Navegando por Assunto "Greisen"

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Geologia, petrografia e geoquímica do granito Bom Jardim, região de São Félix do Xingu, Província Mineral de Carajás
(Universidade Federal do Pará, 2009-05-22) PINHO, Sabrina Cristina Cordovil; LAMARÃO, Claudio Nery; http://lattes.cnpq.br/6973820663339281; https://orcid.org/0000-0002-0672-3977
In the Carajás mineral province, located in the Amazonia Central geochronologic province, three important granite suites (Fig 1) were individualized on geologic, petrochemical, geochronological and isotope data in Jamon, Velho Guilherme and Serra dos Carajás (CPRM/DNPM 1999, Teixeira et al. 2002a, Dall´Agnol et al. 2005). These three suites are composed of ~1.89 to 1.86 Ga (Tab. 1) nondeformed stocks and batholiths. The absence of deformation along the massifs, the discordant character of the plutons and the presence of micrographic intergrowths suggest a high level of emplacement. All granites display characteristics of A-type granites and within-plate signature. The plutons of the Jamon suite show oxidized character, while the plutons of the Serra dos Carajás and Velho Guilherme suites are moderately reduced and reduced, respectively. In the domain of the Velho Guilherme suite, São Felix do Xingu region, the massifs Antonio Vicente, Serra da Queimada, Velho Guilherme, Mocambo, Ubim-sul and Benedita are tin (±W)-mineralized granites. The mineralization is related to evolved granitic facies affected by late to postmagmatic alteration or hosted in small bodies of greisens. The granitic rocks of the Velho Guilherme suite are dominantly hololeucocratic to leucocratic, monzogranite to syenogranite with subordinate alkali-feldspar granite. Show low contents of TiO2, Al2O3, CaO, MgO, P2O5, Sr, Ba and Cl and moderate of Hf, U and Zr. Rb, Y, F, Li, Th, Nb, Ta, Ga, and Rb/Sr, Rb/Ba, F/Cl ratios are systematically high. The Bom Jardim granite, inserted into Velho Guilherme suite, is a rounded Sn-W mineralized massif that cover ~ 400 km2, outcrops south of São Felix do Xingu city, and is intrusive in intermediate to felsic volcanic rocks of the Uatumã Group. It is dominantly constituted by isotropic, pink and medium-to coarse grained monzogranite and syenogranite. Chloritized biotite is the main primary mafic phase. Aplite dikes are common cross cutting the granite in different directions. These rocks are affected in different intensities by late- to posmagmatic alteration. Greisenized rocks containing small primary concentrations of cassiterite+wolframite, besides quartz veins with milimetre to centimetre-sized crystals of wolframite+pyrite+fluorite, were identified in pervasively altered cupolas. Magnetic Susceptibility (MS) data showed values of 5,34x10-4 SIv (minimum valor) to 9,18x10-4 SIv (maximum valor), similar to those found in the syenogranitic rocks of the Antonio Vicente pluton. Scanning Electron Microscopy (SEM) studies indicate that the Sn and W mineralization are dominantly associated with syenogranite and greisenized rocks. The zircons of the different facies of the Bom Jardim granite are characteristically enriched in Hf, Y, Th and U, and display low Zr/Hf ratios, such as zircon crystals of other specialized tin-granites of the Amazonian Craton. The obtained results demonstrate that a preliminary evaluation of the potential for Sn and W (±Nb+Ta) in evolved granites may be based on zircon compositional studies and in EDS semi-quantitative analyses obtained with a SEM. Geochemically, the rocks of the Bom Jardim granite have low contents of TiO2, MnO, MgO, CaO e P2O3. The Al2O3 is near of 13%, with RG1 displaying higher values (14%). Na2O e K2O show medium contents, except in the RG. The Fe2O3 rarely is superior to 2.0%, but it is higher in the greisenized rocks 1 (GR1), with ~7% (medium value) and greisenized rocks 2 (GR2), with ~3.7% (medium value), probably resulting from the presence of syderophyllite. Rb and Ba show inverse behavior. Rb increase in the sense of MzG-LMzG-SGRG1-RG2, with average of 294, 711,790, 874 and 1835 ppm, respectively. Ba display, in this same sense, medium contents of 670, 87, 18, 9 and 2 ppm, respectively. Sr shows similar behavior of the Ba, except in the GR, originating an increase in the Rb/Sr ratios in the sense of the more fractionated rocks. Sn and W show enrichment from less evolved to the more evolved rocks, with the GR2 displaying higher concentrations. F and Li are high in all studies facies (1400 to 3000 ppm and 15 to 134 ppm, respectively), but the higher values are found in the GR1 (21000 ppm and 698 ppm). The Rare Earth Elements (REE) show similar fractionated patterns, with negative Eu anomalies increasing from monzogranite to syenogranite rocks. The GR have similar patterns, but the GR1 are more enriched in REE than the GR2. The geochemical diagrams indicate that the fractionated crystallization was the dominant process during the magmatic evolution of the Bom Jardim granite, such as observed in others granitic bodies of the Velho Guilherme suite. Based in the obtained data and in the petrographic, geochemical and geochronological similarity with others granites of the Velho Guilherme suite, the Bom Jardim granite can be included in the context of this important Paleoproterozoic granitic suite.
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Petrologia e química mineral dos greisens associados ao Granito Água Boa - Mina Pitinga (AM): um estudo dos processos de formação de greisens
(Universidade Federal do Pará, 1997-04-23) BORGES, Régis Munhoz Krás; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
This Master's Dissertation has as its main objective the petrographic and mineral chemistry study of the greisens associated with the Água Boa Granite, in the Pitinga mine (AM), based on core samples from a drilling network carried out on the west edge of the body by the Grupo Paranapanema S/A. The Pitinga mine is one of the world's largest producers of tin, in addition to containing considerable mineralization of cryolite and rare metals, such as Zr, Nb, Ta, Y, REE, etc. The tin-bearing granites of the region present Paleoproterozoic ages, and are inserted in the geological-geotectonic context of the Central Amazon Province. The data accumulated until today show that the Pitinga Tin Province has three primary sources for mineralizations, the albite-granite associated with the Madeira Granite, the greisens and epi-syenites associated with the Água Boa Granite. The host granites of the greisens are predominantly alkali-feldspar-granites, with local variations for syenogranites, exhibit a medium to coarse serial texture, gray to grayish-pink colors, are isotropic, and locally have rapakivitic features. Hornblende and biotite are the varietal mafics; Alanite, opaque minerals, zircon, apatite and fluorite are accessories and pistacite, chlorite and carbonates are secondary minerals. The greisens studied are endogreisens, being located essentially in the apical portions of the granite, being controlled by joints. In the petrographic study carried out, two main typologies were distinguished based on their mineralogical and textural characteristics: Greisen Gs1: this type of greisen is the one with the greatest areal expression, forming continuous zones of up to 5 meters, interdigitated with greisenized granites. It is a black rock, with a medium granular texture and essentially composed of quartz, reddish brown siderophyllite and topaz, accompanied by variable amounts of sphalerite, pyrite, chalcopyrite, cassiterite, zircon, fluorite, siderite and anatase. In facies where topaz is more abundant, there is a noticeable decrease in the content of siderophyllite, sphalerite and pyrite, as well as textural evidence of replacement of the latter. Cassiterite preferentially associates with partially chloritized siderophyllites, or forms thin crowns around pyrite and sphalerite. Sphalerite is an important phase in this greisen, associating with chalcopyrite. Greisen Gs2: this greisen normally occurs as bands or veins of up to 3.5 meters thick, interdigitated with greisenized granites. Topaz, sphalerite, zircon, fluorite, anatase, pyrite, chalcopyrite, galena, cassiterite and, locally, light green siderophyllite, siderite and beryl, complement the mineralogy. Fluorite predominates markedly over topaz. The association of phengitic muscovite with green chlorite is characteristic of this greisen. Both are interdigitated and, sometimes, there is evidence of substitution of chlorite by muscovite. The petrographic data indicate the presence of greater volumes of cassiterite in this greisen. The mineral chemistry studies were carried out from quantitative analyzes (WDS) in electronic microprobe of the minerals that form the main paragenesis of greisens, such as in the siderophyllites of greisens Gs1 and Gs2, and in the phrengites and chlorites of greisen Gs2, in addition to some analyzes in biotites, amphiboles and primary plagioclase from the host granite. Greisens Gs1 and Gs2 occur in different domains in the Guinho-Baixão grid, establishing a well-defined geographical-mineralogical zonation. Data relating to the chemistry of the main minerals forming these greisens indicate that the paragenesis of Gs1 was formed at relatively higher temperatures than those forming Gs2. This is corroborated by the greater amount of Cu and Pb sulfides existing in the latter, typical of lower temperature associations in the evolution of hydrothermal processes. Sphalerite may have been superimposed at lower temperatures in Gs1, not having formed in equilibrium with the higher temperature fluids. Furthermore, F apparently played an important role in the evolution of greisens, since its contents are lower in the Gs2-forming minerals, which may have been decisive for the formation of different paragenesis. The hydrothermal alteration halo associated with greisens, Gs2 is greater than that associated with Gs1, given that in their areas of occurrence there is a greater volume of intensely transformed granitic rocks, with obliterated primary textures and radically reduced quartz contents, in addition to the formation of dissolution cavities, generated by hydrothermal leaching. These features may be associated with episyenitization processes, detected east of the Guindo-Baixão grid. The greisens associated with the Água Boa Granite are different from the majority of the greisens studied in the literature, since their paragenesis are called siderophyllites and chlorites, with muscovites in smaller quantities, contrary to world examples. The petrographic study confirmed the close association of greisens with tin mineralizations. The analysis of petrographic and mineral chemistry data opens up some possibilities to explain the contrasting nature of greisens Gs1 and Gs2, and the evolution of their generating fluids: a) a hydrothermal fluid of the same global composition reacting with granitic rocks of different composition at the time of formation of greisens. This hypothesis is not supported by the variations of the greisens host observed on a microscopic scale; b) a hydrothermal fluid of identical initial composition, but which differed at some point in its evolution, which would have generated the geographic-mineralogical zonation observed in greisens. In this case, it is considered that the host rock has the same global composition, and that local physicochemical conditions have caused its differentiation. c) in addition to possible variations in the nature of the fluids over time, it is likely that the more intense hydraulic fracturing observed in the areas where Gs2 occurs has contributed to the differences between greisens Gs1 and Gs2. It would lead to a dispersion of fluids over a larger volume of host rock, facilitating hydrothermal alteration at first, but making it difficult later on, as it requires an exceptionally high volume of fluids, not available in the hydrothermal system in question.