Browsing by Author "Silva, M.M.V.G."
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- Caracterização geoquímica de minerais de jazigos da região de Segura (Castelo Branco)Publication . Antunes, I.M.H.R.; Neiva, A.M.R.; Silva, M.M.V.G.A região de Segura é uma área mineira actualmente abandonada, tendo sido explorada para Sn, W, Ba, PB e Zn entre 1942 e 1953. Os jazigos minerais são filões de pegmatito granítico estanífero-litinífero, filões de quartzo com cassiterite e volframite e filões de quartzo com barite, galena e blenda. Os pegmatitos graníticos e os filões de quartzo mineralizados intruíram dominantemente o Complexo Xisto-Metagrauváquico, de idade Câmbrica, embora alguns atravessem também os granitos hercínicos. A cassiterite dos pegmatitos estanífero-litiníferos é zonada e possui exsoluções de mangano-columbite e de magano-ferrocolumbite, particularmente nas zonas escuras. A cassiterite dos filões de quartzo com cassiterite e volframite não é zonada e não apresenta exsoluções. A cassiterite destes filões de quartzo é mais rica em Ti e mais pobre em Nb e Nb+Ta do que a cassiterite dos pegmatitos. A volframite ocorre apenas nos filões de quartzo com cassietrite e volframite e é enriquecida na componente ferberite. Nestes filões de quartzo, foram, ainda, encontrados moscovite, pirrotite, arsenopirite, pirite, blenda, calcopirite, estanite, matildite e schapbachite. Nos filões de quartzo com barite, galena e blenda, ocorrem conjuntamente cristais de apatite, moscovite, clorite, cobaltite, pirite e calcopirite. A barite tem composição homogénea.
- Contaminação ambiental provocada pela antiga mina de urânio de Pinhal do Souto, centro de PortugalPublication . Neiva, A.M.R.; Carvalho, Paula; Antunes, I.M.H.R.A.; Silva, M.M.V.G.; Santos, A.C.T.; Pinto, M.M.S.C.; Cunha, Pedro ProençaA mina de urânio de Pinhal do Souto foi explorada subterraneamente entre 1978 e 1989, tendo produzido 93091 kg de U3O8. Na área ficaram duas escombreiras, parcialmente cobertas por vegetação natural. Nesta mina foi explorado um filão de quartzo contendo autunite e torbernite, que corta um granito de duas micas. A mina está a causar contaminação em U, As, Cd, Cr, Cu, Fe, Ni e Pb nas águas, em Cr, Mn, Pb, Cd, Co, Fe, Th, Zn e, por vezes, em U nos sedimentos de corrente e em Cu, Pb, Zn, As, Cd e Sb nos solos. Os valores da mediana de Fe, As, Cd, Pb, Sb, Th, U, W e Zn destes solos são superiores aos dos solos dos países europeus dos dados FOREGS. forma complexos com nas águas que têm pH neutro a alcalino. As concentrações de U na água são superiores aos limites para uso humano nas épocas húmida e seca, mas são maiores na época húmida por os minerais secundários de U se dissolverem e o U ser libertado na água. Os teores de metais e As são maiores na água da época seca devido à evaporação. Os solos têm vermiculite e, por isso, retêm maiores concentrações de metais do que os sedimentos de corrente que possuem caulinite. Devido aos valores de pH ligeiramente ácidos a neutros das águas, há a ocorrência de precipitados de óxido de Fe, sobretudo nas saídas de galerias subterrâneas, que são ricos em óxidos-hidróxidos e matéria orgânica e que retêm grandes quantidades de metais U, Th e As.
- Contaminated water, stream sediments and soils close to the abandoned Pinhal do Souto uranium mine, central PortugalPublication . Neiva, A.M.R.; Carvalho, Paula; Antunes, I.M.H.R.; Silva, M.M.V.G.; Santos, A.C.T.; Pinto, M.M.S.C.; Cunha, Pedro ProençaThe Pinhal do Souto mine exploited a quartz vein containing uranium minerals, mainly autunite and torbernite. This vein intersects a two-mica granite containing 10 ppm U and uraninite. The mine was exploited underground and produced 93091 kg U3O8 between 1978 and 1989 and was then closed down. Two dumps were left in the mine area and these are partially covered by natural vegetation. Groundwater and surface water have a similar slightly acid-to-alkaline pH. The 2 2 UO is abundant and complexed with 2 3 CO , under neutral to alkaline pH. Metals and arsenic concentrations in the water increase during the dry season due to the evaporation. Uranium concentration in the water increases (up to 104.42 g/l) in the wet season, because secondary uranium minerals are dissolved and uranium is released into the water. Soils tend to retain a higher concentration of several metals, including U (up to 336.79 mg/kg) than stream sediments (up to 35.68 mg/kg), because vermiculite from the former could adsorb it more easily than could kaolinite from the latter. The Fe-oxides precipitate retains the highest concentrations of several metals, including U and Th (up to 485.20 and 1053.12 mg/kg, respectively) and the metalloid As, because it is richer in oxyhydroxides and organic matter than stream sediments and soils. The median concentrations of Fe, As, Cd, Pb, Sb, Th, U, W and Zn in soils from this area are higher than in European soils of the FOREGS data. Waters from dry and wet seasons, stream sediments and soils are contaminated and must not be used. This area was compared with another Portuguese abandoned uranium mine area. The former mine caused a lower environmental impact attributable to it having lower sulfide concentration and mineral alteration than in the latter.
- Contamination of soils and waters related to cassiterite-wolframite and baryte-galena-spharelite mineralization from Segura, PortugalPublication . Antunes, I.M.H.R.; Neiva, A.M.R.; Silva, M.M.V.G.Contamination of soils and waters related to cassiterite-wolframite and baryte-galena-spharelite mineralization from Segura, Portugal.
- Geochemical and isotopic constraints on the petrogenesis of Early Ordovician granodiorite and Variscan two-mica granites from the Gouveia area, central PortugalPublication . Neiva, A.M.R.; Williams, I.S.; Ramos, J.M.F.; Gomes, M.E.P.; Silva, M.M.V.G.; Antunes, I.M.H.R.A biotite granodiorite and seven Sn-bearing two-mica granites crop out in the Gouveia area, central Portugal. A SHRIMP U–Th–Pb zircon age from the granodiorite, and monazite ages from four of the two-mica granites, show that they are of Early Ordovician (~480 Ma) and Permo-Carboniferous, i.e. Variscan (~305 and 290 Ma) age respectively. The Variscan two-mica granites are late- and post-D3. Major and trace element variation in the granitic rocks and their biotite and muscovite indicate mainly individual fractionation trends. The granitic rocks are mostly depleted in HREE relative to LREE. The biotite granodiorite is probably derived from igneous lower crust, as evidenced by low initial 87Sr/86Sr (0.7036), high εNdT (+2.5) and moderate δ18O (8.8‰). The two-mica granites are probably derived by partial melting of heterogeneous mid-crustal metasediments, mainly metapelite and some metagraywacke, as evidenced by their high initial 87Sr/86Sr (0.7076–0.7174), δ18O (10.7–13.4‰) and major element compositions. However, variation diagrams for major and trace elements from two of the muscoviteNbiotite granites and their micas define fractionation trends. Rb–Sr whole-rock analyses from the two granites are perfectly fitted to a single isochron and the rocks have subparallel REE patterns; the younger granite is derived from the older by fractional crystallization of quartz, plagioclase, biotite and ilmenite (tested by modelling major and trace elements). Most of the Sn-bearing granites are derived from distinct magma batches. They result from partial melting of a heterogeneous midcrustal metasediment. They do not represent a crustal anomaly in tin. Fractional crystallization is responsible for the increase in the Sn contents of the granites and their micas. Muscovite has a higher Sn content than coexisting biotite and is the principal host mineral for Sn in these rocks.
- Geochemistry and isotopic data of the granitic rocks from Castelo Branco pluton (Central Portugal)Publication . Antunes, I.M.H.R.; Neiva, A.M.R.; Silva, M.M.V.G.The Castelo Branco pluton is exposed over an area of 390 Km2 and consists of five late- to post-tectonic Hercynian granitic rocks, which intruded the Cambrian schist-metagraywacke complex. They are concentrically arranged. A medium- to fine-grained muscovite>biotite granite (G1) crops out in the pluton’s core and is surrounded by a medium- to fine-grained, slightly porphyritic biotite>muscovite granodiorite (G2), encircled by a medium- to coarse-grained porphyritic biotite>muscovite granodiorite (G3), grading into a medium- to coarse-grained porphyritic biotite = muscovite granite (G4). A coarse-grained muscovite>biotite granite (G5) forms only external parts of the pluton in the N and NE.
- Geochemistry and petrogenesis of biotite granodiorite and its microgranulares enclaves from Oledo-Idanha-a-Nova pluton (Central Portugal)Publication . Antunes, I.M.H.R.; Neiva, A.M.R.; Silva, M.M.V.G.The lower Ordovician medium-grained biotite of 479-480 Ma predominates in the Oledo-Idanha-a-Nova pluton and is deformed. It contains different biotite tonalitic and biotite granodioritic microgranular enclaves concentrated on the pluton’s west part. Microgranular enclaves and host biotite granodiorite are I-type granitic rocks and contain amphibole, sphene, allanite, ilmenite and magnetite.
- Geochemistry of granitic rocks and their feldspars and micas from Segura, Central PortugalPublication . Antunes, I.M.H.R.; Silva, M.M.V.G.Geochemistry of granitic rocks and their feldspars and micas from Segura, Central Portugal
- Geochemistry of granitic rocks from Gouveia area, Central PortugalPublication . Neiva, A.M.R.; Ramos, J.M.F.; Gomes, M.E.P.; Silva, M.M.V.G.; Antunes, I.M.H.R.Geochemistry of granitic rocks from Gouveia area, Central Portugal
- Geochemistry of granitoids from Castelo Branco, Central PortugalPublication . Antunes, I.M.H.R.; Neiva, A.M.R.; Silva, M.M.V.G.Geochemistry of granitoids from Castelo Branco, Central Portugal