UNIVERSIDADE DE BRASÍLIA - INSTITUTO DE GEOCIÊNCIAS
UNIVERSIDADE DE BRASÍLIA -
INSTITUTO DE GEOCIÊNCIAS
ABSTRACTS
DE CONTRIBUIÇÕES CIENTÍFICAS DO IG/UnB
ANO 2000
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THE PIRINEUS SYNTAXIS: AN EXAMPLE OF THE INTERSECTION OF TWOBRASÍLIANO FOLD-THRUST BELTS IN CENTRAL BRAZIL AND ITS IMPLICATIONSFOR THE TECTONIC EVOLUTION OF WESTERN GONDWANA
JOSÉ OSWALDO DE ARAÚJO FILHO
Revista Brasileira de Geociências(2000), 30(1): 144-148. (Contribution IGC-159)
Keywords: Fold-thrust belt, Brasíliano orogeny, syntaxis, structural analysis, accreted terranes.
The Pirineus Syntaxis (PSX) is a concave to the foreland (toward the craton) curve in the otherwise NS-trending structural grain of the erogenic province bordering the west side of the São Francisco craton. Its geometry reflects the intersection between two non-parallel foldthrust belts of different ages and style, formed during two separate non-coaxial deformation episodes during the Brasíliano Orogenic period of Neoproterozoic (probably around 900-600 Ma). These ages are not well constrained, since very little radiometric dating is available. The two limbs of the syntaxis are here interpreted as the younger and narrower Northern Brasília belt (NBB), and the older and more complete Southern Brasília belt (SBB). Rocks involved in the deformation that formed the PSX include the Paranoa, Canastra and Araxá Groups, generally accepted as passive margin environment and more restricted to the SBB. The Rio do Peixe Group is a volcanic-sedimentary sequence that prevalently occurs in the SBB, while the Intrusive Rock Suite and Related Metasedimentary Rocks predominantly occur in the NBB. The Granite-gneiss complex of the Goiás Massif (GCM) to the west, comprises the metamorphic core of the syntaxis. The collision of the Rio de la Plata (Paranápanema?) craton with the São Francisco craton formed the SBB, that preceded the final collision and accretion of the GCM with the SFC that formed the NBB. A weak collision of GCM with the southern border of the Amazonian craton determined the last deformation phase in the syntaxis. This is the first attempt to subdivide the Brasíliano of central Brazil and thus contributed to a better understanding of the evolution of western Gondwana. The conclusions of this paper were primarily based on field structural analysis.
STRUCTURALLY-CONTROLLED GOLD OCCURRENCES IN THE SOUTHERN BRASÍLIA FOLD-THRUST BELT OF CENTRAL AND SE-GOIÁS
JOSE OSWALDO DE ARAÚJO FILHO
RAUL MINAS KUYUMJIAN
Revista Brasileira de Geociências(2000),30(2): 289-292. (Contribution IGC-169)
Keywords: gold, shear zones, structural control, Goiás
The study of occurrences of hydrothermal alteration products (tin, emerald, barite, and gold), in an area of approximately 28,7000 km2 in the Southern Brasília fold-belt of Goiás, Central Brazil, reveals a strong structural control for these occurrences. Despite the lack of major deposits in the study area, gold has been the main ore mined both in primary lodes and placer deposit, since the 17th century. E-W-trending intermediate-angle (30°-60° dip angle) thrust-sense shear-zone and NW- and NE-trending high-angle (>60° dip angle) strike-slip shear-zone veins are the main Brasíliano host structures of gold deposits. We propose that gold mineralizations were generated as primary deposits mostly in pegmatite and quartz veins associated with the emplacement of pre-Brasíliano S- and early-Brasíliano A-type granitoids. Pervasive Brasíliano deformation dispersed and relocated gold in preferential EW-trending shear zones associated with lateral ramps of E-verging thrusts, frontal ramps of S-SE-verging thrusts, and with NW- and NE-trending shear zones, associated with late-Brasíliano high-angle strike-slip-sense shear zones. These structural controls may indicate targets for further gold exploration in the study area.
a Maira Barberi
b Maria Lea Salgado-Labouriau
c Kenitiro Suguio
a Centro de Estudos e Pesquisas
Biológicas, Universidade Católica de Goiás, CP 86, Goiânia, GO 74605-010, , ,
Brazil
b Instituto de Geociências, Universidade de Brasília, Campus
Universitario, 70910 900 Asa Norte Brasília, DF 70910-900, , , Brazil
c Instituto de Geociências, Universidade de São Paulo, CP 11348,
São Paulo, SP 05422-970, , , Brazil
Journal of South American Earth
Sciences(2000),13(3):241-254.
Texto Integral - pdf
A análise palinológica de uma extensa área de vereda no topo de um platô próximo à cidade de Brasília (15°34'S e 47°35'W) mostra uma seqüência de mudanças de vegetação e clima nos últimos 30.500 anos. A área é a nascente de alguns afluentes das principais bacias hidrográficas do Brasil: Amazonas, Paraná e São Francisco.
A parte basal do testemunho de sondagem é constituída de argila e não contem palinomorfos. Há 30.480±100 anos AP começou a deposição de turfa na área. Entre 25.790±70 e ca. 24.200 anos AP grãos de pólen e algas são abundantes na turfa, indicando a presença de vegetação palustre cercada de cerrado aberto e manchas de mata de galeria no cume do platô. Entre ca. 24.000 e 21.450±100 anos AP a vegetação era mais densa que a atual, com plantas aquáticas em abundância e com algumas árvores de clima mais frio, o que sugere um clima mais úmido e provavelmente mais frio que o presente. A ausência de pólen de Mauritia durante todo esse tempo indica que não havia vereda na área.
A diminuição drástica da concentração e diversidade de pólen, algas e esporos entre ca. 21.000 e 7.220±50 anos AP sugere uma fase seca que resultou em um clima mais sazonal e na desertificação do cume do platô. Pouco depois de 7.000 anos AP a quantidade de pólen e esporos aumenta e o pólen de Mauritia começa a ser encontrado nos sedimentos, o que mostra que a vegetação principiou novamente a crescer na área; de ca. 5.600 anos AP até o presente, vários tipos de vegetação ocorrem no cume do platô, da mesma forma que hoje: cerrado, matas secas e vereda com buritis. O clima nesta fase era semelhante ao atual.
Partículas de carvão são encontradas em todo o sedimento, porém, são muito mais freqüentes depois de 2.600 anos AP sugerindo que nesse tempo houve pelo menos três grandes queimadas na vegetação local.
A seqüência de vegetação e dos eventos climáticos encontrados nesta análise segue o padrão geral assinalado para outros lugares do Brasil Central. Porém, a fase seca está muito mais marcada na Vereda de Águas Emendadas, provavelmente devido à sua posição sobre um platô.
The palynological analysis from an extensive palm swamp on top of a plateau, close to the city of Brasília (15°34'S and 47°35'W), has shown a sequence of vegetational and climatic changes in the last 30,500 years. The area is the headwater of tributaries of the main Brazilian hydrographic basins: Amazonian (north), Paraná (south), and São Francisco (east).
At the base of the analysed core there is a clay deposit devoid of palynomorphs. At 30,480±100 yr BP, peat started to form in the area. Between 25,790±70 and ca. 24,200 yr BP, pollen grains and algal remains are abundant, indicating a marsh surrounded by open cerrado and patches of gallery forest on top of the plateau. Between ca. 24,000 and 21,450±100 yr BP, vegetation was denser than at present, with abundant marsh plants and the occurrence of some trees of a cooler climate, suggesting a climate more humid than at present and probably cooler. The absence of Mauritia pollen during that time indicates the palm swamp ("vereda") characteristic of the present-day area did not occur.
The drastic decrease in concentration and diversity of palynomorphs from ca. 21,000 yr BP until 7,220±50 yr BP suggests a dry climatic phase that resulted in drier, more seasonal climate and the desertification on top of the plateau. Shortly after 7,000 yr BP, the abundance of pollen grains and spores increased and Mauritia pollen occurred in the sediments, suggesting increased humidity and the beginning of the palm swamp; from ca. 5,600 yr BP onwards, the present types of vegetation grew on top of the plateau: arboreal cerrado, gallery forest, and palm swamp. The climate of this phase was similar to that of the present.
Charcoal particles are found throughout the record but they increase mainly after 2,600 yr BP, suggesting at least three extensive episodes when the local vegetation was burned.
The sequence of events found here follows, in general, the same pattern described for other areas in central Brazil. However, the dry climatic phase is more strongly marked in Vereda de Águas Emendadas, probably due to its location on top of a plateau.
GRAVITY INVERSION ON GRANITE-GREENSTONE BELT AREAS: CRIXÁS, GUARINOS AND PILAR DE GOlÁS, BRAZIL
MARCELO DE LAWRENCE BASSAY
BLUM
AUGUSTO CESAR BITTENCOURT PIRES
ROBERTO ALEXANDRE VITORIA DE MORAES
Revista Brasileira de Geociências(2000), 30(3): 547-550. (Contribution IGC-156)
A gravity survey was carried out in the central and northern state of Goías portions, covering an area of 9000 km2 of granitegreenstone belt rock associations. Using a fixed density of 2.67 g/cm3 and removing a second-degree trend surface for modeling and inversion, we determined the residual anomalies. Each anomaly was associated to well-known geological surface information, and airborne magnetic data, and density determinations. Final solution was obtained in by interactive inversion. The initial models were generated in 2-D, for latter 2.5-D inversion, interpretation and geological sections sketching based on geologic knowledge. The anomalies were also inverted in 3-D in an automatic process. Estimated depths are coherent with those obtained in the 2.5-D inversion, indicating the targets can reach 2 km deep.
THE KAMAFUGITE-CARBONATITE ASSOCIATION IN THE ALTO PARANAÍBA IGNEOUS PROVINCE (APIP) SOUTHEASTERN BRAZIL
JOSE AFFONSO BROD
SALLY A. GIBSON
ROBERT N. THOMPSON
TEREZA CRISTINA JUNQUEIRA-BROD
HILDOR JOSE SEER
LÚCIA CASTANHEIRA DE MORAES
GERALDO RESENDE BOAVENTURA1
Revista Brasileira de Geociências(2000),30(3):404-408. (Contribution IGC-110)
Keywords: kamafugite, carbonatite, phlogopite.-picrite, ultrapotassic, APIP
The Late-Cretaceous Alto Paranaíba Igneous Province (APIP) comprises a variety of ultrapotassic rock-types, including kimberlites, lamproites and large volumes of kamafugites, in addition to a number of carbonatite-bearing plutonic alkaline complexes. Phlogopite-picrites are ultramafic fine-grained rocks typically composed of olivine phenocrysts set in a groundmass of phlogopite, carbonate, perovskite, apatite and chromite. They occur as dyke swarms in the carbonatite complexes, but are also scattered throughout the Province. The phlogopite-picrites represent the peralkaline, ultrapotassic, carbonate-rich, silicate magmas parental to the carbonatite-bearing complexes, and have strong chemical affinity with kamafugites. Together with petrographic similarities observed between silicate rocks from the carbonatite complexes and xenoliths occurring in APIP kamafugites, this provides a strong link between kamafugitic and carbonatitic magmatism in the Province.
THE SEDIMENTOLOGY, STRATIGRAPHY AND TECTONIC CONTEXT OF THE SÃO FRANCISCO SUPERGROUP AT THE SOUTHWESTERN DOMAIN OF THE SÃO FRANCISCO CRATON, BRAZIL
PAULO DE TARSO AMORIM CASTRO
MARCEL AUGUSTE DARDENNE
Revista Brasileira de Geociências (2000), 30(3): 439-441. (Contribution IGC-106)
Keywords: Bambuí Group - São Francisco Craton - fan delta system - foreland basin
Neoproterozoic metasedimentary rocks of the São Francisco Supergroup occur in the Southwest part of São Francisco Craton. Three informal lithostratigraphic units composed mainly by psephites, pelites and carbonates represent the São Francisco Supergroup. The last two units formed in storm influenced muddy shelf and platform/ramp systems, respectively. The psephitic unit crops out as isolated bodies nearby the external zone of the southernmost part of Brasilia Fold and Thrust Belt. Its origin is related to fan delta systems that developed eastward, in a foreland basin, as a result of erosion of metamorphic rocks of Brasilia Fold and Thrust Belt.
Condições de cristalização dos charnockitos ígneos no Nordeste do Brasil
Celino,J.J.
Botelho,N.F.
Geología Colombiana,Santa Fé de Bogotá, 1(25): 199 - 220.
Palavras chave: Petrologia, Química mineral, Charnockitos, Termo-barometria
Las rocas máficas y félsicas con ortopiroxeno (hiperstena) son los componentes importantes de la zona de transición que separa el Cráton Arqueano de San Francisco y el Cinturón Móvil Costero Neoproterozóico, en el extremo sur del estado de Bahia, Brasil. Las charnockitas presentam texturas relictas magmáticas. Los cálculos termobarométricos hacen pensar en temperaturas entre 700 - 800 C y pressiones de 4,5 a 5,5 kbar (12 a 17 km de profundidad) para la generácion de estas rocas. Evidencias de campo, complementadas por datos mineralógicos y texturales, hacen pensar en un origen asociado a la fúsion parcial de corteza continental en las condiciones de la facies granulita.
GENESIS OF NEOPROTEROZOIC GRANITOID MAGMATISM IN THE EASTERN ARAÇUAÍ FOLD BELT, EASTERN BRAZIL: FIELD, GEOCHEMICAL AND Sr -Nd ISOTOPIC EVIDENCE.
JOIL JOSE CELINO
NILSON FRANCISQUINIBOTELHO
MARCIO MARTINS PIMENTEL
Revista Brasileira de Geociências(2000), 30(1):135-139. (Contribution IGC-157)
Keywords: granites, petrogenesis, geochemistry, isotopes, Neoproterozoic.
The Neoproterozoic granitoid magmatism of the Araçuaí Fold Belt (AFB) is an important element for the discussion of the evolution of this belt and its relationships with the African counterpart, the West Congo Belt. In the eastern part of the AFB, four different granitoid suites were recognized. The Nanuque Suite (NQS) comprises syn-tectonic peraluminous cordierite-bearing monzogranites. The São Paulinho Suite (SPS) consists of Th-rich peraluminous two mica or biotite- only granitoids. Calc-alkalic granitoids with magmatic epidote were grouped into the Itagimirim Suite (ITS) and post-tectonic charnockitic rocks were grouped into the Salomão Suite (SLS). Sm-Nd mineral isochrpn and Rb-Sr isochron yielded ages of respectively 761 Ma and 714 Ma for the Nanuque and São Paulinho suites. The general Sr-Nd isotopic characteristics of the granitoid suites and some country rocks indicate that the parental magmas were mostly the product of melting of the Paraíba do Sul metasediments. The chronological and genetic evolution of the Neoproterozoic plutonism can be envisaged in a model of east-dipping subduction zone, followed by a continental collision between the Brasiliano/Pan-African São Francisco (Brazil) and Congo (Africa) cratons and final episodes of uplift and collapse.
Nd ISOTOPE SYSTEMATICS RELATED TO PROTEROZOIC EVOLUTION OF THE CENTRAL RIBEIRA BELT IN THE STATE OF SÃO PAULO, SE BRAZIL
ELTON LUIZ DANTAS
PETER CHRISTIAN HACKSPACHER
ALLEN HUTCHESON FETTER
KEI SATO
MÁRCIO MARTINS PIMENTEL
ANTONIO MISSON GODOY
Revista Brasileira de Geociências (2000),30(1):140-143. (Contribution IGC-158)
Keywords: Nd signature, Ribeira Belt and Isotopic Evolution
New Sm/Nd data from different lithologic units of the Ribeira Belt (RB), southeastern Brazil, provide important information about the crustal architecture and geologic evolution of the region. Nd data of basement gneisses of the RB indicate that they represent Paleoproterozoic crustal growth, whereas those of the supracrustal sequences suggest provenance from both Paleoproterozoic and younger sources. On the basis of their Nd signatures, felsic brasiliano plutons derived largely from Paleoproterozoic basement, but systematic variations in their T model ages as a function of their chemistry may reflect differences in fractionation, vertical lithospheric heterogeneity, or possibly some contributions from the asthenospheric mantle.
FLUIDS RELATED TO ENDOSKARN FORMATION PROCESSES IN THE GUANHÃES AREA, MINAS GERAIS STATE, BRAZIL
MARIA LOURDES SOUZA FERNANDES
KAZUO FUZIKAWA
JOSE MARQUES CORREIA NEVES
ESSAID BILAL
CRISTINA MARIA WIEDEMANN-LEONARDOS
Revista Brasileira de Geociências(2000),30(2): 327-330. (Contribution IGC-044)
Keywords: Late Paleoproterozoic anorogenic granites, Borrachudos granite, endoskarn formation, fluid inclusions
The Borrachudos Suite consists of a group of A-type granitic bodies intruding Archean and Paleoproterozoic rocks outcropping in the central part of Minas Gerais State. Their age range from 1.77 to 1.67 Ga and the emplacement is related to the opening of the Espinhaco rift. On the eastern border of Morro do Urubu plutonite which is part of the Suite and crops out for an extension of 2 km to the southeast of Guanhães town, occurs a greenish crystalline rock. A continuous transition from Borrachudos granite to these rocks can be observed. Five different alteration stages of transformation are defined on the basis of mineral suites, mineral and total rock chemistry. These features allowed the interpretation of these rocks as endoskarnites. Their microscopy showed the presence of areas with abundant fluid inclusions (FI) in quartz. FI distribution, their morphological and compositional characteristics are the same in all five stages, except that the frequency drops rapidly from stage I to V. Compositionally the inclusions are carbonic, aqueo-carbonic, and aqueous and they are all associated in parallel planes. Either in the first or second types of FI the carbonic phases present similar microthermometric behavior and values: Th(CO2)=+27.0 to +29.5°C and Tm(CO2)=-56.7 to -57.6°C. The latter indicates an almost pure CO2 which was confirmed by micro-Raman spectroscopy. The former defines the 0.7 to 0.75g/cc isochores, which at 800°C temperature, calculated from amphibole crystallization at magmatic stage as the upper limit for the metassomatic process, indicate a pressure of 3.0 to 3.5 kbar. This range of pressure points to the process having occurred at maximum depth of roughly 10 km. The monophase aqueous inclusions presented Tm(ice)=-1.3°C indicating a solution of low salinity (~2.2wt% NaCl equivalent) which might be even lower as this Tm is dependent on the pressure developed when the fluid is frozen. The three types of inclusions are interpreted as products of post-formational changes of a homogeneous aqueo-carbonic fluid trapped above the solvus of H2O-CO2 -NaCl(<~2.6wt%) system. The occurrence of deuteric carbonates and their dissolution by fluids from granites may have been the source of CO2 in metasomatized rocks.
Ferreira Filho & Pimentel,2000
C.F. Ferreira Filho
M.M. Pimentel
Journal Of South American Earth Sciences(2000),13(7):647-659
Keyword(s): Isotope systematics; High-grade metamorphism; Layered complex; Mafic; Ultramafic; REE
RESUMO
Os complexos acamadados de Barro Alto, Niquelândia e Cana Brava constituem grandes intrusões de idade proterozóica localizadas nos estados de Goiás e Tocantins. Os complexos acamadados foram afetados por metamorfismo de alto grau com deformação dúctil associada durante o Neoproterozóico (770--795 Ma). Estudos recentes mostram que o Complexo de Niquelândia constitui-se de dois sistemas magmáticos distintos e tectonicamente justapostos; denominados de Série Acamadada Superior (US) e Série Acamadada Inferior (LS), sendo o primeiro mais jovem e situado a oeste enquanto o segundo é mais antigo e situado a leste. Estudos geocronológicos anteriores em rochas da LS sugerem uma idade Paleoproterozóica (ca 2,0 Ga) para este evento magmático. Novos dados de elementos traços combinados com isótopos Sm--Nd de rochas e minerais da US fornecem dados geocronológicos inéditos para o Complexo de Niquelândia. A idade de 1,35 Ga obtida neste trabalho para o magmatismo da US indica um evento muito mais jovem que o relacionado com a LS. A idade confirma a existência de dois complexos acamadados petrologicamente distintos e tectonicamente justapostos. O contato tectônico entre os dois sistemas magmáticos pode ser agora elevado à categoria de uma importante descontinuidade crustal entre o Paleo e o Mesoproterozóico.
The Barro Alto, Niquelândia, and Cana Brava Complexes are major Proterozoic layered intrusions in central Brazil that were affected by high-grade metamorphism with associated ductile deformation during the Neoproterozoic (770--795 Ma). Recent studies recognized that the Niquelândia Complex comprises two petrologically distinct and tectonically juxtaposed magmatic systems: a younger Upper Layered Series to the west and an older Lower Layered Series to the east. Previous geochronological studies on Lower Series rocks suggested a Paleoproterozoic (ca 2.0 Ga) age for the Lower Series magmatic event. New trace element data matched with Sm--Nd isotope data for Upper Series samples yielded well-constrained and original geochronological information. The 1.35 Ga age of the Upper Series magmatism reported in this paper indicates a much younger age of the Upper Series compared with the Lower Series. The tectonic contact between these two distinct magmatic systems is now raised to the category of a major Paleo-Mesoproterozoic crustal discontinuity.
Au/Ag RATIO VARIATIONS AT MINA III, MINA NOVA AND MINA INGLESA GOLD DEPOSITS, CRIXÁS GREENSTONE BELT, BRAZIL
PAULO DE TARSO FERRO DE
OLIVEIRA FORTES
ROQUE FERNANDES COELHO
GASTON GIULIANI
Revista Brasileira de Geociências (2000), 30(2): 246-250. (Contribution IGC-114)
Keywords: Au/Ag ratio, greenstone belt, Crixás, Brazil
The Mina HI and Mina Nova gold deposits occur at the transition between metabasic volcanic rocks and chemical and detrital metasedimentary rocks, while Mina Inglesa occurs within metakomatiites, all in the Archean Crixás Greenstone Belt. Mina III contains three main mineralized zones: the Upper (UOZ-ms), Lower (LOZ-qv and LOZ-cs) and Garnet (GOZ) Ore Zones. The UOZ-ms is composed of massive sulfide lenses within Fe-dolomitic marbles and Fe and Al-rich rocks. A concordant quartz vein within carbonaceous schists represents the LOZ-qv, while the LOZ-cs consists of an arsenopyrite-pyrrhotite-bearing carbonaceous schist close to the quartz-vein contact. The GOZ comprises concordant quartz veins within quartz-chlorite-muscovite-garnet schist. At Mina Nova, two main ore zones occur: the Orebody 1 (OB1-cs), consisting of an arsenopyrite-pyrrhotite-bearing carbonaceous schist and the Orebody 2 (OB2-cm), represented by a carbonate-muscovite schist lenses within carbonaceous schist). Mina Inglesa has one mineralized concordant quartz vein (MOB) within talc schists. Mineralogical characterization of the orebodies indicates gold association with arsenopyrite, pyrrhotite, chalcopyrite, carbonate, quartz, micas and oxides at the UOZ; with quartz, carbonaceous matter and arsenopyrite, and less frequently with carbonate, pyrrhotite and micas at the (LOZ-qv and LOZ- cs); mainly with quartz, micas and arsenopyrite, and more rarely with chalcopyrite and oxides at the GOZ; with arsenopyrite, pyrrhotite and carbonate at the OB1 ; with carbonate, muscovite and arsenopyrite at the OB2; and with quartz and galena at the MOB. Microprobe analysis in individual gold grains of all ore zones indicate very low values of other elements (less than 0.5 wt. %). The identified Au/Ag ratio variations are intimately associated with host minerals or mineral associations of each ore zone. Initial Au/Ag ratios in source-rocks, transport (hydrosulfide and chloride complexes) and deposition mechanisms (fluid-rock interactions and fluid immiscibility) may have had great importance in the observed chemical variations.
The Sm-Nd isotopic method in the geochronology laboratory of the University of Brasília
Gioia,S.M.C.L.
Pimentel,M.M.
Palavras-chave: Sm-Nd method; isotope dilution; ion exchange
chromatography; TIMS.
Anais da Academia Brasileira de Ciências(2000),72(2):219-245.
Abstract
Nd isotopes represent one of the best tools to investigate the processes involved in the evolution of the continental crust and mantle. This is due mainly to the similar geochemical behaviour of Sm and Nd, both light rare earth elements, which inhibits their fractionation during most varied geological processes. In order to carry out crustal evolution studies in central Brazil, the Sm-Nd isotopic method was implanted at the Geochronology Laboratory of the University of Brasília. The Sm-Nd separation methodology is basically that described in Richard et al. (1976), with the addition of some improvements. In this study we describe in detail the methodology used in Brasília. Precision and accuracy were checked with the international standards such as JB-3, BCR-1, BHVO-1 and La Jolla, and the following results were found: JB-3 (Nd=15.74 ppm and Sm=4.28 ppm), BCR-1 (143Nd/144Nd=0.512647±8, Nd=28.73 ppm and Sm=6.66 ppm), BHVO-1 (Nd=24.83 ppm and Sm=6.2 ppm) and La Jolla (143Nd/144Nd=0.511835±14).
DIAGENETIC/ANCHIMETAMORPHIC CHANGES ON THE PROTEROZOIC GLAUCONITE AND GLAUCONY FROM THE PARANOA GROUR MID-WESTERN BRAZIL
EDI MENDES GUIMARAES
BRUCE VELDE
STEVE HILLIER
ELIZABETH NICOT
Revista Brasileira de Geociências(2000), 30(3): 363-366.
Keywords: glauconite, diagenesis, anchizone, Proterozoic, mineralogy, chemistry, sedimentology.
Petrographic, X-ray and chemical studies were made on glauconies from sandstones belonging to the same tectonic and stratigraphic level of the Paranoá Group, a Proterozoic sedimentary unit of Mid-West of Brazil. Three types of mineralogically different glauconitic sandstones were recognized, i.e., white feldspatic, green-reddish ferric and green-grayish calcitic sandstones, with different content of glauconies. In each sandstone, glauconite shows a non-expansible structure, as well as high contents of Al and K, but other chemical features and mineralogical assemblages vary from one rock to another. Differences found in chemical composition of the glauconies from different rocks and their mineralogical evolution are attributed to lithologically dependent changes since all the studied sandstones have the same origin, and have undergone the same diagenetic/anchimetamorphic conditions.
EVIDENCE OF NEOPROTEROZOIC BACKARC BASIN DEVELOPMENT IN THE CENTRAL RIBEIRA BELT, SOUTHEASTERN BRAZIL: NEW GEOCHRONOLOGICAL AND GEOCHEMICAL CONSTRAINTS FROM THE SÃO ROQUE - AÇUNGUI GROUPS
PETER CHRISTIAN HACKSPACHER
ELTON LUIZ DANTAS
ÂNGELO SPOLADORE
ALLEN HUTCHESON FETTER
MARCOS AURÉLIO FARIA DE OLIVEIRA
Revista Brasileira de Geociências(2000),30 (1): 110-114. (Contribution IGC-134)
The Ribeira Belt (RB) of southeastern Brazil represents an important manifestation of the Brasiliano Orogeny formed during the assembly of West Gondwana. Contemporaneous sedimentation and volcanism within the RB provide a basis for helping understand its tectonic evolution and paleogeography. U-Pb monazite data from the basal metavolcanic rocks of the São Roque Group indicate a crystallization age of 628 Ma and the upper sequence is cut by a 605 Ma (U-Pb zircon) rhyolite intrusion. Zircon and monazite analyses of metavolcanic (mafic) rocks and from metagabbros of the lower Açungui Supergroup yield crystallization ages of 614 and 617 Ma, respectively. This supergroup is intruded by a 607 Ma granite. Geochemical signatures of basal mafic units in both sequences are characteristic of E-MORB subalkaline tholeitic basaltic rocks. Nd isotopic signatures of the metamafic rocks indicate that they were derived in part from the asthenospheric mantle (consistent with emplacement in an extensional setting), whereas the felsic bodies appear to have come from the melting of Paleoproterozoic lithosphere. The paleogeographic reconstruction of part of the RB suggests that the São Roque/Açungui groups represent extensional sequences, with features of backarc basins, which evolved during the syn-collisional phase of the Brasiliano Orogeny. These data support the hypothesis that we have a rapid evolution (10-20 Ma) between extensional and compressional tectonics during the geological history of the São Roque/Acungui Backarc.
THE MESOPROTEROZOIC VOLCANO-SEDIMENTARY SERRA DO ITABERABA GROUP OF THE CENTRAL RIBEIRA BELT, SÃO PAULO STATE, BRAZIL: IMPLICATIONS FOR THE AGE OF THE OVERLYING SÃO ROQUE GROUP
Juliani,C.
Hackspacher,P.
Dantas,E.L.
Fetter,A.H.
Revista Brasileira de Geociências(2000),30: 82-86. (Contribution IGC-112)
Keywords: U-Pb geochronology, metandesite, metarhyolite, Serra does Itaberaba Group, São Roque Group
One of the fundamental problems to understanding the evolution of volcano-sedimentary sequences in southeastern Brazil is constraining their depositional ages. Brasiliano tectonic and metamorphic overprinting either obscured or destroyed primary features, such as unconformities, as well as other geologic relationships. This problem is exemplified by the Serra do Itaberaba and São Roque groups, where the lack of data about the timing of their deposition has prevented resolution of proposed one- and two-stage geotectonic/depositional models. Recent U-Pb zircon data obtained from metavolcanic rocks in the São Roque Group indicate that it was deposited between 628 and 607 Ma. New U-Pb zircon data of 1395 + 10 Ma for a metandesite in the basal Morro da Pedra Preta Formation (Serra do Itaberaba Group) indicate the maximum age for the beginning of the deposition of the pelites overlying MORB-like basalt. A metarhyolite of the upper unit, the Nhanguçu Formation, contains two zircon populations. One yielded an age of 619 + 3 Ma, which defines the crystallization age of the rock, and the other an age of 1449 ± 3 Ma, interpreted as inherited xenocrystal grains from older units of the Serra do Itaberaba Group. The younger metarhyolite was affected only by the S, foliation, generated during the Brasiliano orogenesis, whereas the Middle Proterozoic metavolcano-sedimentary sequence records additional metamorphic and deformational events, confirming the presence of two different geotectonic cycles.
Mineral-chemistry of kamafugites and related rocks from the Águas Emendadas region, Goiás State.
TEREZA CRISTINA JUNQUEIRA-BROD
JOSE AFFONSO BROD
SALLY A. GIBSON
ROBERT N. THOMPSON
Revista Brasileira de Geociências(2000), 30(3):399-403. (Contribution IGC-108)
Keywords: Kamafugite; carbonatite; kalsilite; diatreme
In the Águas Emendadas Region, Goiás State, Brazil, Late Cretaceous alkaline magmatism produced a series of diatreme structures of kamafugitic affinity. These occurrences consist of a central breccia body, surrounded by lavas and intruded by dykes. Compositionally, the rocks vary from kamafugites, through melaleucitites and leucitites to basanites. The results of mineral chemistry study of olivine, clinopyroxene, feldspathoids, feldspars, perovskite, opaque minerals and carbonates confirms the kamafugitic character of the studied rocks and suggests a link with carbonatite magmatism.
MAGMATIC ARC AND ASSOCIATED GOLD, COPPER, SILVER, AND BARITE DEPOSITS IN THE STATE OF GOIÁS, CENTRAL BRAZIL: CHARACTERISTICS AND SPECULATIONS
RAUL MINAS KUYUMJIAN
Revista Brasileira de Geociências(2000), 30(2): 285-288. (Contribution IGC-168)
Keywords: mineral deposits, magmatic arc, Goiás, Central Brazil.
In the western portion of the State of Goiás, Central Brazil, base-met al deposits typical of volcanic-plutonic arcs occur. Upto-date studies suggest that porphyry Cu-Au, (volcanogenic ?) hydrothermal exhalative Cu-Au and stratiform, submarine exhalative Au-Ag-Ba are present. Although restricted, the present state of knowledge on the geology and mineralizations of the magmatic arc indicates potential resources for met allic mineral deposits in the Brasilia Belt.
MATRIX ROCK TEXTURE IN THE PITINGA TOPAZ GRANITE, AMAZONAS, BRAZIL
SARA LAIS RAHAL LENHARO
PETER JOHN POLLARD
HELMUT BORN
Revista Brasileira de Geociências(2000), 30(2): 238 - 241. (Contribution IGC-109)
Keywords: topaz granite, matrix rock texture, Pitinga
The porphyritic topaz granite of the Água Boa batholith at the Pitinga Mine contains rounded quartz and rectangular white and pink feldspar phenocrysts. The fine-grained groundmass is composed of quartz, white and pink feldspar, micas (biotite and Li-siderophylite) and topaz. Accessory minerals include zircon, fluorite, Fe-Ti oxides, RE-minerals, thorite, columbite-tantalite, cassiterite, xenotime and sulphides. The topaz granite is peraluminous, similar to low P2O5 topaz granites, shows some features resulting from post-magmatic metassomatic processes and is considered as a likely source of tin mineralizing fluids in the Água Boa Granite. Three subtypes of topaz granite have been identified, based on modal composition and petrographic features: 1) end-member with around 20% angular phenocrysts; 2) intermediate member with approximately 36% phenocrysts exhibiting dendritic margins; and 3) matrix rock containing around 68% phenocrysts and phenocryst aggregates with irregular and dendritic margins. The estimated amount of K-feldspar phenocrysts decreases at the expense of quartz and plagioclase phenocrysts from the matrix rock to the end-member type. Petrographically, the matrix rock contains relict quartz, K-feldspar and plagioclase aggregates of the older, medium- to coarse-grained granite facies. These aggregates are surrounded by irregular and anastomosing films of topaz granite displaying magmatic flow textures. Individual phenocrysts of quartz show evidence of partial absorption and subsequent dendritic growth. The model for the formation of the matrix rock at the Pitinga Mine considers that the partially crystallized, earlier facies were intruded by the more evolved topaz granite crystal mush, which underwent decompression during emplacement, causing compositional desequilibrium and later re-equilibration at low pressure associated with late fluid stage separation.
Ludka & Wiedemann-Leonardos,2000
FURTHER SIGNS OF AN ENRICHED MANTLE SOURCE UNDER THE NEOPROTEROZOIC ARAQUAI-RIBEIRA MOBILE BELT
ISABEL PEREIRA LUDKA
CRISTINA MARIA WIEDEMANN-LEONARDOS
Revista Brasileira de Geociências(2000), 30(1): 95-98. (Contribution IGC-129)
Keywords: mantle geochemistry, gabbro-norites, olivine coronas.
Isolated small bodies of Neoproterozoic coronitic gabbros crop out in the states of Espírito Santo (Jacutinga and Itaoca) and Rio de Janeiro (Amparo), in the central portion of the Araçuaí-Ribeira Mobile Belt. The rocks from Jacutinga and Itaoca are mainly gabbronorites and show a wide range of modal/textural varieties recording crystal-setting processes normally found in layered intrusions. The Amparo intrusion is formed by a coronitic rock, also present as layers in Jacutinga and Itaoca. The primary mineral assemblage consists of olivine, opx, cpx, plagioclase, ilmenite, Ti-magnetite and sulphides. Coronitic overgrowths related to the olivine-plagioclase reaction form concentric rims of opx, amphibole and simplectitic amphibole-spinel around olivine cores. They are all very similar in shape and composition pointing towards similar physical-chemical crystallization conditions. Different gepthermometric measurements yielded crystallization temperatures around 800°C, which are close to those calculated for primary opx-cpx pairs (800/940°C). The absence of regional deformational and metamorphic paragenesis is compatible with their intrusion into the middle/ lower crust during a late collisional phase. A long lasting cooling environment at a late magmatic stage induced the sub-solidus reactions. Low K sub-alkaline signatures with clear enrichment in some incompatible elements such as Ba, Sr and LREE associated with high 87Sr/86Sr initial ratios (around 0.706-0.708) are anomalous-chemical characteristics of these gabbros. Enriched mantle types (EM1, EM2 and HIMU) with high Sr-initial ratios are frequently found in the Southern Hemisphere. The chemical anomalies obtained in this study could be related to largescale phenomena, similar to the Dupal anomaly.
POST-COLLISIONAL MULTISTAGE MAGMATISM IN THE RIBEIRA MOBILE BELT: GEOCHEMICAL AND ISOTOPIC STUDY OF THE VARZEA ALEGRE INTRUSIVE COMPLEX, ESPÍRITO SANTO, BRAZIL
SILVIA REGINA DE MEDEIROS
CRISTINA MARIA WIEDEMANN-LEONARDOS
JULIO CEZAR MENDES
Revista Brasileira de Geociências(2000),30 (1):30-34.
Keywords: post-collisional, high K calc-alkalic, multistage magmatism
The Varzéa Alegre Intrasive Complex (VAIC) corresponds to a post-collisional (late erogenic) pluton, related to the Brasiliano cycle, situated in the central part of the Espírito Santo State. It intrudes amphibolite to granulite facies metamorphic rocks of the Ribeira Belt. Two distinct domains were recognised in this zoned pluton: an inner domain with opx-gabbro, monzogabbro, diorite, quartz-diorite and megaporphyritic granite, and an outer one comprising an irregular and large ring of charnockitic rocks. Geochemical data from the former reveal medium to high-K calc-alkalic rocks, enriched in incompatible elements, mainly Ba, Sr, La, Ce and Pb and partially depleted in HFS elements. The charnockitic rocks show a high-K alkali-calcic signature; they are rich in Ba, K and some HFS elements, such as Zr, P and Nb. The incompatible element enrichment detected in the rocks of the VAIC has been reported for several intrusions from this part of the Ribeira Belt. A Rb-Sr isochronic age of 508±12 Ma was determined for the megaporphyritic granite. The TDM. model age varies from 1.3 Ga (opx-gabbro) to ca. 1.67 Ga (charnockitic rocks), which can be related to an important Mesoproterozoic crustal event. The calculated T model ages of the cogenetic opx-gabbros and intermediary rocks is ca. 1.0 Ga, interpreted as the time when the basic magma was extracted from the source.
THE TOPAZ-ALBITE GRANITE AND RELATED ROCKS FROM THE Sn-In MINERALIZED ZONE OF MANGABEIRA GRANITIC MASSIF (GO, BRASIL)
MARCIA ABRAHAO MOURA
NILSON FRANCISQUINIBOTELHO
Revista Brasileira de Geociências(2000), 30(2): 270-273. (Contribution IGC-160)
Keywords: Topaz-albite granite, topazite, Indium, geochemistry, Goiás Tin Province
The Mangabeira Massif is part of the Goias Tin Province and is constituted of an A-type Paleoproterozoic biotite granite, evolved granites and greisens. The Main Greisenized Zone of the massif (MGZ) is composed of the less evolved Li-siderophyllite g2d granite, the topaz-albite granite (TAG), Sn-mineralized greisens and a quartz-topaz rock, with anomalous indium contents (up to 0.4 wt%). Micas are important for the characterization of each facies and define a phengite-zinnwaldite series, so far only described in the Parana Subprovince rocks. Late/post-magmatic processes affected the MGZ rocks, causing the mobility of many elements, except TiO2, Ta and Th . The topaz-albite granite (TAG) is similar to the rare topaz granites and has derived from g2d granite by magmatic differentiation. TAG is subsolvus, leucocratic, equigranular, medium-grained and contains essential quartz, albite, topaz, microcline and zinnwaldite. Topaz occurs either as inclusions (20mm) in albite or as large subhedral grains. The accessory minerals are monazite, cassiterite, zircon and magnetite. In the Qz-Ab-Or system, TAG samples plot near the minimum for 0 wt. %F. Chemically TAG is characterized by molar Na/K mainly >1, A/CNK ratio between 1.3 and 1.5, high contents of F, Li, A12O3, Rb, Zn and Sn and low TiO2, MgO, P2Oy CaO, Zr, Ba and Sr. The granites REE pattern is flat with a pronounced Eu anomaly. The quartz-topaz rock occurs associated with the TAG and resembles topazites, interpreted as magmatic rocks in the literature, but it can also be the product of topaz-albite granite hydrothermal alteration. The rock also contains sphalerite, wolframite, lollingite, chalcopyrite, bismutinite, galena, stannite, tennantite, In minerals (yanomamite, roquesite and dzhalindite) and hydrated arsenates of Sn, U, Ba, K, Pb and Bi.
THE ARENOPÓLIS-MARA ROSA GOLD-COPPER BELT, NEOPROTEROZOIC GOIÁS MAGMATIC ARC
CLAUDINEI GOUVEIA DE OLIVEIRA
CLAUDIA LIMA DE QUEIROZ
MÁRCIO MARTINS PIMENTEL
Revista Brasileira de Geociências(2000), 30(2): 219-221. (Contribution IGC-007)
Keywords: Goiás magmatic arc, gold and copper-gold mineralization, orogenic evolution.
The Goiás Magmatic Arc hosts several gold and copper-gold deposits, which were originated during several stages of the tectonic, magmatic and metamorphic evolution of this Neoproterozoic Arc, between ca. 0.9 and 0.6 Ga. The nature of these deposits, their spatial distribution, and timing of mineralization agree with the model involving the continuous evolution of a collisional belt. They also display some of the characteristics of the orogenic gold deposits. The Zacarias Au-Ag-Ba Deposit was originated by volcano-exhalative processes in an intra-oceanic subduction setting, whereas the Chapada Cu-Au Deposit (porphyry copper-type mineralization) is connected with plutonic rocks also associated with an intra-oceanic arc. The Posse Au Deposit was formed during the syn-collisional phase, shortly following the metamorphic peak of the Brasiliano Orogeny (ca. 630 Ma). Finally, the genesis of the Mundinho Au-Cu-Bi Mineralization is related with the emplacement of granite intrusions associated with post-orogenic extension.
THE MARA ROSA GOLD DISTRICT, CENTRAL BRAZIL
NELY PALERMO
CLAUDIO GERHEIM PORTO
CARLOS NOGUEIRA DA COSTA JR.
Revista Brasileira de Geociências (2000), 30(2): 256-260. Contribution IGC-127
Keywords: Gold, Volcanic Sequence, Mara Rosa, Brazil
In the Mara Rosa District, three different magmatic series have been identified and all of them host gold mineralization. The Mara Rosa Metavolcano-Sedimentary Sequence constitutes a poorly differentiated calc-alkaline magmatic series typical of arc settings. It is arranged into three belts, forming synclinal structures and was affected by high-grade amphibolite facies metamorphism. Early intrusive rocks of the Posse Unit show an alkaline affinity and host the epigenetic Posse Gold Deposit, which is associated with a late hydrothermal event, preferentially where earlier shear zones are present. Early hydrothermal alteration has produced an aluminous, sulphide rich greisen rock, which is barren of gold. Slices of an allochthonous and more magnesian sequence are associated with metachert and graphitic schists that host the syngenetic Au-Ag Zacarias mineralization and extensions. Gold-bearing quartz veins were emplaced before the second main isoclinal folding event. Supergene ore has played an important role in the mining activity of the Mara Rosa Region, since it represents the majority of the ore exploited. It is possible that yet undiscovered deposits lay hidden underneath the extensive lateritic cover.
THE NEOPROTEROZOIC GOIÁS MAGMATIC ARC, CENTRAL BRAZIL: A REVIEW AND NEW Sm-Nd ISOTOPIC DATA
MÁRCIO MARTINS PIMENTEL
REINHARDT ADOLFO FUCK
SIMONE MARIA COSTA LIMA GIOIA
Revista Brasileira de Geociências(2000), 30(1): 35-39. (Contribution IGC-067)
Keywords: Goiás Magmatic Arc, Neoproterozoic, Brasiliano
In this study we review the main characteristics and geochronological/isotopic data of metaigneous rocks of the juvenile Neoproterozoic Goiás Magmatic Arc in central Brazil. Some new Sm-Nd isotopic data are also presented for both the southern (Arenopolis) and northern (Mara Rosa) sections of the arc. In the south, granitoids of the Choupana-Turvania area yielded a Sm-Nd whole-rock isochron age of 863 ± 97 Ma and Nd(T) of +4.1. T model ages vary between 0.94 and 1.13 Ga. Metavolcanic rocks in the Pontalina region have a Sm-Nd whole-rock isochron age of 762 ± 77 Ma and Nd(T) of+2.9. TDM values are between 0.96 and 1.10 Ga. In the northern section of the Goiás Arc, mylonitic gneisses of the Serra Azul ridge, an important N30E snear zone, were investigated and have a Sm-Nd isochron age of 3058 ± 120 Ma and initial Nd value of ca. +2.1. This data suggests that the Serra Azul ridge might represent either a mylonitized fragment of the Archaean terranes exposed just to the south, or the sialic basement of the Araguaia Belt supracrustals, along the eastern margin of the Amazon Craton. The geochronological data available so far indicate a long history of arc formation and amalgamation on the western margin of the São Francisco- Congo continent during the Neoproterozoic. The history of convergence of continental masses is partially coeval with the fragmentation of Rodinia, indicating that the western margin (present geographic reference) of that continent occupied a peripheral setting in the Rodinia supercontinent.
RE-APPRAISAL OF THE SANTA RITA GREENSTONE BELT STRATIGRAPHY, CENTRAL BRAZIL, BASED ON NEW U-Pb SHRIMP AGE AND Sm-Nd DATA OF FELSIC METAVOLCANIC ROCKS
MARCIO MARTINS PIMENTEL
HARDY JOST
RICHARD ARMSTRONG
REINHARDT ADOLFO FUCK
SERGIO LUIZ JUNGES
MARCELO GONÇALVES RESENDE3
Revista Brasileira de Geociências(2000), 30(1): 157-160. (Contribution IGC-176)
Keywords: Goiás, Greenstone Belts, U-Pb SHRIMP, stratigraphy
The Santa Rita greenstone belt represents one of the supracrustal belts of the Archaean terranes of Goiás, central Brazil. The stratigraphic sequence of this greenstone belt comprises a lower unit made of komatiites and basalts and an upper metasedimentary unit made of carbonaceous schist, chert, iron formation and marble, unconformably overlain by clastic metasedimentary rocks. Felsic metavolcanics occur at the interface between the metabasalts and the upper metasedimentary pile. U-Pb SHRIMP age for zircons from the felsic metavolcanics reveal that it is not part of the Archaean sequence, but represents the product of a Mesoproterozoic (1580 ± 12 Ma) magmatic event. Sm-Nd isotopic data (initial eCHUR values between -10.5 and -14.9) and T values of 3.0 and 3.2 Ga, within the range of the surrounding TTG terranes, indicate that the original felsic magmas were produced by re-melting of Archaean crust. The data demonstrate that the Goiás greenstone belt contains infolded and imbricated Proterozoic rocks, as previously suggested by Sm-Nd isotopic analyses of some of the upper detrital metasedimentary rocks.
MÁRCIO M. PIMENTEL
DANIELLE P. FISCHEL
REINHARDT A. FUCK
Instituto de
Geociências, Universidade de Brasília, Brasília.
Anais da Academia Brasileira de Ciências(2000),
72(4): -
ABSTRACT
The Tocantins Province represents
a large Neoproterozoic orogen in central Brazil. The Brasília Belt consitutes
the eastern half of the province, along the western margin of the São Francisco
Craton. High-grade rocks occur in three mafic-ultramafic complexes (Barro Alto,
Niquelândia and Cana Brava), as well as in the Anápolis-Itauçu Complex (AIC), in
the central-southern part of the belt.
The AIC includes: (i) orthogranulites represented by mafic-ultramafic complexes,
charnockites, tonalitic gneisses, and (ii) supracrustal granulites including
sillimanite-garnet gneisses, marbles and quartzites. Narrow volcano-sedimentary
belts made of amphibolites and micaschists are also recognized. Intruded into
the high grade rocks are a large number of peraluminous granites, some of which
also display high grade mineral assemblages. The rocks of the AIC have been
traditionally been interpreted as part of the ancient
(Archaean/Paleoproterozoic) sialic basement of the Brasília Belt sediments.
The studies have demonstrated that the high-grade metamorphic event which
affected rocks of the mafic-ultramafic complexes occurred at ca. 760-780 Ma
(U-Pb zircon and Sm-Nd garnet ages). Preliminary Sm-Nd data for the AIC
granulites, on the other hand, indicate that the metamorphic mineral assemblages
were formed at ca. 620 Ma.
New U-Pb zircon ages for the AIC rocks indicate that: (i) the granites
crystallized at ca. 630-640 Ma (U-Pb ages of igneous zircon crystals), and (ii)
the metamorphism happened roughly at the same time (ca. 630 Ma).
The data available suggest, therefore, that two events of high-grade
metamorphism took place in the Brasília Belt during the Neoproterozoic. The
earlier event (ca. 760-780 Ma) is well documented in granulitic rocks of the
northern segment of the belt (Barro Alto, Niquelândia and Cana Brava complexes),
whereas the younger (ca. 630 Ma) is registered in granulites of the AIC, in the
southern part of the Brasília Belt. The 760-780 Ma event is preliminarily
interpreted as the result of the collision between the Goiás Magmatic Arc and
the western margin of the São Francisco continent, and the ca. 630 Ma as
indicative of final ocean closure.
METAKOMATIITIC AND META-ULTRAMAFIC ROCKS FROM THE RIO MANSO REGION, MINAS GERAIS: GEOLOGY TEXTURES AND METAMORPHISM
Pinheiro,S.O.
Nilson,A.A.
Revista Brasileira de Geociências(2000),30(3):421-423. (Contribution IGC-171)
Keywords: Komatiite, spinifex, Morro da Onça, ultramafic rocks, metamorphism.
The study area is located in the southwestern part of the "Quadrilátero Ferrífero", south of Serra do Curral, Minas Gerais (Brazil). Three major units occur in the area: Granite-Gneiss Complex, Nova Lima Group and intrusive bodies. The Nova Lima Group consists of metakomatiite and metakomatiitic basalt, meta-ultramafic rocks devoid of spinifex texture, rare iron formation, amphibolites and metasedimentary rocks. It comprises mainly meta-ultramafic rocks without primary igneous texture relicts and subordinate metakomatiitic rocks. Metakomatiites occur only in Morro da Onga, and display spinifex texture, especially of the random type. Thin layers interpreted as probable metacumulate parts of flows (B zone) occur locally. A structure that may correspond to a komatiite lava tube is also exposed, Metakomatiites are typically peridotitic komatiites and subordinately komatiitic metabasalts; the latter may exhibit pillow-lava structures. Progressive metamorphic evolution began with a Mg-chlorite + tremolite-actinolite paragenesis. In a second stage orthopyroxene and olivine porphyroblasts were developed in a Mg-hornblende groundmass. Progressive increase of Al in chlorite contributed to olivine and rare anthophyllite blastesis in the initial phases of the progressive metamorphism. Late metasomatism led to serpentinization and talcification. Amphibole plays an important role when the modal/textural relations of the highest-grade metamorphic parageneses are considered. Al enrichment of Ca-amphibole through tschermak and edenite replacements led to consumption of olivine to yield pyroxene and to spinel consumption through its incorporation as the aluminous component of amphibole. Thus the area consists of abundant meta-ultramafic rocks devoid of igneous texture relicts and subordinate komatiitic flows with thin cumulate zones. The former may partly correspond to an ultramafic layered sequence.
POLYMETAMORPHIC HISTORY OF THE CRIXÁS-AQU GNEISS, CENTRAL BRAZIL: SHRIMP U-PB EVIDENCE FROM TITANITE AND ZIRCON
CLAUDIA LIMA QUEIROZ
NEIL J. MCNAUGHTON
R. FLETCHER
HARDY JOST1
MARK E. BARLEY2
Revista Brasileira de Geociências (2000),30(1):40-44. (Contribution IGC-073)
Keywords: Archean, Central Brazil, gneiss complexes, geochronology, titanite, SHRIMP
A sample of the Crixás-Açu gneiss in Central Brazil contains protolith and metamorphic zircons, and two generations of metamorphic titanite. SHRIMP U-Pb data of these different mineral generations indicate the following temporal sequence: tonalitic magmatism at 2817 ± 9 M.y derived from an older source region (3050 to 2930 M.y zircon cores); Archaean metamorphism at 2772 ± 6 M.y (from zircon) with cooling to the blocking temperature of titanite (at 2711 ± 34 Ma); followed by Palaeoproterozoic metamorphism and weak fabric development at 2011 ± 15 Ma, and a possible Neoproterozoic metamorphism. The field relations and these age data indicate the poly metamorphic history of the area and demonstrate the value of in situ age determinations on well-characterized rocks.
HYDROTHERMAL FEATURES IN THE CATALÃO I CARBONATITE COMPLEX, GOIÁS
CARLOS C. RIBEIRO(*)
JOSÉ C. GASPAR (*)
(*) Instituto de Geociências,
Universidade de Brasília
Anais da Academia Brasileira de
Ciências(2000),72(4): (Resumo de Comunicações)
ABSTRACT
The Catalão I Carbonatite Complex is situated in southwestern Goiás, about 10Km from the Catalão town. Its emplacement occurred in the Cretaceous and it is essentially composed of phlogopitites and carbonatites. In the central part of the complex, in the open pit of the Ultrafértil Mine, there is one remarkable system of hydrothermal activity, related to the final magmatic stages. Compared to other carbonatite complexes the system is unique due to its intensity and singular features. The hydrothermal rocks are mainly composed of apatite, ilmenite, monazite, and barite, which vary from 20 up to 30m thick. Their most important structural features include horizontal layering and banding, dykes, veins, veinlets, and breccia pipes. The system was clearly multistage as demonstrated by crosscutting veins, dykes, and pipes. Whenever the fluid intruded carbonatites a reaction between fluid and rock resulted in important assimilation and erosion of the walls but, if in contact with phlogopitites, no visible reaction is detected. In the first case, the product is a horizontal layered rock composed of apatite and ilmenite that can reach about 0.3 up to 2.0m in thickness, which is differentiated upwards to layers mostly composed of ilmenite and monazite. In the second case, the hydrothermal rocks are mainly composed of apatite and barite. Dyke feeders and pipe roots are often observed. The breccia pipes present a cone shape with a cross section that is roughly circular and that varies from 1cm up to 1.0m in diameter. The fragments in the breccias are derived directly from the wall rocks and show small displacements. Fragments are angular and present a large range of grain size. Few pipes are composed of three main portions: a) a base that is filled with more equigranular fragments (millimeter to centimeter sizes) and that is almost massive; b) a medium portion that is finer (sizes up to few millimeters) and very well layered. In this portion the pipe widens up; c) the uppermost portion that is filled with the breccia that is typical of these pipes. Horizontal fractures, probably due to caldera collapse, were the main conduits to the fluids resulting in structures largely determined by conduit flow mechanisms. Increasing pressure was released through vertical fracture intersections originating the breccia pipes.
a R.V. Santos
a C.J.S. de Alvarenga
a M.A. Dardenne
b A.N. Sial
b V.P. Ferreira
a Instituto de Geociências,
Universidade de Brasília, Campus Universitário, Asa Norte, CEP: 70910-900,
Brasília, DF,Brazil
b Departamento de Geologia/NEG-LABISE, Universidade Federal de
Pernambuco, Caixa Postal: 7852, CEP: 50732-970,Recife-PE, Brazil
Precambrian Research
(2000),104(3-4):107-122.
Keyword(s): Neoproterozoic; Carbon isotopes; Oxygen isotopes; Brazil; Bambuí
group; Paranoá group
We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambuí group in central Brazil. This unit covers an area of more than 300000 km2 and comprises carbonate--silicliclastic sediments at the base that grade into siliciclastic sediments towards the top. The Bambuí group overlies by unconformity the Paranoá group, which consists mostly of siltstone, quartzite and minor limestone. The data presented here improve the stratigraphic correlation within the Bambuí basin and show that it evolved in an environment significantly different from that of the Paranoá basin. Our data show large fluctuations of δ13CPDB in limestones from the Bambuí Group (from +0.8 to +13.5‰) in all the three studied areas. Some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale. This observation is relevant considering the lack of fossil record and other stratigraphic markers in Neoproterozoic sequences. We also present the first isotopic profiles along the Paranoá--Bambuí transition, which shows that the δ13CPDB values grade from +1.0‰ in the Paranoá group, to +2.6‰ in the lower portion of the Bambuí group, increasing up to +12‰ in the upper part of this unit. Based on our carbon isotope data, as well as other geological, mineralogical and Nd isotope studies, we argue that the sediments of the Paranoá group were deposited on an open platform that was fully connected to the ocean. On the other hand, the sediments of the Bambuí group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790--600 Ma. This unit displays an increased amount of clastic sediments upwards. We argue that the high carbon isotope values observed in limestones and marlstones from the Bambuí group are correlated to worldwide high carbon isotope values reported for the Neoproterozoic. However, we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the Bambuí carbonates.
TECTONOSTRATIGRAPHIC TERRANE ANALYSIS ON NEOPROTEROZOIC TIMES: THE CASE STUDY OF ARAXA SYNFORM, MINAS GERAIS STATE, BRAZIL - IMPLICATIONS TO THE FINAL COLLAGE OF THE GONDWANALAND.
HILDOR JOSE SEER
MARCEL AUGUSTE DARDENNE
Revista Brasileira de
Geociências (2000),30:78 - 81. (Contribution IGC-102)
Keywords: Brasilia Belt,
Neoproterozoie, Brasiliano orogeny, Tectonic Evolution, Structural Geology
The Araxá Synform is a regional fold with gently WNW plunging. The outcrops of the Araxá, Ibia and Canastra Groups occur at their limbs. The region is the type locality of these units. These Groups belong to the Brasilia Fold Belt, a Neoproterozoic tectonic unit evolved at the western margin of the Sao Francisco-Congo Craton. Geological mapping, structural analysis, whole rock geochemistry, mineral chemistry, petrography and geochronology are the main tools to understand the tectonic evolution of these geological units. In this paper we apply tectonostratigraphic terrane analysis to the solution of stratigraphic problems in the Brasilia Belt and Gondwanaland collage.
BRAZILIAN KAMAFUGITES
PATRICIA BARBOSA DE
ALBUQUERQUE SGARBI
JOSE CARLOS GASPAR
JOEL GOMES VALENCA
Revista Brasileira de Geociências (2000), 30(3): 417-420. (Contribution IGC-145)
Keywords: kamafugites, mafic alkaline volcanism, Cretaceous, Brasil
Brazilian kamafugites occur in the Mata da Corda (Minas Gerais state) and Santo Antonio da Barra (Goias state) sub-provinces, which belong to Minas-Goias Alkaline Province, one of the Cretaceous alkaline provinces that are located around the Parana Basin in Brazil and Paraguay. The MC and SAB kamafugites are mafurites and ugandites. The rocks are all feldspar-free, with abundant clinopyroxene (mostly diopside) and Ti-magnetite in very fine- to medium-grained porphyritic to seriated textures. Perovskite is abundant in all MC kamafugites but in SAB kamafugites this mineral is accessory or absent. Kalsilite is generally interstitial, often altered. Mineralogically, the ugandites are similar to the mafurites, with the exception that they contain leucite (subhedral pseudomorphs) as the major felsic phase. Also, the mafurites have much more olivine phenocrysts (5-19% modal) than the ugandites (0-3% modal) while the ugandites have much more feldspathoid. The Brazilian kamafugites here studied are all undersaturated in SiO2 (36.6 to 44.9 wt% in MC and 39.9 to 44.2 wt% in SAB), the alkaline characteristic of the rocks being reflected in the contents of TiO (4.1 to 7.8 wt% in MC and 2.8 to 4.1 wt% in SAB), K O (1.2 to 6.8 wt.% in MC and 0.5 to 4.1 wt.% in SAB), and Na2O (0.49 to 2.0 wt% in MC and 1.0 to 4.3 wt.% in SAB). The presence of normative feldspathoid and olivine and lack of normative quartz are significant. MgO contents (6.0 to 18.0 wt% in MC and 5.6 to 15.2 wt% in SAB) are low when compared to those of other ultrabasic rocks, the highest values being associated with samples of high modal olivine content. The clinopyroxene/magnetite isotopic temperature data of the MC and SAB kamafugites are in the range of 830-1060° C (Sgarbi et al. 1998). The U-Pb perovskite ages of Brazilian kamafugites are in the range of 68-89.6 Ma (Sgarbi et al. 2000).
a P.B.A. Sgarbi
b J.C. Gaspar
c J.G. Valença
a Dep. de Geologia, IGC, UFMG,
Av. Antônio Carlos 6627, 31270-901 , Belo Horizonte, MG, Brazil
b Dep. de Miner. E Petrol., IG, UnB, Asa Norte, 70.910-900, ,
Brasília, Distrito Federal, Brazil
c Dep. de Geologia, IG, CCMN, UFRJ, Ilha do Fundão, 21910-900, ,
Rio de Janeiro, RJ, Brazil
Lithos (2000),53(2):101-116.
Keyword(s): Clinopyroxene; Kamafugites; Alkaline rocks; Cretaceous volcanism
The composition of pyroxenes from the two
occurrences of kamafugites in Brazil --- Santo Antônio da Barra and Mata da
Corda --- was investigated with the aim of comparing the localities and
characterizing kamafugitic pyroxenes in general. Kamafugitic pyroxene is always
diopside, except for late phases that can be richer in aegirine. In relation to
their silica content, Mata da Corda and Santo Antônio da Barra pyroxenes are
different: the former are similar to Italian kamafugitic pyroxenes, lamproitic
and orangeitic pyroxenes, having higher Si contents in the tetrahedral site;
whereas the Santo Antônio da Barra pyroxenes are similar to African kamafugitic
pyroxenes. Both are more enriched in IVAl than other kamafugitic
pyroxenes. In Santo Antônio da Barra and Mata da Corda pyroxenes, the Al content
is not always sufficient to fill the tetrahedral site and Si substitution by Fe3+
is required. There is a general trend of enrichment in Ti, Na and Fe3+
with the evolution of crystallization in both Mata da Corda and Santo Antônio da
Barra pyroxenes, especially in ugandites. The main substitutions present in
these Brazilian pyroxenes are Ca(Mg,Fe)Si2O6 by CaTiAl2O6,
NaTiAlSiO6 and Na Fe3+ Si2O6
end-members.
The main conclusion is that it is not possible to distinguish the compositional
range of clinopyroxene from orangeites, lamproites, and kamafugites.
PREDICTIVE GEOPHYSICAL MODEL FOR GOLD MINERALIZATION IN THE QUADRILATERO FERRIFERO, BRAZIL: THE CASE OF CUIABÁ MINE
ADALENE MOREIRA SlLVA
ANNE MCCAFFERTY
AUGUSTO CESAR BITTENCOURT PIRES
LUÍS CLAÚDIO RIBEIRO RODRIGUES
Revista Brasileira de Geociências(2000), 30(3): 543-546. (Contribution IGC-090)
Keywords: Quadrilatero Ferrífero, Rio das Velhas Greenstone Belt, Cuiabá Mine, Cuiabá-type BIF, mineralized environments.
Geophysical and radiometric signatures of the immediate area around the Cuiabá Mine is determined using the probability ratio mapping technique, a new statistical approach to map favorable host geologic units and mineralized environments. Airborne magnetic, electromagnetic and radiometric data collected as part of the Rio das Velhas Project were processed to characterize the geophysical and radiometric signatures at the well-known Cuiabá deposit. Areas with similar signatures to that of Cuiabá were then located in the survey area to delineate possible unknown deposits and occurrences related to carbonate-sulfide-greenstone-hosted mineralization. This approach allows for an objective exploration strategy to link geological and geophysical information over large survey areas. The results of this modeling procedure infer areas of exposed and subsurface Cuiabá-type BIF that are important to gold exploration in the area and require ground followup studies.
Comparative analysis of sulfides for gold using sxrf and sims
Ian M. Steele
Louis J. Cabri
José C. Gaspar
Greg McMahon
Marco A. Marquez
Marcos A.Z. Vasconcellos
Canadian Mineralogist(2000),38:1-10.
Key words: gold, sulfides, secondary-ion mass spectrometry (SIMS),
synchrotron X-ray fluorescence, São Bento deposit, Brazil.
Abstract
Sulfides may be analyzed for gold using Secondary Ion Mass Spectrometry (SIMS) and Synchrotron X-Ray Fluorescence (SXRF); we have compared the two methods with respect to instrumental requirements, time, analysis volume, and detection limits. A limitation in this comparison is the inhomogeneous distribution of Au within arsenopyrite, as demonstrated using SIMS imaging for samples from the São Bento deposit, Brazil. Analysis of the same grains of arsenopyrite produces good agreement between the two techniques; discrepancies are attributable to slight shifts in analysis locations between the two techniques or the different volumes being analyzed. Whereas SIMS has imaging capability and sub-ppm detection limits, quantitation is difficult. SXRF spectra are relatively simple, and quantitative analysis is not as matrix-dependent as with SIMS. The detection level for Au in arsenopyrite by SXRF is limited by the background from AsKa to about 50 ppm, but will be reduced to about 15 ppm using more intense synchrotron sources.
THE IGARAPÉ BAHIA Cu-Au MINERALIZATION, CARAJÁS PROVINCE
FERNANDO HENRIQUE BUCCO
TALLARICO
CLAUDINEI GOUVEIA DE OLIVEIRA
BERNARDINO RIBEIRO FIGUEIREDO
Revista Brasileira de Geociências (2000), 30(2): 230-233. (Contribution IGC-078)
Keywords: Carajás Province, Igarapé Bahia, copper, gold, uranium, rare earth elements
The IGARAPÉ Bahia Cu-Au mineralization is hosted by an Archean, low-grade metavolcano-sedimentary sequence. The orebodies define an ellipsoidal structure and are associated with subvertical breccia units located at the contact between two distinct units of the host sequence. Mineralized breccias include fragments of both footwall and hanging wall, which are cemented by variable amounts of chlorite, siderite, magnetite, chalcopyrite, K-bearing phases and minor U-REE-minerals. Quartz diorite dikes that disrupt the host rocks show a variety of textures, ranging from weakly altered granophyric terms to intensely venulated and brecciated rocks. Hydrothermal alteration of dikes includes propylitization, potassification and local albitization. Based on ore petrology and geochemical data (major elements, REE, oxygen and carbon stable isotopes) it is proposed that the progressive interaction of a hot saline and acid, deep-seated fluid with a low-temperature less saline and oxidizing meteoric solutions is the most likely depositional mechanism of the IGARAPÉ Bahia mineralization. The resemblance of the alteration styles of mineralized breccias and dioritic dikes suggest that both have interacted with the same hydrothermal fluid. Thus, the dioritic dikes could have been the source of heat and of magmatic fluids during the final stages of epicrustal emplacement.