire area showed a preferential direction in N40E, comprising structures related to the Transbrasiliano Lineament and its reactivations. The basin magnetic signature trends N50-80E, with structures generated essentially in the Brasiliano Orogeny and those related to extensional rift faults reactivations (belonging to São Jorge-Alto Paraíso-Cormari, Cavalcante-Teresina, and Teresina-Nova Roma systems). These fault systems exhibit a magnetic signature at the average depth of 6 km, converging to a single fault detected up to 21.8 km. The main rift faults are in accordance with the Euler solutions and the matched filter interpretation, confirmed by mapped alluvial fans conglomerates. The current structural framework of the basin goes back to the Brasiliano Orogeny, when its sedimentary filling was limited by crustal blocks with distinct compositional, rheologic, and magnetic characteristics. These intrinsic characteristics of each block along with the resulting kinematics were responsible for delimiting the shape and depth of the basin.

Paleorift structure constrained by gravity and stratigraphic data: The Statherian Araí rift case

Gravimetric and stratigraphic data were used to investigate the Paleoproterozoic Araí Paleorift, a failed Statherian continental rift located in the western margin of the São Francisco Craton, where basement and cover were affected by the Neoproterozoic Brasiliano Orogeny. Euler deconvolution, tilt, total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift's main faults location, direction and depth, allowing to identify its main horsts, grabens, volcanic and plutonic centers. We found that rift faults occur to a maximum depth of ca. 38 km, but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km, probably the brittle-ductile transition zone at the time of rifting, practically disappearing at 20 km. Stratigraphic data and basement mapping were used in order to constraint gravimetric results. We classify the Araí Rift as a passive, three-armed failed rift, narrow to divergent type, that produced preferably anorogenic rapakivi-related magmas, most of it still lodged in the crust from surface down to ca. 19 km deep and subsidiary mafic magmatism. The results indicate the deep occurrence of low-density magmas beneath the rift's main axis, detected up to 20 km deep. Correlation to other global Statherian rifts show that the São Francisco Craton was strongly affected by taphrogenesis during the Statherian, together with Siberia, North America and North China cratons. Finally, by comparing our results to recent rifts we found that the Ethiopian rift's morphology is quite similar to the Araí. Surrounding the Tanzanian craton, the Cenozoic East Africa rift system morphology is compared to the Araí-Espinhaço rift system, which surrounds the São Francisco craton. The major contribution of this paper is the recognition of Araí Paleorift surface and subsurface morphology, up to now unknown, over an area of ca. 45.000 km2.

Metamorphic imprints on units of the southern Araçuaí belt, SE Brazil: The history of superimposed Transamazonian and Brasiliano orogenesis

The extensive Neoproterozoic Brasiliano belts that surround the São Francisco craton in southern Brazil are a consequence of tectonic processes that resulted in the formation of the supercontinent Gondwana in the Ediacaran to Cambrian. One of these Brasiliano-age orogens, the Araçuaí belt, overprints an earlier orogen developed during the so-called Transamazonian event at the transition between the Rhyacian and Orosirian periods. Archean and Paleoproterozoic basement units in the southern Araçuaí belt have been shown to have experienced both the Transamazonian and Brasiliano orogenies and have the potential to have preserved the P-T conditions of both events.This study investigates a series of granulite- and amphibolite-facies samples from the southern Araçuaí belt. Granulite-facies samples from the Acaiaca Complex and Pedra Dourada Granulite give monazite U-Pb ages in the range: 2015–1960Ma. Both the granulite samples preserve two metamorphic textures. Coarse-grained garnet+biotite+orthopyroxene+K-feldspar+plagioclase+quartz+ilmenite+inferred melt and later, fine-grained garnet+biotite+hornblende+quartz that grows at the expense of orthopyroxene. Peak conditions for the earlier event were c. 750–800°C and 6kbar, whereas those of the later event are poorly constrained. A garnet-bearing amphibolite-facies sample from the Mantiqueira Complex contains monazite included within garnet, which gives U-Pb ages of c. 1940Ma, whereas matrix monazite grains give ages of c. 590–540Ma. Garnet core compositional zonation indicates P-T conditions of 6kbar and 700°C for the c. 1940Ma metamorphism, while the matrix assemblage records a P-T path culminating in peak conditions of 8–9kbar at 650°C. A second amphibolite-facies sample was obtained from the Dom Silvério Group, whose depositional age post-dates the Transamazonian event. This sample gives two monazite U-Pb age populations, 597±3Ma and 547±5Ma. Garnet core zonation indicates a prograde P-T evolution with peak conditions of c. 8–9kbar and 650°C, although it is uncertain at this time what age population this event corresponds to. The results of this study provide better constraints on the chronology and metamorphic regimes of both the Transamazonian and Brasiliano events in the southern Araçuaí belt.

The Pico do Itapeva Formation: A record of gravitational flow deposits in an Ediacaran intracontinental basin, southern Brasília Orogen, SE Brazil

The Pico do Itapeva Formation is a key metasedimentary unit to the understanding of the extensional events that occurred between the late stage of the southern Brasília Orogen collision and the main collision in the central Ribeira Orogen. The formation crops out in a 20 km long NE-trending narrow belt in the Mantiqueira mountain range in eastern São Paulo State, Brazil. It is located in the interference zone of the southern Brasília and the central Ribeira orogens and records deformation and greenschist facies metamorphism (biotite zone) related to the Brasiliano orogeny. The Pico do Itapeva Formation rests unconformably on a metaigneous substratum of the Socorro-Guaxupé Nappe/Embu Terrane and, on the southern side, is truncated by a steep SE-dipping dextral reverse shear zone. It consists of a coarsening- and thickening-upward succession, with minimum thickness of 800 m, composed of lutite, arkose and conglomerate. These rocks constitute three distinct lithofacies associations: LAI- arkose, arkose-lutite composite beds, lutite and fine conglomerate beds; LAII- arkose, pebbly arkose and scarce lutite and; LAIII- conglomerate and pebbly arkose. Most of the beds are massive; graded beds, dish and convolute structures occur locally. Bed thickness varies from thin to very thick and amalgamated bodies constitute up to 30m thick strata. Three mappable units at scale 1:20,000 were recognized based on different proportions of the three lithofacies associations. The deposits are interpreted as the record of mass flows and associated processes in a fan delta setting developed in an intermontane rift basin. U-Pb LA-ICP-MS detrital zircon ages suggest the maximum depositional age at ca. 611 Ma and the basin evolution is interpreted in the range between 611 and 580 Ma during an inter-orogenic stage between the Brasília and Ribeira orogenies.

Granulite accretion to Rio de la Plata Craton, based on zircon U-Pb-Hf isotopes: Tectonic implications for Columbia Supercontinent reconstruction

The paleogeographic reconstruction of the Rio de la Plata involved either allocthonous or autochthonous process reflecting directly the Paleoproterozoic connection of the craton to Columbia Supercontinent. Santa Maria Chico Granulite Complex is a significant fragment of Rio de la Plata intensely affected by the Brasiliano Orogeny. Zircon U-Pb-Hf isotopes by LA-ICP-MS, mineral and whole-rock chemistry and a pseudo-section are presently interpreted. U-Pb-Hf isotopes characterize two main accretionary and metamorphic events: oceanic juvenile crustal accretion (i) 2430–2290 Ma (εHf(t) = −3.17 to +7.00), with arc related metamorphism (830–870 °C, 6.7–7.2 kbar) at ~2.3 Ga; and continental arc accretion (ii) 2240–2120 Ma (εHf(t) = −4 to +2.4), with continental collision metamorphism (770–790 °C, 8.7–9.1 kbar) at 2.1–2.0 Ga. Alkaline granitic dikes related to crustal extension at 1.8 Ga cut the granulitic rocks after the stabilization of this crustal segment. The present data point to formation of Paleoproterozoic granulitic rocks of the Santa Maria Chico Granulite Complex and adjacent Nico Pérez and Rivera terranes in multi-stage volcanic arcs to continental collision environment over 370 Ma (2430 to 2060 Ma). These terranes were amalgamated in the Paleoproterozoic to the core of the Rio de la Plata Craton as part of Columbia Supercontinent and intensely reworked during the amalgamation of Western Gondwana in the Neoproterozoic.

The Proterozoic Vazante Hypogene Zinc Silicate District, Minas Gerais, Brazil: A Review of the Ore System Applied to Mineral Exploration

The Proterozoic Vazante zinc silicate district in Minas Gerais, Brazil, hosts world-class hypogene willemite deposits in dolomitic rocks interbedded with siliciclastic rocks deposited in subtidal to supratidal environments. Willemite ore bodies are structurally controlled along regional NE-trending structures which are interpreted as being active during the Neoproterozoic Brasiliano orogeny. The hydrothermal alteration is characterized by an early stage of Fe-dolomite, which replaced the host dolomitic rocks, followed by precipitation of minor sphalerite and then hematite and willemite. Elements commonly enriched in the zinc ore include As, Ba, Be, Bi, Cd, Co, Fe, Ge, In, Mo, Ni, Sb, Se, U, V and W. Mineralogical, fluid inclusion and isotopic data indicates that mixing of S-poor metalliferous saline fluids with meteoric water favored the formation of willemite ore. Carbonaceous phyllites from the underlying thick siliciclastic sequence show evidence of early enrichment in zinc (and ore-related metals) and remobilization, respectively, prior to and during the Brasiliano orogenic event. This unit is interpreted as a possible source of ore-related elements. It is proposed that during the Brasiliano orogeny, hot (T > 170 °C) saline fluids (>15 wt % eq. NaCl) leached metals from siliciclastic source rocks and precipitated willemite ore in the overlying dolomitic sequence along structures that favored mixing with oxidizing meteoric water.

Tectonic and metallogenic evolution of the Curaçá Valley Copper Province, Bahia, Brazil: A review based on new SHRIMP zircon U-Pb dating and sulfur isotope geochemistry

More than three hundred mafic-ultramafic bodies of different sizes, containing copper sulfide mineralization, are present in the Curaçá River Valley Copper Province, northeastern São Francisco Craton, Bahia State, Brazil, covering an area of about 1700 km2. The basement, supracrustal sequence, and mafic-ultramafic intrusive rocks underwent granulite facies metamorphism and were reworked during the evolution of the Itabuna-Salvador-Curaçá Orogeny, resulting in retrograde metamorphism to the amphibolite facies, partial migmatization, and intrusion of granitic and syenitic rocks. During the slow and progressive rise of the terrain, hydrothermal reactions mainly along shear zones retrogressed part of these rocks to assemblages characteristic of the greenschist metamorphic facies. The main orebodies with copper sulfide minerals are associated with pyroxenite, mica-rich (glimmerite) and noritoid rocks. The present investigation focuses on the major Caraíba, Sussuarana, Surubim, and Vermelhos deposits. Chalcopyrite and bornite compose the ore, in a ratio of 70–30%, with copper grade varying between 0.69% and 1.4%, and associated gold and silver. The sulfide minerals are invariably accompanied by magnetite. Two mineralization styles are distinguished: (i) disseminated in non-altered mafic-ultramafic rocks; and (ii) tectonic controlled related to hydrothermal alteration zones. New geochronological data show that quartz-microcline metasomatites of the Caraíba Mine contain Neoarchean inherited zircons (upper intercept at 2723 ± 51 Ma), related to the early stages of magmatism, whereas Orosirian ages (2047 ± 12 and 2042 ± 15 Ma) are related to the main hydrothermal mineralization activity. Norite from the Surubim Mine has a Paleoproterozoic age (2047 ± 11 Ma), and the dated gabbro shows Pb loss in the Neoproterozoic, probably related to the Brasiliano Orogeny (670–490 Ma). Previous studies have demonstrated that the main hydrothermal event is Paleoproterozoic. Phlogopite Ar-Ar dating from the Caraíba Mine yields plateau-ages around 2000–2100 Ma. A cluster of δ34S values approaching zero is related to interstitial sulfide minerals in the least-altered ultramafic rocks, while values above and below zero are related to remobilized sulfides. These data support the hypothesis of the existence of a primary (magmatic) mineralizing event, alleged to have precipitated disseminated, interstitial sulfide minerals, and a second (hydrothermal) event, responsible for the economically more important mineralization, which is structurally controlled and related to significant potassic alteration. The sulfide minerals are associated with different generations of magmatic and hydrothermal magnetite, as well as distal plagioclase and microcline alteration halos. The combination of these features reaffirms the concept that the Curaçá Valley deposits may be part of the same alteration system, as described in IOCG deposits, with the deepest deposits being located further south. Based on the data obtained during the present study, it is possible to establish a tectono-metallogenic evolutionary model to explain the genesis of the copper mineralization in the Curaçá Valley. The mafic-ultramafic bodies were emplaced as multiple intrusions in Neoarchean arc-related gneisses and chemical sediments, along with bornite and chalcopyrite mineralization. The sulfide minerals that preserve the characteristics of this stage are chalcopyrite or bornite, both with δ34S values around 0‰V-CDT.A continental collision affected the Caraíba arc and the related sequences between 2100 and 2080 Ma. During this Rhyacian-Orosirian orogeny, deformation and amphibolite-granulite facies metamorphism, the rocks underwent partial melting, and sapphirine formation developed. Between 2050 and 2020 Ma, the main mineralization stage took place at Caraíba, Vermelhos, and Surubim, which corresponds to an earlier stage in the Sussuarana deposits. The southern deposits were formed deeper than those to the north, resulting in less extensive potassic alteration and related hydrothermal mineralization. The sulfide minerals of the deeper deposits have negative δ34S (between 0‰ and −6.81‰), to slightly positive values, where the igneous protoliths are best preserved. This alteration took place near the brittle-ductile boundary, mainly associated with ductile structures.The latest stage of mineralization took place between 1950 and 1920 Ma, in the Caraíba, Vermelhos and Surubim deposits, along with major sulfide remobilization in Sussuarana. This stage was characterized by the participation of oxidizing fluids, possibly meteoric waters, which raised the δ34S values up to +11.77‰.

P-T evolution of metasedimentary rocks of the Santa Filomena Complex, Riacho do Pontal Orogen, Borborema Province (NE Brazil): Geothermobarometry and metamorphic modelling

We present in this paper petrologic data and discuss the pressure-temperature (P-T) metamorphic history of the neoproterozoic metasedimentary rocks of the Santa Filomena Complex, Riacho do Pontal Orogen, which is inserted in the southern portion of the Borborema Province (Northeast Brazil). Therefore, the data provide constraints on metamorphic evolution during Neoproterozoic Brasiliano Orogeny in Northeast Brazil. The rocks studied are aluminous schists and paragneisses. Silver-gray and red pelitic schists are intensely deformed, biotite-muscovite rich, contain centimeter-sized garnet, staurolite and kyanite porphyroblasts, and subordinately plagioclase and quartz. Paragneisses are from light gray to dark gray colored, medium to coarse-grained and display a well-spaced foliated matrix of biotite, and kyanite and garnet porphyroblasts. Locally, the schists and paragneisses are migmatized. Pressure-temperature modelling based on thermobarometric calculations indicate that metamorphism reached 643 °C with pressures estimated in 12 kbar. Pre-peak and post-peak metamorphic conditions are constrained by mineralogical and textural relationships: garnet inclusion-rich and inclusion-free (possible of higher T) are documented and the inclusion-rich core probably indicates a Sn-1 foliation that was transposed by Sn. The pre-peak stage most probably occurred close to 500 °C and 8 kbar, in upper greenschist to lower amphibolite facies metamorphism along kyanite stability field. We also propose that post-peak stage was associated with isothermal decompression along a possible path of tectonic exhumation in conditions of 600 °C and 7 kbar. To further evaluate the equilibrium condition, pressure-temperature pseudosections were calculated for the metasedimentary rocks. Thus, the estimated metamorphic peak took place in the upper amphibolite facies. A suggested clockwise pressure-temperature path is compatible with the regional tectonic setting of continent-continent collision which occurred in the Late Neoproterozoic of Borborema Province, during the Brasiliano Orogeny.

Fluid evolution in the southern part of the Proterozoic Vazante Group, Brazil: Implications for exploration of sedimentary-hosted base metal deposits

The Proterozoic Vazante Group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical, fluid inclusion and isotopic studies of the Upper Morro do Pinheiro and Lower Pamplona members from the Serra do Poço Verde Formation in the southern part of the paleo-basin. These units are the main hosts for the hypogene, structurally-controlled zinc silicate deposits in the Vazante Zinc District, including the Vazante mine, which is considered to be the largest willemitic (Zn2SiO4) deposit in the world, with estimated total resources of 40–60Mt at 20% Zn. Five hydrothermal alteration types were identified in the Southern Extension of the Vazante Group. (I) Early stage alteration comprises dolomite substitution bands and nodules, associated with moderate salinity H2O-NaCl-CaCl2 fluids, with temperatures around 90°C, interpreted as late-diagenetic. The calculated C and O isotopic compositions of the fluids suggest meteoric and/or connate origin and interaction with organic carbon. (II) Pre-ore stage alteration is evidenced by dog-tooth dolomite and quartz with minor Fe-oxi/hydroxides which fills dissolution voids formed by H2O-NaCl-CaCl2±MgCl2 fluids at temperatures around 100° to 150°C. The C isotopic data also indicate interaction with organic carbon. The two first stages are poor in ore-related elements. (III) Ore stage encompasses four phases of mineral precipitation. The first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite. The second is the main phase of the ore stage, composed of massive red dolomite, massive hematite and willemite with enrichment mainly in Fe2O3, SiO2, Ag, As, Cd, Cu, Ni, Pb, Se and Zn. The third phase comprises white dolomite, hematite and traces of willemite with enrichment in MnO, Cd, Ni and Pb. The calculated C and O isotopic compositions of the fluids (at 180°C) associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock. The fourth phase corresponds to Zn-chlorite and quartz which are associated with H2O-NaCl fluids of variable salinities and distinct temperatures (90–140°C and 170–190°C). (IV) Pyrite-bearing vein characterized by pyrite, sphalerite, white dolomite, fine hematite and late quartz, with C and O isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage. These data suggest a co-genetic relationship of these two alteration styles. (V) The late sulfide stage characterized by rare galena-bearing stringers with sphalerite, chalcocite, greenockite, covellite and white dolomite that cross-cut the main ore stage phases. The ore-related fluid compositions associated with the silicate zinc mineralization in the southern part of the Vazante Group are also similar to the fluids reported in previous work for the sulfide zinc-lead deposits in the northern part, indicating favorable conditions for metal transport during the Brasiliano Orogeny. Sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water. This finding implies that carbonate sequences in other districts with sulfide lead-zinc deposits may also host hypogene silicate zinc deposits.

Two-stage terrane assembly in Western Gondwana: Insights from structural geology and geophysical data of central Borborema Province, NE Brazil

Combined geophysical and structural data from the Transversal Subprovince of the Borborema Province (NE Brazil) highlight the internal structure of, and interrelationships between, the constituent terranes. Radiometric and magnetic maps show distinctive signatures for the Archean-Paleoproterozoic Alto Moxotó and Meso-to Neoproterozoic Alto Pajeú and Pernambuco-Alagoas terranes. Mapped radiometric and first and second order magnetic lineaments, associated with Euler deconvolution, enable correlation between geophysical data and major structures. In addition to early related deformation of the Alto Moxotó Terrane, combined analysis of late transposition foliations, lineations and kinematic criteria reveal a complex structural evolution marked by two distinct assembly stages. The first phase is characterized by thrust tectonics with top-to-the-south vergence, resulting in the juxtaposition of the allochtonous Alto Pajeú Terrane with the structurally underlying Alto Moxotó Terrane. The terrane boundary is delineated by the Serra de Jabitacá Shear Zone, which is associated with low dipping collisional granitic sheets and ca. 1.0 Ga mafic-ultramafic rocks interpreted as obtudcted ophiolite remnants. Later strike-slip movements strongly folded and obliterated thrust-related markers, and the continental scale E-W Pernambuco Lineament is interpreted as the result of lateral assembly between the composite Alto Pajeú-Alto Moxotó terranes and the Pernambuco-Alagoas Terrane during the metamorphic peak of the Brasiliano orogeny. Evolution of the Borborema Province reflects accretionary processes during the assembly of Western Gondwana.

Proterozoic evolution of part of the Embu Complex, eastern São Paulo state, SE Brazil

This paper presents detrital zircon ages obtained in rocks of the Embu Complex, southeastern São Paulo State, Brazil. The Embu Complex encompasses a Paleoproterozoic basement represented by migmatitic hornblende and biotite orthogneisses covered by (kyanite)-(sillimanite)-(garnet) bearing biotite-muscovite schists and paragneisses with decametric intercalations of quartzites and calcsilicate rocks. In the studied area this metasedimentary sequence is intruded by the porphyritic Serra do Quebra Cangalhagranite. Through field and microstructural studies, four ductile deformational phases wereidentified. Metamorphic events related to the Brasiliano Orogeny that affected the studied rocks were dominantly under medium temperature and pressure conditions, from greenschist to middle amphibolite facies. Detrital zircon crystals from a ∼10 m thick quartzite layer were analyzed by LA-ICP-MS resulting in a wide range of ages between 2100 and 600 Ma that fall mainly in four groups: the first group between 2.1 and 1.6 Ga, with apex at 1.7 Ga; two less expressive Mesoproterozoic groups with values between 1.6 and 1.2 Ga; and a fourth group with values between 1.2 and 0.6 Ga. Considering the geochronological data, the sedimentation of the basin began after 852 ± 40 Ma (the youngest igneous grain) and finished before ∼786 Ma (metamorphic rim). The age of the intrusive Serra do Quebra Cangalha granite (∼680 Ma) is consistent with this minimum age. The opening of the basin could be related to the break-up of Rodinia, which resulted in several small continents, among them the Paranapanema and São Francisco paleocontinents. Comparing these data with similar provenance data from the Apiaí terrane, itseems probable that the Embu Complex was physically connected with it during most of their evolution.

Stratigraphy, tectonics and detrital zircon U-Pb (LA-ICP-MS) geochronology of the Rio Preto Belt and northern Paramirim corridor, NE, Brazil

ABSTRACT: Two important Proterozoic metasedimentary sequences, the Rio Preto and Santo Onofre Groups, crop out along the northwestern margin of the São Francisco craton and in northern Paramirim corridor, respectively. The Rio Preto Group, involved in the eponymous fold-thrust belt along the northwestern cratonic boundary, comprises the Formosa (garnet schist, quartz-mica schist, quartzite, chlorite-sericite schist and ferriferous quartz schist) and Canabravinha (quartzite, micaceous quartzite, metarhytmite, phylite, schist and metaturbidite) formations. The Santo Onofre Group occurs exclusively in the Paramirim corridor, and is composed of quartzite and minor carbonaceous or Mn-rich phylite. These units record sedimentation in shallow to deep-water marine settings related to rift basins, and were deformed and metamorphosed under greenschist facies conditions during the Brasiliano orogeny. Here we present 427 new detrital zircon U-Pb ages, which constrain the maximum depositional ages of ca. 971 Ma for the Santo Onofre Group, ca. 912 Ma for the Canabravinha Formation, and ca. 965 Ma for the Formosa Formation of the Rio Preto Group. Our data suggests that the Santo Onofre and the Rio Preto Groups accumulated in two distinct basin settings. The latter, composed mostly of sandy rocks, would represent a relatively stable, shallow-marine shelf environment. The Rio Preto Group, with metadiamictite, quartzite, pelitic and rhythmitic rocks, represents a shallow to deep marine environment influenced by gravity flows. Both groups were probably deposited in the Late Tonian, and are potential correlatives of the lower (pre-glacial) units of the Macaúbas Group of the Araçuaí belt.

Granulite-facies metamorphism at ca. 570-580 Ma in the Porangatu Granulite Complex, central Brazil: implications for the evolution of the Transbrasiliano Lineament

The Porangatu Granulite Complex is exposed in the central part of the Neoproterozoic Tocantins province in central Brazil, along the boundary between the Brasilia Belt to the east and the Araguaia Belt to the west. This is part of the transcontinental Transbrasiliano-Kandi shear system. The complex includes garnet-rich enderbite and charnockite, high-grade gneisses as well as lenses of garnet-bearing mafic granulite or amphibolites, and in situ anatectic charnockite, elongated in the NNE-SSW direction along the Talisma Shear Zone (TSZ). These rocks represent suites of ortho-derived rocks of calc-alkaline affinity and small contributions of tholeiitic basalts and aluminous paragneisses. The structural framework records thrust components probably related to the early stages of an oblique collision during the evolution of Neoproterozoic Brasiliano orogens, and can be understood as involving a collisional system of two crustal blocks, initially with thrust components which in its final stage evolved to a transcurrent system with dextral movement. This led to intense imbrication, generation of mylonitic foliation, stretching lineation, tectonic banding and rotation of structures and minerals. The heterogeneous and progressive ductile deformation was accompanied by metamorphic re-equilibrium in late Neoproterozoic time. Granulite facies conditions reached a metamorphic maximum at temperature and pressure above 850°C and 10 kbar, in an almost anhydrous environment, with or without anatexis. Zircon U-Pb SHRIMP analyses for two selected rock samples indicated the combined age of 580 ± 7 Ma for a charnockite and 548 ± 48 Ma for a mafic granulite from which the charnockite is throught to have been derived. The mafic granulite contains zircon grains of ca. 2.1 Ga, indicating Paleoproterozoic igneous protoliths involved in Neoproterozoic high-grade metamorphism. In addition, older inherited zircon grains of ca. 3.1 and 2.0 Ga ( 207 Pb/ 206 Pb ages) in charnockite also confirm the existence of older Archaean and Paleoproterozoic material in this region, possibly derived from the Goias Massif. A 0.88 Ga inherited zircon grain is suggestive of derivation from the Goias Magmatic Arc. This Neoproterozoic age for the high-grade metamorphism is substantially younger than those reported for other granulites in the Brasilia Belt (ca. 0.65 Ga), suggesting that the Porangatu Granulite Complex is more probably associated with the evolution of the younger Araguaia Belt. The new field, structural, petrographic and geochronological data suggest that the Porangatu Granulite Complex was involved in a high-temperature ductile strike-slip shear zone juxtaposing terrains of different ages (Archaean, Paleoproterozoic, Neoproterozoic), crustal nature and level (lower and middle continental crust), strongly reworked during the final stages of the Brasiliano orogeny, and represents the exposed roots of the Tocantins orogen.

Phanerozoic low-temperature evolution of the Uruguayan Shield along the South American passive margin

The crystalline basement of Uruguay was assembled during the Brasiliano Orogeny in the Neoproterozoic Era and was later affected by discrete tectonic activity. A new multi-method low-temperature dataset including (U–Th)/He ages from both zircon and apatite, T – t modelling and K–Ar dating of fine sericite fractions and fault gouge reveal a detailed post-orogenic geological history spanning the Phanerozoic Eon. The juxtaposition of the terranes that compose the area was achieved in the Ediacaran Period, and post-collision was marked by intense exhumation, in which the crystalline basement reached near-surface conditions by the early to mid-Palaeozoic. Regional subsidence promoted sedimentation in the Paraná Basin until the Permian, covering and reheating much of the basement that is at present exposed. Afterwards, deposition and volcanism were mostly confined to its current limits. Regional exhumation of the shield during the Permo-Triassic exposed much of the northern portion of the basement, and the south was further affected by the opening of the South Atlantic Ocean during the Mesozoic. Little exhumation affected the Uruguayan Shield during the Cenozoic, as reflected in its modest topography. The reactivation of inherited Neoproterozoic structures influenced the development of Mesozoic basins and the present-day landscape. Supplementary material: Supplementary data (sample locations, (U-Th)/He data and K-Ar data) are available at https://doi.org/10.6084/m9.figshare.c.3702043

Hydrothermal karst and associated breccias in Neoproterozoic limestone from the Barker-Villa Cacique area (Tandilia belt), Argentina

In the Barker-Villa Cacique area (Tandilia belt), remarkable megabreccias, limestone breccias and phosphate-bearing breccias hosted in black limestone and along the contact with the upper section of the sedimentary succession are exposed. These rocks are the result of extensive hydrothermal alteration of the original micritic limestone and other fine-grained clastic sediments. Typical alteration minerals are sericite, chlorite, interstratified chlorite/K-white mica, kaolinite, dickite, pyrite, chalcopyrite, goethite, quartz, calcite, Fe-calcite, dolomite, ankerite, fluor-apatite, barite and aluminium-phosphate-sulfate (APS) minerals. Quartz and calcite cements from hydraulic breccias in the limestone contain low-salinity aqueous fluid inclusions. Corresponding homogenization temperatures display 200-220 °C and 110–140 °C in hydrothermal quartz, and 130-150 °C in late calcite cement.Carbon and oxygen stable isotope analyses of carbonates from the Loma Negra quarry (LNQ) support the major role of hydrothermal activity. A significant difference was found between δ18Ocar values from unaltered micritic limestone (ca. 23.8‰ SMOW) and secondary calcite (ca. 18.5‰ SMOW). The lower δ18Ocar values are interpreted as a result of calcite precipitation from hot hydrothermal fluids. At a late stage, the hydrothermal fluid containing H2S mixed with descending and oxidizing meteoric waters. Circulation of the ensuing acid fluids resulted in the partly dissolution and collapse brecciation of the Loma Negra Formation. The hydrothermal stage can be tentatively dated ca. 590–620 Ma corresponding to the Brasiliano orogeny.