2019-09-032019-09-032012SOUZA, Sulsiene Machado de. Petrografia e mineralogia de epissienitos potássicos estaníferos e da alteração hidrotermal associada, na Borda Oeste do albita-granito, suíte Madeira, Província Pitinga (AM). Orientador: Régis Munhoz Krás Borges. 2012. 123 f. Trabalho de Curso (Bacharelado em Geologia) - Faculdade de Geologia, Instituto de Geociências, Universidade Federal do Pará, Belém, 2012. Disponível em: http://bdm.ufpa.br/jspui/handle/prefix/1903. Acesso em:.http://bdm.ufpa.br/jspui/handle/prefix/1903The Pitinga Province, located in the northeastern region of the Amazonas state, is the largest tin producer in Brazil. The petrographic study in the drill cores from the borehole 250S/1200W, which intercepts the contact zone between border albite-rich granite (ABGB) and porphyritic hypersolvus alkali feldspar granite (FAGHP), in the western part of the Madeira pluton, allowed the characterization of three rock types: 1) porphyritic hypersolvus alkali feldspar granite, 2) episyenitized border albite-rich granite (AGbEp) and 3) hybrid episyenitized granite (GEpsH). The FAGHP is composed mainly by phenocrysts of perthitic alkali feldspar and quartz, embedded in a fine quartz-feldspar matrix, and accessory contents of zircon, fluorite, hematite, pyrite and galena. It locally occurs slightly albitized, greisenized or episyenitized. The AGbEp is composed mainly by quartz and alkali feldspar, and subordinate cassiterite, zircon, thorite, fluorite, sericite and phengite, and in some places shows sulfides enrichment. The GEpsH have porphyritic texture, and consists primarily of alkali feldspar and quartz, and accessory amounts of zircon, fluorite, hematite, pyrite, galena, thorite and cassiterite, and is intensively hematitized and silicificated. The main hydrothermal process identified in this study is the potassic episyenitization, which modified significantly the rocks of the region. Other processes were also characterized, such as albitization, greisenization, sulfidation, silicification and hematitization, and all of these contributed to the formation of the episyenitized rocks. The early alteration stage of potassic episyenitization was the dequartzification of granitic rocks, which induced the quartz leaching and the vugs formation. The intensive fluid circulation during all the hydrothermal activity was triggered by connection of these cavities. The complete dissolution of albite crystals from ABGB, as well as its replacement by hydrothermal microcline, was caused by potassic metassomatism. The magmatic alkali feldspar was partially replaced by phyllosilicates, mainly at greisenization stage. In some places, the perthitic alkali feldspar phenocrysts of the porphyritic hypersolvus alkali feldspar granite are partially replaced by albite at the margins. The hybrid episyenitized granite is composed by mineral phases inherited from both granitic facies (ABGB and FAGHP), and is the more representative rock of the interaction (mingling?) between the two magmatic liquid. Besides the microcline formed in subsolvus conditions, this rock contains a fine population of hydrothermal microcline crystals, which generally filling open 9 spaces among the magmatic minerals. The Sn orebody associated to these rocks is composed by both magmatic cassiterite crystals derived from the ABGB and crystals precipitated from hydrothermal fluids, as demonstrated by mineralogical and textural features obtained by MEV-CL-EDS studies. The cassiterite crystals which filling vugs and/or fractures show almost pure composition and lesser Nb contents than that formed at magmatic stages. The fluids involved in the potassic episyenitization should have been alkaline and subsaturated in silica to have induced the dequartzification, but with low grade of alkalinity, since highly alkaline fluids would tend to promote sodic metasomatism, with stabilization of albite. Based on obtained data and geologic context of the area, two hypotheses has been proposed to the origin of an alkaline, oxidized and K-rich fluid: (1) separation of a fluid phase from the magmatic liquid associated to FAGHP; and (2) the same fluid which caused the autometassomatic alteration of the albite-rich granite core facies. Whichever it may be the origin, this fluid probably would be modified along the system cooling and promoted sulfidation, hematitization and silicification processes, with the filling of vug and cavities by Fe-, Cu-, Pb-, Zn-sulfides, and hematite and quartz, associated to cassiterite in same places. Whereas the main Sn deposit is associated with magmatic evolution of the Madeira pluton, the cassiterite crystals formed during potassic episyenitization are tin-richer.Acesso AbertoPetrologiaMineralogiaProcessos hidrotermaisEpissienitos potássicosProvíncia PitingaAmazonas (AM)CNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOLOGIATrabalho de Curso - Graduação - Monografia