Navegando por Assunto "Greisens"

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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)
Mineralogia dos greisens da área Grota Rica, Plúton Água Boa, Pitinga, Amazonas
(2011-09) FEIO, Gilmara Regina Lima; DALL'AGNOL, Roberto; BORGES, Régis Munhoz Krás; COSTI, Hilton Túlio; LAMARÃO, Cláudio Nery
The topaz-alkali-feldspar-granite, the most evolved facies of the Água Boa pluton, was affected by hydrothermal alteration, represented by greisens and quartz veins, the main host for Sn- and subordinated Zn mineralization. The greisens are classified as quartz-topaz-siderophyllite-greisen, topaz-siderophyllite-greisen and quartz-topaz-quartz-greisen. They are composed essentially of quartz, topaz and siderophyllite, accompanied by variable amounts of fluorite, zinnwaldite, sphalerite, cassiterite, zircon and anatase and locally Ce-monazite, galena, pyrite, chalcopyrite and native bismuth. EMPA studies allowed identifying three types of micas: (1) brown siderophyllite from topaz-granite; (2) the green siderophyllite of greisens and (3) zinnwaldite, weakly colored, found as thin and discontinuous rims around green siderophyllite, and quartz vein. The siderophyllite composition of the granite towards greisens shows an evolution of siderophyllite to Li-siderophyllite with increase of VIAl, Li and Si contents. On the other hand, the siderophyllite of the greisen was partially replaced by zinnwaldite, with increase of VIAl, Li and Si contents. The cassiterite in the greisens forms euhedral to subeuhedral, twin and zoned crystals, with strong pleochroism. It occurs as aggregates together with topaz, quart and fluorite. The pure composition and the low content of the Nb and Ta of cassiterite indicate hydrothermal conditions.
Acesso aberto (Open Access)
Petrografia e geoquímica dos greisens associados ao topázio-granito do pluton Água Boa, Província Estanífera de Pitinga (AM).
(Universidade Federal do Pará, 2007-03-26) FEIO, Gilmara Regina Lima; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
The Pitinga Tin Province is situated 300 km north of the city of Manaus (state of Amazonas, Brazil) and is characterized by important world-class Sn, Nb, Ta and Zr deposits, related to the Madeira Suite, comprised by Paleoproterozoic (~1.82 Ga) A-type granites. The primary magmatic mineralization occurs in an albite-granite of the Madeira pluton, and the hydrotermal mineralization occurs in episyenites and greisens associated with the Água Boa pluton. Plutons of the Madeira Suite intrude the Paleoproterozoic acid volcanic rocks of the Iricoumé Group (~1.88 Ga). The Água Boa pluton has elliptic form, is alongated along NE-SW strike, covers near 350 km2 and is composed of four facies. The earlier facies is a metaluminous amphibole biotite alkali feldspar granite, locally showing rapakivi texture. It was followed, in order of emplacement, for porphiritic biotite granite and an equigranular to seriate biotite granite, both metaluminous to peraluminous; the later facies is a peraluminous, porphyritic topaz-bearing biotite alkali feldspar granite, named topaz granite. The tin mineralization in the Água Boa pluton occurs in altered metasomatic zones, formed by episyenites and greisens. The host rocks of the studied greisens are the topaz-bearing biotite alkali feldspar granite facies of the Água Boa pluton. Two textural variations were distinguished: a gray to pink fine- to medium-grained phase to pink porphyritic phase and albitized granite. Cassiterite-bearing leucogranite pegmatites, weakly albitized, occur transitionally between granite and greisens. Greisen formation is controlled by fractures and greisens occur as continuous zones up to 6 meters thick (F06Gr Grota Rica drill core), transitional to greisenized granites. Greisens are inequigranular light to dark grey, medium- to coarse-grained. They are composed essentially by quartz, green siderophyllite and topaz, with additional variable amounts of fluorite, zinnwaldite, sphalerite, cassiterite, zircon and anatase. Greisens are classified as quartz topaz siderophyllite greisen, topaz siderophyllite quartz greisen and topaz quartz greisen. Scanning electron microscopy studies indicate that these rocks also contain trace Ce-monazite, galena, pyrite, chalcopyrite and native bismuth. Coarsegrained quartz-only or sphalerite ± zinnwaldite bearing quartz veins cross-cut the greisens. Geochemistry data, including mass balance calculations, supported by petrographic observations, show that greisenization processes took place without changes in volume. These processes resulted in gain of Fe2O3t and more significant loss of Na2O, MgO, CO2 and K2O. Almost complete removal of Na2O and partial removal of K2O are related to the destabilization of feldspar and are the main characteristic of greisen formation. The distinct behavior of K2O is due to the retention of K in newly formed micas. Apparent immobility of Ca can be explained by low host granite Ca contents and by its retention in secondary fluorite. During greisenization S, F, Zn, Cu, Sn, Pb, Ta, Rb and U were enriched, while other elements declined. Increases in Fe, S, Zn, Cu and Pb are related to sulfide formation. Among the lithophile elements, Rb is strongly enriched in the greisen due to its retention in the siderophyllite structure, whilst Ba and Sr are removed during feldspar alteration. The rare earth elements (REE) reveal little mobility and patterns very similar to the granites. In general, they present similar patterns and slightly lower contents in greisens in comparison to the granites. REE depletion occurred during the formation of the quartz-topaz-siderophyllite-greisen and greisen relatively rich in quartz shows greater losses in light REE relative to heavy REE. Mineral chemistry allowed for classification of the brown micas within the topaz-bearing granite as annite transitional to siderophyllite, green micas from greisen as siderophyllite, and the late pale micas from greisens and quartz veins as zinnwaldite. The evolution of mica from the granite to greisen is given by annite → siderophylite, showing increasing content of VIAl, VI, Li and Si. The greisen siderophyllite was, in turn, partially replaced by zinnwaldite, also with increased content of VIAl, VI, Li and Si. The Kbearing feldspar phase analysed in granite and leucogranite pegmatite is orthoclase (Or93-98). Albite (Ab95-99) occurs as lamellae within perthites and intergranular growths. Cassiterite forms strongly pleochroic, twinned and zoned euhedral to subhedral crystals with low Nb and Ta contents. Topaz-bearing biotite alkali feldspar granite was postdated by localized formation of cassiterite-bearing leucogranite pegmatites and both were affected by post-magmatic alteration in form of intergranular albite. This process was followed by strong hydrotermal alteration represented by greisenization, and later local silicification that culminated in the formation of greisen and quartz veins, the main hosts for Sn-mineralization, and subordinated Zn-mineralization.
Acesso aberto (Open Access)
Quartzo e zircão como marcadores da evolução magmático-hidrotermal do Granito Antônio Vicente, Suíte Intrusiva Velho Guilherme, Província Carajás
(2013-06) LAMARÃO, Cláudio Nery; ROCHA, Kellen Katucha Nogueira; MARQUES, Gisele Tavares; BORGES, Régis Munhoz Krás
Four morphological and textural types of quartz, informally labeled Qz1, Qz2, Qz3 and Qz4, were identified in the different facies of the Antônio Vicente Granite, Carajás Province, by scanning electron microscope-cathodoluminescence (SEM-CL) images. In the less evolved rocks, containing amphibole and biotite, well developed anhedral to subhedral, luminescent and intensely fractured crystals dominate, named Qz1. Hydrothermal fluids that percolated the granite modified the magmatic quartz (Qz1) into Qz2 and Qz3 through processes of alteration, dissolution and recrystallization, with these changes much more evident in the intensely altered syenogranite rocks. Qz4 constitute medium-to-coarse grained crystals, usually luminescent and comparatively little fractured. Its occurrence is restricted to strongly hydrotermalized syenogranite rocks and bodies of greisens, suggesting the beginning of the greisenization process. In the greisens, medium-to-coarse grained euhedral, concentrically zoned quartz crystals dominate, with typical features of hydrothermal origin (Qz5). Fine crystals of zoned cassiterite (≤ 100 µm) are common and fill cavities in the types Qz4 and Qz5. Zircon crystals dominantly anhedral, corroded, with the highest contents of Hf and the lower Zr/Hf ratios belong to more evolved and hydrothermally altered rocks and to associated greisens, both carriers of Sn mineralization. This fact suggests that the geochemical signature of zircon, especially Zr/Hf ratio, can be used for the preliminary assessment of metallogenic potential of tin granites.