Enriquecimento de elementos terras raras e ítrio adsorvidos em argilas (ion-adsorption clay) em perfis de intemperismo de granitos (sudeste do Estado de São Paulo)
Data
2018-11-29
Autores
Faria Júnior, Ilio Rodarte [UNESP]
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ISSN da Revista
Título de Volume
Editor
Universidade Estadual Paulista (Unesp)
Resumo
A natureza da mobilização, enriquecimento e fracionamento dos elementos terras raras e o ítrio (ETR) nos processos de intemperismo ainda é pouco compreendida. Diversos trabalhos seguiram o entendimento de que esses elementos fossem imóveis perante o intemperismo, o que foi refutado pela descoberta dos depósitos de ETR do tipo ion-adsorption clay (IAC) no sudeste asiático na década de 60, onde se observou o transporte desses elementos no perfil laterítico seguido de acumulação por fixação em minerais neoformados. Os depósitos do tipo IAC se desenvolvem quando biotita e/ou muscovita sienogranitos portadores de minerais de ETR susceptíveis ao intemperismo são submetidos ao intemperismo tropical, onde os ETR são liberados de sua mineralogia original e acumulados no horizonte saprolítico ao serem adsorvidos por argilominerais cauliníticos neoformados. Com base nisso, foi feito um processo prospectivo, tomando como base as características químicas e mineralógicas dos granitos mineralizados em ETR do tipo IAC no sudeste asiático, onde se identificou corpos graníticos no Terreno Apiaí-Guaxupé com características prospectivas para esse modelo de mineralização. Os granitos São Francisco e Capão Bonito, corpos cálcio-alcalinos compostos por biotita sienogranitos metaluminosos a peraluminosos e portadores de minerais (fluor)carbonáticos de ETR, muito susceptíveis ao intemperismo, foram tomados como base para estudo da mobilidade desses ETR no meio exógeno. Nesses corpos foram identificados perfis lateríticos com profundidades médias de 6 a 7 metros, compostos por horizontes pedogenético e saprolítico, com extensa neoformação de argilominerais cauliníticos. O balanço de massa evidenciou a mobilidade desses metais em decorrência da variação da intensidade de intemperismo a que esses perfis foram submetidos, com maior acumulação dos ETR nos perfis com anomalias negativas de Ce/Ce*. A extração química de amostras dos saprolitos demonstrou que as argilas cauliníticas representam a principal fase portadora dos ETR nos perfis enriquecidos, com a retenção ocorrendo por adsorção desses metais. Nos perfis enriquecidos se identificou proporções desses metais adsorvidos em argilas superiores a 50%, com valores absolutos de ETR adsorvidos variando entre 332,8 e 901,67 ppm. Esse conjunto de resultados demonstra exemplares de enriquecimento exógeno de ETR compatível com o modelo do tipo ion-adsorption clay. Com isso, se descreve aqui o primeiro exemplo de enriquecimento de ETR do tipo IAC no sudeste do Brasil, contribuindo, assim, para evidenciar o potencial prospectivo para esse modelo de concentração exógena de ETR no Brasil, país portador de variada granitogênese e de extensos perfis lateríticos.
The nature of the mobilization, enrichment and fractionation of the rare earth elements and yttrium (REE) in weathering processes are still poorly understood. These elements have been considered immobile in weathering profiles, taking into consideration their behavior in other geological processes. In the 1960‟s, the discovery of the ion-adsorption clay (IAC) REE deposits in the southeast Asia, and subsequent studies at other locations, has pointed out to the mobilization of REE in weathered granites, with the possibility of accumulation and retention of these metals neoformed minerals. The IAC deposits are developed by weathering of biotite and/ou muscovite syenogranites that contain weakly resistant REE-bearing minerals. At those conditions, the REE are leached from weakly resistant minerals and accumulated in the saprolite horizon, where they are adsorbed onto the surface neoformed clayminerals, particularly those from the kaolinite group. In this study, prospecting REE IAC work in the southeast of Brazil began with a compilation of published geochemical data on granite occurrences, in order to compare them with the granites that originate IAC deposits in south of China. Results show that that tardi- to post-tectonic granites on the Apiaí-Guaxupé Terrain, especially those from the Itu Rapakivi Province, have geochemical and geological aspects that are similar to those of exogenous REE mineralized granites. In the present work, the exogenous mobility of REE is evaluated by the characterization of two weathering profiles in the São Francisco Granite and two in the Capão Bonito Granite, both from the Itu Rapakivi Province. Those batholiths are calc-alkaline and are composed of metaluminous to peraluminous biotite syenogranites with allanite and titanite. They also show pervasive hydrothermal alteration (greisen) that generate fluorite and REE (fluorine)carbonate minerals. These weathered granite profiles show an average depth of 6 or 7 meters, with a soil horizon, at top, and a thick saprolite horizon, where extensively kaolinite clay neoformation has occurred. The major elements content is combined with mineralogical characterization to assess the weathering process. Mass balance along these profiles has evidenced the REE mobility as result of the weathering intensity. Those profiles with negative anomalies of Ce/Ce* have presented systematic REE enrichment in the saprolite horizon. The sequential extraction indicates that kaolinite it is an important mineral to retain these metals in the weathering profile, even in those profiles with depletion of REE. The REE enriched profiles shows more than 50% of the total horizon REE content adsorbed by neoformed minerals in the saprolite horizon, with absolute contents of REE adsorbed between 332,8 and 901,67 ppm, with slightly tendency of preferential adsorption of the light REE (La – Sm) above heavy REE (Eu – Y). Although the pH range of the enriched profiles is higher than those of the Asian REE exogenous deposits, an IAC type REE enrichment is recorded in this study. Therefore, this work supports REE mobilization in weathered granite profiles in southeastern Brazil and pushes forward the prospective potential of this enrichment model.
The nature of the mobilization, enrichment and fractionation of the rare earth elements and yttrium (REE) in weathering processes are still poorly understood. These elements have been considered immobile in weathering profiles, taking into consideration their behavior in other geological processes. In the 1960‟s, the discovery of the ion-adsorption clay (IAC) REE deposits in the southeast Asia, and subsequent studies at other locations, has pointed out to the mobilization of REE in weathered granites, with the possibility of accumulation and retention of these metals neoformed minerals. The IAC deposits are developed by weathering of biotite and/ou muscovite syenogranites that contain weakly resistant REE-bearing minerals. At those conditions, the REE are leached from weakly resistant minerals and accumulated in the saprolite horizon, where they are adsorbed onto the surface neoformed clayminerals, particularly those from the kaolinite group. In this study, prospecting REE IAC work in the southeast of Brazil began with a compilation of published geochemical data on granite occurrences, in order to compare them with the granites that originate IAC deposits in south of China. Results show that that tardi- to post-tectonic granites on the Apiaí-Guaxupé Terrain, especially those from the Itu Rapakivi Province, have geochemical and geological aspects that are similar to those of exogenous REE mineralized granites. In the present work, the exogenous mobility of REE is evaluated by the characterization of two weathering profiles in the São Francisco Granite and two in the Capão Bonito Granite, both from the Itu Rapakivi Province. Those batholiths are calc-alkaline and are composed of metaluminous to peraluminous biotite syenogranites with allanite and titanite. They also show pervasive hydrothermal alteration (greisen) that generate fluorite and REE (fluorine)carbonate minerals. These weathered granite profiles show an average depth of 6 or 7 meters, with a soil horizon, at top, and a thick saprolite horizon, where extensively kaolinite clay neoformation has occurred. The major elements content is combined with mineralogical characterization to assess the weathering process. Mass balance along these profiles has evidenced the REE mobility as result of the weathering intensity. Those profiles with negative anomalies of Ce/Ce* have presented systematic REE enrichment in the saprolite horizon. The sequential extraction indicates that kaolinite it is an important mineral to retain these metals in the weathering profile, even in those profiles with depletion of REE. The REE enriched profiles shows more than 50% of the total horizon REE content adsorbed by neoformed minerals in the saprolite horizon, with absolute contents of REE adsorbed between 332,8 and 901,67 ppm, with slightly tendency of preferential adsorption of the light REE (La – Sm) above heavy REE (Eu – Y). Although the pH range of the enriched profiles is higher than those of the Asian REE exogenous deposits, an IAC type REE enrichment is recorded in this study. Therefore, this work supports REE mobilization in weathered granite profiles in southeastern Brazil and pushes forward the prospective potential of this enrichment model.
Descrição
Palavras-chave
Granito São Francisco, Granito Capão Bonito, Minerais (fluor)carbonáticos de ETR, Enriquecimento laterítico, Caulinita, São Francisco granite, Capão Bonito granite, REE (fluorine)carbonate minerals, Exogenous enrichment, Kaolinite