G?nese, distribui??o e caracteriza??o dos espongilitos do noroeste de Minas Gerais.

Este estudo est? relacionado aos dep?sitos de espongilito que ocorrem em lagoas rasas associadas a superf?cie de eros?o desenvolvidas sobre rochas do Supergrupo S?o Francisco (Neoproteroz?ico). Coberturas Cenoz?icas de sedimentos silicicl?sticos est?o associados ? esta superf?cie que tem nas adjac?ncias as rochas do Grupo Areado indiviso (Mesoz?ico). Estudos de campo associados ? tratamentos de imagens multiespectrais e SRTM contribu?ram para o conhecimento do contexto geol?gico geomorfol?gico no qual inserem as lagoas com os dep?sitos de espongilitos. A evolu??o da paisagem de cerrado na regi?o de Jo?o Pinheiro desenvolveu se a partir de processos erosivos significativos que dominaram a regi?o ap?s o Cret?ceo, gerando a forma??o de um planalto dissecado onde 4 morfodom?nios foram identificados: i) morfodom?nio 1, representado por plat?s associado aos arenitos do Grupo Areado, apresenta as maiores altitudes da ?rea; ii) morfodom?nio 2 que constitui uma ?rea dissecada associada aos pelitos do Grupo Areado; iii) morfodom?nio 3, representado por superf?cies de eros?o associadas ?s rochas do Grupo Bambui e Pr? Bambu?, sendo sobrepostas por sedimentos terci?rios/quatern?rios, onde encontram se as lagoas; iv) morfodom?nio 4, constitui vales em calha que cont?m as principais drenagens da regi?o (rios da Prata e Paracatu) segundo um padr?o meandrante, com fei??o geomorfol?gica fluvial de rios decapitados. Estes vales cortam a superf?cie de aplainamento (morfodom?nio 3) que cont?m sedimentos pleistoc?nicos, caracterizando o morfodom?nio mais recente. A dire??o preferencial de SE NW para o sistema de drenagens, sugere que os arenitos do Grupo Areado constituem a ?rea fonte de sedimentos hospedeiros dos dep?sitos de espongilito. A caracteriza??o mineralogica e textural por microscopia ?ptica, difratometria de raios X (DRX), an?lise t?rmica diferencial e termogravim?trica (ATD/ATG), espectroscopia do infravermelho (IR), microscopia eletr?nica (MEV), de amostras de sete lagoas de espongilito (Avi?o, Carvoeiro, V?nio, Pregui?a, Divisa, Severino, Feij?o) mostrou uma similaridade sedimentol?gica entre os dep?sitos. Esses apresentam forma lenticular e s?o caracterizados por f?cie arenosa na base, areno argilosa rica em esp?culas na por??o intermedi?ria e f?cie areno argilosa rica em mat?ria org?nica no topo. Petrograficamente, os sedimentos hospedeiros dos espongilitos e os sedimentos silicicl?sticos do Grupo Areado apresentam fases detr?ticas com caracter?sticas mineral?gicas e texturais semelhantes, como a presen?a de gr?os de quartzo bem selecionados e superf?cies de abras?o, t?picas de retrabalhamento e?lico. Esse retrabalhamento ocorre no ambiente deposicional em que os arenitos xxii do Grupo Areado formaram se. Minerais pesados como a estaurolita, turmalina e zirc?o e argilominerais como caolinita, tra?os de ilita, chlorita, interestratificados clorita/esmectita e illita/esmectita ocorrem tanto nos sedimentos formadores dos dep?sitos de espongilito quanto nos arenitos Areado. Essas caracter?sticas mineral?gicas e texturais sugerem que os sedimentos do Grupo Areado constituem a principal fonte dos sedimentos das lagoas que formam os dep?sitos de espongilito. A caracteriza??o microsc?pica das esp?culas levou ? identifica??o espec?fica das seis esp?cies de esponjas de ?gua doce que comp?e a t?pica espongof?cies dos dep?sitos de espongilitos brasileiros: Metania spinata (Carter, 1881), Dosilia pidanieli Volkmer Ribeiro, 1992, Trochospongilla variabilis Bonetto & Ezcurra de Drago, 1973, Corvomeyenia thumi (Traxler, 1895), Heterorotula fistula sp. n. Volkmer Ribeiro & Motta (1995) e Radiospongilla amazonensis Volkmer Ribeiro & Maciel 1983. A varia??o da composi??o dessas assembl?ias, o est?gio de forma??o das esp?culas e sua conserva??o nos distintos horizontes, al?m da presen?a de diatom?ceas e outros compostos abi?ticos sugerem uma varia??o ambiental na ?poca da forma??o destes dep?sitos, alternando um clima mais ?mido e frio e clima mais seco com chuvas torrenciais. Data??es de C14 e ?13 C estabeleceram idades de forma??o destes dep?sitos entre o Pleistoceno Superior/Holoceno. Os dep?sitos de espongilito apresentam de 3 a 80% de s?lica amorfa (opala), que corresponde principalmente a esp?culas de esponjas. Entretanto, os padr?es morfol?gicos/granulom?tricos das esp?culas assim como a presen?a de fases detr?ticas associadas controlam as diferen?as em qualidade dos espongilitos, refletindo a necessidade de an?lises f?sicas e mineral?gicas de modo a qualificar industrialmente estas varia??es. Particularmente para os dep?sitos Avi?o e Pregui?a, as f?cies intermedi?rias apresentaram os maiores teores com esp?culas mais robustas caracterizando espongilitos de alta qualidade. Apesar das an?lises granulom?tricas evidenciarem varia??es de tamanho (240 a 450gm) estas varia??es podem ser consideradas negligenci?veis para sua aplica??o industrial.This study relates to the spongillites deposits that occurs in shallow ponds on a karstic planation surface developed on rocks of the Neoproterozoic S?o Francisco Supergroup. Cenozoic siliciclastics sediments are related to this surface and Mesozoic Areado Group rocks occur adjacent to the deposits. Field studies associated with the analysis of multispectral images have contributed to the knowledge about the geological and geomorphological context in which the lakes with the spongillites deposits occur. The Cerrado landscape evolution from the Jo?o Pinheiro region where the spongillite deposits occur have been controlled by important erosive processes after the Cretaceous that led to the formation of a disssecated high plain where 4 geomorphological domains were identified: i) domain 1, represented by plateaux related to sandstones from the Areado Group that show the highest altitudes in the area; ii) domain 2 that constitutes a dissecated area related to pelites from the Areado Group; iii) domain 3, comprising of a karstic planation surface associated to the carbonatic rocks from Bambui Group and Pr? Bambu? unit covered by Cenozoic sediments where the lakes occur; iv) domain 4 is composed of trough shaped valleys enclosing meandering underfit streams (rivers Prata and Paracatu). These valleys cut through the karstic planation surface (domain 3) where Pleistocene sediments occur and characterize the more recent morphological domain. The preferential direction of SE NW of the drainage system suggests that the sandstones of the Areado Group constituted the source area of the spongillite hosting sediments. Mineralogical and textural characterization by optical microscopic analysis, X ray diffraction (XRD), differential and gravimetric analysis (DTA GTA), infrared spectroscopy (IR), and scanning electron microscopy (SEM) of seven open pit deposits (Avi?o, Carvoeiro, V?nio, Pregui?a, Divisa, Severino, Feij?o) showed a sedimentological similarity between the deposits. They are lens shaped and are characterized at the bottom by sand facies, in the middle by spicules rich muddy sand facies and at the top by organic matter rich muddy sand facies. Petrographically, the spongillite hosting sediments and the siliciclastic sediments of the Areado Group show detrital phases with similar mineralogical and textural features, such as the presence of well sorted quartz grains and surface features of abrasion typical of Aeolian reworking that occurred in the depositional environment in which the sandstone of the Areado Group were formed. Detrital heavy minerals, such as staurolite, zircon, tourmaline, and clay minerals, such as xxiv kaolinite, low amounts of illite, scarce chlorite and muxed layer chlorite/smectite and illite/smectite occur in the spongilite hosting sediments and in sandstones from the Areado Group. These mineralogical and textural features show that the sediments of the Areado Group constitute the main source of the pond sediments that host spongillite. The examination of the spicules by light microscopy showed spongofacies of six sponges typical of Brazilian spongillite deposits: Metania spinata (Carter, 1881), Dosilia pidanieli Volkmer Ribeiro, 1992, Trochospongilla variabilis Bonetto & Ezcurra de Drago, 1973, Corvomeyenia thumi (Traxler, 1895), Heterorotula fistula sp. n. Volkmer Ribeiro & Motta (1995) e Radiospongilla amazonensis Volkmer Ribeiro & Maciel 1983. The variation in composition of the assemblages and the stage of formation and preservation of the spicules in the different facies, as well as the presence of diatoms and abiotic components, suggest variation at the time of their formation and deposition. C14 radiocarbon and stable isotope ratio ?13 C analysis showed an age for the formation of the deposits between the Late Pleistocene and Holocene. The spongillite deposits show percentages between 3 to 80% of amorphous silica (opal) mainly represented by spicules. The morphological/grain size patterns of spicules as well as the associated minerals point towards a quality difference in the spongillites formed, reflecting the need for physical and mineralogical analysis in order to qualify industrially these differences. Particularly for the Pregui?a and Avi?o deposits the intermediate facies showed the highest content of robust spicules characterizing prime quality spongillites. The grain size variations of the spicules (240 a 450gm) observed in these facies were negligible for their industrial applicatio