Clóvis Vaz Parente

O SISTEMA ORÓS-JAGUARIBE NO CEARÁ, NE DO BRASIL

The Oros-Jaguaribe System is a peculiar geotectonic unit in the Borborema Province (NE Brazil) framework. It contains two linear mobile belts, whose evolution began c.a. 1.9 Ga ago, overlying an older basement characterized by high amphibolite metamorphism and tangential tectonics. In contrast with its basement, the System contains essentially metavolcano-sedimentary sequences intruded by acid and basic magmatic rocks metamorphosed under low greenschist to granulite facies and displays a strike-slip deformation pattern of brasiliano age (c.a 600 Ma). The metasediments are essentially pelitic with important intercalation of semi-mature lenticular quartzite, carbonatic quartzite, calcsilicate rocks and calcitic/dolomitic/magnesitic marbles. The Oros Belt exhibitsan inverted distribution: volume and extension of detritic sediments, essentially quartzite, decrease south-westward, whereas carbonates, essentially calcitic marbles, show a significant increase. These ones become more, magnesitic north-eastward by a differentiation chemical process (elimination of Ca that precipitate as calcic, calci-magnesitic carbonates or sulfates). These relationships are consistent with a paralic/deltaic system. The acid metaplutonic and metavolcanic rocks are dominantly alkaline or sub-alkaline, characteristic of an intracratonic setting.The lithologic association and distribution of the metasediments (essentially the basal ones) are shelf type, preceding rift stage, which is associated with an intense magmatism.

Geology and geochemistry of paleoproterozoic magnesite deposits (~1.8Ga), State of Ceará, Northeastern Brazil

Five important magnesite mines and small occurrences in the state of Ceará form a sequence of lenses that extend, discontinuously, for over 140km. The magnesite rocks are hosted by metadolomites with lutecite, sulphate nodules pseudomorphosed by fiber-radiated quartz, scapolite and dissolution breccias. This metacarbonate sequence is more voluminous and more calcic in the southwestern extremity of the belt, but less voluminous and more magnesian towards the northeastern portion of the belt, the highest Mg contents being observed in the Alencar-Orós region. Magnesite deposits pass gradually into metadolomites and then to almost pure calcitic marbles westward the belt. This group is hosted in a greenschists to amphibolite-facies metavolcanic-sedimentary sequence crosscut by basic sills and granitic intrusions of variable size, form, composition and age (middle to late Proterozoic ∼550Ma).Two types of magnesite marble may be distinguished: (1) medium-grained (1 to 9mm) and (2) sparry-magnesite marble (1 to 15cm). The latter displays wide textural variation (porphyric, rosette, layered and palisadic types) and exhibits remnants of the original sedimentary features. The sparry crystals, in spite of their deformation, are hypidiomorphic and pinolitic. The colors vary from white to light or dark gray, or even red. The dark banded term presents traces of microfossils and stromatolites structures. The medium-grained magnesite marble, on the other hand, shows more homogeneous texture and color and anhedral crystals are more abundant.The sparry-magnesite marble is less enriched in SiO2, Fe2O3, Al2O3 and CaO and richer in MgO than the medium-grained ones. The magnesitic marbles display positive Ce, and negative Eu anomalies. This suggests a shallow marine platform environment with continental, reduced influence (lagunar). These magnesitic marbles also show positive Ce and Eu anomalies in relation to the present seawater, suggesting compositional differences, materialized mainly by an enrichement of these elements in the past seawater.Fluid inclusions in the medium-grained magnesites of the Riacho Fundo deposit are aqueous inclusions, while in the sparry magnesite deposits, particularly in Cabeça de Negro, aquo-carbonic inclusions dominate. Hydrocarbons, however, are found in both cases.The calcitic marble, deposited on open-marine environment, displays positive13C (+0.5 to 2.1 ‰ PDB), while dolomitic to dolostone marbles, deposited on less open marine environment, display negative shift (0.7 to −3.2‰ PDB). This13C fluctuation is within the range for carbonates deposited around 1.8Ga.The depositional environment of these carbonates is close to a paralic system, lagoon, with strong evaporitic conditions. The depressions were of variable depth and lengths and could be isolated, in particular by stromatolitic barriers, dried and flooded by seawater with continental inflows. The largest and deepest ones correspond to the sparry magnesite deposits. These magnesites are located in a lateral way to almost pure calcitic marbles, through dolomitic marbles. This points to a process of chemical differentiation within a paralic system, from open sea to landwards, more calcic in the first to more magnesian in the confined environment, in which lagoon waters are progressively purified from Ca through preferential precipitation of calcic carbonates and secondly of gypsum/anhydrite, which increases the Mg/Ca ratio, and lead carbonate deposits towards the magnesite pole. These metacarbonates also reveal a fractionation of the light REE, starting from the calcitic, through the dolomitic to magnesitic marbles. In the calcitic marbles, REE are in association with the clay fraction, while in magnesitic marbles, they seem to be in association with organic and/or soluble complexes. The magnesitic marbles are of sedimentary origin and underwent important diagenetic evolution before being metamorphosed during the Brasiliano orogenic cycle.

GEOLOGIA E GEQQUÍMICA DOS ELEMENTOS MAIORES DOS DEPÓSITOS DE MAGNESITA PRÉ-CAMBRIANA (~1.8Ga) DA FAIXA MÓVEL ORÓS ( CEARÁ)

The five most important magnesite mines and some small occurrences of the Ceara State form an array of discontinuous lenses extending for about 140 km. The magnesite-rich rocks are hosted in metadolomites with lutecite, fibroradiated quartz pseudomorph after sulfate nodules, scapolite and dissolution breccias. The marbles are hosted by a metavolcanosedimentary sequence of the greenschist to amphibolite facies, and were intruded by basic sills and granites of size, form, composition and age that vary from the Meso to Neoproterozoic. Two types of magnesite marbles may be distinguished: (1) magnesite marble of medium grain-size (1 to 9mm) and (2) sparry magnesite marble (1 to 15cm). The sparry magnesite marble has porphyric, rosette, layered, and palisade textures, the latter having remains of the original sedimentary features. In spite of deformation, the sparry crystals are hipidiomorphic and pinolitic. The rocks vary from white to light gray or dark gray or even red. The dark banded terms have traces of microfossils and stromatolitic structures. The texture and color of the medium-grained magnesite marbles are more homogeneous and the crystals are more anhedral. The sparry magnesite marble is in general poorer in SiO 2 , Fe 2 O 3 , Al 2 O 3 and CaO and richer in MgO as compared to the medium-grained types. The paleogeographic interpretation suggests that these rocks formed near a paralic system (lagoon), with strong evaporitic conditions. The depressions were of variable depth and length and probably isolated by stromatolite barriers, dried and flooded by seawater with continental inflows. The largest and deepest deposits correspond to the sparry magnesite. Regionally, the magnesite deposits give place to dolomitic marbles, which grade into almost pure calcitic marbles, indicating a chemical differentiation within the paralic system, by which the lagoon waters turned progressively poorer in Ca by the precipitation of calcium carbonate, thus increasing the Mg/Ca ratio and leading to the precipitation of magnesite. The magnesite marble is than interpreted as of sedimentary origin, and underwent strong diagenesis before metamorphism and Neoproterozoic deformation.

COMPORTAMENTO GEOQUÍMICO DOS ELEMENTOS TERRAS RARAS DA SEQUÊNCIA METACARBONÁTICA MAGNESIANA PRÉ-CAMBRIANA (~1.8Ga) DA FAIXA MÓVEL ORÓS

Geochemical studies of the Rare Earth Elements (REE) in magnesian carbonate sequences are still rare, mainly in Brazil. The reasons for this are numerous and vary from the low content of these elements in this kind of rocks to the lack of knowledge about the their distribution in carbonate rocks. The increase of sensibility of the analytical methods (ICP) made it possible to detect extremely low values and with better knowledge of these elements in sedimentary environments, its use has been a valuable tool to the understanding of magnesian carbonate deposits. The study of these elements in the magnesian metacarbonatic sequence of the Precambrian Oros Mobile Belt (~1.8Ga), which is characterized by an association of carbonates that vary from calcitic marbles (open sea) to dolomitic and magnesitic marbles (paralic marine) showed that there is a fractionation of the LREE from the calcitic marbles to the dolomitic and to the magnesitic marbles. In calcitic marbles, the REE are associated with the clay fraction, while in magnesitic marbles they seem to be associated with organic complexes and/or soluble complexes. The magnesitic marbles also present positive Ce and negative Eu anomalies, probably as a result of a shallow marine shelf environment under the influence of a continental reduced environment (lagunar). The magnesitic marbles also show a positive anomaly of Ce and Eu relative to the modern seawater, suggesting that the Proterozoic seawater was richer in these elements than its modern equivalent. seawater.

Geologia e geoquímica dos elementos maiores da seqüência carbonática magnesiana da faixa móvel Orós (Ceará)

Os cinco principais jazimentos e algumas pequenas ocorrencias de magnesita do Ceara formam um rosario de lentes que se estende por cerca de 140 km. As rochas magnesiticas sao encaixadas em metadolomitos com lutecita, quartzo fibrorradiado pseudomorfo sobre nodulos de sulfates, brechas de dissolucao e escapolita. O conjunto esta hospedado em uma sequencia metavulcanossedimentar de facies xisto verde a anfibolito, com intrusoes de sills basicos e granitos de tamanho, forma e composicao variada do Mesoproterozoico (1.78Ga) ao Neoproterozoico. Dois tipos de marmores magnesiticos podem ser distinguidos: os marmores de grao medio (l a 9mm) e os espaticos (l a 15cm). Os espaticos apresentam grandes variacoes texturais, com predominio dos tipos porfiritico, em roseta, bandado e palissadico. Os ultimos ainda preservam estruturas dos sedimentos originais. A sua cor varia entre o branco, cinza claro, cinza escuro e vermelho tijolo. O termo bandado escuro apresenta tracos de microfosseis e estruturas estromatoliticas. Os marmores de grao medio, por sua vez, sao mais homogeneos em textura e cor e os cristais sao, em geral, xenomorficos. Em geral, os marmores espaticos sao mais pobres em SiO2, Fe2O3, Al2O3 e CaO e mais ricos em MgO que seus equivalentes de grao medio. Uma reconstituicao paleogeografica sugere que o ambiente de formacao dessas rochas se aproxima de um sistema paralico, lagunar, com tendencias evaporiticas marcantes. As depressoes deveriam ter profundidade e comprimento variados e poderiam ser isoladas, em particular por barreiras estromatoliticas, ressecadas e inundadas por aguas marinhas e, episodicamente, por aguas continentais. As depressoes maiores e mais profundas correspondem as ocorrencias de magnesita espatica. Regionalmente, esses marmores gradam para marmores dolomiticos e estes, por sua vez, para marmores calciticos quase puros, o que sugere uma diferenciacao quimica no sistema paralico, segundo a qual a precipitacao de carbonato de calcio resultaria em aumento da razao Mg/Ca, favorecendo a precipitacao de magnesita. Os marmores ricos em magnesita sao, portanto, interpretados como de origem sedimentar e foram submetidos a importantes modificacoes diageneticas antes do metamorfismo e deformacao neoproterozoica.

Comportamento geoquímico dos elementos Terras Raras da Seqüência magnesiana pré-cambriana (~ 1.8 Ga) da faixa móvel Orós

O estudo geoquimico, atraves dos Elementos Terras Raras (ETR) em sequencias carbonaticas magnesianas, e ainda raro, sobretudo no Brasil. As razoes sao varias e envolvem desde o baixo teor desses elementos juntos a essas rochas, outrora, aquem do limite de deteccao dos equipamentos analiticos ate a falta de conhecimentos sobre a distribuicao desses elementos ao seio das rochas carbonaticas. Todavia, com a evolucao da instrumentacao analitica (ICP), capaz de detectar valores extremamente baixos e com o melhor conhecimento sobre a distribuicao desses elementos em ambientes sedimentares, a sua utilizacao tem sido uma ferramenta indispensavel a compreensao de sequencia de rochas carbonaticas magnesianas. O conhecimento desses elementos, na sequencia metacarbonatica magnesiana pre-cambriana da Faixa Movel Oros (~l,8Ga), que e caracterizada por uma assembleia de rochas metacarbonaticas, que vai de marmores calciticos (mar aberto) a marmores dolomiticos e magnesiticos (ambientes marinhos paralicos), revelou que ha um fracionamento de elementos terras leves (ETRL), indo dos marmores calciticos, atraves dos marmores dolomiticos, aos marmores magnesiticos. Nos marmores calciticos, os ETR estao em associacoes com as fracoes argilosas, enquanto nos marmores magnesiticos parecem ocorrer associados aos complexos orgânicos e/ou complexos soluveis. Os marmores magnesiticos apresentam ainda uma anomalia positiva de Ce e negativa de Eu. Isso pode indicar um ambiente plataformal marinho raso com influencia redutora e continental (lagunar). Os marmores magnesiticos mostram tambem uma anomalia positiva de Ce e Eu em relacao a agua do mar atual, sugerindo diferencas composicionais, materializadas, sobretudo, por um maior enriquecimento desses elementos nas aguas dos mares primitivos.