UNIVERSIDADE DE BRASÍLIA - INSTITUTO DE GEOCIÊNCIAS

ABSTRACTS
DE CONTRIBUIÇÕES CIENTÍFICAS DO IG/UnB
ANO 2003

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Bühn & Trumbull,2003

Comparison of petrogenetic signatures between mantle-derived alkali silicate intrusives with and without associated carbonatite, Namibia  

B. Bühn1,2
R.B. Trumbull3

Lithos, 2003, 66: 201-221

Keywords: Liquid immiscibility; Carbonatite; Ijolite; Nepheline foidite; Nepheline syenite; Namibia

1 Institut für Geowissenschaften und Lithosphärenforschung der Universität, Senckenbergstrasse 3, D-35390 Giessen, Germany 
2 Instituto de Geociências, Universidade de Brasília, 70910-900 Brasília DF, Brazil, e-mail: berbu@unb.br 
3 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 

ABSTRACT

We compare the petrogenetic and chemical signatures of two alkali silicate suites from the Cretaceous Damaraland igneous province (Namibia), one with and one without associated carbonatite, in order to explore their differences in terms of magma source and evolution. The Etaneno complex occurs in close spatial proximity to the Kalkfeld bimodal carbonatite-alkali silicate complex, and is dominated by nepheline (ne)-monzosyenites and ne-bearing alkali feldspar syenites. The Etaneno samples have isotopic compositions of 87Sr/86Sr(i)=0.70462-0.70508 and Nd=-0.5 to -1.5, with the highest 87Sr/86Sr(i) and lowest Nd values observed in evolved samples. The magma differentiated via olivine, feldspar, clinopyroxene, and nepheline (ne) fractionation in a F-rich system, which fractionated Zr from Hf, and Y from Ho. Partly glassy, recrystallized inclusions in some samples are less evolved than their host rocks and contain a cumulate component (nepheline, plagioclase). The Kalkfeld ne-foidites (ijolites) and ne-syenites have 87Sr/86Sr(i)=0.70285-0.70592 and Nd=0.5 to 1.1. The isotope ratios show no consistent variation with rock composition, and they are in the same range as the associated carbonatites. The Kalkfeld silicate magma fractionated nepheline and alkali-feldspar in a CO2-dominated, F- and Ca-poor system. As a result, the rocks display some major and trace element trends distinctly different from those of the Etaneno samples.
We suggest that the Etaneno and the Kalkfeld magmas represent different melt fractions of a heterogeneous mantle source, resulting in different compositions especially with respect to CO2 contents of the primitive, parental magmas. In this scenario, the carbonated alkali silicate Kalkfeld parental melt contained a critical CO2 concentration and underwent liquid separation of carbonate and silicate melt fractions at crustal depths. The resulting silicate melt fraction experienced a very different mode of differentiation than the carbonate-poor Etaneno parental magma. Thus, the Kalkfeld rocks are depleted in Ca and other divalent cations, as well as F, rare-earth elements (REE), Ba, and P relative to the Etaneno syenites. We interpret these differences to reflect the partitioning of these elements into the carbonate melt fraction during immiscible separation. 


Bühn et al., 2003

Fluid/rock interactions and element fractionation during progressive migration of carbonatitic fluids, derived from small-scale trace element and Sr,Pb isotopic distribution in fluorite

B. Bühn
J. Schneider
P. Dulski
A.H. Rankin

Geochimica et Cosmochimica Acta(2003),67:4577-4595.

Keywords: Carbonatite, hydrothermal fluorite, Sr-Pb isotopes, fluid-rock interaction, element fractionation.
 
Abstract
 
Associated with the Cretaceous Okorusu carbonatite complex (Namibia) is a hydrothermal fluorite mineralization hosted in Pan- African country rock marbles, which resulted from fluid-rock reaction between the marbles and orthomagmatic, carbonatitic fluids expelled from the carbonatite. Yellow fluorite I was deposited in veins up to 5 cm away from the wallrock contact, followed by purple and colourless fluorite II, smoky quartz and barite, a Mn-rich crust on early calcite, and pure calcite. This clear-cut sequence of mineral growth allows an investigation into fluid-rock interaction processes between the marble and the migrating carbonatitic fluid, and element fractionation patterns between the fluid and subsequent hydrothermal precipitates.
Fluorite I shows a progressive change in colour from dark yellow to colourless with purple laminations over time of deposition. Subsequent fluorite I precipitates show an increase in Ca, and a continuous decrease in F, Sr, REE, Y, Th, U and Pb contents. The ratios (Eu/Eu*)cn, Th/Pb and U/Pb increase whereas Y/Ho, Th/U and (La/Yb)cn decrease. The Sr-isotopic composition remains constant at 87Sr/86Sr = 0.70456-0.70459, but with varying, highly radiogenic Pb (206Pb/204Pb = 32-190, 238U/204Pb = 7-63). Fluorite II has 87Sr/86Sr = 0.70454-0.70459, 206Pb/204Pb = 18.349, and 207Pb/204Pb = 15.600, and a chemical composition similar to youngest fluorite I. The Mn-rich crust on early calcite accumulated REE, Ba, Pb, Zr, Cs, Th and U, developing into pure calcite with a prominent negative Ce anomaly and successively more radiogenic Sr. The calculated degrees of fluid-rock interaction, f = weight fraction of fluid/(fluid + marble), decrease from fluorite I and most fluorite II (f = 0.5) to calcite (f = 0.2-0.3) and hydrothermal quartz (f << 0.1). A crush-leach experiment for fluid inclusions in the hydrothermal quartz yielded a Rb-Sr isochron age of 103±12 Ma. Crush-leach analysis for the carbonatitic fluid trapped in the wallrock yielded a trend from the fluid leachate to the host quartz (206Pb/204Pb = 18.224 and 18.602, 207Pb/204Pb = 15.616 and 15.636, respectively) extending from carbonatite towards crustal rocks.
Calculated trace element distribution coefficients fluorite/fluid are below unity throughout, and increase from La to Yb. Elements largely excluded from fluorite (Ba, Pb, LREE relative to HREE) were incorporated later into the Mn-rich crust on calcite. The trace element patterns of the hydrothermal minerals are related to changing aCO2 and aF- in the fluid during continued fluid-marble reaction. A predominance of carbonate over fluoride complexing in the fluid as reactions proceded controlled the Y/Ho, Th/U and REE patterns in the fluid and the crystallizing phases. Deviations from these trends indicate discontinuous processes of fluid-rock reaction.

Castañeda et al.,2003

FENÔMENOS DE ORDEM-DESORDEM LOCAL EM TURMALINAS NATURAIS ETRATADAS DA SÉRIE SCHORLITA-ELBAÍTA

CRISTIANE CASTAÑEDA
SIGRID GRIET EECKHOUT
EDDY DE GRAVE
NILSON FRANCISQUINI BOTELHO
ANTÔNIO CARLOS PEDROSA SOARES

Revista Brasileira de Geociências 33(1):75-82, março de 2003

Palavras-chave: turmalina, espectro Mössabuer, ordem-desordem.

Resumo

Análises de Fe-turmalinas provenientes de pegmatitos graníticos foram feitas usando microssonda eletrônica, ressonância paramagnética eletrônica e espectroscopia Mössbauer. A caracterização química indica que todas as amostras são espécies próximas da schorlita. Os espectros de ressonância paramagnética eletrônica indicam alta concentração de ferro com linhas largas de aproximadamente 1000 Gauss, diagnosticando interações spin-spin e desordem estrutural. Espectros Mössbauer à temperatura ambiente de schorlitas verdes naturais apresentam contribuição de dois Fe2+ em sítios Y não equivalentes enquanto nos espectros das azuis, três sítios Y podem ser observados. Esses aspectos registram um maior grau de desordem nas amostras de cor azul e um menor grau, nas verdes. Durante o aquecimento em ambientes oxidante e redutor, observa-se reajuste estrutural dos sítios do Fe2+, processos de desidroxilação e oxidação e a geração de uma fase mineral adicional.

LOCAL ORDER-DISORDER PHENOMENA IN NATURAL AND TREATED TURMALINE OS THE SCHORLITEELBAITE SERIES

Keywords: tourmaline, Mössbauer spectra, order-disorder

Abstract

 We report a comparative study between natural and treated green, blue and black color tourmalines from granitic pegmatites of the Minas Gerais state, Brazil, using Electron Microprobe, Electron Paramagnetic Resonance (EPR) and Mössbauer spectroscopy. Electron Microprobe analysis confirms that all studied samples are schorl species. The Electron Paramagnetic Resonance spectra of green and blue tourmaline are characterized by line widths of about 1000 Gauss. These unusual broad lines are probably due to spin-spin interactions caused by the high Fe concentrations and structural disorder. Mössbauer spectra collected at room temperature confirm that iron is dominantly present as Fe2+. It is incorporated at two or three different Y sites for the green and blue tourmalines, respectively. From those features, we conclude that blue tourmalines are characterized by a high degree of disorder, whereas a smaller degree of disorder is observed in green tourmalines. In addition, heat treatments in oxidizing and reducing atmospheres yielded structural readjustments at the Fe sites, de-hydroxylation, oxidation and generation of a second, different phase.


Fortes et.al.,2003

Sm–Nd studies at Mina III gold deposit, Crixás greenstone belt, central Brazil: implications for the depositional age of the upper metasedimentary rocks and associated Au mineralization

Paulo de Tarso Ferro de Oliveira Fortes
Márcio Martins Pimentel
Roberto Ventura Santos
Sérgio Luiz Junges

Keywords: Central Brazil; Crixás; Gold; Model ages; Nd isotopes

Journal of South American Earth Sciences (2003); 16(6): 503-512
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Instituto de Geociências, Universidade de Brasília, Brasília DF 70910-900, Brazil

Abstract

The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe- and Al-rich metasedimentary rocks of the upper unit.

Sm–Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00±0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show TDM model ages of mostly 2.33–2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite–garnet schist yields an age of 505±7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.

Resumo

O greenstone belt de Crixás, Brasil Central, é uma seqüência vulcano-sedimentar constituída, na base, por rochas metavulcânicas ultramáficas e máficas e, no topo, por rochas metassedimentares detríticas e químicas. A seqüência é parte do bloco siálico arqueano exposto na região central da Faixa Brasília, de idade Neoproterozóica, e foi, portanto, fortemente retrabalhada pela orogenia Brasiliana. Importantes depósitos auríferos, como a Mina III, estão associados a rochas metassedimentares carbonosas e ricas em Fe e Al da unidade de topo.

Dados Sm–Nd de metakomatiitos e metabasaltos da unidade basal indicaram idade de 3.00±0.07 Ga. Entretanto, as idades modelo TDM de rochas metassedimentares do topo situam-se predominantemente entre 2.33 e 2.49 Ga, demonstrando que as mesmas, assim como a mineralização, não são arqueanas. Adicionalmente, uma isócrona Sm–Nd construída para granada e rocha total de clorita-granada xisto, hospedeiro da mineralização, forneceu idade de 505±7 Ma, sugerindo idade neoproterozóica para a mineralização. As idades modelo das rochas metassedimentares de topo sugerem que as mesmas sejam parte de uma ou mais lâminas de cavalgamento formadas por rochas supracrustais Proterozóicas posicionadas tectonicamente sobre os terrenos arqueanos, provavelmente durante a orogenia Brasiliana.

Palavras chave;Brasil central; Crixás; Idades modelo; Isótopos de Nd; Ouro


Hollanda et.al.,2003

Paleoproterozoic subduction-related metasomatic signatures in the lithospheric mantle beneath NE Brazil: inferences from trace element and Sr–Nd–Pb isotopic compositions of Neoproterozoic high-K igneous rocks

Maria Helena Bezerra Maia de Hollanda
Márcio Martins Pimentel
Emanuel Ferraz Jardim de Sá

 Journal of South American Earth Sciences (2003), 15(8): 885-900.
(Ver texto integral)

Keywords: Mafic-intermediate rocks; Subduction-modified lithospheric mantle source; Transamazonian tectonics

ABSTRACT

Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/YbCN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (rounded small epsilon, GreekNd from -9.3–-20.1) isotopic compositions. TDM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (206Pb/204Pb=16–17.3, 207Pb/204Pb=15.1–15.6, 208Pb/204Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust.


Marques et.al.,2003

Re–Os and Sm–Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira–Medrado Sill, Bahia, Brazil

J. C. MARQUES
C. F. FERREIRA FILHO
R. W. CARLSON
M. M. PIMENTEL

Journal of Petrology ( 2003), 44(4): 659-678

Key words: mafic–ultramafic sill; isotopes and trace elements; chromite deposit

ABSTRACT

The chromite deposit of the Paleoproterozoic Ipueira–Medrado sill is hosted in a single, thick (5–8 m), massive layer, which sets severe constraints for the origin of chromitites. It is divided from bottom to top into: (1) a Marginal Zone (5–20 m); (2) an Ultramafic Zone (<250 m) consisting of dunite and harzburgite that host the chromitite layer, in which intercumulus amphibole is important and more abundant toward the top; (3) a Mafic Zone (<40 m). The parental magma was large ion lithophile element and light rare earth element enriched and high field strength element depleted. Sm–Nd isotopic compositions are consistent with a 2 Ga age, but suggest a variable initial Nd isotopic composition that correlates with the abundance of amphibole. The more negative Nd (mean –6·5) of the amphibole-rich intervals argues for crustal contamination, although the Nd (mean –4·4) of the amphibole-free samples suggests an old, enriched, subcontinental lithospheric mantle source. Chromite separates have initial Os values that range from –4·6 to +3. The negative Os values are typical of old, Re-depleted, lithospheric peridotitic mantle and give Re-depletion model ages of up to 2·75 Ga. An integrated assessment suggests that the very high-Mg parental magma probably originated from Archean, subcontinental, metasomatized, peridotitic lithospheric mantle and was subsequently contaminated with up to 30% of crust, which triggered the chromitite crystallization.


Moraes et.al.,2003

Geochemistry and Sm–Nd isotopic characteristics of bimodal volcanic rocks of Juscelândia, Goiás, Brazil: Mesoproterozoic transition from continental rift to ocean basin

Renato Moraesa
Reinhardt A. Fuckb
Márcio Martins Pimentelb
Simone Maria Costa Lima Gioiab
Ana Maria Graciano Figueiredoc

Precambrian Research (August 2003), 125(3-4):317-336
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Keywords: Bimodal volcanism; Juscelândia Sequence; Brazil; Continental rift-ocean basin transition; Mesoproterozoic; Sm–Nd isotope geochemistry

a Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Ilha do Fundão, Rio de Janeiro, RJ 21949-900, Brazil
b Instituto de Geociências, Universidade de Brasília, Brasília, DF 70910-900, Brazil
c IPEN-CNEN/SP, Supervisão de Radioquímica, Caixa Postal 11049, Pinheiros 05422-970, São Paulo, SP, Brazil

Abstract

The ca. 1.28 Ga Juscelândia Sequence, in Goiás, Brazil, comprises a unit of bimodal metavolcanic rocks, amphibolite and felsic gneiss, intercalated with metapelite and metachert. The bimodal nature of the magmatism is evident as the metavolcanic rocks have basalt and rhyolite compositions. Bimodality is also shown by the trace elements and Nd isotope data. Amphibolites yield positive rounded small epsilon, GreekNd(T) values that can be separated into two groups. In the first group, amphibolite samples from lower and middle portion of the sequence are characterized by rounded small epsilon, GreekNd between +2.8 and +5.4. These fairly positive values and enrichment in LREE and LILE suggest they were derived from a depleted mantle source with subsequent assimilation of continental crustal material. The second group is formed by amphibolite of the upper portion of the sequence that have rounded small epsilon, GreekNd around +6.0 and their low LILE and LREE contents resemble normal mid-ocean ridge basalts (N-MORB). Initial 143Nd/144Nd ratios represent the composition of the depleted mantle at 1.28 Ga. Felsic rocks yield negative rounded small epsilon, GreekNd(T) values, between ca. −2.8 and −4.5, and Paleoproterozoic model ages (TDM values in the interval between 1.8 and 2.1 Ga). The original felsic magmas represent the product of melting of older (Paleoproterozoic) continental crust with trace element chemistry indicating that the felsic magmas were the product of continental crustal melts with small mantle contribution. Previous models proposing an oceanic setting for the origin of magmatism of the Juscelândia rocks, within a mid-ocean ridge or back-arc basin are not supported by the data presented here. Transition from a continental rift to an ocean basin represents the most likely tectonic setting for the origin of the Juscelândia rocks.


Oliveira & Santos,2003

Isotopic domains controlled by transtensional and transpressional sectors in the auriferous Diadema shear belt, northern Brazil

Claudinei Gouveia de Oliveira
Roberto Ventura Santos

Instituto de Geociências, Universidade de Brasília, Brasília-DF 70910-900, Brazil

Journal of South American Earth Sciences (2003); 16(6): 513-522
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Keywords: Carbon isotopes; Gold mineralization; Oxygen isotopes; Shear zone; Structural domains

Abstract

This paper presents carbon and oxygen isotope data from the auriferous Diadema shear belt, Carajás mineral province, northern Brazil. This area was affected by large-scale deformation accompanied by metamorphism and carbonate alteration of the metavolcanic rocks. Carbonate carbon-isotope data (small delta, Greek13CPDB=−2.2 to −3.6‰, n=11) reveal that the hydrothermally altered zone within the shear zone has a different isotopic composition than that of the country rock (+0.4 to −1.7‰, n=12). Similarly, initial strontium isotope compositions reveal different isotopic compositions between the country rock of the shear zone (87Sr/86Sr=0.70454–0.70642) and the transpressure and transtensional zones (87Sr/86Sr=0.71286–0.71841), which indicates that the alteration zone resulted from large-scale fluid flow derived from an external reservoir. In addition to these differences, there are small variations in the small delta, Greek13CPDB values (from −2.8 to −2.0‰) in the shear zone, which may be related to preferential carbonate decomposition (and CO2 devolatilization) in the transpressional domains. Evidence of carbonate decomposition is supported by calc-silicate rocks, which derived from the reaction between calcite and silicate minerals.

The hydrothermal alteration and gold mineralization at the Diadema shear belt comprised both fluid infiltration from an external source and contemporaneous fluid generation in the shear zone. Compared with other gold deposits related to shear zones, the Diadema gold deposit developed at hypozonal crustal levels in the transition between greenschist and amphibolite metamorphic facies mineralogical assemblages.


Pimentel et.al.,2003

SHRIMP AND CONVENTIONAL U-PB AGE, SM-ND ISOTOPIC CHARACTERISTICS AND TECTONIC SIGNIFICANCE OF THE K-RICH ITAPURANGA SUITE IN GOIÁS, CENTRAL BRAZIL

PIMENTEL, MÁRCIO M.
DANTAS, ELTON L.
FUCK, REINHARDT A.

Anais da Academia Brasileira de Ciências (2003), 75(1):97-108.

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Palavras chave: Faixa Brasília; Brasiliano; Itapuranga; Uruana; U-Pb SHRIMP; Sintaxe dos Pireneus.

RESUMO

O alcali-granito de Itapuranga e o quartzo-sienito de Uruana são corpos alongados na direção E-W na porção central da Faixa Brasília. Eles são associados com o lineamento dos Pireneus, que cortam as estruturas regionais NNW da faixa. Dados U-Pb (SHRIMP e convencional) para as intrusões de Itapuranga e Uruana indicaram idades de 624 ± 10 Ma e 618 ± 4 Ma, respectivamente. Núcleos de três cristais de zircão do granito Itapuranga têm idades entre 1.79 e 1.49 Ga. Idades modelo Sm-Nd são de 1.44 Ga e valores de eNd(T) são -5.1 e -5.7, indicando participação de crosta siálica mais antiga (Paleo- a Mesoproterozóica). Feições diagnósticas de mistura de magmas indicam a co-existência de líquidos máficos e félsicos, de forma que os membros-finais félsicos das intrusões possivelmente representam líquidos derivados predominantemente da fusão de crosta continental, formados em resposta à intrusão de magmas máficos de natureza alcalina há ca. 620 Ma. Essas intrusões são contemporâneas, ou umpouco mais jovens, que o pico metamórfico Brasiliano na porção sul da Faixa Brasília. Seu alojamento ao longo do lineamento dos Pireneus sugere tratar-se de corpos sin-tectônicos em relação a essas estruturas, tendo os magmas originais cristalizado em zonas de transtensão ao longo das falhas.

Key words: Brasília Belt; Brasiliano; Itapuranga; Uruana; U-Pb SHRIMP; Pirineus Syntaxis.

ABSTRACT

The Itapuranga alkali granite and Uruana quartz syenite are large K-rich EW-elongated intrusions, in the central part of the Neoproterozoic Brasília Belt, central Brazil. They are associated with Pireneus lineaments, which cut the regional NNW-SSE structures of the southern part of the belt. SHRIMP and conventional U-Pb data for the Itapuranga and Uruana intrusions indicate crystallization ages of 624 ± 10 Ma and 618 ± 4 Ma, respectively. Three zircon cores from the Itapuranga granite yielded U-Pb ages between 1.79 and 1.49 Ga. Sm-Nd TDM ages for both intrusions are 1.44 Ga and eNd(T) values are -5.1 and -5.7, suggesting the input of material derived from older (Paleo- to Mesoproterozoic) sialic crust in the origin of the parental magmas. Magma mixing structures indicate co-existence of mafic and felsic end-members. The felsic end-member of the intrusions is dominantly represented by crust-derived melts, formed in response to the invasion of Paleo/Mesoproterozoic sialic crust by alkali-rich mafic magmas at ca. 620 Ma. These intrusions are roughly contemporaneous with, or perhaps slightly younger than, the peak of regional metamorphism in the southern Brasília Belt. Their emplacement along the Pireneus lineament suggest a syn-tectonic origin for them, most probably in transtensional settings along these faults.


Pimentel et.al.,2003

Geochronology and Nd isotope geochemistry of the Gameleira Cu–Au deposit, Serra dos Carajás, Brazil: 1.8–1.7 Ga hydrothermal alteration and mineralization

Márcio M. Pimentel
Zara G. Lindenmayer
Jorge H. Laux
Richard Armstrong
Janice Caldas de Araújo

Journal of South American Earth Sciences (2003), 15(7): 803-813
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Keywords: Carajás; Cu-Au; Gameleira

ABSTRACT

The Gameleira deposit is one of several important Cu–Au deposits associated with the late Archean (ca. 2.7 Ga) volcanic rocks of the Itacaiúnas supergroup in the Carajás mineral province, southeastern Pará. It comprises mainly biotite- and sulphide-rich veins and quartz–grunerite–biotite–gold hydrothermal veins that cut andesitic rocks. It is interpreted as representative of the Fe oxide Cu–Au class of deposit.
Sm–Nd isotopic data indicate an age of 2719±80 Ma (MSWD=3.0) and rounded small epsilon, GreekNd(T) of -1.4 for the host meta-andesites. Metavolcanic rocks and cogenetic gabbros give an age of 2757±81 Ma (1small sigma, Greek) with rounded small epsilon, GreekNd(T) of -0.8. This is considered the best estimate for the crystallization age of the Gameleira volcanic and subvolcanic rocks. Negative rounded small epsilon, GreekNd(T) and Archean TDM model ages (mostly between 2.8 and 3.1 Ga) suggest some contamination with older crustal material. The andesitic/gabbroic rocks are cut by two generations of granite dykes. The older has striking petrographic and geochemical similarities to the ca. 1.87 Ga alkali-rich Pojuca granite, which is exposed a few kilometers to the northwest of the deposit. The younger is a leucogranite with a U–Pb SHRIMP age of 1583+9/-7 Ma. Neodymium isotopic analyses of the two generations of granites indicate a strong crustal affinity and possible derivation from reworking of the Archean crust.
The quartz–grunerite–gold hydrothermal vein yields a Sm–Nd isochron (MSWD=.83) age of 1839±15 Ma (1small sigma, Greek) with rounded small epsilon, GreekNd(T) of -9.2. Pervasive potassic alteration, represented by the widespread formation of biotite in the country rocks, is dated by Ar–Ar at 1734±8 Ma, and a similar age of 1700±31 Ma (1small sigma, Greek) is indicated by the Sm–Nd isochron for the biotite–sulphide veins. Similar to that for the quartz–grunerite vein, the rounded small epsilon, GreekNd(T) value for the sulphide-rich veins is strongly negative (-8.2), thereby suggesting that the original fluids percolated through, leached, or were derived from igneous rocks with an Archaean Nd isotopic signature. The geochronological data suggest that the Gameleira Cu–Au mineralization is related to a Paleoproterozoic (ca. 1.83 Ga) episode of hydrothermal activity and is not Archaean. The younger ages of ca. 1.70–1.73 might be interpreted as products of the lower blocking temperatures of biotite in relation to the Ar–Ar and Sm–Nd isotopic systems. Combined with previous geochemistry and stable isotope data, the Nd isotopic data suggest that the mineralizing fluids were derived from, or strongly interacted with, a Paleoproterozoic crustal granite, possibly similar in age and composition to the Pojuca granite.

RESUMO

Palavras chave: Carajás; Cu-Au; Gameleira; Geocronologia

O depósito Gameleira é um dos importantes depósitos de Cu–Au associados com rochas vulcânicas de ca. 2.7 Ga do Supergrupo Itacaiúnas, na Província Mineral de Carajás, sudeste do Pará. É constituído dominantemente de veios de biotita e sulfetos e veios hidrotermais de quartzo–grunerita–biotita–Au que cortam rochas andesíticas do Grupo Salobo–Pojuca. O depósito é interpretado como representativo da classe de depósitos de óxido de Fe–Cu–Au. Dados isotópicos Sm–Nd indicam a idade de 2719±80 Ma (MSWD=3.0) e rounded small epsilon, GreekNd(T) of -1.4 para os meta-andesitos encaixantes. Quando plotados juntos, as rochas metavulcânicas e gabbros associados indicam a idade de 2757±81 Ma (1small sigma, Greek) e rounded small epsilon, GreekNd(T) of -0.8. Essa idade é interpretada como representativa da época de cristalização das rochas ígneas encaixantes do depósito. Valores negativos de rounded small epsilon, GreekNd(T) e idades TDM arqueanas (predominantemente entre 2.8 e 3.1 Ga) sugerem contaminação com material crustal mais antigo. As rochas andesíticas/gabróicas são cortadas por duas gerações de diques graníticos: o mais antigo apresenta marcante similaridade petrográfica e geoquímica com o granito Pojuca com idade de ca. 1.87 Ga, exposto a noroeste do depósito, e o mais jovem é um leucogranito com idade U–Pb SHRIMP de 1583+9/-7 Ma. Composições isotópicas de Nd revelam o caráter crustal dos granitos e sugerem que sejam derivados da refusão de crosta continental arqueana.
Amostras do veio hidrotermal de quartzo–grunerita–Au produziram uma isócrona Sm–Nd bem definida (MSWD=0.83) com a idade de 1839±15 Ma (1small sigma, Greek) with rounded small epsilon, GreekNd(T) of -9.2. A alteração hidrotermal potássica, representada pela formação de biotita nas rochas encaixantes, foi datada pelo método Ar–Ar em 1734±8 Ma, e uma idade similar, de 1700±31 Ma (1small sigma, Greek)(rounded small epsilon, GreekNd(T) of -8.2), é indicada pela isócrona Sm–Nd para amostras dos veios de biotita e sulfeto. Os valores fortemente negativos de rounded small epsilon, GreekNd(T) para os veios hidrotermais sugerem que os fluidos originais percolaram/lixiviaram ou, alternativamente, foram inteiramente derivados de rochas ígneas com assinatura isotópica arqueana.
Os dados geocronológicos sugerem que a mineralização de Gameleira está relacionada com evento(s) hidrotermal(is) há aproximadamente 1.83 Ga, e não com alteração hidrotermal arquena. As idades mais jovens, de aproximadamente 1.70–1.73 Ga podem ser atribuídas às mais baixas temperaturas de bloqueio da biotita em relação aos sistemas isotópicos Ar–Ar e Sm–Nd. Combinados com dados prévios de geoquímica e isótopos estáveis, os dados isotópicos de Nd são compatíveis com derivação (ou forte interação) dos fluidos mineralizantes com granitos crustais paleoproterozóicos do tipo Pojuca.


Piuzana et.al.,2003

Neoproterozoic granulite facies metamorphism and coeval granitic magmatism in the Brasilia Belt, Central Brazil: regional implications of new SHRIMP U–Pb and Sm–Nd data

Danielle Piuzanaa
Márcio Martins Pimentela
Reinhardt A. Fucka
Richard Armstrongb

Keywords: Anápolis–Itauçu; Granulite; Brasília Belt; U–Pb SHRIMP; Sm–Nd; Neoproterozoic

Precambrian Research (August 2003), 125(3-4):245-273
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a Instituto de Geociências, Universidade de Brasília, Brasília DF 70910-900, Brazil
b Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

Abstract

New SHRIMP U–Pb zircon ages combined with Sm–Nd isotopic characteristics of granulites and associated granitic rocks of the Anápolis–Itauçu Complex in the central-southern part of the Brasília Belt are presented and discussed in this study. Igneous crystallization ages obtained in zircon grains of orthogranulite and granites vary between 760 and 650 Ma. Growth of new zircon at ca. 650–640 Ma dates the high-grade metamorphism. Zircon cores from paragranulites and granites give U–Pb ages between 2.0 and 0.8 Ga.
TDM model ages of the granulitic rocks fall into two age intervals: 2.3–1.9 and 1.7–1.4 Ga. rounded small epsilon, GreekNd(T) values are negative, varying between −9.29 and −1.42. SHRIMP U–Pb ages of zircon cores indicate that the granulite sedimentary protoliths were deposited after 800 Ma ago, contradicting previous models which assigned Paleoproterozoic or Archean ages for these rocks. Their Nd isotopic signature indicate that Paleoproterozoic sources (within the São Francisco Craton?), as well as younger sources, such as the Goiás Magmatic Arc, contributed to the sediment infilling of the former basin.
The intrusive granites display Nd model age pattern similar to that of the granulitic rocks, with most TDM model ages ranging between 1.45 and 1.2 Ga. rounded small epsilon, GreekNd(T) values are between −2.61 and −7.96, indicative of assimilation of older material by the original magma. SHRIMP U–Pb data for the granite intrusions indicate that granitoids metamorphosed under amphibolite facies show strong inheritance (between 2.1 and 0.8 Ga) and magmatic(?)/metamorphic age at ca. 660–650 Ma. The syn-granulite facies granitoid ANA 1 gave magmatic and metamorphic ages of ca. 650 Ma, without any discernible inheritance. The growth of new zircon occurred at the same time of the high-grade metamorphism, probably related with the collision event between the Goiás Magmatic Arc to the west–southwest and the São Francisco Craton, to the east.
The data suggest that the Anápolis–Itauçu Complex, instead of representing the ancient sialic basement of the supracrustal rock units of the Brasília Belt, constitutes a Neoproterozoic high-grade metamorphic complex related with the evolution of the Brasiliano orogen.


Piuzana et.al.,2003

SHRIMP U–Pb and Sm–Nd data for the Araxá Group and associated magmatic rocks: constraints for the age of sedimentation and geodynamic context of the southern Brasília Belt, central Brazil

Danielle Piuzanaa
Márcio Martins Pimentela
Reinhardt A. Fucka
Richard Armstrongb

Keywords: Araxá Group; Brasília Belt; Neoproterozoic; Provenance; SHRIMP U–Pb

Precambrian Research (July 2003), 125(1-2):139-160
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a Instituto de Geociências, Universidade de Brasília, Brasília-DF 70910-900, Brazil
b Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia

Abstract

SHRIMP U–Pb and Sm–Nd isotopic data for Araxá Group micaschist, associated amphibolite and intrusive tonalite were used to estimate the age and tectonic setting of deposition of the original detrital sediments, in the southern part of the Neoproterozoic Brasília Fold Belt. One amphibolite sample from a layer interleaved tectonically within the metasedimentary rocks has the U–Pb zircon age of 838±20 Ma. Its tholeiitic composition and depleted mantle isotopic signature (rounded small epsilon, GreekNd=+5.1) suggests that it represents a tectonic slice of the former oceanic lithosphere separating the São Francisco and Amazon continents. U–Pb ages of detrital zircon grains and TDM model ages of the Araxá Group micaschists show a bimodal pattern, interpreted as the result of provenance from two contrasting sources: (i) young juvenile sources, probably represented by igneous rocks of the Goiás Magmatic Arc in the west and (ii) Paleoproterozoic sialic sources, most probably belonging to the São Francisco Craton, in the east. The detrital population is largely dominated by ca. 666–682 Ma old zircon grains derived from both mafic and felsic igneous source rocks. Detrital metamorphic grains and rims dated at ca. 715 and 643 Ma indicate that the original basin received sediments from Neoproterozoic sources with a previous metamorphic overprint, implying that young metamorphic rocks have been uplifted, exposed and eroded, providing sediment for at least part of what is presently mapped as the Araxá Group. The youngest detrital zircon age of ca. 643 Ma represents the upper limit for deposition of the original detrital sediments. Their minimum age is constrained by the crystallisation of an intrusive tonalite at 638±11 Ma. The data suggest, therefore, that the Araxá Group metasediments were deposited and metamorphosed within a very short time interval, most probably during the tectonic events related with the accretion of the Goiás Magmatic Arc to the eastern part of the Brasília Belt and final closure of the Goiás Ocean.


Santana et.al.,2003

 Estudio de inclusiones fluidas en el stockwork del sector La Union. Provincia de Camagüey, Cuba.

SANTANA, M. U.
MOURA, Márcia Abrahão
BOTELHO, N. F.

Pal. chave:ouro; inclusões fluidas; granito; alteração hidrotermal
V Congreso Cubano de Geología e Minería, 2003, Havana. Memorias Geomin 2003. Havana: Centro Nacional de Informacíon Geológica, 2003. p. 148-156.

Abstract:

Study of fluid inclusions in La Unión stockwork. Province of Camagüey. Cuba. La Unión is located in the center-oriental region of Cuba, municipality of Najasa, in Camagüey province. Alteration rocks are represented by zoned quartz-sericite with quartz veins, related to intrusive bodies and locally La Unión stockwork. The porphyry copper (Cu-Au) stockwork mineralization was defined by studies of ground geochemistry, occurring a close correlation between gold and copper contents. Petrography studies based on phases number, nature and proportions at room temperature and preliminary microthermometric studies of the mineralized stockwork's quartz veins fluid inclusions (IF) allowed us to identify three types of fluid inclusions, type 1 (V), type 2 (V+L), type 3 (V+L+Ss). The association of vapor rich IF (type 2) and high-salinity inclusion(type 3 THL) suggests the relationship with an immiscible magmatic fluid (L+V+H), hosted in a subvolcanic environment, related to granitic rocks (quartz diorite porphyries), where the primary fluid boiled in two phases, one low density aqueous and another of high salinity (30 to 40 % wt eq. NaCl) at a temperature around 300 - 400 C, typical condition of fluids associated to porphyry type mineralization. .


Trumbull et.al.,2003

The petrology of basanite-tephrite intrusions in the Erongo Complex and implications for a plume origin of Crateaceous alkaline complexes in Namibia 

R.B. Trumbull1
B. Bühn2,*
R.L. Romer1
F. Volker2 

1 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 
2 Institut für Geowissenschaften und Lithosphärenforschung der Universität, Senckenbergstrasse 3, D-35390 Giessen, Germany 
* Present address: Bernhard Bühn, Instituto de Geociências, Universidade de Brasília, 70910-900 Brasília DF, Brazil, e-mail: berbu@unb.br 

Journal of Petrology, 2003, 44: 93-112
Keywords:
Sr–Nd–Pb isotopes; Namibia; plume; basanite; petrogenesis

ABSTRACT

Basanite intrusions from the Early Cretaceous Erongo complex, Namibia, have compositions consistent with near-primary mantle melts derived from a depth of at least 100 km. These rocks provide a key reference for the mantle component(s) involved in breakup-related magmatism in this region. Initial Sr–Nd–Pb isotope ratios of the Erongo basanites and associated tephrites and phonotephrites (87Sr/86Sr = 0•70425–0•70465; Nd = +1•8 to +2•7; 206Pb/204Pb = 18•63–18•91) are independent of the degree of differentiation and correspond closely to an estimated range for the Tristan plume at 130 Ma. Incompatible trace element ratios also overlap with ratios of ocean island basalt (OIB) from the South Atlantic islands of Tristan da Cunha, Gough and Inaccessible associated with the modern Tristan hotspot. The Tristan plume signature of Erongo basanite–tephrite intrusions is shared by at least six other Early Cretaceous mafic alkaline complexes in Namibia, whereas the associated flood basalts in general lack a plume signature. We attribute the contrast in mantle sources for the flood basalts and alkaline complexes to their relative timing with respect to lithospheric thinning. Thick lithosphere during the main flood basalt event prevented direct melting of the Tristan plume and magmas were generated mostly from the lithosphere. The alkaline complexes intruded later, when the lithosphere was sufficiently thinned to allow decompression melting of the underlying plume mantle.


Van Schmus et.al.,2003

The Seridó Group of NE Brazil, a late Neoproterozoic pre- to syn-collisional basin in West Gondwana: insights from SHRIMP U–Pb detrital zircon ages and Sm–Nd crustal residence (TDM) ages

W. R. Van Schmusa
B. B. de Brito Nevesb
I. S. Williamsc
P. C. Hackspacherd
A. H. Fettera
E. L. Dantasa, d
and M. Babinskia, b

Keywords: Seridó Group; Neoproterozoic; Detrital zircons; Geochronology; Brazil; Borborema Province

Precambrian Research(2003); 127(4): 287-327

a Department of Geology, University of Kansas, Lawrence, KS, USA
b Inst. Geociências, Univ. de São Paulo, São Paulo, SP, Brazil
c Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia
d Inst. de Geociências e Ciências Exatas, Univ. Estadual Paulista, Rio Claro, SP, Brazil

Abstract

The Seridó Group is a deformed and metamorphosed metasedimentary sequence that overlies early Paleoproterozoic to Archean basement of the Rio Grande do Norte domain in the Borborema Province of NE Brazil. The age of the Seridó Group has been disputed over the past two decades, with preferred sedimentation ages being either Paleoproterozoic or Neoproterozoic. Most samples of the Seridó Formation, the upper part of the Seridó Group, have Sm–Nd TDM ages between 1200 and 1600 Ma. Most samples of the Jucurutu Formation, the lower part of the Seridó Group, have TDM ages ranging from 1500 to 1600 Ma; some basal units have TDM ages as old as 2600 Ma, reflecting proximal basement. Thus, based on Sm–Nd data, most, if not all, of the Seridó Group was deposited after 1600 Ma and upper parts must be younger than 1200 Ma.
Cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca. 600 Ma Brasiliano deformation and metamorphism, so that SHRIMP and isotope dilution U–Pb ages must represent crystallization ages of the detrital zircons. Zircons from meta-arkose near the base of the Jucurutu Formation yield two groups of ages: ca. 2200 Ma and ca. 1800 Ma. In contrast, zircons from a metasedimentary gneiss higher in the Jucurutu Formation yield much younger ages, with clusters at ca. 1000 Ma and ca. 650 Ma. Zircons from metasedimentary and metatuffaceous units in the Seridó Formation also yield ages primarily between 1000 and 650 Ma, with clusters at 950–1000, 800, 750, and 650 Ma. Thus, most, if not all, of the Seridó Group must be younger than 650 Ma. Because these units were deformed and metamorphosed in the ca. 600 Ma Brasiliano fold belt during assembly of West Gondwana, deposition probably occurred ca. 610–650 Ma, soon after crystallization of the youngest population of zircons and before or during the onset of Brasiliano deformation.
The Seridó Group was deposited upon Paleoproterozoic basement in a basin receiving detritus from a variety of sources. The Jucurutu Formation includes some basal volcanic rocks and initially received detritus from proximal 2.2–2.0 Ga (Transamazonian) to late Paleoproterozoic (1.8–1.7 Ga) basement. Provenance for the upper Jucurutu Formation and all of the Seridó Formation was dominated by more distal and younger sources ranging in age from 1000 to 650 Ma. We suggest that the Seridó basin may have developed as the result of late Neoproterozoic extension of a pre-existing continental basement, with formation of small marine basins that were largely floored by cratonic basement (subjacent oceanic crust has not yet been found). Immature sediment was initially derived from surrounding land; as the basin evolved much of the detritus probably came from highlands to the south (present coordinates). Alternatively, if the Patos shear zone is a major terrane boundary, the basin may have formed as an early collisional foredeep associated with south-dipping subduction. In any case, within 30 million years the region was compressed, deformed, and metamorphosed during final assembly of West Gondwana and formation of the Brasiliano–Pan African fold belts.