Rio Itapicuru Greenstone Belt Research Articles

Emplacement of the paleoproterozoic Itareru tonalite (NE São Francisco Craton, Brazil) during arc-continent collision revealed by magnetic fabrics

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.

The Serrinha granite-greenstone terrane revisited: A reevaluation of its evolution based on the current U–Pb age database

This paper aims to summarize the current knowledge of the Serrinha Nucleus and add new U–Pb geochronological data for this area. New ages were obtained for granitoids and a metadacite belonging to the Felsic Volcanic Unit of the Rio Itapicuru Greenstone Belt (RIGB). The age of 3185 ± 9 Ma obtained at the Pedra Alta granodiorite-tonalite pluton corresponds to the interval of emplacement (2.9–3.2 Ga) of the TTG granites in the Serrinha Nucleus during the Archean. As expected for granites belonging to the Paleoproterozoic TTG/calc-alkaline group, Barrocas and Lagoa dos Bois plutons yielded U–Pb zircon ages of 2168 ± 12 Ma and 2158 ± 13 Ma, respectively. These 2.15–2.17 Ga ages are attributed to a syn-orogenic evolution of the calc-alkaline felsic volcanism of the RIGB during the Rhyacian period, which is related to the closure of the Itapicuru basin. A sample of a biotite-granite dyke collected at the Araci TTG Pluton, which also is part of the Archean TTGs group of granites, yields an age related to the post-orogenic Morro do Lopes magmatism. Its age of 2076 ± 6 Ma also coincides with those attributed to the hydrothermal event that resulted in the gold mineralization at the RIGB and reinforces the widespread efects of this thermal post-orogenic alkaline magmatic event. The metadacite age of 2147 ± 38 Ma is considered the minimum crystallization age for the felsic volcanic arc of the RIGB, being compatible with regional ages obtained for other metavolcanic rocks and granitoids from this area. The ages obtained in the present work, added to the geochronological database of Serrinha Nucleus, show that the closure of the Itapicuru basin occurred between approximately 2.15 and 2.17 Ga, period corresponding to the first deformation phase that affected the orogen. The age obtained for the intrusive Barrocas Pluton (2168 ± 12 Ma), which outcrops at the southern tip of the greenstone, suggests that the volcanic-sedimentary rocks of Itapicuru should be older and highlights a new model for the evolution of Serrinha Nucleus and the concept of the “Rio Itapicuru Greenstone Belt”.

Northeast São Francisco Craton and West-Congo Craton linked before the Rhyacian (2.10–2.04 Ga) orogeny: Evidences from provenance and U-Pb ages of supracrustal rocks from the Rio Capim greenstone belt, Serrinha Block

The period between the Rhyacian and Orosirian is known for the development of many orogenic belts and supracrustal sequences, which can be found in different cratons worldwide. In the northern part of the São Francisco Craton, there occur several Rhyacian-Orosirian sedimentary sequences associated with the orogenies that amalgamated the precursors of the São Francisco and West-Congo Cratons. However, little is known about the time and tectonic setting of sediment deposition. In this study we present U-Pb detrital zircon ages and provenance studies of sedimentary successions from the Paleoproterozoic Rio Capim greenstone belt, Serrinha Block, São Francisco Craton, Brazil. Detrital zircon data reveal a main sedimentary source of ages between 2.20 and 2.12 Ga, and indicate protoliths deposition between 2106 and 2086 Ma in a pro-foreland type basin. The main age interval matches the arc-related magmatism of the Rio Capim and Rio Itapicuru greenstone belts. Additionally, an age interval of 2.35–2.25 Ga indicates a sedimentary contribution of unknown source, while ages between 2.8 and 3.2 Ga suggest the Archean basement source. Field features, U-Pb ages and major and trace elements data support the interpretation that the Rio Capim and Rio Itapicuru greenstone belt both could make up an arc complex. Lithostratigraphic and tectonic matches allow to suggesting that the Serrinha Block may have an African counterpart, namely the Ntem-Chaillu Block, West-Congo Craton.

Metal accumulation in soils derived from volcano-sedimentary rocks, Rio Itapicuru Greenstone Belt, northeastern Brazil

Many countries and some Brazilian regions have defined the guideline values for metals in soils. However, the local geological features may be so heterogeneous that global or even regional guideline values cannot be applied. The Greenstone Belts are worldwide geological formations of vast extension, containing mineralization of various metals (e.g., Au, Cr, Ni, and Ag). Natural concentrations of soils must be known to correctly assess the impact of mining. We studied the soils of the Rio Itapicuru Greenstone Belt (RIGB), of Paleoproterozoic age, sampling at 24 sites (0–0.20m) in the areas not or minimally human impacted, equally distributed in the three units of the RIGB: Volcanic Mafic Unit (VMU), Volcanic Felsic Unit (VFU), and Volcano-clastic Sedimentary Unit (SU). The natural pseudo-total concentrations of Cr, Ni, Cu, Zn, Pb, Fe, and Mn were obtained by acid digestion (EPA3050b) both in the soil and the particle-size fractions (sand and clay+silt). The concentrations of metals in RIGB soils, especially Cr and Ni, are generally higher than those reported for other regions of Brazil or other countries. Even the sedimentary soils have relatively high metal values, naturally contaminated by the VMU of the RIGB; a potential impact on Mesozoic and Cenozoic sedimentary rocks located near the study region is highly expected. Metals are concentrated (80%) in the fine particle-size fraction, implying an easy availability through surface transport (wind and runoff). We introduced a new index, called the Fe-independent accumulation factor – AF-Fe, which reveals that 90–98% of the dynamics of the trace metals is associated with the iron geochemical cycle. We primarily conclude that determining the guideline values for different soil classes in variable geological/geochemical environment and under semiarid climate is meaningless: the concentration of metals in soils is clearly more related to the source material than to the pedogenesis processes.

Petrology of Teofilândia granitoids: An example of 2.1 Ga crustal accretion in the São Francisco Craton (Bahia, Brazil)

Teofilândia granitoids are representative of the Paleoproterozoic plutonic rocks, which intruded the Serrinha block, an Archean crustal fragment of the Sao Francisco Craton (Bahia, Brazil). Three plutons were emplaced, the Teofilândia granodiorite, the Barrocas trondhjemite and the Santa Rosa granite, respectively dated at 2130, 2127 and 2073 Ma. The two first plutons are calc-alkaline rocks following a trondhjemitic trend. They resemble Archean TTGs (tonalites-trondhjemites-granodiorites) by their major and trace element compositions and especially by their fractionated REE patterns, with very low HREE contents. These juvenile magmas resulted from partial melting of a young mafic protolith, likely represented by the nearby Rio Itapicuru greenstone belt. Barrocas trondhjemite and Teofilândia granodiorite derive from similar sources, possibly at different depths and with a different degree of melting. The rocks were deformed at high temperature during the Trans-Amazonian collision and are therefore pre-collisional and ascribed to a subduction stage. The younger Santa Rosa pluton is a small, syn-to post-collisional granite that derived from anatexis of the Archean crust. It is representative of a second, volumetrically minor, plutonic episode of potassic, shoshonitic or alkaline affinities. The large amount of 2.1 Ga granitoids emplaced in Brazil as well as in the West African craton, suggests that, at that time, a global event of possible mantle origin was responsible for the intense magmatic activity that involved both crustal accretion and crustal reworking in many places of the world.

Anatomy of the Alto Alegre gneiss dome, São Francisco Craton, Brazil: A geological record of transpression along a Palaeoproterozoic arc-continent collision zone

Gneiss domes are crustal-scale structures that can be formed in different tectonic settings. In recent collisional orogens they occur along terrane boundaries either within, or on the edge of deformation belts. The Alto Alegre gneiss dome was recognized in the northern São Francisco Craton along the limit of Archaean-Palaeoproterozoic terranes. It consists of a large N-S-elongated elliptic structure that is composed of migmatite-gneisses and concentric, metre-sized amphibolite sheets. In its central zone, the structure is intruded by irregular pink granite stocks. Geochemical data show that the amphibolites are similar in composition to mafic dykes that crosscut the Archaean Santa Luz high-grade Complex (paleocontinent) and unlike basalts of the Palaeoproterozoic Rio Itapicuru greenstone belt (arc). SHRIMP U–Pb zircon geochronology reveals ages of 3012±25Ma and 3073±51Ma for gneisses and 2082±5Ma for pink granite within the Alto Alegre dome, and 3061±44Ma for Santa Luz gneiss. Field, structural, geochemical, and geochronological data support a model in which the Alto Alegre dome is part of the Santa Luz gneiss complex. The dome was formed by large-scale folding and transpression during the late stage of Palaeoproterozoic arc-continent collision.

Provenance of zircon xenocrysts in the Neoproterozoic Brauna Kimberlite Field, São Francisco Craton, Brazil: Evidence for a thick Palaeoproterozoic lithosphere beneath the Serrinha block

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.

Sediment provenance in the Palaeoproterozoic Rio Itapicuru greenstone belt, Brazil, indicates deposition on arc settings with a hidden 2.17–2.25Ga substrate

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, Sao 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 (TDM), most in the range 2.0–2.2 Ga, and the positive ɛNd values for the proposed deposition time (ca. 2115 Ma) 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 2125–2237 Ma, with age populations clustering at 2164 ± 4 Ma and 2209 ± 4 Ma, and minor outliers at ca. 2112 Ma, 2270 Ma, and 2415 Ma. 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 2.17 Ga and 2.24 Ga 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.

The Fazenda Gavião granodiorite and associated potassic plutons as evidence for Palaeoproterozoic arc-continent collision in the Rio Itapicuru greenstone belt, Brazil

Abstract Several granitic plutons have intruded the Palaeoproterozoic Rio Itapicuru greenstone belt, Sao Francisco craton, Brazil, in the time interval 2163-2080 Ma, but their tectonic significance is poorly understood. The Fazenda Gaviao 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 eNd(t) of FGG and coeval K-rich plutons of the Rio Itapicuru greenstone belt contrasts markedly with the positive eNd(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.

Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil, India, and Uruguay

Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ 13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ 13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ 13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.

Caracterização de vulcânicas adakíticas e cálcio-alcalinas no greenstone belt do rio Itapicuru, Bahia: petrogênese e implicações geodinâmicas

New geochemical data for andesites and dacites of the Maria Preta Unit, Rio Itapicuru greenstone belt, revealed two distinct geochemical groups: one of adakitic affinity and another calc-alkaline. The adakitic suite, localized to the west in the unit, is characterized by high values of Sr/Y, Ti/Y N >;1; it shows enrichment in LFSE (Low Field Strength Elements) relative to HFSE (High Field Strength Elements) and moderate rare earth elements fractionation. The calc-alkaline suite shows low Sr/Y ratios, lower rare earth elements fractionation and negative Eu anomalies. It also shows high abundances of incompatible elements (Y, Zr, Th) and high Cr and Ni concentrations. The most likely mechanism of formation for the calc-alkaline rocks is partial melting of mantle peridotite hydrated by fluids released from the subducting slab, followed by plagioclase fractionation. The two suites cannot be linked to each other by fractional crystallization and have probably derived from distinct magmas (different initial Nd ratios). The adakite geochemical features require partial melt of hydrous metabasalts at depths high enough to stabilize garnet+hornblende as the residual phase. This suite does not show any significant interaction with the mantle wedge as inferred from its low Cr and Ni abundances when compared to Cenozoic adakites. Sm-Nd isotope data indicated high positive εNd(T) for the adakites (+4.8 to +2.5) and slightly lower values for the calc-alkaline suite (εNd(T) +1.9 to +1.4). These data indicate a juvenile magma input to form the UMP volcanics. If the two volcanic suites are of the same age, then their geographical position suggests subduction of the oceanic slab to the west.

Mesoarchaean to Palaeoproterozoic growth of the northern segment of the Itabuna–Salvador–Curaçá orogen, São Francisco craton, Brazil

AbstractThe geology of the northern segment of the Itabuna–Salvador–Curaçá orogen, São Francisco craton, is reviewed, and new U–Pb ages, and Nd isotope and major and trace element data are combined to improve understanding of its tectonic evolution. The results indicate that oceanic crust and island arc sequences accreted at about 3.30 Ga to form the Mundo Novo greenstone belt, and between 2.15 and 2.12 Ga to form the Rio Itapicuru and Rio Capim greenstone belts. At about 3.08–2.98 Ga, mafic crust underwent partial melting to form the Retirolândia and Jacurici tonalite–trondhjemite–granodiorite belts of the Serrinha block. From 2.69 to 2.58 Ga an Andean-type arc with ocean crust remnants formed the Caraiba complex possibly at the Gavião block margin. Between 2.11 and 2.105 Ga, the Rio Itapicuru arc collided with the Retirolândia–Jacurici microcontinent, possibly involving slab breakoff. Oblique convergence between 2.09 and 2.07 Ga led to collision of the Serrinha microcontinent with the Caraíba–Gavião superblock and reworked the Caraíba arc to granulites, locally at ultrahigh-temperature conditions. At the same time, arc dacites spread over the Rio Itapicuru greenstone belt, and the 3.12–3.0 Ga Uauá terrane, crosscut by 2.58 Ga mafic dykes, extruded from south to north, possibly together with the 2.15 Ga Rio Capim greenstone belt.

The Santa Luz chromite-peridotite and associated mafic dykes, Bahia-Brazil: remnants of a transitional-type ophiolite related to the Paleoproterozoic (>2.1 Ga) Rio Itapicuru greenstone belt?

The Santa Luz chromite-rich peridotite complex crops out in the Serrinha Block (NE Sao Francisco craton), approximately between grey gneisses of the Archaean basement and supracrustal rocks of the Palaeoproterozoic Rio Itapicuru greenstone belt. It comprises serpentinite, serpentinised harzburgite, massive chromitites, gabbroic pockets and amphibolite dykes/veins with epidote-rich margins, all intruded by pegmatite and aplite dykes. The mafic-ultramafic rock association is interpreted as basaltic melt percolation through mantle peridotites. Massive chromitites show major element chemistry and platinum group elements signature similar to ophiolite chromitites. The complex is older than 2085 ± 12 Ma and younger than 2983 ± 8 Ma based on U-Pb ages obtained respectively on aplite dyke and host banded gneiss. The Santa Luz peridotite complex and associated mafic dykes may be related to rifting of a magma-poor continental margin that later evolved into an ocean, the closing of which eventually formed the Palaeoproterozoic Rio Itapicuru greenstone belt.

The Santa Luz chromite-peridotite and associated mafic dykes, Bahia-Brazil: remnants of a transitional-type ophiolite related to the Paleoproterozoic (>2.1 Ga) Rio Itapicuru greenstone belt?

The Santa Luz chromite-rich peridotite complex crops out in the Serrinha Block (NE São Francisco craton), approximately between grey gneisses of the Archaean basement and supracrustal rocks of the Palaeoproterozoic Rio Itapicuru greenstone belt. It comprises serpentinite, serpentinised harzburgite, massive chromitites, gabbroic pockets and amphibolite dykes/veins with epidote-rich margins, all intruded by pegmatite and aplite dykes. The mafic-ultramafic rock association is interpreted as basaltic melt percolation through mantle peridotites. Massive chromitites show major element chemistry and platinum group elements signature similar to ophiolite chromitites. The complex is older than 2085 ± 12 Ma and younger than 2983 ± 8 Ma based on U-Pb ages obtained respectively on aplite dyke and host banded gneiss. The Santa Luz peridotite complex and associated mafic dykes may be related to rifting of a magma-poor continental margin that later evolved into an ocean, the closing of which eventually formed the Palaeoproterozoic Rio Itapicuru greenstone belt.

Sulfur Isotope Characteristics of Mesothermal Gold Deposits: Results of an Investigation of the Rio Itapicuru Region, Bahia, Brazil

Stable isotope analyses were carried out on sulfides from three mesothermal gold deposits of the Rio Itapicuru greenstone belt (Fazenda Brasileiro, Maria Preta, and Ambrosio mines) to constrain the source of the hydrothermal solutions. The ores are hosted in a volcanic-sedimentary sequence that evolved in an Archean cratonic area reworked during the Transamazonic event (ca. 2.1 Ga). The ore bodies were deposited by hydrothermal solutions in polycyclic shear zones generated by Paleoproterozoic dynamothermal metamorphism. In the Fazenda Brasileiro deposit, δ34S values for arsenopyrite from mineralized quartz veins range from 5.5‰ to +1.0‰; pyrites from Au-rich mineralized veins from the Maria Preta deposit yielded δ34S values ranging from +8.9‰ to +6.9‰, in agreement with published δ34S data (+0.1‰ to +6.6‰). A narrow spread in the δ34S values for Au-rich sulfides from both deposits indicates that gold deposition occurred under conditions involving a small range of fO2 and fH2O. A barren, galena-rich vein from the Ambrosio deposit yielded δ34S values in the -7.6‰ to -9.9‰ range, probably reflecting mixing of different sources. These results, compared to stable sulfur isotope modeling of lode gold deposits, indicate that the studied deposits formed from hydrothermal solutions of metamorphic origin followed by duluite solutions of magmatic origin.

Age constraints on felsic intrusions, metamorphism and gold mineralisation in the Palaeoproterozoic Rio Itapicuru greenstone belt, NE Bahia State, Brazil

U–Pb sensitive high resolution ion microprobe mass spectrometer (SHRIMP) ages of zircon, monazite and xenotime crystals from felsic intrusive rocks from the Rio Itapicuru greenstone belt show two development stages between 2,152 and 2,130 Ma, and between 2,130 and 2,080 Ma. The older intrusions yielded ages of 2,152±6 Ma in monazite crystals and 2,155±9 Ma in zircon crystals derived from the Trilhado granodiorite, and ages of 2,130±7 Ma and 2,128±8 Ma in zircon crystals derived from the Teofilândia tonalite. The emplacement age of the syntectonic Ambrosio dome as indicated by a 2,080±2-Ma xenotime age for a granite dyke probably marks the end of the felsic magmatism. This age shows good agreement with the Ar–Ar plateau age of 2,080±5 Ma obtained in hornblendes from an amphibolite and with a U–Pb SHRIMP age of 2,076±10 Ma in detrital zircon crystals from a quartzite, interpreted as the age of the peak of the metamorphism. The predominance of inherited zircons in the syntectonic Ambrosio dome suggests that the basement of the supracrustal rocks was composed of Archaean continental crust with components of 2,937±16, 3,111±13 and 3,162±13 Ma. Ar–Ar plateau ages of 2,050±4 Ma and 2,054±2 Ma on hydrothermal muscovite samples from the Fazenda Brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition. The Ar–Ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism, or episodically between 2,080 Ma and 2,050 Ma, during uplift and exhumation of the orogen.

Química mineral e condições de colocação do batólito trondhjemítico nordestina, Núcleo Serrinha, Bahia

The Nordestina Trondhjemitic Batholith (NTB; 720 km 2 ) is a ellipsoidal intrusion, exposed in the central part of Serrinha Nucleus, an Archaean-Paleoproterozoic segment of the Sao Francisco Craton at the eastern of the Bahia State. It intrudes the gneissic-migmatitic Archaean- Paleoproterozoic basement along its western margin and the Paleoproterozoic volcano-sedimentary sequences of the Rio Itapicuru Greenstone Belt along the southern and eastern contacts. Its north-south elongated shape, associated with concentric internal foliation, which becomes more intense toward the border, and crystallization age of 2.15 Ga, are interpreted as result of syn-tectonic emplacement during the compressional Transamazonic event and concomitant to the closure of the Rio Itapicuru Basin (2.2-2.1 Ga). The NTB is composed of zoned oligoclase (An 17 -An 26 ), quartz, calcic amphibole (Fe-hornblende and Mg-hornblende), biotite (Fe-rich), microcline, epidote (pistacite Ps 23 mole %), titanite, zircon and apatite. Iron oxides are rare to absent and the epidote occurs as euedral to subhedral magmatic grains included in biotite. Magnetic susceptibility, electron microprobe analyses of Al content in hornblende (aluminumin- hornblende barometry), coexisting hornblende-plagioclase (hornblende-plagioclase thermometry) and the presence of magmatic epidote were used to constrain the oxygen fugacity ( f O 2 ), P and T during the crystallization of the magma. The NTB crystallized from low f O 2 magmas (between NNO and FMQ buffers), at ~ 5kbar pressure under narrow range of temperature (651-673°C).

Evolução do segmento norte do orógeno Itabuna-Salvador-Curaçá: cronologia da acresção de arcos, colisão continental e escape de terrenos

U-Pb and Pb-evaporation zircon ages, together with whole-rock Sm-Nd and Pb-Pb data, are used to unravel the tectonic evolution of the northern segment of the Itabuna-Salvador-Curaca Orogen, northeastern Sao Francisco Craton. The results of this geochronologic study indicate that oceanic crust and island arc sequences accreted at 3.3 Ga, forming the Mundo Novo Greenstone Belt, and that between 2.2 - 2.12 Ga the Rio Itapicuru and Rio Capim Greenstone Belts were formed in a similar fashion. Age data and Nd isotopes also suggest that Andean-type continental margins are likely to have formed in the span of time 3.08 - 2.98 Ga, giving rise to the TTG belts of Retirolândia and Jacurici river, and to the Caraiba Complex between 2.69 - 2.63 Ga. Around 2109 Ma, continent-continent collision closed the Rio Itapicuru volcano-sedimentary basin, as inferred from the syntectonic emplacement of the Itareru Tonalite between the Rio Itapicuru Greenstone Belt to the east and the banded gneisses of the Serrinha Block to the west. This collision culminated in a major shift from frontal or oblique convergence to orogen-parallel block-escape about 2084 Ma ago and continued at least till 2039 Ma. During this event several elongated granitic and syenitic bodies were emplaced in the continental crust. The Itiuba Syenite, dated at 2084 Ma and stretching over 150 km east of the Caraiba Complex, is an example of one of these intrusive bodies.

Sm-Nd age of the fazenda brasileiro gabbro, Bahia, Brazil: example of robust behavior of the Sm-Nd isotopic system under extreme hydrothermal alteration

The Fazenda Brasileiro gold mineralization is hosted by a gabbroic sill, intrusive into metavolcanicmetasedimentary rocks of the Rio Itapicuru Greenstone Belt, São Francisco Craton. The 2.05 Ga old mineralization is associated with intense shearing and hydrothermal alteration, and the host gabbro is altered to a series of rocks rich in sericite, chlorite, actinolite, carbonate and quartz. Twelve whole-rock samples of the gold mineralization, representing varied degrees of alteration, from rocks with preserved igneous textures to the ore (quartz-carbonate-sulfide-chlorite), were studied by the Sm-Nd method. All analytical points resulted in an isochron (MSWD = 1.9) indicating the age of 2142 +/- 47 Ma (1s) and Epsilon Nd (T) of +1.2. Chlorite-sericite-carbonate rich hydrothermal rocks indicate the age of 2148 +/- 57 Ma and Epsilon Nd (T) of +1.1. The positive Epsilon Nd (T) suggest limited or no contamination with older continental crust, compatible with an oceanic setting for the tholeiites. Combined withREEdata, the Sm-Nd isotopic results reveal that the hydrothermal alteration, although intense, was unable to alter significantly the Sm/Nd ratios of the original igneous rocks and did not cause important scatter of the analytical points, providing a rare example of robust behavior of the isotopic system, even under intense hydrothermal alteration.

Geologia, petrografia e litogeoquímica do batólito trondhjemítico nordestina, Núcleo Serrinha, nordeste da Bahia

The Nordestina Batholith (NB; 720 km 2 ) is a Paleoproterozoic (2.1 Ga) intrusion located in the central part of Serrinha Nucleus, Bahia, Brazil. It intrudes the gneissic-migmatitic Archaean basement and the volcano sedimentary sequences of the Rio Itapicuru Greenstone Belt. Its north-south elongated shape, associated with its concentric magmatic foliation, wich become more intense toward the border, is interpreted as a result of a syn-tectonic emplacement and correlated with the compressional Transamazonic event responsible for the closure of the Rio Itapicuru Basin. In this trondhjemitic batholith there are two predominant lithologies: (i) medium-grained phaneritic rocks; and (ii) porphyritic rocks, the latter limited to the central portion of the massif. Granitic dykes and dioritic enclaves are subordinate features of this batholith. The studied trondhjemitic rocks show a narrow range of SiO 2 contents (68-72 wt. %), low FeO, MgO and TiO 2 , high Al 2 O 3 (15-17 wt. %), fractionated REE patterns, weak and variable Eu anomalies, and La and Yb contents similar to those reported for the Archaean trondhjemites of East Finland. Major and trace element petrogenetic modelling for the NB allows explaining the porphyritic facies as a result of fractional crystallization of from magmas compositionally similar to the borders unit. Correlation with available data from Serrinha Nucleus bibliography indicates that this trondhjemitic magma is correlated with the calc-alkaline volcanism of the Rio Itapicuru Greenstone Belt, and the result of partial melting of a Paleoproterozoic subducted ocean slab.