G?nese e evolu??o mineral?gica, micromorfol?gica e geoqu?mica da bauxita de Espera Feliz, MG.

Esse trabalho tem como principal objetivo o estudo da evolu??o macromorfol?gica, mineral?gica, micromorfol?gica e geoqu?mica da ocorr?ncia de bauxita pr?xima ? cidade de Espera Feliz, no extremo sul da Serra do Capara?. A pesquisa foi desenvolvida em uma ?nica vertente, cuja morfologia ? caracterizada por diversas cavidades decorrentes de fei??es de escorregamento. Ap?s o levantamento macromorfol?gico de dois perfis de altera??o, expostos em uma cascalheira, e de 10 perfis de sondagem a trado, em que foram definidas e amostradas as diferentes f?cies de altera??o, procedeu-se as an?lises laboratoriais. Estas inclu?ram an?lises petrol?gicas e micromorfol?gicas ao microsc?pio ?ptico; mineral?gicas por difra??o de raios X; geoqu?micas por fluoresc?ncia de raios X; morfol?gicas e microqu?micas em microsc?pio eletr?nico de varredura acoplado com EDS e em microssonda eletr?nica. O manto de altera??o se desenvolve sobre rochas charnock?ticas com enclaves m?ficos da Su?te Capara?. As modifica??es da rocha devido aos agentes do intemperismo iniciam-se com a forma??o de um c?rtex de altera??o incipiente onde as solu??es encontram as primeiras descontinuidades caracterizadas, principalmente, por microfissuras. O ferro, exsudado dos minerais ferromagnesianos, precipita em vazios interminerais e intraminerais. A altera??o incipiente evolui para o c?rtex interno onde o plagiocl?sio, por alitiza??o direta, se altera em gibbsita, os ferromagnesianos iniciam a forma??o de boxworks goeth?ticos com n?cleos caulin?ticos e o ortocl?sio inicia a transforma??o indireta para gibbsita, formando uma fase intermedi?ria de illita e caulinita. No c?rtex externo restam principalmente remanescentes de ortocl?sio, envolvidos por gibbsita e goethita na forma de alteromorfos, que se reproduzido no manto de altera??o formar? um horizonte baux?tico isalter?tico. No manto de altera??o foram identificados 3 horizontes: i) horizonte argiloso caulin?tico, ii) horizonte baux?tico e iii) s?lum. Em um perfil, designado perfil A, o horizonte argiloso ? constitu?do pela f?cies isalterita caulin?tica; o horizonte baux?tico pelas f?cies bauxita isalter?tica, bauxita laminar e bauxita fragmentada. O horizonte argiloso ? formado devido a limita??o da atua??o do processo de alitiza??o, provavelmente pela presen?a de um enclave m?fico. O segundo perfil, designado perfil B, o horizonte argiloso ? constitu?do por uma f?cies aloter?tica caulin?tica e o horizonte baux?tico ? formado essencialmente pelas f?cies bauxita laminar desorientada e bauxita fragmentada, devido a sua mobiliza??o e degrada??o. Estes horizontes, com as respectivas f?cies, mostram correla??o com os furos de sondagem situados na mesma vertente. A morfologia da vertente e as f?cies est?o diretamente relacionadas, na borda da vertente, onde n?o h? depress?es causadas por escorregamentos ocorrem as f?cies sem evid?ncias de remobiliza??o (bauxita isalter?tica e bauxita laminar). J?, a jusante das cavidades, os perfis apresentam uma f?cies de argila aloter?tica sobreposta por um espesso horizonte de bauxita fragmentada onde os fragmentos de diferentes f?cies encontram-se em condi??es de instabilidade qu?mica. Regionalmente, a geomorfologia da ?rea apresenta sucessivas corredeiras e meandros abandonados em cotas mais altas que o curso do rio atual evidenciando uma reativa??o tect?nica, com um importante soerguimento, provavelmente no Oligoceno, seguido por um abatimento de blocos e eros?o. No Mioceno, per?odo mais est?vel e com clima quente e ?mido, formou-se o manto de altera??o baux?tico. Posteriormente, no Plioceno, per?odo ?rido, ocorreu a exposi??o da bauxita e os escorregamentos na vertente, formando as cavidades e, consequentemente, as f?cies baux?ticas remobilizadas situadas ? jusante das mesmas.The main purpose of this work is to characterize the macromorfological, mineralogical, geochemical and micromorphological evolution of the bauxite close to the Espera Feliz city, at the southern edge of the Capara? Ridge. The research was conducted in a slope, morphologically characterized by several cavities resulted from landslide features. The alteration facies were identified and sampled, for the laboratory analysis, after the macromorfological description of two representative alteration profiles and 10 auger boreholes. The laboratory analysis included petrological and micromorphological study by optical microscope; mineralogical characterization by X-ray difraction; geochemical analysis by X-ray fluorescence; micromorphological and microchemistry analysis by scanning electron microscopy coupled with EDS and microprobe. The alteration surface developed over charnockitic rocks with mafic enclaves from the Capara? Suite. The rock modifications begin with the formation of an horizon of incipient alteration due to the percolation of weathering solutions in the discontinuities, mainly characterized by micro-fissures. The iron exuded from ferromagnesian minerals, precipitate in the inter-minerals and intra-minerals voids. The horizon of incipient alteration evolves into an inner cortex where the plagioclase, changes in gibbsite by direct alitisation, the ferromagnesian minerals initiate the formation of goethitics boxworks with kaolinitics cores and the orthoclase initiates indirect transformation to gibbsite, forming an intermediate phase of illite and kaolinite. In the outer cortex remain mostly traces of orthoclase, surrounded by goethite and gibbsite as alteromorphics that can be related to the isalteritic bauxitic horizon from the alteration surface. In the alteration surface were identified 3 horizons: i) kaolinitic clay horizon, ii) bauxitic horizon, iii) solum. The profile A, is constituted from the bottom to the top by a clayey horizon that is characterized by kaolinitic isalteritic facies; a bauxitic horizon that is constituted of an isalteritic bauxite facies, a laminar bauxite and a fragmented bauxite. The kaolinitic clay horizon was formed because the action of the alitisation process was limited by the presence of a mafic enclave. The second profile, profile B, the clayey horizon is constituted by a kaolinitic aloteritic facies and the bauxitic horizon is essentially formed by the laminar disoriented bauxite facies and fragmented bauxite facies, due to its mobilization and degradation. All these horizons, with the respective facies, show correlation with the auger boreholes located on the same slope. The slope morphology and the facies are directly related. On the edge the slope, where there are no depressions caused by landslides, the facies occur without evidence of remobilization (laminar bauxite and isalteritic bauxite). Downstream of the cavities, the profiles have a clay aloteritic facies overlapped by a thick fragmented bauxite horizon where fragments of different facies are in conditions of chemical instability. Regionally, the geomorphology of the area shows consecutive waterfalls and meanders at higher altitudes than the river current, showing a tectonic reactivation, with a major uplift, probably in the Oligocene, followed by a block abatement and erosion. In the Miocene, period more tectonically stable and with warm and humid climate, formed the bauxitic alteration surface. Later, in the arid period of the Pliocene, the bauxite was exposure, contributed to the landslide in the slope, forming cavities and, consequently, the mobilized bauxite facies located downstream of them