Navegando por Assunto "Granito Água Boa"

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Acesso aberto (Open Access)
Greisens e Epi-sienitos potássicos associados ao granito água boa, Pitanga (AM): um estudo dos processos hidrotermais geradores de mineralizações estaníferas
(Universidade Federal do Pará, 2002-10-23) BORGES, Régis Munhoz Krás; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
Three stanniferous greisen types were characterized in the western border of Água Boa pluton, Pitinga mine (AM), associated with the rapakivi granite facies: greisen 1 (Gsl), composed mainly by quartz, topaz, brown siderophyllite and sphalerite; greisen 2 (Gs2), composed essentially by quartz, phengite and chlorite; greisen 3 (Gs3), composed of quartz, fluorite and phengite, with minor green siderophyllite. Besides these rocks, a potassic episyenite (EpSK) was identified associated with the Gs2. In spite of the compositional and petrographic differences, all of these hydrothermal rocks derived from a same protholith, a hornblende biotite aikali feldspar granite to syenogranite. The Gsl shows an inner mineralogical zoning defined by topaz or siderophyllite predominance. Along drill cores, the siderophyllite-rich zone occurs near the contact with the greisenized grafite and the topaz-rich zone is situated far from the grafite contact. The brown siderophyllite displays moderated Al contents, and its compositional changes can be explained by Fe+2 substitution for A1+3 and Li in octahedral sites, with a coupled Al+3 substitution for Si+4 in tetrahedral sites. The mineralogical zones in the Gs2 are physicaliy separated in leveis with phengite or chlorite predominance. The mica of Gs2 is a phengite, whose chemical variation is due to substitution of viAl for Fe+2, coupled with Si+4 enrichment. The calculated Li contents in phengites are lesser than those estimated in siderophyllite. The green siderophyllite from Gs3 is VIAl richer and F poorer than Gs1 brown siderophyllite, and the phengite displays two compositional types: an early Fe+2-poor aluminous phengite and a later Fe+2- F-rich one whose chemical variation is similar to that of Gs2 phengite. The chlorite from the three greisen is a Fe-rich daphnite, and its compositional range is due to VIAl substitution for R+2 cations, coupled with Si+2 enrichment. The aluminous chlorite displays higher temperature formation than ferrous one, according to the geothermeter proposed in the literature. The Pitinga greisens were formed by different processes of interaction among three main fluids: (1) low salinity, F-rich, aquo-carbonic fluid, with initial temperatures between 400° -350°C, present during Gsl and Gs3 formation; (2) low salinity aqueous fluid, with a temperature around 300°C, which during a progressive salinity increasing process, originates a moderate to high salinity residual fluid, with temperatures between 200° - 100°C, present during the Gs2 formation and silicification stage of EpSK; (3) low salinity aqueous fluid, with temperatures between 200° - 150°C, which interplayed with the others two fluids in differents grades, contributing to the formation of ali the hydrothermal rocks. The first two fluids has seemingly an orthomagmatic origin while the latter has a surface characteristic (meteoric water?). Moreover, the data suggests that the fluid responsible by the initial stage of the episyenitization process was not registered in the studied samples. These fluids were trapped in pressure conditions around 1 Kbar, representing high crustal levels conditions, similar to that of the stanniferous granites from Pitinga. Both episyenitization and greisenization processes occurred without volume changes in the granitic protholith, and the density differences of the altered rocks were caused by the mass variations along the alteration processes. The greisenization process caused a extensive loss of Na2O and K2O, while SiO2 showed a immobile behaviour in Gsl but was parcially removed in Gs2. The Al2O3 was depleted during the Gs2 formation but added in Gsl. The Fe2O3 (Fe total), Sn, S, volatiles LOl and F were the responsible by the mass increase at greisenization. In the Gsl, the chemical changes in the fiuids were caused by F activity decrease and fO2 increase during cooling. These changes also originated the differentiation between the ZT, in the inner portions of the fratures/conducts, and the ZS, nearest to surrounding gravite. The Gs3 was formed in more oxidizing conditions by F-poorer fiuids than those trapped in the ZS. The dissolution cavities generated during the episyenitization process increased the permeability of the altered rocks, providing an increase of fluid/rock ratios in the EpSK and Gs2 sites. The interaction between aqueous fluid and EpSK feldspar, during the Gs2 formation, caused a continuous salinity increase. The ZF was formed in the early stages of this interaction, at higher temperatures, while the ZC was originated by the more cold and saline, residual fluid. The latter was also trapped in the quartz filling cavities in the EpSK during the later silicification stage. In this way, the greisens and the potassic episyenites were generated from interactions among, at least, three fluids of seemingly independent origin, from a same protholith, in shallow crust conditions. The fO2, F activity and salinity variations, during the hydrothermal system cooling, and the contrast in fluid/rock ratios caused by permeability differences, were very important factors to greisen differentiation. These factors controlled greatly the fluids compositional changes, and caused the cassiterite and sulphides precipitation in the greisens and the Sn- S-enrichment during later greisenization of EpSK.
Acesso aberto (Open Access)
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.