EVIDÊNCIAS DE ENRIQUECIMENTO DE ETR DO TIPO ION-ADSORPTION CLAY NO PERFIL DE INTEMPERISMO DO GRANITO SÃO FRANCISCO (SÃO PAULO, BRASIL): Evidences of ion-adsorption clay-type REY enrichment in a weathered crust of the São Francisco Granite (São Paulo, Brazil)

The rare earth elements and yttrium (REY) ion-adsorption clay deposits (IAC) are important in the global supply chain of these metals, and originated by the weathering of biotite and/or muscovite syenogranite carrying REY weatherable minerals, with enrichment of REY occurring by adsorpotion in neoformed clay minerals, mainly kaolinite and halloysite. Evidences of REY IAC enrichment is reported, for the first time, in the 6,1 to 7,8-m-thick weathering profile on the São Francisco Granite (SFG - shoutheastern Brazil), a late-tectonic metaluminous to peraluminous greisenized syenogranite. Petrography and SEM/EDS analysis indicates that the parent rock, a peraluminous synogranite, hosts REY-(fluor)carbonate minerals originated by alteration process. X-ray diffraction of the weathering profile, with soil and saprolite horizons, indicates the presence of neoformed kaolinite and halloysite. Chemical analysis of the weathered profile indicates REY content modification along the weathering horizons. The enrichment factor presents REY loss in horizons with positive cerium anomalies and a high chemical index of alteration (CIA>90). Meanwhile, REY gain was observed in saprolite horizon with negative cerium anomalies and lower chemical alteration index (CIA<90 and >70). Mass balance calculation demonstrates the REY mobility and mass gain in the weathering profile in similar conditions to the Asian REY-IAC deposits.Os depósitos de elementos terras raras e ítrio (ETR) do tipo ion-adsorption clay (IAC) apresentam grande importância para o suprimento global desses metais e se originam pelo intemperismo de biotita e/ou muscovita sienogranitos portadores de minerais de ETR susceptíveis ao intemperismo, cujos ETR são enriquecidos por adsorção em caulinita e halloysita neoformadas. Evidências desse tipo de enriquecimento de ETR foram reconhecidas em perfil de intemperismo, com espessuras de 6,1 a 7,8 metros, no Granito São Francisco (GSF – sudeste do Brasil), corpo tardi-tectônico metaluminoso a peraluminoso submetido a greisenização. Petrografia e MEV/EDS indicaram a presença de minerais (fluor)carbonáticos de ETR, susceptíveis ao intemperismo, originados por processo de alteração no sienogranito. Difrações de raios X demostraram neoformação de caulinita no horizonte saprolítico, enquanto análises químicas revelaram modificações de conteúdo de ETR ao longo do perfil de intemperismo. O fator de enriquecimento indicou perda de ETR nos horizontes com Ce/Ce*>1 e valores de CIA>90, enquanto se identificou enriquecimento de ETR no saprolito com Ce/Ce*<1 e valores de 70<CIA<90. O balanço de massa demonstrou a mobilidade dos ETR e ganho de massa desses metais no perfil de intemperismo, condição essa similar aos depósitos de ETR do tipo IAC identificados no sudeste asiático

CARACTERIZAÇÃO TEXTURAL E MINERALÓGICA DAS ROCHAS DO MORRO DO NÍQUEL (MG) E CORRELAÇÕES COM O PROCESSO DE SERPENTINIZAÇÃO: Textural and mineralogical analisys of Morro do Níquel rocks (MG) and correlations with serpentinization

Morro do Níquel, a serpentinitic massif located in the southern extension of the Southern Brasilia Belt, in the Passos Nappe context, was the subject of a detailed textural and mineralogical study, developed on a drillhole, aiming to characterize the rocks present there. This massif has an ellipsoidal shape, with dimensions of 400 x 800 meters and thickness greater than 300 meters, composed only by serpentinite. This serpentinite has a predominance of mesh, mesh-ribbon and ribbon textures, consisting of lizardite / chrysotile and pseudomorphs that refer to preterit olivine crystals. The homogeneity of the studied samples indicates that the protolith was a rock with granular texture, medium to coarse granulation (> 0.5 cm) and constant mineralogy, possibly a dunite. The conditions described for that massif indicate two metamorphic events, an older one that developed in amphibolite facies, and a younger one, developed in prehnite-pumpellyite facies, but that partially preserved features of the past event. The presence of awaruite and heazlewoodite suggests a serpentinization developed in low O2 and S2 fugacity. Petrographic, lithological and metamorphic analysis shows that this body has distinct characteristics from serpentinites of the Greenstone Belt Morro do Ferro, an archean metavolcanossedimentary unit to which it is currently associated.O Morro do Níquel, maciço serpentinítico situado na extensão sul da Faixa Brasília Meridional, no contexto da Nappe de Passos, foi objeto de estudo mineralógico e textural detalhado, desenvolvido com base em testemunho de sondagem, visando caracterizar os litotipos ali presentes. Esse corpo apresenta formato elipsoidal, com dimensões de 400 x 800 metros e espessura superior a 300 metros, constituído, unicamente, por serpentinito. Esses serpentinitos apresentam predominância de texturas mesh, mesh-ribbon e ribbon, constituídas de lizardita/crisotilo e pseudomorfos que remetem a cristais reliquiares de olivina. A homogeneidade das amostras estudadas indica que o protólito potencialmente era composto de rocha com textura granular, granulação média/grossa (>0,5cm) e mineralogia constante, correspondendo, possivelmente, a um dunito. Nesse maciço são descritas condições indicativas de dois eventos metamórficos, um mais antigo que se desenvolveu em fácies anfibolito, e um mais jovem, em fácies prehnita-pumpellyita, mas que preservou, parcialmente, feições do evento pretérito. A presença de awaruita e heazlewoodita descreve uma condição de serpentinização em condições de baixa fugacidade de O2 e S2. A análise petrográfica, litológica e metamórfica aponta que esse corpo guarda características distintas dos serpentinitos do Greenstone Belt Morro do Ferro, unidade metavulcanossedimentar arqueana à qual é atualmente associado

On the link between soil hydromorphy and geomorphological development in the Cerrado (Brazil) wetlands

Mound fields (MF), which consist of numerous isolated mounds, are characteristic of the Cerrado wetlands of Brazil. While there are numerous studies on soil redistribution by termites and by differential particulate erosion, a new hypothesis, supported by metal accumulation in these areas, needs to be tested. This study aimed to investigate the extent to which soil saturation by water, and the associated chemical erosion, might be a key driver of the geomorphological development of the Cerrado wetlands. The study was performed in a typical wetland of Minas Gerais that shows a typical succession from well drained Ferralsols on the plateau to Gleysols on the lowland and the presence of MF at the transition. Soil morphological description was used together with optical microscopy, X-ray fluorescence and physical analyses (grain size and bulk density) to discriminate between the biological (bioturbation) and geochemical origins. Additionally, geochemical mass balance was calculated to estimate the total amount of chemical erosion that had occurred in the area. The morphological observations and the results on soil texture did not point to material redistribution in the MF. In contrast, the results showed that iron-depleted aggregates in depressed zones were sensitive to collapse. Soil saturation by water as the main cause of geochemical erosion and geomorphologic development was confirmed by mass balance calculation. The results fill an important information gap on the formation and evolution of cerrado soil landscapes, which have important consequences for biodiversity and water conservation