EM - Escola de Minas

URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/6

Notícias

A Escola de Minas de Ouro Preto foi fundada pelo cientista Claude Henri Gorceix e inaugurada em 12 de outubro de 1876.

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Filtros

2006 - 200912010 - 20201

Configurações

Tem arquivos: SimAssunto: search.filters.subject.Hydrothermal alteration

Resultados da Pesquisa

Agora exibindo 1 - 2 de 2
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    Stable isotopes and fluid inclusion constraints on the fluid evolution in the Bacaba and Castanha iron oxide-copper-gold deposits, Carajás Mineral province, Brazil.
    (2020) Pestilho, André Luiz Silva; Monteiro, Lena Virginia Soares; Melo, Gustavo Henrique Coelho de; Moreto, Carolina Penteado Natividade; Juliani, Caetano; Fallick, Anthony Edward; Xavier, Roberto Perez
    The evolution of the Bacaba and Castanha iron oxide-copper-gold deposits, located in the Carajás Mineral Province, Brazil, is discussed based on petrography, scanning electron microscopy, stable isotopes, and fluid inclusion analyses. The Castanha deposit is mainly hosted by ca. 2.75 Ga subvolcanic and volcanic rhyodacitic rocks, and gabbros. Early sodic (albite, scapolite) alteration was followed by high-temperature calcic-iron (actinolite-magnetite), potassic (biotite), and minor chlorite and sericite alteration. Calcite, REE carbonate, and epidote represent a late and proximal alteration to ore bodies. Ore breccias with Durchbewegung structure comprise chalcopyrite + pyrrhotite + pyrite ± cobaltpentlandite ± sphalerite ± marcasite and are notable due to their nickel-(zinc) enrichment. The Bacaba Deposit is hosted by the ca. 3.00 Ga Bacaba Tonalite, 2.85 Ga Serra Dourada Granite, and gabbro bodies. Early (ca. 2.70 Ga) alteration at Bacaba includes sodic (albite, scapolite), iron (magnetite), and potassic-iron (K feldspar-magnetite, biotite) associations. Well-developed late chlorite, albite, sericite, calcite-hematite-(musketovite) alteration formed during a Paleoproterozoic overprinting (ca. 2.06 Ga). The Bacaba ore is composed of (I) chalcopyrite ± magnetite ± bornite, and (II) chalcopyrite ± pyrite ± hematite/musketovite, related to early potassic-iron and late alteration, respectively. The Castanha deposit was formed from magmatic fluid (δ18OH2O = 9.5 ± 0.5‰ to 5.2 ± 1.0‰, at 500 to 400 °C) and sulfur (δ34S = 0.1–3‰) sources, with a limited contribution of externally-derived fluids during its evolution. Ore precipitation progressed under considerably low fS2 and fO2 conditions, at relatively high temperatures (> 370 ± 50 °C). Fluid inclusion analyses indicate greater proximity of the Castanha deposit to the source of a hot overpressured magmatic fluid, suggesting its formation in a high-temperature hydrothermal center. Fluidrock interaction coupled with increasing pH might have been the critical factors in destabilizing the metalchloride complex in the Castanha deposit. The Bacaba deposit evolved from a fluid-mixing between hot (> 450 °C) hypersaline CaCl2-NaCl-bearing magmatic brine (> 30 mass % equivalent) and a less saline, colder, and 18O-depleted and D-enriched fluid (e.g., seawater or low-latitude meteoric water). Mixing resulted in an oxidizing environment, dilution (salinities between 35 and 4 mass % equivalent), and temperature drop (160–190 °C), triggering the ore precipitation. At Bacaba, the slightly higher δ34S values (1.3–5.4‰) may reflect an additional contribution of externally-derived sulfur through the thermochemical reduction of oxidized sulfur species. In the Southern Copper Belt, the regional spatial distribution of the sulfur isotope compositions shows the highest δ34Ssulfide values close to the Paleoproterozoic Sossego Orebody and the Alvo 118 deposit. This might suggest significant involvement of externally-derived components (e.g., diluted fluids and sulfur) during late stages of a protracted hydrothermal evolution in the Carajás IOCG deposits.
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    Chemical zoning of muscovite megacrystal from the Brazilian Pegmatite Province.
    (2006) Viana, Rúbia Ribeiro; Evangelista, Hanna Jordt; Stern, Willem B.
    Um grande cristal de muscovita, macroscopicamente homogêneo, procedente do Pegmatito Cruzeiro, localizado na Província Pegmatítica Oriental, em Minas Gerais, exibe padrão de distribuição complexa para alguns elementos traços. Em estudos geocronológicos e petrológicos, como, por exemplo, na separação entre micas magmáticas e pós-magmáticas, a causa de zoneamento deve ser levada em consideração. O complexo zoneamento químico no cristal de mica estudado é melhor explicado pelo crescimento em um magma evoluído, seguido pela alteração, proveniente da percolação de fluidos hidrotermais. O enriquecimento de Rb nas bordas é interpretado como resultado da evolução química do magma residual durante o crescimento do cristal. A diminuição em (IVAl+VIAl), bem como o aumento de (Fe+Mg) e Si ao longo da fratura é explicado pela substituição hidrotermal celadonítica da muscovita. A alteração hidrotermal causou, também, a diminuição nos conteúdos de Rb, Ga, Y, Nb, Sn e Zn ao longo desta fratura, além da concentração residual de Ti. Elementos tais como, Ga, Y, Nb, Sn, e Zn, pouco considerados em discussão de diferenciação ou processos de alteração, mostraram significância tanto quanto os elementos alcalinos.