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Unravelling the complexity of tonian-cryogenian (950–680 Ma) evolution of ophiolites requires the search for rare mineral systems and their quantification with varied techniques. Ophiolites in the Brasiliano Orogen are widely distributed over 2,000 km along the eastern half of South America. We selected two ophiolites from different geotectonic settings of the Sul-Riograndense Shield, southern Brasiliano Orogen, to delimit the evolution of the oceanic phase of the orogen. The southern portion of the Bossoroca ophiolite is inserted in the Sao Gabriel juvenile terrane and contains rare metasomatic tourmaline in chloritite close to serpentinite and metamorphosed Cr-spinel. The southern Bossoroca ophiolite was intruded by Cerro da Cria and Ramada Granites and the U-Pb-Hf isotopic study of zircon from these rocks constrains the crustal evolution of the Sao Gabriel juvenile terrane. Capane ophiolite has similar age (793–715 Ma) as the Bossoroca ophiolite and was inserted in the Porongos fold-thrust belt with preserved Cr-spinel of mantellic composition. Integrated use of Cr-spinel mineral chemistry, B isotopes in tourmaline in the Bossoroca ophiolite and zircon U-Pb-Hf isotopes of granites associated with the southern Bossoroca ophiolite revealed several steps in the evolution of the ophiolites in the Dom Feliciano Belt. Capane Cr-spinel cores have mantle-derived compositions (Mg# 0.66 – 0.69; Cr# 0.51 – 0.53), tourmaline from the Bossoroca ophiolite is dravite and has δ11B = 0 to + 3, and granites crystallization ages are 578 ± 3.2 and 612 ± 12 Ma (eHfzrn = −10 to −25). Zircon from other dravite occurrences of the Bossoroca ophiolite were previously dated at 920 Ma. We unraveled the main steps in the evolution of ophiolites from the southern Brasiliano Orogen, with emphasis on the Bossoroca and Capane ophiolites, during their trajectory from mid-ocean ridge (920 Ma), formation of dravite in oceanic crust, preservation of mantellic cores in Cr-spinel, and intrusion of craton-generated granites at 612–578 Ma.

Understanding the accretion of an oceanic island arc to a craton is a major issue in unravelling the evolution of the continents. P-T-t conditions registered in metamorphic rocks, the boron isotopic compositions of tourmaline provide essential information to delimit the accretionary processes. The Tonian-Cryogenian Sao Gabriel Terrane is a prominent juvenile structure in the southern Brasiliano Orogen that records key information about accretionary processes. Limited previous studies have investigated the Sao Gabriel Terrane, with little presently known about the metamorphic conditions. To pursue the objective, we combined multiple methods, including pseudosection modeling, U-Pb garnet geochronology and δ11B isotopes of tourmaline applied to one key sample from the Serrinha Formation of the Sao Gabriel Terrane. The recorded peak P-T conditions occurred at 4.5–5.1 kbar and 536–555 °C. U-Pb dating of low-U (

U–Pb and Lu–Hf data obtained from detrital zircon of 21 samples from four wells were used to determine the source terrains of the Cretaceous sedimentary rocks of the Pendencia, Pescada and Alagamar Formations (rift and post-rift sections) of Potiguar Basin, northeast Brazil. More than 1500 U–Pb concordant ages allowed the determination of the main age peaks in Paleoproterozoic (ca. 2.2–2.0 Ga and 1.9–1.7 Ga) and Neoproterozoic (ca. 0.62–0.55 Ga), matching the ages of rocks from the basement adjacent to the basin (northern portion of Borborema Province). eHf values obtained from 61 zircon grains range from −0.53 to −8.86 for Paleoproterozoic and from 7.67 to −25.43 for Neoproterozoic zircons, consistent with published data from rocks of the adjacent basement. Neoproterozoic zircons shows eHf values more negative from bottom to top, and presents a trend in U–Pb age vs eHf where the older grains show juvenile signature and the younger, continental crustal signature. This trend mimics the evolution of granitoid generation in the basement adjacent to the Potiguar Basin during the Neoproterozoic orogeny. The age pattern varies vertically in most wells, reflecting the contribution of distinct source areas following the rift evolution. Furthermore, the age pattern in the wells located at southeast of the grabens and horsts system of the onshore Potiguar Basin show Paleoproterozoic zircons concentrated in a single peak age, few or no zircon grains of 1.8–1.6 Ga and lesser amount of 0.6–0.7 Ga zircons, in contrast with the wells located at northwest, with Paleoproterozoic ages distributed in more than one peak, significant presence of 1.8–1.6 Ga zircons and proportionally more 0.6–0.7 Ga zircon grains. It suggests that this system acted as a barrier to sediment transport, delimiting the basin in two sectors fed by two different sources, one being the northeast portion of Borborema Province and the other, the northwest portion, both in the rift and post-rift sections. Such differences can result in distinct quality for the potential reservoirs in these sectors and would be an important guide for hydrocarbon exploration in Potiguar basin.

The integrated evaluation of soil geochemistry, aerogammaspectrometry (eTh), geological and structural mapping associated with the description of boreholes and outcrops in the Cacapava do Sul region, southernmost Brazil, led to the discovery of two carbonatite bodies. They are located near the eastern and southeastern border of Cacapava do Sul Granite and intrude the Passo Feio Complex. The carbonatite system is composed of early pink-colored alvikite followed by late white beforsite dikes. The carbonatites are tabular bodies concordant with the deformed host rocks. Petrographic and scanning electron microscopy show that the alvikites are dominantly composed of calcite with subordinate apatite, magnetite, ilmenite, biotite, baddeleyite, zircon, rutile, pyrochlore-like and rare earth element minerals. Beforsite is composed of dolomite and has the same minor and accessory minerals as the alvikite. U-Pb zircon geochronology via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed on a beforsite sample, yielding a 603.2 ± 4.5 Ma crystallization age. The carbonatite was emplaced an Ediacaran post-collisional environment with transpressive tectonism and volcanic activity marked by shoshonitic affinity.

Carbonatites were recently discovered in Southern Brazil, which increased the interest to evaluate the economic potential of these uncommon rocks, especially the Três Estradas Carbonatite. Carbonates are the dominant minerals of fresh rock followed by apatite, but the weathering process makes apatite abundant. We focused on apatite from the carbonatite using conventional petrography and electronic microscopy associated with microprobe, micro-Raman and Fourier-transform infrared spectroscopy. Results demonstrate the existence of four types. The primary type is associated with the rock crystallization and the subsequent three others are associated with weathering processes. The alteration mechanism was favorable for initial carbonate leaching and subsequent increase of phosphate with late precipitation of three new apatite generations. The deduced model involves long exposure during polycyclic climate changes, intercalating periods of warm dry with humid climate. The apatite types differ chemically and morphologically and have distinctive characteristics that are suitable to be used to differentiate them. These properties should be considered in future planes of industrial processes to transform apatite into single superphosphate, a basic input for fertilizer production.

This paper proposes the generation and closure of a proto-ocean and formation of an island arc in the Neoproterozoic Passo Feio Metamorphic Complex (PFMC), Cacapava do Sul region, southern Brazil. The PFMC, Sao Gabriel Terrane, consists of a Neoproterozoic supracrustal metavolcano-sedimentary sequence intruded by the Cacapava Granite. The study uses petrography, whole-rock geochemistry and mineral chemistry to identify the geochemical affinity and tectonic environment of the amphibolites contained in PFMC, the basalts and pillow lavas of Arroio Mudador Formation and dikes intruded in nearby volcanogenic rocks of Hilario Formation. The applied methods allow the metamorphic facies classification of ­prehnite-pumpellyite for the basalts and pillow lavas, greenschist for the dikes and lower amphibolite for the amphibolites. The discriminant diagrams indicate from tholeiitic to alkaline affinities for the Hilario Formation dikes. The basalts, pillow lavas and amphibolites have basaltic to basaltic-andesite composition and predominant tholeiitic affinity, interpreted as derived of the same protolith. The geotectonic diagrams mostly indicate volcanic arc basalts. The proposed evolution model starts with a proto-ocean opening through the Bossoroca Arc rifting around 760Ma, where an island arc is formed (PFMC rocks), precipitation of carbonates and marls, subsequent closure, two metamorphism events and the Cacapava Granite intrusion around 562 Ma.

The Ibaré Lineament is a shear zone that sections the Western portion of Sul-riograndense Shield, affecting the orthogneissic rocks of Santa Maria Chico Granulitic Complex, acting as a conduct for hydrothermal fluids. The studied hydrothermal mineralogy reflects a propylitic alteration paragenesis that is composed of Fe-clinochlore, epidote, albite, calcite and quartz, typical low-pressure paragenesis (< 2,000 bar). This mineral association reflects an equilibrium in a neutral to slightly alkaline pH. Using the new approach for chlorite geothermometry, an average formation temperature of 274°C was obtained, compatible with the stability of polytype IIb chlorite and with the irregular textures of twinning in calcites. The fluid inclusions in quartz establish a low-salinity system (2.45 wt.% NaCl eq.) and an average temperature of 175°C. The obtained temperatures are interpreted as a continuum cooling model with meteoric fluids entrance. The system pressure is evaluated from isochores, reaching an average value of 1,560 bar. This is the first temperature and pressure estimate to the Ibaré Shear Zone, which enabled the establishment of a shallow hydrothermal system (epithermal) with fluid interaction from magmatic and meteoric sources, confirmed by the behavior of carbon and oxygen stable isotopes, which indicated an the origin from mixed sources.

Integrated study of Tonian ophiolite melanges and intra-oceanic arc of the Sao Gabriel juvenile terrane exposes a key geotectonic unit in the evolutionary history of Gondwana during Brasiliano Orogeny. LA-ICP-MS U-Pb geochronology in rutile and zircon were combined with Lu-Hf and rare earth elements in zircon. We report the first rutile dating of ultramafic rock in the Brazilian Shield of 787.6 ± 2.6 Ma for the Cambaizinho ophiolite magnesian schist. Dating of rutile corresponds to the metamorphic age of the mantle section of the ophiolite. Sao Gabriel arc infrastructure is represented by the Cambai Complex, including the Vila Nova Suite metatonalite with zircon U-Pb age of 724.6 ± 3.2 Ma and late Sanga do Jobim Suite monzogranite intrusions with two main pulses at 698.9 ± 4.2 Ma and 673.9 ± 6.8 Ma. Zircon eHf(t) values are positive for both rocks and range between +10.1 and +7.4 in metatonalite and +9.2 to +2.5 in monzogranite. Detrital zircon from metasedimentary rocks support superstructure arc investigation. Ages range from 817 to 650 Ma, with strong concentration between 750 and 730 Ma. Positive eHf(t) values and Rare Earth Elements (REE) signature point to main source of metasedimentary rocks in the arc, with ophiolitic contribution. Therefore, geotectonic processes in the Brasiliano Orogeny include the construction of Sao Gabriel terrane with Tonian oceanic crust evolving to intra-oceanic subduction until 650 Ma to final development along the margin of continental crust. Overthrusting of the Sao Gabriel terrane onto the Rio de La Plata Craton occurred at 650–620 Ma. Our data correlate with Tonian intra-oceanic arcs from Brasilia belt (Goias arc ca. 862–630 Ma) and Ribeira belt (Serra da Prata arc – 856–838 Ma; Rio Negro arc – 790–620 Ma). Oceanic crust generation combined with subduction to form Tonian arcs along the Brasiliano Orogen were most significant for construction of West Gondwana.

O Bloco São Gabriel, localizado na zona central do embasamento cristalino do Rio Grande Sul, apresenta extensos registros do Neoproterozóico do Ciclo Brasiliano, contidos em complexos ígneos e metamórficos. Esses complexos também guardam registros sedimentares, com sequências carbonáticas metamorfisadas, cuja evolução tectônica é pouco conhecida. Por essa razão, buscou-se nesse trabalho investigar a origem e a evolução das rochas carbonáticas ocorrentes no Bloco São Gabriel por meio da caracterização geoquímica de elementos maiores, traço, ETR e isótopos de Sr, C e O. Para tanto, foram escolhidas como áreas de pesquisa no Bloco São Gabriel a Formação Passo Feio, o Complexo Cambaí e o Complexo Coxilha do Batovi. As rochas carbonáticas da Formação Passo Feio compreendem dolomita mármores impuros, as amostras do Complexo Cambaí, calcita mármores impuros e os litotipos do Complexo Coxilha do Batovi, mármores calcíticos e calcissilicáticos, alguns ricos em grafita e com percentuais de até 10% de quartzo demonstrando contribuição siliciclástica no protólito. Idades de zircões detríticos de clorita xistos associados com os mármores, obtidos em trabalhos anteriores, indicam um intervalo entre 770 – 700 Ma para a deposição dos carbonatos do Bloco São Gabriel. Os carbonatos da Formação Passo Feio sofreram dolomitização durante ou logo após a deposição, enquanto os carbonatos do Complexo Cambaí não foram afetados pela dolomitização. A geoquímica dos elementos traço e terras raras indica que os mármores analisados preservaram as composições isotópicas originais do protólito nos diferentes graus metamórficos: Complexo Coxilha do Batovi (grau baixo), Formação Passo Feio e Complexo Cambaí (grau médio). A análise integrada dos isótopos de Sr, C e O da Formação Passo Feio e do Complexo Cambaí revelou a preservação de valores isotópicos primários ou muito próximos da composição isotópica original. Na Formação Passo Feio os valores obtidos são: 0,7074 (87Sr/86Sr), -0,26‰ e 2,44‰ (δ13CPDB) e -5,68‰ (δ18OPDB). No Complexo Cambaí, estes valores são: 0,7069 (87Sr/86Sr), 5,75‰ (δ13CPDB) e -11,64‰ (δ18OPDB). Quando comparadas com a curva de variação da razão 87Sr/86Sr e dos valores de δ13C e δ18O da água do mar ao longo do Neoproterozóico, verifica-se que os mármores da Formação Passo Feio e do Complexo Cambaí estão situados no intervalo entre 770 e 730 Ma. A contextualização dos dados isotópicos na cronologia dos eventos tectônicos reconhecidos no Bloco São Gabriel permitiu a estimativa de períodos de deposição das rochas carbonáticas da Formação Passo Feio entre 770 e 730 Ma, e entre 740 e 730 Ma para o Complexo Cambaí.

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300 million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The Sao Gabriel block in the west is characterized by c.760–690 Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880 Ma old igneous rocks. Both igneous and metasedimentary rocks have positive eNd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the Sao Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D3) yield an U–Pb zircon age (LA-ICP-MS) of 701 ± 10 Ma giving the approximate minimum age of the Sao Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789 ± 7 Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234 ± 28 Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100–2,000 Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800–700 Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and Sao Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880 Ma (Passinho event) and (ii) accretion of the 760–700 Ma Cambai/Vila Nova magmatic arc (Sao Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW–NE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590 Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530 Ma and can be assigned to final stages of the amalgamation of West Gondwana.

The provenance of the Maastrichtian deep-water reservoir sandstones from the Jubarte oil field (Campos Basin, eastern Brazil), was studied using an integrated approach that included quantitative petrography, conventional heavy mineral analysis, garnet mineral chemistry and zircon geochronology. The reservoirs are predominantly coarse, poorly-sorted sandstones with feldspathic composition derived from uplifted basement terrains. The fourth- and fifth-order depositional sequences analyzed show no major variations in composition or in provenance through time. However, variations in apatite:tourmaline presents potential to be used as a parameter for sandstone correlation within the field. The composition of heavy minerals indicates derivation from high temperature and low-to-medium pressure aluminous metapelitic rocks, from granites and subordinate mafic rocks, derived from the Cabo Frio Tectonic Domain and the Oriental terrain of Ribeira orogen, characterizing a supply route from SW to NE. The low ZTR index, as well as the absence of low-grade stable heavy minerals and of metasedimentary rock fragments, suggest that by the end of Cretaceous all supracrustal, low-grade terrains had already been totally eroded, and that plutonic, infracrustal rocks were exposed, similarly to the present situation. The integration of the compositional and textural aspects suggests a tectonically active source area, located relatively close to the basin. The active tectonic uplift of the basement terrains provided a large volume of coarse, poorly-sorted sediments carried directly from mountain streams and alluvial fans to deep water. The high frequency shifts in apatite:tourmaline values support a direct derivation of the deep-water sandstones from adjacent alluvial systems with short but differential exposure to weathering. The voluminous deposition of such coarse-grained and immature turbidites under the general transgressive conditions of the Maastrichtian indicates a tectonic, rather than eustatic control on their deposition, with important implications for the exploration of similar reservoirs in Campos Basin and in similar basins of eastern Brazilian margin.

Turbidite sandstone reservoir rocks in the Campos Basin, offshore eastern Brazil, show evidence of a number of interactions among organic and mineral phases during diagenesis. These include extensive dissolution of plagioclase, precipitation of kaolinite into intergranular and intragranular pores, dissolution of carbonate cements and grains, as well as selective deposition of organic films on kaolinite aggregates, dissolved plagioclase and carbonate grains. These processes are connected with the biodegradation of hydrocarbons within the reservoir, related to the influx of meteoric fluids possibly during sea-level lowstands, through major stratigraphic sequence boundaries and large-scale faults. Oil biodegradation generated organic solvents responsible for mineral alteration, as well as heavy oils rich in asphaltenes. The action of organic solvents in the reservoirs is indicated by the extensive dissolution of feldspars, quartz, garnets and kaolinite, and the negative mass balance between dissolved feldspar and precipitated kaolinite, indicating aluminum mobilization. Asphaltene films and coatings may affect the wettability of the reservoir rocks and the evaluation of hydrocarbon saturation in the reservoirs by resistivity logs. These various inorganic and organic processes are apparently interrelated in the diagenesis of the Campos Basin turbidite reservoir sandstones. These interactions should be considered in future models of the basin and its petroleum system, as well as for the creation of geochemical models capable of predicting the quality of the reservoirs and the occurrence of degraded heavy oils.

Precambrian rocks of southernmost Brazil display a complex tectonometamorphic evolution since the Archean. Neoproterozoic juvenile rocks in the Sao Gabriel block offer a unique opportunity to study unequivocal Brasiliano fabric and tectonics. The stratigraphic succession comprises the Palma Group, which consists of metasedimentary rocks and juxtaposed (ultra-)mafic to intermediate volcanic and volcanoclastic rocks formed mainly in a subduction zone environment, as well as the juvenile metadiorites, -tonalites and -trondhjemites of the Cambai Complex, which represent relics of magmatic arcs. Four deformational events, the first three characterized by ductile deformation, can be distinguished in the Sao Gabriel block. The first deformation, D1, led to the formation of a first foliation and metamorphic layering, possibly related to a first folding event. The D2 event reached amphibolite facies metamorphic peak conditions and corresponds to southeast-directed thrusting. D1 and D2 can be observed only in the lower Palma Group rocks. D3 led to the tectonic juxtaposition of different lithologic units along major SW–NE-striking shear zones, which defines the final overall geometry of the area represented by a dextral transpressive, southeast-vergent ductile thrust stack. Ductile SW–NE-oriented strike-slip shearing occurred predominantly in syntectonic (syn-D3) plutonic rocks of the Cambai Complex due to the lower viscosity and strength of the magmatic rocks, whereas deformation in the wall rocks (Palma Group) was accommodated mainly by southeast-vergent folding. The main part of the D3 deformation took place under upper greenschist to lower amphibolite facies metamorphic conditions. The last deformation, D4, is characterized by semi-brittle, southeast-directed thrusting. A plate tectonic model for the Sao Gabriel block comprises (1) eastward subduction of oceanic crust and approximately 880 Ma accretion of the intraoceanic Passinho arc to the eastern border of the Rio de la Plata Craton; (2) west-dipping subduction beneath the newly formed active continental margin, development of a 750–700 Ma magmatic arc, and related sedimentation and their imbrication in the forearc and backarc regions; and (3) collision with another continental block (Encantadas block) at approximately 700–670 Ma. A change from southeast-directed thrusting (D2) to localized strike-slip tectonics and oblique folding during D3 occurred during progressive structural evolution due to oblique subduction. Oblique convergence and collision was accommodated by orogen-parallel movements, facilitated by syntectonic magmatic activity. The Sao Gabriel block represents a major Brasiliano collision zone. The oceanic basin facing the eastern margin of the Rio de la Plata Craton might be traced to the north and could be linked to Neoproterozoic juvenile oceanic crust in the western Brasilia belt.

The Lavras do Sul intrusive complex (LSIC) and coeval volcanics, formed during the Late Neoproterozoic in the Sul-Riograndense Shield, are important because they contain historic Au-Cu occurrences. Field relationships, and gravity and magnetic data are consistent with the LSIC as nested intrusions of a volcano-plutonic subsidence system. These data also show the LSIC evolved from north to south, so we divided it into two sectors: the Tapera monzonite (TM) in the north, and the granitic body in the south. The Arroio do Jacques monzodiorite (AJM) occurs between the two sectors, surrounding the north-northeast border of the latter. TM is an intrusion emplaced during the formation and stabilization of an upper-crustal magma chamber linked to the coeval volcanics. Granites with distinct geochemical affinities forma concentric body that probably represents a resurgent pluton emplaced after some caldera collapse. Alkali-calcic granites are in the core above the main root zones, whereas alkaline granites occur as semicircular and peripheral bodies. 207 Pb/ 206 Pb and 206 Pb/ 238 U ages for several LSIC-units are comparable, and confirm that the intrusive complex is a multicyclic body, probably formed over a period of ca. 23 m.y. (606-583 Ma). Nonetheless, most of LSIC-units (TM, AJM and the core granites) have similar crystallization ages, of ca. 601 and 599 Ma, suggesting that this is the major period of igneous activity forming the volcano-plutonic system. Two distinct ages were obtained for alkaline granites, 207 Pb/ 206 Pb age of 598± 3 Ma (2σ) and 206 Pb/ 238 U age of 586.0 ± 2.8 Ma (2σ) respectively. Such difference in ages maybe resolved if the alkaline granites were emplaced in increments. The 207 Pb/ 206 Pb age of 567 ± 4 Ma (2σ) for the Jaguari Granite, situated to the west of LSIC, confirms it represents another younger alkaline event.

Este trabalho investiga os processos controladores da mobilidade e concentração do níquel em rochas ultramáficas serpentinizadas no Escudo Sul-Riograndense (ESrg). Foram selecionadas quatro unidades ultramáficas: Complexo Serrinha, Sequência Cerro Mantiqueira, Maciço Pedras Pretas e Complexo Cambaizinho. Com o uso de técnicas convencionais foi possível classificar quimicamente as rochas e relacionar os aspectos texturais com a variação da intensidade dos diferentes eventos que modificaram a mineralogia dos protólitos. Por meio da microssonda eletrônica foram analisadas olivinas e serpentinas para a determinação das concentrações de níquel e suas variações nas unidades investigadas. Foram identificados os minerais portadores de níquel e a relação dos eventos com a mobilidade e concentração dos elementos ao nível da rocha. As olivinas do Maciço Pedras Pretas contêm baixos teores em níquel, com valores variando de 0,08 a 0,29%, e um valor médio de 0,17%, enquanto as olivinas da Sequência Cerro Mantiqueiras contêm teores mais elevados de NiO (máximo 0,44%), estabelecendo um valor médio de 0,35%. Em relação à composição, as olivinas do Maciço Pedras Pretas e da Sequência Cerro Mantiqueiras são homogêneas, enquanto aquelas do Complexo Cambaizinho mostram variações composicionais entre Fo92 e Fo84 e teores de NiO entre 0,17 e 0,30%. No Complexo Serrinha não foram identificadas olivinas devido à intensa serpentinização. No Complexo Cambaizinho e Sequência Cerro Mantiqueiras as serpentinas tendem a apresentar concentrações de níquel mais elevadas que o determinado para as olivinas do protólito, porém, nesta última sequência, os teores também empobrecem em algumas serpentinas. Com a comparação das concentrações de níquel entre as unidades foi possível identificar que a Sequência Cerro Mantiqueiras apresentou os maiores valores de NiO nas olivinas, enquanto o Maciço Pedras Pretas, as menores concentrações. Outra importante constatação é que os teores de níquel nas unidades estudadas do ESrg são equivalentes aos valores observados em outras rochas ultramáficas no mundo.

Two Brasiliano schist belts (Porongos belt and Sao Gabriel block) of contrasting geotectonic setting exhibiting a distinct structural evolution are exposed in the southern Brazilian shield east of the Rio de la Plata craton. Trace-element and isotope data (Sm-Nd, Rb-Sr) demonstrate that the Sao Gabriel block and Porongos belt represent two distinct tectonostratigraphic blocks, with the former consisting of Neoproterozoic juvenile rocks, and the latter being characterized by intense reworking of old continental crust. The Sao Gabriel block in the west consists of Neoproterozoic relics of two Brasiliano magmatic arcs, an intra-oceanic arc, and a younger continental arc or active continental margin. The Porongos belt located on the Encantadas block, in contrast, formed in a passive-margin setting on thinned continental crust in an extensional or transtensional regime. Basin formation was accompanied by volcanism due to partial melting of the stretched continental crust. Comparison of the structural evolution...

Precambrian metaplutonic rocks of the Sao Gabriel block in southernmost Brazil comprise juvenile Neoproterozoic calc-alkaline gneisses (Cambai Complex). The connection with associated (ultra-)mafic metavolcanic and metasedimentary rocks (Palma Group) is not well established. The whole complex was deformed during the Brasiliano orogenic cycle. Both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the Cambai Complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the Sao Gabriel block and its role during the assembly of West-Gondwana. The major element compositions of the igneous rocks (Palma Group and Cambai Complex) indicate a subalkaline character; most orthogneisses have a calc-alkaline chemistry; many metavolcanic rocks of the Palma Group show signatures of low-K tholeiitic volcanic arc basalts. Trace element data, especially Ti, Zr, Y, Nb, of most igneous samples from both the lower Palma Group and the Cambai Complex indicate origin at plate margins, i.e., in a subduction zone environment. This is corroborated by relative enrichment in LREE, low contents of Nb and other high field strength elements and enrichment in LILE like Rb, Ba, and Th. The data indicate the possible existence of two suites, an oceanic island arc and a continental arc or active continental margin. However, some ultramafic samples of the lower Palma Group in the western Sao Gabriel block indicate the existence of another volcanic suite with intra-plate character which possibly represents relics of oceanic island basalts (OIB). Trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks. The patterns of chondrite- and N-MORB-normalized spider diagrams resemble the patterns of the igneous rocks, i.e., LILE and LREE enrichment and HFS depletion. The geochemical signatures of most igneous and metasedimentary samples and their low (87Sr/86Sr)t ratios suggest only minor contribution of old continental crust. A geotectonic model for the Sao Gabriel block comprises east-ward subduction and following accretion of an intra-oceanic island arc to the eastern border of the Rio de la Plata Craton at ca. 880 Ma, and westward subduction beneath the newly formed active continental margin between ca. 750 and 700 Ma. The Sao Gabriel block represents relics of an early Brasiliano oceanic basin between the Rio de la Plata and Kalahari Cratons. This ocean to the east of the Rio de la Plata Craton might be traced to the north and could possibly be linked with Neoproterozoic juvenile oceanic crust in the western Brasilia belt (Goias magmatic arc).

Juvenile Neoproterozoic dioritic, tonalitic, trondhjemitic and granodioritic gneisses in the Sao Gabriel block, southern Brazil, have been identified by geochronologic studies. Age proposals for associated (ultra-)mafic metavolcanic and metasedimentary rocks, however, range from Archean to Neoproterozoic. Whole rock Sm–Nd analyses presented here support a Neoproterozoic age for these rocks. TDM model ages of the (ultra-)mafic metavolcanic rocks range between 0.65 and 1.35 Ga with ɛ Nd( t ) positive values between 3.16 and 6.87; TDM model ages of metasedimentary and metavolcanoclastic rocks vary between 0.77 and 1.19 Ga with ɛ Nd( t ) values between 1.2 and 6.23; tonalitic calc-alkaline gneisses show ɛ Nd( t ) values of 4.34 and 6.3 and TDM model ages of 0.89 and 0.72 Ga, respectively. A late-kinematic granite (Santa Zelia granite) display slightly negative ɛ Nd( t ) values (−1.6) and a higher TDM model age of about 1.4 Ga. These data support the existence of Meso/Neoproterozoic juvenile oceanic crust and island arc rocks during the Brasiliano orogenic events. The main source rocks of the metasedimentary units are previously formed juvenile rocks. The data also indicate minor assimilation of older crustal material and/or contamination of the melts by radiogenic Nd released from older rocks on the subducting slab. Existence of widespread old sialic crust in the subduction zone environment, however, can be ruled out indicating important orogenic accretion between 0.9 and 0.7 Ga. A geotectonic model for the Sao Gabriel block and the eastern margin of the Rio de la Plata craton comprises eastward subduction and following accretion of an intra-oceanic island arc between 0.9 and 0.8 Ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the Rio de la Plata craton between 0.8 and 0.7 Ga. We postulate that the juvenile rocks of Sao Gabriel block represent relics of a Neoproterozoic ocean between the Rio de la Plata craton and a continental block (Encantadas block) possibly derived from the Kalahari craton. Subduction and arc accretion began roughly coeval with the initial stages of the break-up of Rodinia (0.9 Ga) and indicate a peripheric Rio de la Plata craton in relation to the Rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the Neoproterozoic Brasiliano orogenic events.

To obtain the chemical Th ∗ –Pb isochron ages and surface maps of monazite crystals in igneous and metamorphic rocks from the southern Brazilian Shield, we employ Th–U-total Pb dating by an electron probe microanalyzer. The ages of two Trans-Amazonian metamorphic events are given by a felsic, garnet-bearing granulite from the Santa Maria Chico granulitic complex. The age of the first event, at approximately 2.35 Ga, was obtained by surface mapping in a grain included in garnet. The dating of the second event, 1899±43 Ma, is in agreement with previous data obtained in zircon crystals with sensitive high-resolution ion microprobe. Other determinations belong to the Brasiliano cycle. In the Sao Gabriel block, an age of 643±129 Ma was obtained on monazite from a staurolite-garnet schist of the Cambaizinho Formation, whereas a staurolite-bearing schist from the Passo Feio complex yielded a 510±68 Ma age. Several units in the Dom Feliciano belt were dated, including the biotite-sillimanite gneisses of the Varzea do Capivarita complex (552±90 Ma), the sillimanite-garnet gneisses of Camboriu complex (565±77 Ma), the Tres Figueiras granite (558±57 Ma), and the Plaza Itapema granite (545±55 Ma). The ages presented in this study, obtained through monazite chemical dating, are confirmed through comparison with previous data regarding zircon crystals from the same geological units.

Significant improvements, both in understanding the evolution of zircons and in understanding the geotectonic and metallogenetic evolution of the complex terrain of southern Brazil, are obtained from a SHRIMP geochronology study and reviewed in this paper. The use of backscattered electron and cathodoluminescence images, prior to SHRIMP isotopic determinations, proved of enormous fundamental value for this technique. Zircon is a domainal open‐system mineral in many geological conditions; very old domains may be preserved, but the same crystal may show ages of younger tectonic events. Zircons may recrystallise inwards from the rims or outwards from the cores, and also along euhedral high‐U or metamict thin zones. Zircons also may be recrystallised during gold‐related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the identification and dating of magmatic zircons. Precambrian orogenies are identified along with the intervening intracratonic ...

Base-metal deposits in the Cacapava do Sul Copper Province are hosted by both volcanosedimentary rocks of the Bom Jardim Group and by metamorphic rocks of the Passo Feio Formation, and show a spatial relationship to the Cacapava Granite. These associations have led to much controversy about the genesis of the base-metal deposits, which has been at least partly resolved by precise dating using SHRIMP (Sensitive High Resolution Ion Microprobe) U/Pb zircon studies combined with S, Pb, and Sr isotope trace studies. The Passo Feio Formation is Neoproterozoic in age and was derived from a complex continental source, as shown by the presence of xenocryst zircons of Archaean, Paleoproterozoic, and Neoproterozoic ages. It was metamorphosed at ca. 700 Ma. The syntectonic Cacapava Granite that intruded the supracrustal rocks of the Passo Feio Formation at 562 Ma was derived from an old sialic basement. Lead-isotope data are consistent with a 562 Ma age for the base-metal sulphide deposits sited in the Passo Feio Formation. The least-radiogenic compositions lie between the field of the isotopic compositions of the Cacapava Granite and rocks of the Passo Feio Formation, suggesting that Pb in the sulphide deposits may have been derived from both sources. The Pb, like that in the Cacapava Granite and Passo Feio Formation, was derived from a primitive crustal source. Sulphur isotope data from the base-metal sulphide deposits in the Passo Feio Formation are compatible with a mixed sedimentary and magmatic source. The most logical model for ore genesis, based on the isotopic data and spatial relationships, is that magmatic metal-bearing fluids from Cacapava Granite leached metals from the Passo Feio Formation and that the deposited sulphides therefore show mixed isotopic signatures. However, there is also some isotopic evidence from the Cacapava Granite itself that suggests assimilation of S-bearing rocks of the Passo Feio Formation during emplacement. Thus, isotopic signatures could have been inherited from assimilated metal sulphides at this stage, and deposition could have been entirely from Cacapava Granite-derived magmatic fluids. Importantly, the inferred 562±8 Ma age for the deposits in the Passo Feio Formation is younger than the well-constrained age of 594±5 Ma for the Camaqua/Santa Maria deposits. Thus, the epigenetic sulphides in the Passo Feio Formation cannot be the source of these deposits as previously suggested. Other isotopic data also argue against such a model.

The Camaqua Cu (Au, Ag) and Santa Maria Pb-Zn (Cu, Ag) deposits are the largest base-metal deposits discovered so far in the sedimentary clastic sequences of the Neoproterozoic-age Camaqua Basin. The origin of the Camaqua-Santa Maria deposits has been the centre of dispute, with three alternative genetic hypotheses proposed: a syngenetic model, a diagenetic model, and a magmatic hydrothermal model. In detail, this mineralization has been suggested to be related to sedimentary clastic-diagenetic processes, volcanic-related processes, or deep granitic magmatism. Reevaluation of previous data and new studies in the area yield the following conclusions: (1) mineralization is fracture-controlled and magmatic-hydrothermal in origin rather than stratiform syngenetic or diagenetic; (2) the temperature of deposition of the main ore minerals was 210 to 300°C; (3) the ∂ 34 S of sulphides of around 0% indicates an external magmatic-hydrothermal source of sulphur; (4) Pb isotope ratios of sulphides indicate that metals were derived at the end of the Brasiliano Cycle from a large crustal source with very primitive Pb and that (5) the age of mineralization is 594 Ma, as constrained by U/Pb SHRIMP (Sensitive High Resolution Ion Microprobe) determinations on zircons of the Lavras Granite. Thus, the Camaqua and Santa Maria deposits are interpreted to be of magmatic-hydrothermal origin, with the metals derived from an old crustal-basement source during the end of Dom Feliciano Collisional Orogeny, at 594 Ma, late in the Brasiliano Cycle. The interpretation above is critical for base-metal exploration in the Sul Riograndense Shield. Previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits. However, the occurrence of mineralization along fractures within specific wall-rocks requires consideration of alternative exploration parameters. These include: (1) ancient EW- and NW-trending regional fractures and their intersections, which are potential structural sites for base-metal mineralization, and (2) preferential wall-rock sites, either rocks with high initial porosity or secondary fracture-induced porosity or soluble rocks which are susceptible to replacement processes. Post-collisional plutonism of the Dom Feliciano Orogeny was the most likely heat source, and also the source of sulphur and base metals. Gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry-style Cu-Au deposits.

The Neoproterozoic Bossoroca juvenile Volcanic Arc of southernmost Brazil contains arc-related gold deposits. The Bossoroca gold deposit consists of veins and stockworks of quartz-gold ores with minor pyrite, chalcopyrite, galena and tellurides. Carbonate, chlorite, sericite and tourmaline are the main gangue minerals. The ore shoots are contained in calc-alkaline pyroclastic andesites and dacites with minor basalts and epiclastic rocks of the Campestre Formation. SHRIMP U/Pb investigations of zircon show that the island-arc volcanogenic sequence was formed ca 757 m.y. ago in the early Brasiliano Cycle and metamorphosed into transitional greenschist/amphibolite facies of low-pressure regional metamorphism at ca 700 Ma. Nearby, the post-tectonic Sao Sepe Granite was intruded into the volcanic arc at ca 550 Ma. The mineralising fluids have been related either to metamorphism or to solutions derived from post-tectonic intrusive granites. Lead isotopic analyses, carried out on galena from the gold ore, on feldspar and total rock from the associated volcanic pile, and also on feldspar and total rock from the Sao Sepe Granite, indicate that gold mineralisation is related to the volcanogenic rocks, and that the deposit should be considered to be of an epizonal orogenic type.

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The southern Brazilian Shield comprises a number of tectonostratigraphic blocks representing two terranes. The Sao Gabriel block consists of relics of two Brasiliano juvenile magmatic arcs; the Porongos belt located on the Encantadas block formed in a passive margin setting. Plate tectonic evolution started with opening of an oceanic basin to the east of the Rio de la Plata craton since at least 0.9-1.0 Ga. An intra-ocean- ic island arc formed due to eastward subduction and was subsequently accreted to the eastern margin of the Rio de la Plata craton. Westward subduction beneath the newly formed active continental margin occurred between ca. 850 and 700 Ma. At the same time, the Porongos basin formed on stretched continental crust of the Encantadas passive margin. Collision of the two terranes took place at ca. 700-660 Ma followed by left-lateral ductile shear along the Dorsal de Cangucu Shear Zone between 670 and 620 Ma and 630- to 610-Ma sinistral shearing in the Dom Feliciano belt farther east. The episodic character of orogenic evolution can be observed throughout Brazil. The Brasiliano belts cannot be directly linked with pan-African belts in southwestern Africa because main deformation in the latter occurred 50-70 Ma later. The assembly of Gondwana comprises a series of collisions of cratons and microcontinents over a time span of nearly 400 Ma; however, a number of orogenic episodes can be discrimi- nated. Their synchroneity suggests that temporally equivalent episodes are coupled with the global plate tectonic framework, which, however, is far from resolved.

Durante cerca de 40 anos o curso de geologia da UFRGS formou profissionais com base em um currículo cujo trabalho de conclusão dava-se por meio de um mapeamento geológico básico. A partir dos anos de 1990, deu-se início a uma profunda mudança curricular que culminou, no início dos anos 2000, com a implantação de nova modalidade de trabalho de conclusão do curso, denominada de Projetos Temáticos em Geologia. O propósito do presente artigo é discutir o contexto histórico da mudança de paradigma na formação profissional depois de um longo período de autoritarismo institucional que foi vigente durante o regime militar. Além disso, expõe-se o modo de funcionamento e as características dos Projetos Temáticos e os principais resultados alcançados em 13 anos de desenvolvimento em uma instituição universitária pública e democrática. Por fim, estabelecem-se algumas reflexões sobre os avanços possíveis na formação profissional de qualidade em contextos técnico-científicos complexos e democráticos.

Cerro do Sandi and Bela Vista granites occur in the Piratini region and belong to Dom Feliciano Suite. This suite is the most recent magmatic episode of the Pelotas batholith construction. The granitic massifs have elongate forms along NE-SW direction and constitute a positive relief of aligned crysts. Mapping of geological, structural and superficial deposits indicated the presence of outcrops and areas dominated by boulders with potential for exploration of blocks for dimensional stones. Petrography and technological essays confirmed favorable results to this utilization. Bela Vista Granite presents high mechanical resistance and low abrasion, qualities that recommended its utilization as external and internal covering. Cerro do Sandi Granite showed physical indexes and ultrasonic propagation velocity characteristic of rocks without weathering alteration. However, in the alterability essay this granite showed incipient modification of color and minimal loss of material. These characteristics limit the utilization of this granite as external and internal floor.

The SW–NE-striking Porongos belt, located between juvenile Neoproterozoic rocks in the west and the Dom Feliciano belt, characterized by intense reworking of older crust, in the east, comprises a greenschist to amphibolite-facies metavolcano-metasedimentary succession (Porongos sequence) of unknown age with some exposures of Palaeoproterozoic gneisses (Encantadas gneisses). High-temperature ductile deformation of the basement gneisses comprises at least two magmatic events followed by three deformational phases including folding and shearing (DT1–DT3) and can be attributed to the Palaeoproterozoic Trans-Amazonian orogeny. The deformation of the Porongos sequence occurred during the Neoproterozoic Brasiliano orogeny and comprises four ductile deformation phases (DB1–DB4), including two phases of isoclinal folding associated with shearing recorded in mylonitic layers, followed by closed NW-vergent folding and thrusting leading to formation of a thrust stack. Uplift of the basement and formation of late tectonic sedimentary basins occurred as a result of semi-ductile to brittle block faulting in a sinistral strike-slip regime. The Porongos sequence can be subdivided into a southeastern and a northwestern part. Trace element analyses as well as Sm–Nd and Rb–Sr geochemical data indicate partial melting and significant contamination by old continental crust for the metavolcanic rocks. The metavolcanic rocks show εNd(t=780 Ma) values of −20.64 and −21.72 (northwestern units) and −6.87 (southeastern sequence). The metasedimentary rocks were derived from late Palaeoproterozoic to Archaean sources, and the data indicate different sources for the northwestern and southeastern rock units of the Porongos sequence. εNd(t=780 Ma) are −6.25 and −6.85 in the southeastern units, with TDM model ages between 1734 and 1954 Ma, and vary between −14.72 and −17.96 in the northwestern parts, which have TDM model ages between 2346 and 2710 Ma. High 87Sr/86Sr(t) values between 0.7064 and 0.7286 confirm reworking of older crust. Isotopic signatures of the Porongos sequence do not show indications for a significant contribution from a Neoproterozoic juvenile source. A passive margin or continental rift environment is suggested for the tectonic setting of the Porongos belt, which is compatible with both deposition of shallow marine to deep marine sediments and stretching of continental crust leading to volcanism which is characterized by significant contamination by old continental crust.

Este artigo discute a origem dos depósitos arenosos atuais do rio Jacuí, no sul do Brasil. O Jacuí constitui o principal rio do Estado do Rio Grande do Sul e contém os principais depósitos de areia utilizados na indústria de concreto do estado. O rio Jacuí e seus afluentes drenam as mais diversas áreas-fonte, com diferentes idades e associações litológicas, incluindo as rochas do Escudo Sul-riograndense (Pré-Cambriano), da Bacia do Paraná, onde está incluída a Formação Serra Geral (Cretáceo), e da Formação Tupanciretã (Terciário). As amostras foram coletadas de sedimentos fluviais em transporte para a análise de minerais pesados, ao longo do curso principal do rio e de seus principais afluentes. Os resultados demonstram uma grande diversidade de minerais pesados, incluindo: granada, turmalina, hornblenda, zircão, apatita, estaurolita, opacos (magnetita, ilmenita, hematita e pirita) e, minoritariamente, rutilo, piroxênio (hiperstênio principalmente), cianita, sillimanita, andaluzita, titanita, actinolita e tremolita. Esta assembleia de minerais pesados indica que a área-fonte principal dos sedimentos é o Escudo Sul-rio-grandense, proveniente dos afluentes da margem direita do Rio Jacuí. As rochas sedimentares da Bacia do Paraná também constituem uma importante área-fonte para os sedimentos. As rochas vulcânicas da Formação Serra Geral e as sedimentares fluviais da Formação Tupanciretã fornecem uma menor contribuição. Uma redução pequena, mas consistente, dos índices ATi e GZi ao longo do curso principal do rio, de montante para jusante, indica que os sedimentos sofreram decréscimo destes minerais ao longo da planície aluvial possivelmente devido ao intemperismo. Os valores do índice ZTR e ARi são relativamente constantes ao longo do rio principal. Além disso, o ARi é relativamente baixo. Os valores consistentes destes índices indicam uma forte contribuição dos afluentes do rio Jacuí. A assinatura de minerais pesados do rio Jacuí poderá ser utilizada para avaliar a contribuição dos sedimentos do rio no preenchimento da porção mais jovem da Bacia de Pelotas.

Os complexos máfico-ultramáficos do Rio Grande do Sul foram reavaliados, com ênfase nos dados disponíveis para elementos químicos da série 3d de transição. Os complexos apresentam geralmente três componentes, quais sejam, ultramáfico (principalmente peridotitos, serpentinitos e xistos magnesianos), máfico e metassomático. Este último componente está bem desenvolvido na maioria dos complexos e foi identificado mas não estudado em detalhe. A suíte gabróica está presente em diversas áreas, tal como Pedras Pretas, e mostra uma química da série 3d de transição desde menos evoluída até mais evoluída, podendo ser classificada da mesma forma que em outras séries do mundo. Os serpentinitos são semelhantes entre si, independente da área avaliada, normalmente mostrando características residuais exceto em Pedras Pretas, onde a química do protólito gabróico foi mantida. Os peridotitos do Cerro da Mantiqueira são harzburgitos e a sua química da série 3d não pode ser distinguida de serpentinitos. Na região do Arroio Cambaizinho, os xistos magnesianos contendo várias proporções de tremolita + clorita + magnetita + ilmenita foram testados em camadas com um a cinco metros de espessura e que apresentam continuidade lateral de afloramento de centenas de metros. Eles mostram química uniforme na mesma banda e parecem corresponder a lavas magnesianas, de afinidade possivelmente komatiítica, metamorfizada na fácies anfibolito. As lavas foram derivadas de um manto enriquecido em elementos LILE ou contaminadas por material crustal.

This paper presents the geology, chemical data, Pb-Pb zircon ages, sulfur, carbon, oxygen, strontium isotopes and fluid inclusions of Cerro dos Martins copper deposit. The deposit is hosted by the Neoproterozoic volcano-sedimentary sequence of the Camaquã Basin in the Rio Grande do Sul Shield, southern Brazil. The ores consist of a set of Cu-sulfide NW-trending veins and disseminations within of the Bom Jardim Group. Chalcocite and bornite are the main ore minerals, whereas carbonates, barite, quartz and hematite are the gangue. The volcanic host rocks show an alkaline affinity. An intrusive quartz-diorite body, also shown alkaline affinity, yield a 550±5 Ma zircon age (initial 87Sr/86Sr of 0,704), which is considered the minimum age of the copper mineralization. The δ34SCDT of sulfides ranges from –6.2 to +0.9‰. Hematite in the mineral paragenesis suggests oxidizing conditions, which would shift the original magmatic sulfur isotopic compositions (δ34S ~ 0‰) to negative values. Barite with δ34SCDT from +9.25 to +10.65‰ may also indicate oxidation due to mixing of a hot rising fluid with cold meteoric water. The δ13CPDB of calcite from gangue, in the range of – 1,90 to – 4,45% is interpreted as originated from mixing sources – magmatic and fluids contaminated by basement marbles. Studies on primary aqueous biphasic fluid inclusions show homogenisation temperatures with average of 215°C. Water in equilibrium with calcite from hydrothermal fluid show calculated values of δ O18 SMOW between 3 and 14‰, suggesting derivation from a magmatic source with some contributions from meteoric water. 87Sr/86Sr from these calcite show values between 0, 7068 – 0, 7087, typical from upper crust. Hence, the Cerro dos Martins copper deposit is interpreted to have had origin by a mixing between a magmatic-hydrotermal fluids related to the alkaline-shoshonitic, post-collisional magmatic event (595-550 Ma) of the Dom Feliciano Orogeny and basement rocks. Exploration models for copper deposits in this shield should consider the alkaline magmatism in the genesis of deposits.

The State of Rio Grande do Sul has a complex Precambrian/Cambrian shield, which has been investigated for four decades. This complexity involves ages ranging from 2.55 Ga (possibly 3.3 Ga) to 550 Ma (and even 470 Ma). The three major juvenile accretionary episodes occurred at 2.55 Ga, 2.26-2.02 Ga and 900-700 Ma, while a continental-scale crustal reworking (collisional) orogeny occurred from 780 to 550 Ma. The three accretionary orogenies are known as the Jequié, Transamazonian and Brasiliano Cycles, respectively. The Brasiliano Cycle includes the collisional orogeny. Magmatism was tholeiitic low-K bimodal basic-acid in the Archean (Santa Maria Chico granulites), and evolved to tonalitic-trondhjemitic-granodioritic in the Paleoproterozoic (Encantadas Complex). During the Paleoproterozoic/Archean transition, komatiites and basalts were formed in greenstone belts (Passo Feio Sul Formation). The end of the Transamazonian Cycle was the beginning of a long period of tectonic quiescence, and the region remained in the interior of the Atlantica Supercontinent until the beginning of the Brasiliano Cycle at ca. 900 Ma (Passinho Diorite). This Neoproterozoic cycle displays two classical orogenic types, namely the São Gabriel accretionary orogeny in the western part of the State and Dom Feliciano collisional orogeny in its eastern part. Accretion generated juvenile tonalite-trondhjemite-granodiorite associations with related ophiolites (Cerro Mantiqueiras Ophiolite), while the collision formed the voluminous and mostly peraluminous and high-K calcalkaline granites of the Dom Feliciano orogeny. The waning stages of the orogeny were responsible for the outpouring of a very expressive silica-saturated volcanism and eventually finished with the Rodeio Velho basalts at 470 Ma. Comparable Paleoproterozoic/Neoproterozoic Precambrian terranes surround the shield in Uruguay, in Santa Catarina and in western Africa. Comparable Neoproterozoic juvenile and reworked terranes occur in NE Africa. Widespread indications of metals are a good sign of possible deposits, but the two major types of deposits are the orogenic epizonal Bossoroca gold deposit and the distal magmatichydrothermal Lavras/Camaquã copper-gold deposits.